CdTe Mole Fraction Research Articles

Methodological and instrumental problems in high-precision in situ ellipsometry diagnostics of the mercury cadmium telluride layer composition in molecular beam epitaxy

Problems of high-precision in situ ellipsometry diagnostics of the composition of a mercury cadmium telluride (MCT) solid solution in the process of its growth using the molecular beam epitaxy are considered. The required precision was estimated for ellipsometry measurements aimed at determining the MCT composition with a permissible dispersion of ±0.003 mole fraction of CdTe. It has been revealed that for ellipsometers based on the static photometric scheme the instability of measurements is mainly caused by a random change in the directivity of the laser radiation. In combination with polarization nonuniformity over the area of the optical-section elements, this results in continuous drift of measured ellipsometric parameters. Based on these investigations, a high-stability laser ellipsometer has been designed. When used to monitor the in situ MCT layer growth by the molecular beam epitaxy, it allowed a decrease in the dispersion of the MCT composition by an order of magnitude from experiment to experiment and its precision to be maintained at a level of ±0.003 mole fractions of CdTe.

Hole Transport in Arsenic-Doped Hg1−x Cd x Te with x ≥ 0.5

Hole transport in arsenic-doped p-type Hg1-xCdxTe epitaxial layers with x ≥ 0.5 has been studied employing Hall-effect measurements and theoretical modeling of hole scattering mechanisms. The hole transport parameters extracted from four different Hg1-xCdxTe films with x = 0.50, 0.56–0.58, 0.65, and 0.80, were analyzed using an iterative solution of Boltzmann’s transport equation. Hole mobilities in the samples with x values of 0.5 and 0.56–0.58 were found to be predominantly limited by ionized impurity scattering, and exhibited relatively high impurity compensation ratios ≥2. The sample with x = 0.65 exhibited the highest hole mobility, a low compensation ratio of 1.05, and mobility characteristics were limited predominantly by polar optical phonon scattering at temperatures ≥200 K. Hole mobility in the sample with x = 0.80 was found to be limited by polar optical phonon scattering and ionized impurity scattering (compensation ratio 1.20–1.56). Although the sample temperatures employed were not sufficiently low to unambiguously discriminate the scattering strength of static strain and dislocations, the experimental hole mobility characteristics cannot be adequately modeled if these two mechanisms are neglected. The ionization energy of the arsenic acceptor impurities was found to exhibit a quadratic dependence on the CdTe mole fraction.

Thermophysical properties of HgTe and Hg0.9Cd0.1Te melts

Thermophysical properties, namely, density, viscosity, and electrical conductivity of HgTe and Hg0.9Cd0.1Te melts were measured as a function of temperature. A pycnometric method was used to measure the melt density in the temperature range of 948 to 1073K for the HgTe melt and 1011 to 1131K for the Hg0.9Cd0.1Te melt. The density results show a maximum at 1023 and 1020K, respectively, for the HgTe and Hg0.9Cd0.1Te melts. The viscosity and electrical conductivity were simultaneously determined using a transient torque method from 944 to 1098K for the HgTe melt and from 1016 to 1127K for the Hg0.9Cd0.1Te melt. The measured electrical conductivity decreases as the mole fraction of CdTe, x, increases at a specific temperature, and increases as a function of temperature, which shows a semiconductor-like behavior. The measured viscosity decreases as the mole fraction of CdTe, x, increases at a specific temperature and decreases as the temperature increased. The analysis of the electrical conductivity of the melts and the relationship between the kinematic viscosity and density implied a structural transition in these melts. No relaxation phenomena were observed in the density, electrical conductivity, and viscosity of the HgTe and Hg0.9Cd0.1Te melts.

