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Fabrication and study of low-dimensional structures based on GaN


Author: Volciuc Olese
Degree:doctor of engineering
Speciality: 01.04.10 - Semiconductors physics and engineering
Year:2011
Scientific adviser: Ion Tighineanu
doctor habilitat, professor, Institute of Mathematics and Computer Science of the ASM
Institution: Technical University of Moldova
Scientific council: DH 30-01.04.10-23.04.09
Moldova State University

Status

The thesis was presented on the 4 March, 2011
Approved by NCAA on the 31 March, 2011

Abstract

Adobe PDF document0.72 Mb / in romanian

Keywords

GaN, nanostructuring, photoelectrochemical etching, focused ion beam, nanowires, membranes, radiation hardness, gas sensors, chatodoluminescence, photoluminescence, Raman spectroscopy.

Summary

The thesis presents the development of the maskless technology, the so-called surface charge lithography, for the fabrication of the GaN based meso- and nanostructures. The proposed novel maskless approach is based on direct “writing” of radiation defects by focused ion beam on GaN surface that trap negative charge shielding the material against photoelectrochemical etching. Compared with commonly used lithography followed by RIE or FIB etching approaches for patterning of GaN, the surface charge lithography enables one to fabricate high-aspect ratio micro- and nanostructures and mitigates the need for additional mask layers on the surface prior to etching, and is much faster than FIB etching alone reducing furthermore the ion exposure of material and therefore reducing ion beam induced damage.

We demonstrate that using surface charge lithography technology it is possible to reach lateral dimensions as small as 100 nm for GaN nanowalls and nanowires and to fabricate suspended structures with the thickness defined by the main projection range of implanted ions.

Cathodoluminescence (CL) microscopy and spectroscopy have been used to investigate the optical properties of GaN microstructures patterned by Ar+ ion irradiation and subsequent photoelectrochemical (PEC) etching. Monochromatic CL images and CL spectra reveal an enhancement of several defect-related emission bands in a 10 μm wide area around each microstructure. CL emission of the nanocolumns is dominated by free electrons from conduction band to acceptor transitions, while excitonic luminescence prevails in the rest of the etched GaN layers. Investigation of the sidewalls of the microstructures reveals that a CL emission band centered at about 3.41 eV, attributed to excitons bound to structural defects, is effectively suppressed after PEC etching only in the observed nanocolumns.

The utility of electrochemical technologies of nanostructuring GaN for the purpose of enhancing its radiation hardness is demonstrated by both PL and RRS analyses. The near band gap excitonic PL dominates the emission spectra of nanostructured GaN layers up to the dose of 1012 cm−2 for high energy heavy ions, while the luminescence is totally quenched in the as-grown layers at this irradiation dose. At higher doses, the intensity of the RRS lines is much higher in the nanostructured samples and the mechanism of RRS is different in comparison with that inherent to as-grown layers. A nanostructuring induced enhancement of the GaN radiation hardness by more than one order of magnitude was derived from the PL and RRS analyses. These findings show that electrochemical nanostructuring of GaN layers is a potentially attractive technology for the development of radiation hard devices.

Pronounced chemical and radiation stability of GaN and high temperature operation stimulated one to elaborate gas sensors based on nanostructured GaN epilayers. The sensor exhibits high sensitivity towards CO. It was demonstrated that subjection of nanostructured GaN samples to high energy heavy ions irradiation considerably reduces the gas sensitivity, while post-irradiation rapid thermal annealing proves to result in sensitivity restoration of about 50 %, the effect being dependent upon the dose of irradiation and annealing temperature.

The SCL technology was implemented for the fabrication of gas sensors based on GaN nanowalls, capable to detect reducing and oxidative gases. We demonstrated the possibility to increase the sensitivity of elaborated gas sensors by using Pt dots.

The main results of the Doctoral Thesis were published in 12 scientific papers. The Thesis is written in Romanian and consists of 120 text pages, 70 figures, 6 tables and 204 references.