Waldemar Gawron

New generation of near-room-temperature photodetectors

The major drawback of IR photodetectors is the need for cooling to suppress thermal generation of free carriers resulting in noise. New ways to improve the performance of infrared photodetectors operated without cryogenical cooling are discussed, including the optimum design of the devices, the use of optical immersion of photodetectors to high refraction index lenses, and the optical resonant cavity. Another and very promising way, however, is the suppression of thermal generation, which is governed by the Auger mechanism by depletion of the semiconductor in charge carriers. The stationary depletion can be achieved by the use of exclusion, extraction, and magnetoconcentration effects. The combination of various methods would eventually enable us to achieve near-background-limited photodetection (near-BLIP) performance of IR detectors without cooling.

Uncooled photovoltaic Hg1-xCdxTe LWIR detectors

We report an advanced Hg1-xCdxTe photovoltaic detector based on monolithic Hg1-xCdxTe heterostructure with 3-dimensional architecture. It consists of a narrow gap, p-type Hg1-xCdxTe small size (approximately equals 10x10x7 micrometers ) absorber of infrared radiation buried in a graded gap Hg1-xCdxTe layer surrounding absorber and heterojunction contacts obtained by selective doping of the graded gap Hg1-xCdxTe layer surrounding the absorber region. The heterostructure is passivated with a ZnS layer and coated with contact metallization to nPLU and p-type regions. The device is supplied with 50x50 micrometers immersion microlens formed directly in the CdZnTe substrate. These two layers also play a role of a mirror that improves quantum efficiency for weakly absorbed infrared radiation. In addition, the mirror eliminates backside incidence of thermal radiation, which prevents generation of dark current. The design of the device is optimized to achieve the best compromise between requirements of good absorption and collection efficiency; low thermal generation; and low parasitic impedance. Test devices have been prepared using the modified isothermal vapor phase epitaxy of Hg1-xCdxTe on profiled CdZnTe substrates, negative epitaxy of Hg1-xCdxTe to widen band gap of surface regions, selective doping, multiple chemical etching and ion milling, vacuum deposition of dielectric and metal layers.

MOCVD HgCdTe heterostructures for uncooled infrared photodetectors

Recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg1-xCdxTe (HgCdTe) multilayer heterostructures on GaAs/CdTe substrates for uncooled infrared photodetectors is presented. The optimum conditions for the growth of single layers and complex multilayer heterostructures have been established. One of the crucial stages of HgCdTe epitaxy is CdTe nucleation on GaAs substrate. Successful composite substrates have been obtained with suitable substrate preparation, liner and susceptor treatment, proper control of background fluxes and appropriate nucleation conditions. The other critical stage is the interdiffused multilayer process (IMP). The growth of device-quality HgCdTe heterostructures requires complete homogenization of CdTe-HgTe pairs preserving at the same time suitable sharpness of composition and doping profiles. This requires for IMP pairs to be very thin and grown in a short time. Arsenic and iodine have been used for acceptor and donor doping. Suitable growth conditions and post growth anneal is essential for stable and reproducible doping. In situ anneal seems to be sufficient for iodine doping at any required level. In contrast, efficient As doping with near 100% activation requires ex situ anneal at near saturated mercury vapors. As a result we are able to grow multilayer fully doped (100) and (111) heterostructures for various infrared devices including photoconductors, photoelectromagnetic and photovoltaic detectors. The present generation of uncooled long wavelength infrared devices is based on multijunction photovoltaic devices. Near-BLIP performance is possible at ≈ 230 K with optical immersion. These devices are especially promising as 7.8-9.5-μm detectors, indicating the potential for achieving detectivities above 109 cmHz1/2/W.

Uncooled or minimally cooled 10μm photodetectors with subnanosecond response time

We report fast and sensitive long (10 μm) wavelength photodetectors operating at near room temperature. The devices are based on HgCdTe multilayer heterostructures grown by MOCVD on (211) and (111) GaAs substrates. Device-quality heterostructures are obtained without any post growth anneal. The recent improvements of MOCVD growth were: optimized design of the device architecture to increase speed of response, better IMP growth parameters selection taking into account interdiffusion time changes during growth, stoichiometry control during growth by the layer anneal at metal rich vapors during each IMP cycle, precursor delivery to the growth zone monitored with IR gas analyzer, additional metal-rich vapor anneal at the end of growth and passivation of detector structures with wide gap HgCdTe overgrowth deposition. Monolithic optical immersion of the detectors to GaAs microlenses has been applied in purpose to improve performance and reduce RC time constant. The response time of the devices have been characterized using 10μm quantum cascade laser, fast oscilloscope with suitable transimpedance amplifier as a function of detector design, temperature and bias. Detectivity of the best thermoelectrically cooled optically immersed photodiodes approaches 1⋅1010 cmHz1/2/W at ≈10 μm wavelength. The response time of small area decreases with reverse bias to response achieving <100 ps with weak reverse bias.

Mid and long infrared detection modules for picosecond range measurements

Sensitive and broadband detection of MWIR and LWIR radiation with any wavelength within the 2 to 16 μm spectral range and bandwidth from DC to GHz range is reported. Recent efforts have been concentrated on the extension of useful spectrum range above 13 micrometers. This was achieved with improved architecture of the active element, use of monolithic optical immersion technology, enhanced absorption of radiation, dedicated electronics, series connection of small cells and applying more efficient Peltier coolers.

