Growth and characterization of gallium nitride ...

Growth and characterization of gallium nitride nanowires produced on different sol-gel derived catalyst dispersed in titania and polyvinyl alcohol matrix A. Chatterjee Center for Condensed Matter Sciences, National Taiwan University, Taipei-106, Taiwan

S. Chattopadhyaya) Institute of Atomic & Molecular Sciences, Academia Sinica, Taipei-106, Taiwan

C.W. Hsu Department of Chemistry, National Taiwan Normal University, Taipei-116, Taiwan

C.H. Shen Institute of Atomic & Molecular Sciences, Academia Sinica, Taipei-106, Taiwan

L.C. Chen Center for Condensed Matter Sciences, National Taiwan University, Taipei-106, Taiwan

C.C. Chen Department of Chemistry, National Taiwan Normal University, Taipei-116, Taiwan

K.H. Chen Institute of Atomic & Molecular Sciences, Academia Sinica, Taipei-106, Taiwan

H.Y. Lee Research Division, Synchrotron Radiation Research Center, Hsinchu, Taiwan (Received 4 October 2003; accepted 8 March 2004)

Sol-gel derived catalyst systems of cobalt, nickel, and iron were used in the growth of gallium nitride (GaN) nanowires by thermal chemical vapor deposition. A diffusion barrier matrix of titania (TiO2) has been used in which the catalysts were dispersed to have control of the catalyst particle sizes and hence on the size and morphology of the GaN nanowires. This single-step and cost-effective processing of the catalyst bed produced good-quality GaN naowires with comparable structural and optical properties with those previously reported. In a particular case, a stress-induced cubic admixture to the otherwise hexagonal structural symmetry was observed. The samples were characterized by high-resolution scanning electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and cathodoluminescence studies.

I. INTRODUCTION

There has been substantial development in the field of gallium nitride (GaN) research during the past three decades, and the community is well aware of most of the potential optoelectronic and microwave applications of the wurtzite (hexagonal) and zinc blend (cubic) phases. Some fundamental and technological hurdles still exist before the material becomes commonplace. The problem of synthesizing the material to the structure, quality, and size of our choice and whatever lack of understanding of

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0220 1768

J. Mater. Res., Vol. 19, No. 6, Jun 2004

the defects associated with the material continue to provide the impetus for the ongoing research. Several deposition techniques have been adopted to produce different forms1–13 of the GaN material that may have optoelectronic uses. The advent of the nanotubes underlined the utility of the one-dimensional (1D) nanostructures in understanding the effect of dimensionality on the various physical, optolectronic, and other general properties of matter. GaN also occupied its due place in the 1D world with its nanowires/nanorods form14–17 that also showed the lasing action.18 Catalyst-assisted growth of GaN nanowires involving the vapor–liquid–solid (VLS) mechanism12,15,17 has been abundant due to the relative ease in the control of the dimensions. The selection of the catalyst material, the process of dispersing them with © 2004 Materials Research Society

A. Chatterjee et al.: Growth and characterization of GaN nanowires produced on different sol-gel derived catalyst

uniform and controllable sizes on substrates of our choice is the mother step for subsequent nanowire growth and study. The diameter, the aspect ratio, and the density of the nanostructures are controlled by such catalyst particle synthesis and its distribution on the substrate on which the nanowires are to be grown. Selective area growth of InN has been demonstrated on gold-patterned silicon substrates.19 Carbon nanotubes, with controlled dimensions, have also been selectively grown on lithographically patterned substrates, however, the limitations of lithography lie in the low yield of the process. Most studies on GaN growth by catalyst-assisted chemical vapor deposition (CVD) use catalyst particles in the absence of any diffusion barrier. This will result in a loss of control of the catalyst size and also the growth of the nanowires when the temperature is raised during the deposition. The utility of using a diffusion barrier has been described in a recent review.20 In this report, we prepared different catalyst systems, by the age-old sol-gel technique, with titanium dioxide or polyvinyl alcohol-6000 (PVA) as a diffusion barrier matrix to arrest any growth of the catalyst particles and dispersed them on a silicon substrate. This, however, was done in a single step and did not complicate the whole process of the growth of GaN nanowires. The objective was the control of the nanowire dimensions and their optoelectronic and microstructural properties by tailoring the catalyst systems.

