Resistivity CdTe Research Articles

Comparison of undoped and doped high resistivity CdTe and (Cd,Zn)Te detector Crystals

CdTe and (Cd,Zn)Te crystals were grown to study the compensation mechanism and the influence on the transport properties. Undoped and doped crystals with Sn, In, and Ge were grown. The crystals showed resistivities up to 10/sup 9/ /spl Omega/cm and higher. The transport properties depended strongly on the dopant and the compensation mechanism. For the doping with a deep donor, the mobility-lifetime product of electrons were 2/spl times/10/sup -5/ cm/V and 4/spl times/10/sup -4/ mcm/sup 2//V for Ge and Sn doped, respectively. The highest values were obtained for In doped (Cd,Zn)Te with 3.3/spl times/10/sup -3/ cm/sup 2//V.

Defect structure of high resistive CdTe:In prepared by vertical gradient freeze method

High resistive and photosensitive CdTe doped with In aimed for fabrication of X- and gamma-ray detectors was produced by Vertical Gradient Freeze method. A complex investigation of defects and compensation by a number of optical and photoelectrical mapping methods was performed. A model of energy levels dominating the recombination processes in the material was elaborated, where the role of In, and related complexes as well as native defects (Cd vacancy and its complexes) is discussed.

Photomagnetoelectric effect of high resistivity CdTe

Photomagnetoelectric (PME) effect of high resistivity CdTe and CdZnTe for nuclear detector applications is measured. Ambipolar diffusion lengths of 9 μm and 25 μm for CdTe and CdZnTe respectively are obtained at room temperature. From the temperature dependence of PME short circuit current measurements, a peak at T = 240 K is found for THM CdTe when the excitation light is chopped at the frequency of 135 Hz. The behaviour is explained by the reemission of trapped carrier at the recombination centre by optical excitation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of High Resistivity CdTe and (Cd,Zn)Te Crystals

AbstractFor Abstract see ChemInform Abstract in Full Text.

Growth of high resistivity CdTe and (Cd,Zn)Te crystals

AbstractThe growth of high resistivity CdTe and (Cd,Zn)Te is successfully performed with different kinds of doping. In the literature intentionally undoped as well as doped crystals are presented with resistivities up to 1010 Ωcm. In this paper we review the growth of high resistivity CdTe and (Cd,Zn)Te. The mechanism of compensation is discussed regarding the different dopants and deep levels, which seem to be responsible for the high resistivity. A common compensation model explains the high resistivity by deep levels. The doping and the influence on the compensation mechanism is compared for several elements like tin, germanium and chlorine. The material properties and the crystal quality of undoped and doped CdTe as well as (Cd,Zn)Te are shown.

New approach for high resistivity CdTe preparation

Cadmium telluride (CdTe) is one of the important members of II–VI semiconductor compounds. In various applications, for example, X-ray detectors, photo-refractive and electro-optical devices, semi-insulating (SI) materials are necessary. Though extensive studies have been devoted to the selection of a suitable doping element for obtaining SI crystals, only a few have dealt with intrinsic electronic properties. For this reason, a systematic exploration of the intrinsic properties of undoped CdTe with the aim of growing nominally pure CdTe crystals has been carried out. Results are reported on: (i) direct synthesis of 7N pure elements; (ii) stoichiometry correction procedure based on a specially devised post-synthesis heat treatment; (iii) evaluation of the stoichiometry deviation of CdTe by means of partial vapour pressure measurements; (iv) crystal growth carried out from vapour phase (sublimation) and from the melt (normal freezing); (v) electrical characterization. In particular it is found that in high-purity crystals the electrical resistivity critically depends on the stoichiometry deviation of the material source.

Crystal defects and optical transitions in high purity, high resistivity CdTe for device applications

The structural and optical properties of high purity undoped semi-insulating (SI) Cadmium Telluride (CdTe) crystals, grown by Physical Vapour Transport (PVT) and by modified Vertical Bridgman (BG) techniques have been studied. The samples were obtained from 7N source elements by direct synthesis followed by heat treatment to adjust the stoichiometry. The experimental measurements have been systematically performed using high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL) techniques, before and after the thermal annealing procedures. The CL emissions from low and high resistivity samples have been studied in order to correlate the CL bands to the presence and concentration of native defects in the crystals. Two broad luminescence bands have been found in addition to the expected emission related to the excitonic recombination. The nature of the defects involved in the transitions was studied by analysing the position of the energy levels and the relative intensity variation of the CL peaks. The effect of the thermal treatments, performed at different temperatures, on the optical and electrical properties of the specimens was also analysed.

