composition and bandgapgraded semiconductor alloy nanowires · 2019-12-17 · taneously alloy...

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Xiujuan Zhuang, C. Z. Ning, and Anlian Pan*

Composition and Bandgap-Graded Semiconductor Alloy Nanowires

Semiconductor alloy nanowires with spatially graded compositions (and bandgaps) provide a new material platform for many new multifunctional optoelectronic devices, such as broadly tunable lasers, multispectral photo-detectors, broad-band light emitting diodes (LEDs) and high-efficiency solar cells. In this review, we will summarize the recent progress on composition graded semiconductor alloy nanowires with bandgaps graded in a wide range. Depending on different growth methods and material systems, two typical nanowire composition grading approaches will be presented in detail, including composition graded alloy nanowires along a single substrate and those along single nanowires. Furthermore, selected examples of applications of these composition graded semiconductor nanowires will be presented and discussed, including tunable nanolasers, multi-terminal on-nanowire photo-detectors, full-spectrum solar cells, and white-light LEDs. Finally, we will make some concluding remarks with future perspectives including opportuni-ties and challenges in this research area.

1. Introduction

Semiconductor nanowires have been proposed[1–3] and demon-strated as versatile building blocks for future optical and elec-tronic devices, such as broadly tunable nanolasers, white light emitting diodes (LEDs), full-spectrum solar cells and high effi-ciency photodetectors.[4–11] For example, individual nanowires intrinsically can be used simultaneously as good waveguides[12] or laser cavities[13–18] and optical gain materials,[19,20] and can act as nanoscale lasers.[16–18,21–25] Bandgap is one of the most impor-tant parameters of semiconductor materials and it decides the basic spectral features, including the operating wavelength, the absorption and emission processes of a semiconductor based optoelectronic device. Since the available bandgaps of natural semiconductors is very limited, alloying of semiconductors with different bandgaps have usually be used to achieve new

© 2012 WILEYVCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2012, 24, 13–33

Dr. X. Zhuang, Prof. A. PanCollege of Physics and Microelectronics Science Key Laboratory for MicroNano Physics and Technology of Hunan Province Hunan University Changsha, 410082, China Email: [email protected]. C. Z. NingSchool of Electrical, Computer and Energy Engineering Arizona State University AZ, Tempe, 85287, USA

DOI: 10.1002/adma.201103191

and especially tunable bandgaps. It is dif-ficult and even impossible to grow simul-taneously alloy semiconductor materials with different or tunable bandgaps on a single substrate by the conventional planar epitaxial methods, because of the restric-tion of lattice match between the substrate and the semiconductor alloys to be grown. This need of lattice match in material growth has long been the main obstacle of developing semiconductor-based multi-functional, broad spectrum or wavelength-tunable optoelectronic devices. However, using the newly developed metal cata-lyzed vapor-liquid-solid (VLS)[26,27] growth technology, semiconductor nanowires can be grown on all kinds of substrates and even an amorphous substrate just for a mechanical support, without considering of the lattice match. Such VLS based nanowire growth technology brings the

chances to realize the growth of semiconductor materials with largely different compositions or bandgaps,[9,10,28–31] and even alloy nanowires with gradient composition (graded bandgap) on a same substrate.

Composition-graded alloy semiconductor nanostructures on a single substrate (so called single substrate based spa-tial semiconductor composition grading) can work as a new material platform used in super broadly tunable nanolasers, color engineered display and lighting, multispectral detectors and full spectrum solar cells. For example, semiconductor composition grading can potentially allow bandgaps tailored to match the full solar spectrum for maximum conversion efficiency.[6,7] Spatial alloy composition engineering and con-trol on a single substrate can lead to on-chip color design for display applications; Bandgap-graded semiconductor struc-tures can also help to realize new multispectral detection or spectrometer-on-a-chip. In addition to alloy composition grading on a single substrate, another more challenging task is to achieve controlled composition grading along the length of a single nanowire. Different from the well reported axial nanowire hetero structures which involves two or several dif-ferent semiconductor materials or compositions,[32–39] com-position grading on a single wire involves gradually changing multi-composition/energy gaps. Such new nanowire struc-tures have novel physical properties and show fascinating application potential of achieving multi-functional optoelec-tronic devices within a single wire such as nanoscale multi-terminal photodetectors, on-nanowire white light LEDs, full spectrum solar cells and so on.

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her PhD from University of Science and Technology of China in 2008. From 2008 to 2011, she worked as a Postdoctoral Fellow in Arizona State University. Early in 2011, she joined the Key Laboratory for MicroNano Physics and Technology of Hunan Province in Hunan Univerisity as an

Associate Professor. Her current research interests lie in the synthesis and physical properties of semiconductor nanostructures.

Cun-Zheng Ning received PhD in physics from the University of Stuttgart in1991. He was a senior scientist, Nanophotonics Group leader and Nanotechnology Task manager at NASA Ames Research Center from 1997 to 2007, and an ISSP Visiting Professor at University of Tokyo in 2006. He is a Full Professor of Electrical

Engineering at Arizona State University since 2006. His current interests include nanowires and plasmonic nanophotonics. Further information can be found at http://nanophotonics.asu.edu.

Anlian Pan received his PhD in Condensed Mater Physics from the Institute of Physics, Chinese Academy of Sciences, in 2006. From 2006 to 2007, he worked as a Humboldt Research Fellow with Prof. Ulrich Gösele at the Max Planck Institute of Microstructure Physics. Later in 2007, he joined Arizona State University

as a Postdoctoral Fellow, where he became a Research Assistant Professor. He is currently a Full Professor of Physics in Hunan University, and is the director of the Key Laboratory for MicroNano Physics and Technology of Hunan Province. His current research interests lie in the synthesis and physical properties of semiconductor nanostructures, with an emphasis on composition gradient nanomaterials.

In this review, we will summarize the recent progress on bandgap-graded semiconductor alloy nanowires. Different growth methods and material systems of semiconductor nanowires with composition grading on a single substrate or on a single wire will be presented and summarized in detail. The applications of these composition graded alloy semicon-ductor nanowires in new optoelectronics devices will also be discussed. Finally, concluding remarks are made with future perspectives in the last section.

2. Spatially Graded Bandgap Engineering on a Single Substrate

Since the VLS-based nanowire growth technology can remove or greatly relax the requirement of lattice matching between the grown materials and the substrate, and nanowires can even be grown on an amorphous substrate, this growth technology pro-vides the possibility of spatially grading the bandgaps through compositions on a single substrate. Here we will summarize the recently developed growth strategies for achieving single substrate based nanowire composition grading. These strategies include the Temperature Gradient method, the Spatial Source Reagent Gradient method, and the Dual Gradient method.

2.1. Temperature Gradient

Due to the differences in condensation points and growth kinetics, semiconductor alloys with different compositions or bandgaps usually require different deposition temperatures.[40] During the catalyst-assistant VLS growth of alloy nanowires, wires with different alloy compositions could deposit at dif-ferent positions with different local temperatures in the growth zone. Especially, during the VLS nanowire growth with chemical vapor deposition (CVD) route, the grown wires with gradually changed compositions are often deposited along the length of the tube, where a temperature gradient exists in the axial direction. The temperature-dependent composition vari-ations have been observed in the nanowire growth of many alloy systems, such as the II–VI group chalcogenide and the V group silicide, including CdSSe, GeSi, ZnCdS etc. Using this principle, it is possible to realize the growth of a spatial alloy composition grading, i.e. composition or bandgap-graded alloy nanowires grown on a single substrate with a temperature gra-dient along its length.

Through the temperature gradient controlling in the growth with a CVD route, Pan et al. achieved a spatial nanowire com-position grading covering the complete composition range of ternary alloy CdSSe on a single substrate.[4] The exceptional composition graded sample was grown by a specially designed short furnace with a short heating zone and an efficient cooling system equipped. Before the growth, the source mixture of CdS and CdSe powders (mole ratio 1:1) was placed onto a ceramic plate at the center of a quartz tube (1′′ diameter) inside a small horizontal tube furnace. A substrate (glass, silicon, or silicon dioxide) of ∼1 cm in length, coated with Au film ∼1 nm in thickness, was placed 3 cm downstream from the center of the source-material-carrying ceramic plate. High-purity He gas

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was introduced into the quartz tube with flow rate of 200 sccm to purge the O2 for 100 min and then adjust the He flow rate down to 50 sccm before heating up the furnace tube rapidly to

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Figure 1. (a) Typical measured temperature profile along the furnace axis for the nanowire growth. Dotted lines indicate the temperature and space intervals for the substrate placement for optimized sample growth. The realcolor photographs of a quartz substrate with asgrown spatially compositiongraded CdSSe nanowires under room lighting is shown in the inset. (b) Substrate temperature (left coordinate) and Smolar fraction (right coordinate, extracted from the EDX measurement at different points along the substrate length.) (c) MicroPL spectra detected at different locations along the substrate at cryostat temperature of 77 K. (d) PL peak wavelengths as a function of substrate length coordinate. Reproduced with permission.[4] Copyright 2009, American Chemical Society.

1000 °C in the central heating zone. The deposition process was maintained at this temperature for 60 min before cooling down naturally. Figure 1a shows the temperature distribution profile of the furnace. A large temperature variation from ∼580 to ∼690 °C exists along the length of the substrate with only a spatial interval of 1.2 cm, as shown in the inset of Figure 1a. The temperature range along the substrate covers the required growth temperature for each single composition of CdSSe alloys.[9,10] The composition of the as-grown CdSxSe1-x alloy nanowires along the substrate gradually changed from CdSe to CdS within temperature interval of 580–690 °C.

