HgCdTe Photovoltaic Detectors Research Articles

Numerical study on infrared detectors cooling by multi-stage thermoelectric cooler combined with microchannel heat sink

Lightweight, noiseless, and constant low-temperature refrigeration is crucial for cooled infrared (IR) detectors, especially short-wave-infrared (SWIR) detectors. However, some SWIR detectors, such as HgCdTe photovoltaic detectors are working below 210 K. For this application, multi-stage thermoelectric coolers (TEC) are preferable to single-stage thermoelectric coolers because they can generate larger temperature differences. This paper presents a refrigeration design that integrates SWIR and TEC into a vacuum Dewar package and combines them with a microchannel heat sink (MHS) to improve the detection performance and better responsiveness of SWIR. Two different refrigeration models, SWIR + TEC + AHS (air-cooled heat sink) and SWIR + TEC + MHS, were established to evaluate the refrigeration performance of the proposed SWIR + TEC + MHS scheme using the finite element method. The influences of multi-stage TEC leg length and current, inlet water temperature, and water velocity on the refrigeration performance of the TEC are analyzed. The results show that the SWIR + TEC + MHS design scheme has the best comprehensive performance and minimum volume. The influences of all the factors are obvious while the current has the most important effect on the performance. The temperature of SWIR reached 201.69 K at standard working conditions. It shows that the cooling scheme can not only meet the needs of flexible deployment of SWIR but also provide better thermal security. The proposed cooling design has an excellent prospect for short-wave-infrared detectors.

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.

Ohmic metal/Hg1-xCdxTe (x ≈ 0.3) contacts

Some technological features of p-type Hg 1-x Cd x Te (x ≈ 0.3) liquid phase epitaxy layers grown on Cd 1-y Zn y Te (y ≈ 0.04) substrates are briefly discussed. Energy dispersive analysis of mercury-cadmium-telluride (MCT) layers and metal contacts (Au(In)/Cr(Mo,Ti)) to MCT layers together with their current-voltage characteristics at T ≈ 80 and 300 K are considered. It is shown that Cr(Mo,Ti)/MCT contacts have ohmic characteristics or close to them at T ≈ 80 and 300 K for both n- and p-type MCT layers. This gives the opportunity to form ohmic contacts to n- and p-type pads in one technological cycle. The contact resistance Rc is much smaller as compared to the HgCdTe p-n junction resistance R0[ RcA (< 10–2 Ω cm2) ≪ R0A (> 103 Ω cm2), where R0 is the zero bias diode resistance at T = 80 K, and A is the MCT (x ≈ 0.3) junction area]. So, such contacts are appropriate for the fabrication of photovoltaic HgCdTe detectors.

Analysis injection area-dark current characteristics for mid-wavelength HgCdTe photodiodes

In this paper, we study the relationship between dark current mechanism and the injection area of mid-wavelength infrared (MWIR) HgCdTe photovoltaic detectors. A simultaneous-mode nonlinear fitting program for n-on-p mid-wavelength HgCdTe infrared detectors is reported. It is found that the impact of diffusion mechanism gradually weakens and the effect of generation-recombination mechanism becomes more significant as the area of injection increasing under forward bias. The effect of trap-assisted tunneling mechanism gradually weakens as the area of injection increasing under middle reverse bias and band-to-band tunneling mechanism has less impact on dark current of MWIR HgCdTe photodiodes. And as the area of injection increasing, the effect of surface leak mechanism is gradually decrease. Finally, we find the reversed welding pressure and the arrangement of common electrode for MWIR HgCdTe Photodiodes also impacts diffusion mechanism and generation-recombination mechanism under forward bias.

