HgCdTe Photoconductors Research Articles

Performance evaluation of an Ultra-Fast IR Detector for Astronomy Transients

In this work, we present the design of an ultra-fast IR (InfraRed) detector for the search of astronomical transients. Time-domain astronomy is a field in continuous growth in the last years, with interest in detectors with nanosecond performance, in order to observe potential fast transients with the time scale of GRBs (Gamma Ray Bursts) and FRBs (Fast Radio Bursts). An ultra-fast temporal detector based on HgCdTe photoconductors is under development in Italy, at the Frascati National Laboratory of the National Institute of Nuclear Physics, by tests at SINBAD, the DAFNE infrared beamline. The detector will be used for mid-IR observations together with a ground-based reflecting telescope or launched on a scientific balloon.

THz polarization-dependent response of antenna-coupled HgCdTe photoconductors under an external constant electric field

The photoresponse of antenna-coupled Hg1-x Cd x Te (x ≈ 0.201) narrow-gap epitaxially grown photoconductors with normal band ordering having large spin–orbit coupling and being irradiated by the normal to the photoconductor surface terahertz (THz) radiation (linearly or circularly polarized), is investigated at T = 80 and 300 K. The oscillations of polarized photoresponse in biased and unbised structures were revealed. In experiments, Hg1-x Cd x Te layers (point group ) grown on (013) GaAs substrates were used. In biased structures, to the ordinary charge current and Rashba spin splitting current in zero electric field, an additional polarized current, which grows with the external electric field strength, arises. Presumably, the oscillating photoresponses under polarized THz radiation in these unbiased and biased structures are connected with the circular and linear photogalvanic effects without and with an external electric field. The observed features of the polarized dependent photoresponse can be important for spintronics applications at elevated temperatures.

Trends in Performance Limits of the HOT Infrared Photodetectors

The cryogenic cooling of infrared (IR) photon detectors optimized for the mid- (MWIR, 3–5 µm) and long wavelength (LWIR, 8–14 µm) range is required to reach high performance. This is a major obstacle for more extensive use of IR technology. Focal plane arrays (FPAs) based on thermal detectors are presently used in staring thermal imagers operating at room temperature. However, their performance is modest; thermal detectors exhibit slow response, and the multispectral detection is difficult to reach. Initial efforts to develop high operating temperature (HOT) photodetectors were focused on HgCdTe photoconductors and photoelectromagnetic detectors. The technological efforts have been lately directed on advanced heterojunction photovoltaic HgCdTe detectors. This paper presents the several approaches to increase the photon-detectors room-temperature performance. Various kinds of materials are considered: HgCdTe, type-II AIIIBV superlattices, two-dimensional materials and colloidal quantum dots.

A Study of Sidewall Effects in HgCdTe Photoconductors Passivated with MBE-Grown CdTe

In order to evaluate the effectiveness of CdTe surface passivating layers, HgCdTe photoconductors with and without CdTe sidewall passivation were fabricated. As expected, photoconductors with CdTe sidewall passivation demonstrated significantly higher responsivity in comparison with those without sidewall passivation, indicating the effectiveness of molecular-beam epitaxially (MBE)-grown CdTe as a passivation layer in reducing surface recombination velocity. Characterization of the responsivity differences between photoconductors with and without sidewall CdTe passivation offers a potential method for measuring the interface/surface recombination velocity. This has been demonstrated in this paper by extracting the value of the surface recombination velocity using the Synopsys Sentaurus commercial modeling package to fit experimental responsivity data for fully and partially passivated devices.

Materials and Processes for MEMS-Based Infrared Microspectrometer Integrated on HgCdTe Detector

The materials and processes for fabrication of monolithically integrated microelectromechanical systems-based microspectrometers operating in the short-wavelength IR range is presented. Using low-temperature surface micromachining techniques, compatible with a range of IR sensor technologies, silicon-nitride-based tunable Fabry-Perot filter structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been monolithically integrated with HgCdTe photoconductors. The stress within and between the many layers of the structure has been eliminated or compensated by stress tuning of the deposition conditions. The demonstrated microspectrometers have a tuning range of 1.8-2.2 mum with relative peak transmission of 70% and full-width at half-maximum of 80plusmn10 nm.

A monolithically integrated HgCdTe short-wavelength infrared photodetector and micro-electro-mechanical systems-based optical filter

A monolithically integrated low-temperature micro-electro-mechanical systems (MEMS) and HgCdTe infrared (IR) detector technology is introduced, implemented, and characterized. The ultimate aim of this project is to develop a MEMS-based optical filter, integrated with an IR detector, that selects narrow wavelength bands within the short-wavelength IR (SWIR) region of the spectrum. The entire fabrication process is compatible with two-dimensional IR focal plane array technology, and needs to be compatible with a proposed electrically tunable MEMS filter based on a Fabry-Perot optical cavity. The fabricated device consists of an HgCdTe SWIR photoconductor, distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, and a silicon nitride membrane for structural support. Mirror stacks fabricated on silicon, identical to the structures that will form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full-width at half-maximum (FWHM) was 34 nm at the center wavelength of 1,780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1,950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8×104 V/W. The experimental results are in good agreement with the optical model, which takes into account mirror reflectivity, absorption within the cavity by the spacer material, and absorption by the silicon nitride support structure.

Photo-enhanced native oxidation process for Hg/sub 0.8/Cd/sub 0.2/Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg/sub 0.8/Cd/sub 0.2/Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R/sub /spl lambda//, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer.

