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Journal of Luminescence 130 (2010) 334–337

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Journal of Luminescence

0022-23

doi:10.1

� Corr

E-m

hsdzhe@

journal homepage: www.elsevier.com/locate/jlumin

Synthesis and optical properties of N–In codoped ZnO nanobelts

Lili Wu a,b, Zhengguo Gao c, E. Zhang a,�, Hong Gao a, Hua Li a, XiTian Zhang a,�

a Heilongjiang Key Laboratory for Advanced Functional Materials and Excited State Processes, School of Physics and Electronic Engineering,

Harbin Normal University, Harbin 150025, PR Chinab Center for Engineering Training and Basic Experimentation, Heilongjiang Institute of Science and Technology, Harbin 150027, PR Chinac Research Institute of Electronic Information Products Superintending and Inspecting, Heilongjiang Province, Harbin 15001, PR China

a r t i c l e i n f o

Article history:

Received 24 February 2009

Received in revised form

14 July 2009

Accepted 11 September 2009Available online 18 September 2009

PACS:

78.67.Lt

78.30.Fs

81.07.Vb

81.15.Gh

Keywords:

ZnO nanobelts

Raman

Photoluminescence

13/$ - see front matter & 2009 Elsevier B.V. A

016/j.jlumin.2009.09.013

esponding authors. Tel./fax: +86 4518806062

ail addresses: xtzhangzhang@hotmail.com (E.

126.com (X. Zhang).

a b s t r a c t

N–In codoped ZnO nanobelts were successfully synthesized via high-temperature chemical vapor

deposition for the first time, using the mixture of In/ZnO as a precursor. The EDX spectrum showed that

In was introduced into ZnO nanobelts. In order to better understand the optical properties of N–In

codoped ZnO nanobelts, the Raman and low-temperature PL spectra of the undoped, In-doped and N–In

codoped ZnO nanostructures were measured. By contrasting, N is incorporated into the nanobelts. The

temperature dependent photoluminescence (PL) spectra were investigated. Their PL spectra in the

temperature from 9 K to room temperature were dominated by an AoX emission of excitons bound to

2No–InZn acceptor complexes. The dissociation energy of the acceptor complexes is estimated to be

89–112 meV.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

ZnO has attracted considerable attention due to its potentialapplications in optoelectronic devices operating in the visible andnear ultraviolet. It is necessary to control the concentration of thecarriers and its conduction type for its practical application. For n-type ZnO, its concentration of the carriers is easy to be controlled,but for p-type ZnO, it has proved more difficult to find suitableacceptor dopants due to the existence of self-compensation effect.To eliminate the self-compensation effect, many efforts have beenmade to obtain p-type ZnO films by using various acceptordopants such as nitrogen, arsenic, and phosphorus [1]. Amongthem, N was commonly regarded as a relatively better choice,which was also confirmed theoretically by Park et al. [2]. Recently,many researchers reported the fabrication of p-type nitrogen-doped ZnO [3–8]. Unfortunately, the quality of the p-type ZnO:Nfilms still remained an obstacle to the development for ZnO. Thereason was that the N acceptor was more strongly compensated inZnO, i.e., the hole-carrier density was not high enough forindustrial optoelectronic applications, and a high N dopingconcentration must be achieved, which unavoidably produced

ll rights reserved.

9.

Zhang).

the N-related defects. Recently, the codoping method usingacceptors and reactive donors simultaneously was proposed toincrease the solubility of nitrogen in ZnO [9]. Several groupsreported the preparation of p-type ZnO films by the codopingmethods, such as the codoping of N and gallium [10], N and Al[11], N and beryllium [12], N and indium [13]. Until now,successful preparation of one-dimensional N–In codoped ZnOnanostructures was not reported. In this letter, N–In codoped ZnOnanobelts were successfully synthesized via high-temperaturechemical vapor deposition. The presence of N was confirmed byboth PL and Raman properties of the nanobelts. Since the PL andRaman scattering are a powerful and nondestructive method toexplore the characteristics of doped nanostructures, especially thepresence of the trace element such as N dopant in pure ZnO.Temperature dependent PL can show the dissociation processes ofimpurities introduced by doping, and provide useful informationfor the realization of p-type ZnO.

