The paper compares the achievements of quantum well infrared photodetector (QWIP) technology with those of competitive technologies, with the emphasis on the material properties, device structure, and their impact on focal plane array (FPA) performance. Special attention is paid to two competitive technologies, QWIP and HgCdTe, in the long-wavelength IR (LWIR) and very-long-wavelength IR (VLWIR) spectral ranges. Because so far, the dialogue between the QWIP and HgCdTe communities is limited, the paper attempts to settle the main issues of both technologies. Such an approach, however, requires the presentation of fundamental limits to the different types of detectors, which is made at the beginning. To write the paper more clearly for readers, many details are included in the Appendix. In comparative studies both photon and thermal detectors are considered. Emphasis is placed on photon detectors. In this group one may distinguish HgCdTe photodiodes, InSb photodiodes, and doped silicon detectors. The potential performance of different materials as infrared detectors is examined utilizing the α/G ratio, where α is the absorption coefficient and G is the thermal generation rate. It is demonstrated that LWIR QWIP’s cannot compete with HgCdTe photodiodes as single devices, especially at higher operating temperatures (>70 K). This is due to the fundamental limitations associated with intersubband transitions. The advantage of HgCdTe is, however, less distinct at temperatures lower than 50 K due to problems inherent in the HgCdTe material (p-type doping, Shockley–Read recombination, trap-assisted tunneling, surface and interface instabilities). Even though QWIP is a photoconductor, several of its properties, such as high impedance, fast response time, long integration time, and low power consumption, comply well with the requirements imposed on the fabrication of large FPA’s. Due to a high material quality at low temperatures, QWIP has potential advantages over HgCdTe in the area of VLWIR FPA applications in terms of array size, uniformity, yield, and cost of the systems. The performance figures of merit of state-of-the-art QWIP and HgCdTe FPA’s are similar because the main limitations come from the readout circuits. Performance is, however, achieved with very different integration times. The choice of the best technology is therefore driven by the specific needs of a system. In the case of readout-limited detectors a low photoconductive gain increases the signal-to-noise ratio and a QWIP FPA can have a better noise equivalent difference temperature than an HgCdTe FPA with a charge well of similar size. Both HgCdTe photodiodes and QWIP’s offer multicolor capability in the MWIR and LWIR range. Powerful possibilities offered by QWIP technology are associated with VLWIR FPA applications and with multicolor detection. The intrinsic advantage of QWIP’s in this niche is due to the relative ease of growing multicolor structures with a very low defect density.

Quantum well photoconductors in infrared detector technology

The authors review the current status of minority carrier lifetime in n-type and p-type (Hg, Cd)Te. This review includes a discussion of the relevant (Hg, Cd)Te recombination mechanisms and measurement techniques. The reported experimentally determined lifetimes were related to (Hg, Cd)Te material properties of carrier concentration, Shockley-Read-Hall centres, non-uniformities and dislocation densities.

https://iopscience.iop.org/article/10.1088/0268-1242/8/6S/005/pdf

Minority carrier lifetime in mercury cadmium telluride

In this paper, we present a physics-based full three-dimensional (3-D) numerical simulation of simultaneous two-color medium-wave infrared long-wave infrared (MWIR-LWIR) and LWIR-very-long-wave infrared (VLWIR) detectors. The present approach avoids geometrical simplifications typical of one- or two-dimensional models that can introduce errors which are difficult to quantify. We include all the relevant material physics and the drift-diffusion equations are solved on a 3-D finite element grid. We simulate device structures that have been fabricated and characterized for operation in the MWIR-LWIR spectral regions and compare the numerical results with the measured values. Furthermore, we apply the same model to predict the performance of similar detector structures intended for operation in the LWIR-VLWIR spectral regions.

Numerical analysis of HgCdTe simultaneous two-color photovoltaic infrared detectors

Heterostructure infrared photovoltaic detectors

Theory and measurements of the conductivity and the (transient) photoconductivity in the microwave frequency range are presented. The theory is tested on noninvasive measurements of semiconductors with known properties, i.e., Si wafers, in a simple apparatus. Quantitative agreement between theory and experiment is found without the use of adjustable parameters. A contactless and accurate determination of the conductivity of Si wafers in a restricted conductivity range is proposed. The quantitative evaluation of photoconductivity measurements makes a detailed discussion of nonuniform photoconductivity possible. The requirements for reliable measurements of nonhomogeneous charge carrier kinetics are discussed.

Charge‐carrier kinetics in semiconductors by microwave conductivity measurements

We report the first data for a new two-color HgCdTe infrared detector for use in large dual-band infrared focal plane arrays (IRFPAs). Referred to as the independently accessed back-to-back photodiode structure, this novel dual-band HgCdTe detector provides independent electrical access to each of two spatially collocated back-to-back HgCdTe photodiodes so that true simultaneous and independent detection of medium wavelength (MW, 3–5 μm) and long wavelength (LW, 8–12 μm) infrared radiation can be accomplished. This new dual-band detector is directly compatible with standard backside-illuminated bump-interconnected hybrid HgCdTe IRFPA technology. It is capable of high fill factor, and allows high quantum efficiency and BLIP sensitivity to be realized in both the MW and LW photodiodes. We report data that demonstrate experimentally the key features of this new dual-band detector. These arrays have a unit cell size of 100 x 100 μm2, and were fabricated from a four-layer p-n-N-P HgCdTe film grown in situ by metalorganic chemical vapor deposition on a CdZnTe substrate. At 80K, the MW detector cutoff wavelength is 4.5 μm and the LW detector cutoff wavelength is 8.0 μm. Spectral crosstalk is less than 3%. Data confirm that the MW and LW photodiodes are electrically and radiometrically independent.