纳秒激光辐照下HgCdTe光伏探测器的瞬态响应特性退化

纳秒激光辐照下HgCdTe光伏探测器的瞬态响应特性退化

Long wavelength interband cascade infrared photodetectors operating at high temperatures

We report on a comparison study of long wavelength infrared interband cascade infrared photodetectors (ICIPs) with the goal of an improved understanding that will lead to further increases in the operation temperature. We studied four sets of detectors including single absorber barrier detectors and multi-stage ICIPs with four, six, and eight discrete absorbers. The 90% cutoff wavelength of these detectors was between 7.5 and 11.5 μm from 78 to 340 K. Multiple stage ICIPs were able to operate with monotonically increasing bias-independent responsivity up to 280 K, while the responsivity of the one-stage detectors decreased at 200 K with bias dependence. The advantages of the multi-stage ICIPs over the one-stage device are demonstrated in terms of lower dark current density, higher detectivity (D*), and higher operating temperatures. The one-stage detectors operated at temperatures up to 250 K, while the ICIPs were able to operate up to 340 K with D* higher than 1.0 × 108 cm·Hz1/2/W at 300 K. The D* for these ICIPs at 200 K was larger than 1.0 × 109 cm·Hz1/2/W at 8 μm, which is more than a factor of two higher than the corresponding value for photovoltaic HgCdTe detectors at similar cutoff wavelengths. Interestingly, negative differential conductance (NDC) was observed in these detectors at high temperatures. The underlying physics of the NDC was investigated and correlated with the number of cascade stages and electron barriers. With the enhancement of the electron barrier in the multiple-stage ICIPs, the NDC was reduced, and the overall device performance, in terms of D*, was improved.

Electrical Characteristics of Mid-wavelength HgCdTe Photovoltaic Detectors Exposed to Gamma Irradiation

The study of electrical characteristics of mid-wavelength HgCdTe photodiodes irradiated by steady-state gamma rays has been carried out. The measurement of the current–voltage curves during irradiation revealed an abnormal variation of zero biased resistance R0, and it didn’t tend to change monotonically as observed in the case of post irradiation measurement. The irradiation effect was dominated by bulk effect inferred from the fitting calculations, and the generation-recombination current in the depletion region was drastically affected by gamma irradiation. Another irradiation effect was the linear increase of the series resistance with irradiation dosage which was related with the change of transportation parameters of carriers. The influence of hydrogenation on the gamma irradiation effects was also studied for comparison with the same batch of HgCdTe photodiodes, and it was found that R0 for the hydrogenated devices showed similar change to those without hydrogenation. The series resistance, however, gave a totally different irradiation effect from the non-hydrogenated detectors and showed little change up to nearly 1 Mrad(Si) of gamma irradiation, which may be explained by the annihilation of hydrogen radicals with the defects caused by gamma irradiation.

HgCdTe e-avalanche photodiode detector arrays

Initial results on the MWIR e-APD detector arrays with 30 μm pitch fabricated on LPE grown compositionally graded p-HgCdTe epilayers are presented. High dynamic resistance times active area (R0A) product 2 × 106 Ω-cm2, low dark current density 4 nA/cm2 and high gain 5500 at -8 V were achieved in the n+-υ-p+ HgCdTe e-APD at 80 K. LPE based HgCdTe e-APD development makes this technology amenable for adoption in the foundries established for the conventional HgCdTe photovoltaic detector arrays without any additional investment.

Analytical modelling of carrier transport mechanisms in long wavelength planar n+–p HgCdTe photovoltaic detectors

The dark electrical characteristic of n+ on p long wavelength Hg1−xCdxTe photovoltaic detectors has been studied as a function of applied bias voltages. The diodes under study were given different surface treatments prior to their passivation. The dark current components have been identified using the already known carrier transport mechanisms across the junction. It is reported that the diodes under investigation have an excess leakage current component that exhibits quadratic dependence on the applied reverse bias voltage across the diode. The origin of this excess current is found to be closely associated with the ohmic currents. The diodes with higher ohmic current exhibit higher excess leakage current.

Dependence of Ion-Implant-Induced LBIC Novel Characteristic on Excitation Intensity for Long-Wavelength HgCdTe-Based Photovoltaic Infrared Detector Pixel Arrays

In this paper, experimental results of laser-irradiance-dependent polarity inversion of laser beam induced current (LBIC) for As-doped long-wavelength HgCdTe pixel arrays grown on CdZnTe are reported. Models for the ion-implant-induced junction transformation are proposed, and demonstrated using numerical simulations. The novel trap-related p-n junction transformation induced by ion implantation is observed under typical laser irradiances for low temperature. The implantation-induced traps and Hg interstitial diffusion are key factors for inducing the LBIC coupling, polarity reversion, and junction broadening at different laser irradiances. The trap type, trap density, and junction configuration are extracted from the measured experiment data. The results provide the near room-temperature HgCdTe photovoltaic detector with a reliable reference on the junction reversion and broadening around implanted regions, as well as controlling the n-on-p junction formation for very long wavelength HgCdTe infrared detector pixels.

