Full band structure calculation of minority carrier lifetimes in HgCdTe and thallium-based alloys

We have calculated the full band structures-based minority carrier lifetimes in small-gap semiconductor alloys. The contribution from first-order Coulomb interactions and second-order electron-electron interactions coupled through optical phonons are included. Our results agree reasonably well with experiments in Hg_0.78Cd_0.22Te. Similar calculations were carried out for lifetimes in In_0.67Tl_0.33P, In_0.85Tl_0.15As, and In_0.92Tl_0.08Sb. The minority carrier lifetimes in In_0.67Tl_0.33P and In_0.92Tl_0.08Sb are shorter than that in Hg_0.78Cd_0.22Te at all temperatures. However, the low-temperature minority carrier lifetime in In_0.85Tl_0.15 As is an order of magnitude longer than that in Hg_0.78Cd_0.22Te. Our calculations further suggest the possibility of increasing the lifetimes of minority carriers either by decreasing the density of states inside a critical energy and momentum region or by increasing the total hole population outside that critical region. Experimental observations that substantiate this suggestion are discussed.     

Minority carrier lifetime and noise in abrupt molecular-beam epitaxy-grown HgCdTe heterostructures

The steady-state lifetime of photogenerated minority carriers has been investigated in heterostructure HgCdTe devices fabricated on molecular-beam epitaxy (MBE) grown material. A wider bandgap capping layer (Hg_(1−x)Cd_(x)Te, x = 0.44) was grown on a narrower bandgap absorbing layer (Hg_(1−x)Cd_(x)Te, x = 0.32, λ_co,80K = 4.57 μm) material in an uninterrupted MBE growth to create an abrupt heterointerface. Steady-state lifetime as a function of temperature over the range 80–300 K was extracted from photoconductive responsivity at an optical wavelength corresponding to the peak responsivity at that temperature. At 80 K, the photoconductors exhibit a specific detectivity of 4.5 × 10¹¹ cm Hz^−1/2W⁻¹ (chopping frequency of 1 kHz). For each measurement temperature, the steady-state excess carrier lifetime determined experimentally was compared to the theoretical bulk lifetime for material with x = 0.32 and effective n-type doping density of 3.7 × 10¹⁴ cm⁻³. Theoretical calculations of the Auger-1 lifetime based on expressions developed by Pratt et al. were not able to account for the reduction in lifetime observed at temperatures above 180 K. Two approaches have been attempted to resolve this discrepancy: A semiempirical expression for Auger lifetime attributed to Meyer et al. was used to fit to the data, with the Auger coefficient γ as a fitting parameter. However, the resulting Auger coefficient found in this work is more than an order of magnitude higher than that reported previously. Alternatively, the reduction in effective lifetime above 180 K may be understood as a “loss” of carriers from the narrow bandgap absorbing layer that are promoted across the potential barrier in the conduction band into a low lifetime, wider bandgap capping layer. The reduction in lifetime as a function of inverse temperature for temperatures above 180 K may be fitted by a “cap lifetime” that has an activation energy equal to the change in bandgap across the heterostucture and scaled by a fitting constant.     

Electronic structure, absorption coefficient, and auger rate in HgCdTe and thallium-based alloys

The band structures, absorption coefficients, and Auger recombination rates in narrow-gap alloys HgCdTe, InTIP, InTlAs, and InTlSb in the zinc blende structure, along with those of GaAs, are calculated using a hybrid pseudopotential and tight-binding method. The composition-dependent band gaps of the thallium-based alloys are reported along with those of several other semiconductor alloys. Within 50 meV from the absorption edge, the absorption coefficient of In_xTl_1−xP is found to have about the same magnitude as that of Hg_xCd_1−xTe and GaAs, while that of In_xTl_1−xAs and In_xTl_1−xSb is much smaller. In agreement with previous theories, the calculated Auger lifetimes in Hg_0.78Cd_0.22Te with unit or k • p overlap agree very well with experiments. Among the thallium alloys studied, the Auger lifetimes are longest in In_0.33Tl_0.67P, but still shorter than those in Hg_0.78Cd_0.22Te by an order of magnitude. In addition, realistic overlaps produce lifetimes one to two orders of magnitude larger than those observed.     

Minority carrier lifetime in indium-doped HgCdTe(211)B epitaxial layers grown by molecular beam epitaxy

We have studied the minority-carrier lifetime on intentionally indium-doped (211)B molecular beam epitaxially grown Hg_1-xCd_xTe epilayers down to 80K with x ≈ 23.0% ± 2.0%. Measured lifetimes were explained by an Auger-limited band-to-band recombination process in this material even in the extrinsic temperature region. Layers show excellent electron mobilities as high as ≈2 x 10⁵ cm²v^-1s^-1 at low temperatures. When the layers are compensated with Hg vacancies, results show that the Schockley-Read recombination process becomes important in addition to the band-to-band processes. From the values of τ_n0 and τ_p0 of one sample, the obtained defect level is acceptor-like and is somewhat related to the Hg vacancies.     

