10.1149/1.2731219, The Electrochemical Society Wet Chemical Etching of Wide Bandgap Semiconductors-GaN, ZnO and SiC S.J. Pearton 1, J.J. Chen 2, W.T. Lim 1, F. Ren 2 and D.P. Norton 1 1 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 USA 2 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 USA Wide bandgap semiconductors have many properties that make them attractive for high power, high temperature device applications. In this paper we review wet etching of three important materials, namely ZnO, GaN and SiC. While ZnO is readily etched in many acid solutions including HNO 3 /HCl and HF/HNO 3, and in the nonacid acetyleacetone, the group III nitrides and SiC are very difficult to wet etch and generally dry etching is used. Various etchants for GaN and SiC have been investigated, including aqueous mineral acid and base solutions, and molten salts. Wet etches have a variety of applications to wide bandgap semiconductor technology, including defect decoration, polarity and polytype (for SiC) identification by producing characteristic pits or hillocks, and device fabrication on smooth surfaces. Electrochemical etching is successful at room temperature in some situations for GaN and SiC. In addition, photo-assisted wet etching produces similar rates independent of crystal polarity. Introduction The wide bandgap semiconductors GaN, SiC and ZnO are attractive for many emerging applications. For example, development of AlGaN/GaN high electron mobility transistors (HEMTs) and monolithic microwave integrated circuits (MMICs) promises high frequency operation. In addition, GaN is used for ultraviolet wavelength optoelectronic devices. It has a high breakdown field, greater than 50 times that of Si or GaAs, which allows for its use in high power electronic applications. GaN s wide band gap allows for its use in blue/ultra-violet light-emitting diodes (LEDs) and laser diodes (LDs) as well as allowing it to operate at very high temperatures due to its low intrinsic carrier concentration. The high electron mobility and saturation velocity allow its use in high speed electronics. In addition, heterostructures such as AlGaN/GaN allow for the manufacture of high speed devices such as HEMTs. ZnO is a direct, wide bandgap material with a wurtzite crystal structure which can be used in gas sensor, transparent electrodes, Liquid Crystal Display, solar cell, piezoelectric transducers, ophotoelectronics material device, blue, UV LEDs and laser diode. ZnO is of strong interest for blue/uv LEDs and thin film transistors (TFTs). Compared to GaN, ZnO has the advantage of a relative low growth temperature on cheap glass and much higher excition binding energy (~ 60meV) than GaN (25meV). This means ZnO has a more stable exciton state at room temperature since the heat energy is around 26meV. The excitons in the ZnO semiconductor will not dissociate into free electrons or holes due to the heat at room temperature or the scattering between the excitons. Additionally, commercial ZnO substrates are available. The ZnO system also has a simpler processing relative to GaN which cannot be wet-etched in conventional acid mixtures at safe temperatures. SiC is another attractive semiconductor for high temperature, high power, and high frequency electronic devices due to its wide bandgap (3.08 ev for 6H and 3.28 ev for 4H), high 501
breakdown electric field and high electron saturation velocity. (1) The total percentage of the microelectronics market occupied by all compound semiconductor devices and circuits is ~5%, but they do fill important niches unavailable to Si. There are a number of challenges when processing compound semiconductors, including the relatively high vapor pressures of the group V and VI elements compared to the group III and II elements and the difficulty in forming highly reliable Ohmic and rectifying contacts. It is necessary to develop highly selective, as well as non-selective, etch processes for the different materials in the heterostructure systems available with both GaN (ie. InGaN/GaN/AlGaN) and ZnO (namely ZnMgO/ZnO/ZnCdO). Much effort is devoted to achieving lattice-matched compositions, to avoid the introduction of threading dislocations which degrade the electrical transport and optical qualities of devices subsequently fabricated. To some extent the InGaN/AlGaN system represents an exception, since highly luminescent light emitting diodes (LEDs) and laser diodes have been demonstrated. (2,3) For LEDs the resultant reliability is sufficient for commercial applications, but the high dislocation density in heteroepitaxial material limits the lifetime of laser diodes, where the much higher current density leads to metal migration that shorts out the p-n junction. In material grown on quasi-gan substrates, this mechanism is absent (4), and the laser diodes have much longer lifetime. In the following sections, we will give a review of wet etching approaches for some common wide bandgap semiconductor materials systems. Wet Etching Typically the wet etching of III-V materials involves use of an oxidant to oxidize the surface, followed by dissolution of a soluble reaction product. (5-8) The resultant etching tends to be basically isotropic in nature, proceeding as shown in the schematic of Figure 1. This illustrates the selective etching of layer 1 from layer 2, and the undercutting of a mask on layer 1. In the case of III-V compounds, differential etch rates for crystallographic directions containing predominantly one or the other elements can lead to a degree of anisotropy and different sidewall shapes. (7) Figure 1. Schematic of selective wet etching of one layer from another, showing undercut of mask. 502
The etch rate may be limited by the diffusion of the active etchant species to the semiconductor surface, or by the diffusion away of the soluble product. (5) In this case the etching is termed diffusion-limited, and its characteristic include a square root dependence of etch depth on etch time, an activation energy 6 kcal.mol -1 and a strong dependence of etch rate on solution agitation. This mode of etching is not desirable for device fabrication, because of the difficulty in obtaining reproducible rates. The other rate-limiting step may be the chemical reactions at the surface. In this case, the etch depth depends linearly on time, the activation energy is 6 kcal.mol -1 and the rate is independent of solution agitation. This is the preferred mode of etching for device fabrication, since only temperature and solution composition need to be controlled. Since wet etching tends to be isotropic in nature, the undercutting of the mask makes it unsuitable for pattern transfer of small (<2 µm) features. There are a number of other disadvantages relative to dry etching, including an increased safety hazard due to potential exposure to chemicals and fumes, and bubble formation during the etching which can lead to local non-etched regions. (i) SiC Due to its hardness (H=9 + ), SiC is one of the most widely used lapping and polishing abrasives for metals, metallic components and semiconductor wafers. However this very property makes it difficult to etch in typical acid or base solutions. In its single crystal form, SiC is not attacked by single acids at room temperature. Indeed the only techniques for etching SiC employ molten salt fluxes, hot gases, electrochemical processes or plasma etching (9-19). Table 1 shows a list of the molten salt solutions and the temperatures needed for successful etching of SiC. The disadvantages of these high temperature, corrosive mixtures include the need for expensive Pt beakers and sample holders (which can withstand the molten salt solutions) and the inability to etch masked samples because few masks hold up to these mixtures. While one can conceivably use Pt masks, the wet etching is isotropic and therefore undercuts the mask. Photoelectrochemical etching can be successfully employed for SiC (20). The dissolution rate of semiconductors may be altered in acid or base solutions by illumination with above bandgap light. The mechanism for photo-enhanced etching involves the creation of e-h pairs, the subsequent oxidative dissociation of the semiconductor into its component elements (a reaction that consumes the photogenerated holes) and the reduction of the oxidizing agent in the solution by reaction with the photo-generated electrons. Generally, n-type material is readily etched under these conditions, while p-type material is not due to the requirements for confining photogenerated holes at the semiconductor-electrolyte interface (i.e., the p-surface is depleted of holes because of the band-bending). This allows for selective removal of n-sic from an underlying p-sic layer (20). Under conditions of no illumination, it is often possible to get the reverse selectivity if the sample is correctly biased, since n-sic requires photogeneration of carriers for etching to proceed. Etching over large areas can be achieved using Hg lamps and some degree of anisotropy is obtained because of the shadowing effect of the metal masks (typically Ti) allowing carriers to be generated only in unmasked regions. Some of the disadvantages of the technique include fairly rough surface morphologies (due to enhanced dissolution rates for areas around crystal defects), inability to pattern very small dimension features and poor uniformity of etch rate. For these reasons,most attention is now focussed on dry etching methods for SiC, most of 503