The real-color photographs under regular room lighting of the as-grown nanowires on a quartz substrate were shown in the inset of Figure 1a. The color of the as-grown sample was observed changing gradually from light-yellow (corresponding to the color of pure CdS source powder) to dark (corresponding to that of pure CdSe source powder) along the substrate length, while a real color photograph obtained under a UV–visible laser (266 nm) illumination exhibits the color variation from green to red, con-sistent with the micro-photoluminescence (micro-PL) measure-ments (Figure 1c). The substrate was covered with nanowires with typical diameter of ∼200 nm and length up to several tens of micrometers. The measured substrate temperature and the measured S-molar fraction at the corresponding positions along the full length of the substrate were plotted in Figure 1b. All the nanowires along the substrate were composed of Cd, S, and Se, and the Se-fraction was complementary to that of S, which

© 2012 WILEYVCH Verlag GmbH & Co. KGaA, WeinhAdv. Mater. 2012, 24, 13–33

directly confirmed that the obtained sample is the spatially composition graded nanowires on the single substrate. The composition graded CdSxSe1-x alloys was also obviously confirmed by micro-PL measurements. The micro-PL spectra taken at fifteen different points along the substrate at 77 K were shown in Figure 1c, where no surface/defect state related mid-gap emissions were observed for any measured positions or compositions, which indicate the high quality of these as-grown nanowires. The PL peaks gradually varied between 498 nm and 692 nm, covering a large por-tion of the visible spectrum. The gradually varied emission wavelengths indicated the formation of composition gradient CdSxSe1-x (0 ≤ x ≤ 1) alloys along the substrate. A more refined spatially resolved measurement of the PL peaks was shown in Figure 1d. From the comparison between Figures 1b and 1d, the PL peak wavelength is directly related to the spa-tial dependence of the alloy composition. To grow CdSSe nanowires in the full range of alloy composition on a single substrate in a single run of growth, comprehensive optimi-zation involving substrate locations, flux rates of semiconductor vapors, and most impor-tantly, control of the temperature gradient are very important.[11] The temperature gradient distribution is the dominant factor to pro-duce this composition graded alloy CdSxSe1-x nanowires.

Composition varying ternary alloys CdSxSe1-x and ZnyCd1-yS nanoribbons were also synthesized due to the temperature dependent growth, using a laser ablation assistant thermal evap-oration route.[11,29,41] The as-synthesized CdSxSe1-x nano ribbons was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and micro-area X-ray diffraction (XRD). The nanoribbons have a uniform cross section with typical width of 0.4–10 μm, thickness of 65–80 nm, and length up to several hundred micrometers. These ribbons also have smooth surface and little variation of their morphologies with the temperature variation of 450–600 °C along the substrate. High-resolution transmission electron microscopy (HRTEM) investigations con-firm the single-crystal nature of all these composition graded nanoribbons. The PL emission band was observed varying con-tinuously between ∼510 nm and ∼710 nm as a function of the nanoribbon growth position on the silicon substrate, which indicated that the composition x value of these CdSxSe1-x rib-bons can be continuously tuned in the entire composition range of 0 < x < 1.

Synthesis of ternary alloy ZnyCd1-yS single-crystal nanorib-bons was also achieved similarly.[29] The experimental approach is similar to that of CdSxSe1-x nanoribbons described above.[41] In the growth process of ZnyCd1-yS nanoribbons, ZnS and CdS powder were used as the Zn and Cd source, respectively. The PL spectra of the achieved ZnyCd1-yS nanoribbons with varied Cd concentration have near-bandedge emission covering from

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∼340 nm (pure ZnS)[42] to ∼515 nm (pure CdS)[43] as the Cd
concentration increased. Compared to the continuously varied PL emission wavelength from 510 to 710 nm in the entire composition of CdSxSe1-x nanoribbons, two continuously varied emission regions of 485–515 nm and 340–390 nm were achieved in these ZnyCd1-yS nanoribbons, which corresponding to the composition y values in the range of 0 ≤ y ≤ 0.25 and 0.75 ≤ y ≤ 1, respectively. The position of the maximum PL peak as a function of composition y (determined from XRD meas-urement) is consistent with a previously determined composi-tion dependent bandgap energy (Eg) of ZnyCd1-yS films.[44] The near-bandedge emission in ZnyCd1-yS nanoribbons is in good agreement with the bandgap values determined from the com-position. The high crystalline quality of the ZnyCd1-yS nanorib-bons in these two regions is confirmed by the full width at half maximum (FWHM) values (∼13.5 nm and 9.2 nm for samples close to ZnS and CdS, respectively) of the near-bandedge emis-sion. For the intermediate Cd composition region with 0.25 ≤ y ≤ 0.75, the PL shows a broad band emission, attributed to defect-related emission.

Another example of composition varying nanowires, ConSi (n = 1–3), grown using the temperature gradient method was reported by Seo et al.[45] They realized a simultaneous and selec-tive synthesis of single crystal ConSi nanowires with different Co compositions. The single-crystalline nanowires were syn-thesized in a horizontal hot-wall two-zone furnace with a 1′′ inner diameter quartz tube, as shown in Figure 2a, where the upstream zone and downstream zone were used for the vapori-zation of precursor and the nanowire growth, respectively. The Si source comes from a rectangular Si wafer located at the downstream zone and no catalyst was used for the synthesis. Temperatures at the upstream and downstream zone were maintained at 610 and 900 °C, respectively, and the reaction

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Figure 2. Experimental setup (in left panel) and the change of the morphologrowth environments (in right panel). (a) Horizontal tube furnace with twothat the center of the downstream zone is at 900 °C, and the substrates arpanel a. A rectangular Si wafer (50 mm × 15 mm) kept at the downstream zgrown on the sapphire 1 substrate, CoSi nanowires on the Si, and Co3Si N(g) sapphire substrate 1, and (j) sapphire substrate 2 in (c). The lowresoluare shown in the second and third column. Reproduced with permission.[45

process was maintained under this temperature profile for 20 min. The CoSi nanowires were grown on the Si substrate,[46] while the Co2Si and Co3Si nanowires were grown on c-plane sapphire substrates placed on the Si wafer.[47] The sapphire sub-strate only works as supporting plate on which the nanowires grow through gas phase reaction. The composition ratio of Co-Si was determined by the concentration of SiCl4, which was dependent on the reaction temperature. According to the tem-perature profile in the downstream zone shown in Figure 2b, the concentration of SiCl4 could be varied by adjusting the posi-tion of the sapphire substrate. So the formation of Co2Si and Co3Si is indirectly dependent on the temperature. The Co-Si ratio of the nanowires grown on sapphire 1 was lower than that on sapphire 2, because the temperature on sapphire 1 was higher. So the ConSi nanowires with different element compo-sition grow simultaneously on the substrate along the tempera-ture gradient, as shown in Figure 2c.

The morphologies and elemental composition of as-synthesized ConSi nanowires examined by TEM, SEM and energy-dispersive X-ray spectroscopy (EDX) were shown in Figures 2d–2l. The Si concentration of nanowires on different substrate (thus different temperature distribution) was measured to be 50%–15%. Three kinds of crystalline structures, which depend on the temperature gradient in this experimental approach, were confirmed by TEM examinations as simple cubic CoSi,[46] orthorhombic Co2Si, and face centered cubic (fcc) Co3Si. The CoSi nanowires were syn-thesized on a Si wafer[48] with single crystalline structure and Si concentration of 33%, as shown in Figure 2i. The orthorhombic Co2Si with single crystalline structure was characterized and con-firmed by XRD and HRTEM measurement. This experimental approach is a valuable strategy to grow nanowires of different compositions and crystalline structures using temperature pro-file inside the tube reactor.


gy and elemental concentrations of Si in the nanowires grown at different independently controlled heating zones. (b) Temperature profile indicates e at 820–890 °C. (c) Tilted view illustration of the substrate placement in one played a role of Si source for nanowire synthesis. Co2Si nanowires are Ws on the sapphire 2. In the right panel, naowires grown on (d) Si wafer, tion TEM images and TEMEDS spectra corresponding to (d), (g), and (j) ] Copyright 2009, American Chemical Society.


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Temperature gradient is a versatile strategy to achieve spa-tial composition grading of semiconductor nanowires in alloy systems with composition selected deposition temperature, such as CdSxSe1-x, ZnyCd1-yS and ConSi. This strategy is easy to manipulate and applicable to many other systems. However, this method does not work well in alloy systems with similar deposition temperature of the reagent vapors.

2.2. Spatial Source Reagent Gradient

Spatial Source Reagent Gradient method was firstly used to grow InxGa1-xN alloy nanowires, with the full range of compositions

© 2012 WILEYVCH Verlag GAdv. Mater. 2012, 24, 13–33

Figure 3. (a) Experimental setup. The reactor consists of three inner quartand an outer quartz tube, which supplies inert gas (N2) and houses the retapes were used to tune the vapour pressure of the InCl3 and GaCl3 precInGaN compositional gradient. Shown below the furnace is the temperatuwhereas the substrates are at ∼550 °C. Inset: Photograph of an asmade sa(right). The Optical characterization of the InGaN nanowires from (b) Colospectra (x = 0–1.0) of the InxGa1-xN nanowire arrays taken at intervals acrsion.[49] Copyright 2007, Nature Publishing Group.

between InN and GaN achieved on a single substrate.[49] In their experiments, they used spatially configured mini-tubes as chan-nels of different reagent vapors. When the reagent vapors from different channels were transported independently to different spatial areas of the substrate, they form spatial composition gra-dient along the substrate. More specifically, a horizontal tube furnace with two independently controlled heating elements was used to form four temperature zones in their experimental setup, as shown in Figure 3a, and In, Ga, N elements were produced from InCl3, GaCl3 and NH3, respectively. The InCl3 and GaCl3 precursors were transported by N2 through sepa-rate quartz tubes (1/4′′ inner diameter) and were mixed with NH3, which was carried through the third tube, about 0–2 cm

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z tubes, which supply the reactive gases, InCl3, GaCl3 (N2 carrier) and NH3, action in a horizontal tube furnace. Two independently controlled heating

ursors. The positioning of the reactive gas outlets results in the observed re profile, indicating that the centre of the furnace is maintained at 700 °C, mple on quartz (left) and a colour image from PL of a section of substrate ur CCD images, (c) visible PL emission (x = 0–0.6), (d) optical absorption oss the substrates with varying concentration x. Reproduced with permis

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Figure 4. (a) Setup and tube configuration for the growth of the 1D graded sample, (b) temperature profile at the sample growth zone, and (c) setup and tube configuration or the growth of the 2D graded sample. Reproduced with permission.[51] Copyright 2010, American Chemical Society.

downstream of the outlets. The Si or sapphire substrate without catalyst was placed per-pendicular to the outlets, so that the com-positions of the nanowires were a result of precursor-mixing (gradient), while tempera-ture across the substrate was not intention-ally varied. The tubes were stacked in a tripod configuration and positioned in order to keep the outlets in the appropriate temperature region of the furnace. The temperature at the center of furnace and on the substrate was 700 °C and 575–525 °C, respectively.

