Rapid Growth of m-plane Oriented Gallium Nitride Nanoplates on

Jun 12, 2013 - Crystal Growth Centre, Anna University, Chennai-600 025, India. §. School of Mechanical Engineering, Beijing Institute of Technology, ...
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Rapid Growth of m‑plane Oriented Gallium Nitride Nanoplates on Silicon Substrate Using Laser-Assisted Metal Organic Chemical Vapor Deposition P. Thirugnanam,† W. Xiong,† M. Mahjouri-Samani,† L. Fan,† R. Raju,‡ M. Mitchell,† Y. Gao,† B. Krishnan,‡ Y. S. Zhou,† L. Jiang,§ and Y. F. Lu*,† †

Department of Electrical Engineering, University of Nebraska−Lincoln, Lincoln, Nebraska 68588-0511, United States Crystal Growth Centre, Anna University, Chennai-600 025, India § School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China ‡

ABSTRACT: The m-plane-oriented gallium nitride (GaN) nanoplates were successfully grown on silicon (Si) substrates at 450 °C, using laser-assisted metal organic chemical vapor deposition (L-MOCVD). The morphology and orientation of the nanoplates were confirmed using highresolution electron microscopes. GaN nanoplates served as seed crystals for the subsequent growth of m-plane-oriented interlinked GaN nanoplates at a longer growth time. The strong A1 (TO) mode in Raman spectra and the (101̅0) peak in X-ray diffraction confirmed the m plane orientation of the nanoplates. The interlinked GaN nanoplates showed a highgrowth rate of ∼38 μm/h. The results suggest that L-MOCVD is a promising technique for the rapid growth of m-plane-oriented GaN nanoplates on the Si substrates at low-growth temperatures.

1. INTRODUCTION The growth of GaN nanostructures (e.g., nanowires and nanorods) is predominant along the ⟨0001⟩ direction due to the high surface free energy of the c plane.1−3 Several attempts were made to grow the gallium nitride (GaN) nanostructures oriented along the direction or along the m plane.1,2 The oriented GaN nanostructures can be utilized to overcome the quantum-confined stark effect (QCSE) [i.e., separation of the electron and hole wave functions within the quantum wells (QWs)], which degrades the recombination efficiency of GaN-based photonic devices.4−6 However, the growth of oriented GaN nanostructures requires expensive lattice-matched substrates, including lithium aluminate (γ-LiAlO2), m- and r-plane sapphire (Al2O3), and m-plane silicon carbide (4H-SiC).4,5 In addition, the growth of GaN involves high-temperature methods, including metal organic chemical vapor deposition (MOCVD, ∼930 °C), molecular beam epitaxy (MBE, ∼800 °C), and hydride vapor phase epitaxy (HVPE, ∼750 °C).7−9 All of these methods require a long processing time of ∼2 h.1−10 The growth rates of GaN were reported to be 200, 4, and 1 μm/h in HVPE, MOCVD, and MBE techniques, respectively.10,11 The potential applications of GaN demand a simple approach for the rapid and selective growth of GaN nanostructures. Laser-assisted MOCVD (L-MOCVD) is a suitable method for enabling the rapid growth of materials at a low temperature in a predefined area.12−16 In addition, another obvious advantage of the L-MOCVD over conven© XXXX American Chemical Society

tional MOCVD is the ability to generate patterned material deposition on substrates without involving complex lithography steps.14,17 Several semiconductor materials, including gallium arsenide (GaAs), cadmium telluride (CdTe), indium phosphide (InP), indium antimonide (InSb), and aluminum nitride (AlN), were grown using the L-MOCVD technique.15 Similarly, GaN can be grown at low substrate temperatures using L-MOCVD compared to conventional growth methods. The growth rate can be remarkably enhanced due to the rapid processing nature of L-MOCVD. There are few reports available discussing the growth of GaN using L-MOCVD.16 In this study, the m-planeoriented GaN nanoplates have been successfully grown on lattice-mismatched (Si) substrates using L-MOCVD. The rapid growth of GaN was realized by pyrolytically decomposing the source precursors at 450 °C.