Generation mechanism of two different over-saturation phenomena of photovoltaic HgCdTe detectors irradiated by CW band-in laser

We have studied two batches of photovoltaic HgCdTe detectors irradiated by CW band-in laser, and discovered, two different over-saturation phenomena. It is shown that the over-saturation phenomenon associated with the open-circle voltage signals which decreases with increasing light intensity is of universal existence in the PV HgCdTe detectors irradiated by intense light. The regular and special phenomena of PV HgCdTe detectors under intense light radiation are definite and obvious. The generation conditions for the two typical over-saturation phenomena are analyzed in terms of the equivalent circuit model. These two kinds of over-saturation phenomena have been numerically simulated. Numerical results are in good agreement with experimental data. It is found that the two generation mechanisms of the over-saturation phenomena of PV detector under irradiation of the above-band gap laser do exist. One is the increasing dark current due to thermal effectm, and the other is the leak current due to the bugs in the detector material.

Polarity inversion and coupling of laser beam induced current in As-doped long-wavelength HgCdTe infrared detector pixel arrays: Experiment and simulation

In this paper, experimental results of polarity inversion and coupling of laser beam induced current for As-doped long-wavelength HgCdTe pixel arrays grown on CdZnTe are reported. Models for the p-n junction transformation are proposed and demonstrated using numerical simulations. Simulation results are shown to be in agreement with the experimental results. It is found that the deep traps induced by ion implantation are very sensitive to temperature, resulting in a decrease of the quasi Fermi level in the implantation region in comparison to that in the Hg interstitials diffusion and As-doped regions. The Hg interstitial diffusion, As-doping amphoteric behavior, ion implantation damage traps, and the mixed conduction, are key factors for inducing the polarity reversion, coupling, and junction broadening at different temperatures. The results provide the near room-temperature HgCdTe photovoltaic detector with a reliable reference on the junction reversion and broadening around implanted regions, as well as controlling the n-on-p junction for very long wavelength HgCdTe infrared detector pixels.

Temperature dependence on photosensitive area extension in mercury cadmium telluride photodiodes using laser beam induced current

We report on the temperature-dependent extension of n-type inversion regions in mercury cadmium telluride (HgCdTe) photodiodes at low temperatures (87 K) compared to inversion regions at room temperature (300 K). Laser-beam-induced-current (LBIC) measurement techniques are used to obtain the photosensitive area extensions of n-type inversion in HgCdTe photodiodes for typical n+-on-p HgCdTe photovoltaic IR detectors. The effect of temperature on the extension of n-type conversion region is investigated by considering the sign of the LBIC signal. Theoretical results show that the hole concentration decreases in multidoped HgCdTe due to the freeze-out effect as the temperature decreases. Consequently, hole concentration is much lower than electron concentration at 87 K. The n-type inversion region extension is shown to be caused with the p-to-n type conversion.

Absolute linearity measurements on a PV HgCdTe detector in the infrared

The linearity-of-response characteristics of a photovoltaic (PV) HgCdTe detector were investigated at a number of wavelengths in the infrared, using the NPL linearity of detector response characterization facility. The measurements were performed with the test detector operating under conditions identical to those in which the detectors will be used in typical infrared radiometric applications. The deviation from linearity in the generated photocurrent was shown to be strongly dependent on the area of the detector being illuminated. Plots of the linearity factor versus generated photocurrent for different illuminated wavelengths were shown to overlap. The linearity factor was shown to be a function of the photon irradiance of the illuminating beam. This behaviour was similar to that exhibited by photoconductive (PC) HgCdTe detectors, indicating that Auger recombination was the dominant source of the deviation from linearity observed in the test detector.

