Hossein Lotfi

Narrow-bandgap photovoltaic devices operating at room temperature and above with high open-circuit voltage

Narrow-bandgap ( 5 μm) infrared photons from relatively low-temperature radiation sources (<1000 K) into electricity. Detailed characteristics of these PV devices are presented and discussed.

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

Abstract. 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  μm at 78 K, extending to 8  μm 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  μm. This demonstrates the advantage of the interband cascade structures to achieve high-temperature operation for long-wave IR photodetectors.

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.

Interband cascade infrared photodetectors with InAs/GaSb superlattice absorbers

We present a study of the temperature-dependence of the performance metrics of a set of five GaSb-based MWIR interband cascade infrared photodetectors employing InAs/GaSb superlattice absorbers. The cutoff wavelengths of the detectors varied from 4.3 μm at 78 K to 5.1 μm at 300 K. In this study, the number of stages and absorber thicknesses were varied between the samples. Two of the samples were single-stage devices with long (> 1.0 μm) absorbers, while the other three were multiple-stage detectors with short (< 1.0 μm) absorbers. The detectors were designed so that the incoming signal was traveling in the same direction as the flow of the photo-excited electrons. We experimentally show that multiple-stage detectors with shorter absorbers are able to achieve higher values of RoA and are have a photoresponse that is less sensitive to temperature. This confirms their potential utility for high-temperature detector operation. For the particular samples in this study, the multiple-stage devices were able to achieve better sensitivities above 250 K than the single-stage samples. It is notable that for most of the samples, a fit of the temperaturedependence of the dark current yielded an activation energy slightly larger than half the zero-temperature bandgap. This suggests that there may be an electric field and depletion region in the absorber and the interband transport in this series of detectors is governed by generation-recombination current, even at high temperature. Also, preliminary results of interband cascade infrared photodetectors at longer wavelengths (> 12 μm) are reported.

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.

Quantum-engineered interband cascade photovoltaic devices

Quantum-engineered multiple stage photovoltaic (PV) devices are explored based on InAs/GaSb/AlSb interband cascade (IC) structures. These ICPV devices employ multiple discrete absorbers that are connected in series by widebandgap unipolar barriers using type-II heterostructure interfaces for facilitating carrier transport between cascade stages similar to IC lasers. The discrete architecture is beneficial for improving the collection efficiency and for spectral splitting by utilizing absorbers with different bandgaps. As such, the photo-voltages from each individual cascade stage in an ICPV device add together, creating a high overall open-circuit voltage, similar to conventional multi-junction tandem solar cells. Furthermore, photo-generated carriers can be collected with nearly 100% efficiency in each stage. This is because the carriers travel over only a single cascade stage, designed to be shorter than a typical diffusion length. The approach is of significant importance for operation at high temperatures where the diffusion length is reduced. Here, we will present our recent progress in the study of ICPV devices, which includes the demonstration of ICPV devices at room temperature and above with narrow bandgaps (e.g. 0.23 eV) and high open-circuit voltages.

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.

Long-wavelength interband cascade infrared photodetectors towards high temperature operation

High temperature operation of long wavelength interband cascade infrared photodetectors (ICIPs) has been demonstrated with a working temperature above 300 K. We conducted a comparison study of three sets of ICIP structures, which comprise single absorber barrier detectors and multi-stage ICIPs with four, six and eight discrete absorbers. The 90% cutoff wavelength of these detectors was between 7.5 and 11.5 μm from 78 to 340 K. Advantages of the multi-stage ICIPs over the one-stage devices are demonstrated in terms of lower dark current density, higher detectivity (D*) and higher operating temperatures. Multiple stage ICIPs were able to operate at temperatures up to 340 K with a monotonically increasing bias-independent responsivity up to 280 K, while the one-stage detectors operated at temperatures up to 250 K with the responsivity decreased at 200 K with bias dependence. The D* values for these ICIPs at 200 and 300 K were higher than 1.0×109 and 1.0×108 cmˑHz1/2/W at 8 μm, respectively, which is more than a factor of two higher than the corresponding values for photovoltaic HgCdTe detectors with similar cutoff wavelengths. Interestingly, negative differential conductance (NDC) was observed in these detectors at high temperatures. The underlying physics of the NDC was investigated and correlated with the number of cascade stages and electron barriers. With enhanced electron barriers in the multiple-stage ICIPs, the NDC was reduced, and the device performance, in terms of D*, was improved.

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.

Recent developments in interband cascade infrared photodetectors

We investigate high-temperature and high-frequency operation of interband cascade infrared photodetectors (ICIPs)-two critical properties. Short-wavelength ICIPs with a cutoff wavelength of 2.9 μm had Johnson-noise limited detectivity of 5.8×109 cmHz1/2/W at 300 K, comparable to the commercial Hg1-xCdxTe photodetectors of similar wavelengths. A simple but effective method to estimate the minority carrier diffusion length in short-wavelength ICIPs is introduced. Using this approach, the diffusion length was estimated to be significantly shorter than 1 μm at high temperatures, indicating the importance of a multiple-stage photodetector (e.g., ICIPs) at high temperatures. Recent investigations on the high-frequency operation of mid-wavelength ICIPs (λc=4.3 μm) are discussed. These photodetectors had 3-dB bandwidths up to 1.3 GHz with detectivities exceeding 1x109 cmHz1/2/W at room temperature. These results validate the ability of ICIPs to achieve high bandwidths with large sensitivity and demonstrate the great potential for applications such as: heterodyne detection, and free-space optical communication.