Interband Cascade Infrared Photodetectors Research Articles

Detectivity enhancement by double radiation pass in interband cascade infrared photodetectors

The detectivity enhancement in a double-pass architecture in an interband cascade infrared photodetector (ICIP) is investigated. The ICIP consists of twelve identically thick absorbers composed of an InAs/GaSb type-II superlattice. To achieve a double-pass of radiation, a multilayer backside mirror was sputter-deposited after thinning the undoped GaSb substrate. At room temperature, a differential resistance area product of R0A= 1.37 Ω cm2 was achieved and the ICIP has a cutoff wavelength of 6.5 μm. By adding a backside mirror to the device, the responsivity was increased by 47% caused by the double radiation pass through the absorber. This resulted in a room temperature zero-bias detectivity of 8.3 × 108 Jones at a wavelength of 5.27 μm. Furthermore, oscillations are observed in the detectivity due to light interference from the reflected beam at the semiconductor/metal interface, which manifests the reflected double-pass device functionality unequivocally in the device characteristics.

Shot and Johnson noises in interband cascade infrared photodetectors

Shot and Johnson noises are often incorrectly thought of as two independent noise sources. This incorrect picture has affected the evaluation of detectivities in interband cascade infrared photodetectors (ICIPs). In this work, a unified picture of shot and Johnson noises is developed for ICIPs based on a fundamental framework to understand the origin of Johnson noise and clarify the possible confusion between Johnson and shot noises. General, yet concise expressions are derived to evaluate the current noise power spectral density and detectivity for ICIPs even with complicated structures. Also, simple expressions for the signal current due to absorption of photons and the corresponding photon noise are derived, consistent with the previous results derived from alternative methods. Furthermore, a formula is derived to correctly evaluate the detectivity for conventional photodetectors under a reverse bias. The derived formulas with discussion are expected to improve the understanding of noises in ICIPs and other types of photodetectors and help us to appropriately evaluate their detectivities.

Equivalent Circuit Model of the RF Characteristics of Multi-Stage Infrared Photodetectors

Multi-stage infrared photodetectors such as quantum cascade detectors (QCDs) and interband cascade infrared photodetectors (ICIP) have been demonstrated with high detectivity and high-frequency operation. In the previous work published by Rui. Q. Yang [Applied Physics Letters 2021, 119 (14), 141107], an equivalent circuit model is used to derive the relation among quantum efficiency, collection efficiency, and particle conversion efficiency. The ultimate detectivities in multi-stage infrared photodetectors were discussed based on that model. However, the RF performance of multi-stage infrared photodetectors was rarely studied theoretically so far. In this work, a generalized equivalent circuit model is proposed with the AC term included. The detailed calculations and relevant discussion are provided to show how the normalized frequency response function of each stage impacts the overall 3 dB performance of the device. In the limit of weak absorption, such as QCDs or ICIP with very weak and identical absorption in each stage, the multi-stage infrared photodetectors show significant improvement in RF performance. Moreover, how the current mismatching issue between stages impacts 3 dB bandwidth performance quantitatively is derived and discussed for identical absorption stages architecture. The result and insights gained from this work can be beneficial for the understanding and optimization of high-speed multi-stage detectors.

Equivalent circuit and fundamental limit of multi-stage infrared photodetectors

Based on an equivalent circuit model, a signal current in a multi-stage infrared (IR) photodetector is derived and used to discover a meaningful relation among quantum efficiency, collection efficiency, and particle conversion efficiency. Based on such a relation, it is demonstrated theoretically that the ultimate detectivities in multi-stage interband cascade infrared photodetectors (ICIPs) with identical discrete absorbers are the same as that in a conventional single-absorber detector in the limit of perfect collection (i.e., independent of the number of cascade stages) but higher than that in the single-stage detector with a finite diffusion length. Detailed derivations and calculations, along with relevant discussion, are provided to show how ICIPs are optimized for maximizing the detectivity and to understand the underlying physics. Multi-stage ICIPs with identical discrete absorbers are robust and durable against structural variations without being restricted by current matching and, therefore, are of more significance for practical applications such as those that require high-speed response or circumventing the diffusion length limitation. The results obtained for ICIPs with identical discrete absorbers can also be applied for quantum cascade detectors and photovoltaic quantum well infrared photodetectors. The results and insights gained from this work will further improve the understanding of multi-stage IR photodetectors and generate increased interest in the development of ICIPs and related devices for useful applications.

