Longwave and bi-color type-II InAs/(In)GaSb superlattice infrared detectors

Infrared (IR) photodetectors are useful for a variety of military and civil applications such as target acquisition, medical diagnostics, pollution monitoring, to name just a few. Presently photonic IR detectors are based on interband transitions in low bandgap semiconductors such as mercury cadmium telluride (MCT) or InSb or in intersubband transitions in hetero-engineered structures such as quantum well or quantum dot infrared photodetectors (QWIPs or QDIPs). These detectors operate at low temperatures (77 K-200 K) in order to obtain high signal to noise ratio. The cooling requirement limits the lifetime, increases the weight and the total cost, as well as the power budget, of the whole infrared system. There is a concerted effort to develop photonic detectors operating at higher temperatures. In the past few years, interband transitions in type II InAs/GaSb strain layer superlattices (SL)have emerged as a competing technology among other IR systems. Although MCT and QWIP technologies are relatively more mature than the SL technology, the SL technology has potential to enhance performance in several key areas. One of the main advantages of this system lies in the fact that the effective band gap of the SL can be tailored over a wide range (3 μm \u3c λc \u3c 30 μm) by varying the thickness of two mid bandgap\u27 constituent materials, namely GaSb and InAs. Tunneling currents in SL are reduced due to a larger electron effective mass. Large splitting between heavy-hole and light-hole valence subbands due to strain in the SLs contributes to the suppression of Auger recombination. Moreover, the band structure of the SL can be engineered to enhance carrier lifetimes and reduce noise at higher temperatures. SL based IR detectors have demonstrated high quantum efficiency, high temperature operation, and are suitable for incorporation in focal plane arrays (FPA) by tapping into the mature III-V based growth and fabrication processes. The recently proposed nBn heterostructure design has demonstrated a 100 K increase in background-limited infrared photodetection (BLIP) for InAs-based device, by decreasing Shockley-Read-Hall generation currents and by suppressing surface currents using specific processing. Third generation IR detectors have three main emphases, high operating temperature (HOT), multicolor capability, and large format arrays. This work concentrates on multicolor and HOT IR detectors based on nBn design. Contributions of this thesis include 1. Development of design and growth procedure for the long-wave (LW) SL detectors leading to an improved detector performance: 13 MLs of InAs and 7 MLs of GaSb with InSb strain compensating layer were designed and optimized for LW SL detectors. LWIR pin and nBn detectors were introduced and their optical and electrical properties were compared. LW nBn detectors show higher device performance in terms of lower dark current density and higher responsivity as compared to the LW pin detectors. The reduction in dark current in LW nBn detector is due to reduction of SRH centers as well as surface leakage currents. The increase in responsivity for LW nBn detectors is due to reduction non-radiative SRH recombination. 2. Design, growth and characterization of bi-color nBn detectors: Present day two color SL detectors require two contacts per pixel leading to a complicated processing scheme and expensive read out integrated circuits (ROICs). The nBn architecture was modified to realize a dual-band response by changing the polarity of applied bias using single contact processing. The spectral response shows a significant change in the LWIR to MWIR ratio within a very small bias range ( 3c100 mV ) making it compatible with commercially available ROICs. 3. Investigation of background carrier concentration in SLs: The electrical transport in SLs was investigated in order to improve the collection efficiency and understand SL devices performance operating at ambient temperature. For this purpose background carrier concentration of type-II InAs/GaSb SLs on GaAs substrates are studied. The hall measurements on mid-wave SLs revealed that the conduction in the MWIR SLs is dominated by holes at low temperatures (\u3c 200 K) and by electrons at high temperatures (\u3e 200 K) and is dominated by electrons at all temperatures for LWIR SLs possibly due to the thicker InAs (residually n-type) and thinner GaSb (residually p-type) layers. By studying the in-plane transport characteristics of LW SLs grown at different temperatures, it was shown that interface roughness scattering is the dominant scattering mechanism at higher temperatures (200 K- 300 K).\u2

P-Compensated and P-Doped Superlattice Infrared Detectors

Barrier infrared detectors configured to operate in the long-wave (LW) infrared regime are provided. The barrier infrared detector systems may be configured as pin, pbp, barrier and double heterostructrure infrared detectors incorporating optimized p-doped absorbers capable of taking advantage of high mobility (electron) minority carriers. The absorber may be a p-doped Ga-free InAs/InAsSb material. The p-doping may be accomplished by optimizing the Be doping levels used in the absorber material. The barrier infrared detectors may incorporate individual superlattice layers having narrower periodicity and optimization of Sb composition to achieve cutoff wavelengths of.about.10.mu.m