Growth and Deep Level Characterisation of Undoped High Resistivity CdTe Crystals

High resistivity CdTe crystals were grown by Bridgman and by physical vapour transport techniques without intentional doping. Novel synthesis and pre-growth treatment procedures were developed for controlling background impurities concentration and stoichiometry of the source material. A detailed characterisation of the deep levels responsible for the high resistivity behaviour is reported.

Defects introduced in cadmium telluride by γ irradiation

The properties peculiar to high resistivity CdTe:Cl are of great interest because of its application as a radiation detector. The compensation process responsible for the materials semi-insulating character implies the presence in the lattice of impurities and defects which have not yet been thoroughly characterized. The use of CdTe:Cl as a detector exposes the material to high fluxes of ionizing radiation which alter the crystal stoichiometry and affect the resulting electrical and optical properties, but few and scattered experimental data are available about radiation effects on this compound. In this work we have carried out an extensive investigation of the effects of γ irradiation on CdTe:Cl by photoinduced current transient spectroscopy analyses. We have identified the deep levels with activation energies up to midgap and we have followed their evolution with increasing irradiation doses up to 50 kGy, the dose which totally degrades the material detecting properties.

Time-resolved photocurrent and electric field measurements in high resistivity CdTe

We studied current and electric field transients in high resistivity CdTe:In crystals by means of nanosecond time-resolved photocurrent and electro-optic sampling measurements. Electron and hole dynamics have been investigated for different excitation regimes (incident photon fluence and wavelength). A numerical solution for a drift-diffusion model in presence of several defect centers is given which allows to interpret experimental results. Current decay can be described by two time constants of about 10 and 100 ns, the first one being predominant for low incident photon energy (<1 eV). By comparison with the numerical model, the fast and slow time constants are related to hole and electron trapping, respectively. Electric field decays in a millisecond time scale. This is related to charge trapped in a deep recombination center located at 0.75 eV from the conduction band.

Improved spectrometric performance of CdTe radiation detectors in a p-i-n design

CdTe radiation detectors were fabricated using a p-i-n design and a significant improvement in the spectral properties was obtained during room temperature operation. An iodine doped n-CdTe layer was grown on the Te faces of the (111) oriented high resistivity CdTe crystals at the low substrate temperature of 150 °C. An aluminum electrode was evaporated on the n-CdTe side for the n-type contact, while a gold electrode on the opposite side acted as the p-type contact. Very low leakage currents, typically 60 pA/mm2, were attained at room temperature (25 °C) for an applied reverse bias of 250 V. Detectors exhibited excellent spectral responses with an energy resolution of 1.42, 1.7, and 4.2 keV FWHM at 59.5, 122, and 662 keV γ peaks, respectively.

Excimer laser doping technique for application in an integrated CdTe imaging device

CdTe is an attractive semiconductor material for applications in solid-state high-energy X-ray and γ-ray imaging systems because of its high absorption coefficient, large band gap, good mobility lifetime product of holes and stability at normal atmospheric conditions. We propose a new concept for fabricating an integrated CdTe with monolithic circuit configuration for two-dimensional imaging systems suitable for medical, research or industrial applications and operation at room temperature. A new doping technique has been recently developed that employs excimer laser radiation to diffuse impurity atoms into the semiconductor. Accordingly, heavily doped n- and p-type layers with resistivities less than 1 Ω cm can be formed on the high resistive CdTe crystals. We have further extended this technique for doping with spatial pattern. We will present the laser doping technique and various results thus obtained. Spatially patterned doping is demonstrated and we propose the use of these doping techniques for the fabrication of integrated CdTe two-dimensional imaging systems.

Picosecond time-resolved luminescence studies of recombination processes in CdTe

High resistivity CdTe:In samples were studied by means of continuous and picosecond time-resolved photoluminescence (PL). The detected PL signal is affected by non-radiative trapping, while radiative recombination has an excitonic character. Two recombination lifetimes describe the measured time-resolved PL spectra: the first (30–60 ps) takes into account fast non-radiative recombination; the second lifetime is in the range 200–300 ps and can be related to exciton decay.