Interfacial Intermixing and Its Impact on the Energy Band Structure in Interband Cascade Infrared Photodetectors.

Multiple-stage interband cascade infrared photodetector (ICIP) is a new class of semiconductor infrared photodetector that exhibits improved device performance in terms of responsivity and detectivity. The design of the device structure and the electronic structure on superlattices and quantum wells assume abrupt interfaces. However, the emergence of possible interface segregation and atom exchange can only be determined experimentally, impacting the device performance. In this work, the interface atom intermixing and their effects on the energy band structure in a molecular beam epitaxy grown ICIP are studied. Scanning transmission electron microscopy (STEM) reveals atom diffusion and intermixing between the constituent layers of the cascade structure, causing a shift in the quantum state energy levels of the layers and the consequent misalignment of the cascade structures. Combining the STEM observation with high-resolution X-ray diffraction, the alloy composition profiles of the layers are determined. Using the "real" graded composition profiles, the effective band gap of the superlattice absorber and the energy levels of the relaxation region and the tunneling region are recalculated showing a cutoff wavelength of the superlattice absorber 4.93 μm, which is 0.78 μm smaller than that calculated using the nominal step composition profile. However, its agreement is greatly improved with the measured cutoff wavelength of 5.03 μm. The energy level of the narrowest quantum well in the relaxation region is 0.091 eV higher than the conduction miniband of the absorber, which is also consistent with the experiments that the pho-response exits a "turn on" voltage of 0.1 V. The results reported here will help optimize the energy structure design of future ICIP with improved device performance.

High-speed interband cascade infrared photodetectors: photo-response saturation by a femtosecond oscillator.

Interband cascade infrared photodetectors (ICIPs) combine interband optical transitions with fast intraband transport to achieve high-frequency and broad-wavelength operation at room temperature. Here we study the bias-dependent electronic impulse response of ICIPs with a mid-infrared synchronously pumped optical parametric oscillator (OPO). Since the OPO produces ultrashort 104-fs pulses, it is possible to probe the impulse response of the ICIP. From this impulse response, we identify two characteristic decay times, indicating the contribution of electron as well as hole carriers. A reverse bias voltage applied to the ICIP reduces both time scales and leads to an increased electrical cut-off frequency. The OPO emits up to 500mW average power, of which up to 10 mW is directed to the ICIP in order to test its saturation characteristics under short-pulse illumination. The peak of the impulse response profile as well as the average photocurrent experience a gradual saturation behavior, and we determine the corresponding saturation powers by measuring the photo-response as a function of average power directed to the ICIP. We demonstrate that an increasing reverse bias increases the saturation power as well as the responsivity of the ICIP.

Mid-wavelength interband cascade infrared photodetectors with two and three stages

In this paper, we report design, fabrication and characterization of mid-wavelength interband cascade infrared photodetectors (ICIP) with two- and three-stage. The effects of stage number and absorber thickness on the optical and electrical properties of the device are studied. Dark current analysis shows that the shunt current of the three-stage ICIP is lower than that of the two-stage ICIP. The temperature-dependent quantum efficiencies of the two devices are analyzed and the diffusion lengths are simulated. The optimal absorber thicknesses of the ICIP structure are calculated for the further improvement of the device performance.