Independently accessed back-to-back HgCdTe photodiodes: a new dual-band infrared detector

An enhanced quantum well infrared photodetector (EQWIP) with lower dark current and improved performance relative to a conventional QWIP is described. Dark current reduction and external quantum efficiency improvements are achieved by novel structural enhancements that involve patterning the GaAs/AlGaAs multiple quantum well into a diffraction grating and reducing the number of wells. A 64×64 long wave infrared EQWIP array with 60 μm pixel pitch and peak D*∼8×1010 cm Hz1/2/W was demonstrated at 77 K. The low bias current permits hybridization to conventional readout circuits. Test results for pixel pitches down to 30 μm show that high EQWIP performance is achievable in the small pixels required for large focal plane array formats.

Enhanced quantum well infrared photodetector with novel multiple quantum well grating structure

We report new results on metalorganic chemical vapor deposition (MOCVD)in situ growth of long wavelength infrared (LWIR) P-on-n and medium wavelength infrared (MWIR) n-on-P HgCdTe heterojunction photodiodes using the interdiffused multilayer process (IMP). The n-type regions are doped with iodine using the precursor ethyl iodide (El). I-doped HgCdTe using El has mobilities higher than that obtained on undoped background annealed films and are comparable to LPE grown In-doped HgCdTe. The p-type layers are doped with arsenic from either tertiarybutylarsine (TBAs) or a new precursor,tris-dimethylaminoarsenic (DMAAs). The substrates used in this work are lattice matched CdZnTe oriented (211)B or (100)4°→«110». Junction quality was assessed by fabricating and characterizing backside-illuminated arrays of variable-area circular mesa photodiodes. This paper presents four new results. First, carrier lifetimes measured at 80K on arsenic doped single HgCdTe layers are generally longer for films doped from the new precursor DMAAs than from TBAs. Second, we present data on the first P-on-n HgCdTe photodiodes grownin situ with DMAAs which have R0A products limited by g-r current at 80K for λco = 7–12 μm, comparable to the best R0A products we have achieved with TBAs. Third, we report the first experimental data on a new HgCdTe device architecture, the n-on-P heterojunction, with a wide gap p-type layer which allows radiation incident through the substrate to be absorbed in a narrower gap n-type layer, thereby eliminating interface recombination effects. With the n-on-P architecture, MWIR photodiodes were obtained reproducibly with classical spectral response shapes, high quantum efficiencies (70-75%) and R0A products above 2 x 105 ohm-cm2 for λco = 5.0 μm at 80K. Fourth, we report 40K data for LWIR P-on-n HgCdTe heterojunction photodiodes (using TBAs), with R0A values of 2 x 104 ohm-cm2 for λco = 11.7 μm and 5 x 105 ohm-cm2 for λco - 9.4 μm. These are the highest R0A values reported to date for LWIR P-on-n heterojunctions grownin situ by MOCVD.

Metalorganic chemical vapor deposition of HgCdTe p/n junctions using arsenic and iodine doping

Highly effective donor doping during metalorganic chemical vapor deposition interdiffused multilayer growth of Hg1−xCdxTe is demonstrated using a novel precursor of iodine, ethyl iodide. It is shown that the doping level can be controlled in the range of 7×1014–2×1018 cm−3 with abrupt profiles and without any memory effect. Activation of the iodine as a singly ionized donor is shown to be near 100% at concentrations <1×1017 cm−3. Electron mobilities are ≥1×105 and ≥2×105 cm2/V s at 80 and 20 K, respectively, for concentrations of (1–3)×1015 cm−3 and x values ∼0.22. Auger limited lifetimes of ∼1 μs are observed for these layers at 80 K.

Donor doping in metalorganic chemical vapor deposition of HgCdTe using ethyl iodide

A non-contact lifetime screening technique for measuring excess carrier lifetime in HgCdTe (MCT) using transient millimetre-wave reflectance (TMR) is presented. The TMR system described is capable of measuring lifetimes ranging from or approximately=1*1016 cm-3) p-type material to >1 mu s found in low-doped (<or approximately=1*1015 cm-3) n-type material. In TMR the reflected 90 GHz millimetre-wave (mm-wave) signal is proportional to a transient change in photoconductivity induced by a short (1.06 mu m) YAG laser pulse. The details of the TMR test system including the mm-wave circuit design are described. The system uses a computer controller for 80-300 K temperature control, x-y wafer translation and data acquisition and analysis. The TMR technique is shown to be equivalent to standard photoconductive decay. Applications of the technique, including MCT epilayer characterization by both frontside and backside illumination and in-process wafer characterization, are discussed. Spatial mapping of lifetime over an MCT wafer is demonstrated.

T. R. Schimert

Papers

Independently accessed back-to-back HgCdTe photodiodes: a new dual-band infrared detector

Enhanced quantum well infrared photodetector with novel multiple quantum well grating structure

Metalorganic chemical vapor deposition of HgCdTe p/n junctions using arsenic and iodine doping

Donor doping in metalorganic chemical vapor deposition of HgCdTe using ethyl iodide

Non-contact lifetime screening technique for HgCdTe using transient millimetre-wave reflectance