Abstract
III-nitrides semiconductors attract numerous attention in the last decade, especially for the optical devices such as blue laser diodes as well as the high frequency, high power and high temperature electronic devices such as high electron mobility transistors (HEMTs). Owing to the inert chemical characteristics of III-nitride, difficulty incurs in etching nitrides for the device applications. Especially the formation of laser cavity facets must resort to the well-developed etching techniques; on the other hand, low-damage etching techniques are indispensable for the fabrication of recessed-gate HEMTs. In this study, we compare three etching techniques and examine the feasibility of producing the vertical sidewall and low-damage etch surface for real device applications.. Among these etching techniques, photo-assisted cryogenic etching produces very smooth (~1.7 nm as determined from the atomic force microscopy (AFM)) and nearly damage-free etch surface (as-evidenced from the elimination of yellow luminescence (YL)). Photoelectrochemical (PEC) etching provides an economic means to attain smooth sidewall with a satisfactory etch rate. Due to these specific characteristics in PEC etching of GaN, we identified the dislocation-related structures that remain on surface after the etching. All the dislocation-related structures possessed weak bonding characteristics and can thus be removed after a short-duration hot KOH immersion.By varying the ion incidence angles, we identified that ion channeling occurs during the Ar+ ion beam etching in both structures of AlGaN/GaN heterostructures for the HEMTs devices and GaN/InGaN/GaN quantum well (QW) structures for the blue laser diodes. After rapid thermal annealing (RTA) at 800 oC for 30 sec, most etch-damage created by ion channeling could be removed.For a GaN etched at 250 V accelerating voltage, an increase in VB was observed with the subsequent N2 plasma treatment on the etched surface, due to an excess nitrogen supply on the nitrogen-deficient GaN surface. However, an etched GaN showed a decrease in reverse leakage current after the N2 plasma treatment with rf power added, indicating that an additional type of damage was introduced under the energetic ion bombarding conditions.Utilizing these specific etching techniques, and in conjunction with photoluminescence and cathodoluminescence spectroscopy, we have identified some defect-related optical transitions frequently encountered in a PL spectrum on the unintentionally doped GaN epilayer.