which have been developed for high power, high temperature electronics in this materials system Table 1. Molten flux and other wet etchants for SiC. Solution Material Temperature Reference NaF/K 2 CO 3 SiC(0001) 650 o C [10] H 3 PO 4 a-sic(h) 180 o C [11] NaOH SiC(111) 900 o C [12] Na 2 O 2 SiC(0001) >400 o C [13] NaOH/Na 2 O 2 SiC(0001) 700 o C [14] Borax/Na 2 CO 3 epi-sic 855 o C [15] NaOH/KOH bulk 6H 480 o C [16] Na 2 O 2 /NaNO 2 bulk 6H >400 o C [17] KOH/KNO 3 bulk 6H 350 O C [18] (ii) Nitrides Relatively little success has been obtained in developing wet etch solutions for III-V nitrides. (21,22) For AlN, a number of different solutions have been reported for amorphous or polycrystalline material. For example, hot ( 85 o C) H 3 PO 4 has been found to etch AlN deposited on Si by plasma enhanced chemical vapor deposition at low rates ( 500 Å min -1 ). (23,24) A variety of other solutions, including hot (~100 o C) HF/H 2 O (25-27), HF/HNO (28) 3 or NaOH (29) can etch sputtered or reactively evaporated amorphous AlN. For GaN, there were several early reports of wet etching in NaOH that progressed by formation of an insoluble gallium hydroxide (GaOH) coating. (30,31) This film had to be removed by continual jet action. Others have reported that H 3 PO 4 will remove GaN at a very slow rate. For InN, aqueous KOH and NaOH solutions were found to produce etch rates of a few hundred angstrom/min at 60 o C. (32-36) There has been particular difficulty in finding reliable wet etchants for single crystal nitrides. We did not find any etchant for GaN or InN at temperatures below ~80 o C. However, strong base solutions (KOH, NaOH or photoresist developer, in which active ingredient is KOH) were found to etch single crystal AlN at controllable rates whose magnitude was strongly dependent on material quality. Figure 2 shows a plot of etch rates in AZ400K photoresist developer as a function of the temperature for three different AlN samples. 504
1. Data designated by triangles is from polycrystalline AlN grown on GaAs. Etch rates for this material are much faster than for two single crystal samples grown on Al 2 O 3. (34,35) 2. Data designated by squares is from a 1µm thick layer with a double crystal X- ray diffraction peak width of 4000 arcsec. 3. Data designated by circles is from material with a peak width ~200 arcsec. Figure 2. Etch temperature of different AlN samples in KOH-based solution, as a function of temperature. The etching is thermally activated with an activation energy of ~15.5 kcal/mol in each case. This is consistent with reaction limited etching, and the etch depth was also found to be a linear function of time with an absence of dependence on agitation. If the etching was diffusion limited we would expect an activation energy below ~6 kcal/mol, a t dependence of etch on time, and a strong dependence of etch rate on degree of solution agitation. Higher rates for lower crystalline quality materials are expected on the basis of greater number of dangling or defective bonds that be attacked by the OH - ions in solution. Therefore the successful attempt frequency is higher under these conditions and etch rate R is higher. The process is well described by the relation Ea R = R o exp kt 505
where R o is the successful attempt frequency for breaking of an Al-N bond and formation of a soluble etch produce, E a is the activation energy (15.5 kcal/mol), k is Boltzmann s constant and T is absolute temperature of etch solution. We have observed a strong effect of annealing on subsequent wet etch rate of sputtered AlN films in KOH solutions, with over an order of magnitude decrease in rate after annealing at 1100 o C. (36) Similarly the etch rate for In 0.2 Al 0.8 N grown on Si was approximately three times higher in KOH based solutions than for material grown on GaAs, which is consistent with the superior crystalline quality of the latter. Etching of In x Al 1-x N was also examined as a function of In composition, with etch rate initially increasing up to 36% In and then decreasing to zero for InN. (36) Other researchers have found that only molten salts (KOH, NaOH, P 2 O 5 ) will etch GaN at temperatures above 300 o C, making handling and masking of material impractical. Minsky, White and Hu (37) reported laser enhanced, room temperature wet etching of GaN using dilute HCl/H 2 O or 45% KOH/H 2 O, with rates up to a few thousand angstrom/min for HCl and a few thousand angstrom/min for KOH. The mechanism is believed to be photoenhancement of oxidation and reduction reactions in what amounts to an electrochemical cell. Etch rates were linearly dependent on incident HeCd laser power. Zory et al. (38) have employed a pulsed electrochemical cell combining 40 parts ethylene glycol, 20 parts water and 1 part 85% H 3 PO 4 to etch p-gan and InGaN epitaxial layers at rates up to 1.5 µm/h. Cell voltage (220 V) was pulsed at 100 Hz (300 µm/sec pulse width). This