Małgorzata Kopytko

InAs/GaSb type-II superlattice infrared detectors: three decades of development

Recently, there has been considerable progress towards III-V antimonide-based low dimensional solids development and device design innovations. From a physics point of view, the type-II InAs/GaSb superlattice is an extremely attractive proposition. Their development results from two primary motivations: the perceived challenges of reproducibly fabricating high-operability HgCdTe FPAs at reasonable cost and theoretical predictions of lower Auger recombination for type-II superlattice (T2SL) detectors compared to HgCdTe. Lower Auger recombination should be translated into a fundamental advantage for T2SL over HgCdTe in terms of lower dark current and/or higher operating temperature, provided other parameters such as Shockley-Read-Hall lifetime are equal. Based on these promising results it is obvious now that the InAs/GaSb superlattice technology is competing with HgCdTe third generation detector technology with the potential advantage of standard III-V technology to be more competitive in costs and as a consequence series production pricing. Comments to the statement whether the superlattice IR photodetectors can outperform the “bulk” narrow gap HgCdTe detectors is one of the most important questions for the future of IR photodetectors presented by Rogalski at the April 2006 SPIE meeting in Orlando, Florida, are more credible today and are presented in this paper. It concerns the trade-offs between two most competing IR material technologies: InAs/GaSb type-II superlattices and HgCdTe ternary alloy system.

Response time improvement of LWIR HOT MCT detectors

Theoretical and experimental investigations on the response time improvement of biased and unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T = 230K were presented in this paper. MOCVD technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping and without post grown ex-situ annealing. Donor doping efficiency in (111) and (100) oriented HgCdTe layers has been discussed. The time constant is lower in biased detectors due to Auger suppression phenomena and reduction of diffusion capacitance related to wider depletion region. The relatively high bias currents requirements and excessive low frequency noise which reduces the detectivity of biased detectors inspire researches on the time constant improvement of unbiased detectors. The response time of high-operating temperature (HOT) LWIR HgCdTe detectors revealed complex behavior being dependent on the applied the reverse bias, the operating temperature, the absorber thickness and doping, the series resistance and the electrical area of the devices.

Uncooled middle wavelength infrared photoconductors based on (111) and (100) oriented HgCdTe

Abstract. We present progress in metal organic chemical vapor deposition (MOCVD) growth of (100) HgCdTe epilayers achieved recently at the Institute of Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an excellent tool for the fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping, and without post grown ex-situ annealing. Surface morphology, residual background concentration, and acceptor doping efficiency are compared in (111) and (100) oriented HgCdTe epilayers. At elevated temperatures, the carrier lifetime in measured p-type photoresistors is determined by Auger 7 process with about one order of magnitude difference between theoretical and experimental values. Particular progress has been achieved in the growth of (100) HgCdTe epilayers for medium wavelength infrared photoconductors operated in high-operating temperature conditions.

Electrical and optical performance of mid-wavelength infrared InAsSb heterostructure detectors

In this work we investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cut-off wavelengths near 5 μm at 230 K. The devices have been fabricated with different type of the absorbing layer: nominally undoped absorber, and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer doping. Generally, p-type absorber provides higher values of current responsivity than n-type absorber, but at the same time also higher values of dark current. The device with nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D° of non-immersed devices varies from 2×109 to 7×109 cmHz1/2/W at 230 K, which is easily achievable with a two stage thermoelectric cooler.

Optical microlenses for MOEMS

A new concept of the fabrication process for glass microlenses (external diameter ED<1 mm, focal length a few millimeters), based on the silicon master mask-less anisotropic wet etching in KOH, vacuum anodic bonding and re-flow of borosilicate glass, followed by the precise wafer-scale polishing and DRIE has been presented. A single spherical microlens as well as an array of spherical microlenses with focal length between 44.8 and 8.6 mm and external diameter 0.35 to 0.985 mm have been repeatable manufactured.

