A simple setup for electrodeless photo-assisted electrochemical (PEC) etching was discussed from the viewpoint of the experimental geometry, in which the sample was dipped into the electrolyte under ...ultraviolet (UV) irradiation. Sulfate radicals ( SO 4 − ) were produced from K2S2O8 with UV light as the oxidizing agent; this consumed the extra UV photogenerated electrons, making it electrodeless. The transmittances were measured for various concentrations of K2S2O8 (aq.) to adjust the electrolyte depth. The effect of tetramethylammonium hydroxide post-treatment was also examined. The results indicate that damage-free PEC etching is feasible for everyone, even those who are not familiar with electrochemistry.
Optical characteristics of Mg-ion implanted GaN layers with various fluence ranges were evaluated. Mg ion implantation was performed twice at energies of 30 and 60keV on n-GaN layers. The first ...implantation at 30keV was performed with three different fluence ranges of 1.0×1014, 1.0×1015 and 5.0×1015cm−2. The second implantation at an energy of 60keV was performed with a fluence of 6.5×1013cm−2. After implantation, samples were annealed at 1250°C for 1min under N2 atmosphere. Photoluminescence (PL) spectrum of the GaN layer with the Mg ion implantation at the fluence range of 1.0×1014cm−2 at 30keV was similar to the one of Mg-doped p-GaN layers grown by MOVPE (Metal-Organic Vapor Phase Epitaxy) on free-standing GaN substrates and those at the fluence ranges over 1.0×1015cm−2 were largely degraded.
In the mass production of GaN-on-GaN vertical power devices, the wafer-level uniformity of net donor concentration, N D - N A , of the n - -drift layer in around 10 15 cm -3 is an important factor ...because it determines the breakdown voltage. A nondestructive simple inspection is also required. In this paper, we demonstrated the wafer-level nondestructive inspection of a GaN Schottky barrier diode epi-layer and improved the wafer-level net donor uniformity by controlling the off-angle of GaN substrates. Epi-structures were grown by metal-organic vapor phase epitaxy on free-standing GaN substrates with various off-angles and deviations. The variation in N D - N A was carefully analyzed using non-contact capacitance-voltage measurement and photoluminescence. Silicon and carbon concentrations were confirmed by secondary ion mass spectrometry. We found that the normalized yellow luminescence peak intensity is almost linearly related to the acceptor concentration. A carbon related variation in the acceptor concentration (N A ) resulted in the non-uniformity of N D - N A , which is found to be related to the substrate off-angle of the wafer. The N D - N A uniformity can be improved by minimizing variation in the off-angle. Criteria of the GaN substrate off-angle deviation for power applications are discussed.
Photoelectrochemical (PEC) etching was used to fabricate deep trench structures in GaN-on-GaN epilayers grown on n-GaN substrates. The width of the side etching was less than 1 μm, with high ...accuracy. The aspect ratio (depth/width) of a 3.3-μm-wide trench with a PEC etching depth of 24.3 μm was 7.3. These results demonstrate the excellent potential of PEC etching for fabricating deep trenches in vertical GaN devices. Furthermore, we simplified the PEC etching technology to permit its use in a wafer-scale process. We also demonstrated simple contactless PEC etching technologies for the manufacture of power and RF devices. A trench structure was fabricated in a GaN-on-GaN epilayer by simple contactless PEC etching. The role of the cathodic reaction in contactless PEC etching is discussed in relation to the application of a GaN HEMT epilayer on a semi-insulating substrate. Fortunately, the GaN HEMT structure contains an ohmic electrode that can act as a cathode in contactless PEC etching, thereby permitting the recess etching of a GaN HEMT epilayer grown on a semi-insulating SiC substrate. These results indicate that PEC etching technologies are becoming suitable for use in the fabrication of practical GaN power and RF devices.
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