R3C  Defects in CdTe and CdZnTe

Wednesday, Nov. 4  14:00-16:00  California

Session Chair:  Henry Chen, Brimrose, United States

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(14:00) R3C-1, invited, Study of Surface Recombination in CdTe and CdZnTe Radiation Detectors by Laser Induced Transient Current Technique

R. Grill, E. Belas, P. Praus, J. Pekarek, J. Franc, J. Zazvorka, A. Musiienko, J. Bok, P. Hoschl

Charles University, Institute of Physics, Prague 2, Czech Republic
The characterization of radiation detectors by strongly absorbed radiation is disturbed by surface recombination (SR) of photo-carriers, which reduces the collected charge and apparently degrades the charge collection efficiency. Consequently, the mobility-lifetime product evaluated with obvious Hecht relation is debased. The generalized Hecht relation involving SR by means of SR velocity (s) improves the model. The simplified approach, however, does not describe processes occurring in illuminated detector near the surface in the entirety and principal features of the charge transport remains unexplored. The description of the charge transport in radiation detector is complicated by multiple effects that occur in parallel near the surface. (1) Enhanced density of point defects and dislocations comparing to the bulk values causes strong trapping and detrapping of photo-carriers. (2) Band bending induced by metal-semiconductor contact or by uncompensated shallow defects results in the space charge formation. (3) The appearance of surface photovoltage influences collected charge at low bias. In this contribution we report on the complex research of charge transport in radiation detectors involving phenomena mentioned above. Simulations of the laser-pulse-excited current waveforms are performed solving numerically the drift-diffusion and Poisson's equations. Free carriers trapping/detrapping is described by the Shockley-Read-Hall model both in bulk and in surface layer. We show, how surface damage affects current transients and how the principal surface properties may be deduced from the current waveform shape. Results are compared with analogous procedure using obvious model of SR and with experimental data. In addition, the determination of surface and interface properties by means of study of SR is conveniently used at the research of the influence of contacts on the detector's performance like carriers depletion, space charge formation and detector polarization.

(14:20) R3C-2, Resolving Electrical Inhomogeneity in CdZnTe Bulk Crystal via Scanning Microwave Impedance Microscopy

Y. Xu1,2, Y. Gu1, X. Han2, J. Jia1, R. Guo1

1State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi, China
2Laboratory of Solid State Microstructures, Nanjing University, Nanjing, Jiangsu, China
The use of CdZnTe (CZT) detectors in the fields of medical imaging, nonproliferation, astrophysics and homeland security, etc., has grown considerably due to the improvement of CZT crystal growth and detector fabrication. However, their wide deployment is hampered by the low availability of high-quality CZT crystals. It has been widely reported that the presence of Te-rich secondary phase particles (Te inclusions) in CZT crystals tends to affect the structural uniformity and account for the nonuniformity of charge collection. As we all know, the charge carrier transport behaviors in CZT devices are determined by the electricity distribution. Currently, it is still unclear as to the specific electrical potential distribution that induces the enhanced carrier trapping or recombination surrounding Te inclusions. The aim of this work is to continue our studies of Te inclusions and the effects on electrical distribution in CZT detectors. Firstly, the microstructural defects induced by Te inclusions in CZT single crystal were investigated through the defect-selective etching and cathodeluminescence (CL). Then, local electrical imaging using scanning microwave impedance microscope (SMIM) is performed on CdZnTe in different modalities at room temperature. Significant contrasts of the conductivity and capacitance are observed in the local area surrounding Te inclusion. It is suggested that Te inclusions act as the negative potential centers, according to the analysis of potential difference. Finally, the mechanism therein is proposed based on the corresponding energy band structure analysis.

(14:35) R3C-3, invited, Electronic Structure of Point Defect-Decorated Dislocations in CdTe and Nucleation of Te Precipitates

V. Lordi

Materials Science Division, Lawrence Livermore National Lab, Livermore, CA, USA
Cadmium telluride (CdTe) and related alloys such as CdZnTe are leading candidates for high energy-resolution, room-temperature semiconductor gamma and X-ray radiation detectors. The performance of detector material is dependent on the presence of various defects that reduce carrier lifetimes. Most notably, Te-rich secondary phases and crystal dislocations have been observed to degrade performance. In addition, dislocations may serve as sinks for impurities and point defects and, indeed, often have been observed to be associated with the presence of Te precipitates. We previously showed using first-principles atomistic simulations of CdTe that native point defect migration and reactions, specifically involving Cd vacancies, Te interstitials, and Te anti-site defects, can lead to local Te enrichment and a mechanism for nucleation of Te precipitates. Here, we present recent first-principles calculations of the electronic structure and carrier trapping propensity of dislocations in CdTe and compare bare dislocations with those decorated with various point defects. In addition, we compute the binding energies between the point defects and the dislocation cores as a function of distance, as well as the kinetics of point defect migration toward and away from the dislocations. In this light, we discuss a possible mechanism for nucleation of Te precipitates near dislocation cores, along with the implications for carrier capture and lifetimes. We consider the prevalent 60° mixed dislocation in both glide and shuffle configurations, as well as several 90° pure edge dislocations and a screw dislocation.