II. EXPERIMENTAL

The catalyst (Co, Ni, Fe) preparation was done from a solution of respective nitrates in 1-propanol and nitric acid to keep the pH of the solution equal to 1. Only for gold (Au) catalyst a salt (HAuCl4) was used. Titaniumn-butoxide was added dropwise to this solution with constant stirring, and care was taken to avoid the fast hydroxylation of the titanium alkoxide. An aqueous solution of PVA powders was used to get the PVA matrix in which the catalysts were dispersed. In a particular case, a noncatalytic Mg was introduced in the catalyst bed by mixing Mg(NO3)2·6H2O in the solution to lower the active catalyst (Ni) concentration. The solution was spincoated onto the silicon substrates, which were then ready for the reduction treatments under hydrogen atmosphere and subsequent growth of GaN nanowires. The growth of GaN was done in presence of molten gallium and ammonia gas in the thermal CVD chamber (quartz tube) described elsewhere.15 Two substrates, one containing molten gallium and the other containing the catalyst system, were introduced in the quartz tube and kept at a separation of 1–10 cm from each other, with the substrate having the catalyst being placed downstream with respect to the flow of the ammonia gas. The quartz tube

was kept inside a furnace. The deposition temperature was 900 °C and the deposition time was 3 h. The morphology of the GaN nanowires was studied by a high-resolution scanning electron microscopy using a JEOL (Tokyo, Japan) 6700F field-emission scanning electron microscopy (SEM) system. This SEM system had a Gatan (Oxford, UK) Mono CL attached with it that can perform temperature-dependent cathodoluminescence studies. The microstructure was studied by a combination of Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and x-ray diffraction (XRD). Raman spectroscopy was done in a Renishaw (Gloucestershire, UK) Micro-Raman spectroscope fitted with a 532-nm laser. FTIR spectra were obtained with a Bomem (Hartmann-Braun, Switzerland) MB series FTIR spectrometer in the reflection mode. The resolution of the spectrum was 4 cm−1. The XRD studies were done in an in-house system that uses a Mac Science (Yokohama, Japan) 18-kW rotating anode x-ray generator. III. RESULTS AND DISCUSSION

The different catalyst systems show substantial growth of nanowires but with certain differences in their morphologies. The GaN nanostructures grown on Au-PVA [Fig. 1(a) or Co-PVA systems [inset, Fig. 1(a)] were morphologically more defective with polycrystalline nature of the thick wires (>100 nm). Thin single-crystalline nanowires were not observed. This may be due to the rapid dissociation and subsequent removal of the PVA matrix at high deposition temperatures (900 °C) allowing the catalysts to coalesce to bigger microclusters. This instability of PVA at high temperatures was known but still intentionally used to produce pure GaN nanowires with controlled dimensions but without any remnant diffusion barrier matrix after the nanowire growth. By changing the dispersing matrix to TiO2, a dramatic improvement in the morphologies of the GaN nanowires was observed. Now the GaN nanowires were mostly single crystalline with much smaller diameters (30– 100 nm) and lengths in the scale of micrometers and overall high aspect ratios as in the case of GaN nanowires on Co–TiO2 catalyst system [Fig. 1(b) and inset]. Only the Fe–TiO2 catalyst system produced short and irregular GaN nanowire morphology as shown in the two low- and high-magnification micrographs [Fig. 1(c) and inset]. The GaN nanowires prepared in Ni–TiO2 system with a noncatalytic Mg produced exceptionally straight and faceted nanowires [Fig. 1(d) and inset] of much lower density. The nanowires carried at their tip catalyst particles [Fig. 1(d)] confirmed by energy dispersive analysis of x-rays, whereas the bulk of the wire is purely GaN clearly signifying the VLS growth mechanism. Highresolution transmission electron microscopy (HRTEM) studies confirmed the single-crystalline predominantly hexagonal nature of the nanowires prepared with the