A variety of colors ranging from clear to light yellow on the GaN side to reddish-black on the InN side was observed on substrates, as shown in the inset of Figure 3a. The Ga-rich and In-rich nanowires have diameters of 10–50 nm and 100–250 nm, and lengths of 0.5–1 μm and 1–2 μm, respectively. The wurtzite crystal structure was confirmed from the XRD patterns. The nanowires are single crystalline through the TEM analysis, and no phase separations were observed due to the absence of multiple sets of peaks in XRD patterns.[50] A near-linear relationship was observed between the lattice spacing and the spatially graded indium concentration in the alloyed InxGa1-xN nanowires. The optical properties of the nanowires were character-
ized by several techniques and the results clearly demonstrate a shift in bandgap as a function of composition along the length of the substrate. The color CCD images in Figure 3b, displaying the light emissions in the visible region, were taken at regular intervals along the substrate under the pumping of a contin-uous-wave He-Cd laser at 325 nm. Figure 3c shows the normal-ized PL spectra of the InxGa1-xN nanowires in the wavelength range of 325–850 nm, and the PL peaks shift to longer wave-lengths with increasing In concentration. The optical absorp-tion spectra for the nanowires plotted as a function of photon energy were shown in Figure 3d, where the absorption edge also shifts systematically to lower energy as In concentration increasing. All those results demonstrated that the composition and energy bandgap gradient (from the near-ultraviolet to the near-infrared region) was achieved based on the synthesized single-crystalline InGaN nanowires across the entire composi-tion range. The composition gradient is sharp near the outlets, and it tends to be smooth when the sample was grown at down-stream far away from the outlets, due to the nature of spatially precursor mixing.
The low growth temperature (∼550 °C) and the strain-relaxed growth accommodation ability were believed to promote the formation of the non-thermodynamically-stable product. Although the produced temperature gradient of ∼50 °C cov-ered the whole range of the substrate, little differences in wire morphology and other properties were observed. In this experimental approach, the spatially graded composition (thus spatial graded bandgap) and the corresponding optical proper-ties of the nanowires were a result of the spatial source reagent gradient.


2.3. Dual Gradient

Pan et al. combined their previously used temperature gra-dient route with the precursor gradient method,[49] developing a Dual Gradient CVD growth strategy, and achieved a contin-uous spatial composition grading of a single-crystal quaternary ZnxCd1-xSySe1-y alloy nanowires over the complete bandgap range along the length of a substrate.[51] The bandgap grading spans between 3.55 eV (ZnS) and 1.75 eV (CdSe) on a single substrate, with the corresponding light emission tuned over the entire visible range.

The Dual Gradient CVD growth system for one dimen-sional (1D) graded growth was schematically shown in Figure 4a, where the source materials (ZnS and CdSe powders) were loaded in two mini-tubes arranged in a carefully designed spatial configuration to achieve spatial reaction reagent gra-dient. At the same time, the substrate was tilted into a proper angle to achieve a temperature gradient along the length of the substrate. This spatial reagent configuration and the additional temperature profiling across the substrate provide dual gradi-ents for the growth of ZnCdSSe alloy nanowires with graded composition. More specifically, two ¼′′-diameter quartz mini-tubes were placed horizontally inside a 1′′-diameter quartz tube for transporting the reaction reagents independently to the reaction zone. The high-purity ZnS powder in one mini-tube was placed at the center of the furnace, while the CdSe powder in another mini-tube was placed upstream and 10 cm from the center of the furnace. A quartz substrate (0.5 cm × 2 cm) pre-sputtered with Au film of ∼2 nm thickness was mounted downstream facing the ends of the mini-tubes. The distance



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between the substrate and the two mini-tube ends are main-tained at ∼3 mm and a proper angle between the tiled substrate and furnace axis was formed in order to keep the different dis-tances from the large tube reactor. The two ends of the tilted substrate were spaced 15.6 mm (low-temperature end, near to the mini-tube for CdSe) and 14.2 mm (high-temperature end, near to the mini-tube for ZnS) from the center of the furnace, respectively. The temperature of the locations where ZnS and CdSe powder was placed, was 960 and 900 °C, respectively, and the two ends of the tilted substrate were measured as 815 °C and 700 °C, respectively, as shown in Figure 4b. The as-grown 1D composition grading sample is shown in Figure 5a and 5b.

Figure 5a shows a real-color photograph under room lighting of the as-grown 1D composition-graded sample, where color changes from dark (ZnS side) to light (CdSe side). The same 1D sample under a UV laser illumination along the central stripe shows much richer color distribution from red, through yellow, green and blue, to purple as shown in Figure 5b, indi-cating the gradually varied bandgap of the materials along the substrate. The substrate covered by nanowires with dia meter distribution of 100–200 nm and lengths of several tens of micrometers. The Zn, S, Cd, and Se elements were observed in all the single nanowires by in situ EDX measurement, and the Cd-Se concentrations are complementary to those of Zn and S along the substrate. The single-crystal and wurtzite hexa-gonal structure was verified by the HRTEM images of these wires. A small-area XRD measurement was applied to examine the position-dependent crystal quality/structure across the overall length of the sample. Figure 5c shows the XRD map-ping along the length of the sample with a scanning step of 2 mm. The standard 2θ values of single crystalline structure of


Figure 5. (a,b) Realcolor photograph of the asgrown singlecrystal quateroom lighting and under a UV laser (266 nm) illumination along the cencompositiongraded sample with a step of 2 mm, as well as the standard 20PL spectra (normalized) along the length of the 1D sample. (e) The line wpermission.[51] Copyright 2010, American Chemical Society.

ZnS and CdSe are also shown for comparison. The diffraction peaks shift gradually from the Zn-rich end to the CdSe-rich end toward smaller angles, indicating the formation of the alloys with intermediate compositions with their lattice constants gradually increased. The position-dependent lattice constants a and c obtained from XRD measurements, HRTEM analysis, as well as the Vegard’s fit[52–56] of the ZnCdSSe quaternary alloys are also in good agreement. The composition variables, x and y, are determined from the element profile measurements along the length of the substrate. All these compositional and structural results unambiguously demonstrate the realization of quaternary alloy nanowires and their spatial composition grading on a single chip. The position-dependent normalized PL spectra in Figure 5d show that each spot along the sub-strate has a single-peak light emission with the peak wavelength gradually tuned from near UV (350 nm) at the ZnS-rich end to 710 nm at the CdSe-rich end, which covered the entire visible range. The wavelength variation of the PL emission is consistent with the color distribution of the sample under the UV light illumination as shown in Figure 5b. The position/composition-tunable PL spectra came from the band-edge emission rather than the defect-related emission, which also confirmed the high crystal quality of the alloy wires. The emission energy of the com-position x-dependent PL and position-dependent PL along the entire sample substrate was analyzed and compared to the alloy bandgap values of ZnS (3.6 eV), ZnSe (2.8 eV), CdS (2.44 eV), and CdSe (1.72 eV). The good agreement between the continuously varied PL peak energy and the graded band-gap values is a further validation of the spatial composition gradient in single chip.

A plot of linewidth (full width at half maximum) from the position-dependent PL spectra in Figure 5d is shown in Figure 5e. The PL linewidths of ∼30 and ∼35 nm were detected


rnary ZnxCd1-xSySe1-y alloy nanowires with 1D composition grading under tral stripe, respectively. (c) Spatial XRD scan along the length of the 1D

values for the ZnS and CdSe wurtzite single crystal. (d) Positiondependent idths (FHHM) of positiondependent PL spectra in (d). Reproduced with


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Figure 6. (a) Realcolor photograph of the sample with 2D composition grading under UV light illumination. Positions A, B, and C in panel (a) are close to the downstream ends of the minitubes loaded with ZnS, CdSe, and CdS powder, respectively. (b) The positiondependent PL peak wavelength/energy along each side of the triangle ABC of the 2D graded sample. (c) The compositional x-y relations for the 1D composition graded sample (green stars), and the 2D graded sample along the three sides of the triangle ABC (black rectangles, red circles, and blue triangles), respectively. Reproduced with permission.[51] Copyright 2010, American Chemical Society.

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closing to the pure ZnS and CdSe ends, respectively. However, the detected PL linewidths (∼50–55 nm) of the alloy wires in the intermediate section along the substrate are apparently broader than those of the pure binary compounds at the ends. This observation has a similar trend with that of the composition gradient single wires shown in Figure 8E and will be discussed later.

The successful realization of 1D composition grading in qua-ternary alloy nanowires provides a feasible strategy of designing two dimensional (2D) alloy composition grading. To achieve a 2D grading based on ZnCdSSe quaternary alloy nanowires, one more mini-tube loaded with CdS powder was added to the experimental setup of 1D grading system, as shown in Figure 4c. The three mini-tubes were arranged triangularly, and a quartz substrate (3 cm × 3 cm) was mounted with a proper angle at the downstream ends of three mini-tubes. The growth procedure is similar to that of 1D sample described above.

