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Optoelectronics & Communications

Toward high-performance infrared imaging

Better design of barriers in antimonide-based superlattice photodetectors extends their wavelength and temperature range.
7 April 2011, SPIE Newsroom. DOI: 10.1117/2.1201103.003423

Focal plane arrays (FPAs) operating in the 3–5μm and 8–12μm atmospheric transmission windows have many applications, such as imaging spectroscopy of land and coastal surfaces. Currently, the commercial market for FPAs operating in the 3–5μm mid-wavelength infrared (MWIR) spectral region is dominated by indium antimonide (InSb), which enjoys cost and large-format advantages over mercury cadmium telluride (MCT), but operates only below 80K. In the 8–12μm long-wavelength infrared (LWIR) spectral range, MCT grown on cadmium zinc telluride (CZT) substrates can produce excellent imaging FPAs, but high-quality, large-area CZT substrates are expensive and in short supply. Alternative material systems are under development, including lead salts, indium arsenide antimonide (InAsSb), and a type-II indium arsenide/gallium antimonide (InAs/GaSb) superlattice (SL), but so far none has demonstrated the desired performance.

In contrast, the nearly-lattice-matched antimonide material system offers tremendous flexibility in realizing high-performance infrared detectors. Antimonide-based SL detectors, where the absorber is a periodic structure of layers of two or more materials, can be tailor-made to have cutoff wavelengths in the range 1–11.5μm. SL detectors have been predicted to have suppressed Auger (i.e., electron-hole) recombination rates and low interband tunneling, reducing dark currents (that is, the current that flows through the photodetector when not exposed to light and a source of noise in the detectors that limits their performance).1 Moreover, this material system, consisting of InAs, GaSb, aluminum antimonide (AlSb), and their alloys, allows for the construction of SL heterostructures. In particular, it is possible to implement unipolar barriers in their design. These block one carrier type without impeding the flow of the other, improving performance in LWIR.2

In MWIR, barrier detectors, often known as nBn (n-barrier-n) or XBn photodetectors,3–5 are an attractive alternative class of device. They are robust, easy to manufacture, and could perform better than InSb and MCT detectors. However, research is still required to optimize their design and understand important factors affecting their performance. We worked to improve the performance of MWIR and LWIR barrier photodetectors and understand their properties.

We incorporated nanostructures into the absorber of nBn barrier photodetectors, to extend their absorption wavelength range in MWIR. These detectors use a wide bandgap barrier layer sandwiched between the top contact and the absorber layers. This barrier blocks the flow of majority carriers (electrons), but not minority ones. This architecture suppresses the Shockley-Read-Hall (SRH) recombination process, so it reduces dark current. However, these detectors are limited by the availability of barrier and absorber materials that have the appropriate band alignment and are lattice-matched to a suitable substrate. Existing high-performance barrier photodetectors with an InAsSb/aluminum arsenide antimonide (AlAsSb) absorber-barrier combination have cutoff wavelengths of up to about 4μm. By incorporating nanostructures, such as quantum dots (QDs), into the detector absorber, we modified the band structure of the device and enabled light absorption at longer wavelengths. Our new nBn self-assembled QDs integrated into an InAs0.91Sb0.09 absorber exhibited a cutoff wavelength exceeding 6μm and showed responsivity above T = 225K (see Figure 1).

Figure 1. Spectral responsivity of barrier photodetectors without quantum dots (QD) (A) and with QD (B) at temperature T = 225K at applied bias Vb = -0.1V. The photoluminescence (PL) intensity for sample B (dashed blue line) shows a correspondence between the two photoluminescence peaks and the short/long wavelength sections of the photoresponsivity curve.

In LWIR, we developed an antimonide SL detector based on a complementary barrier infrared detector (CBIRD) design, consisting of a 600-period InAs/GaSb absorber SL sandwiched between an InAs/AlSb hole-barrier SL and InAs/GaSb electron-barrier SL. We grew the device structure on a GaSb (100) substrate by molecular beam epitaxy. We used standard contact-mode optical lithography to fabricate large-area (220×220μm2) devices for dark current and responsivity measurements. Under 0.2V applied bias at 77K, the spectral response we obtained shows that the device (without anti-reflection coating) has a 9.9μm cutoff (defined by 50% peak responsivity), with a peak responsivity of 1.5A/W (see Figure 2), and a dark current density of 0.99×10−5A/cm2. The detector reaches 300K background-limited infrared photodetection (BLIP) operation at 87K, with a black-body BLIP D* value of 1.1×1011cm-Hz1/2/W for f/2 optics under 0.2V bias.

Figure 2. Measured spectral responsivity of our complementary barrier infrared detector (CBIRD). λ: wavelength. Vb: applied bias voltage. T: temperature.

We also studied the noise and gain characteristics of high-performance SL photodetectors. We have recently shown6that SL heterodiode can have an electrical gain much larger than unity, but it was not known whether the observed gain was due to the device structure or an intrinsic property of the SL. We compared the performance of two SL heterodiodes with identical absorber regions but with different barriers. We found that the gain arises from a difference between electron and hole transports caused by the heterostructure, rather than from the intrinsic properties of SL absorber.

In addition, we made direct measurements of the noise spectra of high-performance SL heterodiodes at different operational conditions. These revealed the absence of intrinsic 1/f noise in these structures, but showed that an additional frequency-dependent noise can be generated by side-wall leakage current. We analyzed gain and noise measurements and found that exact dependence of the shot noise on the dark current in these SL heterodiodes can be different from that in the diffusion-limited diode homojunction.7 These results advance our understanding of SL photodiode operation and are essential for improving the design and fabrication of state-of-the-art SL heterostructures.

The antimonide material system and barrier infrared photodetector architecture have great potential for large-format, high-performance focal plane arrays. We extended the cutoff wavelength of MWIR photodetectors from 4.2μm to 6μm at temperatures up to 225K by incorporating self-assembled InSb QDs into the active area of the detector.8 In LWIR, we developed a 10μm cutoff SL device based on a CBIRD design that exhibits reduced dark current characteristics.9 In addition, we studied the noise spectra of these SL heterostructures at different operating conditions.10 We are currently working to further improve the detector device design, optimize material growth and fabrication processes, and advance technology of SL focal plane arrays.

The authors thank S. Bandara, E. R. Blazejewski, D. R. Rhiger, R. E. DeWames and J. N. Schulman for helpful discussions, M. McKelvey for technical assistance, and M. Tidrow, R. Liang, M. Herman, E. Kolawa, and P. Dimotakis for encouragement and support. The research described in this publication was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged.

Alexander Soibel, David Z.-Y. Ting, Cory J. Hill, Jean Nguyen, Sam Keo, Michael L. Lee, Jason M. Mumolo, Anna Liao, Linda Hoglund, Arezou Khoshakhlagh, Sarath D. Gunapala
Jet Propulsion Laboratory (JPL)
California Institute of Technology (Caltech)
Pasadena, CA 

Alexander Soibel received MSc and PhD degrees in physics from the Weizmann Institute of Science, Israel. He joined Bell Laboratories in 2001 as a postdoc and in 2004 he became a senior member of engineering staff at JPL, National Aeronautics and Space Administration/Caltech where he works on development of mid-IR lasers and detectors.

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