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FIG. 1. Scanning electron micrographs for GaN nanowire samples prepared on (a) Au–PVA, (b) Co–TiO2, (c) Fe–TiO2, and (d) Ni:Mg–TiO2 catalyst system. Inset in (a) shows GaN nanowires grown on Co–PVA. Insets in (b) and (c) show the respective high-magnification micrographs. Inset in (d) shows low-magnification micrograph of GaN nanowires grown on Ni:Mg–TiO2 catalyst.

Co–TiO2 catalyst system (Fig. 2). The long axis of the nanowires were found to be along the [100] direction. The transmission electron diffraction image (inset, Fig. 2) corresponds to the hexagonal symmetry also. The typical phase pure hexagonal GaN is signified in the XRD by the (100), (002), and (101) triplet at 2␪ values of 32.4°, 34.6°, and 37.0° with intensity ratios of 70:50:100, respectively (JCPDS File No. 2-1078). XRD

FIG. 2. HRTEM image of a single-crystalline GaN nanowire produced with the Co–TiO2 catalyst system. The inset shows the corresponding diffraction pattern for hexagonal GaN. 1770

studies revealed that the GaN nanowires produced by the Au–PVA or Co–PVA systems could be indexed to the hexagonal structure [Fig. 3(a)] with a ⳱ 3.184 and c ⳱ 5.180 Å.3,21 The peak at a 2␪ value of 44.6° originated from the glue used in the XRD apparatus. The GaN nanowires prepared from the Ni:Mg–TiO2 catalyst system showed predominantly hexagonal features in its diffraction pattern [Fig. 3(b)], but compared to the GaN nanowires on the Ni–TiO2 system [inset, Fig. 3(b)], its (002) reflection was relatively large indicating some preferred orientation along the [002] direction as the long axis of the nanowires deposited in our system is always along [100] direction and grown parallel to the substrate. A shape effect contributing to the enhanced (002) cannot be ruled out, because the GaN nanowires prepared with Ni:Mg–TiO2 catalyst system produced exceptionally straight nanowires [Fig. 1(d)] in comparison to the morphologically more curved nanowires produced by the Ni–TiO2 catalyst system. A greater increase in the (002) reflection was seen in the Co–TiO2 case [Fig. 3(c)]. This may be due to a cubic (111) admixture in the hexagonal structure, because the hexagonal (002) and cubic (111) (2␪ ⳱ 34.45°) coexist around the same 2␪ values [Fig. 3(c)]. The hexagonal (110) and the cubic (220) also coexist around 2␪ values of 58° [Fig. 3(c)].22 The fingerprint peak for the cubic GaN was observed at diffraction angles of 2␪ ⳱ 40.8° corresponding to the cubic (002) plane [Fig. 3(c)].23 To verify this observation, FTIR studies were done, and the presence of this mixed cubic and hexagonal



FIG. 4. Fourier transform infrared transmission spectra for GaN nanowires prepared on Au–PVA, Ni:Mg–TiO2, and Co–TiO2 catalyst systems.

