ANALYSIS OF STEP-VELOCITY DEPENDENCES OF CARBON CONCENTRATION IN c- AND m-PLANE GaN HOMOEPITAXIAL LAYERS

Reported step-velocity dependences of carbon concentration in c- and m-plane GaN homoepitaxial layers were successfully reproduced based on a step-edge segregation model with the following assumptions: 1) the diffusion coefficient of carbon in GaN is 2×10^-13 cm^2/s (@ 1000℃−1100℃); 2) the length of time before the carbon concentration at the step-edge site reaches its equilibrium value is sufficiently shorter than the meantime until a carbon atom incorporated at the kink site moves through the step-edge site to the surface site

ESTIMATION OF EXCITATION DENSTITY OF REPORTED TWO-PHOTON PHOTOLUMINESCENCE INTENSITY DISTRIBUTIONS AT A DISLOCATION IN AN n-GaN LAYER ON A M-3D SUBSTRATE

We estimated the excitation density of the reported two-photon photoluminescence from an n-GaN layer on an M-3D substrate by fitting the intensity distributions at a dislocation. We found that all the data points of the intensity distributions were reproduced with the effective dislocation radius of 8 nm. Considering the range of the excess hole concentration (Δp) within which almost all data points fell, we estimated Δp to be 10^16±1 cm^−3

Electrodeless photo-assisted electrochemical etching of GaN using a H3PO4-based solution containing S2O82- ions

Electrodeless photo-assisted electrochemical etching was successfully demonstrated using a H-3 PO4-based solution containing S2O82- ions. The pH value of the solution changed under UVC illumination, clearly showing that SO4- radicals were produced from S2O82- ions by absorbing UVC light. The production rate of SO4- radicals maintained a constant value over the wide pH range of the solution, leading to etching rates and surface roughness comparable to those obtained in KOH-based solutions. The positive-type photoresist was applicable as the etching mask for the H3PO4-based solution. This finding will contribute to the development of a simple wet etching process suitable for the manufacturing of GaN-based devices

High-k Dielectric Passivation for GaN Diode with a Field Plate Termination

Vertical structured Gallium nitride (GaN) p-n junction diodes with improved breakdown properties have been demonstrated using high-k dielectric passivation underneath the field plate. Simulation results at a reverse voltage of 1 kV showed that the maximum electric field near the mesa-etched p-n junction edges covered with film of dielectric constant k = 10 was reduced to 2.0 MV/cm from 3.0 MV/cm (SiO2 (k = 3.9)). The diodes were fabricated using the high-k mixed oxide of SiO2 and CeO2 with k = 12.3. I–V characteristics of the diode with a field plate showed a breakdown voltage above 2 kV with an increased avalanche resistance. This means that the electric field reduces at the periphery of the mesa-etched p-n junction and is uniformly formed across the whole p-n junction. It is clear that high-k dielectric film passivation and filed plate termination are essential techniques for GaN power devices

Photoelectrochemical Etching Technology for Gallium Nitride Power and RF Devices

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 mu m, with high accuracy. The aspect ratio (depth/width) of a 3.3-mu m-wide trench with a PEC etching depth of 24.3 mu m 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

Self-terminating contactless photo-electrochemical (CL-PEC) etching for fabricating highly uniform recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs)

Contactless photo-electrochemical (CL-PEC) etching was used to fabricate recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). Self-termination of etching was observed during CL-PEC etching on an AlGaN barrier layer whose residual thickness had a uniform value of 6 nm overall on the same chip. After tetramethylammonium hydroxide post-treatment, the root-mean-square roughness of the etched surface was around 0.4 nm, which had smoothness comparable to that of the unetched surface. Recessed-Schottky HEMTs showed a positive shift in V-th, the suppression of drain leakage currents, and an improvement in the subthreshold-slope value as compared with planar-gate HEMTs. By applying a metal-insulator-semiconductor (MIS)-gate structure, the gate and drain leakage currents were significantly reduced, leading to an increased input dynamic range. Furthermore, the standard deviations (sigma) of the V-th of CL-PEC-etched recessed-Schottky HEMTs and recessed-MIS HEMTs were very small, 5.5 and 16.7 mV, respectively. These results showed that the CL-PEC etching process is promising for the fabrication of recessed-gate AlGaN/GaN HEMTs having excellent uniformity for normally-off device operations