Linear HgCdTe IR FPA 288×4 with bidirectional scanning

The long wavelength (8–12 μm) IR FPA 288×4 based on a hybrid assembly of n+-p diode photosensitive arrays (PA) of HgCdTe (MCT) MBE-grown structures and time delay integration (TDI) readout integrated circuits (ROIC) with bidirectional scanning have been developed, fabricated, and investigated. The p-type MCT structures were obtained by thermal annealing of as-grown n-type material in inert atmosphere. The MCT photosensitive layer with the composition 0.20–0.23 of mole fraction of CdTe was surrounded by the wide gap layers to decrease the recombination rate and surface leakage current. The diode arrays were fabricated by planar implantation of boron ions into p-MCT. The typical dark currents were about 4–7 nA at the reverse bias voltage of 150 mV. The differential resistance R was up to R0 = 1.6×107 Ω zero bias voltage, which corresponded to R0A ∼70 Ω ·cm2 and to the maximal value Rmax = 2.1 × 108 Ω. The bidirectional TDI deselecting ROIC was developed and fabricated by 1.0-μm CMOS technology with two metallic and two polysilicon layers.

Thermodynamic description of the Hg–Te binary system

The (Hg,Cd)Te is a traditional material for infrared optoelectronic devices. It is possible to tune its energy gap practically to any infrared wavelength by varying CdTe mole fraction. The (Hg,Cd) Te is using as an epitaxial layers which have been grown on CdTe substrate from pseudobinary Hg-rich and Te-rich solutions by PLE. The knowledge of the phase diagram of Hg–Cd–Te system as well as of the Cd–Hg, Hg–Te and Cd–Te systems is essential for understanding and planning technological processes. In this work the binary system Hg–Te has been modeled using ionic liquid, associate liquid and subregular models. Results of calculations were compared and discussed.

Linear HgCdTe IR FPA 288×4 with bidirectional scanning

AbstractThe long wavelength (8–12 μm) IR FPA 288×4 based on a hybrid assembly of n+-p diode photosensitive arrays (PA) of HgCdTe (MCT) MBE-grown structures and time delay integration (TDI) readout integrated circuits (ROIC) with bidirectional scanning have been developed, fabricated, and investigated. The p-type MCT structures were obtained by thermal annealing of as-grown n-type material in inert atmosphere. The MCT photosensitive layer with the composition 0.20–0.23 of mole fraction of CdTe was surrounded by the wide gap layers to decrease the recombination rate and surface leakage current. The diode arrays were fabricated by planar implantation of boron ions into p-MCT. The typical dark currents were about 4–7 nA at the reverse bias voltage of 150 mV. The differential resistance R was up to R0 = 1.6×107 Ω zero bias voltage, which corresponded to R0A ∼70 Ω ·cm2 and to the maximal value Rmax = 2.1 × 108 Ω. The bidirectional TDI deselecting ROIC was developed and fabricated by 1.0-μm CMOS technology with two metallic and two polysilicon layers.The IR FPAs were free of defect channels and have the average values of responsivity Sλ = 2.27×108 V/W, the detectivity Dλ * = 2.13 × 1011 cm × Hz1/2 × Wt1, and the noise equivalent temperature difference NETD = 9 mK.

Low-temperature annealing of (Hg,Cd)Te

Many methods for the employment of (Hg,Cd)Te alloys employ anneals at temperatures <300°C to convert the p type left over from the growth process or to adjust the concentration of the native acceptors. An investigation of the kinetics of this annealing process has been performed as functions of (1) vacancy concentration; (2) composition, or the CdTe mole fraction in the alloy; and (3) temperature. If these anneals are carried out under mercury-saturated conditions, the tellurium precipitates in the material, which result either from the growth process or from specific thermal histories, are annihilated by in-diffusing mercury, which results in a significant multiplication of dislocations. This interface, delineated by defect etching, has been employed to investigate the kinetics of the annealing process. These results will be unaffected by the uncertainties introduced in determining this interface by electrical measurements, which arise from incomplete ionizations of the metal vacancies at 77 K for Hg1−xCdxTe with CdTe mole fractions exceeding ∼0.26. The annealing rate appears to be strongly dependent on both the temperature and composition of the alloy, decreasing with increasing CdTe mole fraction, within 0.15 and 0.5, with the behavior resembling a composition-dependent, activated-diffusion process. The depth of the interface appears to vary inversely as the root of the metal-vacancy concentration.