Comparison of the performance of quantum well and conventional bulk infrared photodetectors

Investigations of the performance of quantum well infrared photodetectors (QWIPs) as compared o other types of semiconductor infrared detectors are presented. Two types of QWIPs are considered: GaAs/AlGaAs intersubband quantum well photoconductors and type II staggered InAs/InGaSb photodiodes. In comparative studies the HgCdTe photoconductors and photodiodes, PbSnTe photodiodes, Schottky barrier photoemissive detectors and doped silicon detectors are considered. It is assumed that the performance of HgCdTe and PbSnTe photodiodes is due to thermal generation governed by the Auger mechanism in the base regions. Investigations of the fundamental physical limitations of HgCdTe photodiodes indicate better performance of this type of detectors in comparison with GaAs/AlGaAs QWIPs operated in the range 35 to 77 K. In practice however, in temperature range below 50 K the performance of the HgCdTe photodiodes is determined by trap-assited tunneling. As a result, advantage of GaAs/AlGaAs QWIPs increases in the wider spectral range (below 14 μm) and temperature below 50 K. This observation plus the maturity of GaAs/AlGaAs technology and its radiation hard characteristics promise that QWIPs technology can produce high quality focal plane arrays (FPAs) for space applications. The operating temperature for HgCdTe detectors is higher than for other types of photondetectors. HgCdTe detectors with background limited performance operate with thermoelectric coolers in the medium wavelength range, instead the long wavelength detectors operate at ≈ 100 K. HgCdTe is characterized by high absorption coefficient and quantum efficiency and relatively low thermal generation rate compared to extrinsic detectors, silicide Schottky barriers and GaAs/AlGaAs QWIPs. However, the cooling requirments for GaAs/AlGaAs QWIPs with cutoff wavelengths below 10 μm are less stringent in comparison with extrinsic detectors and Schottky barrier devices. The theoretically predicted performance of long wavelength InAs/GaInSb photodiodes are comparable with HgCdTe photodiodes. The high performance of InAs/InGaSb detectors is due to suppression of band-to-band Auger recombination rate.

Ambient temperature HgCdTe photoconductor can achieve detectivity higher than 1×10 8 cm Hz 1/2 /W at 10.6μm

The authors have calculated the performance of ambient temperature long wavelength HgCdTe photoconductors as a function of material composition and doping. It has been shown that the figures of merit, both calculated and measured, are several times higher than the ones reported previously. It is concluded that an optimised ambient temperature HgCdTe photoconductor can achieve detectivity higher than 1*10/sup 7/ cm Hz/sup 1/2//W at lambda =10.6 mu m.

On 1/f noise characteristics for 0.1 eV HgCdTe photoconductors

Sensitivities on 0.1 eV HgCdTe photoconductors with new electrode configuration and in different sizes were measured at 77K and under 1014phcm−2s−1 photon background conditions. After data for responsivity, generation-recombination noise (g-r noise) and minority carrier lifetime were reproduced by solving a one dimensional diffusion equation on excess minority carrier, discussions on 1/f noise were made and the following characteristics were concluded: (1) 1/f noise does not originate near electrodes for bias current. (2) 1/f noise hardly depends on sensor size and temperature in the 77–95K range, while g-r noise does. (3) 1/f noise is proportional to bias electric field, i.e. current density. (4) 1/f noise does not depend on photon background. From characteristics (2) and (4), it was concluded that 1/f noise has nothing to do with g-r noise. Finally, a new empirical formula was proposed for 1/f noise.

Effects of blocking contacts on generation-recombination noise and responsivity in intrinsic photoconductors

A theory is presented of generation-recombination (g-r) noise, responsivity, and detectivity D* in intrinsic photoconductors, including for the first time the effects of recombination at the contacts as a variable parameter. The contacts are described using a contact recombination velocity. Increasing the blocking at the contacts can significantly increase the responsivity, and also push the detectivity toward the background limited value since blocking contacts increase responsivity and the background contribution to the noise voltage faster than the thermal contribution. Blocking contacts also cause both the responsivity and g-r noise voltage to roll off at lower frequencies. Numerical calculations are presented for X∼0.2 HgCdTe photoconductors. Blocking contacts are found to increase the range of material parameters which can be used to make detectors. For example, under typical conditions, to reach responsivities of 105 V/W and detectivities of 6×1010 cm(Hz)1/2/W at T=77 °K, blocking contacts can increase the maximum allowed carrier density from about 4×1014 to about 2×1015 cm−3, and enlarge the required composition window from about 0.200<x<0.212 to about 0.188<x<0.212 for typical device geometries.

Effect of diffusion and drift on generation–recombination in HgCdTe photoconductors

We present calculations and measurements of the bias field dependence of generation– recombination (g–r) noise and D* in x∠0.2 HgCdTe photoconductors. The field dependence of the noise and D* depend on whether the detector noise is background or thermal dominated. For the background dominated case, the noise voltage increases linearly with field at low field and saturates at high field. D* initially increases with bias field and saturates at high field. For the thermal dominated case, the noise voltage initially increases linearly with bias field at low field and continues to increase like the square root of the field at large fields. D* decreases with increasing field in the thermal dominated case. Comparison is made between the theoretical and experimental results and good agreement is found.

Geometrical enhancement of HgCdTe photoconductive detectors

Ohmic contacts pose a severe lifetime degradation problem for photoconductive detectors whose length is comparable to a minority carrier diffusion length. A photoconductive device is described which greatly reduces the effect of ohmic contacts on effective photoconductive lifetime. The theory of operation is presented together with experimental data indicating significant enhancement in photoconductive responsivity, effective minority carrier lifetime, and l/f noise performance in n-type HgCdTe photoconductors.

Minority-Carrier Sweepout in 0.09-eV HgCdTe

High-field measurements of decreased response time, gain saturation, and enhanced signal-to-noise ratio are reported as evidence of minority-carrier sweepout in 0.09-eV HgCdTe photoconductors.