2. Experimental details

For the synthesis of the N–In codoped ZnO nanobelts, an In/ZnO mixture was loaded into one end of an alumina boat and theSi substrate coated with Au was placed at the other end,downstream of the mixture. The boat was placed inside analumina tube that was inserted into a horizontal furnace and the

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L. Wu et al. / Journal of Luminescence 130 (2010) 334–337 335

mixture was located at the center of the furnace. The furnace washeated to a preset temperature (1400 1C) under a flow of30 ml/min of nitrogen gas and a pressure of 50 Pa. The furnacetemperature was kept for 20 min. Then, the furnace was naturallycooled down to room temperature. The synthesized productswere found covering the entire substrate. They were characterizedby X-ray powder diffraction (XRD, Rigaku RU-300 with Cu Ka

radiation), scanning electron microscopy (SEM, LEO, 1450 VP),equipped with energy dispersive X-ray spectrometer, and trans-mission electron microscopy (TEM, Philips CM 120) and high-resolution transmission electron microscopy (HRTEM, PhilipsTecnai 20). Room-temperature Raman spectra of the N–Incodoped ZnO nanobelts were measured using a micro-Ramanspectrometer (Renishaw, RM1000, UK) in a backscattering con-figuration, employing the 514.5 nm line of an Ar+ laser as theexcitation source. Their PL spectra were taken using the 325 nmline of a He–Cd laser as the excitation source.

Fig. 2. A typical HRTEM image of a nanobelt, the inset represents the

corresponding fast Fourier transform.

300 400 500 600 700

617

578

330

(c) Undoped

(b) In-doped

(a) N-In codoped

588

520

436

376279

Inte

nsity

(a.

u.)

3. Results and discussion

A typical SEM image of the product is shown in Fig. 1. A highdensity of wool-like nanobelts is clearly observed over the entireSi substrate. The top inset of Fig. 1 shows a low-magnification TEMimage of a single nanobelt. From the TEM studies, the nanobeltshave a rectangular cross section. They are very thin andtransparent, about 10mm in length and 50 nm in width. Thereare some dark and bright stripes on the surface of the nanobelt,revealing the presence of strain inside the nanobelt. The bottominset of Fig. 1 shows an EDX spectrum of the nanobelts in theSEM. The peaks O, Zn and In at atomic ratios of 37.5:61.9:0.6(In/(In+Zn+O)�0.6 at%) are observed in which, revealing that Inwas introduced into ZnO nanobelts. Detailed microstructure wasstudied by HRTEM. A high-resolution lattice image of the ZnOnanobelt along the /0 0 0 1S zone axis is shown in Fig. 2, whoseinset represents the fast Fourier transform of the lattice. Thelattice spacing of the planes is 0.28 nm (marked in Fig. 2), agreeingwell with that of the {0 110} planes of wurtzite ZnO, and themicrostructure is defect-free. The image and the transform showthat nanobelts have a single crystalline wurtzite internal structureand grow along the /10 10S direction. In terms of the HRTEMresults, the top surface of the nanobelt in the top inset of Fig. 1should be the {0 0 0 1} crystal plane and the side surface {11 2 0}crystal plane. In addition, the positions of all peaks in the XRDpattern (not shown) agree with those of ZnO powder.

Fig. 1. SEM image of N–In codoped ZnO nanobelts; the top inset shows a TEM

image of a single nanobelt; the bottom inset shows a EDX spectrum of nanobelts.

Raman Shift (cm-1)

Fig. 3. Raman spectra of (a) N–In codoped ZnO nanobelts, (b) In-doped, and (c)

undoped ZnO nanostructures.

Raman and PL spectroscopies are a very sensitive andnondestructive tool for investigating impurities in semiconductornanostructures. Raman scattering was measured on the synthe-sized nanobelts, as illustrated in Fig. 3(a). The peaks at 376, 436 and588 cm�1 were observed. They are attributed to the A1 (TO), E2

(high) and E1(LO) modes, respectively [14]. The E1 (LO) mode istheoretically forbidden according to Raman selection rules in thebackscattering geometry configuration [15]. However, we stillobserve the weak E1 (LO) mode, which could be attributed to adisorderly pile of nanobelts. The peak at 279 cm�1 could be foundonly in the N-doped ZnO film. Such a similar Raman shift hasbeen observed in the N-doped ZnO thin films by many authors[8,16–20]. On the basis of a calculation related to a local phonondensity of states, they suggested that the additional modeoriginates from localized vibration of the N-related complexes

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D20X-2LO

D20X-LO

BX

BX

D20X

D10XD

10X-LO

DAxP

A0X-LO

DAP2

DAP1

(a) Undoped

(b) In-doped

PL I

nten

sity

(a.

u.)

Photon Energy (eV)

A0X

(c) N-In codoped

2.8 3.0 3.2 3.4

Fig. 4. PL spectra at 9 K of (a) undoped, (b) In-doped ZnO nanostructures, and

(c) N–In codoped ZnO nanobelts.

