MDL News & Trends
Infrared Detector & Focal Plane Array Research at JPL
Quantum Well Infrared Photodetectors (QWIPs)
The idea of using a quantum well to detect light can be understood by the basic principles of quantum mechanics. An electron in a square quantum well is the classic particle-in-a-box of basic quantum mechanics. The electron cannot have just any energy inside the well; rather, it is constrained to reside in certain discrete energy levels, i.e., its energy is quantized. These allowed energy levels, which depend on the electron mass and the size and shape of the quantum well, can be calculated by solving the time-independent Schrödinger wave equation. For a square quantum well, the energy levels depend in a straightforward way on the well dimensions (width and potential depth). At low temperatures, an electron will reside in the lowest energy level (called the ground state) of the well. When a photon strikes the well, it can impart its energy to the ground state electron and excite it to the next allowed energy level (called the first excited state), a process called intersubband absorption. In order for this to happen, the photon energy must equal the energy separation between the ground state and the first excited state. A photon with a different energy (i.e., light of a different wavelength) is not absorbed because there is no allowed energy state available for the photon to excite the ground state electron. The simplest QWIP design uses a simple square quantum well designed to hold just two states: a ground state deep inside the well, and the first excited state near the well top. A voltage bias can now be applied to sweep out the photoelectron; under constant illumination a steady-state photocurrent (a measure of the incident photon flux) thus flows through the detector. The spectral response of the QWIP is therefore quite narrow, its sharpness determined by the sharpness of the two energy states involved. Making a QWIP to detect light of a different wavelength is then simply accomplished by changing the width and potential depth of the well in such a way that the two energy states are separated by the corresponding photon energy.
JPL’s Infrared Photonics Technology group has done pioneering work on QWIP focal plane array technology and it is well documented on over 100 journal articles and seventeen patents. All initial single color QWIP camera work is already commercialized via Caltech. QWIP Technologies and OmniCorder Technologies are based on JPL QWIP technology. In 2001, the Food and Drug Administration approved OmniCorder Technology’s BioScan System based on QWIP technology for commercial applications. Over the last several years, JPL’s Infrared Photonics Technology group has been actively engaged in the development of multi-color QWIP focal plane arrays. Recently, they have demonstrated the first megapixel dual-band QWIP focal plane array.
Benefits of QWIP focal plane arrays for thermal imagery
- 99.99% pixel operability
- 99.99% array uniformity
- Large format arrays
- Low 1/f noise
- High thermal sensitivity < 10 mK
- No frequent array calibrations
- Low cost
Quantum Dot Infrared Photodetectors (QDIPs)
JPL’s Infrared Photonics Technology group has exploited the artificial atomlike properties of epitaxially self-assembled quantum dots for the development of high operating temperature long wavelength infrared (LWIR) focal plane arrays. Quantum dots are nanometer-scale islands that form spontaneously on a semiconductor substrate due to lattice mismatch. QDIPs are expected to outperform quantum well infrared detectors (QWIPs) and are expected to offer significant advantages over II-VI material based focal plane arrays. QDIPs are fabricated using robust wide bandgap III-V materials which are well suited to the production of highly uniform LWIR arrays. We have used molecular beam epitaxy (MBE) technology to grow multi-layer LWIR quantum dot structures based on the InAs/InGaAs/GaAs material system. JPL is building on its significant QWIP experience and is basically building a Dot-in-the-Well (DWELL) device design by embedding InAs/InGaAs quantum dots in a QWIP structure. This hybrid quantum dot/quantum well device offers additional control in wavelength tuning via control of dot-size and/or quantum well sizes. In addition the quantum wells can trap electrons and aide in ground state refilling. Recent measurements have shown a 10 times higher photoconductive gain than the typical QWIP device, which indirectly confirms the lower relaxation rate of excited electrons (photon bottleneck) in QDIPs. Subsequent material and device improvements have demonstrated much higher quantum efficiency than previously reported in the literature. Dot-in-the-well (DWELL) QDIPs were also experimentally shown to absorb both 45° and normally incident light. Thus we have employed a reflection grating structure to further enhance the quantum efficiency. JPL has demonstrated wavelength control by progressively growing material and fabricating devices structures that have continuously increased in LWIR response. We have fabricated the first long-wavelength 640x512 pixels QDIP focal plane array. This QDIP focal plane array has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60K operating temperature.
Furthermore, we have recently demonstrated the first megapixel QDIP focal plane array based on sub-monolayer (SML) quantum dots. The use of SML QDs instead of regular Stranski–Krastanow QDs has the advantage that whereas typically 2–3 ML of InAs is needed for a single layer of regular QD formation, only 1/3 to 1/2 monolayer is needed for SML QD. The reduction in the amount of lattice mismatched material InAs used per layer of QD formation means that the material is less strained, and therefore more stacks of QD layers can be formed. In principle, for SML QDs, all of the strained material could be used for 3D structure formation, while for regular QDs a significant fraction is used in forming the two dimensional wetting layer. SML QDs can be realized in a variety of insert/host matrix semiconductors. The lateral dimensions of SML QDs can be quite small 5–10 nm, and the dot areal density can be quite high. Multiple SML QD layers can be stacked with vertical alignment by controlling the interlayer spacer thickness, yielding considerable device design flexibility.
Sb-based Superlattice Infrared Detectors
The closely lattice-matched material system of InAs, GaSb, and AlSb, commonly referred to as the 6.1Å material system, has emerged as a fertile ground for the development of new infrared detectors. The flexibility of the system in simultaneously permitting type-I, type-II staggered, and type-II broken-gap band alignments has been the basis for many novel, high-performance heterostructure devices in recent years, including the GaInSb/InAs type-II strained layer superlattice infrared detectors. The type-II superlattice design promises optical properties comparable to HgCdTe, better uniformity, reduced tunneling currents, suppressed Auger recombination, and normal incidence operation. The antimonide material system also allows for the design of high performance barrier structures, often referred to as nBn or xBn structures. JPL’s Infrared Photonics Technology group has already demonstrated nBn and superlattice detector arrays. We have demonstrated both the MWIR and LWIR superlattice imaging arrays. Recently David Ting et al. at JPL have demonstrated RoA values over 14,000 Ohm cm2 for a 9.9µm cutoff device by incorporating electron-blocking and hole-blocking unipolar barriers . Furthermore, this device has shown 300K BLIP operation with f/2 optics at 87 K with blackbody D* of 1.1x1011 cm Hz1/2/W.
- David Ting, Cory Hill, Alex Soibel, Sam Keo, Jason Mumolo, and Sarath Gunapala, “A high-performance long wavelength superlattice complementary barrier infrared detector” Appl. Phys. Lett., Vol. 95, pp. 023508 (2009).