V.E. Kutny

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.

Transport properties and spectrometric performances of CdZnTe gamma-ray detectors

We investigated the influence of the ratio of the electron and hole mobility-lifetime products, (μτ)e,h and (μτ)e/(μτ)h, on the resolution of CdZnTe planar radiation detectors via Monte-Carlo simulations. Preliminary results show that this ratio exercises a larger effect than that of any other parameter on the detector’s peak-to-valley ratio and resolution. We determined the range of values of the ratio (μt)e/(μτ)h where the fast degeneration of the photopeak in CdZnTe detectors takes place at a gamma-ray energy 661.7 keV (137Cs). We offer an explanation, based on the results of some of our experimental data, on the spectrometric performance of CdZnTe detectors.

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.

The energy dependence of the sensitivity for planar CdZnTe gamma-ray detectors

Considerable variations in the charge-carrier transport parameters necessitate individual calibration of CdZnTe gammaray detectors for many applications. We carried out a set of experiments wherein we determined that the main region of interest for the energy dependence of CdZnTe-detectors’ sensitivity lay in the gamma-quantum energy range of 0.03- to 3-MeV. This finding was satisfactorily verified and reconstructed from our measurements of pulse-height distributions using 241Am-, 137Cs-, and 60Co-sources. We discuss our comparison of the quality-of-fit of the approximation formulae with our detailed calculations of the sensitivity of CdZnTe detectors via a Monte-Carlo method.

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.

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.

Effect of side-surface passivation on the electrical properties of metal-Cd(Zn)Te-metal structures

We explored the influence of Cd(Zn)Te detectors on the detector’s dark current for different methods of contact formation and passivation of the side surfaces. Our findings suggest that the dark current of a homogeneous detector with ohmic contacts is limited by the detector’s resistivity and the operating voltage. Detectors with a rectifying barrier have a markedly lower dark current at the same voltage and contact geometry than those without such a barrier, and their sides have a larger space charge than those of untreated ones. The major factor lowering the detector’s dark current is the formation of a rectifying barrier that occurs while creating contacts to the detector; the role of passivation of the lateral surface in this case is minimal. However, passivation plays the main role in the formation of leakage current in homogeneous detectors with ohmic contacts, where the uniformity of the electric field is important inside the detector, or in other studies used for determining the bulk resistivity of the detector material. We formed a surface-barrier structure on a semi-insulating Cr-Cd(Zn)Te-Cr crystal (n-type) with a resistivity of 1010 Ohm-cm at room temperature. The measured leakage current of this detector was less than 3 nA at 1500 V. We discuss our findings on this detector’s structural properties.

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.

Mechanisms of the passage of dark currents through Cd(Zn)Te semi-insulating crystals

We investigated the passage of dark currents through semi-insulating crystals of Cd(Zn)Te with weak n-type conductivity that are used widely as detectors of ionizing radiation. The crystals were grown from a tellurium solution melt at 800 оС by the zone-melting method, in which a polycrystalline rod in a quartz ampoule was moved through a zone heater at a rate of 2 mm per day. The synthesis of the rod was carried out at ~1150 оС. We determined the important electro-physical parameters of this semiconductor, using techniques based on a parallel study of the temperature dependence of current-voltage characteristics in both the ohmic and the space-charge-limited current regions. We established in these crystals the relationship between the energy levels and the concentrations of deep-level impurity states, responsible for dark conductivity and their usefulness as detectors.