Minority carrier lifetime in mid-wavelength interband cascade infrared photodetectors

A simple and effective electrical method is developed to extract the thermal generation rate and minority carrier lifetime in type-II (T2) InAs/GaSb/Al(In)Sb superlattices (SLs) by taking advantage of the features of interband cascade infrared photodetectors (ICIPs). This method is more generally applicable, considers the parasitic shunt and series resistances found in practical devices, and can account for various transport mechanisms including Auger and Shockley-Read-Hall processes. The investigated ICIPs have cutoff wavelengths in the close vicinity of 4.3 μm at 300 K. The carrier lifetimes in these ICIPs at high temperatures (200–340 K) were extracted and found in the range of 8.5–167 ns, with values correlated with the material quality. The temperature dependence of the extracted carrier lifetime exhibited different characteristics from previously reported behavior obtained using other methods for T2SL detectors, especially at high temperatures.

Electrical gain in interband cascade infrared photodetectors

In order to achieve improved understanding and gain insights into the device operation of interband cascade infrared photodetectors (ICIPs) and ultimately to optimize the design, we present a comparative study of five long-wavelength (LW) ICIPs based on a type-II InAs/GaSb superlattice. This study shows how the device responsivity is affected by the individual absorber thicknesses and the number of cascade stages, through the impact of light attenuation. Additionally, this study further validates that the electrical gain universally exists in non-current-matched ICIPs. With multiple cascade stages to suppress noise, these LW ICIPs achieved superior device performance at high temperatures, in terms of Johnson-noise limited detectivities, compared to commercial MCT detectors. Furthermore, a theory is developed to quantitatively describe the electrical gain in ICIPs and our calculations are in good agreement with the experimental results. Based on the theory, the optimal number of stages for maximizing the device detectivity D* is identified with inclusion of the electrical gain. Our calculation shows that this optimal number of stages is relatively large in the presence of the gain and the maximized D* has a relatively weak dependence on the absorber thickness when it is sufficiently thin.

Resonant tunneling and multiple negative differential conductance features in long wavelength interband cascade infrared photodetectors

We report on an investigation of multiple negative differential conductance (NDC) features in long wavelength interband cascade infrared photodetectors (ICIPs) at and above 300 K. Using ICIPs with various structures and carrier concentrations, we employ several approaches to demonstrate that the observed multiple NDC features and their unusual temperature dependence are related to the sequential turn off of resonant tunneling of minority carriers through the electron barriers at high temperatures.

Midwavelength interband cascade infrared photodetectors with superlattice absorbers and gain

We report on a comparison study of the electrical and optical properties of a set of device structures with different numbers of cascade stages, type-II superlattice (T2SL) absorber thickness, and doping variations, as well as a noncurrent-matched interband cascade infrared photodetectors (ICIP) structure with equal absorbers. Multistage ICIPs were demonstrated to be capable of operating at high temperatures at zero-bias with superior carrier transport over comparable conventional one-stage detectors. Based on the temperature dependence and bias sensitivity of their responsivities with various absorber thicknesses, the diffusion length is estimated to be between 0.6 and 1.0 μm for T2SL materials at high temperatures (>250 K). A comparison of responsivities between current matched ICIPs with varied absorber thicknesses and noncurrent-matched ICIPs with equal absorbers shows that the current-matching among cascade stages is necessary to maximize responsivity. Additionally, electrical gain exceeding unity is demonstrated in these detectors in the reverse-illumination configuration.

Current-matching versus non-current-matching in long wavelength interband cascade infrared photodetectors

We report on a comparative study of two sets of different long-wavelength (LW) interband cascade infrared photodetectors (ICIPs) based on a type-II InAs/GaSb superlattice. The devices in one set have a current-matched configuration while those in the other set are non-current-matched. It is shown that carrier transport in these LW ICIPs at high temperatures is diffusion limited and the diffusion length is estimated to be longer than or comparable to 0.5 μm at various temperatures. By comparing the two sets of devices, we demonstrate the necessity of current-matching to maximize utilization of absorbed photons for an optimal responsivity and the correlation of reduced responsivity with light attenuation in the optically deeper stages for non-current-matched ICIPs. However, the device performances for both sets are comparable in terms of detectivity. Electrical gain exceeding unity is observed, which is more significant in the last stage of non-current-matched ICIPs than in any other stages in order to obtain photocurrent continuity. The significant electrical gain enabled an appreciable responsivity in non-current-matched ICIPs and resulted in Johnson-noise limited detectivities as high as 2.4 × 108 Jones at 300 K, which substantially exceeds the reported value (e.g., 4.0 × 107 Jones) for commercial uncooled mercury cadmium telluride detectors.