technique was used in fabricating a double heterostructure p- GaN/InGaN QW/n-GaN light-emitting diode using a liquid contact. (iii) ZnO and Related Compounds Numerous wet etchants have been reported for ZnO and it soluble in most single acids, mixed acids, alkalies and ammonium chloride. Some previously reported mixtures for achieving controlled etch rates include HNO 3 /HCl and HF (39,40). In most device fabrication schemes, wet etching is needed for isolation or mesa formation if nonconductive substrates are used. The etching is mostly reaction-rate limited with the usual characteristics, ie. no dependence of etch rate on agitation of the acid mixture,high activation energy(>6 kcal.mol -1 ) with a linear dependence of etch depth on time. In the case of viscous acids such as H 2 SO 4, the etching may be diffusion-limited with a low activation energy (<6 kcal.mol -1 ), strong dependence on agitation of the mixture and a square root dependence of etch depth on time. The binary ZnO is readily etched in many acid solutions, including HNO 3 /HCl and HF (40-42). In most cases, the etching is reaction limited, with typical thermal activation energies of >6 kcal٠mol -1. In preliminary work, we have found that the etching of the ZnO is strongly dependent on material quality. If the ZnO is very thin, the wet etch rates are high in all acid solutions. A particular problem encountered with the wet etching of ZnO/AlGaN-based LED structures was the presence of a very significant undercut (as much as around 10 µm), which occurred mainly at the end of the selective removal of the ZnO from the underlying AlGaN. There are no reports to date on the wet etching of ZnCdO and ZnMgO and in particular it is important to develop selective etches for ZnCdO/ZnO and ZnMgO/ZnO systems. We demonstrated the achievement of selective etching of ZnCdO over ZnO using both dilute HCl and H 3 PO 4 mixtures. We also report on the selective etching of 506
Zn 0.9 Mg 0.1 O relative to ZnO, both grown with similar thicknesses on sapphire substrates by pulsed laser deposition (PLD) to ensure similar crystal quality. In Zn 0.9 Mg 0.1 O/ZnO system, wet etch selectivities over 400 for ZnMgO over ZnO can be achieved with HCl at high dilution factors with water. The wet chemical etch was performed with HCl/H 2 O or H 3 PO 4 /H 2 O solutions as a function of both solution concentration and temperature. A photoresist mask (AZ 1045) was used for creating features whose depth was measured by stylus profilometry after postetch removal of the mask in acetone. Figure 3 and 4 show the etch rates of Zn 0.95 Cd 0.05 O and Zn 0.9 Mg 0.1 O, respectively, as a function of solution concentration for HCl/H 2 O or H 3 PO 4 /H 2 O at 25 C. For Zn 0.95 Cd 0.05 O, controllable etch rates in the range (<100 nm min 1 ) are desirable for mesa formation and were obtained over this set of solution concentrations. For Zn 0.9 Mg 0.1 O, the etch rates were significantly faster with HCl/H 2 O at all concentrations (120 1100 nm min -1 ). Note, high dilution factors of the acids with water were used to obtain controllable etch rates. The use of pure HCl or H 3 PO 4 produced very high rates and extensive bubbling in the solutions that led to nonuniform and rough surfaces. To determine the rate-limiting step in the etching, we measured the etch rate as a function of solution temperature over the range 25 75 C. In the dilute mixtures used here, it is common to have an etch rate of semiconductor is limited by the diffusion of the active etchant species to the ZnCdO and ZnMgO surface, or by the out-diffusion of the soluble product, i.e., a diffusion-limited etch rate. Further characteristics include a square root dependence of etch depth on etch time, an activation energy 6 kcal mol 1, and a strong dependence of etch rate on solution agitation. This mode of etching is not desirable for device fabrication because of the difficulty in obtaining reproducible rates. Arrhenius plots of ZnCdO (~3 10 3 M) and ZnMgO (2.4 10 2 M) etch rate in the two solutions of HCl and H 3 PO 4 at high dilution factors with water, respectively, showed the etch activation energies are in the range ~0.4 kcal mol 1 for ZnCdO and 2-3 kcal mol 1 for ZnMgO, values that are consistent with diffusion-limited etching. By contrast to the case of ZnMgO, the etch rates are significantly faster with H 3 PO 4 /H 2 O at all concentrations. Note that lower acid dilution factors were used to obtain controllable ZnO etch rates, when compared to those used for ZnMgO. Once again, the use of pure HCl or H 3 PO 4 produced irreproducible etch rates and nonuniform, rough surfaces. The etch rates for ZnO were significantly slower than ZnCdO (2 10 3-3.5 10 3 M) and ZnMgO (5 10 3-2.5 10 2 M) under the same conditions, and we had to employ lower dilution factors (2 