Simple Wet Etching of GaN

G. Parish*, P.A. Scali, S.M.R. Spaargaren, B.D. Nener Dept. of Electrical and Electronic Engineering, The University of Western Australia

ABSTRACT

We discuss investigations into a contactless UV-enhanced wet etching technique for GaN. The technique utilises the oxidising agent potassium persulfate to consume photogenerated electrons, thus avoiding the need for an electrical contact to an external cathode. The etch rate is strongly dependent on illumination intensity and uniformity and on the pH of the KOH solution, as is the roughness of the etched surface. The implementation of a dual illumination scheme whereby an additional UVC lamp was used to illuminate only the solution and not the wafer, resulted in an increased etch rate and smoother etched surface. Finally, the ohmic nature of contacts deposited on n-type GaN that had been etched in this manner was found to be improved compared to contacts on the unetched surface. Keywords: GaN, gallium nitride, wet etch, UV, PEC, processing, surface

1. INTRODUCTION GaN and its alloys with InGaN and AlGaN form a wide-bandgap semiconductor material system with numerous optical and electronic device applications. The wavelength range spans from 1.9eV (InN) to 6.2eV (AlN), covering the technologically important ultraviolet (UV) and visible spectral ranges. Also, due to the wide bandgap and high bond strength the material has a high chemical and radiation resistance. With no semiconductor material previously satisfying commercial demands for blue, green and UV lasers and light-emitting devices this was the first immediate focus of GaN research and progress, and these are now commercially available. With a high breakdown field and large predicted electron saturation velocity, GaN-based materials are also suited for high power, high frequency transistors for microwave applications. In this area commercial potential is close to being realized, using modulation doped field effect transistors. The spectral range, which has been utilized with success for light emitting devices, is also being exploited for detectors in the UV and visible spectrum. Although some device applications have been or are close to being commercially realised, there still remain many impediments to the full realisation of the potential of this material system. Material quality has been hindered by lack of a suitable substrate1 and doping difficulties2. This has meant that other, equally promising, GaN-based devices such as bipolar junction transistors (BJTs), heterojunction bipolar transistors (HBTs) and solar-blind UV photodetectors, so far demonstrate insufficient performance to challenge more traditional market solutions to their applications. A significant challenge in GaN research is the chemical and physical robustness of (Al,In)GaN, due to the high bond strengths and wide bandgap. This is advantageous for application of GaN-based devices in harsh environments and under extreme operating conditions, however it causes great impediments to device processing. Perhaps one of the most difficult areas has been etching3. The most common technique used has been chlorine-based reactive ion etching (RIE), however due to the high plasma voltages required this results in damage to the etched surface3,4. Ohmic contacts have been another area of processing complexity. Reasons for this include the wide bandgap, surface oxides, and the prevalence of point defects, which compensate doping and affect the surface potential5,6. Ohmic contacts to RIE-etched surfaces pose particular problems due to the etch damage sustained. An attractive alternative to ion etching that has been investigated for some time is photoelectrochemical (PEC) etching. In this technique, chemical etching using a base (such as KOH) or acid (such as HCl) is enabled using UV light of photon energy greater than the bandgap of the material being etched. Photogenerated holes at the surface excite the surface Ga to higher oxidation states and this oxide is then dissolved. This technique was first demonstrated for GaN by Minsky et al.7 and has since been investigated by many researchers. Recently, Zhu et al.8 compared GaN n--n+ rectifiers * giap@ee.uwa.edu.au; phone 61 8 9380 3390; fax 61 8 9380 1095; Dept. of Electrical and Electronic Engineering/The University of Western Australia, 35 Stirling Hwy Crawley WA 6009 Australia

which had been etched by RIE to those etched by PEC technique. The latter had superior properties, confirming the advantages of the PEC process. A somewhat modified PEC process was introduced by Bardwell et al. in 19999. They employed the oxidising agent potassium persulfate to consume photogenerated electrons. This circumvented the need for the electrical contact used in conventional photoelectrochemical techniques. Maher et al10 later reported success in optimising the technique, achieving minimal increase in roughness (1nm increase in RMS roughness compared to pre-etch surface) with a moderate etch rate of 2nm/min. This paper will detail the investigations at UWA of this contactless UV-enhanced wet etching technique for GaN. We will discuss the investigation of the effect of etch parameters such as illumination intensity and wavelength, pH of the solution, and method of illumination. Atomic force microscopy (AFM) of the surface before and after etching was used to assess both the etch rate and the quality of the etched surface. Finally, the quality of ohmic contacts deposited on n-type GaN that has been etched in this manner will be addressed.

