Adel Najar*a,
Muhammad Shafaa and
Dalaver Anjumb
aDepartment of Physics, College of Science, United Arab Emirates University, Al Ain 15505, United Arab Emirates. E-mail: adel.najar@uaeu.ac.ae
bImaging and Characterization Lab, King Abdullah University of Science and Technology (KAUST), 23599-6900, Thuwal, Kingdom of Saudi Arabia
First published on 18th April 2017
Herein, we report on the studies of GaN nanowires (GaN NWs) prepared via a metal-assisted photochemical electroless etching method with Pt as the catalyst. It has been found that etching time greatly influences the growth of GaN NWs. The density and the length of nanowires increased with longer etching time, and excellent substrate coverage was observed. The average nanowire width and length are around 35 nm and 10 μm, respectively. Transmission electron microscopy (TEM) shows a single-crystalline wurtzite structure and is confirmed by X-ray measurements. The synthesis mechanism of GaN NWs using the metal-assisted photochemical electroless etching method was presented. Photoluminescence (PL) measurements of GaN NWs show red-shift PL peaks compared to the as-grown sample associated with the relaxation of compressive stress. Furthermore, a shift of the E2 peak to the lower frequency in the Raman spectra for the samples etched for a longer time confirms such a stress relaxation. Based on Raman measurements, the compressive stress σxx and the residual strain εxx were evaluated to be 0.23 GPa and 2.6 × 10−4, respectively. GaN NW synthesis using a low cost method might be used for the fabrication of power optoelectronic devices and gas sensors.
Virtually, to date, all reported synthetic schemes for GaN-based nanowires have employed either laser ablation,12 chemical vapor transport,9,11,13,14 or hybrid vapor epitaxy.15 Most of these processes use Ga metal as the high-temperature vapor source. Metals such as Ni, Fe, and Au have been used as initiators for vapor–liquid–solid (VLS) nanowire growth,12 although self-catalytic VLS or a direct vapor-solid process can also generate nanowires in this system.16 A recently developed photoetching system for n-type GaN, in KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K2S2O8), was studied by Weyher et al.17,18 The development of the metal-assisted electroless etching to synthesise silicon nanowires was well established during the last few years.19,20 However, a few reports explored the synthesis of GaN NWs using metal-assisted electroless etching, but the etching mechanism was not well established.21 Furthermore, the optical and mechanical properties of the GaN NWs, generated via the metal-assisted photochemical electroless method, are not well developed.
In this study, we report on the fabrication of GaN NWs, using a metal-assisted photochemical electroless method. The morphological structures of GaN NWs prepared under different etching times were investigated. The etching mechanism of GaN NWs was developed. The optical properties, the compressive stress σxx, and the residual strain εxx were investigated using PL and Raman measurements.
Fig. 2 shows the high-resolution TEM images, and the corresponding calculated fast Fourier transform (FFT) patterns (inset image) of individual GaN nanowires matches with the [0001] diffraction pattern. This pattern is single crystalline with symmetric diffraction spots showing the wurtzite structure of GaN NWs. Moreover, the interval of the closest inter-planar distance was measured to be 0.25 nm, which corresponds to that of the crystal plane (1 0 0) of GaN.22,23 The morphology of the GaN NWs synthesized using metal-assisted photochemical electroless etching was different than that of the NWs prepared with the aid of Au–Pd as a vapor–solid–liquid (VLS) catalyst. Typical VLS-grown GaN nanowires exhibited triangular cross-sections, and each of these nanowires was straight, smooth and tipped with a catalyst particle, a hallmark of the VLS process.16 However, GaN NWs synthesized using a metal-assisted photochemical electroless etching method are simple, fast and less expensive compared to other complicated methods: MOCVD system or VLS growth.16
Fig. 2 TEM image of an individual NW, scale bar 100 nm. Inset: FFT image with scale bar of 5 nm−1 and HRTEM with scale bar of 5 nm. |
To confirm the TEM observations, X-ray diffraction measurement was recorded for the samples etched for 300 min, and the results are shown in Fig. 3. The peaks on the spectrum can be indexed to the wurtzite GaN (JCPDS card no. 898624) with lattice constants of a = 0.3186 nm and c = 0.5178 nm, and the diffraction peaks are located at 2θ = 32.3°, 34.5°, 36.7° and 57.8° corresponding to (100), (002), (101) and (110) planes.
