An analysis of carrier transport is performed on multilayer HgCdTe heterostructures with complex composition and doping profiles by combining models for carrier scattering with multilayer modeling of the transport properties obtained through resistivity and Hall measurements of the entire multilayer stack. The analysis is applied to the study of carrier scattering in low-doped HgCdTe multilayers grown on (211) CdZnTe substrates by molecular beam epitaxy. The predictive capability of the modeling and its usefulness as a form of rapid and accurate feedback from simple Hall effect measurements are highlighted. The ability to routinely produce HgCdTe films with free carrier concentration (nd–na) in the range of 1013–1014 cm−3 while preserving carrier mobility with respect to higher concentrations is evident from the dependence of mobility on carrier density for fixed HgCdTe compositions.

It has been reported that the basic electrical properties of n-type long wave length infrared (LWIR) HgCdTe grown on silicon, including the majority carrier mobility (μ e) and minority carrier lifetime (τ), are qualitatively comparable to those reported for LWIR HgCdTe grown on bulk CdZnTe by molecular beam epitaxy (MBE). Detailed measurements of the majority carrier mobility have revealed important differences between the values measured for HgCdTe grown on bulk CdZnTe and those measured for HgCdTe grown on buffered silicon substrates. The mobility of LWIR HgCdTe grown on buffered silicon by MBE is reported over a large temperature range and is analyzed in terms of standard electron scattering mechanisms. The role of dislocation scattering is addressed for high dislocation density HgCdTe grown on lattice-mismatched silicon. Differences between the low temperature mobility data of HgCdTe grown on bulk CdZnTe and HgCdTe grown on silicon are partially explained in terms of the dislocation scattering contribution to the total mobility.

Variable magnetic-field Hall and transient photoconductance-lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy (MBE). Use of quantitative mobility-spectrum analysis (QMSA) combined with multiple carrier-fitting (MCF) techniques indicates that the majority of samples contain an interfacial n-type layer that significantly influences the interpretation of the electrical measurements. This n-type layer completely masks the high-quality electrical properties of undoped or low n-type In-doped HgCdTe, as well as complicating the interpretation of activation in As-doped p-type HgCdTe. Introduction of an intentional n-type background, typically created through doping with In to "recover" high mobility, is actually shown to increase the "bulk" layer conductivity to a level comparable to the interface layer conductivity. Photoconductance-lifetime measurements suggest that In-doping may introduce Shockley-Read-Hall (SRH) recombination centers. Variable-field Hall analysis is shown to be essential for characterizing p-type material. Photoconductance-lifetime measurements suggest that trapping states may be introduced during the incorporation and activation of As. Two distinctly different types of temperature dependencies were observed for the lifetimes of As-doped samples.

We have initiated a joint effort to better elucidate the fundamental mechanisms underlying As-doping in molecular beam epitaxy (MBE)-grown HgCdTe. We have greatly increased the As incorporation rate by using an As cracker cell. With a cracker temperature of 700°C, As incorporation as high as 4×10 20 cm −3 has been achieved by using an As-reservoir temperature of only 175°C. This allows the growth of highly doped layers with high quality as measured by low dislocation density. Annealing experiments show higher As-activation efficiency with higher anneal temperatures for longer time and higher Hg overpressures. Data are presented for layers with a wide range of doping levels and for layer composition from 0.2 to 0.6.

Epilayers of Hg1−xCdxTe (mercury cadmium telluride: MCT) were grown at 300 °C, under atmospheric pressure, in a cold wall annular reactant inlet inverted-vertical reactor chamber. As-grown MCT surfaces (0.126≤x≤0.58) were specular and free of visible stacking faults. Their growth rate and composition were linear functions of the dimethylcadmium flow rate (QCd), for QCd≤0.031 sccm. As-grown epilayers exhibited n-type characteristics at x≤0.40 and p-type characteristics at x=0.580. The mobility (μ) in Hg annealed layers, which were n-type as-grown, decreased by a factor of 3, on average. This reduction in μ, after Hg annealing, is ascribed to the presence of acceptor impurities, such as copper, which propagate into the epilayers during growth process. Hall measurements as a function of temperature, performed on a Hg0.792Cd0.208Te layer, gave a minimum concentration of 1.8×1015 cm−3 and a maximum mobility of 3.8×104 cm2/V s, at approximately 180 K.