Mid-wave interband cascade infrared photodetectors based on GaInAsSb absorbers

In this work, we report the demonstration of quaternary GaInAsSb-based mid-wavelength infrared photodetectors with cutoff wavelengths longer than 4 μm at 300 K. Both interband cascade infrared photodetector (ICIP) with a three-stage discrete absorber architecture and conventional one-stage detector structures have been grown by molecular beam epitaxy and investigated in experiments for their electrical and optical properties. High absorption coefficient and gain were observed in both detector structures. The three-stage ICIPs had superior carrier transport over the one-stage detectors. A detectivity as high as 1.0 × 109 cm Hz1/2 W−1 was achieved at 3.3 μm for both one- and three-stage detectors under zero bias at 300 K. The implications of these results are discussed along with potential of GaInAsSb-based ICIPs for high-speed applications.

Short-wavelength interband cascade infrared photodetectors operating above room temperature

High temperature operation (250–340 K) of short-wavelength interband cascade infrared photodetectors (ICIPs) with InAs/GaSb/Al0.2In0.8Sb/GaSb superlattice absorbers has been demonstrated with a 50% cutoff wavelength of 2.9 μm at 300 K. Two ICIP structures, one with two and the other with three stages, were designed and grown to explore this multiple-stage architecture. At λ = 2.1 μm, the two- and three-stage ICIPs had Johnson-noise-limited detectivities of 5.1 × 109 and 5.8 × 109 cm Hz1/2/W, respectively, at 300 K. The better device performance of the three-stage ICIP over the two-stage ICIP confirmed the advantage of more stages for this cascade architecture. An Arrhenius activation energy of 450 meV is extracted for the bulk resistance-area product, which indicates the dominance of the diffusion current at these high temperatures.

Interband cascade infrared photodetectors with long and very-long cutoff wavelengths

We report our recent studies of interband cascade infrared photodetectors (ICIPs) with 100% cutoff wavelengths of 16.0μm and 9.2μm at 78K. The very-long-wavelength infrared (VLWIR) detectors were able to operate at temperatures up to 143K. Relatively high dark current densities were observed from these VLWIR detectors. This was attributed to the very narrow bandgap and possibly to defects in the materials. In addition, the photo-response of these detectors was strongly dependent on bias, indicating that further research and device optimization are necessary. Two-stage ICIPs in the long-wavelength infrared (LWIR) spectrum (8–12μm) were able to operate at higher temperatures (up to 250K) with an extended cutoff wavelength of ∼12μm. At 78K, these LWIR detectors had a bias-independent photocurrent, with an R0A of 115Ωcm2. This corresponded to a Johnson-noise-limited D∗ of 3.7×1010cmHz1/2/W at 8.0μm.

Interband-cascade infrared photodetectors with superlattice absorbers

Interband-cascade infrared photodetectors (ICIPs), composed of discrete superlattice absorbers, are demonstrated at temperatures up to 350 K with a cutoff wavelength near 5 μm at 80 K to beyond 7 μm above room temperature. The peak responsivity exceeds 200 mA/W, higher than the values reported from early interband cascade laser structures, suggesting a significantly enhanced quantum efficiency of the superlattice absorbers. A theoretical model, originally developed for quantum well infrared photodetectors (QWIPs), is applied to ICIPs to analyze their device performance. The Johnson-limited and background-limited detectivities are extracted and indicate that background-limited performance temperatures for two ICIP structures are 126 and 105 K at 5 μm. It is expected that optimized ICIPs will provide improved performance by combining the advantages of conventional photodiodes and the discrete nature of QWIPs and IC lasers.