Thallium bromoiodide, a tuneable band gap semiconductor system, was investigated as a photodetector for scintillation spectrometers. Extensive zone refining of starting materials, based on numerical simulations, considerably enhanced the electrical resistivity to 1011 Ωcm. In addition, accelerated crucible rotation technique (ACRT) crystal growth and after-growth annealing have improved the charge carrier mobility-lifetime product. However, a relatively low signal-to-noise ratio due to a high dielectric constant and relatively low quantum efficiency continues to be an obstacle to achieving high performance, large area T1BrxI1−xphotodetectors.

Significant developments have occurred in the technology of room-temperature PbI2 nuclear sensors which lead to some improvements in the detection of high energy gamma-rays. Discussion of crystal growth, purification, monitoring purification, and detector processing are reviewed as they relate to device performance.

Variable angle spectroscopic ellipsometry (VASE) measurements were employed to study the optical properties of HgI2. The bulk crystal HgI2 surface was subjected to a 10% KI etching prior to the VASE measurements. SE measurements were performed at room temperature, in air with several different angles of incidence. The uniaxial anisotropic nature of the HgI2 crystal was treated in the VASE analysis. Anisotropic dielectric functions of single crystal HgI2, ε||(wo) and ε⊥(ω), for optical electric field vector oriented parallel and perpendicular to the c axis, respectively, were obtained in the range of ∼2.0 - 5.0 eV. Surface aging effects of the HgI2 crystal, after the 10% KI etching, were characterized by VASE.

Physical properties of large band gap semiconductors such as: HgI2, CdTe, Cd0.8Zn0.2Te, CdSe, Cd0.7Zn0.3Se, GaAs, PbI2 and TlBr are briefly reviewed and discussed in terms of their use as room temperature operating x-ray and gamma ray radiation detectors. It is shown that HgI2 which has the largest drift length for holes, λ=μτE i.e., the product of the mobility μh, life time τh and the electrical field E, is at present the leading material, being followed by the newly developed Cd0.8Zn0.2Te. Chemical defects in HgI2 were enhanced by doping the material with aliphatic, aromatic and oxyhydrocarbons as well as with excess Hg and I2 and the increase in unit cell parameter was studied as a function of the amount of dopant. The value of τ was measured as a function of dopant concentration and it was found that Hg doping causes the most severe trapping defects. Low temperature studies of τh down to 170K allowed the identification of the trapping energy levels and concentration of electrically active defects. Shallow traps of 0.13-0.18 eV stemming from deviation from stoichiometry of H-gI2 were found to be in the ppm level whereas deeper traps of 0.4-0.5 eV stemming from hydrocarbons were found to be in the ppb level. It is concluded that only extensive research on the physical, chemical and structural defects correlated with improved crystal growth and device fabrication methods, would lead, in the future, to improvements in λh also of the other large Eg semiconductor detector materials.

This paper reports our recent work on the crystal processing, structural and optical characterization of HgBrxI2−x nuclear radiation detectors. To understand the electrical and optical properties of the detectors, we measured the energy gap of HgBrxI2−x as a function of the Br/I ratio. The energy band of this ternary semiconductor compound can be modulated from 2. 1eV (HgI2) to 3.4eV (HgBr2) by adjusting its chemical composition. This energy scope covers a wavelength spectrum between 365nm and 596nm, much of the visible spectrum. Nuclear and photoconductive detectors were fabricated from HgBrxI2−x single crystals and the responses of these devices were investigated with different radiation sources (241Am, 137Cs).

The behaviour of the charge collection efficiency of particle detectors fabricated from Semi-Insulating (SI) Liquid Encapsulated Czochralski (LEC) GaAs as a function of applied bias is investigated for alpha particles and it is shown that it cannot be explained by simple models that assume a uniform electric field across the detector. To investigate this the electric field within the SI GaAs has been studied. We have attempted to correlate the measured charge collection efficiency with the observed field distributions. Some correlation is found but other effects may also be present.

GaAs photoconductors -have been tailored to detect ultrafast proton pulses having energies ranging between 4 and 9 MeV. The sensitivity, the linearity and the speed of response of the devices are analyzed as a function of their neutron pre-irradiation treatment. The dependence of the sensitivity on the proton energy and the applied polarization is also studied. Finally, the experimental results are compared with a simple theoretical model.