Suppression of strain-induced cross-hatch on molecular beam epitaxy (211)B HgCdTe

Because the performance of HgCdTe-based photodiodes can be significantly degraded by the presence of dislocations, we have systematically investigated and suppressed lattice-mismatch-induced cross-hatch formation and the associated generation of dislocations in (211)B HgCdTe/CdZnTe. A series of HgCdTe epilayers were deposited simultaneously on pairs of substrates with differing ZnTe mole fractions. Epilayers' CdTe mole fraction and substrates' ZnTe mole fractions were measured using optical-transmission spectra. Lattice mismatch and residual strain were estimated from room-temperature, x-ray diffraction, and double-crystal rocking-curve measurements (DCRC). It was found that cross-hatch patterns were suppressed in epilayers deposited on nearly lattice-matched substrates (|Δa/asub| < 0.02%). Such epilayers exhibited excellent crystalline quality as revealed by defect-decoration etching (etch-pit density (EPD) < 105 cm-2) and x-ray diffraction (full-width at half-maximum (FWHM) ∼ 10 arcsec). In addition to determining the upper limits of lattice mismatch needed to eliminate cross-hatch, we investigated the use of reticulated substrates as a means to suppress cross-hatch. We found that growth on reticulated mesa structures (<100 µm)with edges parallel to [01-1] resulted in epilayers with substantially reduced cross-hatch-line densities despite large lattice mismatch (Δa/asub < 0.04%). The use of reticulated substrates could suppress cross-hatch because of lateral-alloy variation in large substrates and complex multistack epilayers (e.g., multicolor detectors).

Molecular-beam epitaxial growth and high-temperature performance of HgCdTe midwave infrared detectors

Results are reported on the molecular-beam epitaxial (MBE) growth and electrical performance of HgCdTe midwave-infrared (MWIR) detector structures. These devices are designed for operation in the 140–160 K temperature range with cutoff wavelengths ranging from 3.4–3.8 µm at 140 K. Epitaxial structures, grown at 185°C on (211)B-oriented CdZnTe substrates, consisting of either conventional two-layer P-n configurations or three-layer P-n-N configurations, were designed to examine the impact of device performance on variation of the n-type base layer (absorber) thickness and the inclusion or omission of an underlying wide-bandgap buffer layer. Devices were grown with absorber thicknesses of 3 µm, 5 µm, and 7 µm to examine the tradeoff between the spectral response characteristic and the reverse-bias electrical performance. In addition, 5-µm-thick, wide-bandgap HgCdTe buffer layers, whose CdTe mole fraction was approximately 0.1 larger than the absorber layer, were introduced into several device structures to study the effect of isolating the device absorbing layer from the substrate/growth initiation interface. The MBE-grown epitaxial wafers were processed into passivated, mesa-type, discrete device structures and diode mini arrays, which were tested for temperature-dependent R0A product, quantum efficiency, spectral response, and the I-V characteristic at temperatures close to 140 K. External quantum efficiencies of 75–79% were obtained with lateral optical-collection lengths of 7 µm. Analysis of the temperature dependence of the diode R0A product indicates that the device impedance is limited by the diffusion current at temperatures above 140 K with typical R0A values of 2×106 Ω cm2 for a detector cutoff of 3.8 µm at 140 K. An alloy composition anomaly at the absorbing-layer/buffer-layer interface is believed to limit the observed R0A products to values approximately one order of magnitude below the theoretical limit projected for radiatively limited carrier lifetime. Device electrical performance was observed to be improved through incorporation of a wide-bandgap buffer layer and through reduction of the absorbing layer thickness. An optimum spectral response characteristic was observed for device structures with 5-µm-thick absorbing layers.