FIG. 3. XRD spectra (2␪ scan) of GaN nanowires prepared on (a) Au–PVA, (b) Ni:Mg–TiO2, and (c) Co–TiO2 catalyst systems. Inset in (b) shows the XRD spectrum for Ni–TiO2 catalyst system.

phases in the GaN nanorwires were confirmed. The vibrational frequencies in semiconductors depend mostly on the nearest neighbor interactions whereas hexagonal and cubic GaN differ only in the next nearest neighbor, so that both their transverse (TO) and longitudinal (LO) optical phonon frequencies are very close to each other. Infrared studies on hexagonal GaN (h-GaN) revealed the frequencies for the TO and LO bands at 560 and 746 cm−1, respectively.24 However, studies on the cubic structure were far less. A recent experimental study on the optical phonons in cubic and hexagonal GaN by infrared spectroscopy25 revealed a clear shift in the TO frequencies to lower wavenumbers in case of cubic GaN, which agreed with the calculations of Miwa et al.26 The transmission spectra of the TO phonon regime of the hexagonal GaN, around 558 cm −1 , was hence probed.24,25 The Au–PVA catalyst system produced predominantly hexagonal GaN with a symmetric mode at approximately 558 cm−1 (Fig. 4). The GaN nanowires grown on the Ni:Mg–TiO2 catalyst system although shows a predominantly hexagonal TO mode around 558 cm−1, but its asymmetry along the lower wavenumber side was also observed. The presence of a small cubic fraction, for the GaN nanowires on Ni:Mg–TiO2 catalyst system, cannot be ruled out.25 The purely cubic

crystals had the signature of the TO phonon around 552 cm−1.25,26 The TO phonon mode in the FTIR spectra of the GaN nanowires grown with Co–TiO2 system shifted clearly toward the lower wavenumber side and can be deconvoluted for peaks at 558 cm−1 and 552 cm−1 (Fig. 4), accounting for the hexagonal and cubic phases, respectively.25 The GaN nanowires reported in Ref. 15 using various “free-standing” catalysts all had hexagonal symmetries (from XRD), whereas the same deposition system (as used in Ref. 15) produced the mixed phase nanowires when sol-gel derived catalysts embedded in barrier matrix were used. In order to understand the driving force for any creation of cubic phases in GaN nanowires, precision XRD studies were done on commercially available h-GaN powders (∼2 ␮m) (Johnson Matthey, Royston Hertfordshire, UK), purely hexagonal GaN nanowires with approximately 50-nm diameters (prepared by thermal-CVD using chemically preformed gold nanoparticles as catalyst) and GaN nanowires produced by the Co–TiO2 catalyst system where an admixture of the cubic phase has been confirmed. The three most intense XRD peaks within the 2␪ range of 30° to 40° were probed for each sample (Fig. 5). One must note that the XRD reflects the separation of the lattice planes lying along the long axis of the nanowires but not those lying perpendicular to the long axis (growth direction). So the (100), (002), or (101) reflections predominantly come from those nanowires that have these individual planes along their growth direction. A true shift of these reflections toward higher diffraction angles would imply a decrease in the separation of the neighboring lattice planes (biaxial compressive stress) lying along the growth direction. Consequently, the nanowires will experience a tensile stress along the growth direction, and the separation of the lattice planes perpendicular to the growth direction will increase. Clearly, there has been substantial shift in the position of these peaks toward higher 2␪ values when we go from the GaN powder to


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FIG. 5. XRD spectra for GaN powders, h-GaN nanowires, and GaN nanowires grown with Co–TiO2 catalyst system. The arrows indicate the shift in the position of the 〈100〉 and 〈002〉 XRD peaks.

