Date of Graduation

8-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Ware, Morgan E.

Committee Member

Naseem, Hameed A.

Second Committee Member

Chen, Zhong

Third Committee Member

Hu, Jin

Keywords

Graded Structure; InGaN; polarization doping; Relax; Solar Cell; Strain

Abstract

Indium gallium nitride (In_x Ga_(1-x) N) materials have displayed great potential for photovoltaic and optoelectronic devices due to their optical and electrical properties. Properties such as direct bandgap, strong bandgap absorption, thermal stability and high radiation resistance qualify them as great materials for photovoltaic devices. The tunable bandgap which absorbs the whole solar spectrum is the most significant feature which became attractive for scientists. The bandgap for these materials varies from 0.7 eV for InN to 3.4 eV for GaN covering from infrared to ultraviolet. In_x Ga_(1-x) N wurtzite crystal is grown on GaN buffer layer by Molecular Beam Epitaxy (MBE). Epitaxial growth of high quality In_x Ga_(1-x) N material, however, creates great challenges due to lattice mismatch between InN and GaN (up to 11%). This might be the actual reason of partially and fully strain at the interface relating to growth condition which affect optical properties of the materials. Therefore, studying solar cell parameters for different indium compositions (low to high) in the material is significant. In this work, graded composition In_x Ga_(1-x) N (44 nm ramping up followed by 44 nm ramping down) were grown on GaN/sapphire template. The growth was done at different indium compositions (low to high) in plasma-assisted MBE. Additionally, optical and structural characterizations of the materials were done. The results showed that by increasing indium composition, the composition was not linearly graded as expected and was accompanied by strain relaxation along the growth direction. In other words, for low indium composition, the results showed fully strained. However, for high indium composition partially strain relaxation was seen. The optical respond of three samples was studied with photoluminescence. For the first: to study the source of each peak in aspect of either exciton or different kinds of defect states. Second, peaks related to ground state transition. Furthermore, nextnano3 and nextnano+ software were used to simulate optical properties of 100 nm graded structures such as the band structure, ground state wave-function position as well as determine the optical transition probabilities among ground state hole and electrons as well as solar cell parameters for different structures under different strained conditions. Simulation continued for higher alloys (20% to 90%) under strain and (20%-100%) under relaxed condition. An equation like Vegard’s law was created to predict the energy bandgap under strain for different indium compositions. The simulation was performed for 100 nm -graded structure to find the optimum xmax for both conditions for maximum solar efficiency. In addition, the performance of graded structure in a Flat Base Graded (FBG) was studied to compare with Square well and Homojunction structure.

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