negative photoconductance in heavily doped si nanowire...

1 Negative Photoconductance in Heavily Doped Si Nanowire Field-2 Effect Transistors3 Eunhye Baek,† Taiuk Rim,‡ Julian Schütt,† Larysa Baraban,*,†,§ and Gianaurelio Cuniberti†,§

4†Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany

5‡Department of Creative IT Engineering, Pohang University of Science and Technology, 37673 Pohang, Korea

6§Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany

7 *S Supporting Information

8 ABSTRACT: We report the first observation of negative photo-9 conductance (NPC) in n- and p-doped Si nanowire field-effect transistors10 (FETs) and demonstrate the strong influence of doping concentrations on11 the nonconventional optical switching of the devices. Furthermore, we12 show that the NPC of Si nanowire FETs is dependent on the wavelength of13 visible light due to the phonon-assisted excitation to multiple conduction14 bands with different band gap energies that would be a distinct15 optoelectronic property of indirect band gap semiconductor. We attribute16 the main driving force of NPC in Si nanowire FETs to the photogenerated17 hot electrons trapping by dopants ions and interfacial states. Finally,18 comparing back- and top-gate modulation, we derive the mechanisms of19 the transition between negative and positive photoconductance regimes in20 nanowire devices. The transition is decided by the competition between the21 light-induced interfacial trapping and the recombination of mobile carriers,22 which is dependent on the light intensity and the doping concentration.

23 KEYWORDS: Negative photoconductance, hot electron trapping, interfacial trapping, Si nanowire, indirect band gap semiconductor

24 Negative photoconductance (NPC) is a rare effect because25 the photoexcitation of charge carriers normally enhances26 the channel conductivity.1 In order to reach the situation, when27 the channel conductivity is decreased (NPC), additional28 electronic states are required that can compensate a generation29 of photoelectrons. Some of the low-dimensional materials (e.g.,30 nanoparticles, nanowires, and thin film) reveal a negative31 photoconductance due to the surface effects originating from32 the high surface-to-volume ratio.2,3 Thus, the large surface area33 of nanostructured materials can potentially generate high34 density of localized energy states acting as traps for charge35 carriers, sufficient to reverse the type of the channel36 conductivity. For instance, arrays of metal nanoparticles,37 which are capable of surface plasmon excitations upon light38 illumination, can reveal the NPC due to the presence of39 interfacial charges.2 On the other hand, the NPC in40 semiconductors is of different nature and is linked to the41 energy band gap structure. In many cases, the NPC has been42 observed in large band gap semiconductors such as AlN,4 p-43 ZnSe,5 or Ga2O3

6 with sub-band gap excitation where44 photoexcited electrons can be captured by extrinsic (e.g.,45 surface oxygen) and intrinsic (e.g., defects) trap states in the46 middle of the band gap. Moreover, because photoexcited47 electrons are generated via the superband gap excitation, NPC48 requires additional phenomena like scattering at recombination49 centers in InN.7

50Photoconductivity studies of Si have a long history8,9 as well51as numerous industrial realizations10 because of the well-known52electronic properties and performance, e.g., high speed and53efficient signal processing and compatibility with various54electrical platforms by mature integration. The NPC of bulk55Si was observed for the first time in cobalt-doped Si under the56infrared light illumination.8,11 The localized energy states of57dopants in the band gap of Si act as a powerful recombination58center, which is typical sub-band gap NPC phenomena. During59the past decade, Si and Si nanostructures, especially nanowires,60have been studied for various optical applications, such as61photodetectors,12,13 photovoltaics,14,15 and solar cells,16,17 using62advantages from a one-dimensinal structure and relying mostly63on the phonon-assisted photoexcitation, due to its indirect64bandgap, and generating a conventional “positive” photo-65current.66However, despite the well-developed Si photodetectors67offered on the market and the enormous research and industrial68demands of Si nanowires for various optical applications, the69NPC in Si nanowire devices has not yet been reported. In70particular, modern Si nanowire field effect transistors (FETs)71need proper doping in the conduction channel for effective gate