2. EXPERIMENTAL Several different experimental schemes were used during the experiments. Wafers with a Pt mask pattern (Pt thickness 15, 50 or 150nm) were illuminated in a stirred KOH/K2S2O8 solution. Adjustment of molar concentration of the KOH was used to vary solution pH. A variety of light sources were used, either separately or in combination. A HeCd laser was used, with either an intensity of 1W/cm2 or 10mW/cm2 at 325nm. The latter was achieved by using a beam expander consisting of a concave-convex lens combination (Galilean principle). Also used was a 253.7nm low pressure Hg vapour discharge lamp (with a parabolic reflector to produce parallel output) that had an intensity of 1.1mW/cm2 at the distance used in the experiments. Wafers were either held in place using a plastic clip (for illumination from the side) or placed face-up on a perforated plastic basket above the stirrer (for illumination from above). All etches were performed on pieces of GaN taken from the same 2” template. The template (on sapphire substrate) was 2µm thick and Si-doped with a carrier concentration of 1-2x1018cm-3. High-resolution x-ray diffraction of templates grown under similar conditions showed an on-axis (0002) full-width-half-max (FWHM) of approximately 250arcsec and an off-axis (10 1 2) FWHM of approximately 550arcsec. The RMS roughness as measured by AFM was less than 1nm. AFM images were taken on DI Nanoscope (III) operated in contact mode. Optical microscope images were obtained using an Olympus IP11 Digital Microscope and field-effect secondary electron microscope (FE-SEM) images were obtained using a JEOL JSM-6300F.

3. RESULTS AND DISCUSSION 2.1 Illumination The significance of the choice of light source is evident when reviewing the results of Bardwell et al. In their recent paper11, they discuss the particular importance of the wavelength range for this contactless technique. They found generation of OH and/or SO4

- radicals from the K2S2O8 was required to facilitate the cathodic consumption of photogenerated electrons. These radicals are by-products of the photolysis of K2S2O8 by illumination at or below 310nm. They also believe these radicals are produced via a catalytic reaction at the mask surface if Pt is used as the mask. Use of a Pt mask was found to not only enhance the etch rate when illumination below 310nm was used, but to enable etching at higher wavelengths (such as 365nm). The first etch achieved in this research was with the HeCd laser beam at the full intensity of 1W/cm2. The solution pH was 12.2, with 0.02M K2S2O8. The sample was etched for 10 minutes and was found to be etched to the substrate, indicating a etch rate of at least 200nm/min. This rapid etch resulted in an extremely rough etched surface, with an RMS roughness of over 300nm when measured over a 100µm2 area. Figure 1 shows a 30µm by 30µm image of the etch border. The large etch height and roughness meant that a clear image could not be obtained. The 15nm thick Pt mask has not been removed and it can be seen that near the edges the mask appears to be delaminating. This occurred for many of the wafers and the cause is still under investigation.