Impedance or admittance spectroscopy has been shown to be a very convenient tool for the investigation of deep levels in semiconductor junctions. At constant temperature a frequency sweep is performed. After that the impedance signal is analysed by a regularization method based on Tikhonov regularization in order to obtain the thermal relaxation times of the deep levels present in the junction. The high resolution of the regularization method in comparison to conventional techniques is demonstrated using simulated data. The temperature dependence of the thermal relaxation times provides information about the properties of the deep levels such as activation energy or capture cross section. Two donor levels with activation energies dE1 =0.58 eV and dE2 =0.68 eV are observed in our detector diodes. It can be shown that the concentration of level 2 is increased after irradiation.

Gallium Arsenide crystals are among the most efficient detectors of X-rays at room temperature. Different GaAs crystals have been irradiated with photons from radioactive sources, to compare their response in view of a possible application for Digital Radiology. The detectors have been obtained by various manufacturers and with different techniques (LEC and LPE crystals). Experimental results include I-V curves, charge collection efficiency and detection efficiency as a function of bias. A comparison is made with a simulation program.

Single crystals of CdTe or dilute alloys of Cdl−y ZnyTe (y ≤ 0.04) and CdTel−zSez (z ≤ 0.04) with low defect density, high purity and large single-crystal area (>30 cm2) are required as substrates for high-quality epitaxial Hgl−xCdxTe thin films in the infrared (IR) detector industry. Bridgman or gradient freeze is the most common technique used for commercial production of these materials because of its success in producing large area substrates of good quality and reproducibility. For epitaxial growth of Hg1−xZnxTe, which has been of considerable interest in recent years as an IR detector material, the substrate of choice has been Cd0.80Zn0.20Te, for lattice matching with long wavelength Hg1−xZnxTe epitaxial layers (x = 0.13–0.14). The primary focus of this paper is on CdZnTe which is currently the preferred substrate material and most widely used for both HgCdTe and HgZnTe epitaxy. This paper reviews the current status of bulk substrate technology for IR detector applications, highlighting critical issues and essential research areas for further improvement of these materials.

Transient lifetime measurements on p- and n-type Hg1-xCdxTe epitaxial films, grown by the MOCVD-interdiffused multilayer process (IMP) on CdTe and CdZnTe, are reported. Lifetimes have been measured on undoped n-type, vacancy and arsenic doped p-type (HgCd)Te for x-values of 0.20-0.28 over the 25K-300K temperature range. Lifetime characterization has been carried out primarily by non-contact transient millimeter wave reflectance as well as by standard photoconductive decay. It is shown that Auger limited lifetimes are achievable in undoped n-type material. N-type lifetime results are analyzed within the framework of a multilevel Shockley-Read model which provide insight into the nature and density of defect states in the material. In p-type (HgCd)Te, for the same carrier concentration, longer lifetimes are obtained by As-doping than in vacancy doped material.

The incidence of compositional and structural inhomogenieties in MSM detectors based on Hg1−xCdxTe/GaAs and Hg1−xCdxTe/GaAs/Si has been examined by nuclear microprobe. With a 2 MeV He+ beam focussed to ≥5 μm, the microprobe has demonstrated the capability for RBS channelling in the active region of a Hg1−xCdxTe device and the imaging of defects by Channelling Contrast Microscopy (CCM). A series of linear growth defects in some Hg1−xCdxTe devices were identified using CCM. The channelling RBS spectra from these regions have shown an increase in χmin compared with the surrounding high quality crystal. The occurence of these defects was associated with a degradation in the performance of affected devices in an array. RBS spectra have also revealed the presence of an anomalous CdTe layer, correlating with a significant reduction in dark current and increase in breakdown voltage of these devices. RBS channelling of individual devices has identified differences in χmin between arrays which were prepared under equivalent conditions of growth and processing.

Hg1−xCdxTe crystals with x∼0.2 were grown by the traveling heater method (THM), in either the [111]A or [111]B directions, using oriented CdTe seeds. Twins are sometimes formed during the growth of these crystals. In crystals grown in the [11]A direction the twins, of orientation [511]B, are constantly growing at the expense of the original [111]A oriented grain. Growth in the [111]B direction, on the other hand, suppresses the growth of the twin domain. Photodiodes and capacitors realized on the (111)A plane are markedly superior to those on the twin plane, (511)B. The difference is due to higher fixed charge and larger fast surface state densities in the case of the (511)B plane. These effects are explained by the lattice structures in the {111} and {511} planes and their possible influence on surface reactivity.