2. EXPERIMENTAL SECTION GaN nanoplates were grown on Si (100) substrates using a home-built L-MOCVD system at 450 °C. The L-MOCVD chamber was first evacuated to a base pressure of 1 × 10−3 Torr after loading a Si substrate. Trimethylgallium (TMGa) and ammonia (NH3) were used as the gallium (Ga) and nitrogen (N) sources, respectively. The molar flow rate of NH3 was maintained at 2.2 mmol/min. The TMGa was carried into the growth chamber using a nitrogen carrier gas (N2) at a molar flow rate of 446 μmol/min. The chamber pressure was Received: April 12, 2013 Revised: June 5, 2013

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maintained at 450 Torr during the deposition process. Once the chamber pressure was stabilized, the Si substrate was shined and heated with a carbon dioxide (CO2) laser beam of diameter 2.5 ± 0.5 mm through a zinc selenide (ZnSe) window. The power of the laser beam was fixed at 14 W. The temperature of the growth region was monitored using a pyrometer (Omega, OS3752). The GaN nanoplates were grown at different deposition times from 1 to 25 min. After the growth process, the GaN nanoplates were characterized as follows. The morphology of the GaN films was examined using a field-emission scanning electron microscope (FESEM, S4700) and a high-resolution transmission electron microscope (HRTEM, FEI Tecnai). The composition of the nanoplates was analyzed using an energy dispersive X-ray spectrometer (EDX, Oxford X-max 20 mm2). The crystallinity and structure of the GaN nanoplates was studied using a micro-Raman spectroscope (Renishaw Invia, Argon ion laser, λ = 514 nm) and a powder X-ray diffractometer (Rigaku D/Max B diffractometer, Co Kα1 λ = 1.788 Å). The optical properties of the nanoplates were examined using a home-built photoluminescence spectrometer (Spectra Physics, wavelength-tunable Argon ion laser, λex = 244 nm).

The high-resolution transmission electron microscopy (TEM) was applied to understand the growth direction and crystallinity of the GaN nanoplates. Figure 2 (panels a and b)

3. RESULTS AND DISCUSSION 3.1. Morphological Analysis. The FESEM images in Figure 1 (panels a−c) show the micrographs of GaN

Figure 2. TEM images of GaN nanoplates grown at 5 min. (a) GaN nanoplate, (b) lattice fringes of the GaN nanoplate, and (c) schematic diagram showing different planes of the GaN nanoplate.

show the low- and high-magnification TEM images of a GaN nanoplate. The lattice fringes in Figure 2b indicate the single crystallinity of the GaN nanoplate. The interplanar spacing of 0.28 nm matches well with the lattice spacing of the (101̅0) plane, which corresponds to the m plane of GaN.19 The schematic diagram in Figure 2c represents the c and m planes of the GaN nanoplates. Figure 3 (panels a−c) compare the FESEM images of interlinked GaN nanoplates grown at 7, 10, and 15 min,

Figure 1. FESEM images of GaN nanoplates grown at (a) 1, (b) 3, and (c) 5 min. (d) EDX spectrum of GaN nanoplates grown at 5 min.

nanoplates grown at 1, 3, and 5 min, respectively. The GaN film deposited at 1 min revealed the formation of nanoplates, as shown in Figure 1a. The shape of the nanoplates was similar to a semicircle and semihexagon. The arrows in Figure 1a indicate the hexagonal facets of the nanoplates. The GaN nanoplates grown at 3 min had similar shapes compared to the nanoplates grown at 1 min, as shown in Figure 1b. As the growth time was further increased to 5 min, the nanoplates overlapped with the adjacent nanoplates and formed interlinked GaN nanoplates, as shown in Figure 1c. The hexagonal facets of the nanoplates can be clearly seen at 5 min growth. The diameters of the GaN nanoplates were ∼200 ± 50, 250 ± 50, and 900 ± 50 nm at 1, 3, and 5 min, respectively. The thicknesses of the GaN nanoplates were 40 ± 10 nm, 40 ± 10 nm, and 90 ± 10 nm at 1, 3, and 5 min, respectively. The EDX results in Figure 1d confirmed the presence of Ga and N elements in the nanoplates, as well as the Si signal from the Si substrate.18 This proved that the as-grown nanoplates were of the GaN phase. The atomic concentrations of Ga and N were 52.63 and 43.42, respectively.