Figure 6a shows the real-color photograph of the as-grown 2D spatially graded composition sample under UV light illumina-tion. The three spots marking as A, B, and C are corresponding to the ends of three mini-tubes, which were loaded with ZnS, CdSe, and CdS powder, respectively. Along the three sides of the triangle ABC, the percentage profiles of those four ele-ments and the composition x and y values were plotted. These observed spatial compositions confirmed that the whole sub-strate was also composed of quaternary ZnCdSSe nanowires, which annotated the 2D color distribution produced by the spa-tial graded composition in the substrate. The normalized PL spectra of the 2D spatially graded sample along three paths of triangle ABC are given in Figure 6b, which shows a continu-ously varied emission wavelength. Similar to the 1D ZnCdSSe nanowire compositions grading, the strong single-peak band-edge emission was also observed from each spot along the triangle paths in the 2D composition grading. These 2D com-position graded ZnxCd1-xSySe1-y nanowires in their two dimen-sional composition coordinate plane was shown in Figure 6c, in which the four corners represent the four possible binaries, ZnS, ZnSe, CdS, and CdSe, while the four edges represent the corresponding ternaries between two nearby binaries with var-ying composition. The 2D composition coordinate plane rep-resents the possible quaternary ZnCdSSe alloys with gradient compositions (x, y). For example, the 1D composition graded sample shown in Figure 5a has a fixed composition dependence


corresponding to one curve in this composition coordinate plane shown as green stars. However, for the 2D composition graded sample, there are many different composition depend-ences or curves in this composition coordinate, depending on the different path. As shown in Figure 6c, the compositional dependences along three sides of the 2D composition graded triangle sample are given as black rectangles, red circles, and blue triangles, respectively.

The spatial composition graded ZnxCd1-xSySe1-y quaternary alloys were achieved over the entire composition range by com-bining the spatial source reagent gradient method with the proper temperature gradient to produce the dual gradients in one or two dimensions.[49] Here the spatially configured mini-tubes provide multi-channels of reagent vapors, which were transported to separate spatial areas of the substrate to form source reagent gradient, as well as the controlled temperature gradient distribution further allows these different source vapors (ZnS, CdS, and CdSe) to be deposited at different positions of the substrate with optimized local temperatures. The effective spatial separation of different reagent elements, induced by the comprehensive action of the properly configured multiple tubes and the tilting of the substrate, is very crucial to realize the growth of the complete quaternary composition grading. The contrasting study with only one of the gradients was also per-formed in this research, but either the reagent gradient or the temperature gradient alone is not sufficient for achieving the complete quaternary spatial composition grading. This dual gradient method could be used as a general strategy for growing other quaternary or multi-component alloy systems.

3. Graded Bandgaps Based on Single Nanowires

According to the VLS based nanowire growth process,[26,27] elemental composition in the grown nanowires can be directly controlled by the corresponding element concentration in the source materials. Based on this principle, various alloys of nanowire form with new and tunable bandgaps were reported recently, through using mixed source materials with different composition and changing the molar radio of these mate-rials.[9,29,40,41,59–68] Based on the same idea of composition con-trol but applying an in-situ concentration changing of source reagents, composition gradient single nanowires in several



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different alloy systems have been achieved using developed VLS routes. The composition or bandgap is gradually changed along the length of the wires. In addition to the VLS route, a solu-tion route involving the solution-liquid-solid (SLS)[57,58] growth mechanism has also been successfully employed to prepare quantum wires with graded composition. In the following text, some of the recent achievements in composition graded single nanowires will be highlighted.

3.1. CdSSe

Our group and collaborators recently developed a source-moving thermal evaporation route and successfully realized the growth of CdS1-xSex alloy nanowires with graded bandgaps along the length of the wires.[69] As schematically shown in the experi-mental setup in Figure 7, CdS and CdSe powder in separate alumina boats were placed in the center of heating zone and upstream far away from the heating zone, respectively. These two boats with source powders were separated by two empty alumina boats to obtain enough temperature difference for the two different evaporation sources. During the whole growth process, the CdSe boat was continuously shifted along the tube through magnetic force, which was transferred from the quartz push rod driven by a step motor. Nitrogen carrier gas was used to purge the oxygen before heating up. The heating zone center was heated with a increasing rate of 40 °C min-1 to 830 °C, and then maintaining the growth for 40 min. Then the CdSe boat was pushed downstream to the center of the heating zone, and simultaneously cooled down the system to 800 °C. After one more hour of growth at 800 °C, the temperature was reduced to room temperature. The composition graded CdS1-xSex alloy nanowires were deposited on the downstream silicon substrate pre-coated with a 2 nm gold film. The manipulation of moving source during the growth is the determining factor for success-fully producing these spatial composition graded CdS1-xSex wires. Combining the moving source and the temperature dis-tribution in the furnace, a controllable evaporation rate of source materials, and thus a continuously varied vapor concentration was achieved. This continuously varied concentration of vapor leads to the continuously changed element composition of the liquid alloy catalyst droplets, and then results in the changing of the composition in the newly grown nanowire segments. At the beginning of the growth, the CdS powder at the high tem-perature zone is evaporated very fast, while the CdSe powder in the low temperature zone can not be evaporated at this time. Thus only CdS source vapor is available and pure CdS binary alloys start to grow at this initial stage. As the growth proceeds,

© 2012 WILEYVCH Verlag GmAdv. Mater. 2012, 24, 13–33

Figure 7. The sourcemoving thermal evaporation setup for compositiongraded CdSSe nanowires. Reproduced with permission.[69] Copyright 2011, American Chemical Society.

the CdS and CdSe source powder are gradually pushed for-ward by the magnet force to the low and the high temperature zone, respectively, which will lead to the respective decrease and increase of the CdS and CdSe vapor concentrations. Thus, the composition in the newly grown wires will vary continu-ously resulting from the variation of the vapor concentration, with decreasing S and increasing Se gradually along the growth direction of the nanowires. At the last stage, the CdS powder is moved to the low-temperature zone where there is no more evaporation, while the CdSe powder locates at the high- temperature zone with a maximum evaporation rate, which means that pure CdSe vapor is only available in the system at this time, and pure CdSe is grown into the wires. From a side-view of the cross section of the as-grown sample under laser illumination, the light emission was observed gradually changed from green at the substrate, through yellow and orange in the middle, to red at the top along the axial growth direction of these nanowires. This fact is due to the composi-tion gradually tuned from CdS (x = 0) at one end, through the composition gradient ternary alloy CdS1-xSex (0 < x < 1) at the middle, to CdSe (x = 1) at the opposite end of the wires. Accord-ingly, the bandgap gradually changes along the length of the wires from 2.44 eV at one end (pure CdS, emission wavelength at 507 nm, green light) to 1.74 eV at the other end (pure CdSe, emission wavelength at 710 nm, red light). This source-moving growth method could be extended to achieve nanowire compo-sition grading of new alloy systems with different wavelength regions, and even the grading with wavelength tunability cov-ering the entire range from UV to near-infrared (NIR).

The spatially graded CdS1-xSex nanowires synthesized through this source-moving approach have typical length of several hundreds of micrometers and the diameter distribu-tion between 100 nm and 1000 nm, which were demonstrated by the SEM observations. The original sample was dispersed onto a MgF2 substrate for optical measurements. The real-color photograph of a dispersed sample under laser illuminations (405 nm) was presented in Figure 8A, which shows that all the nanowires have multicolor light emissions with color continu-ously varied from green to red, which confirms the realization of graded bandgap from CdS to CdSe along the length of the wires. The gradient composition along the length of a single wire was identified by the EDX measurements, which shows that one end of the nanowire mainly contains elements Cd and S while the opposite end was composed of elements Cd and Se. The microstructure of these composition graded CdS1-xSex nanowires was further investigated by TEM examinations. The fact that neither apparent defects nor phase segregations were observed from the clear lattice profiles, combining with the well arrayed diffraction spots of the SAED patterns, con-firms that the as-grown wires have high crystalline quality with hexagonal wurtzite structures. The wires grow along the [002] lattice direction from the SAED results, while the [002] lattice spacing shows a continuously increasing from left to right along the length of the wires. The variation of the lattice spacing shows a good agreement with the gradually varied concentra-tion from S to Se along the wires. The real-color photo graph of a representative composition gradient CdS1-xSex nanowire was shown in Figure 8B, where the gradual variation of color from green to red was observed under the illumination of a

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Figure 8. (A, B) Realcolor photograph of some CdSSe nanowires dispersed on a lowindex MgF2 substrate under a 405 nm laser illumination. (B1B12) Realcolor photographs of different spots along the length, excited locally with a beam of focused 405nm laser. (C) The corresponding local PL spectra collected from these excited spots in B112, respectively. The PL spectra of pure CdSe and CdS are also shown for comparison, respectively. (D) Positiondependent compositions and bandgap values along the length of the multicolor CdSSe nanowire. Se mole fraction values (black triangles) are obtained from the EDX results. Bandgap values are obtained from both the EDX compositions (green rectangles), and the PL spectra (red circles). (E) Positiondependent PL linewidths (FWHM) along the length of the multicolor CdSSe nanowire. These linewidths values are measured from the PL spectra shown in panel (C). Reproduced with permission.[69] Copyright 2011, American Chemical Society.

divergence of laser beam at 405 nm. Panels B1–B12 in Figure 8 show more clearly that different spot along the single wire have different PL emission when the wire was locally excited by a focused laser beam. The color of those emission changes gradually from green to red. The corresponding locally excited PL spectra shown in Figure 8C demonstrate the relatively single-wavelength emission from each spot of the wire, with emission wavelength continuously varied from 505 nm at one end to 710 nm at the opposite end. The emission peaks at both ends are consistent with the pure CdS and CdSe bandgap emission respectively, which confirmed the complete compo-sition tunability in these CdS1-xSex nanowires. The position-dependent compositions and bandgap values along the length of the multi color CdS1-xSex nanowire, were summarized in


Figure 8D. The agreement between the calculated values from EDX combined with the Vegard’s law (green rectangles)[9] and the values converted from the PL spectra (red spots) indicates that all the observed PL spectra are from the band-edge emis-sion. Neither defect nor structural disorder-related low-energy emission was detected in all the spectra, which further dem-onstrates that all spots along the entire single wires are highly crystallized alloys, consistent with the results of HRTEM investigations.