HgCdTe MBE technology is becoming a mature growth technology for infrared focal plane array applications. The ability to dope HgCdTe with In(n-type) and As(p-type) dopants in-situ provides greater flexibilities for fabricating heterojunction devices. In this paper, we will first discuss the current status of HgCdTe MBE growth and then focus on the key results in the control of In(n-type) doping, various approaches and breakthroughs in the growth of As(p-type) doped HgCdTe and issues related to doping such as memory effects and dopants activation. In addition, device results from double layer heterojunction structure(DLHJ) will be briefly discussed.

Both the cadmium vacancy (Vcd) and the tellurium vacancy (VTe) in CdTe are identified by means of electron paramagnetic resonance (EPR). The VCd is a double acceptor and the EPR spectrum is observed in its single negative charge state. The symmetry is found to be trigonal, which can be explained in a model in which the hole occupies a dangling bond t2 orbital and the orbital degeneracy is removed by a static Jahn-Teller distortion. The hyperfine interaction shows that the hole is localised on one of the four Te neighbours. The EPR spectrum of VTe+ reveals cubic (unperturbed) symmetry and the hyperfine structure shows that the unpaired electron is equally spread over the four Cd neighbours.

Photo-EPR measurements locate the 0/+ state of VTe at Ev + 0.2 eV and the 2−/− acceptor level of VCd to be situated less than 0.47 eV above the valence band.

Cd0.91Zn0.09Te/CdTe multilayers of various period thicknesses were inserted into Cd0.955Zn0.045Te bulk alloys grown on (001) GaAs. The net strain of the multilayer on the underlying Cd0.955Zn0.045Te was designed to be zero. X-ray diffraction full width at half maximum (FWHM) was used as a means to optimize the period thickness of the multilayer. Transmission electron microscopy of the optimum period thickness samples demonstrated four orders of magnitude decrease in the threading dislocation density. Mechanism of bending by equi-strained multilayers is discussed.

Epitaxial films of ZnTe(100) and CdZnTe(100)/ZnTe(100) have been deposited by molecular-beam epitaxy onto Si(100) substrates misoriented from 0-8 degrees towards the [011] direction. The films were characterized with x ray diffraction, photoluminescence spectroscopy, optical microscopy, and stylus profilometry. Through use of ZnTe buffer layers, single crystal CdZnTe(100) films have been demonstrated on both 4° and 8° misoriented Si with structural quality comparable to that obtained with GaAs/Si composite substrates. X ray rocking curves for ZnTe(400) with FWHM less than 300 arcseconds and for CdZnTe(400) with FWHM less than 160 arcseconds have been obtained for as-grown films. The observed surface morphologies are superior to those obtained on GaAs/Si composite substrates. HgCdTe(100) films with x ray FWHM as low as 55 arcseconds and average etch pit densities of 5 x 106 cm−2 have been deposited by liquid phase epitaxy on these MBE CdZnTe/ZnTe/Si substrates.

(111) wafers sliced from a boule of ZnTe grown by horizontal physical vapor transport (PVT) have been characterized using synchrotron white beam X-ray topography. The growth axis was about 6° off [311]. The presence of dislocation slip bands, subgrain structures and [111] axis 180° rotational twins were revealed. The slip bands were observed to break up the ordered dislocation cell structures comprising the subgrain boundaries. The initiation of slip at regions of stress concentration at the junctions of subgrain boundaries and twin boundaries was observed. The asymmetric distribution of slip bands either side of the twinned region of crystal suggests that twin boundaries can act as barriers for slip. Several types of detailed twin boundary configuration were determined from the topographs. Using a combination of white beam X-ray topography and Nomarski interference microscopy, the three dimensional shapes of the twin boundaries were determined. Approximate atomic structures at these boundaries are presented.

The properties of Cadmium Telluride are very sensitive to impurities. The growth method has also a strong impact on the material characteristics. Crystals grown by THM have good X-ray detector properties, while normal undoped Bridgman grown crystals do not detect. We have grown pure and doped samples by Bridgman and THM methods. As doping materials we used V, Zn and Cl, also in different combinations. The samples were characterized by several methods, including resistivity, TSC and nuclear detection measurements.

Radiation detector grade CdTe crystals are characterized by several crystallographic and metallurgical techniques including infrared microscopy, dislocation etch pitting and X-ray diffraction. Results are presented for 50 detectors fabricated from an ingot produced by the high pressure Bridgman method. Data on the temperature dependence of leakage current and pulse height analysis are presented, along with measurements of room temperature charge transport properties. Attempts to relate crystal structure to detector performance will be discussed.