10 3-10 1 M) in order to get practical removal rates (Figure 5). Figure 6 shows an Arrhenius plot of ZnO etch rate in the two acid/water mixtures. Under these conditions the activation energies (~5.6 5.9 ± 0.4 kcal mol 1 ) were consistent with a transition to reaction-limited etching. Also the transport of the reaction species through the etch solution is less of a factor than the very dilute solutions used for ZnCdO and ZnMgO, as the etch rate did not vary with changes in the solution agitation rate. For the reaction-limited etch mechanism, the etch depth depends linearly on time, the activation energy is 6 kcal mol 1, and the rate is independent of solution agitation (41,42). This is the preferred mode of etching for device fabrication, as the temperature and solution composition need to be controlled, without the influence of varying composition gradients throughout the mixture. 507
Figure 3. Etch rate of ZnCdO with different concentrations of HCl and H 3 PO 4 solutions diluted in water. Figure 4. Etch rate of Zn 0.9 Mg 0.1 O in different concentrations of HCl and H 3 PO 4 diluted with water 508
Figure 5. Etch rate of ZnO in different concentrations of HCl and H 3 PO 4 diluted with water. Figure 6. Arrhenius plot of ZnO etch rate in 0.24M HCl and 0.06M H 3 PO 4 diluted with water. The resulting selectivities for etching ZnCdO over ZnO and ZnMgO over ZnO in the two mixtures with the same concentration are shown in Figure 7 and 8, respectively. The ZnO etch rates were in the range 30 90 nm min 1 for dilute acid mixtures. For ZnCdO, 509
the HCl/H 2 O solution provides selectivities in excess of 50 under optimum conditions, while the maximum selectivity with H 3 PO 4 /H 2 O was ~15. ZnMgO had selectivities in excess of 425 with the HCl/H 2 O solution, under optimum conditions. A rule of thumb in device fabrication schemes is that a selectivity of at least 10, and preferably 100. Figure 7. Etch selectivity of ZnCdO to ZnO at room temperature as a function of solution concentration. Figure 8. Etch selectivity of ZnMgO to ZnO as a function of solution concentration 510
Optical microscopic images were taken after selectively etching ZnCdO layers from an underlying ZnO layer on sapphire and ZnMgO layers from an underlying ZnO layer on sapphire. These clearly demonstrate that it is possible to generate a clean pattern transfer using selective etching. In summary, ZnCdO, ZnMgO, and ZnO can be readily etched in dilute solutions of HCl and H 3 PO 4. High dilution factors of these acids with water provides controllable etch rates in the range 30 90 nm min 1 for ZnCdO and 120 1100 nm min -1 for ZnMgO, with adequate selectivity to ZnO grown under the same conditions. Photoresist provides a stable and convenient mask for patterning ZnCdO, ZnMgO, and ZnO in these acid solutions. The availability of simple wet solutions for this heterostructure system simplifies the processing of mesa-type ZnO-based LEDs and avoids the need for plasma etching processes which are known to damage the ZnO surface even at low plasma powers. Detailed summaries of etching of III-V and wide bandgap semiconductors have been published by Clawson (43) and Zhuang and Edgar (44) and these are valuable resources for situations where wet etching is needed in device processing. Conclusions Wet etching of GaN and SiC is difficult and largely impractical for most device applications. In these cases, dry etching is preferred. ZnO is readily etched in most acid solutions and is relatively difficult to dry etch. Acknowledgments The work at UF is partially supported by DOE under grant DE-FC26-04NT42271(Ryan Egidi), AFOSR grant under grant number F49620-03-1-0370, by the Army Research Office under grant no. DAAD19-01-1-0603, NSF (CTS-0301178, monitored by Dr. M. Burka and Dr. D. Senich and DMR 0400416, Dr. L. Hess). References 1. See for example, The Blue Laser Diode, S. Nakamura, G. Fasol and S.J. Pearton (Springer, Berlin, 2000). 2. S. Nakamura, T. Mukai and M. Senoh, Appl. Phys. Lett. 64, 1687 (1994). 3. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kikoyu and Y. Sugimoto, Jap. J. Appl. Phys. 35, L74 (1996). 4. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kikoyu, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano and K. Chocho, Appl. Phys. Lett. 72, 2014 (1998). 5. CRC Handbook of Metal Etchants, ed. P. Walker and W.H. Tarn (CRC Press, Boca Raton, FL. 1991) pp. 1092-1101. 6. G.L. Harris, in Properties of SiC, Ed. G.L. Harris, EMIS Data Review No. 13 (INSPEC, London, UK, 19950, pp. 134-135. 7. D. Buckley, J. Vac. Sci. Technol. A (3) 762 (1985). 8. T.L. Chu and R.B. Campbell, J. Electrochem. Soc. 112 955 (1965). 9. W.K. Liebmann, J. Electrochem. Soc. 12 885 (1964). 10. E.D. Wolley, J. Appl. Phys. 37 1588 (1966). 511
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