Date of Graduation

5-2017

Document Type

Thesis

Degree Name

Bachelor of Science in Electrical Engineering

Degree Level

Undergraduate

Department

Electrical Engineering

Advisor

Ware, Morgan

Reader

Wu, Jingxian

Abstract

For the past several decades, methods to harvest solar energy have been investigated intensively. A majority of the work done in this field has been on solar cells made with silicon – the most mature semiconductor material. Recent developments in material fabrication and processing techniques have enabled other semiconductor materials to attract practical interest and research effort as well. Indium gallium nitride (InGaN) is one such material. The material properties of InGaN indicate that solar cells made with it have the potential to achieve much higher power density than a standard silicon solar cell. High power density InGaN solar cells could replace silicon cells in applications where size and weight are critical, or in environments where silicon devices cannot survive. This is especially true of space, and most InGaN development has been done with that in mind. However, at high enough power densities, InGaN solar cells could begin to compete with silicon devices in commercial applications. The goal of this research is to investigate the effect a novel growth technique for InGaN – graded layer deposition – has on the power density of an InGaN solar cell.

In this research, first a baseline InGaN solar cell was grown, fabricated, and characterized. A standard PiN (P: p-type, i: intrinsic, N: n-type) structure was used for this baseline device. The reference alloy composition was chosen to be 20% indium and 80% gallium (In­0.2Ga0.8N). This sample was grown using molecular beam epitaxy (MBE) under standard conditions for the material. Once the reference crystal was fabricated it was optically and electrically characterized. The material composition was verified through a combination of x-ray diffraction (XRD), photoluminescence (PL), and transmittance/reflectance measurements. The quality of the surface of the crystal was examined using atomic force microscopy (AFM). Once the optical characterization of the material was complete, the crystal was processed for electrical characterization. Individual devices were constructed by etching away much of the p-type and intrinsic layer, leaving behind circular mesas. Each mesa was then given a top and bottom contact, so that it could be connected to test equipment electrically. After the crystal was processed into a solar cell in this way, each device was connected to a test source electrically, and the current-voltage (I-V) curves were taken. This information was used to find the current and power densities of each device.

The second step in this work was fabricating and characterizing a graded layer device that was similar to the reference cell. To this end, the graded layer device was chosen to have a starting composition of 25% indium and 75% gallium, with an ending composition of 15% indium and 85% gallium in place of the intrinsic layer. This new crystal was grown under identical conditions as the baseline cell, except for the graded layer, which required a slightly different approach. The graded layer crystal was then characterized and processed consistent with the reference in an attempt to get as accurate of a comparison between the two as possible. The results of this research could significantly affect the field of III-nitride solar cells.

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