Date of Award

9-2025

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Engineering Physics

First Advisor

Christina L. Dugan, PhD

Abstract

GeSn (germanium tin) alloys are potentially well suited for near-mid infrared space optoelectronic applications. Alloys of GeSn have similar properties to group III-V and mercury-cadmium-telluride semiconductors and are compatible with cost-effective complementary metal oxide semiconductor (CMOS) manufacturing technology. Recent progress in non-equilibrium remote plasma-enhanced chemical vapor deposition (RPECVD) has enabled the crystalline growth of GeSn with Sn concentrations of up to 10% without Sn surface segregation. Several experimental studies in previous literature report CVD- or molecular beam epitaxy (MBE)-grown GeSn alloys achieving a direct bandgap with 6%-9% Sn content. This novel growth technique opens opportunities for a cost-effective, next-generation optical photon detector. In this work, X-ray photoelectron spectroscopy, UV-visible light spectroscopy, current-voltage/capacitance-voltage measurements, and deep-level transient spectroscopy (DLTS) techniques were applied to analyze the electrical characteristics of GeSn alloys. GeSn/Si device material properties to include oxidation states, bandgap energies, and interface defect states were analyzed. To mimic the exposure of unshielded optical detectors in Low Earth Orbit (LEO), GeSn alloys electrical properties were evaluated before and after proton irradiation. The oxidation states study found Ge less susceptible to oxidation than Sn, with oxidation percentages ranging from 25 ± 1.25% to 86 ± 2.35%. The Sn dopant enhanced the oxidation features associated with the Ge 3p peak, aiding surface oxidation and allowing it to penetrate further into the film. The optical bandgap measurements determined that the bandgap energy of RPECVD grown GeSn alloys were similar to conventional CVD- and MBE-grown alloys. This research also revealed p-type Fermi energy pinning occurs close to the valence band, representing highly doped semiconductor behavior. Bulk electrostatic properties from GeSn/Si devices, including ideality factors, barrier heights and rectification ratios displayed ideal diode characteristics. Furthermore, the proton irradiation and annealing effects study characterized the increased deep-level trap concentration at the interface. The radiation-induced defects act as trapping sites, reducing the number of free charge carriers available to contribute to capacitance and increasing the hole-electron trap-assisted recombination rate, hence reducing both diffusion and drift currents. These deep-level traps were identified and attributed to artifacts  of displacement damage often observed in proton irradiated Si and Ge.

AFIT Designator

AFIT-ENP-DS-25-S-004

Comments

An embargo was observed for posting this dissertation.

Approved for public release, distribution unlimited.  PA case number 88ABW-2025-0623

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