Valence band offset in HgTe/Hg1−xCdxTe superlattices

The valence band offset (Λ) between HgTe and CdTe has been determined by means of an optical investigation of (112)B oriented HgTe/Hg_1−xCd_xTe superlattices. Based on the fact that the difference in energy between the first heavy hole and the first light hole subband is to a good approximation due primarily to Λ, it has been shown that Λ=580±40 meV at 5K. In addition Λ has a significant temperature dependence with a linear coefficient of −0.34±0.02 meV/K, i.e., Λ is 480±40 meV at room temperature.     

Development of Molecular Beam Epitaxially Grown Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te for High-Density Vertically-Integrated Photodiode-Based Focal Plane Arrays

Hg_1−x Cd _x Te samples of x ~ 0.3 (in the midwave infrared, or MWIR, spectral band) were prepared by molecular beam epitaxy (MBE) for fabrication into 30-μm-pitch, 256 × 256, front-side-illuminated, high-density vertically-integrated photodiode (HDVIP) focal plane arrays (FPAs). These MBE Hg_1−x Cd _x Te samples were grown on CdZnTe(211) substrates prepared in this laboratory; they were ~10-μm thick and were doped with indium to ~5 × 10¹⁴ cm⁻³. Standard HDVIP process flow was employed for array fabrication. Excellent array performance data were obtained from these MWIR arrays with MBE HgCdTe material. The noise-equivalent differential flux (NEΔΦ) operability of the best array is 99.76%, comparable to the best array obtained from liquid-phase epitaxy (LPE) material prepared in this laboratory.     

An improved method for Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te surface chemistry characterization

Hg_1−xCd_xTe surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg_1−xCd_xTe surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-μm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg_1−xCd_xTe processing steps including photolithography, bromine etching, and photoresist stripping.     

Effect of Hydrogen Free Radicals on Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The effects of atomic hydrogen (H) and Br/methanol etching on Hg_1−x Cd _x Te films were investigated using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Exposure of an as-received Hg_1−x Cd _x Te sample to H + H₂ resulted in H-induced TeO₂ reduction. The oxide reduction was first order with respect to H + H₂ exposure. Exposure to H + H₂ after etching the Hg_1−x Cd _x Te film in a Br/methanol solution induced Hg and C depletion. Hg and C removal was also observed after completely reducing the TeO₂ on the as-received sample. The removal process was hindered by the formation of a Cd-rich overlayer on both etched and unetched surfaces.     

HgCdTe Research at FFI: Molecular Beam Epitaxy Growth and Characterization

This paper presents results from recent work on molecular beam epitaxy growth of HgCdTe at the Norwegian Defence Research Establishment (FFI), including studies of material properties and fabrication of photodiodes and nanostructures. Systematic studies of defect morphology in HgTe and Hg_1−x Cd _x Te have revealed that there is a minimum in the area covered by defects just below the onset of Te precipitation. The shape and density of microvoids in HgTe can be used to determine the deviation from the optimal growth temperature. This can be further related to the optimal growth temperature of Hg_1−x Cd _x Te with any Cd mole fraction by thermodynamic calculations. A mechanism for the formation of microvoids and needles has been presented. Photoluminescence (PL) has been used to study layers without doping and with Hg vacancy, Ag, and In doping. Planar photodiodes with high dynamic resistance and good quantum efficiency were fabricated by ion-milling vacancy-doped mid-wave and long-wave infrared layers. Quantum wells (QWs) with good crystallinity and high PL light output have been grown. Surface patterning has been found to enhance light emission from HgCdTe thin-film and QW samples by ∼30%. Single-crystal HgTe and segmented HgTe/Te nanowires have been grown, and the resistivity of the nanowires has been measured by conductive atomic force microscopy (AFM), where the AFM tip has been used as a mobile electrode.     

