Thesis presented December 17, 2004

Abstract:
The main objective of this work is to study the feasibility of a Group III-nitride semiconductor laser pumped by a microtip cathode and emitting in the far UV (250–350 nm). The laser is a separate confinement heterostructure with an active layer made up of MBE-grown GaN quantum wells (or dots) in an Al_xGa_1-xN barrier. A 10 keV e-beam generated by a microtip cathode is focused onto the laser structure by cyclotron motion using permanent magnets. This study showed the existence of two major technological hurdles. Firstly, the laser threshold is very high (~10 MW/cm² in optical pumping). This is due to the inadequate quality of the Al_xGa_1-xN barrier which results in low carrier collection in the quantum wells. Ambipolar diffusion was studied by combining cathodoluminescence measurements with the results of a simulation program. It was found that the diffusion length varies over three orders of magnitude (~nm–μm) depending on the layer concerned. It is shortest in the Al_0.1Ga_0.9N barrier. Use of carbon-face SiC substrates gave some improvement in the quality of our samples. Secondly, the current emitted by the microtips (~A/cm²) is well below the laser threshold. A cesium deposition device was constructed to reduce the electron work function of the microtips. The current was increased by a factor of 50 for a grid–cathode voltage of 70V. However, the cesium deposit proved to be both inhomogeneous and unstable.

Keywords:
Laser, semiconductor, diffusion, microtips, ultraviolet, nitrides, cathodoluminescence, cesium

On-line thesis.