Growth and characterization of gallium arsenide phosphide and gallium phosphide on silicon for III-V/Si multi-junction solar cells

Hool, Ryan D.

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https://hdl.handle.net/2142/115926 Copy

Description

Title

Growth and characterization of gallium arsenide phosphide and gallium phosphide on silicon for III-V/Si multi-junction solar cells

Author(s)

Hool, Ryan D.

Issue Date

2022-07-14

Director of Research (if dissertation) or Advisor (if thesis)

Lee, Minjoo Lawrence

Doctoral Committee Chair(s)

• Lee, Minjoo Lawrence
• Shim, Moonsub

Committee Member(s)

• Zuo, Jian-Min
• Shoemaker, Daniel P

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Epitaxial III-V/Si integration
• metamorphic growth
• dislocations
• TDD
• GaAsP solar cell
• GaP on Si
• MBE
• DSE
• ECCI
• EBIC
• photovoltaics
• electron irradiation

Abstract

Solar photovoltaic (PV) technologies offer the ability to provide a significant percent of the world’s increasing energy demand while reducing overall greenhouse gas emissions. At the same time, PV technology is critical for the future of power generation in space. Silicon single-junction (1J) solar cells dominate the terrestrial solar market due to high production efficiencies and low material and processing costs from decades of investment. However efficiency is a key driver to further reduce the cost of PV technology and little room remains to improve the efficiency of Si toward its theoretical limit. Multi-junction (MJ) solar cells using a combination of III-V compound semiconductors achieve much higher efficiencies through tailoring bandgaps of sub-cells, leading to their utilization in space technology. Nevertheless, III-V solar cells are hindered by small-diameter, high-cost substrates, excluding their use in terrestrial power generation and placing a barrier to pursuing a greater variety of space missions. Through epitaxial integration of III-V semiconductors on Si, there is great potential to dramatically reduce the cost of III-V MJ cells and to realize competitive, high-efficiency solar cells for both space and terrestrial power. While epitaxial III-V/Si MJ solar cells offer the possibility of a much-desired combination of high efficiency and low cost, the difficulty of growing high-quality III-V materials on Si substrates has left the potential of this PV technology unfilled. The combination of a ~1.7 eV III-V top cell and a ~1.1 eV Si bottom cell is close to the ideal double-junction (2J) bandgap pairing and has a > 25% (relative) higher theoretical maximum efficiency than for Si by itself. However, epitaxial III-V/Si MJ solar cells have yet to demonstrate efficiency higher than Si 1J solar cells. Extended defects resulting from crystal mismatches between III-V materials and Si hamper III-V sub-cell performance, and reducing the densities of these defects is critical to advancing epitaxial III-V/Si technologies. The larger lattice constants for III-V materials than for Si inevitably cause threading dislocations to form that extend through III-V sub-cell(s). Gallium Arsenide Phosphide (GaAsyP1-y) is an ideal partner to Si due to possessing a tunable direct bandgap from 1.42 – 2.01 eV and the smallest lattice mismatch to Si for ~1.7 eV III-V materials (~3.1%). Gallium Phosphide (GaP), with the closest lattice constant to Si for III-V materials, offers a convenient starting point for III-V epitaxial growth on Si before further expanding the lattice constant with compositional grading of GaAsyP1-y. Obtaining low defect densities in ~1.7 eV GaAs0.77P0.23 (GaAsP) has remained challenging with threading dislocation densities (TDD) typically around an order of magnitude higher than desired for high performance (targeting ≤ 1 – 2 × 10^6 cm-2). More troubling is that relaxed GaP on Si can result in TDD > 10^7 cm-2, even though the lattice mismatch is less than an eighth of that for ~1.7 eV GaAsP. In this dissertation, I focus on defect management for relaxed GaP and GaAsP to improve material quality and advance GaAsP/Si 2J solar cell development. To this end, I investigated defects in GaP and determined growth strategies to reduce TDD to the lowest reported values I am aware of for both p- and n-type relaxed GaP on Si. I observed a strong dependence of TDD with doping and demonstrated methods to reach parity for relaxed p- and n-GaP on Si. For the GaAsP top cell I identified additional performance-damaging defects and strategies to mitigate them to realize high-current top cells. As a result, my collaborators and I realized the highest reported efficiencies for both GaAsP/Si 2J and ~1.7 eV GaAsP 1J solar cells on Si. Lastly, I studied radiation hardness and temperature-dependent device performance of ~1.7 eV GaAsP solar cells to broaden the space testing literature for this material.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Ryan D. Hool

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