Effect of successive implantation of Ag+(Cu+) and Xe+ ions on the recombination properties of CdxHg1−x Te crystals

The effect of successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions on the recombination properties of Cd_xHg_1−x Te (0.2<x<0.3) crystals has been investigated. It is shown that after implantation of ions of one chemical element, followed by diffusion thermal annealing at temperatures below 150–200 K, recombination through local levels lying 30±5 meV below the conduction band bottom dominates. Successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions followed by diffusion thermal annealing changes the course of the temperature dependence of the lifetime of the nonequilibrium charge carriers. It was determined that for Cd_xHg_1−x Te crystals with x⋍0.20–0.25 in the temperature interval 700–200 K the lifetime of the nonequilibrium charge carriers is low (τ<0.15 μs) and does not depend on the temperature. For Cd_xHg_1−x Te crystals with x⋍0.3 recombination of nonequilibrium charge carriers occurs through two types of levels: in the temperature range 140–200 K — deep levels E _t1⋍E _c −51 meV and at lower temperatures (77–140 K) — through shallower levels E _t2⋍E _c −(16±2) meV. Fiz. Tekh. Poluprovodn. 31, 786–789 (July 1997)     

Growth of HgTe Quantum Wells for IR to THz Detectors

We zone-engineered HgCdTe/HgTe/HgCdTe quantum wells (QWs) using the molecular-beam epitaxy (MBE) method with in situ high-precision ellipsometric control of composition and thickness. The variations of ellipsometric parameters in the ψ–Δ plane were represented by smooth broken curves during HgTe QW growth with abrupt composition changes. The form of the spiral fragments and their extensions from fracture to fracture revealed the growing layer composition and its thickness. Single and multiple (up to 30) Cd _x Hg_1−x Te/HgTe/Cd _x Hg_1−x Te QWs with abrupt changes of composition were grown reproducibly on (013) GaAs substrates. HgTe thickness was in the range of 16 nm to 22 nm, with the central portion of Cd _x Hg_1−x Te spacers doped by In to a concentration of 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³. Based on this research, high-quality (013)-grown HgTe QW structures can be used for all-electric detection of radiation ellipticity in a wide spectral range, from far-infrared (terahertz radiation) to mid-infrared wavelengths. Detection was demonstrated for various low-power continuous-wave (CW) lasers and high-power THz pulsed laser systems.     

Experimental determination of tie-lines in the Hg-Cd-Te system

Starting with powdered Hg_1−xCd_xTe, several tie-lines at 500 and 560°C were established using an energy dispersive spectrometer on a scanning electron microscope for the quantitative analysis. After holding at 500 or 560°C for time periods based upon the powder size and the published interdiffusion constant, then water quenching to room temperature, the primary grains were found to be uniform in composition and covered with a 5-6 μ layer of HgTe or low x Hg_1−xCd_xTe. The primary grain and overall compositions establish directions for tie-lines that are in good agreement with published experimental and theoreti-cal results.     

Far-infrared detector based on HgTe/HgCdTe superlattices

HgTe/Hg_0.05Cd_0.95Te superlattices (SLs) were grown on (112)B oriented Cd_0.96Zn_0.04 Te substrates using molecular beam epitaxy (MBE). The SLs, consisting of 100 periods of 80-Å-thick HgTe wells alternating with 77-Å-thick Hg_0.05Cd_0.95Te barriers, were designed to operate as detectors in the far-infrared (FIR) region. Infrared absorption spectroscopy, high-resolution transmission electron microscopy (TEM), Hall effect measurements, and x-ray diffraction were used to characterize the superlattice layers. A series of annealing experiments were initiated to quantify the temperature-dependent interdiffusion of the HgTe wells and Hg_0.05Cd_0.95Te barriers and consequently their degradation, which shifts the absorption edges of the SLs to higher energies, since a high-temperature ex situ anneal is normally required in order to produce the p-type material required for a photovoltaic detector. Results from infrared absorption spectroscopy, TEM, and Hall effect measurements for the annealed samples are presented. A FIR SLs single-element photoconductive (PC) device was designed and fabricated. Both material characterization and device testing have established the applicability of the HgTe/Hg_0.05Cd_0.95Te SLs for the FIR region.     

