Stress Relaxation in Porous GaN Prepared by UV Assisted Electrochemical Etching

This study describes the stress relaxation in porous GaN grown on Al2O3 substrates. The results indicate that the stress relaxation has taken place in the samples which increases by increasing the etching current. As compared to the as-grown GaN films, porous GaN exhibits substantial photoluminescence (PL) intensity enhancement with red-shifted band-edge peaks associated with the relaxation of the compressive stress. The red shifted phonon energy peak (E2) in the Raman spectra of the porous GaN films confirms further such a stress relaxation. Scanning electron microscopy (SEM) demonstrates that, the current density has significant effect on the size and shape of the pores.


Introduction
Among all the III-V nitrides, GaN is considerably the most intensely studied. GaN is a direct and wide band gap semiconductor (3.39 eV) at room temperature. With its superior radiation hardness and chemical stability, together with its large band gap characteristic, these properties have made GaN a suitable semiconductor material for device applications in the high-temperature and caustic environment as well as in space applications [1]. Producing and fabricating high quality electronic devices such as LEDs, LDs, photo detectors etcetera, requires growing high quality GaN doped thin films. Selecting appropriate substrate to deposit the film on it is one of the main factors that affect GaN growth techniques. The most common substrates used to grow GaN are; sapphire (Al2O3) and Si substrates. Sapphire is an interesting choice because it is a semi-insulating, high growth temperature and it is relatively cheap, but it is a large lattice mismatch (13.8%) [2], large thermal expansion coefficient mismatch (34%) and low thermal conductivity. The second choice is Si substrate which still has a large thermal expansion coefficient mismatch (56%) and large lattice mismatch (17%) [3]. The film/substrate mismatching produces high dislocation densities and high residual stress. Several techniques have been developed in order to reduce the dislocation density and residual stress including porous method.
One of the reasons for using a porous semiconductor as a substrate for depositing semiconductor layers is that, the nanopatterned porous structure can lead to a reduced extended defect density [4]. The interest in porous semiconductor arises from the fact that they can act as a sink for threading dislocations and accommodate the strain. Hence, they are widely used as a buffer or intermediate layer in epitaxial growth to obtain a subsequent layer with lowered strain and dislocation densities [5,6]. Mynbaeva et al. [7] proposed that the growth of GaN on porous GaN could lead to high-quality strain-released epilayers. Porous GaN is a promising simple method to achieve low dislocation density and low residual stress on the whole area of a wafer. This nanostructure was first developed by Bardwell  additional techniques such as UV-assisted electrochemical etching and electrochemical etching have been developed [9][10][11]. In this method, the important parameters are applied voltage and current, electrolyte composition and illumination. These parameters control etch rate, morphology and optical properties. In this work, we report the stress relaxation of GaN during etching process under different current densities of J= 5, 10 and 20 mA/cm 2 for 20 min. We investigated the surface morphology and the optical properties of porous GaN samples by scanning electron microscopy, and by high-spatial resolution photoluminescence and Raman spectroscopy.

Experiment
The samples used in this study were commercial (Si doped) n-GaN grown by metalorganic chemical vapor deposition (MOCVD) on Al2O3 substrates. The electron concentration obtained by Hall measurements was n=1x10 17 cm 3 . The samples were cleaned first with acetone and methanol, then cleaned in 1:20 NH4OH:H2O for 10 min, followed by cleaning in 1:50 HF:H2O solution to remove the surface oxides. This was followed by cleaning in 3:1 HCl:HNO3 at 80 C o for 10 min. Between the cleaning steps, the samples were rinsed in deionized water. Aluminum was then partly evaporated on the surface of GaN using a thermal evaporation system, which provided an ohmic contact. By using a homemade Tefloncell, we fixed the GaN sample as an anode and Pt wire as a cathode ( fig.1). The electrolyte was a mixture of aqueous HF 49% and ethanol C2H5OH 95%, 1:4 by volume. We used constant current densities of J= 5, 10 and 20 mA/cm 2 for 20 min, (supplied by a Keithly 220 programmable current source) and a UV lamp in the electrochemical etching process. After the etching, the samples were rinsed in deionized water, and dried in ambient air. The optical properties of the porous GaN samples were investigated by high-spatial resolution Raman and PL spectroscopy.  Figure 2 shows the scanning electron micrographs (SEM) of the morphology of the asgrown and porous GaN samples. The as-grown GaN possessed a smooth surface morphology ( Figure. 2a). The evolution of the porous morphology with increase in etching current density is apparent from Figures 2(b-d). The figures show that the etching current density has significant effect on the size and shape of the pores. The pores stated to form in the sample etched with 5 mA/cm 2 as shown in Figure 2b. The sample etched with 10 mA/cm 2 ( Figure  2c) shows that, the pore sizes appears to be around 30 to 40 nm and the distance between