The linewidth of the PL spectra in Figure 8C were meas-ured and shown in Figure 8E. For the composition graded single wire, the diameter along its entire length is identical (∼200 nm). The two ends of the wire have narrow PL linewidth of ∼16 nm and ∼22 nm, corresponding to the pure CdS and



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Figure 9. (a) The schematics of the combinatorial syntheses; the flow rates of GeH4 and SiH4 were systematically modulated for the total growth of 800 s. (b) A representative TEM image of an individual Si1-xGex nanowire. Inset shows the intensity of Ge Kα radiations in energy dispersive ray EDX spectra collected from six different positions (1–6) marked in (b). The intensity of all spectra was normalized respect to Si Kα radiation for comparison. (c) The Ge contents and [022] interplanar distances at six different positions of an individual Si1−xGex nanowire, marked in (b). Open circles are [022] interplanar distances in the bulk limit, extracted from the measured compositions. The inset schematically indicates that some degree of strain, associated with the graded axial heteroepitaxy of Si on Ge, is built up within the individual nanowire due to the lattice mismatch. Reproduced with permission.[70] Copyright 2008, American Institute of Physics.

pure CdSe, respectively, while the locally detected PL linewidths of the alloyed wire segment in the middle are relatively larger (∼25–30 nm). This wavelength dependent linewidth result is similar to that of the composition gradient wires along a single substrate in Figure 5e. The results indicate that alloying have great effects on the linewidth of the detected PL, which can be attributed to the alloying-induced structural disorder.[59] Here the wire size should have very weak effect on the linewidth, since the size of these examined wires are quite large which is beyond the region of quantum size effect. Also, in the examina-tion of the composition related PL linewidths along the single wire (Figure 8E), all the detected regions have the same dia-meter, which directly avoid the effects of size on the observed PL linewidth.

3.2. SiGe

Great progress has been made in the composition graded single Si1-xGex alloy nanowires in recent years.[8,70] Before the realization of composition grading along single nanowires, the bandgap modulation in single crystalline Si1-xGex nanowires were achieved on quartz substrate through separate growths using a Au catalyst-assisted CVD route.[71,72] The gas flow rate of SiH4 and GeH4 precursors and the growth temperature are important factors for achieving a given bandgap in the produced nanowires. For example, wires with a Si:Ge concentration ratio of 65:35 were obtained by keeping the gas flow rate of 80:3:45 (SiH4:GeH4:H2 in sccm) and the growth temperature at 370–390 °C, while wires with Si:Ge concentration ratio of 15:85 could be achieved when the gas flow rate was maintained at a ratio of 10:50:45 and the temperature at 350–390 °C during the growth.

Based on these previous studies of bandgap modulation on SiGe alloy nanowires, an optimized growth was applied to synthesize composition graded Si1-xGex alloy nanowires with bandgap modulation along their lengths.[70] As schematically shown in Figure 9a,[71–73] the chemical vapor deposition was car-ried out under independently varying flows of SiH4 and GeH4 precursors at given temperatures and pressures. The composi-tion gradient along the wires was well controlled by modulating the flow rate ratio of SiH4 and GeH4 within a given growth time and temperature. The growth kinetics of individual Si and Ge nanowires were considered as references for modulating the flow rate ratio of these two precursors.[73] The growth tempera-ture of 390 °C and the total gas pressure of 200 Torr were used in the whole reaction. The flow rate modulation and the dep-osition kinetics of Ge and Si were schematically displayed in Figure 9a. A Ge nanowire segment was grown during the first 180 s by only introducing the GeH4 precursor. Later, the SiH4 was introduced and the relative flow rate of two precursors was tuned for every 40 s to grow the composition gradient Si1-xGex segment. At the last 180 s only SiH4 was maintained in growth. A low flow rate of GeH4 was used in the whole process, because the nanowires tend to be tapered (radial growth of Ge) under the GeH4 rich condition due to the faster thermal decomposition kinetics of GeH4 compared to that of SiH4.[73] Therefore, the low flow rate of GeH4, as well as the H2 gas, was used to maintain the uniform nanowire diameter along the entire axis without tapering.


Figure 9b is the representative TEM images of an individual Si1-xGex nanowire of 20 nm in diameter. The marked posi-tions from 1 to 6 were spaced by ∼400 nm along the entire wire and were selected for characterization. The nanowire is single


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crystalline and has the same crystal orientation along its entire
length, which is demonstrated by the HRTEM images and the corresponding diffraction patterns. The composition gra-dient of Si and Ge along the wire was measured from the EDX spectra by the convergent electron beam in TEM. The probed intensities of Ge Kα line were normalized with respect to those of Si Kα lines, and were shown in the inset of Figure 9b. The gradual decrease of normalized Ge Kα lines (thus the systematic variation of the Ge content) demonstrated that the graded alloy of Si and Ge was produced by the kinetic control of the catalytic decomposition of SiH4 and GeH4 precursors. The Ge concentration and inter-planar distance along the entire Si1-xGex nanowire were summarized in Figure 9c. The relative composition of Ge, as well as the inter-planar distance of [022] planes, decrease along the growth direction. Associated with the graded growth of Si and Ge, strain was produced within the nanowires due to the lattice mismatch, which can be seen from the fact that the [022] planar distance (black squares) decreases more steeply than those values extracted from the bulk when the Ge content decreases from position 6 to 1, as schematically shown in the inset of Figure 9c.

Micro-Raman measurements were conducted on the com-position gradient Si1−xGex nanowires in order to investigate the spatial composition change along their lengths. An epi-confocal Raman microscope equipped with a linearly polarized second harmonic frequency output of Nd: YAG excitation laser at 532 nm was used in the experiment. Three Raman shifts at ∼500 cm-1, ∼400 cm-1, and ∼270 cm-1 correspond to the lat-tice phonon vibration modes of Si-Si, Si-Ge and Ge-Ge bonds, respectively, with frequency shifts represented by nSi-Si, nSi-Ge and nGe-Ge.[74,75] The relative intensities of nSi-Si/nGe-Ge smoothly vary along the wire, which indicates the composition gradient in SiGe nanowires. The corresponding Raman images by moni-toring the respective phonon bands in the identical Si1−xGex nanowire were observed. The continuously varied distribution of nSi-Ge intensity in the images along the entire nanowire also confirms the fact of composition gradient Si1−xGex nanowires. Associate both peak position and spectral width, the observed Raman spectra are similar to those of the bulk Si1−xGex alloys and Si1−xGex nanoparticles with graded Ge concentrations.[76] The local concentrations of Ge at two different positions were determined to be 0.64 and 0.33, respectively, due to the assump-tion that the Si1−xGex nanowire follows the same frequency shift/Ge-concentration rule as empirically found in the bulk and nanospheres.[77]

The growth of bandgap-graded Si/SiGe nanowires through a molecular beam epitaxy (MBE) route was reported by Kanungo and co-workers.[78,79] <111> oriented 5′′ Si wafers cleaned by the conventional RCA procedure were used as the substrates. The MBE system used in their experiment includes three electron-beam guns for the evaporation of Au and Si as well as of Ge.[80] The Au film with a nominal thickness of 2 nm was deposited on the substrate at a substrate temperature of 525 °C. A constant Si and Ge fluxes were controlled at 0.05 nm s-1 and 0.01 nm s-1, respectively. The composition graded Si1−xGex nanowire seg-ment with maximum Ge concentration of 10% was obtained when the growth temperatures of 525 °C and 545 °C were chosen corresponding to the growth of Si and Si/Ge, respec-tively. When the growth temperature of 300 and 545 °C were

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used for graded Si1−xGex segment and top Si segment growing, respectively, the composition gradient Si1−xGex nanowires with maximum Ge concentration of 26% were obtained.

The nanowires were single crystalline as revealed by the TEM investigation. The Ge concentration along the length of the nanowires was measured by multi-beam TEM bright field imaging. The Ge layers show an approximate triangular concen-tration profile and the Ge incorporation increases with a spe-cific gradient. The concentration x in the layers is relatively low, with about 4% in the first layer and 10% in the third layer.[79] The Ge concentration x showed a Gaussian profile with a max-imum value of 26% and FWHM of 7 nm.[78] Electrical measure-ments of these individual composition gradient Si1-xGex alloy nanowires were performed by the technique described previ-ously.[81] The sample was mounted on a copper stage of an SEM by making a back contact with silver paste. The Pt/Ir tips were used to contact the tips of individual nanowires.[82] In order to get direct contacts between Pt/Ir tips and the nanowires, the gold catalyst caps on top of the nanowires were removed by dip-ping in a standard gold etchant.[78] An Ohmic/linear behavior of the Si/SiGe nanowire was observed with a current in the range of 1 mA at 500 mV, and the calculated resistance of 470 kΩ and resistivity of 4 Ωcm. This value corresponds to a carrier con-centration of 1 × 1015 cm-3 for n-type and 3 × 1015 cm-3 for p-type Si with bulk mobility values used.[83] A simulated elec-tron concentration of 5 × 1010 cm-3 and a hole concentration of 2 × 109 cm-3 were obtained following the Fermi-Dirac dis-tribution. These simulated values are 6 orders less than those obtained from the experimental I–V curves. Compared to the simulated carrier concentration, the Si/SiGe heterostructure nanowires were much more conductive than expected. The enhanced electrical conductivity comes from the current con-duction through their surface state of the composition graded Si/SiGe nanowires.

3.3. InGaN

A sandwich-like n-GaN/InGaN/p-GaN p-i-n nanowire hetero-structure with an indium composition gradient in the InGaN alloy nanowire segments was achieved by a plasma-assisted MBE technique.[5] Before the epitaxy process, the native oxide was removed from the substrate surface by a standard tech-nique.[12] Then the temperature of the substrate was lowered down to 800 °C, and Ga monolayers were deposited without N under this temperature. GaN nanowires were grown at a rate of 300 nm h-1 with a constant N flow rate under the Ga flux. The produced seed GaN nanowire of 300 nm was used to the grow the InGaN nanowires with a continuously varied In con-centration, as well as a constant In concentration. The tempera-ture was lowered to 550 °C, and then the composition graded In1-xGaxN nanowires were grown with a continuously varying In flux during the epitaxial growth.