The minority carrier lifetime in doped and undoped p-type Hg0.78Cd0.22Te liquid phase epitaxy films

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg_1-xCd_xTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τ_A7 ⁱ/τ_A1 ⁱ ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 10¹⁵ to 1.5 x 10¹⁷ cm^-3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 10¹⁵ and 8 x 10¹⁶ cm^-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τ_A7 ⁱ/τ_A1 ⁱ, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (E_v+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.     

用微波反射法测HgCdTe中少数载流子寿命

介绍了用微波反射法测量ＨｇＣｄＴｅ中的少数载流子寿命,分析了其测量原理,并与接触式的光电导衰减法进行了比较。     

Behavior of p-type dopants in HgCdTe

Obtaining high concentrations of active p-type dopants in HgCdTe is an issue of much current interest. We discuss the results of our calculations on column IB and VA dopants. The full-potential linear muffin-tin orbital method, based on the local density approximation is used to calculate electronic total energies and localized levels in the band gap. Free energies are predicted and incorporated into a thermodynamical model to calculate impurity and native defect concentrations as a function of temperature, stoichiometry, and total impurity density. Copper, silver, and gold are found to be incorporated nearly exclusively on the metal sublattice and to be 100% active for all near-equilibrium growth and processing conditions. The density of interstitial copper is high enough to impact copper diffusion. In contrast, significant concentrations of phosphorus, arsenic, and antimony are found on the metal sublattice where they behave as n-type dopants, accounting for highly compensated, or even n-type material, depending on the equilibration temperature and equivalent mercury partial pressure.     

n型HgCdTe晶片少子寿命分布的小光点测量

用聚焦的激光光点（宽度约为０．１ｍｍ）测量了ｎ型Ｈｇ１－ｘＣｄｘＴｅ（ｘ＝０．２）长条状薄片样品的少数载流子寿命沿样品长条方向的分布，结果表明分布是不均匀的，讨论了引起这种不均匀的可能原因。     

Measurement of minority carrier lifetime in n-type MBE HgCdTe and its dependence on annealing

Results are presented for minority carrier lifetime in n-type molecular beam epitaxy Hg_1−xCd_xTe with x ranging from 0.2 to 0.6. It was found that the lifetime was unintentionally degraded by post-growth annealing under Hg saturated conditions in a H₂ atmosphere that was both time and temperature dependent. This effect was minimal or non-existent for x∼0.2 material, but very strong for x ≥ 0.3. Hydrogen was identified as responsible for this degradation. Identical annealing in a He atmosphere avoids this degradation and results in neartheoretical lifetime values for carrier concentrations as low 1 × 10¹⁵ cm⁻³ in ≥0.3 material. Modeling was carried out for x∼0.2 and x∼0.4 material that shows the extent to which lifetime is reduced by Shockley-Real-Hall recombination for carrier concentrations below 1 × 10¹⁵ cm⁻³, as well as for layers annealed in H₂. It appears that annealing in H₂ results in a deep recombination center in wider bandgap HgCdTe that lowers the lifetime without affecting the majority carrier concentration and mobility.     

Minority carrier lifetimes in HgCdTe alloys

We present the results of full band structure calculations of Fermi levels, intrinsic carrier densities, and one-photon absorption coefficients in undoped HgCdTe alloys. The full band structure is used in the calculation of majority and minority carrier densities and the minority carrier lifetimes limited by radiative, Auger-1, and Auger-7 mechanisms in both n- and p-doped alloys. The lifetimes we predicted differ substantially from those calculated with widely used analytical expressions, except in cases where the hole density is small. This difference originates from the significantly non-parabolic and anisotropic valence bands, which are of increasing significance as the hole density increases. From a comparison of the calculated and measured lifetimes, we deduce that the lifetimes at low temperatures are limited by the Shockley-Read-Hall (SRH) recombination. We have generalized the SRH expression to include Fermi-Dirac statistics, but we still treat the density, energy level, and cross section as adjustable parameters. We find that the calculated radiative and Auger recombination lifetimes, as well as SRH lifetimes, can fit the measured lifetimes using traps located (a) near the conduction band edge in n-HgCdTe and (b) near the valence band edge for p-HgCdTe. In addition, the movement of Fermi level with respect to the trap level explains the observed temperature-dependence of the lifetimes. We conclude that there is considerable room for improvement in HgCdTe material quality.     

