Electrical and Computer Engineering ETDs

Publication Date

9-5-2013

Abstract

Infrared (IR) detectors today are being utilized for a variety of imaging applications such as medical diagnostics, navigation instruments of automobiles and aircrafts, meteorological imaging, night-vision and fog/smoke imaging, surveillance, target acquisition and astronomical/space imaging. These wide ranging implementations have given rise to varied design requirements for these devices such as the required sensitivity, operating temperature, spectral sensitivity, peak wavelength and cost. Type-II band aligned InAs/Ga(In)Sb strained layer superlattice (SLS) material system proposed for the IR detection in the 1970s has been considered in recent years as an interesting alternative to the present day IR detection technologies. Type-II SLS technology possesses mature growth technologies able to achieve uniformity over large areas along with band-gap tunability which results in IR detectors in mid wave (MW), long-wave (LW) and very long-wave infrared (VLWIR) ranges. The large electron effective mass in SLS help in reducing inter-band tunneling and hence allow for longer-wavelength operation in these IR detectors. Large splitting between heavy-hole and light-hole valence subbands due to strain in the SLSs contributes to the suppression of Auger recombination and this enables significantly higher operating temperatures. The commercialization of type-II SLS technology has been hindered by material defects that promote excess dark currents in the bulk and on detector surfaces. During SLS device fabrication the mesa delineation leads to discontinuity of the periodic crystal structure which results in formation of unsatisfied chemical bonds on the etched surfaces and hence enhancing the surface leakage currents. More-over, with the scaling of single pixel dimensions, the performance of focal plane arrays is strongly dependent on surface effects due to the large pixels surface to volume ratio. Hence the reduction of surface leakage currents in LWIR detectors has become a technological necessity and is the objective of this dissertation. This work focuses on performance improvement of InAs/GaSb SLS LWIR detectors by optimization of mesa delineation and surface passivation techniques. The first part of the dissertation works on the development of optimized etching scheme for the 400μm x 400μm area single-pixel SLS detectors and for 25μm x 25μm area focal plane arrays (FPAs). Firstly, optimization of wet chemical etches were carried out on GaSb substrates and reported here is that HCl: H2O2:H2O (1:1:4) solution rendered the smoothest etched surface with a root mean square (RMS) roughness of 1.59nm. This work also reports the results of comparison of mesa sidewall profiles of InAs/GaSb SLS single-pixel and FPA detectors obtained after (a) a conventional BCl3 gas based inductively coupled plasma dry etch, (b) a HCl: H2O2:H2O (1:1:4) solution wet chemical etch and (c) combination of both. It was found that HCl: H2O2:H2O (1:1:4) solution based wet etch was ideal for single pixel mesa delineation, but not on FPA device proportions. In the etch study experiments, the InAs/GaSb SLS structure was looked at as one single entity and as an amalgamation of its constituents InAs and GaSb materials. The second part of this research deals with the development of efficient surface passivation methods for the long wave infrared (LWIR) SLS detectors with pin, pBiBn, and graded bandgap designs. A comparative study of dielectric passivants (Silicon di-oxide and Silicon nitride) versus organic passivant of SU-8 versus chalcogenide passivants (Zinc Sulfide, Ammonium Sulfide and Electro-chemical passivation i.e. deposition of pure sulphur) were carried out on InAs/GaSb SLS LWIR single-pixel detectors. Ammonium sulfide [(NH4)2S] treatment and electrochemical sulfur deposition (ECP) reduced dark current density (Jd) at 77K and applied bias= - 0.1V by (a) by factor of 25 and 200 in InAs/GaSb SLS LWIR detector with p-i-n design and (b) by factor of 3 and 54 in InAs/GaSb SLS LWIR with pBiBn design. In the comparative study of all the sulphur-based passivants of thioacetamide (acid-based and base-based), (NH4)2S and ECP on graded-bandgap design based single-pixel LWIR SLS detectors, ECP showed superior performance of all with the highest surface resistivity (rsurface) of 1.4x105 \u2126-cm at 77K. In the long term stability study of ECP, it showed degradation over time with Jd=0.09A/cm2 right after passivation and Jd =2.18A/cm2 after 4 weeks of passivation. Hence as an alternative to ECP (i.e. sulphur passivation), we propose the utilization of novel chlorine-doped Zinc Telluride (ZnTe:Cl) as a passivation technique. It is for the first time that ZnTe:Cl has been used as a passivation technique on any IR device. This novel ZnTe:Cl passivation technique is implemented on InAs/GaSb SLS LWIR single-pixel detectors with pin and graded-bandgap designs. Though ECP showed a superior rsurface=105\u2126-cm over ZnTe:Cl passivation with rsurface =6700\u2126-cm, ZnTe:Cl passivation had almost no degradation over time. Ergo, an improved ZnTe:Cl with its robust, uniform passivation layer has the potential to replace ECP in FPA fabrication on InAs/GaSb SLS LWIR detectors and there-of take these high-performing SLS-based IR detectors to commercialization.

Keywords

Infrared Detectors, Type II SLS, Stained Layer Superlattice, InAs/GaSb superlattice, passivation, mesa etch, wet etch, dry etch, electro-chemical passivation, chalcogenide passivation, ZnTe, ECP

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Advisor

Krishna, Sanjay

First Committee Member (Chair)

Balakrishnan, Ganesh

Second Committee Member

Plis, Elena

Third Committee Member

Phillips, Jamie

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