Received: July 1, 2017Revised: August 30, 2017Published: September 29, 2017

Letter

pubs.acs.org/NanoLett

© XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.7b02788Nano Lett. XXXX, XXX, XXX−XXX

mac00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i12 HF02:4458 | 2.0 alpha 39) 2016/10/28 09:46:00 | PROD-JCAVA | rq_11261579 | 10/02/2017 11:32:49 | 8 | JCA-DEFAULT

pubs.acs.org/NanoLetthttp://dx.doi.org/10.1021/acs.nanolett.7b02788

72 modulation and should include insulating layers in contact with73 the channel area for field effect or surface functionalization for74 bio-18,19 or optical20,21 sensor application. In this situation, the75 devices are working in more complex electrical systems76 including the charge transfer via defects and the interfaces.77 Nowadays optoelectronic integrated systems require active78 and passive photonic devices, like lasers or photodiodes, for79 data transmission and conventional CMOS devices for80 computing.22 Those systems are fast and power-efficient81 compared to pure electronic circuits, but the optics are mainly82 dedicated to signal transmission, which needs to be converted83 back to the electronic signal for actual data processing in the84 CMOS devices. In order to actively involve optics into85 processes, hybrid phototransistors or memories, modulated by86 light, have been developed by combining quantum dots or87 organic films on FETs.20,21,23,24 However, several drawbacks88 still exist in the hybrid devices, such as slow switching speed,89 induced from amorphous organic layers and unidirectional90 current changes upon illumination. In this regard, NPC studies91 of Si nanowire devices would be a critical issue, not only for a

92deeper understanding of the optoelectronic properties of one-93dimensional Si systems but also for realizing Si-based optical94processors which have bilateral switching functionality,95preserving the speed of Si in the CMOS technology.96In this study, we report the observation of NPC in the Si97nanowire FETs with different doping concentrations under98visible light illumination. The photoconductivity of the devices99was investigated depending on the doping type (both p- and n-100type) and concentration as well as the intensity and the101wavelength of light. Interestingly, the reduction of the current is102observed only in the special gate bias condition, which is able to103overcome depletion. We also demonstrate that the main104sources of NPC in FET devices is not only the change of the105mobile carrier density in the conduction band of a SiNW but106also the strong threshold voltage shift induced by the modified107depletion and by interfacial charge from the electron capturing108in dopant ions and interface states or, in other words, the109electric field gating effect. Therefore, dopant type and110concentration play a significant role in the emergence of the111 t1NPC effect in SiNW-based FETs. Table 1 shows previous

Table 1. Negative Photoconductance Observed in Different Systems and Its Mechanism

material dim excitation NPC mechanism year ref

metal Au nanoparticle 0D plasmonic change by charged SAMs 2009 2direct semicond. AlN nanowire 1D sub-bandgap hole trapping by surface oxygen 2010 4

ZnSe nanowire 1D 2011 5Ga2O3 nanobelt 1D 2011 6InN thin film 2D superbandgap scattering by recombination centers 2010 7MoS2 monolayer 2D increasing effective mass by trion 2014 34InAs nanowire 1D hot-carrier trapping by surface states 2015 3

indirect semicond. Au-doped Ge Bulk sub-bandgap recombination by positive donor ions 1960 35Co-doped Si Bulk 1966 8

1971 11P-doped Si nanowire 1D superbandgap gating effect by hot electron trapping in dopants and interface states 2016 this work

