Nanoscience and Microsystems ETDs

Publication Date

Fall 11-18-2016

Abstract

Infrared (IR) hybrid detector arrays and discrete detectors operated in the space environment may be subjected to a variety of sources of natural radiation while in orbit. This means IR detectors intended for applications such as space-based intelligence, surveillance, and reconnaissance (ISR) or space-situational awareness (SSA) must not only have high performance (high quantum efficiency, h and low dark-current density, JD, and preferably minimal 1/f noise content), but also their radiation tolerance or ability to withstand the effects of the radiation they would expect to encounter in space must be characterized and well understood. As the effects of proton interactions with hybrid detector arrays can dominate in space, a specific detector’s radiation tolerance is typically characterized by measuring its performance degradation as a function of proton fluence, FP, up to a total ionizing dose (TID) of typically 100 krad(Si), which is 3-5 times the maximum expected on-orbit TID value for typical space-based E/O applications. Now for other applications such as astronomy, planetary science, and imaging associated with nuclear medicine applications, the TID requirement can be much higher. When comparing the performance of novel IR detector technologies, it has also proven valuable to determine the rate of performance degradation induced by radiation, referred to as a damage factor. It has also proven valuable to perform temperature-dependent measurements of JD, which are used to determine the dark current limiting mechanism via an Arrhenius-analysis, and the degree to which any thermal annealing of the irradiation induced defects may occur have provided unique insights. Finally, given the potential sensor/system impact it is of the upmost importance to understand the frequency dependent contributions to the overall noise in IR detectors. This body of work contains in-depth measurements and analysis of these performance metrics for both III-V- and II-VI-based IR detectors of various detector architectures.

In this dissertation, the results of IR III-V-based InAs/GaSb and InAs/InAsSb type-II strained layer superlattice (T2SLS) and bulk detectors that employ unipolar barriers in their detector architecture and II-VI-based HgCdTe IR detectors are characterized in both clear and radiation environments. III-V-based IR detectors that employ unipolar barriers are now being considered for space applications due to their relative advantage in manufacturability as compared with conventional HgCdTe IR detectors that dominant space-based IR E/O imaging. T2SLS detectors are theoretically predicted to have lower Auger-limited dark currents compared with HgCdTe. However, this advantage is yet to be realized due to the lack of reliable passivation schemesand higher bulk defect densities in these materials, which lead to surface- and Shockley-Read-Hall (SRH)-limited dark currents, respectively. Unipolar-barrier architecture detectors, including the nBn, pBp, pBiBn, etc. detectors reported on here, have been introduced in an effort to mitigate these dark current limiting mechanisms. By deliberate choices of the absorber materials and device structure, the potential barriers in these detectors appear only in either the conduction or valence band to block the majority-carrier bulk and surface currents (e.g. in a nBn detector the potential barrier appears only in the conduction band). This results in an elegant detector architecture in which the ideal barrier layer limits the depletion by an external bias to itself so that the absorbing layer remains in the flatband condition, which eliminates Generation Recombination currents due to SRH defects that may be present in the absorbing layer that ultimately limit the diffusion length.

Subjecting IR detectors to proton irradiation may lead to both TID and displacement damage effects, both of which occur on orbit. TID effects occur as incoming protons lose their kinetic energy to ionization of the detector material’s constituent atoms and the additional charges become trapped in oxide layers or surface traps. This additional charging may result in flat-band voltage shifts and increased surface leakage currents. TID effects generally are more visible at lower device temperatures, where charges generated in oxide layers are less mobile, and tend to anneal out at higher temperatures. Displacement damage effects result from the occasional non-ionizing energy loss of an incoming proton due to elastic or inelastic scattering with an atomic nucleus that is sufficient to knock the atom from its lattice site and generate vacancy-interstitial pairs, anti-sites, and defect complexes. In this work these defects were shown to manifest in lower h, due to the consequent reduction in minority carrier lifetime t, and higher JD, due to the SRH mechanism. The proton fluence required to alter the background doping levels, such that the fundamental Auger mechanism is enhanced, when using protons with an energy of 63 MeV is expected to be order’s of magnitude higher than the fluence levels used in this work. Thus, a vital step to characterizing a detector’s radiation tolerance is measuring h and JD as a function of FP, with all irradiation and measurements conducted in-situ stepwise at the detector’s expected operating temperature and bias. In this research, it was found that rate of degradation in quantum efficiency when irradiated with 63 MeV protons for a family of Sb-based MWIR detectors that employed unipolar barrier architectures was greater than 3 times that of conventional p-on-n HgCdTe photodiodes with similar cut-off wavelengths. Likewise, it was found that the rate of degradation in the lateral optical collection length for these same devices was greater than 20 times that of the equivalent MWIR HgCdTe photodiodes. This has been attributed to a degradation in minority carrier lifetime leading to a reduction in the diffusion length. This body of research provides unique insights into the radiation susceptibility and fundamental mechanisms taking place that directly contribute to performance degradation of III-V- and II-V-based IR detectors of various detector architectures.

Document Type

Dissertation

Language

English

Degree Name

Nanoscience and Microsystems

Level of Degree

Doctoral

Department Name

Nanoscience and Microsystems

First Committee Member (Chair)

Sanjay Krishna

Second Committee Member

Ganesh Balakrishnan

Third Committee Member

Danhong Huang

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