Fabrication and characterisation of microscale air bridges in conductive gallium nitride

Abstract

Fabrication and electrical characterisation of mi- croscale 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 fabrica- tion 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. Process- ing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported be- tween anchor posts at both ends. Deposition of Ti-Au con- tact pads onto the anchor posts allowed study of the elec- trical characteristics. At low applied voltages, vertical con- duction 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 re- sistances of 1050 � . The observed resistance values are compared with the predicted value based on materials para- meters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2 × 10 5 Ac m −2 .