The n-GaN/InGaN/p-GaN nanowires were grown on the (001) Si substrate vertically and separated at the top. The areal density of nanowire on the substrate was estimated to be 1–2 × 1011 cm-2. The as-grown nanowires were stripped off from the substrate into an ethanol solution and subsequently dried on a carbon-coated copper grid for TEM study on single wires. A

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Figure 10. (a) A TEM image of a nGaN/InGaN/pGaN pin nanowire heterostructure and the indium composition along the entire length, as measured by EDX. The indium composition was continuously varied in the InGaN section during epitaxy, which is confirmed by the data. Also noticeable is the fact that the diameter of the nanowire increases slightly in going from GaN to InGaN at the nGaN/InGaN heterointerface. This is probably a consequence of the lower growth temperature of InGaN compared to that of GaN. The inset shows a higher magnification TEM image of the nGaN/InGaN interface, where no defects are observed. (b) Roomtemperature photoluminescence spectra of InGaN nanowires with varying In content. The PL is shifted to longer wavelengths by increasing the In composition. The broad spectrum is obtained from a nanowire sample in which the In composition was continuously varied (along length) during epitaxy. The inset shows the temperature dependence of the broad luminescence with emission peak at 580 nm from a 300 nm long nanowire with continuously varying In composition. Reproduced with permission.[5] Copyright 2010, American Chemical Society.

constant diameter of ∼20 nm along the entire nanowire was measured by the TEM obser-vations. The heterostructured nanowire was single crystal with wurtzite structure and the c-plane is normal to the growth direc-tion. Figure 11a shows the TEM image of a n-GaN/InGaN/p-GaN nanowire heterostruc-ture, and a plot of Indium composition along the entire nanowire was presented in the bottom panel. The In concentration of ∼20% in maximum was obtained by EDX analysis. The continuously varied In concentration along the InGaN section confirms the spa-tially graded growth during the epitaxy. The diameter of the nanowire increases slightly from GaN to InGaN segment probably due to the lower growth temperature of InGaN compared to that of GaN. No defects were observed in the n-GaN/InGaN interface, as shown in the inset of Figure 10a.

The optical properties of the nanowires were investigated by temperature-dependent and time-resolved PL measurements. The PL spectra of constant-composition InGaN alloy nanowires, as well as those of composition
gradient In1-xGaxN nanowires were all shown in Figure 10b. For constant-composition InGaN nanowires, a large tunability of the detected spectrum could be obtained by varying the In concentration in these separate wires. The FWHM of emission line progressively increases corresponding to the increasing wavelength, which is mainly due to increased alloy broadening with increased In concentration in the nanowires. Compared to the relatively narrow PL spectra (line width of ∼50 nm) from the constant-composition nanowires, a PL spectrum with broad line width of ∼150 nm was measured from the composition gradient In1-xGaxN nanowires. This broad PL spectrum with peak at 580 nm corresponds to ‘white’ light emission.
3.4. ZnSeTe

A solution route of nanowire growth involving a solution-liquid-solid (SLS) growth mechanism has been successfully employed to prepare ZnSeTe nanowires with composition graded along the length.[57] Compared to the VLS mechanism, this SLS growth can produce nanowires with smaller diameters (≤10 nm) within the quantum-confined regime and narrower diameter distributions, due to the lower synthesis temperatures (180–300 °C) employed in solutions. The composition graded ZnSexTe1-x alloy nanowires were grown in a solution with zinc-stearate precursor (Zn(SA)2), tri-n-butylphosphine telluride (TBPTe), surface-capping ligands, and Bi-catalyst nanoparticles dispersed. The reaction was kept at 290 °C for ∼2 min until the color of solution turned to orange, then the solution was heating up to 300 °C and quickly injected a tri-n-octylphosphine selenide (TOPSe). The composition gradient ZnSexTe1-x alloy nanowires were produced after this procedure. Figure 11a shows the TEM image of a typical graded nanowire with dia-meter of ∼10 nm. The composition of this wire was measured


by EDX and the composition ratios of Se:Te were determined to be 3.9:1, 2.7:1, 1.4:1, and 1.0:1 from spot 1 to spot 4 along the wire, respectively. The gradual decrease of the Se:Te ratio from the Bi tip (Figure 11a inset) to the opposite end reveals a com-position gradient along the wire. The alloyed crystal structure of these graded nanowires was confirmed by XRD measurement. The diffraction peaks were broadened by the graded nature of the wires. No resolvable features were observed in the absorption spectra of these wires, as shown in Figure 11b, which further confirms the graded nature of the obtained wires.

Another ZnSe-ZnTe heterostructure containing a gradient ZnSexTe1-x segment was produced by modifying the reaction process above. Thus additional Zn(SA)2 was added after the ini-tial 2 min reaction period, and the reaction was allowed to pro-ceed for another 1 min before the injection of TOPSe. Figure 11c shows the TEM image of a nanowire (∼10 nm in diameter) pro-duced by this process. The EDX spectra in selected spots along the wire showed that the Bi tip segment (upper-left inset of Figure 11c) and the opposite side of the wire were composed of ZnSe and ZnTe, respectively. The middle segment of 200 nm in length contains Zn, Se, and Te composition (bottom inset of Figure 11c). Various characterizations demonstrated that this segment is a composition gradient ZnSexTe1-x alloy. A cartoon heterostructure of this wire is given in the inset of Figure 11d, where the transitional region of graded ZnSexTe1-x is sandwiched between ZnSe and ZnTe segments. Two resolvable transitions were observed in the absorption spectrum shown in Figure 11d, coming from the ZnSe (λ ∼ 450 nm) and ZnTe segments (λ ∼ 520 nm), respectively. This solution phase synthesis route can be easily extended to other alloy systems, and could be a general strategy to achieve composition graded quantum wires.

The composition gradient CdSSe and ZnCdSSe nanowires we synthesized through the VLS mechanism have very weak size effect due to their large sizes. The tunable bandgaps of


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Figure 11. (a) TEM image of ternary ZnSexTe1-x nanowires. The inset shows a higher magnification TEM image of the selected area highlighted by the square. The black dots on the wire show the spots where EDS spectra were collected. (b) Absorption spectrum of the sample shown in (a). The arrows indicate the bulk bandgap energies of ZnSe and ZnTe, respectively. (c) TEM image of a representative ZnSeZnTe nanowire with a threesegment heterojunction structure. The insets show highermagnification TEM images of the selected areas highlighted by the rectangles. The black dots on the wire represent the spots where EDS spectra were collected. (d) Absorption spectrum of the sample shown in (c). The arrows indicate the firstexcitonic transitions arising from the ZnSe and ZnTe segments, respectively. The inset shows a cartoon depicting the threesegment heterojunction structure. Reproduced with permission.[57] Copyright 2007, American Chemical Society.

those wires mainly come from the spatial composition gradient. However, the ZnSexTe1-x nanowires synthesized by the SLS mechanism are small in diameter and are quantum-confined wires. Size effect can be another way to tune the bandgap of these quantum wires. Furthermore, these composition gra-dient quantum wires provide a reasonable materials platform for further investigating some basic scientific issues in mate-rials science, such as the composition related quantum con-finement effect in alloy semiconductor nanostructures, and the size dependent electrical and optical transport properties along 1D bandgap gradient quantum structures. These composition gradient quantum wires should have potential applications in bio-nanotechnology, biomedical engineering, electro-analytical chemistry, and photo-electron devices et al.

3.5. Other Systems

Composition graded single-crystalline NiSi nanowires were successfully synthesized through a hot-wall CVD route.[48] The

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experimental approach employs the Ni- cata-lyzed decomposition of SH4,[84] which can occur well below the thermal decomposi-tion temperature (above 600 °C) of SH4. The growth temperature and the SiH4 par-tial pressure during the deposition of SiH4 were optimized at around 250–600 °C and 10–100 Torr, respectively. Either NiSi thin films or nanowires of various phases were produced in this reaction.

The NiSi grown at 250, 400, and 600 °C at 50 Torr of SH4 were characterized by SEM and XRD. Within the SH4 vapor-Ni solid reaction, where SH4 decomposition and solid Ni diffu-sion from the pre-deposited Ni film are both thermally activated. First at 250 °C, Ni film was not turned into NiSi film completely after these two reactions, only a very thin NiSi layer was formed on the Ni film. While at 600 °C, the NiSi thick film with various phases were formed due to the sufficient reactions. As for the reactions at 400 °C, NiSi nanowires are formed with a NiSi phase on the top of a Ni-rich phased film. The synthesized nanowires are single crystal, with dia meters of 20–50 nm and length above 20 mm. It is observed that Ni concentration is gradually decreased along the growth direction from ‘a’ to ‘e’ as shown in Figure 12. The major section in the middle region (marked as ‘c’) in the nanowire over several micrometers have a Ni-to-Si concen-tration ratio of approximately 1:1. So the composition is graded only at two end seg-ments of the nanowires. This growth process is based on the one-dimensional diffusion of Ni out of the Ni film, through the inter-mediate NiSi and silicide thin films. The dif-fused Ni reacts with the SiH4 vapor through catalyst and finally forms the NiSi composi-

tion gradient nanowires. These achieved NiSi nanowires exhibit typical metallic properties and promising field-emission proper-ties. The simple and low temperature synthesis route of single-crystalline composition graded NiSi nanowires in this work can provide interesting strategies to fabricate other nanostructured composition grading.

