Epitaxial Growth and Composition-Dependent Optical Properties of

Aug 31, 2010 - logies, structures, and compositions of the synthesized samples were characterized by scanning electron microscopy (SEM, FEI. Quanta 40...
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DOI: 10.1021/cg9014493

Epitaxial Growth and Composition-Dependent Optical Properties of Vertically Aligned ZnS1-xSex Alloy Nanowire Arrays

2010, Vol. 10 4206–4210

Yao Liang, Haiyang Xu, and Suikong Hark* Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China Received November 26, 2009; Revised Manuscript Received August 19, 2010

ABSTRACT: Vertically aligned ZnS1-xSex alloy nanowire arrays covering the entire compositional range were grown on GaAs (111)B substrates by metal organic chemical vapor deposition (MOCVD) using Ga nanoparticles as catalysts. The ZnS1-xSex nanowires obtained are predominantly zincblende in structure and preferentially grow along the [111] direction. They show a narrow distribution in composition, which is the result of good growth controls afforded by MOCVD. The relationship between composition of the nanowires and the precursors was studied and determined; it allows the predetermination of the composition of the alloy nanowires. In low temperature (77 K) cathodoluminescence spectra, the band edge emission of ZnS1-xSex nanowires is found to shift from 3.214 to 2.779 eV, and its line width significantly decreases from 213 to 44 meV, when the Se content increases from 0.45 to 1. This phenomenon is understood in terms of the chemical disorder in the alloy. Recently, one-dimensional (1D) II-VI semiconductor nanostructures have attracted much attention due to their unique electrical and optical properties and potential applications as building blocks for future nanodevices. Compared with their binary counterparts, ternary alloy semiconductors offer continuously tunable band gaps through variations in compositions, which facilitate band gap engineering. ZnS1-xSex, an important member of II-VI alloy semiconductors, has a band gap tunable from 2.7 to 3.8 eV, which is suitable for developing light emitting diodes (LED) and laser diodes operating in the blue-UV region. However, few reports are available on 1D ZnS1-xSex nanostructures, except for nanowires, nanotetrapods, and wurtzite-twinning-induced three-dimensional nanoarchitectures.1 Moreover, none of them show a vertical alignment. Compared with random assemblies of nanowires, nanowire arrays can truly realize their potential applications at the sublithographic scales and are critical for high density nanodevice assembly.2 Furthermore, it is easy to assemble vertically aligned nanowire arrays to form various electronic or photonic devices with direct charge transport pathways connecting top and bottom electrodes, such as field-effect transistors,3 LED,4 dye-sensitized solar cells,5 and piezoelectric generators.6 Much research effort has been spent in growing vertically aligned nanowire arrays.7 However, considerable challenges remain in synthesizing vertically aligned ternary of II-VI semiconductor nanowire arrays over the entire compositional range. The growth of vertically aligned CdZnS nanowire arrays has only recently been reported.8 Vertically aligned ZnS1-xSex nanowire arrays have not been achieved up to now. Among all synthesis methods, metal organic chemical vapor deposition (MOCVD) is one of the commonly utilized methods for large-scale production of vertically aligned semiconductor nanowires, with high reproducibility and controllability.9 In addition, it allows precise and separate control of the flow rates of precursors,10 which is critical in controlling the composition of alloy nanowires. In this paper, we report the epitaxial growth of vertically aligned ZnS1-xSex nanowire arrays, with a predetermined composition, by MOCVD and studies on their optical properties. Vertically aligned ZnS1-xSex nanowire arrays were synthesized on the GaAs (111)B substrates by MOCVD using Ga nanoparticles as catalysts. The substrates were washed ultrasonically *To whom correspondence should be addressed. E-mail: skhark@ phy.cuhk.edu.hk. pubs.acs.org/crystal

Published on Web 08/31/2010

with 1,1,1-trichoroethane, acetone, and ethanol for 15 min each in succession, rinsed by deionized water, and dried by high-purity (4 N) nitrogen gas. After cleaning, the substrates were annealed at 500 C for 1 h in air to form a GaOx thin layer, and then they were

Figure 1. SEM images in a 30 tilted view of vertically aligned ZnS1-xSex nanowire arrays with different compositions: (a) x = 1, (b) x = 0.69, (c) x = 0.48, (d) x = 0.28, and (e) x = 0. (f ) The corresponding XRD patterns. (g) Enlarged XRD patterns showing the shift of the (111) peak. (h) Lattice constant a as a function of composition x; the compositions labeled were determined by SEMEDX quantitative analyses. r 2010 American Chemical Society

