L.N. Davydov

Influence of detector surface processing on detector performance

Abstract Characteristics of gamma ray semiconductor detectors essentially depend on properties of crystal surface. The status of a lateral surface influences surface leakage current of the detector, and the status of a surface, on which the contacts are made, influences properties of contacts and, thus, a volume leakage current and the highest possible bias voltage. In this connection several ways of processing the lateral surface of CdZnTe and CdTe crystals grown by a high-pressure Bridgman method were investigated: chemical etching, ion cleaning, passivation. Influence of a preliminary processing of a crystal surface on the properties of ohmic contacts is investigated. An analysis of electrophysical properties of crystals subjected to surface processing is carried out.

AlSb single-crystal grown by HPBM

Abstract Theoretical prognosis for AlSb places this widegap compound among the best materials for room temperature detectors and spectrometers. However, results experimentally obtained with AlSb are somewhat discouraging showing the necessity for improving the compound quality. The main difficulties connected with single-crystal growth are high reactivity of the melt with crucible material and a high volatility of Sb. In order to counteract the latter obstacle, an attempt was undertaken to synthesize and grow AlSb crystals by HPBM under the inert gas pressure of 40xa0atm. Different crucible materials: aluminum oxide, vitreous carbon, quartz, graphite, beryllium oxide, and zirconium oxide were tested and their comparative analysis was made. The obtained crystals were investigated and some electrophysical properties measured.

Improving and Characterizing (Cd,Zn)Te Crystals for Detecting Gamma-Ray Radiation

Cd0.9Zn0.1Te ingots were synthesized from pure components (6N purity Cd, Zn, and Te with In as the dopant) and subsequently grown from the melt under an argon overpressure. Graphite crucibles (with and without an inner coating of pyrolytic BN) were used. The temperature gradient in the solidification zone was 7-30 K/cm, and the growth rate was 0.6-1.0 mm/hour. We investigated the chemical composition, structure, and electrical properties of the as-grown crystals, and established relationships between the crystal properties and the growth conditions. The bottom, middle, and top of the ingots had n-type conductivity, but slightly different properties. Resistivity reached a maximum in the middle of the ingots ((2.5 - 5.0) ×1010 Ohm-cm), and was less at the edges ~ 0.8 ×1010 Ohm-cm. The value of the bandgap was minimal in the middle of the ingots (~ 1.5 eV), and 1.53-1.55 eV at the edges. The compensation degree ( Nd/Na) of the energy level responsible for the low dark conductivity showed a maximum value at the bottom of the ingots (~ 60 - 90%), and a minimum in the middle part (1-2%). The crystals were then used to fabricate Cd(Zn)Te detectors for gamma-ray radiation.

Registration of high-intensity electron and x-ray fields with polycrystalline CVD diamond detectors

We developed radiation-hard diamond detectors for registering intense fields of high energy electrons and X-rays, and monitoring the mode of operation of electron accelerators. After synthesizing a diamond film of detection quality up to 350-microns thick by chemical vapor deposition (CVD), we analyzed it by infra-red spectroscopy. We also developed techniques for heat treatment of the film, chemical etching, substrate removal, contact application, and priming by an exposure to X-rays and electrons. This work supported the production of detectors with a specific resistance of 1014Ohm×cm. The dependence of the detector signal’s amplitude on the displacement voltage was investigated under exposure to a direct electron beam with a current ranging from 660 to 930 mA. The duration of the leading edge of a detector pulse was 5 μs. Experiments also were undertaken on the registration by diamond detectors of Bremsstrahlung radiation with an end-point energy of 9 to 70 MeV. We also evaluated the dependence of the amplitude of the detector’s signal on the displacement voltage. Our comparison of detectors’ physical properties and detectors’ response to alphaparticle irradiation before and after the exposure to the accelerator beam showed no degradation, even after the absorbed dose exceeded 11.5 MGy.

