Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Microelectronics-Photonics

Advisor/Mentor

Morgan Ware

Committee Member

Shui-Qing Yu

Second Committee Member

Cynthia Sides

Third Committee Member

Douglas Hutchings

Fourth Committee Member

Matthew Leftwich

Keywords

Graded structures, nextnano3, Nitride Materials, Optoelectronics, Polarization doping, Semiconductors

Abstract

Indium gallium nitride (InxGa1-xN) materials have held great potential for the optoelectronic industry due to their electrical and optical properties. The tunable band gap that can span the solar spectrum was one of the most significant features that attracted researchers’ attention. The band gap can be varied continuously from 0.77 eV for InN to 3.42 eV for GaN, covering the solar spectrum from near infrared to near ultraviolet. Additionally, it has a high absorption coefficient on the order of ∼105 cm−1, a direct band gap, high radiation resistance, thermal stability, and so on. Nevertheless, the epitaxial growth of high quality In-rich InxGa1-xN material has faced numerous challenges to date due to lattice mismatch between InN and GaN (up to 11%) and the difference in growth temperatures. This can be the main reason for spinodal decomposition that results in phase separation, relaxation of the strain at the interface, or compositional pulling. Therefore, a high density of defects will be generated in the InxGa1-xN layer, depending on the growth conditions, which can degrade the optical properties of the material. Thus, understanding and improving the growth conditions which also affect the amount of In incorporated in the material is significant.Here, graded composition InxGa1-xN layers have been grown on GaN/sapphire templates. The gradient was accomplished by a continuous increase of the In mole fraction in the atomic flux used to grow the crystal. The growth was done at different temperatures (555 °C to 475 °C) in plasma-assisted molecular beam epitaxy (MBE) to establish good parameters to grow a thick film grading the entire range from x = 0 to 1. Furthermore, this study focused on the optical and structural characterization of the material. It was found that the composition was not linearly graded as expected and was accompanied by strain relaxation along the growth direction. As expected, lower growth temperatures allowed for higher In content. A simple growth model was used to understand the growth conditions and the parameters that affect the In content in the material. Moreover, nextnano3 software was used to simulate the band structure and determine the optical transition probabilities as well as the ground state wavefunctions, which can help in future device development.

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