Electrical properties of HgCdTe epilayers doped with silver using an AgNO3 solution

We employed AgNO₃ solutions for doping Ag in liquid phase epitaxy (LPE) grown Hg_0.78Cd_0.22Te epilayers and found that the minority carrier lifetimes became longer so that the diode properties improved. After annealing LPE grown Hg(1-x)Cd(x)Te layers (x=0.22) in Hg atmosphere, the epilayers were immersed in an AgNO₃ solution at room temperature. The typical carrier concentrations of holes was 3 × 10¹⁶ cm⁻³ at 77K. These values were almost the same as for the nondoped wafers. Also, its acceptor level was 3 to 4 meV. This shows that the Ag was activated. The doped crystals have lifetimes several times longer than those of the nondoped crystals. Numerical fitting showed the lifetime was limited mostly by the Auger 7 process. The Shockley-Read-Hall recombination process was not effective. To examine the Ag-doped wafer, we fabricated photodiodes using standard planar technology. The diodes have an average zero-bias resistance of several MΩ and a shunt resistance of about 1 GΩ for a 10 μm cutoff wavelength at 78K. These values are about four times higher than those of nondoped diodes. The photo current is also two times higher at the same pixel size. This shows that the quantum efficiency is increased. The extension of the lifetime contributes to the high resistance and the high quantum efficiency of the photodiode.     

Si基HgCdTe变面积光伏探测器的变温特性研究

通过变面积Si基HgCdTe器件变温I-V测试和暗电流特性拟合分析,研究了不同偏压下n-on-p型Si基HgCdTe光伏器件的暗电流成分与Si基HgCdTe材料少子扩散长度和少子寿命随温度的变化规律.在液氮温度下,随着反向偏压的增大器件的表面漏电流在暗电流中所占比重逐渐增加.在零偏压下,当温度低于200 K时材料的少子...     

Performance limitation of short wavelength infrared InGaAs and HgCdTe photodiodes

The carrier lifetimes in In_xGa_1−xAs (InGaAs) and Hg_1−xCd_xTe (HgCdTe) ternary alloys for radiative and Auger recombination are calculated for temperature 300K in the short wavelength range 1.5<λ<3.7 μm. Due to photon recycling, an order of magnitude enhancements in the radiative lifetimes over those obtained from the standard van Roosbroeck and Shockley expression, has been assumed. The possible Auger recombination mechanisms (CHCC, CHLH, and CHSH processes) in direct-gap semiconductors are investigated. In both n-type ternary alloys, the carrier lifetimes are similar, and competition between radiative and CHCC processes take place. In p-type materials, the carrier lifetimes are also comparable, however the most effective channels of Auger mechanism are: CHSH process in InGaAs, and CHLH process in HgCdTe. Next, the performance of heterostructure p-on-n photovoltaic devices are considered. Theoretically predicted R_oA values are compared with experimental data reported by other authors. In_0.53Ga_0.47As photodiodes have shown the device performance within a factor often of theoretical limit. However, the performance of InGaAs photodiodes decreases rapidly at intermediate wavelengths due to mismatch-induced defects. HgCdTe photodiodes maintain high performance close to the ultimate limit over a wider range of wavelengths. In this context technology of HgCdTe is considerably advanced since the same lattice parameter of this alloy is the same over wide composition range.     

Enhancement of the steady state minority carrier lifetime in HgCdTe photodiode using ECR plasma hydrogenation

This paper reports the effects of electron cyclotron resonance (ECR)H₂ plasma hydrogenation on the characteristics of HgCdTe devices for the first time. We compared the characteristics of photodiodes and n-channel enhancement type field effect transistors (FETs) in the hydrogenated regions with those in the unhydrogenated regions on the same wafer. From the measurement of the photodiodes, it was found that the steady-state minority carrier diffusion length was increased from 19 to 28 urn by the hydrogenation. The surface mobility of the n-channel FET was about 5800 cm²Vs and was not varied by hydrogénation. From these facts, the steady-state minority carrier lifetime is increased about two times by the ECR H₂ plasma hydrogenation. We believe that the ECR hydrogenation can effectively reduce the surface trap-states which results in increasing the minority carrier lifetime and improving the characteristics of HgCdTe devices.     

The Effect of Wet Etching on Surface Properties of HgCdTe

The surface of HgCdTe, grown by molecular-beam epitaxy and liquid-phase epitaxy, was studied by atomic force microscopy and x-ray photoelectron microscopy after etching in different solutions such as Br:methanol and HBr:H₂O₂:H₂O. Minority-carrier lifetime and surface recombination velocity were measured by photoelectron decay spectroscopy. The same measurements were repeated after exposure to air for periods from 2 h to 2 days. Although these surfaces are rather complicated, the main feature is that Br-based etchants produce elemental Te at the surface, which oxidizes rapidly in air. Without elemental Te, there is less Te oxide, even after longer exposure to air. The existence of elemental Te is correlated with higher surface recombination velocity. This can be explained in terms of band bending and band offsets at Te/HgCdTe and TeO₂/HgCdTe interfaces.     

