Theoretical performance of mid wavelength HgCdTe(1 0 0) heterostructure infrared detectors
Introduction
The mercury cadmium telluride (Hg1−xCdxTe) is almost ideal semiconductor material for high performance infrared (IR) detectors. The composition-dependent energy band gap can be tuned by changing the molar composition of HgTe and CdTe, thus the cut-off wavelength of HgCdTe photodetectors can varied from the visible to the very long wave-length infrared range. Moreover, additional specific advantages of HgCdTe are the large optical coefficients that enable high quantum efficiency, and favorite inherent recombination mechanisms that lead to long carrier lifetime [1].
The performance of photon IR detectors is limited by the dark current, since any fluctuations in dark current (statistical nature of thermal generation-recombination (GR) processes of charge carriers) are directly translated into noise in the detector, which limits the achievable detectivity. There are three main thermal GR processes to be considered in the narrow band gap semiconductors, namely: Shockley-Read-Hall (SRH), radiative, and Auger. Very often, especially for high operating temperatures, the radiative process can be ignored since its contribution is small enough to be neglected due to photon recycling effect [2], [3]. Cooling of IR photon detectors to very low temperatures (often to a temperature of liquid nitrogen) is the most commonly used to reduce thermal generation. However, the increasing demand for high-operating temperature (HOT) detectors, motivated by the need to substantially reduce the size, weight, and power requirements of infrared imaging systems, has outlined trends in IR technology in recent decades.
Various ways to reduce thermal generation have been proposed for HOT operating conditions. Significant improvements have been obtained by the reduction of the absorber volume using optical immersion [4], double or multiple pass of IR radiation [4], suppression of Auger thermal generation in non-equilibrium photoconductors [5], [6], photodiodes [7], [8], [9], [10], photon trapping detectors [11], plasmonic coupling of IR detectors [12], and barrier detectors [13], [14], [15], [16].
Detectors based on the XBnn concept utilizing III-V compound semiconductor technology have translated into either higher temperature operation of the device for the same performance or higher performance for the same temperature [14]. XBnn structure consist of a n- or p-type cap contact and n-type absorber layer separated by a thin wide bandgap barrier (B). An unipolar barrier blocks one carrier type (electrons) but allow the unimpeded flow of the other (holes). The introduction of unipolar barrier in various configurations of photovoltaic structures suppress dark current and noises without impending photocurrent flow. Uniform n-type doping of the barrier and active layer ensures the absence of depletion in the narrow bandgap absorber, what results in eliminating the currents associated with SRH centers in the depletion region in p-i-n design. This is especially important in III–V compound semiconductor technology. Generally, in comparison with HgCdTe ternary alloys, III–V semiconductor bulk materials exhibit more active SRH centers resulting in lower lifetime. The best SRH carrier lifetimes was estimated as ∼400 ns for InSb and InAsSb alloys [17], [18]. The current favorite in the semiconductor family is type-II superlattices (SLs). At the present, InAs/GaSb SLs material shows SRH lifetime below 100 ns [19]. The gallium-free InAs/InAsSb SLs posses much longer lifetimes, up to 10 μs [20], comparable to those obtained for HgCdTe alloys. Measured values for n-type LWIR HgCdTe at 77 K lie in a broad range of value 2–20 μs and are independent of doping concentration for value bellow 1015 cm−3. The MWIR HgCdTe values are typically somewhat longer, in the range 2–60 μs. More recent data for HgCdTe presented by Kinch in Ref. [21] are considerably larger in lower temperature range, and are >200 μs to 50 ms depending on the cutoff wavelength.
In good-quality HgCdTe photodiodes fabricated on molecular beam epitaxial (MBE) and liquid phase epitaxial (LPE) materials, the absence of measurable depletion current component in a p-n junction technology is observed. By applying a low doping semiconductor, so that doping concentration should be considerable lower than intrinsic concentration in a device operating temperature, it is possible to achieve a background limited performance (BLIP condition) with thermoelectric cooling and even at room temperature in the double-layer heterojunction (DLHJ) detector architecture [22], [23]. Despite recent technology improvements, the performance of uncooled HgCdTe devices grown by metal organic chemical vapor deposition (MOCVD) in a joint laboratory run by VIGO System S.A. and Military University of Technology (MUT) has remained below the fundamental limits [4]. This is due to the excess charge carrier generation caused by the point and extended defects. The most likely defects are residual metal site vacancies and dislocations.
Performance improvements of MOCVD grown HOT HgCdTe detectors could have been achieved by applying XBnn architecture. However, band alignment in uniform n-type doping HgCdTe results both in conduction and valence band offset [24], [25]. Thus, a relatively high bias, the so-called “turn-on voltage,” which is typically greater than the bandgap energy, is required to be applied to the device in order to collect all the photogenerated carriers. One method to align the valence band in HgCdTe alloys is a proper p-type doping of the barrier. First experimental results of MWIR HgCdTe barrier detectors with a zero valence band offset have been presented by Kopytko et al. [26], [27]. Devices were MOCVD grown with (1 1 1) B HgCdTe orientation within the interdiffuse multilayer process (IMP) on (1 0 0) GaAs substrates, oriented 2° off toward nearest 〈1 1 0〉. p+BpnN+ design with the doping and compositional gradient at the barrier-absorber heterointerface ensure reduction of valence band offset allowing minority carrier holes photogenerated in the absorber layer to reach the contact layer unimpeded. In principle, p+BpnN+ design combines the concept of a high impedance photoconductor proposed by White [28] with DLHJ device. Experiments indicated the influence of the barrier on detectors’ performances. Zero valence band offset approximation throughout the p+Bpn heterostructure allows flow of only minority holes generated in the absorber, what in a combination with n-N+ exclusion junction provides the Auger suppression. Device optimized at 3.6 μm cut-off wavelength at 230 K exhibited very low dark current densities in the range (2–3) × 10−4 A/cm2 at 230 K and maximum responsivities of ∼2 A/W while operate at zero and reverse bias in a wide range of voltages.