Figure 1. Structure and electrical characteristics of honeycomb Si nanowire FETs under illumination. (a) Schematic diagram of the Si nanowireFETs under light illumination. (b) Microscopic image of Si nanowire devices with source and drain transmission line. Scanning electron microscopy(SEM) image of (c) Si nanowire channel area and (d) honeycomb structure of nanowires. (d) Transmission electron microscopy (TEM) image ofcross-section of nanowire and thermal SiO2 layer. (Scale bar: (b) 100 μm, (c) 10 μm, (d) 1 μm, (e) 20 nm.) (f) Transfer (Id−Vg) and (g) output(Id−Vd) characteristics of an n+-doped nanowire device under illumination ((f) Vd = 0.5 V, (g) Vg = 2.3 V). (h) Photocurrent switchingcharacteristics on the time domain with increasing light intensity, (i) 0.022, (ii) 0.089, (iii) 0.442, (iv) 1.8, (v) 8.9 mW/cm2 respectively. (Vds = 0.5 V,Vg = 2 V) λ = 625 nm for (f−h).

Nano Letters Letter

DOI: 10.1021/acs.nanolett.7b02788Nano Lett. XXXX, XXX, XXX−XXX

B

http://dx.doi.org/10.1021/acs.nanolett.7b02788

112 studies that claim single dominant phenomenon leading to113 NPC, for instance, sub-band gap trapping in direct band gap114 semiconductors or carrier trapping by surface states in metal115 nanoparticles. Unlike those studies, we show that the NPC of116 heavily doped Si nanowire FETs is driven by (i) photoexcited117 electron trapping by dopant ions in the nanowires and (ii)118 competitive electron transient between interfacial trapping and119 fast recombination, which are dependent on the doping120 concentration. Therefore, NPC effect is particularly maximized121 in subthreshold region, which is the electrically most sensitive122 area in the FET operation, that agrees the observations of123 transistor-based sensor applications.25 NPC change of Si has a124 special dependence on the wavelength of visible light area due125 to the strong correlation with the carrier generation rate by126 phonon-associated excitation in indirect band gap. Finally, we127 believe that our observation of NPC can be considered, as a128 universal paradigm, extended to any kind of nanostructure,129 having defect and interfacial states.130 Results. Si nanowire FETs with a honeycomb nanowire131 network were fabricated on an 8-in. SOI wafer using

f1 132 conventional CMOS fabrication technique (see Figure 1a−-133 e). The lithography steps are shown in Figure S1 panels i−viii134 in Supporting Information. The bare SOI wafer consists of a135 top-Si layer doped with 1016 cm−3 of boron on a 400 nm-thick136 oxide layer and p-type Si substrate doped with 1015 cm−3 of137 boron. The Si channel area was heavily doped with phosphorus138 to modify the channel conduction properties from the normal139 inversion mode n-type FET to the accumulation mode n-type140 FET. Therefore, the device is normally in an on-state at the gate141 bias Vg = 0 V, which is advantageous for low power sensor142 applications. As a result, the variations of doping concentration143 were 1018 cm−3 and 1019 cm−3 of phosphorus and 1016 cm−3 of144 boron, respectively (see details in Methods). Hereafter, we will145 use the terms “n+”, “n+2” and “p”-doped device to designate the146 above-mentioned doping concentrations. As a nanowire147 channel region, a honeycomb structure was designed using148 electron beam lithography in order to obtain higher signal-to-149 noise ratio and higher current stability at the subthreshold150 voltage regime26,27 (Figure 1c,d). Figure 1e shows a cross151 section of Si nanowire with 50 nm width and 30 nm height,152 which are covered with a 5 nm layer of thermally grown oxide.153 The devices have Ag contact pads with a Ti adhesion layer on154 the source and drain area, which were heavily doped to form155 ohmic contacts (see Figure S2 in Supporting Information) with156 metal transmission lines (Figure 1b). For illumination, four157 light-emitting diodes (LEDs) emitting in the visible range158 (wavelengths, λ = 405, 470, 530, and 625 nm) were used. The159 transmission line method (TLM) measurement was used to160 verify that there is no plasmonic effect by Ag contact.161 In the following, we demonstrate the influence of the light162 illumination on the conductivity of Si nanowire FET devices.163 Figure 1f,g shows the transfer and output characteristics of the164 n+-doped (Nd = 10

