Group III-vacancies mediated disordering of intrinsic and n-type AlGaAs/GaAs superlattices

Abstract

Thesis (Ph. D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1993.The mechanisms of superlattice disordering, a useful technique for optoelectronic device fabrication and integration, have been investigated. Al-Ga interdiffusion coefficients in AlGaAs/GaAs superlattices were determined using photoluminescence spectroscopy (PLS) and secondary ion mass spectrometry (SIMS). Al-Ga interdiffusion in intrinsic AlGaAs heterostructures is mediated by second nearest-neighbor hopping of group III vacancies throughout the 700 to 1050 °C range and in both Ga- and As-rich annealing ambients. The predicted increase of two orders of magnitude in the Al-Ga interdiffusion coefficient with As pressure between the Ga- and As-rich solidus limits was observed. The depth profiles of AlGa interdiffusion are consistent with group III-vacancy diffusion between the crystal surface and bulk, and yielded a single, reasonable vacancy diffusion coefficient. The activation energy of interdiffusion agreed with that predicted for second nearest-neighbor hopping of vacancies based on theoretical estimates of vacancy formation and migration energies. Additionally, the annealing of silicon nitride encapsulated superlattices indicated very limited Al-Ga interdiffusion even with an As overpressure. This is attributed to the lack of group III vacancies. Conversely, silicon dioxide caps appeared to be very permeable. A quantitative test of the Fermi-level effect on Al-Ga interdiffusion in n-type superlattices was performed using PLS, SIMS, and C-V profiling. A significant variation was observed in the enhancement with annealing ambient in quantum wells that were Si-doped during growth. This attributed to electrical compensation and the As overpressure's effect on group III-vacancy formation at the crystal surface. The predicted Fermi-level enhancement of a factor of forty was observed only when neither excess Ga or As were included. Disordering by the indiffusion of a variety of group IV and VI donors was also investigated. An enhancement in Al-Ga interdiffusion was observed in each case with the disordering being attributed to group III vacancies. However, important differences have been observed in the interdiffusion characteristics induced by Si or Ge, and that by S or Se. Additionally, the depth profiles of deep levels associated with group III vacancy-donor complexes were obtained using cathodoluminescence