A schematic of the synthesis of GaN NWs via metal-assisted photochemical electroless etching is presented in Fig. 4. The proposed GaN NW etching mechanism has three steps. First, photo-generated electron–hole pairs (e–h) are generated at the GaN–electrolyte interface when exposed to ultraviolet light in the presence of (H2O2:HF:CH3OH) as the etching medium, as shown in Fig. 4(a). Due to the band bending at the interface of the GaN substrate–electrolyte, the photo-generated holes support the anode reaction at the surface of GaN for the oxidation reaction, while electrons diffuse into bulk GaN. In Fig. 4(b), the holes oxidize the surface layer to form Ga2O3 with the evolution of N2 gas, as depicted in eqn (a), and play the role of an anode.24
2GaN + 6h+ + 3H2O → Ga2O3 + 6H+ + N2 | (a) |
At platinum, the reduction reaction of H2O2 through eqn (b) takes place as the cathode to generate water.24
H2O2 + 2H+ → 2H2O + 2h+ | (b) |
Then, the second step is the reaction of HF with Ga2O3. This reaction will be on the GaN crystal grain surface, on the surface dislocations and on the grain boundaries of GaN crystallites.25 The dissolution of the Ga2O3 oxide takes place to form GaF3, as depicted in eqn (c).
Ga2O3 + 6HF → 2GaF3 + 3H2O | (c) |
Under oxygen poor conditions, the dissolution of Ga2O3 oxide in HF solution will enhance the dangling bonds and will be responsible for the positive charges in the oxide.26 The repulsion of the H+ charge ions in the solution from the positive oxide surface will reduce the dissolution rate of Ga2O3 on the crystal grain surface through eqn (d).
Ga2O3 + 6H+ → 2Ga3+ + 3H2O | (d) |
However, dislocated sites and defects are negatively charged,26,27 and the dissolution of Ga2O3 can be easily initiated at surface defect sites showing a much faster rate due to HF.28 For short etching times, pores are produced on the surface of the GaN film, as shown in the SEM images in Fig. 4(b). GaN NWs are formed along with porous structures. The last step is for longer etching times, when pores will be etched along the vertical (0001) crystallographic direction in the GaN layer at the dislocation sites, leaving behind NW structures. Then, the GaN NWs will become longer and will collapse after a few microns.
The etching process depends on the presence of a surface charge region (w) at the n-GaN–electrolyte interface, which arises due to the GaN surface Fermi level equilibrium with the electrochemical potential of the electrolyte, as shown in Fig. 5.29 This surface charge region is characterized by the presence of surface electrical fields, which cause an upward bending in the energy bands near the surface. The metal-assisted photochemical electroless etching of n-GaN initiates when the incident UV photons excite electron–hole pairs. If the carriers are excited up to the hole diffusion length, away from the surface charge region,29 holes may diffuse to the surface charge region where they drift towards the interface under the effect of the surface electric fields, or they simply recombine. The photo-generated holes reach the GaN/electrolyte interface and are localized at the surface bonds, allowing the surface gallium atoms to get oxidized, forming gallium oxide (Ga2O3), as depicted in eqn (a).30
The PL measurements shown in Fig. 6 were conducted on the samples etched at different times of 90, 180, 240, and 300 min. The emission peaks are predominated by the bound exciton D0x located at 3.425 eV (362 nm) for the as-grown sample, 3.419 eV (362.6 nm) for the sample etched for 90 min, 3.414 eV (363.2 nm) for the sample etched for 180 min, 3.412 eV (363.4 nm) for the sample etched for 240 min, and 3.41 eV (363.7 nm) for the sample etched for 300 min. The PL spectra of the etched samples show a second peak by a donor–acceptor pair (DAP) at around 3.35 eV (370 nm). Therefore, a red-shift is discerned compared with the bulk GaN. This red-shift was attributed to the relaxation of compressive stress in the GaN NWs. Furthermore, the full width at half-maximum (FWHM) of the GaN NWs is broader compared to that of the as-grown sample, and this increase of FWHM could be attributed to the wide size distribution of the nanowires.31,32