`The demand for high detectivity LWIR IR focal plane arrays that operate at low backgrounds is shown to drive the HgCdTe technology toward increased detector performance. Reduced operating temperature together with advanced material technology and detector design are presented as solutions. High performance MWIR, MLWIR and LWIR HgCdTe detector test arrays and variable area test structures were recently demonstrated through the joint collaboration of Aerojet Electronic Systems Division and Rockwell International. These devices are based on the innovative buried planar heterostructure (BPH) detector architecture grown by liquid phase epitaxy of HgCdTe on II-VI substrates. The major features of the BPH design include planar geometry, heterostructure wide gap p-type on narrow gap n-type HgCdTe and a buried LWIR electrical junction. Excellent 78K median R(omicron )A performance across the IR spectrum from 5.2 micrometers to 12 micrometers is reported and shown to follow the diffusion trend line. Excellent 40K median R(omicron )A performance for devices with cutoffs ranging from 9 micrometers to 19 micrometers are also presented. LWIR R(omicron )A statistical performance data at both 78K and 40K from fanout test arrays are presented with median R(omicron )A values of 100 ohm-cm2 at 78K and > 106 ohm-cm2 at 40K for cutoffs of 10.4 micrometers and 11.4 micrometers respectively. The 90% test array operability was found to exceed 5 x 105 ohm-cm2 at 40K. Devices with median R(omicron )As of 20 ohm-cm2 at 78K and 7 x 105 ohm-cm2 at 40K were measured for cutoffs of 12 micrometers and 13 micrometers respectively. Uniform and high quantum efficiencies were measured at 40K with a median of approximately equals 70%.

Cd1−xZnxTe compounds of different compositions have been prepared at temperatures ranging from 400 to 1000°C by annealing elemental Te in sealed quartz ampoules, in an atmosphere comprising vapors of Cd and Zn whose partial pressures were varied by varying the composition of the binary Cd1−yZny alloys which provided the Cd and Zn vapors in these annealing experiments. The chemical compositions of the resulting Cd1−xZnxTe compounds have been analyzed using electron probe microanalytical techniques. Results indicate that presence of a 0.5%Zn along with Cd in a closed or semi-closed system may prove to be beneficial in preventing decomposition and/or formation of a metal/non metal phase during annealing of Cd0.96Zn0.04 Te substrates. Using the thermodynamic data in the literature for the binary Cd1−yZny alloys and with the assumption that the activities of the Cd and Zn components are weakly dependent on temperature, the partial pressures of Cd and Zn in equilibrium with the Cd1−xZnxTe compounds at various temperatures have been evaluated.

(Cd,Zn)Te wafers containing Te precipitates have been annealed under well defined thermodynamic conditions at temperatures below and above the melting of Te. Results of the examination of the wafers with infrared microscopy before and after the anneals indicate a substantial reduction of the Te precipitates in wafers annealed at temperatures in excess of the melting point of Te compared with those annealed at temperatures below the melting point of Te. These results confirm the thermomigration of liquid Te precipitates to be the principally operative mechanism during annealing in the elimination of these precipitates in (Cd,Zn)Te wafers. The occurrence of Te precipitates in (Hg,Cd)Te epitaxial layers grown on (Cd,Zn)Te substrates containing Te precipitates is also explained on the basis of thermomigration of these precipitates during LPE growth from the substrates to the epilayers. Absence of occurrence of Te precipitates in (Hg,Cd)Te epilayers grown on annealed (Cd,Zn)Te substrates with negligible Te precipitates is also confirmed. Usefulness of annealing (Cd,Zn)Te substrates-to eliminate Te precipitates -prior to epilayer growth is confirmed via demonstration of improved long wavelength infrared (Hg,Cd)Te device array performance uniformity in epitaxial layers grown on (Cd,Zn)Te substrates with negligible Te precipitates after annealing.