MOCVD Grown HgCdTe Barrier Structures for HOT Conditions (July 2014)

In this paper, we report on first midwavelength infrared HgCdTe barrier detectors with a zero valence band offset grown by metal organic chemical vapor deposition on GaAs substrates. Investigated structures have the same capbarrier structural unit, p⁺-B_p, and N⁺ bottom contact layer, but a different nand p-type absorption layers. Initial experiments indicate the influence of the barrier on electrical and optical performances of the devices. Both type of photodetectors exhibit dark current densities in the range (2÷3×10^-4 A/cm² at 230 K and maximum responsivities of ~2 A/W.

Different cap-barrier design for MOCVD grown HOT HgCdTe barrier detectors

Abstract The performance of HgCdTe barrier detectors with cut-off wavelengths up to 3.6 μm fabricated using metaloorganic chemi- cal vapour deposition operated at high temperatures is presented. The detectors’ architecture consists of four layers: cap contact, wide bandgap barrier, absorber and bottom contact layer. The structures were fabricated both with n- and p-type absorbing layers. In the paper, different design of cap-barrier structural unit (n-Bp′, n+-Bp′, p+-Bp) were analysed in terms of various electrical and optical properties of the detectors, such as dark current, current responsivity time constant and detectivity. The devices with a p-type cap contact exhibit very low dark current densities in the range of (2÷3)×10-4 A/cm2 at 230 K and the maximum photoresponse of about 2 A/W in wide range of reverse bias voltage. The time constant of measured de- vices with n-type cap contact and p-type absorbing drops below 1 ns with reverse bias while the detectivity is at the level of 1010 cm・Hz1/2/W.

Modeling of midwavelength infrared InAs/GaSb type II superlattice detectors

Abstract. The performance of midwavelength infrared type II superllatice InAs/GaSb PIN and nBn (AlGaSb barrier) photodetectors in a reverse bias voltage range and a temperature range from 77 to 240 K is described. The PIN and nBn structures are modeled by a bulk based model, i.e., type II superllatice is treated as an artificial semiconductor material where parameters describing its physical properties are extracted from the experimental data. The model assumes that position of the effective trap energy level depends on temperature, what allowed to obtain a very good fitting to the measurements. Temperature and bias dependent dark current and differential resistance area product of the both devices have been analyzed to investigate contributing mechanisms such as: diffusion, generation-recombination, band-to-band and trap-assisted tunneling that limit the electrical performance of both types of the detectors. The I−V and RA(V) product characteristics of both types of type II superllatice InAs/GaSb photodetectors were found to be dominated by diffusion and generation-recombination currents in the nearly zero-bias region. At medium values of reverse bias, the trap-assisted tunneling reveals its significance, while at higher reverse voltages—the band-to-band tunneling is decisive to I−V and RA(V) characteristics. The fitting procedure allowed to extract both generation-recombination and diffusion components of carrier lifetimes pointing out that the carrier lifetimes range from 2 to 10 ns at T=200  K. Detectivity for T=240  K and V=50  mV was estimated to be 109  cmHz1/2/W and 4×109  cmHz1/2/W for PIN and nBn detector, respectively. Finally, type II superlattice InAs/GaSb PIN and nBn structures’ performance is compared to both unipolar barrier nBn HgCdTe detector and bulk HgCdTe photodiodes operated at near-room temperature. It is shown that the performance of SL and HgCdTe photo detectors with a cut-off of about 5 μm is comparable at operation temperatures around 240 K.

Status of HgCdTe Barrier Infrared Detectors Grown by MOCVD in Military University of Technology

In this paper we present the status of HgCdTe barrier detectors with an emphasis on technological progress in metalorganic chemical vapor deposition (MOCVD) growth achieved recently at the Institute of Applied Physics, Military University of Technology. It is shown that MOCVD technology is an excellent tool for HgCdTe barrier architecture growth with a wide range of composition, donor/acceptor doping, and without post-grown annealing. The device concept of a specific barrier bandgap architecture integrated with Auger-suppression is as a good solution for high-operating temperature infrared detectors. Analyzed devices show a high performance comparable with the state-of-the-art of HgCdTe photodiodes. Dark current densities are close to the values given by “Rule 07” and detectivities of non-immersed detectors are close to the value marked for HgCdTe photodiodes. Experimental data of long-wavelength infrared detector structures were confirmed by numerical simulations obtained by a commercially available software APSYS platform. A detailed analysis applied to explain dark current plots was made, taking into account Shockley–Read–Hall, Auger, and tunneling currents.

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The role of generation-recombination (g-r) and diffusion currents in the generation of 1/f noise was investigated in mid-wavelength infrared detectors with InAs/GaSb superlattice (SL) absorber. Modeling of the dark current reveals the region where g-r and/or diffusion currents dominate over the leakage current (shuntand/or trap-assisted tunneling). Measurements of 1/f noise at constant reverse bias versus temperature show that noise intensity follows squared leakage current. There is no contribution to 1/f noise from g-r or diffusion currents or it is too small to be observed. This property should be attributed to InAs/GaSb SL material rather than to device specific features, since the batch of examined devices contained specimens with various architecture, passivation method, and substrate. Results for SL-based devices were compared with the state-of-the-art HgCdTe detectors. In these detectors, dedicated for high operating temperature, correlation between g-r/diffusion currents and 1/f noise is significant.