Figure 3. FESEM images of GaN nanoplates grown at (a) 7, (b) 10, (c) 15, and (d) 25 min.

respectively. As the growth time increased, the thickness of the nanoplates increased from 100 nm to 1 μm. The nanoplates joined with the adjacent counterparts in all directions. The dotted circles in Figure 3 (panels b and c) indicate the overlapping of nanoplates along the c axis, while the continuous circles in Figure 3 (panels b and c) indicate the overlapping of the m and c planes of GaN. At 25 min, large GaN crystals were obtained, as shown in Figure 3d. The orientation of the GaN B

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the c and m planes of GaN, the m-plane-oriented nanoplates were dominant at longer growth times.29 The repeated deposition on the c and m plane of the nanoplates led to the formation of oriented nanoplate networks. The m plane of the nanoplate networks was parallel to the Si substrate, as shown in Figures 5d and 1c. The FESEM images in Figure 5e clearly revealed the growth of nanoplates on the c and m planes of GaN. The c plane of GaN was hardly observed in the interlinked GaN nanoplates, as shown in Figures 1c and 3 (panels a−c). Therefore, the repeated growth on the c and m planes of the nanoplates could lead to the formation of mplane-orientated nanoplate networks. Typically, growth of m-plane GaN requires lattice-matched substrates and high-growth temperatures.1,30 However, in this approach, the m-plane-oriented GaN nanoplates were obtained on a lattice-mismatched substrate at 450 °C, which is a lower growth temperature compared to the MOCVD, HVPE, and MBE techniques.1,11,30 The GaN was grown at a high-growth rate of ∼38 μm/h compared to the MBE (∼1 μm/h) and MOCVD (∼4 μm/h) methods.10,11 In addition, the overall processing time of L-MOCVD was ∼20 min rather than several hours, as required for the MOCVD, HVPE, and MBE techniques.11 Hence, the L-MOCVD is a promising method for growing m-plane GaN nanoplates at low temperatures with high deposition rates.15,17 3.3. Structural and Optical Properties. Wurtzite GaN belongs to the space group C46v with two formula units per primitive cell. In accordance with group theory, the Raman active modes of GaN are A1 + E1 + 2E2, where two nonpolar E2 modes are Raman active and A1 and E1 modes are both Raman and infrared active.31,32 The vibrational modes of the GaN nanoplates were examined using a micro-Raman spectroscope under a Z(X,X)Z̅ backscattering geometry, where, Z and Z̅ represent the projection direction of the incoming and scattered light and X represents the polarization direction of the incoming and scattered light.32,33 Figure 6 shows the microRaman spectra of GaN nanoplates grown at 1, 3, and 5 min, respectively. The GaN nanoplates grown at 1 min showed a strong E2H mode at 569 cm−1 and relatively weak A1 (TO), E1 (TO), and A1 (LO) modes at 530, 560, and 738 cm−1, respectively. The strong E2H mode is characteristic of wurtzite GaN.31 GaN nanoplates grown at 3 min showed a relatively weak E2H mode compared to GaN nanoplates grown at 1 min. The strong A1 (TO) mode was obtained for GaN nanoplates grown at 5 min, as shown in Figure 6. The nanoplates grown at 5 min did not show the E2H mode. In accordance with the Raman selection rule, the c-plane GaN will show a strong E2H mode in the Z(X,X)Z̅ geometry, whereas the m-plane GaN will show a strong A1 (TO) mode.2,31−33 Since the m plane of the nanoplate networks was orienting parallel to the Si substrate, the argon (Ar) ion laser beam interacts only with the m plane. This can be attributed to the absence of an E2H mode and the presence of a strong A1 (TO) mode for GaN film grown at 5 min. The presence of a strong A1 (TO) mode confirmed the interlinked GaN nanoplates, showing its m-plane upward. To further confirm that the A1 (TO) mode is from the m plane of GaN, the Raman spectrum was recorded on a c-plane GaN epitaxial layer under Z(X,X)Z̅ geometry. The c-plane GaN showed a strong E2H mode at 571 cm−1, as shown in Figure 6. This further confirmed the m-plane orientation of the interlinked GaN nanoplates.31,32 Figure 7 compares the X-ray diffraction (XRD) patterns of GaN nanoplates grown at 1 and 5 min. The XRD peaks,