Fig. 5. PL spectra as a function of laser power; the inset shows the peak position

for DAP2 emission plotted against laser power; rectangles represent the

L. Wu et al. / Journal of Luminescence 130 (2010) 334–337336

[16]. The peak at 520 cm�1 is from the Si substrate. To confirm the279 cm�1 peak originating from the incorporation of N in the ZnOnanobelts, the Raman spectra of the undoped [21] and In-dopedZnO nanostructures (synthesis conditions: weight: ZnO 1.0 g;temperature: 1400 1C; growth time: 10 min; growth pressure:9000 Pa; carrier gas: Ar; flow rate: 60 sccm) fabricated at Ar-atmosphere are provided in Fig. 3(b) and (c), respectively. The threeRaman spectra are all dominated by the peak at 436 cm�1. Theother peaks in Fig. 3(c) are located at 330, 376 and 578 cm�1, whichare attributed to the multiphonon process (E2H–E2L), A1 (TO), A1

(LO) modes, respectively [14,22]. And in Fig. 3(b), the peak at617 cm�1 is attributed to the multiphonon process, also [14].However, the peak at 279 cm�1 is both absent in Fig. 3(b) and (c).Thus, together with our observations and other groups’ results[16–20], it would be reasonable to deduce that (1) N is introducedinto In-doped ZnO nanobelts and (2) the N-related complexes maybe formed. The successful incorporation of N in this study mayprovide a route for us to obtain stable and reproducible p-typedoping.

In order to better understand the PL properties of N–Incodoped ZnO nanobelts, low-temperature PL spectra (9 K) of theundoped and In-doped ZnO nanostructures are shown in Fig. 4(a)and (b), respectively. The two spectra are not the same in the peakshape. The spectrum of the undoped ZnO nanostructures(Fig. 4(a)) is decomposed into four emission peaks, located at3.363, 3.343, 3.296 and 3.220 eV, which could be attributed to theradiative recombinations of neutral-donor-bound excitons (D1

0X)[23], excitons bound to a neutral donor or acceptor (BX)[15,24,25], longitudinal optical (LO) phonon replicas of D1

0X anddonor–acceptor pairs (labeled as DAxPs) [26], respectively. TheDAxP transitions were found in some pure ZnO samples by Meyeret al., also [26]. And the acceptor Ax is unknown. The spectrum ofthe In-doped ZnO nanostructures shown in Fig. 4(b) is fitted byfour Gaussian peaks. Compared with Fig. 4(a), the D1

0X emissionpeak at 3.363 eV is disappeared, and a weak shoulder at 3.351 eVappears due to the incorporation of In. It is possible to be anotherkind of neutral-donor-bound exciton (D2

0X) emissions, and In canact as a donor [26]. The BX emission peak in Fig. 4(a) redshifts to3.313 eV and broadens. The peak at 3.313 eV could be the BXemission peak in Fig. 4(a), due to the formation of band-tail stateresulted by the introduction of In dopants, although it has beenpreviously attributed to emissions due to neutral acceptor-bound

excitons or DAPs [27]. The other two peaks at 3.283, 3.205 eV arethe first and second order LO-phonon replicas of D2

0X emission.In order to investigate the PL properties of N–In codoped ZnO

nanobelts, their temperature dependent PL spectra were mea-sured (not shown). The PL spectrum at 9 K is shown in Fig. 4(c). Itis completely different from both PL spectra of the undoped andIn-doped ZnO nanostructures in the peak shape. The shoulder at3.351 eV disappears in Fig. 4(c) while a new and obvious peak at3.056 eV emerges due to the introduction of N–In complex. Thislow-temperature spectrum can be well fitted by four Gaussianpeaks. The dominant sharp peak is located at 3.320 eV, with a fullwidth at half maximum of 58 meV. Since the position of this peakfalls in the range (3.315–3.358 eV) of acceptor-bound excitonsreported for N-, [28] P-, [29] Li–N- [30] and As- [31] doped p-typeZnO thin films, it could be attributed to acceptor-bound exciton(AoX) transition, and a 2No–InZn complex (see below) can act asan acceptor. Other three dominant peaks are located at 3.289,3.245, and 3.056 eV, respectively. The emission line at 3.289 eV islikely attributed to the donor–acceptor pair transition. Similaremission peak was observed in the Li–N-codoped ZnO thin film[31]; the peak at 3.245 eV has an energy 75 meV lower than AoX,which is close to LO-phonon energy (72 meV) in ZnO. Therefore,they are ascribed to DAP1 and the LO-phonon replica of AoX,respectively. The peak at 3.056 eV is associated with another DAPemission (labeled as DAP2). This peak was also observed in N-doped ZnO films [32], further confirming that N is introduced intoZnO nanobelts. In order to confirm the spectral assignment,excitation power-dependent PL spectra were examined at 9 K.Fig. 5 shows an excitation power dependence of PL spectra. Theinset of Fig. 5 illustrates the emission peak energy as a function ofexcitation power. The peak position shifts to higher energies withthe increase of excitation power. This is a typical characteristic ofDAP luminescence. The energy of the DAP2 luminescence isrepresented as follows:

