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.
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.
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.
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.
We present recent studies on a set of three different long wave IR interband cascade infrared photodetectors with Type-II
InAs/GaSb absorbers. Two of these detectors were two- and three-stage structures with regular-illumination
configuration and the other was a two-stage structure with reverse-illumination configuration. The 100% cutoff
wavelength for these detectors was 6.2 μm at 78 K and extended 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 unexpected behavior at these temperatures.
We also found that the zero-bias photo-response increased for temperatures up to 200 K, which was indicative of
efficient collection of photo-generated carriers at relatively high temperatures. Electroluminescence and X-ray
diffraction measurements suggest that all three grown structures had comparable material qualities. However, the twostage
detectors with the reverse-illumination had significantly lower performance than the other two detectors. The
activation energy for the two-stage detectors with the reverse-illumination was 37 meV for T=78-100 K, which was
much lower than the activation energies of the other two detectors (~140 meV). This low activation energy was
attributed to shunt leakage observed in detectors with the reverse-illumination configuration.
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.
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.