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The  (Figure 2d) shows that, the pore sizes were observed to increase which become around 50 to 60 nm and some areas were destroyed, indicating that the GaN layer was very thin [11,12].  Figure 3 shows the room temperature PL spectra and the stress relaxation of porous GaN samples etched with various current densities. The spectra were observed to be slightly redshifted for all samples (relative to the spectrum of the as-grown sample). Similar red-shifted PL from porous GaN has been reported before [13]. The red-shift can be attributed to the relaxation of the compressive stress in the porous samples. The energy shift between porous and the as-grown GaN sample can be used to quantify the amount of stress relaxation. The amount of stress relaxation can be calculated using the expression: Δ=Kσ, where K is the proportionality factor and σ is the stress. Using the K value reported in literature, i.e., K = 21.273.2 meV/GPa [14], using this value of the proportionality factor for the stress-induced PL peak shift, a compressive stress relaxation of about 0.12, 0.17 and 0.32 GPa is estimated in 5, 10 and 20 mA/cm 2 respectively.

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The Small deviations in the peaks position indicating that the change of pore size has no considerable influence on the PL peak shift. However, the PL peak intensity of the porous samples has increased compared to that of the as-grown sample. The PL spectra for porous GaN samples show an increase in the intensity while increasing the current density. For example, at 10 mA/cm 2 and 20 mA/cm 2 current densities, the PL peak intensity was improved by factor of about 7 and 13, respectively, compared to that for the as-grown sample. The intensity of emitted light is proportional to the number of photons emitted. This means that the number of photons emitted is much higher in porous GaN than that in asgrown GaN. This could be attributed to the increase in the anti-reflectivity due to the roughening of the surface and the high surface area of porous structure. Figure 4 shows Raman spectra of the as-grown and the porous GaN samples. The spectra exhibit silent phonon mode E2 (high) and transverse optical mode A1 (LO), and relatively small peak of A1 (TO), also figure shows the increase in peaks intensity with etching current density. The frequencies of all the observed modes were in good agreement with the results of other researchers [15][16][17]. The phonon peak position, intensity and peak shift for GaN samples etched with different current densities are summarized in Table 1.
Circle inset in figure 4 shows the Raman spectra on E2 (high) together with that of the asgrown one. Moreover, the E2 Raman mode of as grown GaN appears at an identical wave number of 566.2 cm -1 . We take this number as a reference for unstressed films.

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The All the porous samples prepared with etching current densities of 5, 10 and 20 mA/cm 2 show an E2 (high) shifted to slightly lower frequency than that of the as-grown one. Using the proportionality factor of 4.2 cm -1 /GPa for hexagonal GaN [16,18], the total stress in the asgrown GaN is 0.85GPa compared to the unstressed sample (free stress). While the relaxation of stress took place in the porous sample, and by using the same proportionality factor, the relaxation of stress was found to be 0.25 GPa for the 5 mA/cm 2 sample, 0.38 GPa for the 10 mA/cm 2 sample and 0.51 GPa for the 20 mA/cm 2 sample. These results indicate the stress relaxation has taken place in the samples and it increases by increasing the etching current density as seen in Figure 5.  From this figure, we can conclude that the etch rate increases with the current density by an exponential proportionality. Another conclusion is that we can obtain from Figure 5 that, for high current densities, the higher etch rate hinders the fine control of the layer thickness in the case of thin layer. The full width at half maximum (FWHM) value of E2 phonon for the as-grown and the porous samples at 5, 10 and 20 mA/cm 2 are also shown in Table 1, indicating decreasing FWHM value with increasing etching current density. The lower value of the FWHM represents good crystalline quality. This suggests an improvement in the crystalline structure of porous GaN with increasing etching current density [19].
Generally, the porous samples exhibited broader FWHM than the as grown sample for (0002) a diffraction planes, this could be due to the relatively wide statistical size distribution of the pores, however for porous samples, no relation between the FWHM and the average pore size was observed.

Conclusions
Porous n-GaN was prepared by photoelectrochemical etching under ultraviolet light using various current densities in electrolytes consisting of aqueous HF and ethanol electrolyte at room temperature. All porous samples showed increased PL and Raman intensities compared with that of the as-grown. PL and Raman results confirms that porous GaN layer possess less compressive stress than that for the as-grown GaN layer and the quality of the porous sample is as good as the as-grown sample. The peaks of E2 (high) in Raman spectra for all the porous samples suggest that the crystallite quality and stress relaxation increases with increasing the current density. The relaxation of stress is 0.25 GPa for the 5 mA/cm 2 sample, 0.38 GPa for the 10 mA/cm 2 sample and 0.51 GPa for the 20 mA/cm 2 sample. These results indicate that the stress relaxation has taken place in the samples and increasing with higher etching current density. This indicates that, it is possible to fabricate high quality porous GaN layer with tunable stress, suitable for growing or depositing further high quality active layers with minimized lattice mismatch.