Resistivity CdTe Research Articles

Influence of Laser Radiation on Optical Properties of High Resistivity CdTe Crystals and Cd1 – xZnxTe Solid Solutions in the Field of the Fundamental Optical Transition E0

Influence of Laser Radiation on Optical Properties of High Resistivity CdTe Crystals and Cd1 – xZnxTe Solid Solutions in the Field of the Fundamental Optical Transition E0

Deep levels in high resistive CdTe and CdZnTe explored by photo-Hall effect and photoluminescence spectroscopy

High resistive CdTe and CdZnTe single crystals were measured by photo-Hall effect spectroscopy (PHES) and photoluminescence spectroscopy (PL) with the aim of discovering the position of deep levels (DLs) in the band gap. Illumination in the range of 0.65–1.77 eV, room temperature, and DC electrical measurements were used in the case of PHES. Low temperature (4 K) photoluminescence spectra were recorded in the spectral range above 0.47 eV. Eight samples, both n-type and p-type, were studied and typical shapes of spectra were collected, compared and interpreted for both spectroscopy methods. It was shown that a simple single-level model of PHES often fails in the interpretation of DLs distant from the midgap. Eight DLs with the energy Ec − 0.65 eV, Ec − 0.8 eV, Ec − 0.9 eV, Ec − (1.10–1.15) eV, Ev + 0.70 eV, Ev + 0.85 eV, Ev + 1.0 eV, and Ec − 1.25 eV were interpreted. A memory effect characterized by a relaxation time of about 60 s was observed at the 0.8 eV level and allowed us to determine the 1.7 × 10−17 cm2 capture cross-section of electrons on this level. It is argued that PHES is a convenient complementary method to identify and characterize DLs, including DLs inaccessible by thermal emission techniques. DLs observed by PHES were consistently verified by PL.

Tellurium Secondary-Phase Defects in CdZnTe and their Association With the 1.1-eV Deep Trap

Defects located at the E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> - 1.1 (eV) level, which are electro-optically active deep traps, generally have been overlooked since their presence cannot be detected except for those in highly resistive CdTe compounds. The origin of this trap is still debated on whether it is from Te vacancies or from dislocations induced by secondary phase defects in Te. We have grown high-resistivity Te-rich CZT ingots to clarify the origin of the 1.1 eV defects and to analyze the defect levels in the CZT samples by current deep level transient spectroscopy (I-DLTS)and photoluminescence (PL). From the analysis, defect levels such as shallow acceptor/donor, A-centers, Cd vacancies and Te antisite appeared to be in both high and low concentrations of Te inclusions, but the level of 1.1-eV defects exhibited dependence on the density of Te inclusion in the CZT samples. We also evaluated the effect of the 1.1-eV deep-level defects on the detector's performance in point view of carrier trapping and de-trapping.

Photoluminescence of high‐resistivity ZnTe crystals doped with gallium and indium

AbstractWe have measured photoluminescence spectra of high‐resistivity ZnTe crystals doped with gallium and indium in the wide energy range from near‐band‐edge energy to deep energy region up to 0.78 eV. In the near‐band‐edge energy, we detected a characteristic excitonic peak at 2.363 eV in both gallium and indium doped high resistivity ZnTe. By comparison with a similar PL peak observed in high resistivity CdTe, we identified the characteristic peak at 2.363 eV as emission from excitons bound to defects complex arising from a Zn vacancy and gallium or indium atoms (VZn‐2D). In lower energy range, a newly strong deep emission band near 1.0 eV has been observed for the first time in high‐resistivity ZnTe crystals. The intensity of the deep emission is two order magnitude stronger than that in undoped one. Furthermore, the intensity and peak position of the deep emission band are almost unchanged by Zn‐saturated anneal at 600‐680 °C. We specualte that this deep emission band is responsible for the high resistivity of ZnTe and prevent conversion of n‐type low resistivity. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Dark Conductivity in Semi-Insulating Crystals of CdTe:Sn

We prepared semi-insulating CdTe for radiation detectors by isothermal annealing of single crystals grown by Bridgeman technique in a sealed quartz container filled with Sn vapor. The resistivity of CdTe:Sn samples thus obtained was of order of 10SUP10/SUP Ohm·cm at room temperature with electrons lifetime of 2´10SUP-8/SUP s, which is appropriate for the applications desired. Analysis of electric transport characteristics depending on temperature, sample thickness and voltage applied revealed the presence of traps with concentration of about (4-5)´10SUP12/SUP cmSUP-3/SUP with the corresponding energy level at 0.8 ? 0.9 eV counted from the bottom of conduction band. The conductivity was determined by electron injection from electrodes in space charge limited current mode.