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Figure 2. (a-c) A TEM image, a magnified TEM image, and an HRTEM image taken from a typical ZnS0.52Se0.48 nanowire, which grows along the zincblende [111] direction. Inset is a FFT pattern generated from (c). (d-f ) A TEM image, an HRTEM image, and a corresponding FFT pattern taken from another kind of ZnS0.52Se0.48 nanowire with [311] growth direction. (g) A typical EDX spectrum. (h) An STEM image of a typical ZnS0.52Se0.48 nanowire. (i) Two dimensional STEM-EDX element mappings for Zn, Se, and S.

put into the reactor chamber. The reactor chamber was pumped to 700 Torr. Then, hydrogen gas (7 N) was imported into the reactor chamber at a flow rate of 975 sccm. Next, the chamber was heated quickly to 530 C, and the substrate was annealed at this temperature for 5 min. ZnS1-xSex nanowire arrays were grown at this temperature for 30 min. Diethylzinc (DEZ), diisopropylselenide (DIPSe), and diethylsulphide (DES) as precursors were introduced simultaneously into the reaction chamber to supply Zn, Se, and S species, respectively. The flow rate of DEZ was kept at 0.18 μmol/min throughout the whole growth process, but those of DES and DIPSe were tuned to fabricate ZnS1-xSex alloy nanowires of different compositions. The morphologies, structures, and compositions of the synthesized samples were characterized by scanning electron microscopy (SEM, FEI Quanta 400F), X-ray diffraction (XRD, Rigaku RU-300, using the Cu KR radiation), high resolution transmission electron microscopy (HRTEM, Philips Tecnai 20), and energy dispersive X-ray (EDX, attached to SEM and TEM). Cathodoluminescence (CL) spectra were measured at low temperature (77K) by using a MonoCL II system (Oxford) attached to an SEM. The ZnS1-xSex nanowire arrays of various compositions (x = 0, 0.28, 0.48, 0.69, and 1) were epitaxially grown on the GaAs (111)B substrates using Ga nanoparticles as catalysts; their compositions were controlled by adjusting the flow rate ratios of S and Se precursors. Figure 1a-e presents their SEM images in a 30 tilted view. ZnS1-xSex nanowires are seen aligned vertically on the whole substrate and have diameters of ∼180 nm and lengths of ∼5 μm. The XRD patterns in Figure 1f are dominated by a prominent diffraction peak, which can be indexed to the

(111) reflection of the zincblende structure and whose compositional dependent position shifts from 28.78-27.55 as the content of Se increases (Figure 1g). This indicates that the ZnS1-xSex nanowire arrays have a preferred [111] orientation. Other than the extraneous diffraction peak at 27.50 from the GaAs (111) substrate, there is no evidence of any other phases or impurities (see Supporting Information, Figure S1). The a-axis lattice constants, calculated from the (111) peak positions, are plotted in Figure 1h as a function of the compositions x, which were determined by EDX quantitative analyses. The linear relationship between a and x is consistent with the Vegard’s law.11 The morphology and internal structure of the ZnS1-xSex nanowires were further examined by TEM. Since there is no obvious difference among all samples, the ZnS0.52Se0.48 sample was chosen as an example (another ZnS0.31Se0.69 sample was shown in Figures S2-S3, Supporting Information). Figure 2a shows a TEM image of a typical ZnS0.52Se0.48 nanowire. The Ga particle (confirmed by the EDX spectrum, Figure S4, Supporting Information) located at its tip indicates that its growth follows a vapor-liquid-solid process. In Figure 2b, a magnified image reveals the presence of some stacking faults, which are perpendicular to the growth direction. Stacking faults were usually observed in ZnSe and ZnS nanowires.12 A stacking fault in the zincblende (wurtzite) structure can be regarded as a single segment of wurtzite (zincblende) structure. Its formation is attributed to the bistability of zincblende and wurtzite structures. Its density depends on the diameter of nanowires and growth conditions.13 Figure 2c and its inset show a HRTEM image and its fast Fourier transform (FFT) pattern along the [011] zone axis,

