Formation of Embedded Nitride Semiconductor Nanocrystals.
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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.Ph.D.PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91523/1/adamww_1.pd