18 cm−3) device under illumination at λ =165 625 nm with various light power intensities. Interestingly, the166 threshold voltage (Vth) of the device increases under167 illumination, which causes a distinct decrease of the photo-168 current as the light intensity increases (see inset, Figure 1f).169 The output characteristics of the device also support the clear170 NPC behavior of Si nanowire FETs (Figure 1g). Figure 1h171 shows reverse switching of the channel current (Id) under light172 illumination. The magnitude of the light-induced reversed173 current switching increases (see stages i−v in Figure 1h) as the174 light intensity increases in the time domain.. Contrary to

175expectations that the current should increase due to increasing176number of channel carriers by photoexcitation,28 the Id is177decreasing when the gate bias and light intensity satisfy certain178conditions.179Next, we demonstrate the condition of gate potential that180 f2leads to the NPC in Si nanowire FETs. Figure 2a shows that

181the subthreshold slope of devices increases as the light intensity182increases. Therefore, the NPC is not able to be observed at low183Vg. In order to determine the dependence between the NPC184and the gate bias, the current change ratio (ΔId (%)) is185extracted from the transfer curve (see Figure 2b). When186exposed to the light of very low power intensity (400 μW/cm2), the off current191of the transfer curve also increases because of photoexcitation.192This leads to the increase of the ΔId (%) at low Vg and NPC is193switched to conventional positive photoconductance (PPC).194Therefore, the best area to observe the NPC in FET devices195covering wide range of light intensities is the subthreshold area196near Vg = Vth,dark.197Because the visible light photons have much higher energy198(1.9 eV < Eph < 3.06 eV) than the band gap of Si (Eg = 1.1 eV),199photoexcited electrons behave as hot electrons in Si nanowires.200In general, hot electrons can create trap states in an oxide201layer29 and these additional trap states increase the202subthreshold slope (SS) accordingly to the following equation

Δ =Δ

−⎛⎝⎜

⎞⎠⎟⎡⎣⎢

⎤⎦⎥N

Cq

q eKT

log( ) SS1it

ox

203(1)

204where Cox is oxide capacitance, q is the elementary charge, K is205Boltzmann constant, T is the temperature, ΔNit is density206change of interface state by light illumination, and ΔSS is the207change of the subthreshold slope.30 The degradation of the208subthreshold slope in FET devices is observed by the interface209trap creation under light illumination.30,31 Hence, the increased210SS by photoinduced hot electrons raises the current at low Vg.211For this reason, the NPC is only observed at high Vg to cause212the channel depletion.

Figure 2. Current change depending on gate bias under lightillumination (a) Transfer characteristics (log scale) of the n+-doped Sinanowire FETs upon illumination. (b) The current change

(Δ =−

I (%)I V I V

I Vd( ) ( )

( )d g d,dark g

d,dark g) in light condition as a function of Vg (λ

= 625 nm).

Nano Letters Letter


C

http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.7b02788/suppl_file/nl7b02788_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.7b02788/suppl_file/nl7b02788_si_001.pdfhttp://dx.doi.org/10.1021/acs.nanolett.7b02788

213 From now on, we will discuss how various factors, such as214 doping concentration and light intensity, influence the NPC of

f3 215 Si nanowire devices (Figure 3). Since strong NPC was observed216 near Vth, we extracted the light-induced threshold voltage shift217 (ΔVth = Vth,light − Vth,dark) from the Id−Vg curve for various218 doping concentration of nanowires (Figure 3a). Using the219 constant current method, Vg at Id = 200 nA was defined as Vth.220 Since Vth was changed under the light illumination, the221 photoconductance in FET devices are mainly due to the shift of222 Vth. Under illumination with light of low intensity light (