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respectively. These results illustrate that the nanowire is of zincblende structure and grows along the [111] direction. Selected area electron diffractions were also carried out and they corroborated these observations. The lattice fringes with an interplanar spacing of 0.32 nm correspond well to the (111) plane of zincblende ZnS0.52Se0.48. A large number of observations further confirm that [111] is the dominant growth direction in our ZnS1-xSex nanowires. We note that the nanowires are bounded by {111} and {100} side facets of low surface energies.14 In a previous paper, it was also observed that Si nanowires grew along [111] and were enclosed by the {111} and {100} side facets.15 Moreover, it was suggested that impurities could be the nucleation sites for the facet growth. In our results, a stacking fault is observed whenever the side facet changes from one to another. Thus, we suspect that the presence of the stacking faults is strongly correlated with the nucleation and growth of the side facets. Figure 2d-f displays a TEM image, HRTEM image, and FFT pattern along the [112] zone axis of another ZnS0.52Se0.48 nanowire, correspondingly. This nanowire is also zincblende in structure, but its growth direction is [311]. This growth direction has been observed in some semiconductor nanowires.16 The nanowires growing along [311] should align along the [311] direction of the GaAs substrates due to their epitaxial relationship. Thus, a few nonvertically aligned ZnS1-xSex nanowires can be found in all samples grown on GaAs(111)B substrates. Moreover, as stacking faults are rarely observed, such nanowires tend to have high crystal quality and smooth surfaces. Thus, it appears that stacking faults could be suppressed by growing nanowires along the [311] direction. HRTEM studies on many nanowires reveal that the zincblende structure is predominant in ZnS1-xSex nanowires, regardless of their composition. This is different from some previous reports, where ZnS1-xSex nanostructures were found to crystallize as a wurtzite phase over the entire compositional range.1 Compared with wurtzite, the zincblende phase shows some advantages due to its higher crystallographic symmetry.17 Thus, it is highly desirable to synthesize zincblende ZnS1-xSex nanowires. In our work, the formation of zincblende nanowires may be associated with the relatively low growth temperature. An EDX spectrum in Figure 2g shows that the synthesized nanowires consist of only Zn, Se, and S elements. The Cu signal originates from the TEM grid. The spatial distributions of Zn, Se, and S in a single nanowire were obtained by scanning TEM (STEM)-EDX element mapping and shown in Figure 2h,i. The uniform images show that the nanowire is a homogeneous random alloy. We have also studied the distribution of compositions of different ZnS1-xSex samples by EDX (attached to TEM) analyses on many individual nanowires. As shown in Figure 3a, the distributions of the alloys exhibit a narrow and nearly Gaussian peak centered at x = 0.30, 0.45, and 0.69, which are very close to the corresponding composition (x = 0.28, 0.48, and 0.69) determined by SEM-EDX. In the growth of the ZnS1-xSex nanowires, the control over the alloy composition was achieved by adjusting the molar flow ratio of DES and DIPSe. A relationship between the compositions of the precursors and the resulting nanowires is plotted in Figure 3b, which shows a nonlinear behavior. The Se content in the nanowires is sensitive to small amounts of it in the precursors, suggesting that Se, relative to S, is preferably incorporated into the nanowires. This preference may be ascribed to the difference of the pyrolysis rates of DES and DIPSe. Unfortunately, their pyrolysis rates are not available in the literature for a direct comparison. However, we know that the radical-hydrogen bond strengths are 98 and 95 kcal/mol for ethyl and i-propyl radicals, respectively.18 The radical-Se bond strength should also be higher for the ethyl radical than the i-propyl one. It is reasonable to expect, for pyrolysis efficiency, DIPSe > diethylselenide (DESe). We also know that the bond strengths of DES and DESe are 65 and 58 kcal/mol, respectively.18 So the pyrolysis efficiency of DESe is higher than that of DES. Thus, from the

Liang et al.

Figure 3. (a) Compositional distributions of the ZnS1-xSex samples of x = 0.28, 0.48, and 0.69. For each sample, statistical data were obtained from TEM-EDX quantitative analyses on 50 nanowires. The red lines show the Gaussian fittings. (b) Source versus nanowire composition. Inset is an enlarged diagram taken from the region marked by the dotted line.