Figure 1: AFM image of GaN:Si etched using 325nm laser illumination of 1W/cm2. The left side of the image is the unetched region, including the 15nm Pt mask. The greyscale is 2.5µm. Another GaN piece was etched using the expanded beam with an intensity of 10mW/cm2, and a similar solution pH (though a slightly stronger K2S2O8 concentration of 0.05M). In comparison to the first etch, an etch rate of around 5nm/min was observed. (Note that this is only an approximation. Exact determination of etch heights by AFM was very difficult, due to the roughness of the etched surface, and because the etch steps were not abrupt.) Thus a two orders of magnitude reduction in intensity resulted in a two orders of magnitude reduction in etch rate. This extends the results of Bardwell et al.9, who reported a linear dependence of etch rate on intensity using an unfiltered Hg lamp with intensity varied between 1 and 25mW/cm2 at 365nm. This dependence has also been seen under certain situations by researchers investigating standard PEC etching of GaN7,12. Youtsey et al.12 noted that it indicates the reaction rate is proportional to the electron-hole pair generation (carrier-limited regime). At higher light intensities they observed a saturation of this relationship. For example, for a solution of 0.01M KOH, saturation was observed at an intensity of 15mW/cm2 at 365nm. They suggested that this saturation was indicative of a diffusion-limited regime, in which the etch rate was limited by the rate of arrival from the solution of reactants needed to dissolve the surface oxide. The fact that a saturation in the intensity-etch rate relationship was not observed over two orders of magnitude variation in light intensity (despite a similar KOH concentration to Youtsey et al.’s experiments) requires careful consideration of the different mechanisms involved in the contactless etching technique. Perhaps it is more instructive to compare the surface oxide formation- and dissolution-limited regimes (which correspond to the carrier- and diffusion-limited regimes in the above discussion). In the case of the contactless technique, the oxide formation mechanism depends not only on the generation of electrons and holes by above-bandgap photon energy, but also on the availability of reactants in the solution to consume the electrons as the holes oxidise the surface. When using the 325nm illumination, the production of the necessary radicals occurs only because the Pt acts as a catalyst. However they would only be produced locally on the illuminated mask, and not in the rest of the solution. Perhaps therefore, the dependence on the intensity is due to a limitation in the production of these radicals which continues to limit the progression of the etch rate even at much higher intensities (at the higher intensity the beam diameter was on the order of 1mm or less, providing an even smaller area of radical production). The etch rates at similar pH and intensity are certainly much slower than those obtained from PEC (at least an order of magnitude lower), which would appear to support this theory. This strong dependence of etch rate on light intensity has tremendous implications for the experimental setup and choice of illumination source. The etch uniformity will obviously be dependent on the uniformity of the light intensity. Thus, the use of the laser beam, even with the beam expander in place, resulted in non-uniform etching across the wafer area (pieces were sized approximately 8mm by 8mm or larger). This was due to the Gaussian distribution of the beam. Furthermore, imperfections in the lenses used for the beam expander resulted in localised areas where there was no etching. Another aspect which affected the etch uniformity was the method of light projection onto the wafer. When the light was projected through the side of the beaker with the wafer held vertically, imperfections in the glass also led to a patterning effect of the etch. This is evident in the image given in Figure 2.

Figure 2: Microscope image of GaN:Si etched using 325nm laser illumination of 10mW/cm2, with light incident through the side of the beaker holding the solution and the wafer. The black areas correspond to the etched regions. The black areas correspond to the (rough) etched surface. It can be seen that the light was patterned in stripes, with some circular diffraction-like patterns also present. It should be noted that the non-uniform light distribution incident on the wafer was not visible to the eye. However, the resulting etch pattern was clearly visible. When a mirror was used to reflect the light directly onto the wafer (held horizontally) from above, this particular patterning did not occur. As already mentioned, several different illumination schemes were investigated in this work. An experiment was devised in which two GaN pieces were etched using the laser light at 325nm incident directly on the (horizontal) wafer, under identical conditions except that for the second piece the solution was illuminated from the side using the 253.7nm lamp. The pH was 12.5 with 0.05M K2S2O8, and a 50nm Pt mask was used. The placement of the lamp on the side meant that it would enable photolysis of the K2S2O8 without contributing to photogeneration of carriers in the GaN. This resulted in two effects. The etch rate was increased nearly threefold from approximately 5nm/min to approximately 13nm/min. Conversely, the RMS roughness was reduced from 130nm to 70nm in a 4µm2 area. These results complement both Bardwell et al.’s results and the above results regarding etch rate dependence on light intensity. The increased production of the necessary radicals via illumination of the entire solution with the 253.7nm light would have increased the etch rate and moved the etching away from the oxide-formation-limited regime to the oxide-dissolution-limited regime. Youtsey et al. had shown that etching in the latter regime (the diffusion-limited regime) resulted in a smoother etch, which is consistent with these results. 2.2 Solution pH Bardwell et al.9 had found that the etch rate was dependent on solution pH, with a peak etch rate occurring at 12.7. However, no information was given as to the variation in etch quality. Our investigations therefore included a series of etches in solutions of different pH, for which both etch rate and etched surface roughness were measured. Figure 3 plots the results of these measurements. For this series the expanded 325nm laser beam and a K2S2O8 molarity of 0.05M were used, with a Pt mask thickness of 150nm. The etch time was 20 minutes. Unfortunately the illumination was through the side of the beaker, resulting in the etch patterns seen in Figure 2. Etch rate therefore had to be estimated by measuring the step heights between the dark (etched) and light (non-etched) regions. There was significant variation in the measured heights on a given wafer, so the etch rates given are representative of the differences in etch rate as pH was varied, rather than being measurements of the absolute etch rate. It can be seen from the figure that the variation in etch rate with pH, which matches that seen by Bardwell et al., is closely matched by the variation in etch roughness. That is, as the pH was decreased, the etch rate decreased, but so did the etch roughness. This again agrees with the PEC etch findings of Youtsey et al., that by moving to the oxide-dissolution- (diffusion-) limited regime the smoothness was improved. The reason for the increase in etch rate at lower solution pH values (less than 12) is yet to be determined.