Experimental study of non-stoichiometry in Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te<Subscript>1±δ</Subscript>

In this communication, the latest experimental results are presented on a composition of Cd_1−xZn_xTe_1±δ, both x and δ, determined by the high-precision, vapor-pressure scanning method. The space arrangement of the single-phase solidus volume of Cd_1−xZn_xTe_1±δ in the pressure-temperature-composition (P-T-X) phase space has been derived from direct vapor-pressure measurements in the temperature range up to 1,375 K. From the experimental data for x=0.0, 0.05, 0.1, 0.15, 0.25, 0.5, 0.75, 0.8, 0.9, and 1, the complete phase diagram of Cd_1−xZn_xTe_1±δ was constructed. Transformation of the diagram was traced for the whole range of existence of the Cd_1−xZn_xTe_1±δ solid solution as a function of ZnTe concentration. Increase of ZnTe in the solid solution results in the extension of the homogeneity range and a shift of the solidus toward Te, so that already for x≥0.15 the solidus does not contain the stoichiometric, (n_Cd+n_Zn):n_Te=50 mol.% plane. From detailed experimental studies, maximum non-stoichiometry as a function of the temperature was determined in a wide temperature range for Te and metal solubility. Temperature dependencies of the partial pressures of vapor-phase species for different crystal compositions, X_S=const, were deduced from the vapor-pressure experiment for the x=0.05 section of the diagram. From these results, relationships are derived between crystal composition, vapor pressure, and composition of the conjugated vapor.     

Accurate Determination of the Matrix Composition Profile of Hg<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te by Secondary Ion Mass Spectrometry

In this report, we present a new secondary ion mass spectrometry (SIMS) analysis technique to provide accurate Cd composition profiles based on the measurement of HgCs⁺ and CdCs⁺ cluster ions. Study of Hg_1–x Cd _x Te samples with different x values shows that x/(1 − x) is linearly proportional to HgCs⁺/CdCs⁺ over the range of x = 0.2 to x = 0.9. This technique allows us to obtain an accurate Cd profile for a multilayer HgCdTe sample with different x values for each layer using a single standard with known x value. (Received 10/15/06; accepted 2/14/07)     

Growth method,composition, and defect structure dependence of mercury diffusion in CdxHg1–xTe

Mercury radiotracer diffusion results are presented, in the range 254 to 452°C, for bulk and epitaxial Cd_xHg_1–xTe, and we believe this to be the first report for metalorganic vapor phase epitaxy (MOVPE) grown Cd_xHg_1–xTe. For all growth types studied, with compositions of x_Cd=0.2±0.04, the variation of the lattice diffusion coefficient, D_Hg, with temperature, under saturated mercury partial pressure, obeyed the equation: D_Hg=3×10⁻³ exp(−1.2 eV/kT) cm² s⁻¹. It was found to have a strong composition dependence but was insensitive to changes of substrate material or crystal orientation. Autoradiography was used to show that mercury also exploited defect structure to diffuse rapidly from the surface. Dislocation diffusion analysis is used to model defect tails in MOVPE Cd_xHg_1–xTe profiles.     

The temperature-composition phase diagram and the miscibility gap of Hg1-xCdxTe solid solutions by dynamic mass-loss measurements

The dynamic mass-loss technique has been employed to measure Hg partial pressures over Te-saturated Hg_1-xCd_xTe solid solutions with x = 0.40, 0.54, and 0.70 in the 10^-1 to 10^-4 atm range. The relative chemical potentials of HgTe in Hg_1-xCd_xTe solid solutions have been calculated using the measured Hg partial pressures at temperatures below 413°C, and fitted into an analytical expression. A Gibbs-Duhem integration yielded the relative chemical potentials of CdTe. By combining the relative chemical potentials of the binary components HgTe and CdTe, an expression for the Gibbs free energy of mixing was derived. The binodal (miscibility gap) and spinodal curves of the Hg_1-xCd_xTe solid solutions have been established with the critical temperature and composition of 221°C and Hg_0.40Cd_0.60Te.     