Charge collection efficiency and space charge formation in CdTe gamma and X-ray detectors

A new extended model for the charge collection efficiency in CdTe gamma and X ray detectors is presented which allows to derive from apparent experimental gamma spectra of a quasi-monochromatic source, an 241Am source in the present case, not only the μρ products of electrons and holes individually but also the sign, spatial distribution, and temporal evolution of the net space charge accumulated in the detector. Resistive CdTe and CdZnTe as well as CdTe Schottky detectors are studied. While the resistive type is stable in time and exhibits higher μτ products, the Schottky type shows space charge accumulation which approaches saturation after about 1 h at several 10 11 cm −3. This is attributed to efficient majority carrier depletion, Fermi level shift, and trap filling. Resistive detectors thus appear optimized to the needs of gamma spectroscopy even at low bias voltage, while Schottky types need higher bias to overcome the space charge. They are suited to both, gamma spectroscopy and X-ray detection in analog current mode, where they operate more stably due ρo the higher bias. From the point of view of materials characterization, gamma spectroscopy with Schottky detectors probes and reveals the trap density near the Fermi level (several 10 12 cm −3 eV −1). We find a basically homogeneous spatial distribution suggesting the trap origin being in crystal growth rather than surface processing. Capture of photogenerated charges in traps is detrimental for current-mode operation under high X-ray flux because delayed emission from traps limits the detector's ability to respond to fast signal changes.

Picosecond time-resolved studies of defect-related recombination in high resistivity CdTe, CdZnTe

AbstractHigh resistivity CdTe:In and CdZnTe samples were studied by means of continuos and picosecond time-resolved photoluminescence (PL). Detected PL signal is affected by non-radiative trapping into defects, while radiative recombination has an excitonic character. Time-resolved PL can be described by two lifetimes: the first (30-60 ps) takes into account fast non radiative recombination, the second in the range 200-300 ps can be related to exciton decay.

Effect of the vacuum annealing on p-type CdTe

The photoluminescence spectra and formation of the complex defects in the Cl doped CdTe p-type layers are investigated. It is supposed that at low Cl concentration complex defects TeiClTe and VCdClTe are responsible for the low p-type resistivity in CdTe. At higher Cl concentration formed neutral complexes Tei2ClTe and VCd2ClTe increased the p-type resistivity. At high Cl concentration excess ClTe caused a high resistivity and photoconductivity of CdTe. It is proposed that Tei and complex defects with Tei caused the 1.4eV PL band quenching and the 1.0eV PL band formation. The unstable 1.25eV PL band is supposed to be due to VCd-VTe pair. Acceptor impurities in initial material as Cu and its complexes can increase the p-type resistivity value in CdTe: Cl.

Electrically active defects in detector-grade CdTe:Cl and CdZnTe materials grown by THM and HPBM

High resistive CdTe is one of the key materials of room-temperature radiation detectors. For these materials, deep levels play an important role on the electrical as well as transport properties and, therefore, drastically affect the performance of the nuclear detectors. In this work, we report the electrical properties of several materials mainly for two types of investigated crystals: THM doped by chlorine with and without additional dopants; HPBM (high pressure Bridgman method). A variety of high sensitivity characterization and analytical methods were used, including TSC (thermally stimulated current), Van der Pauw resistivity analysis as well as nuclear detection. The similarities as well as differences of these two categories of materials with and without controlled chemical impurities will be described with a tentative assignment of each considered chemical to a deep defect level.

Boron implantation into CdTe

The behaviour of 12B implanted into p-type, highly resistive n-type and n-type CdTe is investigated by the β-NMR method. At the Larmor frequency a single line resonance shows up which indicates that a measurable boron fraction is in a diamagnetic charge state and occupies positions with cubic environment. This boron population is about 15% in highly resistive n-type CdTe at room temperature and rises with increasing implantation temperature. In CdTe:Cd and CdTe:Te crystals the cubic boron fraction starts from a value of 10%. Activation energies, which are deduced from the data, are similar for highly resistive n-type CdTe and CdTe:Cd (0.39(5) and 0.44(5) eV, respectively), but are different in case of CdTe:Te (0.71(5) eV) indicating different annealing processes. Only in CdTe:Cd a boron population that is exposed to defects is detected by the Δm = 2 NMR transition. These defects are characterized by a mean electric field coupling constant of 210(20) kHz.

Non-Ohmic Conductivity of High Resistivity CdTe

Non-Ohmic Conductivity of High Resistivity CdTe

Electrical and Structural Properties of Ohmic Contacts to n-Type and High Resistivity CdTe

The interaction between CdTe and In during the formation of an ohmic contact has been investigated. Emphasis is placed on the study of the effect of thermally induced sublimation of cadmium on electrical properties of contacts. Presented results prove the effectiveness of cap annealing and rapid thermal processing in fabrication of improved ohmic contacts with limited Cd losses during the contacting procedure. PACS numbers: 73.40.Ns