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.     

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。     

Si基HgCdTe材料的电学特性研究

文章研究了Si基分子束外延HgCdTe原生材料、P型退火材料和N型退火材料的霍耳参数、少子寿命等材料电学特性。研究发现，晶格失配导致Si基HgCdTe材料原生材料和N型退火材料迁移率低；Si基原生HgCdTe材料属于高补偿材料，但高补偿性并非材料的固有特性，通过P型退火可使材料变为低补偿材料，迁移率得到提高。采用分子束外延方法制备的3in Si基HgCdTe材料电学性能与GaAs基HgCdTe材料相比，性能还有待提高。改进分子束外延生长工艺提高HgCdTe质量，从而进一步提高迁移率，是Si基外延研究的关键。     

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...     

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.     

Optical absorption properties of HgCdTe epilayers with uniform composition

Hg_1−xCd_xTe is an important material for high-performance infrared detection for a wide spectral range, from 1.7 μm to beyond 14 μm. An accurate understanding of the relationship between optical absorption and bandgap energy of this semiconductor alloy is needed for pre-process layer screening, detector design, and interpretation of detector performance. There is currently a disparity among the infrared detector community in relating the optical absorption properties to HgCdTe alloy composition and bandgap energy. This disagreement may stem from a misunderstanding of absorption properties, where existing models were developed decades ago using either bulk material or material with nonuniform composition. In this work, we have initiated an investigation of the optical absorption properties of HgCdTe with uniform composition grown by molecular-beam epitaxy (MBE) with in-situ compositional control via spectroscopic ellipsometry. The absorption properties show unique characteristics in the bandtail region, with insignificant temperature dependence. The absorption properties above the bandgap suggest a hyperbolic bandstructure as opposed to the common assumption of a parabolic bandstructure. The temperature dependence of the bandgap energy shows good agreement to the commonly used expression developed previously by Hansen et al.     

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.     

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.     

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。     

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.     

碲镉汞pn结中少数载流子寿命的测量

采用改进的光致开路电压衰退方法测量了不同组分Hg1-xCd,Te光伏探测器中光生少数载流子寿命。激发光源为脉冲YAG激光泵浦的光学参量产生器／放大器形成的可调谐红外脉冲激光器。为了减小结电容和串联电阻的影响,引入稳态背景光照明。用存储示波器记录pn结两端开路状态下的电压变化。从指数衰退的曲线经拟合得到开路电压的衰退时间常数,认为其反映了碲镉汞pn结中光生载流子的寿命。实验结果表明：四种组分的探测器在液氮温度下其少数载流子寿命值范围在18—407ns之间。     

Comparative Study of HgCdTe Etchants: An Electrical Characterization

A comparative study of wet etchants for both molecular beam epitaxy (MBE) and liquid phase epitaxy (LPE) grown n- and p-type samples was performed using capacitance–voltage (C–V) characteristics and surface recombination velocity (SRV) extracted from photoconductive decay (PCD) measurements. Different wet etchants were divided in two categories, (i) where bromine is a direct reagent in the etching solution and (ii) where bromine is a byproduct after reaction among different reagents. Negative shift of the flat-band voltages were observed for both n- and p-type samples treated with second category of etchants. A decrease in minority carrier lifetimes and an increase in the surface recombination velocities were also observed for the n-type samples treated with second category of etchants.     

Hydrogenation of HgCdTe epilayers on Si substrates using glow discharge plasma

Preliminary results of a study of the hydrogenation of HgCdTe epilayers grown by molecular beam epitaxy on Si substrates using a glow-discharge plasma are presented. The aim of the program is to employ H to passivate the detrimental opto-electronic effects of threading dislocations present in the HgCdTe epilayers. Secondary ion mass spectroscopy depth profiling has been performed to characterize ¹H and ²H incorporation. It has been found that H can be controllably incorporated in HgCdTe epilayers to levels in the 10¹⁴ cm⁻³ to 10¹⁸ cm⁻³ range while maintaining the sample at temperatures lower than 60°C. Profiles indicate that H accumulates in regions of known high defect density or in highly strained regions. Analysis of the H depth profile data indicates that the current density-time product is a good figure of merit to predict the H levels in the HgCdTe epilayer. There are progressive differences in the ¹H and ²H uptake efficiencies as a function of depth. Magneto-Hall measurements show consistently higher mobilities at low temperatures for majority carriers in hydrogenated samples.     

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.     

A detailed calculation of the auger lifetime in p-type HgCdTe

There has been recent experimental evidence that showed, in heavily doped p-type HgCdTe, the lifetime may be limited by the Auger 7 recombination mechanism. We have performed a detailed calculation of both the Auger 7 and Auger 1 lifetimes as a function of Cd composition (x), temperature (T), and doping (N_A). Compared with those done 20 years ago, the depth and breadth of these calculations result in a significant increase in the accuracy of the predictions. We present here the Auguer 7 lifetime for two different compositions, x=0.305 and x=0.226 over a range of temperature extending from 60 K to 300 K and for acceptor doping from 10¹⁵ cm⁻³ to 10¹⁸ cm⁻³. The calculated results for MWIR (x=0.305) are in reasonably good agreement with recent experiments performed on MWIR HgCdTe at 77 K over a range of doping. In addition, we calculated γ (≡ τ_A7 /τ_A1) with the same doping, composition (x ≈ 0.22), for a range of temperatures (40–80 K) and found γ=3–6.     

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.     

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.     

Determination of HgCdTe elasto-plastic properties using nanoindentation

Depth sensing indentation has been used to investigate the elasto-plastic behavior of Hg_0.7Cd_0.3Te prepared by molecular beam epitaxy, liquid phase epitaxy, as well as of bulk Hg_0.7Cd_0.3Te prepared by the modified Bridgman method. It was found that Hg_0.7Cd_0.3Te was characterized by a modulus of elasticity of E _avg ∼ 50 GPa and hardness of H _avg ∼ 0.66 GPa, independent of growth technology. The measured hardness was observed to increase with decreasing size of indentation owing to the nucleation of dislocations within the plastic zone. The elasto-plastic response of the samples to nanoindentation was observed to be purely elastic at low indentation depths and developed into ∼10% elastic and ∼90% plastic response, with an increase in penetration contact depth to above 100 nm exhibiting significant amounts of creep. The transition from purely elastic to elasto-plastic behavior has been observed to be marked by discontinuities, or “pop-in” events in the indenter load-penetration curves, with the indentation zone maximum sheer stress varying with HgCdTe growth method in the range 1.1–1.8 GPa. This onset and subsequent flow of plasticity is postulated to be associated with the spontaneous nucleation and propagation of dislocations.     

利用二次阳极氧化方法降低N型碲镉汞材料表面复合速度

利用微波反射光电导衰退法比较了采用一次阳极氧化和二次阳极氧化的N型碲镉汞材料的非平衡载流子寿命及其随温度的变化,通过与理论值进行比较拟合得到了碲镉汞材料表面复合速度随温度的变化曲线.结果发现,二次阳极氧化方法能够更好地降低材料表面悬挂键的密度,同时减少抛光引入的表面缺陷能级的数量,从而降低材料的表面复合速度,改善材料的非平衡载流子寿命,利于制造出高性能的HgCdTe红外探测器.     

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.