300 was formed on the middle of the honeycomb nanowire array301 covered with an oxide gate dielectric. Pt is chosen to protect the302 nanowire from any additional optical effects like surface303 plasmonic effects in the visible range. The back gate was304 floating in order to electrically isolate the nanowire from the305 substrate. The fabrication steps are shown in Method.306 Figure 4c shows the change of the photoinduced current with307 the various intensities of the light, extracted using the same308 method as for Figure 3b. Even when the substrate effect is309 eliminated, the photocurrent is reduced when the sample is310 illuminated with a low light intensity. The current change ratio311 in the top-gate device is much smaller than that in the back gate312 devices due to the absence of the strong potential drop. Similar313 to the data shown in the inset in Figure 3b, ΔId of the top-gate314 device also decreases when illuminated with light of low315 intensity and increases as the light intensity increases. The ΔId316 is attributed to the combination of the two major phenomena317 such as carrier trapping and photogeneration that are inducing318 NPC and PPC, respectively.319 Figure 4d shows the energy band diagram of the Si and SiO2320 interfaces that describe the physical origin of the NPC and321 PPC. When photons are absorbed by Si, electron−hole pairs322 are created with the optical generation rate g. Due to the high323 energy of visible photons, hot electrons are generated in the324 conduction band with a density of nhot, leaving hot holes, phot in325 the valence band. Mobile hot electrons can thermally transit to326 interface states, Eit and the edge of the conduction band with327 the transit time constant τit and τ′, respectively. The density328 change of electrons at conduction band edge, Δn, combines the329 thermally relaxed hot electrons and the detrapped electrons330 from the interface states with the detrapping time constant, τdit.331 The electrons at the conduction band edge are recombined332 with the holes at the valence band edge, Δp. Because Si is an

333indirect band gap semiconductor, the recombination process334must involve the defect states transition in the band gap with335the recombination time constant τr.336In order to understand the effect of the interfacial trapping337on the change of the photoconductance of the devices, we338consider a model describing the dynamics of the density change339of the photoexcited carriers

τ τ= − − −

′

⎡⎣⎢

⎤⎦⎥

nt

gn n

Nnd

d1hot hot

it

it

it

hot

340(4)

τ τ= − −

⎡⎣⎢

⎤⎦⎥

nt

n nN

ndd

1it hotit

it

it

it

dit 341(5)

τ τ τΔ = + −

Δnt

n n pdd

hot

it

it

dit r 342(6)

τ τΔ

=′

−Δp

t

p pdd

hot

r 343(7)

τ= −

′p

tg

pd

dhot hot

344(8)

345where nit is the density of the interface trapped electrons and346Nit is interface state density. The photoconductivity change can347be expressed with the excess mobile carriers under illumina-348tion3

σ μ μΔ = + Δ + + Δq n n q p p( ) ( )n hot p hot 349(9)

350where μn and μp are the mobility of electrons and holes,351respectively.352In the steady state, we can obtain the solutions of eqs 4−8353depending on the interface trapped electrons.3 If the light354intensity is very low, then most of the interface trap states are355empty (nit ≪ Nit). Therefore, Δσ is expressed as

σ μ ττ ττ

μ τ τΔ ≈ −′

= −⎡⎣⎢

⎤⎦⎥q g q g[ ]n r

dit

itn r b

356(10)

357where τ = τ τ

τ′

bdit

it.3 Since electrons are majority carriers in n-type

358FETs and the mobility of electrons is much higher than that of359hole, the excess electron density in the conduction band is the360major contribution to the light-induced conductivity change in361the nanowire.362If τr < τb, then Δσ < 0 under illumination, which implies the363 t2NPC. From Table 2, estimated τb is in a range of 10 μs to 10364ms. Therefore, τr of each heavily n-doped device (for both n

+

365and n+2) (≤1 μs) is much smaller than τb, which agrees with the366NPC behavior of heavily n-doped devices (Figures 3 and 4c). It367implies that the electron trapping by the oxide interfacial layer

Figure 4. Negative photoconductivity of the Si nanowire devicewithout substrate effect. (a) Schematic diagram of Si nanowire FETswith top platinum electrode. (b) SEM images of the top electrodeconfiguration on a honeycomb nanowire device. (c) Photocurrentchange of the n+-doped top gate devices depending on light powerintensity. The dark current level was 100 nA. (λ = 625 nm) Thedashed lines are fitted curves. (d) The schematic energy band diagramof Si and SiO2 interface explaining the hot carrier generation by lightillumination, interfacial trapping and release of excess electrons andrecombination process via defect states.

