Interband Cascade Structures Research Articles

Theory of Hybrid Microwave–Photonic Quantum Devices

AbstractThe Maxwell–Lindblad transmission line (MLT) equations are introduced as a model for hybrid photonic and microwave‐electronic quantum devices. The hybrid concept has long enabled high‐bandwidth photodetectors and modulators by using waveguide structures supporting microwave–optical co‐propagation. Extending this technique to quantum‐engineered lasers provides a route for greatly improved short‐pulse and frequency comb operation, as impressively demonstrated for quantum cascade lasers. Even more, the self‐organized formation of coupled microwave and optical oscillation patterns has recently enabled photonics‐based ultralow‐noise microwave generation. The MLT model provides a suitable theoretical description for hybrid quantum devices by combining optical and microwave propagation equations with a Lindblad‐type approach for the quantum active region dynamics. A stable numerical scheme is presented, enabling realistic device simulations in excellent agreement with experimental data. Based on numerical results and analytical considerations, it is demonstrated that the functionality of the investigated hybrid quantum devices does not only rely on local coupling effects, but rather benefits from microwave‐optical co‐propagation, settling an ongoing debate. The MLT equations provide the basis for systematic device development and extension of the concept to other quantum‐engineered hybrid devices based on, for example, quantum dot or interband cascade structures, as well as for simplified analytical models providing additional intuitive insight.

Long wavelength interband cascade photodetector with type II InAs/GaSb superlattice absorber

We report on a long wavelength interband cascade photodetector with type II InAs/GaSb superlattice absorber. The device is a three-stage interband cascade structure. At 77 K, the 50% cutoff wavelength of the detector is 8.48 μm and the peak photoresponse wavelength is 7.78 μm. The peak responsivity is 0.93 A/W and the detectivity D* is 1.12 × 1011 cm·Hz0.5/W for 7.78 μm at –0.20 V. The detector can operate up to about 260 K. At 260 K, the 50% cutoff wavelength is 11.52 μm, the peak responsivity is 0.78 A/W and the D* is 5.02 × 108 cm·Hz0.5/W for the peak wavelength of 10.39 μm at –2.75 V. The dark current of the device is dominated by the diffusion current under both a small bias voltage of –0.2 V and a large one of –2.75 V for the temperature range of 120 to 260 K.

Mid-infrared widely tunable single-mode interband cascade lasers based on V-coupled cavities.

We demonstrate widely tunable single-mode V-coupled-cavity lasers emitting at wavelengths near 3 µm based on a type-II interband cascade (IC) structure. The mode selection is achieved using a half-wave V-coupler designed for the IC structure in the mid-infrared range. The laser waveguides and cavity structure are deeply etched in a single etching step, without any grating. By changing the injection current at a fixed heat-sink temperature, a tuning range over 35 nm can be achieved with a side-mode suppression-ratio up to 28 dB. The tuning range can be extended to 60 nm when combined with the adjustments of the heat-sink temperature.

Enhanced collection efficiencies and performance of interband cascade structures for narrow bandgap semiconductor thermophotovoltaic devices

We report on a comparative study of narrow-bandgap (∼0.2 eV at 300 K) thermophotovoltaic (TPV) devices with InAs/GaSb type-II superlattice absorbers. By comparing the characteristics of three narrow bandgap TPV structures with a single absorber or multiple discrete absorbers, it is clearly demonstrated that the device performance of a conventional single-absorber TPV cell is limited mainly by the small collection efficiency associated with a relatively short diffusion length (1.5 μm at 300 K). Furthermore, this study revealed that multi-stage interband cascade (IC) TPV structures with thin individual absorbers can circumvent the diffusion length limitation and are capable of achieving a collection efficiency approaching 100% for photo-generated carriers. Additionally, the open-circuit voltage, the fill factor, the output power, and the power conversion efficiency can be significantly increased in IC TPV devices compared to the conventional single-absorber TPV structure. These results have further validated the potential and advantages of narrow bandgap IC structures for TPV cells.

High quantum efficiency mid-wavelength interband cascade infrared photodetectors with one and two stages

In this paper, we report on mid-wavelength infrared interband cascade photodetectors grown on InAs substrates. We studied the transport properties of the photon-generated carriers in the interband cascade structures by comparing two different detectors, a single stage detector and a two-stage cascade detector. The two-stage device showed quantum efficiency around 19.8% at room temperature, and clear optical response was measured even at a temperature of 323 K. The two detectors showed similar Johnson-noise limited detectivity. The peak detectivity of the one- and two-stage devices was measured to be 2.15 × 1014 cm·Hz1/02/W and 2.19 × 1014 cm·Hz1/02/W at 80 K, 1.21 × 109 cm·Hz1/02/W and 1.23 × 109 cm·Hz1/02/W at 300 K, respectively. The 300 K background limited infrared performance (BLIP) operation temperature is estimated to be over 140 K.