Electrical properties of in situ As doped Hg1−xCdxTe epilayers grown by molecular beam epitaxy

The electrical properties of extrinsic in situ doped mercury cadmium telluride (Hg1−xCdxTe) epilayers grown by molecular beam epitaxy on (211)B CdTe/Si and CdZnTe substrates are studied. The doping is performed with an elemental arsenic source. HgCdTe epilayers with a CdTe mole fraction between 0.23 and 0.36 are grown at substrate temperatures of 175–185 °C. The temperature dependent Hall effect characteristics of the grown samples are measured by the van der Pauw technique. A magnetic field of up to 0.8 T is used in these measurements. An analysis of the Hall coefficient in the temperature range of 40–300 K with a fitting based on a two-band nonparabolic Kane model, a fully ionized compensating donor concentration, and two independent discrete acceptor levels is reported. In addition, the fitting results of a three-band modeling of Hall effect results are compared to published data on p-type Hg1−xCdxTe. Both as-grown and annealed samples are used in this study. All of the as-grown samples showed n-type characteristics whereas annealed samples showed p-type characteristics. The minority carrier lifetimes of arsenic doped epilayers measured by a photoconductive decay method are presented. In this work, an AlGaAs laser of wavelength 850 nm with a pulse length of 10–90 ns is used. The electron lifetimes obtained from this study are compared to published minority electron lifetimes in p-type HgCdTe. Theoretical electron lifetimes of p-type Hg1−xCdxTe material are reported and a comparison to published electron lifetimes is also given.

HgCdTe mid-wavelength IR photovoltaic detectors fabricated using plasma induced junction technology

Preliminary characterization results are presented for mid-wave infrared (MWIR) mercury cadmium telluride n-on-p photodiodes fabricated using a plasma induced type conversion junction formation technology. The diodes have been fabricated on three different vacancy doped p-type epitaxial starting materials, grown by liquid phase epitaxy (LPE) on CdZnTe, LPE on sapphire, and P/p isotype heterojunction material grown by molecular beam epitaxy (MBE) on CdZnTe. All materials had CdTe mole fraction in the active region of the device of ∼0.3. The process uses a H2/CH4 plasma generated in a parallel plate reactive ion etching (RIE) system to type convert the p-type material to n-type. The process is different from previously reported type conversion techniques in that it does not require a high temperature anneal, does not expose the junction at the surface to atmosphere after formation, and requires significantly fewer process steps than other planar processes. Homojunction devices fabricated using this process exhibit R0A values >107 Ω·cm2 at 80 K. The R0A is diffusion limited for temperatures >∼135 K. Results for responsivity, bias dependence of dynamic resistance — junction area product and 1/f noise show that the resulting diodes are comparable to the best planar diodes reported in the literature.

In situ sensors for monitoring and control in molecular beam epitaxial growth of Hg1−xCdxTe

The current status of the implementation and refinement of two wafer state sensors forin situ monitoring and control during molecular beam epitaxial (MBE) growth of Hg1−xCdxTe will be reported. First a rapid scan spectral ellipsometer has been developed and employed for precisely measuring compositions of Hg1−xCdxTe alloys during growth. MBE films in the composition range x = 0.20 to 0.30 have been grown andin situ spectra taken at the growth temperature (180°C) and at room temperature. The MBE films were treated as single layers without the need to invoke any surface film (due to surface roughness, oxide, or of any different composition) as required for exsitu data. The least squares fit over the whole spectral range was used as a measure of the precision. The film composition was also determinedex situ by wavelength dispersive analysis of x-rays and by Fourier transform infrared (FTIR) spectrometry after verifying that there was no lateral variation. A precision of better than ±0.0015 has so far been demonstrated usingin situ spectral ellipsometry for Cd composition or CdTe mole fraction, x, measurements. This compares with ±0.003 for single wavelength ellipsometry. The composition of Hg1−xCdxTe films were also monitored during growth. A spectral pyrometer based on a FTIR spectrometer has also been developed for substrate temperature measurements during growth. The spectral pyrometer measures both the emission and reflectance to give the emissivity of a growing sample over a range of wavelengths spanning the peak of the grey body emission. From the reflectivity measurements, the thickness (in excess of 1 µm) of the growing film is also determined from the interference fringes. The spectral ellipsometer is only capable of measuring thicknesses up to C.a 5000°A (i.e. optically thin). Excellent agreement is obtained between thein situ (at growth temperature) andex situ (at room temperature) thickness measurements. The small discrepancy can be explained by the refractive index of Hg1−xCdxTe being 5% higher at the growth temperature than at room temperature. The combination ofin situ sensors now provides a means of continuously monitoring the composition and thickness of the growing Hg1−xCdxTe film.