(14:50) R3C-4, (Withdrawn), Depolarization under Sub-Bandgap Illumination in Detector Grade CdZnTe

R. Guo1, W. Jie1, Y. Xu1,2, S. Jiang3, G. Zha1, T. Wang1

1State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, P R China
2Laboratory of Solid State Microstructures, Nanjing University, Nanjing, P R China
3Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, P R China
Abstract withdrawn

(15:05) R3C-5, Characterization of Semiconductor and Scintillator Detectors at the Advanced Light Source (ALS)

G. S. Camarda, A. Bolotnikov, Y. Cui, R. Gul, A. Hossain, U. Roy, G. Yang, R. B. James

Brookhaven National Lab, Upton, NY, USA
During the transition period between closure of Beamline X27B at BNL’s NSLS and the opening of Beamline MID at NSLS-II, we began operation of LBNL’s ALS Beamline 3.3.2 to carry out our radiation detection materials R&D. Measurements performed at this Beamline include, X-ray Detector Response Mapping and White Beam X-ray Diffraction Topography (WBXDT), among others. We will introduce the capabilities of the Beamline and present the most recent results obtained on CdZnTe and multiple scintillators. The goal of the studies on CdZnTe is to understand the origin and effects of subgrain boundaries and help to visualize the presence of a higher concentration of impurities, which might be responsible for the deterioration of the energy resolution and response uniformity in the vicinity of the sub-grain boundaries.

(15:20) R3C-6, Evolution of Native Oxide Thickness on CdTe/CdZnTe in Time Dependence after Surface Treatment

J. Zazvorka, J. Franc, L. Beran, J. Pekarek, P. Moravec, M. Veis

Institute of Physics, Charles University in Prague, Prague, Czech Republic
Surface treatment of CdTe/CdZnTe radiation detectors is an important technological operation critically influencing the detector performance and has been studied thoroughly in the last years. However, the results differ and no optimal surface preparation process has been established yet. One of the not largely investigated issues is the time deterioration of the detector, normally assigned to oxide layer formation on the detector surface. Using ellipsometric measurements we have studied the evolution of the native oxide thickness after the surface treatment. Studied samples were chemo-mechanically polished on a silk pad using a 3% Br-ethylenglycol solution for 60 seconds. Afterwards they were chemically etched by immersion into a standard 3% Br-methanol (Br-MeOH) solution for 60 seconds. Ellipsometry of the samples was measured in dependence of time within one month after the treatment. The data were confronted with a new theoretical model of the sample structure, consisting of the substrate, an effective-medium-approximation layer (composed of substrate and CdTe oxide) and a surface roughness layer. The least square minimization was used to evaluate the layer thicknesses. A different dynamic for various CdTe/CdZnTe samples and a semi-saturation of the surface layer growth is observed. The saturation time varies from 8 to 12 days. Dynamics of the native oxide formation on ambient air at room-temperature in five different CdTe/CdZnTe samples were determined. This investigation will be furthermore correlated with I-V measurements of the samples.

(15:35) R3C-7, invited, Multiple Radiation Detection Based on Mercurous Bromine for Nuclear Planetology

H. Chen, J.-S. Kim, S. Trivedi

Brimrose Technology Corp, Sparks, MD, USA
Present day radiation detectors used in space missions, both scintillation detectors and semiconductor detectors, are still mostly based on technologies that have been developed decades ago. Each type of detector can only satisfy some of the desirable properties such as good energy resolution and efficiency, high radiation tolerance as well as compact and light weight, while having many limitations and hence current detector systems have to be designed with two or more different types of detector materials and modules which place a lot of inconveniences for spaceflight where power consumption, weight and volume are at a premium. In this work, we introduce a new class of room temperature semiconductor material, the mercurous bromine Hg2Br2 that has great potential of multiple radiation detection, from x-rays/gamma rays to particle radiations while offering low cost, high performance and long term stability characteristics simultaneously. Initial results via gamma spectroscopy are very promising, for example, < 2% energy resolution @ Cs-137 662 keV at room temperature is achievable. Spectral response from lower energy gammas such as via Am-241 59.6 keV to Co-57 122 keV to higher energies gammas via Cs-137 662 keV to alpha particle at 5.5 MeV will be presented. Advantages of this novel material over existing technologies, from material science to practical wide deployment aspects will also be discussed.