High-frequency operation of a mid-infrared interband cascade system at room temperature

The high-frequency operation of a mid-infrared interband cascade system that consists of a type-I interband cascade laser and an uncooled interband cascade infrared photodetector (ICIP) is demonstrated at room temperature. The 3-dB bandwidth of this system under direct frequency modulation was ∼850 MHz. A circuit model was developed to analyze the high-frequency characteristics. The extracted 3-dB bandwidth for an uncooled ICIP was ∼1.3 GHz, signifying the great potential of interband cascade structures for high-speed applications. The normalized Johnson-noise-limited detectivity of these ICIPs exceeded 109 cm Hz1/2/W at 300 K. These results validate the advantage of ICIPs to achieve both high speed and high sensitivity at high temperatures.

Molecular beam epitaxy of interband cascade structures with InAs/GaSb superlattice absorbers for long-wavelength infrared detection

The interfaces of InAs/GaSb superlattices (SLs) were studied with the goal of improving interband cascade infrared photodetectors (ICIPs) designed for the long-wavelength infrared region. Two ICIP structures with different SL interfaces were grown by molecular beam epitaxy, one with a ∼1.2 monolayer (ML) InSb layer inserted intentionally only at the GaSb-on-InAs interfaces and another with a ∼0.6 ML InSb layer inserted at both InAs-on-GaSb and GaSb-on-InAs interfaces. The material quality of the ICIP structures was similar according to characterization by differential interference contrast microscopy, atomic force microscopy, and x-ray diffraction. The performances of the ICIP devices were not substantially different despite the different interface structure. Both ICIPs had a peak detectivity of >3.7 × 1010 Jones at 78 K with a cutoff wavelength near 9.2 μm. The maximum operation temperatures of both ICIPs were as high as ∼250 K, although the structures were not fully optimized. This suggests that the two interface arrangements may have a similar effect on structural, optical and electrical properties. Alternatively, the device performance of the ICIPs may be limited by mechanisms unrelated to the interfaces. In either case, the arrangement of dividing a thick continuous InSb layer at the GaSb-on-InAs interface into thinner InSb layers at both interfaces can be used to achieve strain balance in SL detectors for longer wavelengths. This suggests that with further improvements ICIPs should be able to operate at higher temperatures at even longer wavelengths.

High-temperature operation of interband cascade infrared photodetectors with cutoff wavelengths near 8 μ m

We present our recent studies on a set of three different type-II InAs/GaSb superlattice interband cascade infrared (IR) photodetectors. Electroluminescence and x-ray diffraction measurements suggest that all the grown structures had comparable material qualities. Two of these detectors were two- and three-stage structures with regular-illumination configurations and the other was a two-stage structure with a reverse-illumination configuration. The 100% cutoff wavelength for these detectors was 6.2 μ m at 78 K, extending to 8 μ m at 300 K. At T =125 K and higher temperatures, we were able to observe the benefits of the three-stage detector over the two-stage device in terms of lower dark current and higher detectivity. We conjecture that the imperfections from the device growth and fabrication had a substantial effect on the low-temperature device performance and were responsible for the unexpected behavior at these temperatures. We also found that the zero-bias photoresponse increased with temperatures up to 200 K, which was indicative of efficient collection of photogenerated carriers at high temperatures. These detectors were able to operate at temperatures up to 340 K with a cutoff wavelength longer than 8 μ m . This demonstrates the advantage of the interband cascade structures to achieve high-temperature operation for long-wave IR photodetectors.

Interband Cascade Photovoltaic Devices for Conversion of Mid-IR Radiation

Interband cascade structures are investigated for applications in the photovoltaic conversion of mid-infrared radiation. We present detailed studies of the performance characteristics and the temperature dependence of seven-stage, single-bandgap devices. These cascade structure devices were able to achieve open-circuit voltages as high as 1.68 V with a cutoff wavelength of 4.0 μm at 80 K. The total power efficiency was broken down into a product of individual efficiencies in order to determine the power loss from each of the physical mechanisms that are involved in the conversion process. We find that at low temperature, the power conversion is limited by incomplete absorption of the incident light and a fill factor below 50%. At higher temperature, the drop in open-circuit voltage limits the efficiency. We comment on how the results of this efficiency analysis will steer the direction of our future efforts toward device improvement.

Interband cascade infrared photodetectors with enhanced electron barriers and p-type superlattice absorbers

We present results on the optical and electrical performance of mid-infrared detectors based on interband-cascade structures. These devices include enhanced electron barriers, designed to suppress intraband-tunneling current between stages, and p-doped type-II InAs/GaSb superlattice absorbers. Within the sample set, we examined devices with different absorber thicknesses and doping levels. Carriers are extracted less efficiently in devices with longer absorbers, which is attributed to more band bending within the absorber due to electric charge accumulation. Also, devices with lower-doped (1 × 1017 cm−3) absorbers are found to have better optical and electrical performances than those with higher levels of doping (3 × 1017 cm−3). The overall performance of these devices was superior to previously reported results, with Johnson-noise limited detectivities, at 4.0 μm, as high as 6.0 × 1012 and 2.5 × 1011 Jones at 80 and 150 K, respectively.

Mid-infrared intersubband electroluminescence in InAs/GaSb/AlSb type-II cascade structures

Mid-infrared intersubband light-emitting diodes based on InAs/GaSb/AlSb type-II cascade structure have been investigated. The observed emission energy is in good agreement with calculation based on the multi-band k·p theory. In contrast to interband cascade structures, dominant polarization of the emitted light is perpendicular to the quantum well layers. Structure dependence of intersubband electroluminescence is also presented.