Compositional dependence of infrared phonon parameters for Hg1-xCdxTe

The full set of Lorentzian oscillator parameters describing the two-mode phonon behavior in Hg_1-x-jCd_xTe is reported. A new analysis of reflectivity spectra combined with existing results gives the most accurate available values for the CdTe-like and HgTe-like transverse optical frequency, strength and (for the first time) damping constant vs CdTe fractionx at room, liquid nitrogen and liquid helium temperatures. Polynomial fits vsx for each parameter are provided for use in characterizing Hg_1-xCd_xTe and the HgTe-CdTe superlattice.     

Growth and properties of Hg1-x Cdx Te epitaxial layers

The growth of epitaxial layers of mercury-cadmium-telluride (Hg_1-xCd_xTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd_1-yZ_yTe and Hg_1-xCd_xTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·10¹⁷ cm⁻³, hole mobility of about 300 cm²·V⁻¹ sec⁻¹ and excess minority carrier life-time of 3 nsec, at 77 K.     

Molecular beam epitaxy of CdTe and Hg1-xCdxTe ON GaAs (100)

Using the molecular beam epitaxial (MBE) technique, CdTe and Hg_1-xCd_xTe have been grown on Cr-doped GaAs (100) sub-strates. A single effusion cell charged with polycrystal-line CdTe is used for the growth of CdTe films. The CdTe films grown at 200 °C with a growth rate of ~ 2 μm/hr show both streaked and “Kikuchi” patterns, indicating single crystalline CdTe films are smoothly grown on the GaAs sub-strates. A sharp emission peak is observed at near band-edge (7865 Å, 1.577 eV) in the photoluminescence spectrum at 77 K. For the growth of Hg_1-xCd_xTe films, separate sources of HgTe, Cd and Te are used. Hg_0.6Cd_0.4Te films are grown at 50 °C with a growth rate of 1.7 μm/hr. The surfaces are mirror-smooth and the interfaces between the films and the substrates are very flat and smooth. As-grown Hg_0.6Cd_0.4Te films are p-type and converted into n-type by annealing in Hg pressure. Carrier concentration and Hall mobility of an annealed Hg_0.6Cd_0.4Te film are 1 × 10¹⁷ cm^?3 and 1000 cm²/V-sec at 77 K, respectively.     

Optically pumped LPE-grown Hg1−xCdxTe lasers

Liquid phase epitaxially-grown HgCdTe crystals, which were cooled to approximately 12 K and optically pumped with a Nd:YAG laser were found to lase continuously at 2.79 Μm. In addition, pulsed laser emission in several HgCdTe crystals was observed at 77 K in the wavelength range from 1. 25 to 2. 97 Μm using a O-switched Nd:YAG laser. This work was sponsored by the Department of the Air Force.     

Oxidation of Hg1−xCdxTe studied with surface sensitive techniques

We report the use of surface sensitive electron spectroscopies to monitor the initial steps of formation of a native oxide on atomically clean cleaved Hg_l-xCd_xTe single crystal surfaces in ultrahigh vacuum (UHV). Here the oxide is formed by oxygen excited by the presence of an operating ion gauge. During the reaction, the composition of the surface region of the substrate was found to change, with a net loss of Hg from the surface. Parallel studies of the compositions of thick anodic oxides using sputter profiling techniques showed only small amounts of Hg in the anodic oxides. On leave from Odense University, Denmark.     

N-Type Hg1−xCdxTe: Undoped x = 0.3 LPE material for sprite IR detectors

The requirement for two color Sprite detectors, with elements sensitive in the ranges 3-5 μn (MW) and 8-14 μn (LW) at 77K, is met using Hg_1−xCd_xTe elements of composition x = 0.3 and x = 0.2, respectively. The need for low defect levels for increased performance indicates the use of liquid phase epitaxy (LPE). While LW material is fairly well characterized, the growth and conversion to n-type of MW LPE has proved more difficult. Reported work shows limited data and limited success in converting MW LPE to n-type, and this primarily in donor-doped material. This paper describes the growth, annealing to n-type and characterization of Hg_0.7Cd_0.3Te. High n-type conversion yields were obtained, with low donor levels (mid-10¹³ to mid-10¹⁴ cm⁻³), high mobility (>10⁴ cm² (Vs)⁻¹) and long minority carrier lifetime (>10 us).     

Open-tube vapor transport epitaxy of Hg1−xCdxTe

A novel set-up for horizontal open-tube vapor transport epitaxy of Hg_1−xCd_xTe films is described. Mirror-like Hg_1−xCd_xTe epitaxial layers with thicknesses up to 40 Μm were grown and characterized. The growth temperature ranged from 380 to 550‡C, with growth rates of the order of 0.5–7 Μm per hour. The concentration depth profiles and the optical and electrical properties of relatively uniform films with x≈0.3–0.4 are reported. The process kinetics are studied. A simple model which takes into account the reactions occurring at the boundaries of the epitaxial layer and the interdiffusion in the epilayer is presented and discussed. The model fits the experimentally observed characteristics of the epitaxial growth process. A constant growth rate leading to a linear dependence of film thickness upon deposition time y–y_i=k_s t is derived. The reaction rate constant k is given by k_s=k_oe^−Ea^/kT with k_o=0.18 cm-sec⁻¹and the energy of activation E_a=1.12 eV.     

