StatusThe thesis was presented on the 5 December, 2005
Approved by NCAA on the 26 January, 2006
Abstract– 0.72 Mb / in romanian
The photoelectrochemical (PEC) etching process of GaN is developed. The influence of such parameters as solution concentration, agitation, light intensity and temperature on morphology and electrophysical properties was investigated.
Systematic research using PEC etching in KOH solution for different etching durations evidenced the mechanism of the decrease of the density of threading dislocations as a function of the epitaxial layer thickness.
By maneuvering with etching parameters and types of solution, it is possible to reach columnar or nanowire morphology, honeycomb or bridge like structures, which can find applications as free-stress templates for high quality GaN epitaxial growth, biosensors with high chemical stability and huge surface, and gas sensors, as demonstrated in the thesis.
The advantage of GaN properties was used for Schottky diode fabrication based on this material. First, we used electrochemical Pt deposition which did not introduce defects on the crystal surface because of low ion energy in comparison with electron beam evaporation commonly used. Second, a study of the influence of passivation techniques upon the diode electrical parameters was carried out.
It was observed and studied for the first time that, along with native defects, artificially introduced defects in GaN are stable to PEC etching. This fascinating property can be used for the purpose of maskless GaN meso- and nanostructuring, using just a preliminary ion beam treatment followed by PEC etching process. This new technology was highly appreciated at INPEX 2005 exhibition (Pittsburg, PA, USA) with a GOLD Medal.
The CL analysis of PEC etched layers revealed the stress distribution along GaN epilayers. The CL peak position of exciton emission in islands with coral-like relief is indicative of the occurrence of compressive stress, while that of the exciton emission in columnar structures evidences the existence of tensile stress.
Pronounced chemical stability of GaN and high temperature operation allowed one to elaborate gas sensors based on nanostructured GaN epilayers. The sensors exhibits high sensitivity towards alcohols, hydrocarbons, ammonia, hydrochloric acid vapors, ozone and hydrogen. Taking into account quite different values of gas sensitivity toward methane and ethylic alcohol of the etched epilayers in KOH and H3PO4 solutions, it was possible to fabricate a selective sensor for methane leakage detection using an external simple electronic differentiator. A prototype device for alcohol concentration monitoring was demonstrated.