Search for the Major Chlorine-Related Defects in CdTe:Cl

ABSTRACTUnderstanding the effect of chlorine-related defects on the CdTe electric properties is important both for obtaining high resistivity CdTe-based detectors and for high efficiency CdTe-based thin-film solar cells. The actual mechanism of the effect of Cl on electric properties of CdTe is not clear and different sometimes contradictory hypotheses appear. For example ClTeVCd shallow acceptor complex defect was proposed both as a reason of increased carrier concentration in CdTe thin film and also as a reason of high resistivity of CdTe:Cl thin films. In the present work we are trying to clarify the effect of Cl on CdTe electric properties and to find the reason of high resistivity of CdTe:Cl crystals using first principles calculations and defect chemistry modeling. For the first time we are trying to develop a model capable to describe experimental data on both high temperature and room temperature conductivity of CdTe:Cl.

Electrical Properties Study of Three Highly Purified CdTe Ingots

Abstract Three high resistivity CdTe ingots have been prepared with highly purified materials. The three starting Te components were purified through three different purification processes: horizontal zone refining, vacuum distillation and by a combination of both processes. The three crystals growths were achieved by the THM method. The purity studies were achieved by the determination of the concentration of 21 impurities, using the Atomic Absorption (AASGF) method. The resistivity ρ measurements along the length of the three CdTe ingots at room temperature showed a nearly constant ρ value within the range of 109 - 1010 Ω.cm. This represents a motivating result, in net advance with the general behavior of classic THM materials, where it began by high ρ at the starting point, and then it decreased by few decades. Values of drifting motilities μ were measured for all ingots and showed interesting high values: μe =1100 cm2/Vs for electrons and μh =90 cm2/Vs for holes. The position in the band gap and eventually the nature of the corresponding electrical defects levels, involved in the trapping process was studied by using the PICTS method. The uniformity of ρ, the high mobility μ and the low concentration values of impurities in all our CdTe ingots confirm the high electronic grade quality of these CdTe materials, which is better than 6N purity.

Why shallow defect levels alone do not cause high resistivity in CdTe

It is usually assumed that deep defect levels are responsible for the high resistivity in detector-grade CdTe, however, it has been recently reported that shallow defects alone can explain high resistivity. In order to resolve this contradiction we analyze different high-temperature compensation regimes and we particularly show that donor–acceptor self-compensation is not a sufficient condition for high resistivity. We also analyze the dependence of the Fermi level on shallow donor concentration at both high temperature and room temperature using analytical solution of the charge neutrality equation and the graphical method based on the formation energy diagrams. We derive the limits of the high-temperature Fermi level values that lead to the high resistivity at room temperature in a system with shallow defect levels only and find that it is theoretically possible to obtain high resistivity using only shallow defects, but only under an extremely narrow range of physical parameters that is unlikely to occur in practice. Finally, we show that the transition levels of cadmium vacancy acceptor are not deep enough to provide high resistivity.

What causes high resistivity in CdTe

Shallow donors are often introduced into semiconductor materials to enhance n-type conductivity. However, they can sometimes also be used to obtain compensation between donors and acceptors, resulting in high resistivity in semiconductors. For example, CdTe can be made semi-insulating by shallow donor doping. This is routinely done to obtain high resistivity in CdTe-based radiation detectors. However, it is widely believed that the shallow donor alone cannot be responsible for the high resistivity in CdTe. This is based on the argument that it is practically impossible to control the shallow donor doping level so precisely that the free carrier density can be brought below the desired value suitable for radiation detection applications. Therefore, a deep native donor is usually assumed to exist in CdTe and pin the Fermi level near midgap. In this paper, we present our calculations on carrier statistics and energetics of shallow donors and native defects in CdTe and illustrate different donor-specific mechanisms for achieving carrier compensation. Our results show that the shallow donor can be used to reliably obtain high resistivity in CdTe without requiring additional deep donors. Since radiation detection applications require both high resistivity and good carrier transport, one should generally use shallow donors and shallow acceptors for carrier compensation and avoid deep centers that are effective carrier traps. This study highlights how the interaction between impurities and native defects intricately affects the Fermi level pinning in the semiconductor band gap and the associated resistivity of the material.

