Mercuric Iodide Single Crystals Research Articles

Photoelectronic properties of HgI2 crystals for nuclear radiation detection

Photoelectronic properties of red mercuric iodide single crystals, grown from its saturated solution in tetrahydrofuran, have been studied for the wavelength range 450–700 nm at temperatures 80,110, 175, 235 and 300 K. Various aspects of the optical generation of charge carriers have been discussed. The computer simulation of the room temperature photoconductivity has generated the optimized values of the mobility-lifetime products μeτe = 5.67 × 10−5 cm2/V, μhτh = 0.18 × 10−5 cm2/V), and surface recombination velocities (Se = 3.2 × 105 cm/s, Sh = 4.5 × 105 cm/s) of the charge carriers in these crystals. The estimated values of the electron and hole drift lengths for typical electric fields suggest that, under the negative electrode illumination, THF α-HgI2 crystals have high potential as regards to their use as photodetectors in most of the scintillation spectrometers.

Growth of mercuric iodide single crystals by physical vapor transport: development of a new “monothermal” furnace: Non-destructive bulk characterization by a laboratory hard X-ray method

A new growth device in vapour phase to obtain large iodide mercuric single crystals is presented here. The temperature field of this furnace is better defined than in the devices derived from the Scholz process. Using a volume diffraction method recently developed at the “Institute Laue Langevin”, we compare the crystalline quality of two crystals produced in our device and in a Scholz type furnace. Both crystals exhibit bad surface layers having a thickness in the millimetre range. But the core of the crystal grown in the new device has a more homogeneous mosaicity than that of the crystal produced in the Scholz type furnace.

Growth kinetics and morphology of mercuric iodide crystals grown by physical vapor transport

The growth kinetics of mercuric iodide single crystals grown by physical vapor transport from synthesized material were measured using an instrumented growth ampule, and in situ crystal size resolution to ± 0.2 μm was achieved. The kinetic coefficients are 2 × 10 −4mm/s and 1.3 × 10 −4mm/s for (0 0 1) and (1 1 0), respectively, as found from extrapolating the measured (apparent) kinetic coefficients to zero crystal size. The kinetic coefficients are nearly independent of growth rate in the practical range, ≈ 1–5 mm/day, indicating linear growth kinetics, and have substantial temperature coefficients of 0.3 × 10 −6mm/(s °C) and 0.4 × 10 −6mm/(s °C), respectively. The results indicate that the growth process is kinetically controlled at small crystal sizes and undergoes a transition to transport control at ≈ 30–40 mm crystal size, depending on the particular face. The results are consistent with a layer spreading process of growth in which adsorbed molecules surface-diffuse with activation energies ≅ 4 kcal/mol and ≅ 8 kcal/mol for (0 0 1) and (1 1 0), respectively.

Mercuric iodide single crystals for nuclear radiation detectors

Large size HgI/sub 2/ single crystals were grown using the modified temperature oscillation method with low dislocation densities in a relatively stable temperature environment. Radiation detectors were fabricated from the single crystals which showed good energy resolution with small polarization.

Layer-by-layer etching of HgI2 films and crystals by scanning force microscopy

Thin films and single crystals of mercuric iodide (α-HgI2) have been investigated by scanning force microscopy. During the scan process, material evaporation is observed proceeding by hole formation and etching at steps. The ratio of etched volume per second and per interaction area between tip and mercuric iodide step edges is found to be constant for hole formation on an HgI2 crystal and for etching at a screw dislocation on HgI2 films. An estimate for the loading of the SFM tip gives a value comparable to the critical resolved shear stress in this material.

A study of trapping levels in some red mercuric iodide single crystals for nuclear radiation detection

Photon-stimulated currents (PSC) and thermally stimulated currents (TSC) have been measured in red mercuric iodide ( alpha -Hgl2) single crystals grown in our laboratory from tetrahydrofurane- alpha -Hgl2 saturated solution, dimethyl sulphoxide-methanol- alpha -Hgl2 saturated solution and from a polymer-controlled vapour phase. PSC measurements helped in determining uniquely the activation energies and nature (electron or hole) of different trapping levels in these crystals. Subsequently, by a comparative study of TSC and PSC measurements, the capture cross sections have been determined uniquely. Finally, after estimating the trap densities, an attempt has been made to predict the influence of different trapping levels on the performance of the detectors fabricated out of the three types of crystal.

Effect of annealing on the electrical properties and refractive index of HgI2 single crystals

The effect of annealing on the electrical properties and the refractive index of red mercuric iodide (HgI2) single crystals is investigated. The ac-impedance and phase angle were measured in the frequency range from 100 Hz to 10 kHz at different annealing temperatures. The ac-conductivity, dielectric constants, loss tangent and the refractive index as a function of frequency are determined. The results are discussed in terms of structural changes taking place as a result of the heat treatment of the HgI2 single crystals.

