Fabrication and characterisation of microscale air bridges in conductive gallium nitride

Abstract

Fabrication and electrical characterisation of microscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN-AlInN-GaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ~17% InN fraction. The start of the fabrication sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ~70% of the etch rate of GaN. Processing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported between anchor posts at both ends. Deposition of Ti-Au contact pads onto the anchor posts allowed study of the electrical characteristics. At low applied voltages, vertical conduction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear current-voltage characteristics at low applied voltages, and had resistances of 1050 Ω. The observed resistance values are compared with the predicted value based on materials parameters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2 x 10 5 A cm-2 .

High-reflectivity GaN/air vertical distributed Bragg reflectors fabricated by wet etching of sacrificial AlInN layers

“…The second type of microstructure consisted of rectangular beams 4 μm wide by 20 μm in length each supported between two larger square anchor posts. The planview geometry was the same as that used to fabricate singlelayer doped GaN microbridges for electrical studies reported in [13]. Figure 3(a) shows an oblique SEM image of a microstructure containing two AlInN layers, which have been exposed by vertical ICP etching, but not yet subjected to wet etching.…”

“…to form resonant cavity light-emitting diodes) and microcavities. Also wider applications of the sacrificial layer technology are expected in fabrication of microelectromechanical sensors and actuators [4,13].…”

“…Epitaxial GaN-AlInN multilayers were grown in an Aixtron 200/4 RF-S metal organic chemical vapour deposition reactor. Deposition conditions for GaN and AlInN were similar to those in [13], apart from the omission of intentional doping, and the use of FS-GaN rather than sapphire substrates. The FS-GaN substrates were from Lumilog (Vallauris, France), and had…”

“…The method of fabricating vertical GaN-air DBRs presented here involved selective wet etching with hot nitric acid of multiple AlInN layers grown close to the latticematched composition of Al 0.17 In 0.83 N. Importantly, the AlInN etch does not show pronounced chemical attack on the reactive GaN (0 0 0 1) crystal face, which is progressively exposed as the AlInN is removed. We have previously applied similar processing sequences to structures containing single sacrificial AlInN layers, in order to fabricate microbridges and planar microcavities [11][12][13][14]. The application of our microfabrication method using lateral etching of AlInN to epitaxial multilayers, and the associated microoptical characterization of vertical air-gap DBRs, constitutes the new results in this report.…”

Fabrication of GaN topological photonic crystal membranes in the visible wavelength region by a combination process of HEATE and AlInN wet etching

“…The HEATE method can be implemented with a simple equipment configuration, and the thin SiO 2 mask enables ultrafine nanofabrication that is tens of nanometers in size. Membrane structures that require strong optical confinement are commonly used for PhCs, and several fabrication techniques such as Si-selective dry etching for GaN/Si structures, 25) photoelectrochemical wet etching of InGaN superlattice sacrificial layers, 26) selective high-temperature pyrolysis of GaN for AlGaN/GaN structures, 27) and selective wet etching of AlInN sacrificial layers by hot nitric acid [28][29][30][31][32][33] have been reported.…”