GaSb and its related alloys possess properties attractive for applications in optoelectronic devices. By replacing the anion with other group V elements, e.g. with N or Bi, the band gap and lattice parameter of the binary material can be tuned, thereby vastly increasing the range of uses for the materials. For a semiconductor material to be suitable for optoelectronic applications, the optical as well as the electrical properties need to be well understood. Due to their localized states in the band gap, point defects in semiconductors have a large impact on such properties. The substitutional element also affects the electrical and optical properties since it modifies the band structure. As non-equilibrium growth conditions are needed for the incorporation of the group V element, higher concentrations of point defects are typically introduced in the lattice further affecting the above-mentioned properties.
In this work, point defects in GaSb and its alloys are studied. Undoped, GaSb is p-type and the results show that the crystallization conditions affect which of the native acceptor-type defects in the material that dominates. Whereas the Ga antisite is the main cause of the p-type conductivity for bulk GaSb, the Ga vacancy plays a more significant role in the case of epitaxially grown thin films. Donor-type Sb vacancies are found to be unstable in GaSb at room temperature. The irradiation induced defect undergoes a transition at a low temperature resulting in two acceptor-type defects. By adding N or Bi to the GaSb lattice, the hole concentration is increased by a magnitude or more. The reason for the increased hole concentration is attributed to an increase in both the total amount and the fraction of negative charged defects due to growth conditions as well as the modified band structure.
Using N-rich GaNSb as a prototype material, a model for predicting the band offsets over the whole composition range is also developed in this work. Results based on the developed model and optical measurements show that the band alignment of the GaNSb alloys are promising for photoelectrochemical water splitting applications. The electrical properties of the N-rich GaNSb were studied as a function of annealing temperature. The films were found to be p-type and the hole concentration to increase while the resistivity decreases with increasing annealing temperature. The developed model indicates that the incorporated Sb produces a narrow partially occupied band above the valence band edge of GaN. This band is found to be responsible for the p-type conductivity.
|Tila||Julkaistu - 2017|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|