Using a quasichemical approach, the total native defect concentration and the minimum deviation in stoichiometry have been calculated in CdTe crystals as a function of the Cd pressure at various temperatures. With this knowledge, CdTe and (Cd,Zn)Te wafers have been subjected to postgrowth step annealing treatment under conditions such that the crystals are in equilibrium with a Cd or (Cd,Zn) vapor corresponding to the minimum in deviation from stoichiometry at each annealing temperature. The step annealed CdTe and (Cd,Zn)Te wafers have been examined under infrared microscopy and have shown significant reduction in the concentration of Te precipitates, whereas the unannealed wafers have had numerous Te precipitates distributed throughout the bulk. HgCdTe epitaxial films have been grown on the step annealed CdTe and (Cd,Zn)Te wafers as well as on unannealed wafers from the same boule. Examination of the cross sections of the epitaxial films indicates appearance of Te precipitates in films grown on unannealed substrates, whereas no Te precipitation was evident in films grown on the annealed substrates leading to the inference that the occurrence of Te precipitates in the (Hg,Cd)Te films is possibly related to the presence of Te precipitates in the substrates. Thermal migration of Te under a temperature gradient during step annealing is suggested as a possible mechanism in the elimination of larger size Te precipitates whereas the extremely fine precipitates (<1 μm) appear to need in-diffusion of metal vapor for their elimination.

Synchrotron x-ray topography studies have been conducted at the National Synchrotron Light Source at Brookhaven National Laboratory to correlate defects in HgCdTe epilayers with those in underlying CdTe family substrates. Infrared detectors have been fabricated on these epilayers to investigate the performance impact of specific defects. This paper describes synchrotron x-ray facilities and methods. Images of substrates and epilayers are discussed and mapping of epilayer/substrate defects, such as microtwins, subgrain boundaries and slip lines, is demonstrated. Efforts to map detector array performance to epilayer and substrate topographs are described.

LWIR photodiodes have been fabricated in (Hg,Cd)Te heterostructures grown liquid phase epitaxially from tellurium rich solutions. R°A values of up to 10 8 ohm-cm2 and quantum efficiencies in excess of 50 percent have been demonstrated at 40K for a cutoff wavelength of λco ≈ 10.0 μm. These diodes have also been found to be extremely hard to nuclear radiation with no perceivable deterioration in the device performance even upon irradiation of up to 0.85 Megarads. We believe that these R°A performance and the levels of tolerance to radiation are the highest anyone has reported in literature to date.

Hg1−xCdxTe films were grown liquid phase epitaxially from tellurium rich solutions containing up to 10 at. % of the group V elements P, As, Sb, and Bi. Chemical analysis of the Te growth solutions and the films was carried out in conjunction with extensive Hall effect measurements on the films subsequent to various annealing treatments under Hg rich and Te rich conditions. Despite the presence of a large concentration of the group V elements in the Te source solution, the maximum concentration of these elements incorporated into the liquid phase epitaxially grown Hg1-xCdxTe appears to vary from 15cm−3 for Bi up to 1017cm−3 for phosphorus and As implying a distribution coefficient varying from −5 for Bi up to 10−3 for P at growth temperature of ∼500° C. This low value of the distribution coefficient for group V elements for growths from Te rich solutions contrasts with the moderately high values reported in the literature to date for growth from Hg rich solutions as well as pseudobinary solutions (Bridgman growth). The widely differing distribution coefficients and hence the solubility of the group V elements for Hg rich and Te rich liquid phase epitaxial solutions is explained on the basis that the activity coefficient of the group V elements in Te rich solutions is probably orders of magnitude lower than it is in Hg rich solutions. Finally, the results of the anneals at 200° C under Hg saturated conditions with and without a 500° C Hg saturated preanneal have indicatedn top conversion in many of the films attesting to the amphoteric behavior of the group V elements in LPE grown Hg1−xCdxTe(s) similar to the previously reported behavior of P in bulk grown Hg0.8Cd0.2Te.

(Hg,Cd)Te heterostructures have been grown liquid phase epitaxially from tellurium rich solutions on CdTe and (Cd,Zn)Te substrates. Both MWIR detectors sensitive in the 3-5 μm spectral region and LWIR detectors sensitive in the 8-14 µm spectral region have been fabricated in the heterostructures. Detectors with high RoA (low noise) and high quantum efficiency (high signal) have been fabricated. For the MWIR detectors, quantum efficiency in excess of 75 percent and RoA values in excess of 107 ohm cm2 at 80K have been demonstrated for λCo ~ 5.5 µm. For the LWIR detectors RoA values of ~ 106 ohm cm2 have been demonstrated at 40K for λCo ~ 11 μm. A correlation of the trap energies established via carrier lifetime and DLTS measurements with the depletion width - capacitance data indicates the p-n junction to be located at the heterostructure interface.