Unipolar Barrier Dual-Band Infrared Detectors

Dual-band barrier infrared detectors having structures configured to reduce spectral crosstalk between spectral bands and/or enhance quantum efficiency, and methods of their manufacture are provided. In particular, dual-band device structures are provided for constructing high-performance barrier infrared detectors having reduced crosstalk and/or enhance quantum efficiency using novel multi-segmented absorber regions. The novel absorber regions may comprise both p-type and n-type absorber sections. Utilizing such multi-segmented absorbers it is possible to construct any suitable barrier infrared detector having reduced crosstalk, including npBPN, nBPN, pBPN, npBN, npBP, pBN and nBP structures. The pBPN and pBN detector structures have high quantum efficiency and suppresses dark current, but has a smaller etch depth than conventional detectors and does not require a thick bottom contact layer

Mid- and Long-IR Broadband Quantum Well Photodetector

A single-stack broadband quantum well infrared photodetector (QWIP) has been developed that consists of stacked layers of GaAs/AlGaAs quantum wells with absorption peaks centered at various wavelengths spanning across the 9- to-11- m spectral regions. The correct design of broadband QWIPs was a critical step in this task because the earlier implementation of broadband QWIPs suffered from a tuning of spectral response curve with an applied bias. Here, a new QWIP design has been developed to overcome the spectral tuning with voltage that results from non-uniformity and bias variation of the electrical field across the detector stacks with different absorption wavelengths. In this design, a special effort has been made to avoid non-uniformity and bias tuning by changing the doping levels in detector stacks to compensate for variation of dark current generation rate across the stacks with different absorption wavelengths. Single-pixel photodetectors were grown, fabricated, and tested using this new design. The measured dark current is comparable with the dark measured current for single-color QWIP detectors with similar cutoff wavelength, thus indicating high material quality as well as absence of performance degradation resulting from broadband design. The measured spectra clearly demonstrate that the developed detectors cover the desired special range of 8 to 12 m. Moreover, the shape of the spectral curves does not change with applied biases, thus overcoming the problem plaguing previous designs of broadband QWIPs

Barrier infrared detector

A superlattice-based infrared absorber and the matching electron-blocking and hole-blocking unipolar barriers, absorbers and barriers with graded band gaps, high-performance infrared detectors, and methods of manufacturing such devices are provided herein. The infrared absorber material is made from a superlattice (periodic structure) where each period consists of two or more layers of InAs, InSb, InSbAs, or InGaAs. The layer widths and alloy compositions are chosen to yield the desired energy band gap, absorption strength, and strain balance for the particular application. Furthermore, the periodicity of the superlattice can be "chirped" (varied) to create a material with a graded or varying energy band gap. The superlattice based barrier infrared detectors described and demonstrated herein have spectral ranges covering the entire 3-5 micron atmospheric transmission window, excellent dark current characteristics operating at least 150K, high yield, and have the potential for high-operability, high-uniformity focal plane arrays

Single-Band and Dual-Band Infrared Detectors

Bias-switchable dual-band infrared detectors and methods of manufacturing such detectors are provided. The infrared detectors are based on a back-to-back heterojunction diode design, where the detector structure consists of, sequentially, a top contact layer, a unipolar hole barrier layer, an absorber layer, a unipolar electron barrier, a second absorber, a second unipolar hole barrier, and a bottom contact layer. In addition, by substantially reducing the width of one of the absorber layers, a single-band infrared detector can also be formed

Minority Carrier Lifetime and Photoluminescence Studies of Antimony-Based Superlattices

In this paper, we have used the OMR technique to study the minority carrier lifetimes in three InAs/GaSb-photoluminescence (PL) structures with different number of periods in the absorber: 300, 400 and 600 periods respectively. The feasibility of using a visible 643 nm laser source with short penetration depth for lifetime measurements was studied by comparing the achieved results to measurements performed on the same samples with a 1550 nm IR laser source, which penetrates much deeper into the sample. Despite the differences in excitation wavelengths and penetration depths, the results from both measurements were very similar. This indicates that the diffusion length is long enough to facilitate a homogeneous distribution of excess carriers in the material

Photoluminescence Study of Long Wavelength Superlattice Infrared Detectors

In this paper, the relation between the photoluminescence (PL) intensity and the PL peak wavelength was studied. A linear decrease of the PL intensity with increasing cut-off wavelength of long wavelength infrared CBIRDs was observed at 77 K and the trend remained unchanged in the temperature range 10 - 77 K. This relation between the PL intensity and the peak wavelength can be favorably used for comparison of the optical quality of samples with different PL peak wavelengths. A strong increase of the width of the PL spectrum in the studied temperature interval was observed, which was attributed to thermal broadening

Minority Carrier Lifetime Studies of Narrow Bandgap Antimonide Superlattices

In this study optical modulation response and photoluminescence spectroscopy were used to study mid-wave Ga-free InAs/InAsSb superlattices. The minority carrier lifetimes in the different samples varied from 480 ns to 4700 ns, partly due to different background doping concentrations. It was shown that the photoluminescence intensity can be used as a fast non-destructive tool to predict the material quality. It was also demonstrated that it is crucial to use a low excitation power in the photoluminescence measurements in order to get a good correlation between the photoluminescence intensity and the minority carrier lifetime