Optical methods for measuring iodine vapor during mercuric iodide crystal growth by physical vapor transport

Optical methods have been established for in-ampoule measurements of the iodine vapor concentration during the growth of mercuric iodide (HgI2) single crystals by physical vapor transport (PVT). Significant concentrations of iodine vapor, which can vary during the growth period, occur in some ampoules. The absorptivity of iodine vapor at 514 nm is 7.7 × 105 cm2 mol−1. Differential scanning calorimetry (DSC) measurements correlate with vapor absorption and indicate the presence of free crystalline iodine in vapor-growth HgI2.

A complete characterization of trapping levels in red mercuric iodide single crystals

Photon stimulated current measurements have been performed on red mercuric iodide single crystals in order to determine the trap energy, trap density, and capture cross section for different trapping levels that may be present in the platelet-shaped single crystals of red mercuric iodide grown by polymer controlled growth technique in vapor phase. The procedure of analysis of these measurements is outlined in brief. The analysis has shown that these crystals in general possess four trapping levels: two electron trapping levels with energies 0.10 and 0.31 eV and two hole trapping levels with energies 0.15 and 0.59 eV. The analysis has further shown that the total trap density is only of the order of 1013/cm3, indicating the superiority of this growth technique over other techniques. The analysis has also shown that all four trapping levels are of retrapping type. Finally, the possible mechanisms of formation of these trapping levels and the ways in which these trapping levels will influence the detector operation at room temperature are discussed.

High Resolution Diffraction Imaging of Mercuric Iodide: Demonstration of the Necessity for Alternate Crystal Processing Techniques for Highly Purified Material

AbstractThe overall crystalline lattice uniformity in recently available, highly purified mercuric iodide single crystals has been shown to be impacted by crystal handling techniques that were previously satisfactory. High resolution diffraction imaging of the surface regularity of crystals of various levels of purity and growth orientation shows: 1) that the newer materials have a generally lower level of precipitates, 2) that the incidence of these precipitates is now closely correlated with growth direction, and 3) that the deformation resistance and resulting sensitivity to crystal handling procedures are also closely correlated with these factors in this soft material.As a result, gentler cutting and polishing procedures have been developed and are shown to be effective in preserving overall lattice regularity in the new material. The polishing required to remove residual surface scratches affects the lattice orientation of the softer, precipitate-free regions, while not affecting those regions with detectible levels of precipitates. These results correlate closely with the electrical properties of devices made from these crystals.

Optical properties and surface morphology studies of palladium contacts on mercuric iodide single crystals

Palladium is chemically suitable for electric contacts on mercuric iodide detectors for photon and nuclear radiation detection, so the understanding of palladium contacts is important for fundamental and practical scientific purposes. A study has been conducted on the surface morphology of evaporated contacts using atomic force microscopy and optical transmission and reflection. Evaporated palladium coatings are typically non-uniform and may deposit selectively on mercuric iodide surface defects. Reflection measurements show that the coating thickness and the surface treatment affect the intensity, position and shape of a reflected peak characteristic of the mercuric iodide structure. The results indicate that the band gap energy in the surface of the mercuric iodide is lowered by palladium contacts.

Growth of Single Crystals of Mercuric Iodide on the Ground and in Space

ABSTRACTA short review will be given of the methods by which mercuric iodide is prepared and purified to obtain material suitable for the growth of single crystals. The method used in our laboratory to grow single crystals up to 1,000 grams in weight from the vapor will be discussed. The effects of gravity on the growth process will be described.A crystal growth system suitable for operation in the reduced gravity environment of space was designed, and crystal growth experiments were performed during the flights of Spacelab 3 (April 1985) and the International Microgravity Laboratory (IML-1) (January 1992). The structural quality and electronic properties of the ground-based and spacegrown crystals were compared, and the results will be presented.

Evidence of Electrical Interactions between Photo-Induced Charge Carriers and Moving Dislocations in Red Mercuric Iodide

ABSTRACTVariations in both the hole and the electron photocurrents in single crystals of mercuric iodide (HgI2) are measured during plastic deformation by shearing over (001) planes. The results demonstrate the presence of electrical interactions between moving “easy glide” dislocations and photo-induced charge carriers in HgI2. It is found that both hole and electron photocurrents are decreased during dislocation motion and that the extent of this effect (and the subsequent recovery after deformation ceases) depends on the applied bias.

Enhanced dark conductivity during the plastic deformation of red mercuric iodide

An enhanced dark conductivity is shown to exist during the plastic shear deformation of red mercuric iodide single crystals. This enhancement is related to the movement of ‘‘easy glide’’ dislocations during deformation.

