Date of Graduation

5-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Yu, Shui-Qing "Fisher"

Committee Member

Salamo, Gregory J.

Second Committee Member

Naseem, Hameed A.

Keywords

Infrared (IR) radiation; Photodetectors; Optical; Wavelength; Bandwidth; Semiconductors

Abstract

Infrared (IR) radiation spans the wavelengths of the windows: (1) near-IR region ranging from 0.8 to 1.0 μm, (2) shortwave IR (SWIR) ranging from 1.0 to 3.0 μm, (3) mid-wave IR (MWIR) region covering from 3.0 to 5.0 μm, (4) longwave IR (LWIR) spanning from 8.0 to 12.0 μm, and (5) very longwave IR extending beyond 12.0 μm. The MWIR and LWIR regions are important for night vision in the military, and since the atmosphere does not absorb at these wavelengths, they are also used for free-space communications and astronomy. Automotive and defect detection in the food industry and electronic circuits also use IR detection as non-contact inspection methods. IR detection is also applied in the medical field. The market of SWIR and MWIR detectors is primarily dominated by mature technology from III-V systems such as indium gallium arsenide (InGaAs and extended InGaAs), indium antimonide (InSb), from II-VI such as mercury cadmium telluride (MCT), lead sulfide (PbS), and from group IV such as silicon (Si) and germanium (Ge) for shorter wavelength. However, the mature IR photodetector technology is expensive, demands to operate at low temperatures, and has complicated fabrication processes. In order to lower cost by mass production, many approaches have been developed towards the hybrid integration of III-Vs or II-VIs on a Si substrate. At the same time, it is desirable to develop an alternative material to reduce the cost and improve the performance for high-temperature operations. The discovery of group IV (Si)GeSn alloys has opened a route for a new generation of IR detectors.

The work in this dissertation set out to develop Si-based Ge1-xSnx photodetectors for low-cost infrared imaging and high-speed detection. A study of effective carrier lifetime and optical properties of Ge1-xSnx materials is presented. The carrier lifetime is then applied to model the Ge1-xSnx photodetectors. For optical properties of Ge1-xSnx materials, two empirical formulae with extracted constants and coefficients were developed: (1) Absorption coefficient. The absorption regarding Urbach tail, indirect and direct bandgap transitions were comprehensively considered; (2) refractive index. The developed formulae could simplify the optoelectronic device design process due to their parameter-based expressions.

A comprehensive study of Si-based GeSn mid-infrared photodetectors is carried out. A set of photoconductors with Sn compositions ranging from 10.5% to 22.3% show the cutoff wavelength to be extended to 3.65 μm. The devices’ peak D* is comparable to that of commercial extended-InGaAs detectors. The GeSn photodiodes are also explored with an in-depth analysis of a dark current. The dark current is suppressed as the photodiode was passivated. Moreover, mid-infrared images were captured using GeSn photodetectors, showing the comparable image quality with that acquired by using commercial PbSe detectors.

The performance of GeSn photodiodes with 6.44 % and 9.24 % Sn is evaluated under high-speed measurements and simulations. The cutoff wavelength is extended up to 2.2 μm and 2.5 μm for 6.44 % and 9.24 % Sn devices, respectively. The photodiodes’ bandwidth is 1.78 GHz, and the simulation shows excellent agreement with measurement results.

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