The InAs/InAsSb superlattices are attractive materials for the replacement of both RoHS restricted bulk HgCdTe and strongly Shockley-Read (SR) generation limited InAs/GaSb superlattices. Two main factors limit the performance of InAs/InAsSb photodiodes: the rate of the SR processes, especially in the depletion region, which is the source of the large dark current and a short vertical diffusion length of charge carriers in superlattice absorbers which results in poor responsivity. In this paper, we report on the status of HOT LWIR detectors based on InAs/InAsSb superlattices at VIGO System S.A. The uncooled and Peltier cooled LWIR photoconductors are the most successful devices developed so far. The practical InAs/InAsSb SL-based photoconductors have been fabricated by MBE heteroepitaxial growth on buffered 3” wafers. The design of the devices, material composition and doping, has been optimized for operation at temperatures from 200 to 300 K at a spectral range up to 18 μm. Some of the detectors were supplied with immersion microlenses formed in the GaAs substrates. The devices were characterized by measurements of the spectral responsivity and frequency-dependent noise density. The measured spectral detectivities of the best SL devices were found to be close or better compared to the HgCdTe counterparts operating at the same conditions. The devices are now offered as commercial products. Vigo present efforts are focused on the development of HOT LWIR photodiodes including monolithic cascade devices and thin absorber devices with the plasmonic enhancement of absorption. The development roadmap of advanced HOT devices is also sketched.
The fabrication and characterization of InAs/GaSb type-II superlattice long-wavelength infrared (LWIR) photodetectors for high operating temperature (HOT) are assessed regarding possible device yield. We investigate laterally-operated photoconductors with a detector cutoff wavelength in the LWIR at an operating temperature accessible with 3-stage thermoelectric cooling, realized by suitably tailoring the layer composition. Type-II superlattices with a layer composition of 14 monolayers InAs and 7 monolayers GaSb are grown on semi-insulating 3-inch GaAs substrates. We report on the growth of three different buffer layer variants that serve as growth templates for GaSb-based layers on GaAs substrates. The characterization of 75 nominally equal single element detectors per sample evidences the reliability of device processing. The electro-optical evaluation of a randomly chosen subset indicates a high uniformity of responsivity and noise of LWIR InAs/GaSb HOT photoconductors. At 210 K, the devices operate at a cutoff wavelength of 10.5 μm and achieve a mean peak spectral detectivity of 3.3 × 108 Jones.
We investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cutoff wavelengths near 5 μm at 230 K. The devices have been fabricated with different types of absorbing layers: nominally undoped absorber (with n-type conductivity), and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer type. Generally, the p-type absorber provides higher values of current responsivity than the n-type absorber, but at the same time also higher values of dark current. The device with the nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D * of nonimmersed devices vary from 2 × 109 to 5 × 109 cm Hz1/2 W ? 1 at 230 K, which is easily achievable with a two-stage thermoelectric cooler. Optical immersion increases the detectivity up to 5 × 1010 cm Hz1/2 W ? 1.
Measurements of low-frequency noise of type-II superlattice detectors designed for mid-IR wavelengths are used to determine noise limitations, calculate the real detectivity, and study 1/f noise-current correlations in these devices. No 1/f noise connected to the diffusion current is found as opposed to the generation-recombination, shunt, and tunneling currents. The contribution from the shunt current to 1/f noise can be so large that shunt-originated noise dominates in the high-temperature region, in which current is limited by the generation-recombination and diffusion components. It is also demonstrated that devices made of type-II superlattice contain traps generating random processes with thermally activated kinetics, and the activation energies of these traps are determined.
The essential steps in simulations of modern separate absorption, grading, charge, and multiplication avalanche photodiode and their results are discussed. All simulations were performed using two commercial technology computer-aided design type software packages, namely Silvaco ATLAS and Crosslight APSYS. Comparison between those two frameworks was made and differences between them were pointed out. Several examples of the influence of changes made in individual layers on overall device characteristics have been shown. Proper selection of models and their parameters as well as its significance on results has been illustrated. Additionally, default values of material parameters were revised and adequate values from the literature were entered. Simulated characteristics of optimized structure were compared with ones obtained from measurements of real devices (e.g., current-voltage curves). Finally, properties of crucial layers in the structure were discussed.
For high-bit rate and long-haul receivers in optical telecommunication systems the avalanche photodiodes are preferred since they offer an improvement of the receiver sensitivity by several decibels. Recently critical sensing and imaging applications stimulated development of modified avalanche photodiodes structures operating in 1.55 μm spectral range. For these devices speed is not further critical. Instead, very low current densities and low multiplication noises are the main requirements. The most advanced structure of avalanche photodiodes is known as Separate Absorption, Grading, Charge and Multiplication (SAGCM). In the present work the performance of uncooled InGaAs/InAlAs/InP avalanche photodiodes operating near 1.55 μm has been studied theoretically. Device modeling based on advanced drift - diffusion model with commercial Crosslight APSYS software has been performed. Conventional SAGCM avalanche photodiodes as well as devices with a relatively thick undepleted p-type InGaAs absorption region and thin InAlAs multiplication layer have been considered. This type of avalanche photodiodes enables to increase device quantum efficiency, reduce dark current and eliminate impact ionization processes within absorbing layer. Extensive calculations allowed for detailed analysis of individual regions of the device and determination of their influence on diode characteristics.