Formation of Embedded Nitride Semiconductor Nanocrystals.

Abstract

In this thesis, the formation, phase selection, and spatial positioning of GaAs:N (InAs:N) nanocomposite layers produced by N-implantation, focused ion beam (FIB) irradiation, and rapid thermal annealing (RTA) of GaAs (InAs) were investigated. To examine nanocrystal formation mechanisms, the influence of annealing temperature and annealing time on phase formation in GaAs:N were examined. For RTA times of 30 s, we observed the nucleation of zincblende (ZB) GaN at temperatures of 650 to 900ºC. For furnace anneal (FA) times of 10 min, wurtzite (WZ) GaN nucleation was observed for anneal temperatures as low as 650ºC. In the case of InAs:N, for RTA at 500-550ºC ZB InN nanocrystals were nucleated. However, RTA temperatures of 600ºC led to the nucleation of WZ InN nanocrystals with a larger average diameter. These results indicated the key role of annealing time and temperature on crystallite nucleation. A TTT diagram was developed for GaN nucleation in ion-implanted GaAs. The TTT diagram provides an annealing schedule for the selective formation of ZB and WZ GaN. The formation mechanisms for nucleation of ZB and WZ GaN (InN) were investigated and a thermodynamic model for the preferential nucleation of the ZB phase was proposed. ZB nanocrystals had a higher density of low-energy surface planes, which drove the adoption of the ZB phase for sufficiently small nuclei. We demonstrated the first nucleation of ZB and WZ InN in InAs using ion-implantation followed by thermal annealing. Finally, a new process for simultaneous nanostructuring and phase selection, termed “directed matrix seeding,” was demonstrated. Broad-area N+ implantation of GaAs followed by rapid thermal annealing led to the formation of nanocrystals at the depth of maximum ion damage. With additional irradiation using a Ga+ focused ion beam, selective lateral positioning of the nanocrystals within the GaAs matrix was observed in isolated regions of increased vacancy concentration. Following rapid thermal annealing, the formation of zincblende GaN was observed in the regions of highest vacancy concentration. The directed matrix seeding process offers a method for precisely controlling the phase and spatial location of embedded nitride nanostructures in a variety of host materials.