crystal on the Si substrate was similar to the GaN nanoplates (i.e., the c axis of GaN was parallel to the substrate surface). The transition of thin GaN nanoplates into large-size crystals resulted by the increased growth time. 3.2. Growth Mechanism. The growth rates of different planes of hexagonal crystals were reported to have the following tendency [0001] > [011̅1̅] > [101̅0] > [011̅1].20,21 Typically, due to the high-growth rate of the c plane, the GaN nanostructures grow along the ⟨0001⟩ direction.20 However, in this study, oriented GaN nanoplates were obtained. Lu et al. reported that the growth rate of the c plane would not necessarily be higher than other planes if the film was grown at nonequilibrium conditions.3 Growth orientations other than [0001] can be achieved for the films deposited at a high-growth rate and/or low temperature.3,22,23 In our study, the growth rate of the GaN film was higher compared to the traditional MOCVD. The cross section of the GaN film was examined using a FESEM to measure the thickness. Figure 4a shows the

Figure 4. (a) Cross-sectional view of GaN nanoplates grown at 5 min. Schematic diagram of diffusion paths in (b) L-MOCVD and (c) traditional MOCVD reactions.

cross-sectional FESEM image of GaN nanoplates grown at 5 min. The estimated growth rate of the GaN film was ∼38 μm/ h. Hersee et al. reported a growth rate of 2 μm/h for GaN nanowires grown by MOCVD.24 The high-growth rate can be attributed to the three-dimensional (3D) diffusion path of the reactive gases in the L-MOCVD (Figure 4b),17,15 while the traditional MOCVD has a one-dimensional (1D) diffusion path (Figure 4c). In addition, the GaN nanoplates were grown at a low substrate temperature (450 °C) compared to the GaN nanostructures grown at 930 °C by MOCVD.7 The highgrowth rate and low-growth temperature can be attributed to the formation of oriented GaN nanoplates.3 The schematic diagram in Figure 5a represents the GaN nanoplates grown at 1 min. Once the substrate was completely covered by the nanoplates, the gas precursors that had reached the substrate had to nucleate either on the c or m planes of the nanoplates, as shown in Figure 5 (panels b and c).25−29 Therefore, at higher growth times, the GaN nanoplates served as seed crystals for the subsequent growth of the oriented nanoplate networks. As the growth time was increased further, the GaN growth continued on the different facets of the nanoplates. Due to the negligible lattice mismatch between C

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Figure 5. Schematic diagram demonstrates the growth of GaN nanoplates. (a) GaN grown for 1 min. Growth of m-plane-oriented nanoplates on the (b) c and (c) m planes. (d) GaN grown for 5 min. (e) FESEM images of GaN nanoplates grown on the c and m planes of the nanoplates.

including (101̅0), (0002), and (101̅1), can be indexed to wurtzite GaN with the hexagonal structure, which are in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 01-070-2546. The GaN nanoplates grown at 1 min showed a strong (0002) peak, which indicates the c-axis orientation of the nanoplates. The degree of orientation can be estimated by the relative texture coefficient,19,34 which is given as TC0002 =

I0002/I 0 0002 I0002/I 0 0002/I10 1̅ 0/I 010 1̅ 0

(1)

where TC0002 is the relative texture coefficient of the (0002) peak over the (101̅0) peak. I(0002) and I101̅0 are the measured intensities of the (0002) and (1010̅ ) peaks, respectively. I0(0002) and I0101̅0 are the intensity values of the (0002) and (101̅0) peaks of GaN powders obtained from the JCPDS card no. 01070-2546, respectively. The textured coefficient of GaN powders with random crystallographic orientation is 0.5.19 The textured coefficient of GaN nanoplates grown at 1 min is 0.923, which indicates a strong c axis orientation of the GaN nanoplates. This is obvious from Figure 1a, which clearly showed the formation of GaN nanoplates and that the c axis of most of the nanoplates was perpendicular to the substrate surface. On the other hand, GaN film grown at 5 min showed a strong (101̅0) peak, as shown in Figure 6b, which corresponds to the m-plane of GaN. The relative texture coefficient was used to estimate the degree of orientation using the following relation:19,34

Figure 6. Raman spectra of GaN nanoplates grown at 1, 3, and 5 min.