光伏型长波HgCdTe红外探测器的数值模拟研究

报道了光伏型长波Hg1-xCdxTe(x=0.224)n-on-p红外探测器数值模拟研究的结果.采用二维模型,在背照射方式下,以p区厚度、两电极间距离、结区杂质浓度分布、少子寿命为参量,模拟计算了R0A积、零偏光电流与这些参量之间的变化关系.计算表明,随p区厚度增加,R0A积下降;R0A积随电极之间距离增大而增加,但量子效率随距离增大而降低,电极之间最佳距离大约为100μm;R0A积和光电流对结区杂质浓度分布形状不敏感;随少子(电子)寿命降低,R0A积和光电流下降.二维模型弥补了一维模型难以计算横向电流、电场的缺点.     

Modeling and Design Considerations of HgCdTe Infrared Photodiodes under Nonequilibrium Operation

The general approach and effects of nonequilibrium operation of Auger suppressed HgCdTe infrared detectors are well understood. However, the complex relationships of carrier generation and dependencies on nonuniform carrier profiles in the device prevent the development of simplistic analytical device models with acceptable accuracy. In this work, finite element methods are used to accurately model the devices, including self-consistent, steady-state solutions of Poisson’s equation and the carrier continuity equations for carrier densities, Boltzmann transport theory, and published models for recombination/generation processes in HgCdTe. Numerical simulations are used to optimize the material structure and doping levels for an Auger suppressed detector with λ _{c =} 5.5 μm at 200 K. The optimized detector structure with step doping and compositional profiles is then compared to a device with realistic gradient doping and compositional profiles.     

Minority carrier lifetime in p-HgCdTe

High operating temperature (HOT) detector concepts using midwave infrared (MWIR) (x∼0.3) p-type HgCdTe operating at temperatures within the thermoelectric cooler range are of significant interest at the present time. However, it is apparent that much work remains to be done in the areas of material, diode passivation, and diode formation technologies before the “holy grail” of photon detection at room temperature for all infrared wavelengths is achieved. Over the years, at DRS, we have developed a technology base for both n- and p-type HgCdTe materials parameters that are relevant to photodiode design and fabrication. This paper will discuss data that we have taken recently on minority carrier lifetime in MWIR and long wave infrared (LWIR) HgCdTe, particularly p type, and how it compares to current theories of Auger 7, radiative, and Shockley-Read recombination in this material. Extrinsic group IB (Cu, Au) and group V (arsenic) p-type dopants were used, together with group III (In) for n-type. The impact of the data on future HOT detector work is discussed.     

A discussion on the utilization of elemental transmutation for efficient p-type doping in HgCdTe and HgCdTeSe: Approach and methodology

For p-type doping of HgCdTe, As is preferred because of its relatively low diffusion coefficient, but it suffers from being amphoteric. In this paper, two possible techniques for incorporating As into HgCdTe that should ensure its presence only on nonmetal sites are presented and discussed. These methods are primarily based on the fact that Se can be readily incorporated into group VI sites and that ⁷⁵Se naturally decays into ⁷⁵As. Because the nuclear recoils associated with this decay are too small to displace arsenic atoms, substitutional p-doping should be ensured. In addition, a methodology for accurately determining the efficiency of these doping techniques and their influence on the electrical and optical properties of the material is presented.     

Transport properties of reactive-ion-etching-induced p-to-n type converted layers in HgCdTe

In this work, the effect of the reactive ion etching (RIE)-induced p-to-n type conversion process on the transport properties of HgCdTe is investigated. Magnetic-field-dependent differential Hall and resistivity measurements have been performed to determine the n-type doping profile and temperature-dependent transport properties of carriers resulting from the RIE-induced type conversion. The study examined Hg_1-xCd_xTe with x=0.23 and x=0.31 for both vacancy-doped and gold-doped p-type epilayers grown by liquid phase epitaxy (LPE) on lattice-matched CdZnTe, which were partly converted to n-type by a RIE process. Analysis using quantitative mobility spectrum analysis (QMSA) reveals that RIE type conversion results in a damaged surface layer characterized by moderate mobility electrons and a bulk n-type region exhibiting higher electron mobility. The surface and bulk electrons show a distinct temperature dependence. It can be observed that, generally, the concentration and mobility of the surface electrons are similar for all samples studied and are independent of temperature in the measured temperature range. In contrast, the bulk electrons exhibit classical behavior with characteristics that are strongly dependent on temperature and consistent with high-quality HgCdTe material. Differential Hall results indicate that the n-type dopants resulting from the RIE process are distributed into the sample via a diffusion process. The results suggest a p-to-n type conversion mechanism involving neutralization of the p-type dopants and diffusion of extrinsic n-type dopants from the surface.