Table 2. Experimental Values of Carrier Life Time in DopedSi

parameter lifetime ref

τdit 0.01−1 s 36τit 0.1−1 nsτ′ 1 ps (at RT) 37

τr Nd = 1019 cm−3 0.1 μs 38

Nd = 1018 cm−3 1 μs

Na = 1016 cm−3 100 μs 39

Nano Letters Letter


E


368 is more preferable than electrons staying at the edge of the369 conduction band and that it limits the mobile carrier density.370 On the other hand, Δσ of the p-doped device could be negative371 or a positive depending on the interface trapping and372 detrapping time constants, which could possibly induce the373 weak NPC or the PPC in the nanowire device. This estimation374 agrees with the strong increase of ΔId and opposite trend of375 ΔVth of the p-doped device under illumination with a low376 intensity of light (Figure 3).377 On the other hand, if the light intensity increases then the378 interface states are fully filled with photoexcited electrons, i.e.,379 nit ≈ Nit. Here we obtain an approximation for the channel380 conductivity

σ μ τΔ ≈ −q g N( )n r it381 (11)382 The derivation of all equations is described in ref 3. First, Δσ383 could positively increase proportional to g and τr when the384 product overcomes the constant negative component, Nit,385 leading to the PPC effect. As a result, NPC is limited and the386 current increases with the increase of the light intensity.387 Second, there is a competition between g and τr depending on388 the doping concentration. As the doping concentration389 increases, g increases and τr decreases. Therefore, the strong390 PPC in the p-doped device is expected and caused by the large391 τr. Explaining of the effect of the PPC on the dopant392 concentration in the heavily n-doped devices is not393 straightforward, due to higher generation rate of heavier (n+2)394 doped device in spite of shorter τr. Consequently, the n

+2-395 doped device shows stronger PPC effect than the n+-doped396 device does in Figure 3b. The detailed logarithmic changes of397 photocurrent of the devices with the back gate are shown in398 Figure S5 in Supporting Information.

f5 399 Figure 5a shows a wavelength dependence of the NPC in the400 n+-doped device upon visible light illumination. When

401 illuminating with light of low light intensity (0.1 mW/cm2),402 the NPC is linearly dependent on the wavelength. It follows the403 generation rate, which is an inverse function of the energy of a404 photon (g = αIph/Eph, where Iph and Eph are light intensity and405 photon energy, respectively). However, under illuminating with406 light of strong intensity (10 mW/cm2), only red light (λ = 625407 nm) is resposnible for the decrease of ΔId, whereas the other408 spectra cause ΔId to increase. The wavelength dependent409 current change with various doping concentration is shown in410 Figure S5 in Supporting Information. It implies that the411 interface states are seldom filled with red light absorption. That

412is because the excitation probability is comparably small due to413the low photon energy of red light as shown in Figure 5b.414Because the photon energy of the red light (1.96 eV) is lower415than the energy band gap on the L-valley in the Si, the excited416electrons are directly injected into the Γ-X band. However, hot417electrons generated by the photon energy above the 2.2 eV can418enter both X- or L-valleys with phonon assisted transition.32,33

