Date of Graduation

8-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science & Engineering (PhD)

Degree Level

Graduate

Department

Materials Science & Engineering

Advisor/Mentor

Yu, Shui-Qing

Committee Member

Salamo, Gregory

Second Committee Member

Salamo, Gregory J.

Third Committee Member

Chen, Zhong

Fourth Committee Member

Naseem, Hameed

Fifth Committee Member

Ware, Morgan E.

Keywords

Gradiang profile; Partially relaxed GeSn; Room temperature PL; Surface relief; Twinning

Abstract

This research presents comprehensive insights into the epitaxial growth of germanium (Ge) on c-plane sapphire substrates using molecular beam epitaxy (MBE). The direct growth of Ge on sapphire typically yields three-dimensional (3D) Ge islands with diverse growth directions, multiple primary domains, and twinned crystals. However, incorporating a thin AlAs nucleation layer proves instrumental in enhancing surface and material quality. With a 10 nm AlAs layer, the achieved surface exhibits improved characteristics, including a smoother surface, single epitaxial orientation, sharper rocking curve, and a single domain. This study surpasses previous works by achieving a single primary domain and reducing twinning. Furthermore, germanium films grown on c-plane sapphire with a 10 nm AlAs buffer layer were investigated to understand the impact of Ge film thickness on surface morphology and crystal structure. The nucleation process involves the formation of (111) oriented 3D islands with rotational twin domains, contributing to strain and tilt grains. Transitioning to a single-grain orientation reduces strain and improves the overall quality of the Ge buffer. This underscores the vital role of thickness in achieving device quality during Ge(111)/Al2O3(0001) epitaxy when utilizing group IV materials on the sapphire platform. The potential of a high-quality Ge buffer on sapphire is highlighted as an effective platform for subsequent growth of GeSn and SiGeSn for microwave photonics applications. Additionally, the successful demonstration of Ge1-xSnx growth on a novel sapphire platform is presented. Utilizing a developed algorithm, GeSn growth on Ge/GaAs layers was achieved. In-situ and ex-situ characterization techniques were employed to understand surface morphology, crystal structure, and overall quality. Ge on GaAs primarily exhibited a two-dimensional (2D) growth mode, indicating layer-by-layer deposition and contributing to enhanced crystal quality with reduced line width. The subsequent growth of Ge1-xSnx with 10% Sn on a graded profile demonstrated controlled and predictable growth, validated by the correlation between SIMS and Reciprocal Space Mapping (RSM) profiles. This research holds implications for optimizing material properties for specific applications in integrated microwave photonics. Lastly, investigation of single-step GaAs epitaxial growth on sapphire substrates using an AlAs nucleation layer was demonstrated. It transitions from a two-step to a single-step growth process. Our primary objective was to understand how growth temperature and arsenic flux affect growth of the GaAs layers' crystal quality, surface morphology, and optical properties. Omega-2-Theta scans confirmed that all samples exhibited GaAs (111) orientation. Sample S1, grown at 700 °C, demonstrated the highest crystal quality with minimal defects and strain, evidenced by narrow FWHM values. Sample S4, grown at 690 °C, also displayed high crystal quality but with slightly more imperfections. In contrast, Sample S5, grown at 720 °C, had the lowest crystal quality, indicated by broader FWHM values and increased defect density. AFM analysis revealed well-ordered (111) facets for S1 and S4, while S5 showed coalescing surfaces with high roughness. Photoluminescence (PL) measurements at room temperature and 77 K further supported these findings, with S1 and S2 exhibiting higher energy peaks, while S3 showed reduced bandgap energy due to increased defects and strain. This study underscores the critical role of arsenic flux and growth temperature in optimizing GaAs epitaxial growth. It also highlights the need for careful control of these parameters to achieve high-quality semiconductor materials.

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