A Distributed Bragg Reflector (DBR) is an important component for semiconductor microcavities and optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), resonant cavity light-emitting diodes (RCLEDs). In the past thirty years, epitaxially grown GaAs-based DBRs have made great achievements of the application of III-V VCSELs in communications and mobile applications. At the same time, III-nitrides have demonstrated excellent performance in solid-state lighting and advanced optoelectronic devices due to the wide bandgap and unique properties. In recent years, GaN-based semiconductors have made great progress in the application of blue VCSELs. However, the absence of high-performance DBRs is a challenge for developing higher-power GaN-based VCSELs.
Currently, the typical epitaxial GaN-based DBRs are limited by a long growth period, low optical performance, and poor quality of growth. Therefore, this project proposes a method to fabricate nanoporous (NP)/GaN-based DBR by electrochemical etching (EC), which are grown using metalorganic vapour-phase epitaxy (MOVPE). The heavily silicon doped GaN layer is transformed into an NP structure by selective etching, resulting in a higher refractive index contrast in each periodic layer. Moreover, a lateral etching method is proposed to further improve the EC etching of DBRs. This method can confine the etching in each sacrificial layer and make the etching aperture directions highly uniform. The corresponding characterizations have been carried out to explore the mechanisms of different etching methods, by optical microscopy, scanning electron microscopy (SEM) and reflectance measurements. It further confirms that the laterally etched NP GaN-based DBRs exhibit a higher reflectivity and wider stopband.
The GaN sacrificial layers required for the EC etching are typically heavily silicon doped (>1019cm-3), resulting in a rough surface and saturated conductivity. On the other hand, the heavily silicon doped AlGaN with a low Al content (≤5%) exhibits an atomically flat surface and an enhanced electrical conductivity. Therefore, in this work, we introduced multiple pairs of heavily doped n++-Al0.01Ga0.99N/GaN to replace the widely used multiple pairs of heavily doped n++-GaN/GaN to fabricate lattice-matched NP DBRs by EC etching. Consequently, the epitaxially grown n++-Al0.01Ga0.99N/GaN-based DBR demonstrates a smoother surface than the n++-GaN/GaN-based DBR. Moreover, the NP-Al0.01Ga0.99N/GaN-based DBR exhibits higher reflectivity and wider stopband after lateral EC etching compared to the NP-GaN/GaN-based DBR. This method has been successfully applied to fabrication of high-performance DBR structures with the wavelength range from blue to deep yellow by modifying the epitaxial growth conditions.
Furthermore, it is found that a very thin Al-Si diffusion layer is formed at the interface between an AlN buffer layer and a silicon substrate when growing the low-temperature AlN buffer layer on the n-doped silicon substrate by MOVPE. The diffusion layer exhibits high conductivity and can be EC-etched and polished as a sacrificial layer. Therefore, this method is proposed for stripping large-area GaN membranes by EC etching. A sample with AlN/AlGaN/GaN layers is first epitaxially grown by MOVPE on an n-doped (111) silicon substrate, and then bonded upside-down to a new glass host substrate and EC etched. Finally, lift-off of a large size GaN-based membrane has been realized with an area of 2.625cm2 and a crack-free and nanoscale smooth surface. Compared to other lift-off methods such as laser lift-off (LLO), chemical lift-off (CLO), and mechanical release techniques, this method does not involve bulky and expensive equipment, which can be used to fabricate high-performance III-nitride devices on the membrane at low cost in the future.