Investigations into the source of 1/f noise in Hg<Subscript>x</Subscript>Cd<Subscript>1−x</Subscript>Te diodes

This work is aimed at elucidating the mechanism of 1/f noise in Hg_xCd_1−xTe diodes in order to reduce 1/f noise in high-performance thermal cameras. Currently, to reach the required signal-to-noise ratio, the operating temperature of thermal cameras is 77 K. If 1/f noise could be reduced, this operating temperature could be raised without reducing the signal-to-noise ratio, allowing cheaper, smaller, and lighter cooling systems to be used. There are still some fundamental aspects of 1/f noise that are not fully understood. One aspect in particular is whether the noise is caused by mobility or number density fluctuations because both could give the conductivity fluctuations observed. This paper explores the noise coupling and current coupling between diodes on an array. It is shown that 1/f noise is present on the photons emitted from a forward-biased diode, suggesting that the rate of radiative recombination fluctuates. The results suggest that these fluctuations are due to carrier number-density fluctuations. From correlation measurements, it is shown that at least 32% of the 1/f noise in the forward-biased diode is correlated with carrier density fluctuations in Hg_xCd_1−xTe diodes.     

Terahertz emission from CdHgTe/HgTe quantum wells with an inverted band structure

The terahertz electroluminescence from Cd_0.7Hg_0.3Te/HgTe quantum wells with an inverted band structure in lateral electric fields is experimentally detected and studied. The emission-spectrum maximum for wells 6.5 and 7 nm wide is near 6 meV which corresponds to interband optical transitions. The emission is explained by state depletion in the valence band and conduction band filling due to Zener tunneling, which is confirmed by power-law current–voltage characteristics.     

Transient behavior of Hg1−xCdxTe films deposited on (100) CdTe substrates by chemical vapor transport

The growth history of Hg_1−xCd_xTe films deposited on (100) CdTe substrates by chemical vapor transport (CVT) has been studied, for the first time, by using a transient growth technique. The observed morphological evolution of Hg_1−xCd_xTe films deposited at 545°C shows a transition behavior from three-dimensional (3D) islands to two-dimensional (2D) layer growth. The experimental results indicate that the so-called critical time needed for the above morphological transition is about lh under present experimental conditions. Based on the chemical bonding properties of Hg_1−xCd_xTe, and on the behavior of the morphological transition, the Stranski-Krastanov growth mode is suggested for the epitaxial growth system. The time dependence of the growth thickness, of the growth rate (R₁₀₀) along the [100] direction, and of the surface composition all reveal a transient behavior. These are related to the nature of the Hg_1-xCd_xTe/ (100)CdTe heterojunction and to the surface reactions. Comparison of the growth rates and of the total mass deposited as a function of time shows the relationship between epitaxial growth and mass flux of the Hg_1−xCd_xTe-HgI₂ chemical vapor transport system.     

Thermoelectric Characteristics in MBE-Grown HgCdTe-Based Superlattices

We present a study on the thermoelectric properties of n-type Hg_0.75Cd_0.25 Te/Hg_0.7Cd_0.3Te superlattices (SLs). This material system was chosen because HgCdTe is the primary material used in high-performance infrared imaging applications. HgCdTe-based devices can be directly grown on Hg_1−x Cd _x Te/Hg_1−y Cd _y Te SL coolers using advanced growth methods such as molecular-beam epitaxy (MBE), making the monolithic integration of infrared sensors and thermoelectric elements possible. Also, the thermoelectric figure of merit ZT for Hg_0.75Cd_0.25Te/Hg_0.7Cd_0.3Te SLs is predicted to reach values of 2.09, more than two times greater than that achieved in the best thermoelectric devices based on bulk Bi₂Te₃. This large ZT is due to the unique and superior electrical and thermal properties of the HgCdTe system, which has not yet been experimentally explored in any great depth as a thermoelectric material. We used a Riber 32P MBE system equipped with a Hg valved cell, reflection high-energy electron diffraction, infrared pyrometer and in situ spectroscopic ellipsometry to grow the thermoelectric structures. MBE was chosen as a growth technique since it allows for the lowest growth temperature compared with other methods, which limits interdiffusion at the interfaces, thereby allowing for a precise control over electrical and thermal properties. Thermal devices were fabricated using standard photolithography and etching techniques. Thermal properties were evaluated using a differential technique. A thermal conductivity of 0.82 ± 0.07 W/m K and a Seebeck coefficient of 811 ± 150 μV/K were measured. Using a measured value of 0.017 Ω cm for the resistivity, an upper bound ZT of 1.4 is estimated. An erratum to this article can be found at     