Large single crystals of mercuric iodide from the vapour phase; preparation of a space experiment

Recently, the vapour growth of large single crystals has been recognized as an attractive new field in materials research under microgravity conditions and mercuric iodide as a model substance for the growth experiments in space. Presently we are involved in the development of a sophisticated growth facility for the European Space Agency in which long-term (up to 6 months) vapour growth (computer controlled) experiments can be performed under microgravity conditions (telescience). There are several prerequisites in the development of this facility, which have been identified as separate technical or scientific tasks of the project and are discussed in this paper: (1) high resolution in situ measurements of the growth rate of the crystals as a function of crystallographic orientation, (2) crystal surface temperature measurements for evaluation of the actual supersaturation, which is the driving force for the growth, (3) a cooling system capable adjusting the cooled area to the increasing crystal size (in order to avoid thermal stresses in the crystal). For automatic control of the growth, the dependence of the growth rate on supersaturation for various starting materials (doping, nonstoichiometry) has to be known in advance; for this purpose the transport rate will be measured in a separate precursor space experiment (DCMF).

Microhardness and microstructure in single crystals of mercuric iodide

Experimental studies were made of microhardness and microstructure on the (001) planes of vapor grown single crystals of HgI 2 using optical microscopy at room temperature and secondary electron emission and cathodoluminescence (CL) at 80 K in a SEM operated at 20 kV. Optical microscopy and etching techniques were used to identify three characteristic microstructures, each of which responded differently to microhardness indentation. These were regions (1) relatively free of etch pits, (2) containing a high density of relatively large, randomly distributed etch pits, and (3) containing orthogonal arrays of relatively small etch pits aligned with 〈100〉 crystallographic axes. The regions with random distributions of etch pits were softer than those without etch pits; the indentations in regions without etch pits exhibited brittle fracture; the regions containing orthogonal arrays of etch pits were particularly hard. Evidently the “orthogonal arrays” are much more detrimental to radiation detector performance than the “random arrangements”, since the very high Knoop hardnesses associated with the orthogonal arrays have been observed only in the worst detectors. The orthogonal arrays are apparently the traces of nested dislocation loops surrounding inclusions. In the SEM studies, it was determined that HgI 2 does exhibit strong CL image contrast at 80 K and does not pose problems of dissociation in the electron beam. Both ingrown electrically active defects (i.e. the orthogonal arrays) and electrically active defects produced by microhardness indentations were clearly resolved in the CL images. The indented samples exhibited well defined dark bands associated with dislocations on slip bands radiating from the indentations. Comparisons of microhardness measurements and microstructural observations suggest that ingrown microstructural defects that quench luminescence in HgI 2 are particularly likely to affect detector quality adversely.

Characterization of ground-based and space-based mercuric iodide single crystals by means of X-ray topography and diffractometry

X-ray topography and rocking curve analysis have been used in order to characterize the crystalline quality of α-HgI 2 single crystals grown by physical vapor transport on the ground and in a microgravity environment during the SL3 mission of Spacelab.

Total organic carbon and gas chromatography-mass spectroscopy methods to determine total carbon and hydrocarbons in mercuric iodide single crystals

Total organic carbon was determined by measuring the CO2 produced by combustion in a sealed quartz vessel. The CO2 was quantified by nondispersive IR and by titration using commercial detectors. The total organic carbon was found to be around 10–100 μg/g in both starting materials and in single crystals. Gas chromatography-mass spectroscopy (GC/MS) measurements were made on hexane extracts of mercuric iodide (HgI2) dissolved in potassium iodide solution. Hydrocarbons starting with C10 (DIENE) and up to C26 were found. In addition, phthalates, such as diethyl and dioctyl phthalate were also found. Some of the organic compounds, for example, such hydrocarbons as branched nC16, nC20, nC21, nC22, nC23, and nC24, were present in some HgI2 materials in quantities of the order of weight ppm, but were eliminated in the purification process and were not found in the single crystals. Other organic compounds such as the phthalates were not always eliminated and were identified in the single crystals. In general, the GC/MS could identify only hydrocarbons of C10 and higher which account for only a few percent of the total organic carbon determined by oxidation.

Degradation of mercuric iodide crystals on aging

Mercuric iodide single crystals were grown and characterized by studying their spectral response of photoconductivity, thermally stimulated current, dielectric constant and di-electric loss as a function of frequency. The as-grown crystals have high resistivity, good photo-sensitivity, low dielectric constant and loss at low frequencies, etc. On aging, the crystals show degradation in their properties when they were stored in air, whereas the crystals stored in an iodine chamber do not exhibit any degradation. The degradation of the properties have been interpreted in terms of surface evaporation of HgI2 and consequently formation of a surface layer rich in mercuric atoms. The effect of a dc bias on aged (degraded) samples shows restoration of the properties. The results could be interpreted in terms of polarization of mobile defects and formation of space charge layers which enhance a high internal field in the crystals, hence the restoration of the properties.

Growth and microhardness studies of mercuric iodide single crystals

Growth and microhardness studies of mercuric iodide single crystals