He et al. reported composition-graded CuSi nanostructures fabricated by an evaporation system using an oblique angle codeposition (OACD) with two sources.[85–88] The schemati-cally experimental setup is shown in Figure 13A and has previ-ously been described in detail.[89] The oblique angle deposition provides a route to form a well-aligned nanorod array, where a large incident angle of vapor was manipulated by a step motor. The deposition rate of the two materials can be monitored and tuned precisely through two sets of independent in situ deposi-tion rate monitor and feedback systems, which were controlled by a computer during the fabrication. Si and Cu powders were used as the evaporation sources in the chamber. The inci-dent angle of the vapor flux was fixed at 88° with respect to normal direction of the Si wafer substrate normal as shown in

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Figure 12. The composition gradient in individual nanowires near the optimum conditions with pressure of 50 Torr and temperature of 350 °C, determined from EDX spectra probed by the convergent electron beam along the entire nanowire. The middle region in a white shade that extends to 15 μm is not shown, because of the scale of the image. The relative compositions of Si and Ni at various positions within the individual nanowire were shown in table. The values at region ‘c’ are averaged from five different positions, which are almost the same within the 5% variation. Reproduced with permission.[48] Copyright 2007, WILEYVCH Verlag GmbH & Co. KgaA, Weinheim.

Figure 13. (A) OACD Setup used for the fabrication of a composition graded CuSi nanorod array. SEM image (B) and elemental mapping of Cu (C) and Si (D) of a bundle of CuSi nanorods lying on the conductive carbon tape. (E) Atomic concentrations of Cu and Si as functions of the sample length. The atomic concentration values own by hollow squares for Cu and hollow circles for Si are obtained by multiple EDX measurements along the samples shown in the SEM image (B) from bottom to top, and the calculated concentration values shown by solid squares for Cu and solid circles for Si are based on their relative deposition rates with the total nanorod length normalized to 3 μm from dCu

QCM+dSiQCM = 4 μm (The dCu

QCM and dSiQCM represent the deposition

process read from quartz crystal microbalance (QCM)). The scale bar in part a denotes 500 nm, and the arrow in part a indicates the growth direction of the nanorods. Reprinted with permission.[85] Copyright 2010, American Chemical Society.

Figure 13A. CuSi nanostructures were fabricated by controlling the gradually varied deposition rate of Cu and Si during the codeposition process. Specifically, a thin layer of Cu nanorods will first be deposited on a substrate and then the Si source start to deposit together with Cu. With increasing the deposition rate of Si, the deposition rate of Cu will gradually be decreased to zero. In the end, only Si was deposited on the substrate. A com-position graded nanorod array with higher Cu composition at the bottom and higher Si composition on the top was thus fab-ricated on the substrate.

The obtained CuSi sample consists of a well-aligned nanorod array as characterized by SEM observations. The nanorods have a width of ∼500 nm, a density of 3.4 rods m-2, and center-to-center separations of 2.2 mm and 0.6 mm. The angle between nanorods and the normal direction of the substrate is 62°, and the average height and length of the nanorods are approximately 1.4 mm and 3.0 mm, respectively. The CuSi nanorods fan out slightly along the perpendicular direction of the incident vapor due to the lack of atomic shadowing effect in this direction.[90] The SEM image of a bunch of composition gradient CuSi nano-rods is presented in Figure 13B, and the elemental mappings of Cu and Si are presented in Figure 13C and 13D respec-tively. The graded atomic concentration of Cu and Si along the entire nanorod length was measured by the EDX and plotted in Figure 13E. At the initial stage of the growth, only Cu vapor source is available for deposition. With the growth proceeding, the concentration of Cu decreased while the concentration of Si increased gradually passing the middle position of the nano-rods. From the TEM observations, one end of these produced nanorods are polycrystalline fcc Cu. With increasing the con-tent of Si, the crystal structure gradually changed from poly-crystalline fcc Cu, through the mixture of fcc Cu, orthorhombic Cu3Si, and amorphous Si at the middle segment, to the mixture of orthorhombic Cu3Si and amorphous Si. At the opposite end of the nanorods, the crystal structure is amorphous Si.


The technique of growing the composition graded CuSi nano-rods in this experiment is a combination of codeposition and oblique angle deposition. The codeposition provides a route to evaporate two sources simultaneously and the composition gra-dient is controlled by the ratio of the deposition rate of the two sources. The nanorod width (w) has a power law dependence on the nanorod length (l), w ≈ lp, where p is the fan-out exponent. The p value is an important factor to affect the shape, size, and spacing of nanorods that produced in this oblique angle deposi-tion. The pure shadowing effect lead to a value of p = 0.50, while the p value will be ≤0.31 when the pure surface diffusion effect is considered only. Related to the value of p ≈ 0.38 in this experi-ment, the growth of CuSi nanorods is controlled by the com-bination of shadowing effect and surface atomic diffusion.[91] This composition graded CuSi nanostructure is promising in the Li-ion battery anode application. Other composition-graded nanostructures could be designed, when replacing the Si and/or Cu by other materials. This growth strategy could be applied in various material systems in the future.

4. Applications

Bandgap-graded semiconductor nanowires provide a material platform for realizing many new optoelectronics devices. A direct application of these graded nanowires is in wavelength-tunable or multi-wavelength light emission devices, such as


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Figure 14. (a) Realcolor photograph of a quartz substrate with the asgrown spatially compositiongraded CdSxSe1-x nanowires under UV laser illumination (266 nm). (b) MicroPL collected at various locations along the substrate maintained at 77 K under high optical pumping condition, showing lasing or multimode lasing. Reprinted with permission.[4] Copyright 2010, American Chemical Society.

tunable nanolasers and white light LEDs. For the spatially con-tinuous bandgap distribution, graded nanowires can also be used as multi-spectrum or broadband light detecting devices and full-spectrum photovoltaic devices. Here we will present several examples of applications of bandgap-graded semicon-ductor nanowires.

4.1. Tunable Nanolasers

Wavelength flexibility, variety, and controllability are impor-tant for almost any applications of a laser. Only a few discrete lasing wavelengths can be achieved through multiple electronic transitions in gas or solution phase molecular materials, doped solid state materials.[92] Producing lasers with large wavelength-tunability is also fundamentally difficult using the conventional thin film planar epitaxial growth technology due to lattice mis-match between the substrate and the materials to be grown. An ultimate wavelength-flexible laser could ideally be tuned to any wavelength in a controllable fashion over a wide wavelength range on a single semiconductor chip. Since the wavelength of a semiconductor laser is determined by its fundamental bandgap, a wavelength tunable semiconductor laser would require bandgap tunable materials with their composition vari-ation in a large range and on a single chip.

Using the spatial composition grading of CdSxSe1-x alloy nanowires achieved by the temperature gradient method, Pan and co-workers realized a super broad wavelength-tunable nanowire laser with an unprecedented tuning range over 200 nm, which is an important technology application of such bandgap-graded semiconductor structures.[4] Since individual semiconductor nanowires can act as both waveguide cavi-ties and optical gain medium, and can be used as nanoscale lasers.[12–25] The bandgap-graded alloy nanowires in the com-position grading could be used as wavelength-tunable nano-lasers. Based on this idea, the composition graded CdSxSe1-x sample was pumped step by step along its length by a pulse laser. Figure 14a shows a real-color photograph of the spatially composition-graded CdSxSe1-x nanowires on a quartz sub-strate under a UV laser (266 nm) illumination. As expected, the composition grading realized sharp lasing emissions under high optical pumping at low temperature. Figure 14b shows the lasing spectra taken from 16 spots along the length of the substrate at 77 K, which indicates that the lasing wavelength can be tuned continuously at a large range from ∼503 nm to 692 nm. The pumping power dependence of the PL spectra indicates the transition from broadband spontaneous emis-sion to sharp lasing. The emission intensity shows a typical threshold-like behavior with increasing power intensity. This spatially tunable semiconductor nanowire lasers could be used in various applications such as novel optical interconnects or multiplexing, multi-agent chemical or biological detections, solid-state lighting, and super bright microdisplays.

Lasing at various wavelengths in the spectral range from ultraviolet (340 nm, 3.65 eV) to near-infrared (710 nm, 1.74 eV) was demonstrated in different samples.[11] This is achieved using CdSxSe1-x and ZnyCd1-yS alloy nanoribbons grown on the Au-coated c-Si substrate. The light emission properties of both CdSxSe1-x and ZnyCd1-yS alloy nanoribbons are dependent


on the optical pumping power intensity. For example, the PL spectra of CdSxSe1-x nanoribbons with emission peak at 585 nm have a weak and broad emission peak when excited in low pumping power, while sharp and intense resonant lasing modes with HWHM of ∼0.50–0.76 nm appear under high pumping power, with the threshold of ∼55 kW cm-2. Under optical pumping, the CdSxSe1-x nanoribbons have lasing emis-sion covering from 710 nm to 510 nm when the composition x value varied from 0 to 1, while the ZnyCd1-yS alloy nanoribbons have lasing emission from 510 nm to 340 nm when y varied from 0 to 1. In addition, the CdSxSe1-x nanoribbons have a fine lasing tuning capabilities arising from the composition varia-tion. The tuning of the lasing emission can be obtained with a relative precision better than 0.1 nm.[41]

4.2. Full-Spectra Solar Cells

The spatial composition graded quaternary ZnxCd1-xSySe1-y alloy nanowires, with bandgaps spanning the entire solar cell spectrum on a single substrate, is an ideal material platform for full-spectrum photovoltaic design and solar cell application.[51] One of the advantages of using the composition graded alloys is that potentially much more junctions can be realized. In order to maximize the absorption efficiency, a lateral multiple junction solar cell (LMJSC) was designed by Ning et al.[6,7] In this LMJSC design, each of the lateral junctions in solar cells could absorb a specific wavelength of the solar radiation cor-responding to the optimized bandgap of these cells. As shown



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Figure 15. (a) Schematic of the wavelengthspecific lateral multijunction solar cells with N lateral junctions shown. (b) Schematics of lateral MJSCs using composition graded nanowires with random orientations and with vertically arrays of pn junction nanowires. For the wires with random orientations, either interdigitated electrodes on the backside or topbottom contacts can be used. Reproduced with permission.[6] Copyright 2009, IEEE Publisher.