419The absorption of the red light leads two effects; (i) no420photoconduction in the L-band and (ii) quasi sub-bandgap421excitation. Regarding the first aspect, (i) the excess of mobile422carriers is generated only in the X-valley, strongly limiting423carrier generation, unlike other visible light-induced excitation,424which allows both valleys conduction. With respect to the425second aspect: though the visible light absorption induces426superbandgap excitation (i.e., Eph > Eg), excited electrons could427be captured by defect states (Ed) in the band gap between the428L-band minimum and the X-band minimum during the429momentum change by phonon absorption or emission. This430is similar to the sub-bandgap trapping reported previously.4−6

431This phenomenon is expected only in indirect band gap432semiconductors. Thus, quasi sub-bandgap trapping would be433highly probable with the red light absorption, which could434enhance the NPC.435In conclusion, we have demonstrated a negative photo-436conductance effect in Si nanowire FETs with different doping437concentrations as well as under illumination conditions, i.e.,438light wavelengths and intensities. It is the first observation of439the NPC, induced by the hot trapped carriers in nanoscaled440semiconductors with indirect gap. It is an important message441for the readership audience working with Si-based nano-442technology (and nanowires particularly), which remains443dominant in semiconductor industry. The main sources of444the NPC are the light induced Vth shift by photoexcited445electrons trapped at the interface (outside of nanowire) and446dopants ions (inside of nanowire). The interfacial trap state447explains the doping concentration dependence, but the dopants448ion trapping becomes important for the doping types (n- or p-449type). The NPC of nanowire devices depends on doping450concentration, such that heavily n-doped devices show a strong451NPC behavior due to its longer interfacial trapping time. Also,452the NPC and PPC occurs by means of light intensity which453determines the carrier generation rate competing with the454carrier recombination lifetime. NPC effect appears differently at455wavelengths in visible light region due to the phonon assisted456excitation to multiconduction bands in the indirect band gap Si.457Finally, the analysis of heavily doped Si nanowire based458devices and comparison with NPC observed in other459nanomaterials, leads to a conclusion that the NPC is a460universal phenomenon for low-dimensional systems, due to the461stronger influence of the surface/interface states. The control-462lable bipolar optical current switching may open novel463possibilities in Si nanostructure-based electronic applications464like optical integrated logic circuits, photonic function465generators or even bio/chemosensor applications. For the466latter, because NPC phenomenon is observed in the467subthreshold regime of the transfer characteristics, bio/468chemical nanosensors, typically driven in this regime, can469potentially benefit from this new discovery. In particular, NPC470can be considered as a measure of biochemical gating of the471nanoscaled FET devices.472Methods. Device Fabrication. Si nanowire FETs were473fabricated on three 8-in. SOI wafers that consist of a 40 nm-474thick top-Si layer (Boron, 1016 cm−3), a 400 nm-thick buried

Figure 5. (a) Wavelength dependence of photocurrent change of then+-doped back gate device with weak and strong light power intensity.(b) The schematic energy band diagram of Si under visible lightillumination. The energy band gap of Γ- and L- valley of Si and theenergy of red (λ = 625 nm) and violet (λ = 405 nm) light are shown inthe diagram.

Nano Letters Letter


F

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475 oxide layer and the 725 μm-thick p-type Si substrate. To476 implant phosphorus ions in the top Si layer, the 20 nm SiO2477 buffer layer was deposited using PECVD at 300 °C. After that,478 phosphorus ions are implanted with an energy of 15 kEV and479 the concentrations of dopants were 1013 cm−2 for 1018 cm−3