EDAP ¼ Eg � ðEAþEDÞþe2=4per ð1Þ

where Eg, ED, EA, e, e, and r are band-gap energy, donor ionizationenergy, acceptor ionization energy, elementary electric charge,dielectric constant, and a donor–acceptor pair-distance. Theaverage coulomb energy DE=e2/4pe/rS can be very roughlyestimated by letting /rS�(3/4pNA)1/3. It is difficult to obtainhigher hole concentration than 1018 cm�3 for ZnO. Thus, we takeNAE1016–1018 cm�3, giving DEE6.75–27 meV. We can find suchdoing approximation in many references, such as Ref. [33]. N–Incodoping in ZnO films results in a deep donor level of 112 meV

experimental data.

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20 40 60 80 100

4

8

PL I

nteg

rate

d In

tens

ity

(a. u

.)

1000/T (K-1)

Fig. 6. PL integrated intensity of AoX emission as a function of temperature;

rectangles represent the experimental data; the solid curve represents a best

fitting using Eq. (2).

L. Wu et al. / Journal of Luminescence 130 (2010) 334–337 337

according to Ref. [25]. EAE275–296 meV, taken Eg=3.437 eV,according to Eq. (1). This value is closed to the zinc vacancy-related acceptor energy level of 270 meV within the uncertaintiesof the measurements [34], indicating that an intrinsic acceptorstate could be introduced in the N–In codoped ZnO nanobelts. Forthe DAP1, In likely acts as donors, ED=63.5 meV [26]. According toEq. (1), EA=89–112 meV. It is in agreement with 112 meV of 2No–InZn acceptor complex [35], further confirming the formation ofacceptor states caused by In–N codoping in ZnO nanobelts.

For a better inspection of AoX emission properties, thetemperature dependent AoX emission intensity of the N–Incodoped ZnO nanobelts is shown in Fig. 6, in which rectanglesshow experimental data and the curve presents fitted resultsusing the following formula [36]:

IðTÞ ¼I0

1þC1 expð�E1=kBTÞþC2 expð�E2=kBTÞð2Þ

where I(T) and I0 are the PL intensities at temperature T and 0 K,respectively. C1 and C2 are constants, and E1 and E2 aredissociation energies. By Eq. (2), i.e., considering two channelscontributing to the quenching of the AoX with temperature, theexperimental data can be fitted very well, as shown by the solidline in Fig. 5. The two-channel fitting yields E1=40.2 meV, andE2=8.4 meV. It is noted that E2 almost equals to the thermalactivation energy of 1072 meV, excitons bound to structuraldefects (Y-line defect) very common in II–VI semiconductors [26].E1 could be attributed to the ionization energy of excitons boundto 2No–InZn acceptor complex. This is consistent with the reported34 meV in Ref. [35]. In the PL spectra, free exciton peak is notobserved. Its absence is attributed to the dissociation of freeexcitons into free electrons and holes, caused by a substantiallyhigh concentration of the dopant In. The similar result was alsoobserved in As-doped ZnO nanowires [19].

4. Conclusions

N–In codoped ZnO nanobelts were successfully synthesized.The appearance of the Raman shift at 279 cm�1 in the Ramanspectrum confirms that N is introduced into In-doped ZnOnanobelts. The temperature dependent PL spectra indicate that2No–InZn acceptor complexes are formed in N–In codoped ZnOnanobelts and the 9 K PL spectrum is dominated by the AoXemission of acceptor complex bound excitons. The N–In codopingin ZnO nanobelts results in the deeper and deeper donor level, andevidently decreases the self-compensation effect. It is expectedthat p-ZnO could be attainable if the shallow donors could bereduced. The incorporation of N in this study may provide a routefor us to obtain stable and reproducible p-type doing.

Acknowledgements

This work was partially supported by the National NaturalScience Foundation of China under Grant no. 60776010; ScienceFoundation for Distinguished Young Scholars of HeilongjiangProvince (JC200805); the Natural Science Foundation of Heilong-jiang (A2007-03, A200807 and F200828); the Education Bureau ofHeilongjiang Province (11531225 and 11531227); the Project ofOverseas Talent, Personnel Bureau, Heilongjiang Province; and theExcellent Leader of Subjects, Bureau of Science and Technology ofHarbin, Heilongjiang Province (2007RFXXG028).

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