in Figure 15a, the broad-band solar light is first dispersed into various wavelength bands by a thin film grating or a dispersive concentrator.[93,94] Different wavelength bands propagating at corresponding angles will reach the absorption layer at different spatial locations or junctions. The use of dispersive concentrator is especially important when many junctions are designed, so that enough carriers are generated to have significant open cir-cuit voltage for each junction. Based on the spatial composition graded alloy nanowires, the entire absorption layer with multiple lateral junctions can be grown in a single run of CVD growth monolithically on a single substrate. This approach provides a unique opportunity of exploring the theoretical efficiency limit of MJSC by using this inexpensive nanowire growth technology. Several MLJ layers designed with the corresponding growth and fabrication processes were shown in Figure 15b, where the wires with random (top panel) and vertical (bottom panel) ori-entations were applied. For wires with random orientations on amorphous substrates shown in the top panel of the Figure 15b, one straightforward approach is to design inter-digitated elec-trodes on the backside of the solar cells. Another approach is to design the top transparent and bottom contacts for each lateral junction, with the advantages of easy and inexpensive growth of wires on amorphous substrates without pattering. Vertical


orientated nanowire growth is typically achieved using single crystal substrates, which cannot be used for varied composi-tion in a wide range such as the composition graded quaternary ZnxCd1-xSySe1-y alloy nanowires described above. Patterned arrays with specially fabricated spatial features can be used to control wire orientations to achieve nanowire arrays for the second design shown in Figure 15b. For solar cell applications, the lack of narrow bandgaps in the ZnCdSSe materials limits the ultimate conversion efficiency of this approach. To add narrow gap materials, it was recently proposed[7] that alloy of CdPbS would be a better material system, since pure PbS has a much narrow bandgap of around 0.4 eV. Alloying PbS and CdS would be able to produce any bandgap of importance to solar cell applications. A detailed design study showed that this is indeed an attractive approach.[7] With a 6 subcell design, this approach showed a conversion efficiency of around 40%, comparable to that of the more expensive vertical triple junction. The lateral multijunction approach is one of the few approaches that could promise both high efficiency and low cost.

4.3. Photodetectors

The single nanowire based composition grading of Si1-xGex alloys was used to fabricate an on-wire high efficiency photo-detectors.[8] Local carrier transport of composition graded Si:Ge alloys were investigated by a multi-terminal contacted bandgap-graded Si1-xGex nanowire. Each segment on the nanowire have an axially compositions variation of ∼15% within a length of ∼3.7 mm. With the Ge concentration gradually increasing, the resistivity of nanowires decreases over three orders of magni-tude due to the nature of bandgap variation in the composition gradient Si1-xGex nanowire.

A super continuum laser source combined a monochro-mator were used to the photocurrent (Iph) measurements. The laser beam was locally focused on a composition graded Si1-xGex nanowire along the entire length. The measured Iph of each spot on the graded Si1-xGex nanowire progressively shifted to the lower photon energy with the decreased content of Ge. Figure 16 shows the representative Iph images measured on the multi-terminal nanowire photodetectors of D1–D3 (device 1, 2 and 3) by scanning the photocurrent, where the composition gradient Si:Ge alloy is modulated axially by the labeled amount. The Iph intensity is observed to be the maximum value at the Ge rich segment, and it is progressively diminished from one end of the nanowire to the other end, as the Ge content is gradually diluted. The Iph magnitude peak value in each channel gradu-ally varies by roughly one order. The Iph intensity profiles shown in Figure 16d were extracted from D1. The Iph peak values are commonly observed in the stems of the nanowire instead of the contact region, which confirms that the photocarrier trans-port is mainly governed by drift along the channel band tilting instead of a local bending at the contact.[95]

4.4. White Light LED

White light-emitting diodes was fabricated with the epitaxial grown of GaN/InGaN/GaN p-i-n composition graded nanowires



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Figure 16. (a) Schematics of the SPCM (spectra photocurrent measurements) setup for the axially composition modulated Si1-xGex NW multiterminal photodetectors. The inset illustrates the expected band diagram of NW channel. (b,c) SPCM images of (b) D1 and (c) D2D3 at V = 1 V overlaid on the reflection images in gray. The marked numbers represent various Si:Ge compositions, as labeled, and the scale bar is 2 μm. (d) Photocurrent line profiles taken along the channel of (b) at 1 V bias voltage. The inset shows the normalized photocurrent by the effectively absorbed photon flux, as detailed in the main text. All scanning photocurrent data were taken by a lockin current measurement, relayed with the chopper of 10 kHz. Reproduced with permission.[8]

by Guo et al.[5] The three segments of the diode devices were fabricated on n-type (001) Si substrate sequentially by a Si-doped n-type GaN, an undoped InGaN and a Mg-doped p-type GaN, as schematically shown in the inset of Figure 17a. The nanowires were planarized by a parylene-insulating layer, and then cov-ering a Ni/Au and indium tin oxide (ITO) layer on top of the insulating layer for ohmic contact to the p-type GaN segment. Aluminum was deposited on the n-type Si to form the bottom electrode. For the ‘white’ LEDs, the InGaN segment contains continuously varied In composition, while in ‘green’ LEDs, a InGaN segment with constant composition was used. Figure 17b shows the current-voltage (I–V) characteristics of the ‘white’ LED. The electroluminescence spectrum and an image of the emitted white light are shown in the inset. The Commission Internationale de I’Eclairage (CIE) chromaticity coordinates of x = 0.31 and y = 0.36 are derived from the electroluminescence spectrum of the ‘white’ nanowire LEDs under a forward-bias current of 300 mA. The broad FWHM of 153 nm of electro-luminescence spectrum resulted from the composition gra-dient of InGaN segment. Comparing with the single bandgap of a constant composition alloyed nanowire, the bandgap dis-tribution of a composition graded nanowire is broadened due to graded alloying of different composition. In another words, comparing to the single wavelength emission of the nanowires with constant composition, the light emission in a nanowire with broad bandgap distribution could cover a larger color region. This was confirmed by comparing to a ‘green’ light LED with an electroluminescence emission peak at ∼520 nm. This

30 wileyonlinelibrary.com © 2012 WILEYVCH Verlag

‘green’ LED is fabricated by a constant composition of InGaN. A line width of ∼50 nm (250 meV) was measured from this elec-troluminescence spectrum. Compared to the narrow FWHM of electroluminescence spectrum in ‘green’ LED, the broad emis-sion in ‘white’ LED have an advantage that could be applied to the white light source study. So the fabrication of a spatial com-position graded nanowire is a feasible strategy to achieve the high efficiency white light emitting.

5. Concluding Remarks and Future Perspectives

Great progress has been made in recent years in search for dif-ferent synthetic strategies of realizing bandgap-graded semi-conductor nanowires. Two kinds of important graded structures have been successfully achieved. One is the single-substrate-based nanowire composition grading with a composition of alloy semiconductor nanowires spatially tuned on a single substrate. Several kinds of CVD growth routes based on metal-catalyzed VLS process have been developed to realize the single-substrate grading, which includes the Temperature Gradient route, the Spatial Source Reagent Gradient route and the Dual Gradient route.[51] The basic idea is to realize a single substrate based grading to get a composition gradient of the reactants across the substrate in 1D or 2D directions during growth. The other reported bandgap-graded structure is the single-nanowire-based composition grading, with the alloy composition or bandgap continuously varying along the length of a single wire. The

GmbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 13–33


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Figure 17. (a) Room temperature currentvoltage characteristics of nanowire LEDs. The fabricated device on (001) Si is schematically shown in the inset; (b) Measured lightcurrent characteristics of “white” InGaN nanowire LED in which In composition varies along the length of nanowires. Insets show the electroluminescence spectrum and microscope image of white light emission. Reproduced with permission.[5] Copyright 2010, American Chemical Society.

main methods used for the growth of single-nanowire compo-sition gradings are also based on metal-catalyzed VLS process, with a dynamically changing of the relative reagent concentra-tion in the vapor during the nanowire growth, resulting from controlling the relative evaporation rate (for solid source) or flux rate (for gas source) of different source materials. Such bandgap-graded semiconductor nanowires have shown prom-ising applications in multifunctional and multispectral opto-electronic devices, including wavelength-tunable nanolasers, full-spectra solar cells, multispectral nanowire photodetectors, and white light LEDs.

The progress achieved in bandgap-graded semiconductor nanowires represents only an initial success in this new field of material science research. Many new opportunities and chal-lenges remain in the coming years. Nanowire composition


grading of new multi-component alloy materials and alloys with broader range of bandgap tunability could be realized. An important breakthrough will be the realization of composition gradings with narrow bandgap alloy materials, such as GaInSb and InAsSb, and the use of these gradings in IR light detectors and tunable lasers. For applications in tunable and multifunc-tional electrical devices, a great challenge in nanowire composi-tion grading is to realize the growth of well-aligned composition graded alloy nanowires on single substrate or along single wires. Future developments of improved growth methods with porous templates could be important in meeting the challenges.

In addition to the growth and corresponding device applica-tions, it is equally important to pursue the novel physical prop-erties of the bandgap-graded nanowires and use them in new nanowire-based functional device designs. For example, using single-nanowire based composition grading, a wavelength-selected optical waveguide could be realized. This novel waveguide property will probably be used in designing a new type of optical logic circuit. In addition, researching the spa-tial bandgap engineering of composition gradient quantum wires will be more important, from both the basic science in these quantum structures and their potential application in novel quantum devices. Finally, research in bandgap-graded nanowires will continue to expand in various directions in the near future, and many new fascinating structures and prop-erties will be realized, together with the corresponding new devices and applications.

AcknowledgementsThe authors are grateful to the NSF of China (Nos. 90923014 and 10974050), the National Basic Research Program of China (No. 2012CB932703), the Hunan Provincial Natural Science Fund for Distinguished Young Scholars (09JJ1009), the Program for New Century Excellent Talents in University (NCET080182) and the Aid program for Science and Technology Innovative Research Team in Higher Educational Instituions of Hunan Province for financial support. The research at Arizona State University is supported by Army Research Office (under Dr. Michael Gerhold).

Received: August 19, 2011 Revised: October 19, 2011

Published online: November 22, 2011

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