Structural characterization of the (111) surfaces of CdZnTe and HgCdTe epilayers by x-ray photoelectron diffraction

HgCdTe epilayers and lattice matched CdZnTe (111) substrates have been investigated by x-ray photoelectron diffraction to characterize their surface structure in regard to termination, reconstruction and relaxation as well as in regard to degradations. Well-ordered CdZnTe surfaces of stoichiometric composition were realized by Ar ion sputtering, while sputter cleaning of the HgCdTe epilayers led to a significant increase of the CdTe mole fraction in the surface region. However, the crystallinity of the HgCdTe surfaces is found to be maintained in spite of their compositional degradation. Ordered and nondegraded HgCdTe(111) surfaces are obtained by electrochemical etching. The examined (111)A and (111)B surfaces can be easily differentiated since their photoelectron diffraction patterns indicates a surprisingly perfect termination. Evidence for any reconstruction or relaxation of the investigated (111) surfaces of CdZnTe and HgCdTe is not found.

Directional solidification of HgCdTe and HgZnTe in a transverse magnetic field

Hg0.80Cd0.20Te crystals were grown vertically in a transverse magnetic field by directional solidification. The effects of a magnetic field on the nature of fluid flow in the melts were investigated by measuring compositional variations along the axial and radial directions of the grown ingots. Magnetic field effects were shown to be significant over the entire field range employed (i.e. 2 to 5 kG). The axial compositional profiles (determined by precision densoty measurements) showed an abrupt decrease in the mole fraction of CdTe when the field was applied. Radial compositional mapping by infrared transmission and X-ray energy dispersion spectrometry indicated that the solid-liquid interface evolved through three stages when the field was applied, i.e. from a radially symmetric concave interface to an off-center concave shape when the field was initially applied, then to a tilted plane, and, finally, to an off-center concave interface. The axial compositional profile of an Hg0.84Zn0.16Te ingot showed similar field effects.

Interdiffusion and related defect mechanisms in the HgTe–CdTe system

Interdiffusion coefficients (D) were measured from 300 to 600 °C using an angle lapping technique and electron microprobe analysis. The influence of the binary composition, temperature, and Hg overpressure was studied. There is a significant difference in the behavior of D in the higher and lower temperature regions. At higher temperatures (T≥450 °C), D is a strong function of composition and temperature and is insensitive to the Hg overpressure. At lower temperatures (T&amp;lt;450 °C), we observe the following trends with decreasing temperature: (i) the activation energy decreases, (ii) D is less dependent on X (X is the CdTe mole fraction), and (iii) D depends on Hg overpressure. Based on an analysis of our interdiffusion data, we propose a dual mechanism for higher temperatures with a vacancy and an interstitial mechanism dominating at low and high X values, respectively, and an interstitial mechanism at lower temperatures.

The Band Structures of Alloy System Hg1-xCdxTe Calculated by the Psedopotential Method

The band structures of alloy system Hg 1- x Cd x Te are calculated at high symmetry points \(\varGamma\), X , L , K and on symmetry lines Δ , \(\varLambda\), Σ by the use of the model potential of Animalu and Heine screened by penn's dielectric function. The parameter E g in Penn's dielectric function is adjusted so as to make the resulting band structure be in agreement with certain known optical transition energies. The band structures of the alloy system change from the inverted band structure to the normal one as mole fraction of CdTe increases from 0 to 1. The critical mole fraction at which the semimetal-semiconductor transition occurs is obtained as x 0 =0.146 at 77K. The agreement with experiment is excellent. The detailed band structure in the vicinity of \(\varGamma\) point is also calculated and is found to be well described by Kane's expression. The effective masses at the extrema of the bands are evaluated in the full range of mole fraction. The values calculated are about twice the experimen...