UC Berkeley

Layered semiconductors, like transition-metal dichalcogenides and III-VI monochalcogenides, possess interesting properties attractive for future opto-electronic applications. Among the III-VI monochalcogenides, gallium telluride (GaTe) possesses a unique monoclinic structure, good p-type transport properties, and contrary to most layered materials, a direct bandgap in the bulk (1.67 eV). This dissertation explores different avenues for the bandgap engineering of GaTe, including access to the bandstructure through the layers’ surfaces, conventional semiconductor alloying and stabilization of alternate metastable phases.

In the presence of air, mechanically exfoliated GaTe develops a deep-level defect band effectively reducing the bandgap in a direct-to-indirect transition to about 0.8 eV. The intercalation and chemisorption of molecular oxygen to the Te-terminated layers was responsible for the behavior. I discuss on how surface defects created by the mechanical exfoliation facilitate the transformation as well as procedures to delay or accelerate such transformation. Contrary to traditional bandgap engineering methods, the partial reversibility of this process can also be achieved.

The alignment of the conduction and valence band edges as well as shallow-defect levels were determined following an ion irradiation study. Based on the amphoteric defect model, the conduction band and valence band edges of GaTe were found to be 3.47 eV and 5.12 eV below vacuum, respectively. Low-temperature spectroscopy found two acceptor levels around 100 and 150 meV above the valence band and a donor level around 130 meV below the conduction band.

Gallium selenide (GaSe) and GaTe alloys (GaSexTe1-x) were grown by vapor deposition. Monoclininc crystals were obtained for x < 0.32, and hexagonal crystals were obtained for x > 0.28. The bandgap of the monoclinic phase increases linearly with Se content from 1.65 eV to 1.77 eV while hexagonal-phase bandgap decreases from 2.01 eV (GaSe) to 1.38 eV (x = 0.28). Finally, the bandgap of hexagonal GaTe was confirmed to be 1.45 eV, by epitaxially growing hexagonal GaTe crystals on GaSe substrates. The results presented here show how the selected bandgap-engineering avenue can affect the structural and opto-electronic properties of GaTe.