Ge clustering effects in Ge doped CdTe: Electrical and structural properties

High resistivity CdTe can be achieved by introducing impurities that create deep levels which, in turn, control the electronic transport properties of the material via a compensation process. We have characterized the effects of thermal annealing of high resistivity CdTe:Ge under either Te- or Cd-rich atmosphere to understand how modifications in the structure of Ge-related defective states and their electrical activity affect the material transport properties. We have investigated the transport properties with current-voltage analyses, the electrically active deep traps by photo-induced current transient spectroscopy and the local environment of Ge atoms by x-ray absorption spectroscopy. By correlating the modifications observed, we determined the occurrence of Ge clustering effects and associated them to the formation of electrically active deep donor traps, one located at EC-0.31 eV and the other one at midgap, with an activation energy of 0.82 eV.

Current simulation of symmetric contacts on CdTe

This article presents the calculated current–voltage characteristics of symmetric Metal–Semiconductor–Metal configurations for Schottky, Ohmic, and injecting-Ohmic contacts on high resistivity CdTe. The results clearly demonstrate that in the wide band-gap, semi-insulating semiconductors, such as high resistivity CdTe, the linearity of the I–V curves cannot be considered a proof of the ohmicity of the contacts. It is shown that the linear I–V curves are expected for a wide range of contact barriers. Furthermore, the slope of these linear curves is governed by the barrier height, rather than the bulk doping concentration. Therefore the deduction of bulk's resistivity from the I–V curves may be false.

Electrical activity of deep traps in high resistivity CdTe: Spectroscopic characterization

The electrical compensation processes of high resistivity CdTe is controlled by deep levels. We have characterized the electrical activity of deep traps by means of three different and complementary spectroscopic methods: photoinduced current transient spectroscopy, surface photovoltage spectroscopy, and space charge limited current analyses. The aim is twofold: to achieve a thorough characterization of the deep trap properties and to assess the potentiality and limitations of the three experimental techniques by a cross correlation of the results obtained with each one of them. We have obtained a direct quantitative estimate of the major deep trap concentration, and we have assessed the sensitivity limit in deep-level detection for surface photovoltage spectroscopy.

Preparation of Semi-Insulating CdTe:In by Post-Grown Annealing After Elimination of Te Inclusions

Annealing under Cd-saturated overpressure within the temperature range of 700-800degC was used for elimination of Te inclusions in high resistive In-doped CdTe. Te inclusions were significantly reduced by annealing and only residual inclusions with dimensions less than 1 mum remained in the material. All annealed samples were converted to the high conductive n-type after Cd-saturated annealing. Annealing finished by quenching at 550degC, 650degC, and 750degC under various Cd/Te overpressures was used to find an annealing conditions leading to the preparation of a semi-insulating material. The range of optimal overpressures was found to be very narrow and keeping these conditions during annealing and quenching is practically impossible. The semi-insulating material was prepared by means of Te-rich annealing finished by slow cooling. The annealing temperature, In doping concentration, and cooling regime were observed to strongly affect the preparation of semi-insulating material.

Simulation of Photoelectric Transport in High-Resistivity CdTe for X-Ray Detectors

Photoelectric transport in high resistivity CdTe at room temperature was numerically simulated based on the self-consistent steady state solution of coupled drift-diffusion equations and the Poisson equation of the photo-excited electron-hole system. Relative influence of recombination and space charge induced distortion of electric field on the reduction of charge collection efficiency was investigated. It was shown that space charge effects play significant role, if concentration of deep levels exceeds 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> at typical values of carriers' capture cross-section. Possibility to reduce the screening effects decreasing a detector thickness to a minimum value guaranteeing full absorption of incident X-ray radiation was investigated.

Screening Effects in High Resistivity CdTe for X-ray and Gamma Ray Detectors

The possibility to estimate an upper limit of concentrations of defects forming deep levels in high-resistivity CdTe is discussed based on evaluation of room-temperature photocurrent spectra. A model explaining the shift of the maximum of photocurrent with the applied electric field as a consequence of screening effects caused by space charge accumulated on deep levels is presented. Theoretical calculations show that a maximum concentration of deep levels leading to the observed shift of the photocurrent maximum is less than ∼1013 cm−3 for typical capture cross sections of electrons and holes. This result supports the models assuming formation of a high-resistivity state with a minimum deep level doping.