Fig. 6 PL spectra of as-grown GaN and GaN NW samples for different etching times: 90, 180, 240 and 300 min. Inset: PL decay of GaN NWs in ethanol from the sample etched for 300 min. |
Time-resolved PL measurements on dispersed nanowires in ethanol prepared from the sample etched at 300 min are studied at room temperature, as shown in the inset in Fig. 6. The PL decay was fitted using a single-exponential decay. The lifetime emission was equal to 10 ns. The measured lifetime is longer than the lifetime measured on GaN NWs grown by catalyst-free molecular-beam epitaxy, which is equal to 2 ns at 275 K.33
Temperature dependent photoluminescence spectra of GaN NWs etched at 300 min are shown in Fig. 7. The near-band-edge (NBE) emission is at 3.474 eV at 78 K. As the temperature increases, the peak position shifts to lower energy, and at 300 K, the peak is around 3.41 eV, as shown in Fig. 7(a). Furthermore, the FWHM is increased with temperature. The crystalline property of these GaN NWs prepared by metal-assisted photochemical electroless etching is quite good compared to that of the external catalyst assisted GaN NWs grown using CVD.34 Using the Varshni formula, the energy versus temperature plot can be fitted by the below equation:35
Eg(T) = Eg(0) − (αT2/(β + T)) |
Fig. 7 (a) Temperature dependent PL spectra of GaN NWs and (b) Varshni fitting for the peak at 3.41 eV with various temperatures. |
Raman spectra for the as-grown sample and etched samples are shown in Fig. 8. The Raman spectra reveal that the E1(TO) peak (560.5 cm−1) is visible for the etched samples. The E2 peak positions are located at 570.0 cm−1 for the as-grown sample, 569.4 cm−1 for the etching times of 90 min and 180 min, and 569.0 cm−1 for samples etched for 240 and 300 min. A shift of ΔE2(TO) = 1.0 cm−1 towards the lower wavenumber was measured for the samples at 240 and 300 min etching duration. The shift of E2 to the lower frequency could be explained by the stress relaxation that takes place for the longer etching time. However, a new shoulder appears for the etched samples at 750.5 cm−1. This shoulder peak is attributed to a Froehlich mode due to the anisotropy etching. This observation has also been reported in columnar GaN prepared by illumination assisted anodic etching.38
Fig. 8 Raman spectra of as-grown GaN, GaN NW samples for different etching times of 90 min, 180 min, 240 min, and 300 min. |
The residual strain in GaN NWs can be evaluated based on these Raman measurements. The compressive stress (σxx) can be estimated by the expression ΔE2(TO) = Kσxx cm−1 GPa−1, where ΔE2(TO) is the shift in the wavenumber from GaN NWs and the as-grown GaN, which is equal to 1.0 cm−1 for the samples etched for 240 and 300 min, and K is a proportionality factor equal to 4.2 cm−1 GPa−1 for GaN.39,40 The calculated σxx in the GaN NWs equal 0.23 GPa. However, the compressive stress values in the literature for the as-grown and porous GaN prepared by the same method were evaluated to be 0.36 GPa and 0.27 GPa, respectively.8 The compressive stress in the GaN NWs is lower than that in the as-grown sample due to the stress release. Furthermore, the residual strain, εxx, in the GaN NWs can be obtained using the expression σxx = Mf. εxx, where Mf is the biaxial modulus of the GaN substrate equal to 449.6 GPa.41 The residual strain, εxx, in the nanowires is determined to be 2.6 × 10−4. Hugues et al. shows that the in-plane strain in the GaN NWs prepared by Metal Organic Vapor Phase Epitaxy (MOVPE) is around 1.0 × 10−3 and demonstrates that the strain decreases during the first 100–200 nm and is fully relaxed when the nanowire height reaches 300 nm.42 Hence, residual strain measurements have been confirmed, which are one order of magnitude smaller due to the length (few microns) of our nanowires.
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