Investigation of the nonlinear response mechanism of photovoltaic HgCdTe detector irradiated by CW band-in laser

The medium wave HgCdTe photovoltaic detector (band gap ～0.33 eV) is irradiated by a CW band-in laser. The experimental results are shown that the detector enter into nonlinear response state as the incidence laser power increase. When the detector entered into nonlinear response state, the open circle voltage (Voc) signal decreased with the laser power. The Voc signal rapidly decreased when the laser turned on and the Voc signal rapidly increased when the laser turned off. The effect of laser induced temperature field and temperature dependence built-in field of pn junction were considered. The laser power variation of laser turn on and turn off were aslo considered. The analytical model of photovoltaic detector irradiated by above-band gap CW laser was built up. The calculation results agree well with the experiment results. It is shown that the temperature dependence built-in field of pn junction is the main mechanism induced the nonlinear response. The magnitude of Voc signals with laser turn on and turn off is decided by light intensity and temperature.

A numerical study of broadband absorbers for visible to infrared detectors

We have studied the electromagnetic response of micro-structured surfaces realized with pillar arrays intended to provide a broadband (0.5–5.0 μm) absorption enhancement for HgCdTe photovoltaic detector arrays. We have considered both square and hexagonal lattice pillar configurations. Using a finite-difference time-domain approach we have found that the absorption enhancement is weakly dependent on the pillar lattice type, but the lattice period does have a significant impact on the enhancement. The use of these micro-structured surfaces makes it possible to eliminate the need for anti-reflection coatings on the detector back-side while maintaining negligible reflectance over a broad spectral band.

Study of photovoltaic effect photovoltaic HgCdTe detector irradiated by spectral unrelated laser

A spectral unrelated laser was induced on the short wave and medium wave photovoltaic HgCdTe infrared detectors respectively, under various power density, the detectors had a serious of output. The results show that the two detectors all had response of spectral unrelated laser, but the response was in the opposite direction. Through the analysis of experimental phenomena, it is found that carrier concentration of detectors before laser irradiation is the reason why the response of detectors was in the opposite direction. It showed valuable clue for future research on photovoltaic HgCdTe irradiated by spectral unrelated laser.

Investigation of the response mechanism of photovoltaic semiconductor with sub-bandgap photons

The photovoltaic HgCdTe detectors with band gaps 0.91 and 0.33 eV are irradiated by 10.6 m laser (0.12 eV photon energy), separately. It is found that output voltage of detector (0.91 eV band gap) is positive, while the response voltage of detector (0.33 eV band gap) is opposite to it. To investigate this phenomenon, the detector with a band gap of 0.91 eV is irradiated by a given power 10.6 m laser under different initial open-circuit voltags. It is experimentally demonstrated that the phenomenon is caused by the initial open-circuit voltage. With further investigation, the open-circuit voltage of the photovoltaic detector is determined by both the thermovoltage caused by thermoexcited carrier and the crystal thermal effect produced by free carrier absorption under sub-bandgap laser irradiation.

Improvement in performance of high-operating temperature HgCdTe photodiodes

One of the key factors determining HgCdTe photodiode quality is acceptor doping efficiency. This paper presents significant progress made over the past 3 years in development of acceptor doping technology in metalorganic chemical vapour deposition (MOCVD) HgCdTe photovoltaic detectors. High acceptor doping is required for P +-contact layers, whereas low doping is necessary for p-type base absorbing layer. Previously, AsH 3 precursor was used as an acceptor dopant. This precursor is partially incorporated as electrically neutral As–H pairs, which are likely to be recombination centres in HgCdTe and in consequence they influence the carriers lifetime lowering. Substituting AsH 3 by TDMAAs resulted in higher carrier lifetimes and thereby about one order of magnitude higher R 0 A product of HgCdTe photodiodes at temperatures close to 230 K. Finally, it is shown that the response time of HgCdTe photodiodes at weak reverse bias condition is mainly limited by the drift time of carriers moving into p–N + junction. Using the reverse bias higher than 50 mV, the transit time across the absorber region limits the response time. The response time of small-area devices decreases in the region of weak reverse bias achieving value below 1 ns.

Influence of TDMAAs Acceptor Precursor on Performance Improvement of HgCdTe Photodiodes

OneofthekeyfactorwhichdetermineHgCdTephotodiodequalityisacceptordopingefficiency. Thispaperpresents significant progress made over the past three years in development of acceptor doping technology inmetalorganic chemical vapour deposition HgCdTe photovoltaic detectors. High acceptor doping is required forP