the purely hexagonal GaN nanowires and then to the GaN nanowires with cubic admixture. The shift [⌬(2␪)] of the (100), (002), and (101) XRD peaks for the GaN nanowires grown on the Co–TiO 2 catalyst system (Fig. 5) as compared to h-GaN powders are 0.18, 0.20, and 0.20, respectively, which is much higher than that reported by Seo et al.27 for their hexagonal GaN nanowires. This shift in the XRD peaks is not the result of decreasing nanowire diameters, as the GaN nanowire diameters in Ref. 27 and our studies are similar. Again, this shift is not due to any internal calibration error, which was taken care of prior to each measurement, and sufficiently high setting accuracy (±0.0025°) and reproducibility (±0.001°) was maintained ensuring a resolution of 0.06° along the 2␪ axis of the XRD spectrum. This shift [⌬(2␪)] of the XRD peaks would then imply a biaxial compressive stress27 in the nanowires as compared to the stress-free powders of GaN. This is expected for 1D nanowires (Fig. 5) that experience a compressive stress in the radial direction and a tensile stress along the growth direction. The lattice constant a and c will decrease and increase, respectively, as a result of this shift and can be calculated27 for the unit cells giving rise to the (100) and (002) peaks in the XRD in Fig. 5. The lattice constants a and c were calculated to be 3.165 Å and 5.20 Å for the GaN nanowires grown with the Co–TiO2 catalyst system. Compared to the strain values calculated for hexagonal GaN nanowires,27 along the x and z axes, the strain values for the GaN nanowires prepared by the Co–TiO2 catalyst system being presented in this work is about 2.5 times more. This is true for both the (100) and (001) unit cells that gave rise to the (100) and (001) peaks, respectively, in the XRD. It was difficult to find the cubic signature by transmission electron microscopy studies, probably due to its small volume fraction. These compressive and tensile stresses, exceeding certain values, can bring about a partial phase change, and a presence of cubic phase can be envisaged in the sample having the maximum stress indicated by the maximum shift 1772

of the XRD peaks with respect to those in h-GaN powders. The origin of this stress is not known at the moment but might arise from the catalyst-diffusion barrier matrix. The Raman spectra (Fig. 6) for the GaN nanowires grown on different catalyst systems resembled those reported earlier.3,15 It had a prominent feature around 570 cm−1 and two comparatively weaker features around 424 cm −1 (zone boundary phonon) and 740 cm −1 [A1(LO)].15 The mode at 424 cm−1 was initially assigned to the zone boundary phonon,15 but a recent study28 showed that it is the overtone of an acoustic phonon although further confirmatory tests are required. The feature around 570 cm−1 is a convolution of several peaks arising due to the A1(TO) around 530 cm−1, E1(TO) around 563 cm−1, and E2(high) around 570 cm−1. Usually, E2 (high) is the strongest signal for GaN reported in the literature and also for most of the samples in this study. However, the GaN prepared with Au–PVA and Fe–TiO2 catalyst systems shows a stronger A1 (TO) line. The GaN prepared with the Au-PVA catalyst were polycrystalline in nature, and GaN prepared with the Fe–TiO2 catalyst system were short with irregular morphology unlike the nanowires prepared with other catalyst systems. The stronger A1 (TO) may be due to some orientation dependence in the GaN nanowire samples. In certain cases, such as Co–PVA [Fig. 6(a)] or Fe–TiO2 [Fig. 6(b)], the broad A1(TO) peak separates out at approximately 524 cm−1. There are reports of a LO phonon-plasmon (LPP) mode around 518 cm−1, which is a function of the free electron concentration in GaN.29 The full width at half-maximum (FWHM) of this mode indicates the possibility of a degenerate A1(TO) and LPP vibration mode. The evident large FWHM values of the 570 cm−1 mode for the GaN on Co–TiO2 systems

FIG. 6. Raman scattering spectra for GaN nanowires: (a) for Au– PVA, Co–PVA, and Co–TiO2 catalyst system; (b) for Fe–TiO2, Ni:Mg–TiO2, and Ni–TiO2 catalyst system.