Influence of radiative recombination on performance of p-i-n HOT long wavelength infrared HgCdTe photodiodes

An enhanced computer program has been applied to explain in detail the influence of different recombination mechanisms (Auger, radiative and Shockley-Read-Hall) on the performance of high operation temperature long wavelength infrared p-i-n HgCdTe heterojunction photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. We distinguish photons in different energy ranges with unequal band gaps. As a result, both the distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. It is shown that photon recycling effect limits the influence of radiative recombination on the performance of small pixel HgCdTe photodiodes. In comparison with two previously published papers in Journal of Electronics Materials (Lee et al., DOI: 10.1007/s11664-016-4566-6 and Schuster et al., DOI: 10.1007/s11664-017-5736-x) our paper indicates an additional insight on ultimate performance of LWIR HOT HgCdTe arrays with pixel densities that are fully consistent with background- and diffraction-limited performance due to system optics.

Theoretical investigation of properties of InAsSb mid-wave infrared detectors

In this work we present the theoretical investigation of the electrical and optical properties of high operating temperature (HOT) mid-wavelength infrared detectors (5 μm at 230 K) based on InAsSb/AlSb heterostructures [1]. In this work the performance comparison of barrier detectors with different doping concentration of n-type absorbing layer is presented. The barrier structure was simulated by commercially available software APSYS. We report on the dependence of the calculated current responsivity on the active layer thickness for a different doping concentration and doping concentration for optimal absorber thickness. Moreover, we show the influence of the bottom contact material on device’s performance.

Experimental determination of leakage current occurring in HgCdTe infrared detectors operating in the mid-infrared

Most of the HgCdTe infrared detectors are fabricated by mesa geometry using a wet chemical or plasma etching techniques. The mesa definition etch process induces undesirable changes in HgCdTe surface properties. In narrow bandgap materials these surface changes could deteriorate a device performance. Uncontrolled band bending occurred on the slopes of the active layer increase of the recombination velocity causing surface leakage current which is a serious problem that affects infrared detectors. Adequate passivation is essential to minimize the effects from the surface states by saturating them. The HgCdTe barrier detectors were investigated for unpassivated and passivated devices. For the unpassivated structure the experimental value of Jbulk (at -0.2 V bias and a temperature of 200 K) was found at the level of 52% of the total dark current for devices with large diameters (500 μm). In the case of detectors with small diameters, the dark current is dominated by the surface leakage current. For a detector with a diameter of 200 μm, the bulk current consists only 28% of the total dark current. After passivation the level of bulk current increase to 58% in cause of 200 μm diameter and almost 75% in cause of large diameter.

Progress in MOCVD growth of HgCdTe epilayers for HOT infrared detectors

In this paper we present progress in MOCVD growth of (100) HgCdTe epilayers achieved recently at the Institute of Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping and without post grown annealing. Particular progress has been achieved in the growth of (100) HgCdTe epilayers for long wavelength infrared photoconductors operated in HOT conditions. The (100) HgCdTe photoconductor optimized for 13-μm attain detectivity equal to 6.5x109 Jones and therefore outperform its (111) counterpart. The paper also presents technological progress in fabrication of MOCVD-grown (111) HgCdTe barrier detectors. The barrier device performance is comparable with state-of-the-art of HgCdTe photodiodes. The detectivity of HgCdTe detectors is close to the value marked HgCdTe photodiodes. Dark current densities are close to the values given by “Rule 07”.

HgCdTe Mid- and Long-Wave Barrier Infrared Detectors for Higher Operating Temperature Condition

In the last decade, a new architecture design such as nBn device or unipolar barrier photodiode has been proposed to achieve high operating temperature condition. This idea has also been implemented into HgCdTe ternary material system. In this chapter, we present the status of HgCdTe barrier detectors grown by metalorganic chemical vapor deposition with emphasis on numerical simulations of their properties. The device concept of a specific barrier bandgap architecture integrated with Auger suppression is a proper solution for high operating temperature infrared detectors. The device performance is comparable with state-of-the-art HgCdTe photodiodes. Theoretical modeling of the HgCdTe barrier detectors has been performed using our original numerical program developed at the Institute of Applied Physics, Military University of Technology (MUT) and the commercially available APSYS platform (Crosslight Inc.). The detector’s performance was assessed taking into account a wide spectrum of generation-recombination mechanism: Auger, Shockley-Read-Hall, and tunneling processes.