Slider LPE of Hg1-xCdxTe using mercury pressure controlled growth solutions

Homogeneous, nearly perfect single crystals of Hg_1-xCd_xTe are extremely difficult to prepare due primarily to the high vapor pressure of mercury. However, epitaxially grown Hg_1-xCd_xTe layers have a high potential for yielding material of a substantially higher quality. Using a new, open-tube, horizontal slider-type liquid phase epitaxial (LPE) growth technique, in which mercury pressure controlled growth solutions are used, a high degree of growth solution compositional control has been demonstrated. LPE layers of Hg_1-xCd_xTe have been grown on CdTe substrates and their high quality has been confirmed by optical, transport and electron microprobe measurements. Layer thicknesses are uniform and have been varied from 5 to 40 μ by changing the degree of supercooling or the growth time. An electron carrier concentration as low as 8.6 × 10¹⁵/cm³ and electron Hall mobilities up to 2.8 × 105 cm²/V-sec at 77K have been measured on in situ annealed samples. This work was sponsored by the Department of the Air Force and the U.S. Army Research Office.     

Minority carrier lifetime in doped and undoped epitaxially grown n-type CdxHg1−xTe

The performance of infrared detectors made from Cd_xHg_1-xTe (CMT) is related to the lifetime of minority carriers in the material. Both photoconductive and photovoltaic devices require long lifetimes for high performance. This paper compares lifetime measurements on epitaxially grown CMT layers which have been isothermally annealed to minimize the vacancy concentration and are n-type due to native defects, residual impurities, or deliberately added dopants. Layers grown by liquid phase epitaxy (LPE), metalorganic vapor phase epitaxy (MOVPE), and molecular beam epitaxy (MBE) have been considered, in all cases the same measurement system was used under the same low injection conditions. All layers were of compositions required for operation in the 8 to 14 μm wavelength range. Comparisons have been made between undoped and indium doped LPE layers and undoped and iodine doped MOVPE layers. It was hoped that a deliberately introduced donor impurity would give better control of the n-type properties. The effect of passivation on the measurement of lifetime has also been considered, the results showing that a surface with a native oxide is required to obtain the true bulk lifetime as has been seen previously for bulk material.     

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.     

Passivation of Hg1-xCdxTe by photochemical native oxidation: Quantitative analysis of oxide composition

The composition of photochemically grown native oxides on Hg_1-xCd_xTe (x = 0.3) has been analyzed and depth profiled using x-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. The oxide films were grown in either N₂O or O₂ ambients, and differences in the oxidation process were examined by varying the time and temperature of oxide growth. Under all growth conditions, oxides grown in an O₂ ambient exhibited a higher Hg concentration in the bulk oxide region when compared to N₂O grown oxides. The Hg/Te ratio of all the oxides was found to be less than the starting Hg_1-xCd_xTe substrates and, in some cases, this may be leading to an accumulation of Hg in the oxide/Hg_1-xCd_xTe interface region. For growths at higher temperatures (∼75°C), the excess Hg was seen to move from the oxide/Hg_1-xCd_xTe interface region to the oxide surface. In O₂ ambients, the Hg accumulated at the surface of the oxide whereas for growths in N₂O, it was lost to the ambient. Previous results on photochemical oxidation of Hg_1-xCd_xTe show an inverse relationship between oxide growth rate and temperature. Evidence obtained in this study from oxide compositions, depth profiles and annealing at higher temperatures, suggest that this relationship between oxide growth rate and temperature is primarily due to temperature induced differences in the oxidizing ambient, and not the result of a change in the film growth mechanism due to changing diffusion characteristics with temperature.     

碲镉汞红外探测器性能调控技术研究

对影响Hg_1-xCd_xTe红外探测器性能的不同调控技术——包括材料调控(组分及温度、掺杂浓度、压强及应力等对材料性能的调控)、器件结构调控(n-on-p、p-on-n、p-i-n、n-B-n等器件结构的调控)和工艺调控(各种工艺调控对材料制备和器件制备等的影响)等——进行了简单介绍,以合理调控器件性能、有效降低器件暗电流、提高器件工作温度等,从而促进Hg_1-xCd_xTe红外探测器在降低成本、减小功耗、提高可靠性等方面的发展。     

Transient behavior of Hg1−xCdxTe film growth on 3° off-(100) CdTe substrates by chemical vapor transport