[Fig. 6(a)] demonstrates the convolution effect. In some case, as in Ni–TiO2 and Fe–TiO2, there was a signal from amorphous silicon, at 480 cm−1 [Fig. 6(b)], that may have formed from the heat treatment of the silicon substrate. No significant signature of the cubic GaN was observed for the Co–TiO2 catalyst system. Room-temperature cathodoluminescence (CL) studies were performed to check the optical properties of the GaN nanowires. The CL emission intensity was probed as a function of the wavelength for the GaN nanowires (40–120 nm diameter) produced on different catalyst systems (Fig. 7). A broad but strong emission at 3.35 eV was found for GaN naowires grown on Co–TiO2 and Ni:Mg– TiO2 catalyst systems, but this emission weakened significantly when the Fe–TiO2 and Ni–TiO2 catalyst systems were used. This 3.35 eV band is believed to be the near band edge (excitonic) emission in hexagonal GaN whose bandgap at 0 K is approximately 3.44 eV.30 Along with the 3.35 eV emission, the GaN nanowires on the Co–TiO2 and Ni:Mg–TiO2 catalyst systems had a clear band at approximately 2.95 eV (Fig. 7) reported to be the LO phonon replica of a free to bound transition in cubic GaN.31 However, the XRD measurements showed the cubic reflections only for GaN on Co–TiO2 catalyst system. Hence, the 2.95 eV band for the GaN grown on Ni:Mg–TiO2 catalyst system may be due to the recombination between compensating deep donors and shallow Mg acceptors.32 This might be indicative of the possibility of Mg incorporation in the GaN nanowire by the method being discussed in this paper. Although a recent work of GaN prepared on sapphire substrates also reports a 2.9 eV CL band, which they claim to be coming from GaN nanorod–buffer interface.33 The yellow luminescence (YL) band around 600 nm, reported to be due to Ga vacancies in GaN,34 was strong and broad when the Ni–TiO2 and Ni:Mg–TiO2 catalysts were used (Fig. 7). The YL band was absent in the Co–TiO2 and Fe–TiO2 catalyst systems. The Au–PVA system that produced the

wurtzite GaN had a broad asymmetric emission around 3.35 eV and a broad and weak YL band. Apart from the cathodoluminescence spectrum where a difference in the hexagonal and cubic GaN near bandedge emission was evident, all the other vibrational techniques employed to characterize the material had minimal difference between the hexagonal and cubic reflections. This is due to the basic structure of hexagonal and cubic GaN where the two differ only in the next nearest neighboring atoms. IV. CONCLUSIONS

The catalyst system in a dispersive matrix produced by the sol-gel technique was efficient in growing GaN nanowires by the thermal CVD technique with qualities as good as those previously reported by similar or different growth techniques. The method of producing catalysts used here, apart from the fact of being easier and cost effective, gives us the option of a diffusion barrier matrix for the catalysts without any extra added step during growth. The TiO2 matrix proved to be an efficient diffusion barrier for the transition metal catalyst system. The addition of a noncatalytic metal, such as Mg, could be used as an additional growth control parameter especially for controlling the density and morphology of the nanowires. The XRD results indicated the presence of predominantly hexagonal GaN for most catalyst systems and a mixed cubic and hexagonal phase when cobalt catalyst was used in the titania matrix. This result was corroborated by the presence of the 2.95 eV peak, reportedly for the free to bound transition from a level about 0.22 eV deep in the gap of cubic GaN, in the cathodoluminescence spectrum. However, the presence of the mixed phase was also illustrated from the FTIR measurements where a clear shift of the TO phonon mode (558 cm−1), of the hexagonal GaN, was observed toward the lower wavenumbers signifying the cubic admixture. We believe by subtle control and choice of the catalyst, diffusion barrier matrix and growth parameters, straininduced phase transformation in GaN nanowire is possible. ACKNOWLEDGMENTS

We thank Prof. H.L. Liu for useful discussions regarding the Raman spectroscopic results. One of the authors (S. C.) acknowledges a postdoctoral fellowship from Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan. The work was done under grants from National Science Council (NSC), Taiwan, Republic of China, and Ministry of Education, Taiwan, Republic of China. REFERENCES FIG. 7. Room-temperature cathodoluminescence emission spectra for GaN nanowires grown on Fe–TiO2, Co–TiO2, Ni:Mg–TiO2, Au–PVA, and Ni–TiO2 catalyst system.

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