Figure 3: Etch rate and roughness of GaN:Si etched using 325nm laser illumination of 10mW/cm2, as a function of solution pH (KOH concentration). Light was incident through the side of the beaker holding the solution. 2.3 Pt mask There are several as-yet unresolved issues regarding the Pt mask. As was evident in Figure 1, the mask frequently appeared to be delaminating at the edges. The FE-SEM image in Figure 4a is an extreme example of cracking and peeling of a mask that was 15nm thick. In another experiment, two pieces with different Pt mask thicknesses were etched under identical conditions using the expanded 325nm laser beam. In that case, the 15nm mask appeared unaffected however peeling occurred on the second piece, which had a mask thickness of 150nm. A microscope image of this is given in Figure 4b. The affected area was limited to where the mask had been illuminated during etching. It is worthwhile to note that there was no difference in etch rate or etched surface roughness for the two mask thicknesses.

Figure 4: a) FESEM image of 15nm Pt mask after 30 minutes etching. b) Microscope image of 150nm Pt mask after 20 minutes etching. In both cases 325nm laser illumination of 10mW/cm2 was used. Another perplexing problem is that in most cases no etching occurred immediately in the vicinity of the mask edge. This can be seen in Figures 5a and 5b. Figure 5a is an optical microscope image of a section of the mask for a wafer etched using the expanded laser beam incident from above. Figure 5b is an AFM image in the vicinity of the mask edge of the wafer etched using both the laser beam and the 253.7nm lamp. (These two wafers were those discussed in subsection 2.1)

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Figure 5: a) Microscope image of GaN:Si etched using 325nm laser illumination of 10mW/cm2. The black area corresponds to the etched region b). AFM image of GaN:Si etched using 325nm laser illumination of 10mW/cm2 and additional illumination of the solution by a 253.7nm lamp at 1.1mW/cm2. The greyscale is 400nm. In both images, it can be seen that there is a distinct gap between the etched region and the mask edge. This gap was not uniform, and not always present. Often there was a gradual increase in etch rate away from the gap, which meant that the steps weren’t abrupt. This further compounded the difficulty in determining etch rates. 2.4 Ohmic contacts Although the etch technique has not yet been perfected, preliminary investigations were made as to the nature of ohmic contacts to the etched surface. The pH=12.13 wafer from the pH series described in subsection 2.2 was used. Al contacts were deposited by thermal evaporation on both the etched and unetched regions. The contacts were not annealed. I-V measurements were made between adjacent contacts in each of the regions. Typical curves for each region are shown in Figure 6. It can be seen from the linearity that the contacts to the etch region are more ohmic. Obviously, the roughness of the etched surface is beneficial rather than detrimental to the nature of the contact. Furthermore there are no compensating defects introduced by the etching process.

Figure 6: I-V characteristics between adjacent Al contacts deposited on etched and unetched regions of GaN:Si.

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4. SUMMARY

We have presented investigations into a contactless UV-enhanced wet etching technique for GaN using a K2S2O8/KOH etch solution. This etch is a two-step process. Above-bandgap UV light creates electron-hole pairs in the GaN, which results in oxidation of the surface atoms by the photogenerated holes. The oxide is then dissolved in the KOH solution. It was found that smoother etching was achieved by moving from an oxide-formation-limited regime towards an oxide-dissolution-limited regime. This required illumination of the entire K2S2O8 solution (rather than local illumination on the wafer) to generate sufficient radicals for consumption of the photogenerated electrons. Smoother etching was also obtained by lowering the pH of the solution. When in the oxide-formation-limited regime, the method of illumination was critical. The etch rate was extremely sensitive to light intensity and therefore to uniformity of the illumination on the wafer surface. Finally, the ohmic nature of contacts deposited on n-type GaN that had been etched in this manner was found to be improved compared to contacts on the unetched surface.

ACKNOWLEDGEMENTS We would like to acknowledge the Australian Research Council for their funding, which made this research possible. We are also grateful to the University of California, Santa Barbara, for the GaN:Si template which was used. Finally we wish to acknowledge the support of the Magnetics Laboratory at UWA, where the AFM measurements were taken.

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