The effects of substrate misorientation on Hg_1−xCd_xTe films, deposited on 3° off-(100) CdTe substrates by chemical vapor transport (CVT), have been studied for the first time using a transient growth technique. The morphological evolution of Hg_1−xCd_xTe films deposited on the vicinal CdTe substrates at 545°C shows a transition from three-dimensional islands to two-dimensional layer growth. The time and thickness required for the above morphological transition is about 0.75 h and 7 μm, respectively, under present experimental conditions. The pronounced long-range-terrace surface morphology of the Hg_1−xCd_xTe films illustrates the strong effects of the misorientation of the CdTe substrates and of the growth kinetics on the CVT growth of this hetero-epitaxial system. The transient behavior of the surface morphology, of the surface composition, and of the growth rate all reveal the influences of the 3° misorientation of the (100) CdTe substrates on the Hg_1−xCd_xTe epitaxy. The experimental mass flux results of the Hg_1−xCd_xTe-HgI₂ CVT system under transient and steady-state conditions can be related to the surface kinetics and to the thermodynamic properties of the system. The combined results show that the interface kinetics are not fixed in the transient regime and that they are coupled to the vapor mass transport.     

Thermodynamically self-consistent approximations to the liquidus and solidus of Hg1−xCdxTe

Based on liquidus and solidus temperatures and associated confidence limits reported by Brice, Capper, and Jones [J. Cryst. Growth, 75, 395 (1986)], we develop polynomial approximations to the liquidus and solidus curves for the pseudobinary material Hg_1−xCd_xTe. These approximations are “thermodynamically consistent” in that they satisfy the requirement that the liquidus and solidus temperatures must agree at x=0 and 1. A linear programming approach is used to find the lowest-degree polynomial approximations to the liquidus and solidus temperatures falling within the confidence limits of Brice et al. at each of their 21 compositions, and also satisfying the convexity requirement imposed by Brice et al. on their own manually-drawn curves. Finally, we present a twoparameter rational approximation that fits the tabulated segregation coefficients for Hg_1−xCd_xTe better than an earlier five-parameter polynomial approximation.     

Passivation of Hg1−xCdxTe by photochemical native oxidation: Growth characteristics

The growth of photochemical native oxides on x = 0.3 bulk wafers of n-type Hg_1−xCd_xTe has been studied with relation to growth temperature, gas flow rate, oxidizing ambient, and the wavelengths of ultraviolet radiation needed to drive the process. The growth was found to proceed with a relationship of thickness ∝ (time)^1/3 for both O₂ and N₂O ambients. An observed inverse relationship between thickness, and both growth temperature and gas flow rate, indicates a growth process that is either competing with parasitic quenching of the excited oxidizing species produced, or a diffusion controlled process, in which different growth conditions produce oxides with a variety of diffusion characteristics. The physical and optical properties of the photochemical oxides were found to be similar to anodic oxides. Rutherford backscattering spectra have shown the composition of the films to be highly dependent on growth conditions with films grown in O₂ ambients generally showing a higher mercury content than those grown in N₂O. Further, for growths in O₂ atmospheres, those carried out at lower temperatures result in more mercury in the oxide and at the oxide/mercury cadmium telluride interface when compared to those grown at higher temperatures.     

Temperature dependence of the optical properties of Hg1−xCdxTe

The alloy composition of Hg_1−xCd_xTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg_1−xCd_xTe by spectroscopic ellipsometry (SE). The spectral data of Hg_1−xCd_xTe with x = 1,0.235, and 0.344 were obtained from room temperature to 800Kin the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E₀, E₀+Δ₀, E₁, E₁+Δ₁, E₂(X), and E₂(Σ). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.     

碲镉汞量子点的离子交换能带调控及其近红外自吸收性质

通过软化学方法制备单分散的CdTe量子点,调节Hg²⁺的浓度离子交换实现从可见到近红外光谱准连续可调的碲镉汞(Hg_xCd_1-xTe)量子点制备。深入分析了近红外Hg_0.33Cd_0.67Te量子点的变温光致发光及其自吸收特性,研究结果表明：碲镉汞量子点的荧光强度随着温度的升高（0～100℃）呈线性降低趋势,谱线展宽,峰位发生红移（12nm）;量子点的吸收和发射光谱部分重叠导致自吸收效应随着量子点的浓度增加而增强,导致量子点荧光强度的降低。     

Characterization of CdTe/Hg1−xCdxTe heterostructures by high-resolution x-ray diffraction

CdTe/Hg_1−xCd_xTe heterostructures variously heat-treated and implanted with boron were studied by means of high-resolution x-ray diffraction. A novel procedure for simulating diffraction spectra was developed, which is based on direct summation of scattered waves across a heterostructure. In that routine, short-range variations of structural parameters, including concentration of the components, can be taken into account. The new approach allows precise characterization of II–VI heterostructures, because it correctly treats atomic diffusion effects in diffraction spectra. As a result, subtle spectral modifications induced by boron implantation could be detected and were attributed to the dynamics of post implantation point defects.     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.