pyrolysis efficiency point of view, we know DIPSe > DESe > DES. The higher pyrolysis efficiency of DIPSe, compared with DES, results in more Se in the nanowires than in the sources. More importantly, the curve in Figure 3b allows us to predetermine the composition of ZnS1-xSex nanowires by choosing an appropriate flow ratio of S and Se precursors. Only because of the precise adjustment (Figure 3b, inset) of the molar flow ratio in MOCVD were we able to control the alloy composition. Plausible growth stages of the vertically aligned ZnS1-xSex nanowires are illustrated in Figure 4a. First, a GaOx thin layer formed on the GaAs surface during substrate annealing in air. When the substrate was heated to 530 for 5 min under the flow of H2 in the MOCVD reactor, GaOx was reduced to Ga, resulting in Ga droplets on the substrate surface. When the precursors passed over the substrate, they were pyrolyzed and Zn, S, and Se dissolved into the droplets. After supersaturation was reached, they coprecipitated out as ZnS1-xSex. By a continuous incorporation of Zn, S, and Se, ZnS1-xSex nanowires nucleate and grow along the [111] direction preferentially. The lattice mismatch between ZnS1-xSex and GaAs is small (0.6% (ZnSe)-5.5% (ZnS), with ZnSe better matched than ZnS). Their epitaxial relationship leads to the vertical growth of nanowire arrays on the GaAs (111)B substrates (Figure 4b). In fact, based on such an epitaxial relationship, we have succeeded in controlling the orientation of nanowire arrays on GaAs substrates by choosing a different crystallographic surface.19 In Figure 5a, we show representative low-temperature (77 K) CL spectra taken from three individual ZnS1-xSex nanowires of different compositions (x = 1, 0.69, and 0.45). However, for the ZnS1-xSex nanowires of x = 0.28 and 0, their band edge

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Figure 4. (a) Schematics showing the growth mechanism of vertically aligned ZnS1-xSex nanowire arrays. (b) An atomic model showing the epitaxial relationship between the ZnS1-xSex nanowire with Æ111æ growth direction and the GaAs (111)B substrate.

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where EZnSe , EZnS g g , and Eg(x) are the band gap energies of ZnSe, ZnS, and their alloy, respectively, and the b is a bowing parameter. The best fit of eq 1 to our data points yields b = 0.45 eV, which is within the reported range of values 0.4-0.63 eV.20 We also note that the band edge emission peak is significantly broadened from 44 to 213 meV with a decrease in x from 1 to 0.45. This broadening is attributed to a chemical disorder. When composition x is near 0.5, the alloy tends to have the largest disorder, thus giving rise to the nearly largest CL line width in the ZnS0.55Se0.45 nanowires. In summary, we have synthesized vertically aligned ZnS1-xSex nanowires with a predetermined composition by MOCVD using Ga nanoparticles as catalysts. The lattice parameter of ZnS1-xSex nanowire depends linearly on the composition x, and the structure of nanowires are mostly zincblende, a form that is not reported in previous literature. Most of the ZnS1-xSex nanowires grow along the [111] direction, with only a few along the [311] direction. Thus, we can epitaxially grow vertically aligned ZnS1-xSex nanowire arrays on the GaAs (111)B substrates. In addition, the composition of ZnS1-xSex nanowires can be precisely controlled by adjusting the flow ratio of DES and DIPSe. Compared to S, Se is easier to be incorporated into ZnS1-xSex nanowires, a result that is attributed to the higher pyrolysis efficiency of DIPSe. At 77 K, the ZnS1-xSex nanowire band edge emission peak shifts from 3.214 to 2.779 eV when x increases from 0.45 to 1, and its full width at half maximum (fwhm) narrows as x approaches 1. Acknowledgment. The work described in this paper was partially supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project Nos. 411807 and 417507) and CUHK direct grants. Supporting Information Available: XRD patterns; HRTEM images and the corresponding FFT patterns; EDX spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. (a) 77K CL spectra of three single ZnS1-xSex nanowires with different compositions x = 0.45, 0.69, and 1. (b) Compositional dependence of the energy and line width of the band edge emission. The band gap energy of ZnS is 3.82 eV at 77 K (blue spot).21 The red solid line is a fitting to the data points using eq 1.

emissions were difficult to obtain. That is, the band edge emissions were only observed in nanowires of high Se-content. Since ZnSe is very well lattice matched to the GaAs substrate (mismatch 0.6%), high crystalline and optical quality Se rich nanowires are easier to obtain. As the Se content x increases from 0.45 to 1, the band edge emission energy decreases from 3.214 to 2.779 eV (Figure 5b). The relation between the band gap energy of ZnS1-xSex and composition x can be described by the following quadratic function, Eg ðxÞ ¼ xEgZnSe þ ð1 - xÞEgZnS - bxð1 - xÞ

ð1Þ

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