HgCdTe Photovoltaic Devices Research Articles

γ辐照导致中波碲镉汞光伏器件暗电流退化的机理研究

γ辐照导致中波碲镉汞光伏器件暗电流退化的机理研究

Different approximations of carrier statistics in middle-wavelength infrared HgCdTe photovoltaic devices with nonparabolic band

The optical bandgap and photoresponse characteristics of middle-wavelength infrared (MWIR) mercury-cadmium-telluride (HgCdTe) photodiodes have been performed based on a self-consistent solution of the Poisson's equation, the electron/hole continuity equations, and three-generation-recombination processes as Auger, Shockley-Read-Hall and optical generation recombination. Three different carrier-density approximations: (i) parabolic conduction-band approximation, (ii) Bebb's nonparabolic expression, and (iii) Harman's nonparabolic approximation, are proposed to calculate the optical bandgap and photoresponse of MWIR HgCdTe photovoltaic devices by considering the carrier degeneracy and the nonparabolic conduction band. It is found that omitting nonparabolic effect can lead to an enormous deviation in the simulation result, especially for heavily doped HgCdTe devices. On the basis of the calculated results of photoresponse, the parabolic conduction-band and Harman's nonparabolic approximations can lead to the response peak shift to short and long wavelengths, respectively.

Effects of carrier degeneracy and conduction band non-parabolicity on the simulation of HgCdTe photovoltaic devices

A new simple carrier density approximation is proposed to take account of carrier degeneracy and conduction band non-parabolicity into the simulation of HgCdTe (Mercury Cadmium Telluride – MCT) photovoltaic devices. It demonstrates a similar accuracy to the numerical calculation of the non-parabolic Fermi–Dirac integration in the range of the reduced Fermi energy at less than 30 for both LWIR and MWIR devices. By using the analytic model of the MCT device simulation, R d– V curve comparisons are made with the carrier density approximations by considering (1) both carrier degeneracy and conduction band non-parabolicity, (2) only carrier degeneracy, (3) only conduction band non-parabolicity, and (4) neither of these two factors. The effects of the omission of carrier degeneracy and conduction band non-parabolicity on each of the four types of dark current mechanisms are evaluated, which is found to lead to an enormous deviation in the simulation of MCT devices, especially for LWIR devices with heavy doping. The deviations of dark current in the latter three cases from the first case under different dominant dark current mechanisms are also demonstrated.

Laser beam induced current as a tool for HgCdTe photodiode characterisation

A non-destructive optical characterisation technique is used for the investigation of Mercury Cadmium Telluride (HgCdTe) photovoltaic devices. The technique uses a scanning laser microscope to obtain Laser Beam Induced Current (LBIC) data from which it may be possible to extract information such as junction depth, array uniformity, and other material and device parameters. LBIC has been previously used only as a qualitative technique, but in this work the procedure is being developed into a quantitative tool. At present the only junction depth profiling techniques are destructive, while array uniformity can only be examined after bonding to readout circuits. In this paper we present both theoretical and experimental results which show that LBIC can be employed as a quantitative tool for device characterisation. The primary measure of performance of IR detectors is the zero bias dynamic resistance junction area product, R0A. LBIC measurements indicate that the peak LBIC signal varies by a factor of ≃2 for long wavelength infrared photodiodes for which the R0A varies between 70Ωcm2 (acceptable for operation and the lower bound of typical values) and 8Ωcm2 (unacceptable and typical for poor quality diodes).

Junction depth measurement in HgCdTe using laser beam induced current (LBIC)

A new, nondestructive junction depth measurement technique for HgCdTe photovoltaic devices is investigated. The technique uses a scanning laser microscope to obtain laser beam induced current (LBIC) data from which information regarding the junction depth is extracted, and is applicable to both homojunction and heterojunction diodes. For implanted heterojunction photodiodes, the position of the n-p junction relative to the heterojunction is an important factor determining completed device performance, with blind photodiodes resulting if the n-p junction is incorrectly placed. At present, the only methods available for junction depth determination (e.g., secondary ion mass spectroscopy and differential Hall) are destructive and not applicable as routine process monitoring techniques. It is envisaged that the development of a nondestructive routine process monitoring procedure will help improve yield and reduce the cost of HgCdTe photovoltaic devices. In this paper, experimental and theoretical results are presented in order to assess the sensitivity of the new technique to the effects of junction doping density, illumination wavelength, frontside/backside illumination, and test structure geometry.

Recent advances in Ga1−xInxSb/InAs superlattice IR detector materials

Development of Ga1−xInxSb/InAs superlattices has been motivated by potential applications to infrared (IR) detectors in the 8–12 μm band and the far IR region 12–20+ μm. Theoretical and experimental results to date support the prospect that very high performance III–V materials-based IR focal plane array (FPA) sensors that can operate at temperatures higher than HgCdTe photovoltaic devices are feasible. The heterostructures under development are grown by molecular beam epitaxy (MBE) and are type-II, broken gap superlattices of alternating layers of Ga1−xInxSb/InAs with compositions chosen to allow the superlattice to be symmetrically strained to GaSb substrates. The details of the interface structure at the GaSb surface, the character of the interfaces between the Ga1−xInxSb and InAs layers, and the chemical nature of the Group V flux used in the MBE growth process are among the critical factors in enabling the growth of superlattices of extremely high optical quality suitable for IR detectors. We review recent advances in the materials and epitaxial processes for growing these superlattices. These processes have been applied to producing IR detector structures tailored to the particularly difficult spectral range of very long wavelength cut-off of 12+ μm.

Laser induced effects in HgCdTe photovoltaic devices

In this paper we report on induced persistent dark currents in HgCdTe photodiodes induced with lasers, and relationships of these effects with those observed with γ rays and high-energy electron irradiations. Front side laser illumination with relatively small photon flux induces changes in the dark currents with thermal recovery characteristics similar to those observed from irradiation experiments with Co60 gammas and high-energy electrons. The induced currents produced by these alternative sources are mostly of tunneling origin. The degree of susceptibility to irradiation and illumination observed on devices of different structures is found to be similar, i.e., devices with relatively hard characteristics when exposed to irradiation with γ rays or high-energy electrons also are observed to be hard to laser illumination. Our results indicate that these metastable effects produced by lasers, γ rays, and high-energy electrons are of a common origin. The laser experiments suggest that these effects are associated mostly with changes in the electronic properties in the vicinity of the semiconductor junction at the insulator interface. The magnitude of the change in the dark current decreases with increasing wavelength of light and band-gap light produces no notable changes. This observation suggests a threshold energy for producing the effects and suggests that the physical processes are excitation of carriers from the semiconductor into the insulator/interface region. Junction parameters and the material features of the insulator/semiconductor interface are shown to be important factors in determining the susceptibility of devices to irradiation and illumination.

High performance epitaxial HgCdTe photodiodes for 2.7 µm applications

High performance photovoltaic devices tailored for 2.7 µm cut-off have been fabricated on liquid phase epitaxial HgCdTe/CdTe. The peak external quantum efficiency is measured to be 67% without any AR-coating. The measured zero-bias resistance-area (R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> A) product is 5 ∼ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> µ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 195K and µ10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> Ω-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 140K. The demonstrated performance of HgCdTe photovoltaic devices tailored for a 2.7 µm cut-off is considerably better than conventional PbS photodetectors, which have appeared in available literature and which are currently in wide use in this spectral range.