Kinetics of impurity segregation at grain boundaries in polycrystals. II. Concentrated solution

The isothermal impurity segregation from a finite-size grain into an intergrain boundary region or at an external free surface is investigated when the impurity concentration in the boundary region is not small. Simple algebraic equations are obtained that describe the impurity concentration for the case of several competing and interacting impurities. The process of segregation of two impurities is discussed in detail, and it is shown that the concentration of one of them can have a maximum as a function of time.

Spectroscopic response of Cd(Zn)Te radiation detectors with a Schottky diode

We investigated the spectroscopic properties of several Cd(Zn)Te detectors with a Schottky contact and simulated them via a computer code. The responses were determined of 0.5-mm-thick surface-barrier Ni/Cd(Zn)Te/Ni detectors to gamma-rays from reference sources of 241Am, 133Ba, 152Eu, 137Cs and 60Co. The best measured energy-resolution at 661.67 keV (137Cs) of these detectors under 800 V of displacement voltage was better than 1.5%. The detectors’ response functions, simulated with Geant4 toolkit, agreed satisfactorily with our experimental data.

Response function of planar Cd(Zn)Te detectors to beta radiation

We investigated the response function of a planar Cd(Zn)Te detector designed for measurement of electron energy spectra and experimentally measured the response of Cd(Zn)Te detector to radiation of 90Sr/90Y reference radiation source. The obtained experimental spectra were compared with the spectra simulated by the Monte-Carlo method with Geant4 package. We managed to agree the simulated response with the experimental one using only two fitting parameters: products of mobility and average lifetime for electrons and holes. Thereby determined transport parameters of charge carriers were independently verified through the measurement of the positions of low energy 133Ba photopeaks of a reference gamma-ray source.

Improving and characterizing the quality of (Cd,Zn)Te crystals for detecting gamma radiation

Cd0.9Zn0.1Te ingots were synthesized from pure components (6N purity Cd, Zn, Te, with In and Fe as dopants) and subsequently grown from the melt under an argon overpressure. Graphite crucibles (with and without an inner coating of pyrolytic BN) were used. The temperature gradient in the solidification zone was ~15-30 K/cm, and the growth rate was 0.6-1 mm/hour. We investigated the chemical composition, structure, and electrical properties of the grown crystals, and established the relationships with their growth conditions. The beginning, middle, and top of the ingot had n-type conductivity, but slightly different properties. Resistivity reached a maximum in the middle of the ingot ((2.5-5)×1010 Ohm-cm), and was less at the edges ~0.8×1010 Ohm-cm. The value of the band-gap was minimal in the middle of the ingot (~1.5 eV), and 1.53-1.55 eV at its edges. The compensation degree (Nd/Na) of the energy level, responsible for the dark conductivity, showed a maximum value at the beginning of the ingot (~60-90 %), and a minimum in the ingots middle part (1-2 %). High-temperature Hall-effect measurements of CdTe:In samples ([In]~2×1017 cm-3) demonstrated the possibility of restoring the initial samples high resistivity (decreased after eliminating inclusions using Cd overpressure annealing) by treatment under a Te overpressure at ~870 K. The crystals were then used to fabricate Cd(Zn)Te detectors for gamma radiation.

Adaptive photodetectors for vibration monitoring

We report on simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of laser vibrometer using GaAs, CdTe and CdZnTe adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force. It enables efficient direct conversion of high-frequency phase modulation of speckle-like optical wave reflected from the vibrating object into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise. The results of measurements of semiconductor materials parameters of CdTe and CdZnTe are presented. The experiments are carried out for diffusion regime of signal excitation at light wavelength &lgr;=1.15 &mgr;m. The sign, conductivity and diffusion length are estimated from the dependencies of the signal on the temporal and spatial frequencies. Preliminary studies at 1.06 &mgr;m showed that it is possible to detect ultrasonic vibrations with the amplitude of 0.2 Å with a signal power of 20 mW and a bandwidth of 15.5 MHz. This optical phase-to-electrical signal converter is not sensible to ambient vibrations, thermal drift, amplitude laser noise and is therefore appropriated for industrial applications.