Metal-Organic Chemical Vapor Deposition of Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Fully Doped Heterostructures Without Postgrowth Anneal for Uncooled MWIR and LWIR Detectors

Uncertainty in precursor pulse delays and shapes has been found to be an important factor in Hg_1−x Cd _x Te metal-organic chemical vapor deposition (MOCVD) growth using the interdiffused multilayer process (IMP). Herein, metal-organic concentration changes in the growth zone are examined using an in␣situ infrared (IR) absorption gas monitoring system, and modifications to the interdiffused multilayer process are applied for in␣situ control of stoichiometry, improved morphology, minimized process length, and consumption of precursors. Dimethylcadmium (DMCd) introduction during IMP flush stages in HgTe was used for stoichiometry control. The final stage of heterostructure formation was optimized to prevent Hg outdiffusion. As a result, vacancy concentration was reduced far below the equilibrium level at the growth conditions so the background of n-type doping was revealed. Acceptor doping with arsine (AsH₃) and trisdimethylaminoarsenic (TDMAAs) was examined over a wide range of compositions, and doping levels of 5 × 10¹⁵ cm⁻³ to 5 × 10¹⁷ cm⁻³ were obtained. The presence of both arsenic dopants significantly increased the CdTe growth rate. This caused an increase of Cd mole fraction in the grown material. Doped heterostructures can be grown without any postgrowth anneal and used for mid- and long-wavelength infrared (MWIR and LWIR) devices operating at near-ambient temperatures. Student paper; supervisors are A. Rogalski, J. Piotrowski and J. Szmidt     

Band gap of CdTe and Cd<Subscript>0.9</Subscript>Zn<Subscript>0.1</Subscript>Te crystals

The band gap E _g of the CdTe and Cd_0.9Zn_0.1Te crystals and its temperature dependence are determined by optical methods. This is motivated by considerable contradictoriness of the published data, which hampers the interpretation and calculation of characteristics of detectors of X-ray and γ radiation based on these materials (E _g = 1.39–1.54 and 1.51–1.6 eV for CdTe and Cd_0.9Zn_0.1Te, respectively). The used procedure of determination of E _g is analyzed from the viewpoint of the influence of the factors leading to inaccuracies in determination of its value. The measurements are performed for well-purified high-quality samples. The acquired data for CdTe (E _g = 1.47–1.48 eV) and Cd_0.9Zn_0.1Te (E _g = 1.52–1.53 eV) at room temperature substantially narrow the range of accurate determination of E _g.     

Potential of using the Cd<Subscript>0.8</Subscript>Hg<Subscript>0.2</Subscript>Te alloy in solar cells

Surface-barrier diodes based on the Cd_xHg_1?x Te alloy (x ～ 0.8) sensitive in the wavelength range 0.3–1.1 μm, which were obtained by etching (bombardment) of the surface of the p-type crystal with argon ions, are studied. Using the measured spectral absorption and reflection curves, as well as the parameters of the diode structure, which were found from electrical characteristics, the spectra of photoelectric quantum efficiency of diodes are calculated. The results of the calculation of photoelectric parameters of the Gd_0.8Hg_0.2Te-based diodes are given in comparison with the CdTe-based and Si-based solar cells. For the AM1.5 solar irradiation conditions, the open-circuit voltage and short-circuit current, as well as values of limiting efficiency, are determined.