480 and 1014 cm−2 for 1019 cm−3 samples. Rapid thermal annealing481 followed at 1000 °C for 20 s in N2 atmosphere to activate482 dopants. Finally, the buffer oxide layer was stripped in 1:100483 dHF for 2−3 min. Consequently, one of the wafers has a boron484 concentration of 1016 cm−3 and the other two wafers have485 phosphorus concentration of 1018 cm−3 and 1019 cm−3,486 respectively. An active area including the channel, the source487 and the drain region was defined for electrical isolation of488 devices using photolithography and inductively coupled plasma489 reactive ion etching (ICP-RIE). The source and the drain490 region were formed using phosphorus ion implantation with a491 concentration of 5 × 1020 cm−3 and dopant activation followed492 using the same recipe above. A honeycomb nanowire was493 patterned on the channel region using electron beam494 lithography and etched with ICP-RIE. The pattern width of495 the nanowire was 50 nm and the length of the nanowire was 8496 μm. A 5 nm thick gate oxide layer was grown on the nanowire497 using a wet oxidation furnace at 850 °C for passivation and498 postprocessing. To form the source and the drain electrodes, a499 500 nm Ag layer on 50 nm Ti adhesion layer was deposited500 using an electron-beam evaporator and liftoff process was501 followed. Finally, the whole wafer area except the nanowire502 channel region and metal contact pad was passivated with 2 μm503 thick SU-8 epoxy-based photoresist to protect the long504 transmission line from unwanted contamination. The Figure505 S1 in Supporting Information shows the schematics of all steps506 for the device fabrication.507 Top Gate Fabrication. Top gate electrodes were fabricated508 by patterning PMMA 950k using electron beam lithography,509 followed by a lift-off process with sputtered platinum. The resist510 was spin-coated on the honeycomb nanowire devices at a speed511 of 1000 rpm for 60 s, resulting in a 120 nm thick PMMA film.512 The top electrode pattern was written by electron beam. Then,513 the samples were immersed into the H2O/IPA (1:3)514 development solution for 3 min and cleaned in isopropanol.515 After, a thin chromium adhesion layer (3 nm) was thermally516 evaporated and a 30 nm platinum layer was sputtered on it. The517 chip was immersed into acetone for 15 min to remove the518 PMMA layer. Applying this protocol allowed connection of the519 gate electrode to the honeycomb nanowires with a thin Pt520 electrode with a width of 650 nm, which covers approximately521 8% of the nanowire area. Finally, the sample was annealed in522 200 °C to reduce the contact resistance.523 Photocurrent Measurement. A 4-channel light emitting524 diode (LED) driver (DC-4100, Thorlabs) which includes four525 visible LEDs (λ = 405, 470, 530, and 625 nm) was used as a526 visible light source. The driver could control the power527 intensity of light illumination and select the wavelength. The528 driver was connected to the collimator through liquid529 waveguide to illuminate the target device area with equivalent530 light power. The collimator was installed on the handmade531 metal dark box with 5 cm height.532 The bias generation and current measurement of FET533 devices were performed using Keithley 2600B at room534 temperature. The Keithley device was controlled by manual535 lab-view program.

536■ ASSOCIATED CONTENT537*S Supporting Information538The Supporting Information is available free of charge on the539ACS Publications website at DOI: 10.1021/acs.nano-540lett.7b02788.

541Additional figures for device fabrication step, substrate542potential measurement, optical current switching, and543wavelength dependent photoconductivity (PDF)

544■ AUTHOR INFORMATION545Corresponding Author546*E-mail: [email protected] Baraban: 0000-0003-1010-2791549Gianaurelio Cuniberti: 0000-0002-6574-7848550Notes551The authors declare no competing financial interest.

552■ ACKNOWLEDGMENTS553This work was supported by the German Excellence Initiative554via the Cluster of Excellence EXC1056 “Center for Advancing555Electronics Dresden” (CfAED) and the MSIP (Ministry of556Science, ICT and Future Planning), Korea, under the “ICT557Consilience Creative Program” (IITP-R0346-16-1007) super-558vised by the IITP (Institute for Information and communica-559tions Technology Promotion). We further acknowledge the560support from the Initiative and Networking Fund of the561Helmholtz Association of German Research Centers through562the International Helmholtz Research School for Nano-563electronic Networks (IHRS NANONET) (No. VH-KO-606).564We would like to give special thanks to Nils Puetz for support565in top-electrode patterning and Anh Thuy Phuong Nguyen for566reading and discussion.

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negative photoconductance in heavily doped si nanowire...

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