Investigation of Cu-containing low resistivity CdTe thin films deposited by the two-source evaporation technique

Cadmium telluride (CdTe) thin films were deposited onto scratch-free transparent glass substrates by the two-source evaporation technique, using Cd and Te as two different evaporants. In the next step, films were heated under vacuum at 500 °C for 1 h and dipped in Cu(NO3)2–H2O solution at room temperature. These films were again heated under vacuum for 1 h at 500 °C to obtain maximum Cu diffusion. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), optically by a Lambda 900 UV/VIS/NIR spectrophotometer and electrically, i.e. dc electrical resistivity, by the van der Pauw method at room temperature, dark conductivity, and activation energy analysis as a function of temperature by the two-probe method under vacuum. The EDX results showed an increase of Cu content in the samples by increasing the immersion time of the CdTe films in the solution. The layer thickness of diffused Cu atoms in CdTe is determined by comparing the difference of absorption between as-deposited and immersed films with the absorption graph of Cu films of varying thicknesses.

Effects of sub‐gap irradiation on the time‐of‐flight current waveforms of high resistivity CdTe

AbstractEffects of IR irradiation on TOF current waveforms of a CdTe:Cl crystal with Pt electrodes on its (111) faces have been investigated. An increase of TOF current and a change in the evolution of DC photocurrent versus applied voltage by IR irradiation are observed. By taking into account an effect of surface recombination, satisfactory fits to theory are obtained for the charge collection efficiency and the DC photocurrent versus applied voltage. These results show that the observed effects are mainly explained by a reduction of surface recombination velocity rather than by an increase of bulk carrier lifetime. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoconductivity spectroscopy of deep levels in CdTe

High resistivity (10/sup 8/-10/sup 9/ /spl Omega/cm) CdTe samples (undoped, Cl and In doped) were investigated by means of spectral and temperature dependent photoconductivity (PC) with the goal to look for correlations of the signal from deep levels and transport properties represented by the mobility-lifetime product. For comparison, low temperature PC measurements were performed on p-type undoped CdTe crystals having a low resistivity at room temperature. It was found out, that the transition dominating the below bandgap PC in these undoped samples has the energy E/sub V/+0.95 eV. We suppose, that the corresponding defect is native, probably related to Cd vacancy or Cd vacancy complex. Doping with shallow donors (In,Cl) results in practical disappearance of this transition. The near midgap PC signal is observed in high resistivity undoped CdTe, although with lower intensity, than in case of low resistivity p-type undoped samples. A correlation between intensity of the E/sub V/+0.95 eV PC transition and the mobility lifetime measured by alpha spectroscopy was observed. We suppose, that this deep level acts as a significant trap influencing performance of X and gamma ray detectors. Doping with shallow donors probably results in physical removing of the corresponding defect by a reaction with In/sub Cd/ or Cl/sub Te/ shallow donors, significantly reducing its ability to trap electrons.

Characterization of transport and charge collection in high resistivity CdTe by photoelectric measurements

A systematic study of photoelectric properties of high resistivity CdTe (undoped and Cl, In doped) has been undertaken in the configuration, where the sample surface is illuminated in a direction parallel to the applied electric field to create experimental conditions similar to those used by measurements of detection properties of X-ray and gamma-ray detectors. A detailed investigation of photoconductivity was performed at above bandgap energy 1.55 eV in dependence of the illumination intensity and the applied voltage. It was observed, that the transport in the material is generally nonlinear in respect to both the illumination intensity and the applied voltage. Drift-diffusion equations for photogenerated electron and hole transport and the Poisson equation were solved numerically using a model including one near midgap energy level. It was found, that this level in detector-grade samples acts as a weak hole trap and has a moderate concentration /spl sim/10/sup 14/ cm/sup -3/. Simulations of electric field in dependence of illumination intensity show the formation of a dead layer extending up to 100 /spl mu/m for a flux of 10/sup 14/ cm/sup -2/ s/sup -1/.

Spatial uniformity of electron charge transport in high resistivity CdTe

Electron charge transport in high resistivity CdTe was investigated in terms of drift mobility, charge collection efficiency, and mobility-lifetime product. CdTe devices were produced from material grown by the Travelling Heater Method. Infrared microscopy was used to assess the quality of CdTe wafers, which showed a concentration of bulk defects and tellurium precipitates around the edges of the wafers. Laser-induced time of flight was used to measure the electron drift velocity, which was linear with respect to electric field at field strengths up to 200 V/cm. The measured electron drift mobility was 1040±20 cm 2/V s. Ion-beam induced charge (IBIC) imaging of the device cathode was carried out to produce high resolution maps of signal amplitude and electron drift time. Excellent spatial uniformity was observed in the sample, and a value of 6×10 −3 cm 2/V was measured for the electron mobility-lifetime product.