TC10 1̅ 0 =

I10 1̅ 0/I 010 1̅ 0 I10 1̅ 0/I 010 1̅ 0/I0002/I 0 0002

(2)

where TC101̅0 is the relative texture coefficient of the (101̅0) peak over the (0002) peak. The estimated TC101̅0 of interlinked GaN nanoplates grown at 5 min was 0.563. This result indicates that the as-grown interlinked GaN nanoplates are preferentially oriented along the direction. The XRD data are in

Figure 7. XRD patterns of GaN nanoplates grown at 1 and 5 min.

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Crystal Growth & Design good correlation with the FESEM, HRTEM, and Raman results. Figure 8 compares the photoluminescence (PL) spectra of the GaN nanoplates grown at 5 min. The GaN nanoplates

ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Science Foundation (NSF CMMI 1068510 and 1129613).

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Figure 8. Photoluminescence spectra of GaN nanoplates grown at 5 min.

grown at 5 min showed a near-band-edge (NBE) emission at 3.4 eV, which is closer to the NBE emission of bulk GaN (3.47 eV).35,36 In addition, GaN nanoplates showed a broad deep level emission (DLE) at 2.21 eV. The DLE emission at 2.21 eV can be attributed to nitrogen vacancies and gallium interstitials of the GaN nanoplates.32,33 This correlates well with the EDX spectra that confirmed the low atomic percent of nitrogen in the as-grown GaN nanoplates. On the other hand, in the nanoregime, due to the high surface-to-volume ratio of the nanostructures, the surface-related defects will also generate emissions in the PL spectrum. The nitrogen vacancy, gallium interstitial, and surface-related defects can be attributed to the DLE emission of GaN nanoplates.7,35−37 The NBE emission peaking at 3.4 eV confirmed the good optical quality of the GaN nanoplates.

4. CONCLUSIONS The interlinked m-plane-oriented GaN nanoplates can rapidly be grown on a Si substrate using the L-MOCVD method. A growth rate of ∼38 μm/h has been achieved using the LMOCVD method. The FESEM and HRTEM images confirmed the formation of GaN nanoplates. The growth direction of GaN nanoplates was found to be the direction. The A1 (TO) mode of Raman further confirmed the m-plane orientation of the GaN nanoplates. The high deposition rate, low-growth temperature, and repeated growth on the different facets of the nanoplates resulted in the formation of m-plane-oriented interlinked GaN nanoplates. Therefore, the L-MOCVD is a suitable technique for the rapid growth of m-plane-oriented GaN nanoplates on Si substrate at low-growth temperatures. This method can be extended to the rapid growth of other III-nitride materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 402-472-8323. Fax: 402-472-4732. Notes

The authors declare no competing financial interest. E

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(34) Yang, H.; Song, Y.; Li, L.; Ma, J.; Chen, D.; Mai, S.; Zhao, H. Cryst. Growth Des. 2008, 8, 1039. (35) Chin, A. H.; Ahn, T. S.; Li, H.; Vaddiraju, S.; Bardeen, C. J.; Ning, C. Z.; Sunkara, M. K. Nano Lett. 2007, 7, 626. (36) Li, Q.; Wang, G. T. Nano Lett. 2010, 10, 1554. (37) Hofmann, D. M.; Kovalev, D.; Steude, G.; Meyer, B. K.; Hoffmann, A.; Eckey, L.; Heitz, R.; Detchprom, T.; Amano, H.; Akasaki, I. Phys. Rev. B 1995, 52, 16702.

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