Recently, advances in MOCVD growth of (1 0 0) oriented HgCdTe was achieved in our laboratory. So far, significant improvements has been obtained in photoconductors operated at near-room temperatures [29], [30]. HgCdTe (1 0 0) layers exhibit better crystalline quality than HgCdTe (1 1 1) B layers and reduction of residual defects which contribute to a lower background doping. HgCdTe (1 1 1) B layers will twin and it was shown that the resulting microtwins exhibit donor-like activity [31]. Moreover, HgCdTe (1 0 0) layers show higher acceptor doping efficiency in comparison to (1 1 1) B. In HgCdTe (1 0 0) arsenic atoms are incorporated effectively into desired tellurium sublattice and act as the acceptors. In HgCdTe (1 1 1)B growing surface offers poorer configuration for arsenic atoms incorporation into tellurium sublattice. As a result part of arsenic atoms can be located in metal sublattice or interstitials, where they may act as the recombination centers.
The residual background concentration is a matter of much concern for each semiconductor laboratory. In particular, it is important during p-type doping at the low level. The mean residual concentration maintained in our lab is in the mid of 1014 cm−3 for (1 1 1) B orientation and for (1 0 0) orientation is an order of magnitude lower [29]. The concentration was determined using Hall measurements at a temperature of 77 K. Hall concentration measured at liquid nitrogen temperature is identified as the residual background concentration (because intrinsic electrons are frozen).
This paper presents a theoretical study of a design of p+BpnN+ detectors with the assumption of all advantages of HgCdTe (1 0 0) layers (lower background concentration and lower residual defects). It is shown that application of HgCdTe (1 0 0) absorbing layers significantly improves the performance of detectors optimized for the MWIR spectral range and HOT conditions. A key to its success is fine control over the defect density.
Section snippets
Analysis details
Theoretical modeling of the HgCdTe detectors has been performed using our original numerical program developed at the Institute of Applied Physics, Military University of Technology (MUT). The program based on the solution of the set of transport equations that are comprised of the continuity equations for electrons and holes, Poisson’s equation, and the heat equation [32], [33], [34]:where Ψ is the electrostatic
Detector design
The analyzed MOCVD grown device consist of four HgCdTe layers – p+BpnN+ (a capital letter denotes wider band; the symbol “+” denotes strong doping): cap contact, wide bandgap barrier, absorber and bottom contact layer. The cap-barrier structural unit consists a highly doped with arsenic p+ cap contact layer and p-type wide bandgap barrier with compositional gradient at the absorber side. Active layer is intentionally undoped with composition optimized at 3.6 μm cut-off wavelength at 230 K. The N+
Results
Fig. 2 presents experimental current-voltage characteristics of HgCdTe (1 1 1) B detector at a temperature of 230 K and a simulated fitting. Calculations took into account all considered mechanisms of thermal generation, tunneling and impact ionization. The Auger and the SRH parts of the dark current are depicted. Current-voltage characteristics were calculated both for dislocation free structures (Fig. 2a), as well as for structures containing dislocations (Fig. 2b). Electrical properties of
Conclusions
The paper presents a theoretical study of a HgCdTe p+BpnN+ design with the assumption of all advantages of (1 0 0) oriented layers, and comparison to its (1 1 1) B counterpart. So far, MOCVD grown HgCdTe (1 1 1) B p+BpnN+ detector shows dark current densities close to the “Rule 07”. HgCdTe (1 0 0) layers exhibit better crystalline quality, low dislocation density and reduction of residual defects which contribute to a background doping in the mid of 1014 cm−3. Modeling shows that by applying a low
Acknowledgements
The work has been done under the financial support of the National Science Centre (Poland) - the grant no. DEC-2013/08/A/ST5/00773.
Małgorzata Kopytko received the M.Sc. degree in electronics and telecommunication from the Department of Electronics, Wrocław University of Technology, Wrocław, Poland, in 2005 and Ph.D. degree in electronics from the Institute of Optoelectronics, Military University of Technology, Warsaw, Poland, in 2011.
She is currently working for Military University of Technology at the Institute of Applied Physics. Her research areas include design, simulation, fabrication, and characterization of
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Małgorzata Kopytko received the M.Sc. degree in electronics and telecommunication from the Department of Electronics, Wrocław University of Technology, Wrocław, Poland, in 2005 and Ph.D. degree in electronics from the Institute of Optoelectronics, Military University of Technology, Warsaw, Poland, in 2011.
She is currently working for Military University of Technology at the Institute of Applied Physics. Her research areas include design, simulation, fabrication, and characterization of HgCdTe-based infrared barrier detectors.