CdTe Samples Research Articles

Microwave measurements of the pulsed photoconductivity and photoelectric effect

A method of pulsed measurements of microwave conductivity and photoelectric effect in a range of 36 GHz is described. This method ensures contactless measurements of the electrical properties of semi-conductors and studies of the kinetics of electron-ion processes with the participation of free current carriers and carriers localized in traps, resulting in data on lifetimes and mobilities of carriers in the nano- and micro-second ranges. The time resolution of the recording system is 5 ns. The method is based on the recording of the kinetics of changes in the resonator quality factor and the shift of the resonance frequency caused by a light-stimulated change in the complex permittivity. The results of measuring the temperature dependences of the microwave photoconductivity in a p-type CdTe sample doped with 0.25 mol % Ag2Te and a CdSe sample are presented.

Comparison of Pixelated CdZnTe, CdTe and Si Sensors With the Simultaneously Counting and Integrating CIX Chip

CIX is a direct converting hybrid pixel detector designed for medical X-ray imaging applications. Its key feature is the simultaneous counting and integrating of absorbed X-ray quanta, which offers, among other benefits, a large dynamic range as well as the opportunity to calculate the average photon energy of the absorbed spectrum. In this work several different Si, CdTe and CdZnTe sensors are characterized and their suitability as sensor materials for a combined counting and integrating system is assessed. The measurements indicate that temporal stability is an issue for the tested CdTe samples, both on short (ms) and long (s) timescales. Furthermore, the homogeneity of the detector response is addressed and interpreted in terms of lateral polarization. The influence of charge sharing and X-ray fluorescence in the sensor materials will be characterized and their impact on the average photon energy reconstruction will be illustrated through simulations and measurements.

Helicity-independent optical pumping of nuclear magnetization in bulk CdTe

We report on the observation of optically pumped nuclear spin polarization in samples of CdTe at high magnetic field using $^{125}\text{T}\text{e}$ NMR. We find strong enhancement both in bulk material as well as in a molecular beam epitaxy grown, $2\text{ }\ensuremath{\mu}$ thick epilayer. Contrary to prior results, however, we observe that the magnitude and orientation of the nuclear spin alignment is independent of laser helicity. Further, we find that after its onset at sub-bandgap energies, the nuclear polarization rapidly increases to reach $\ensuremath{\sim}3%$ and remains virtually unchanged as higher energies are scanned. Comparison with additional data from a semi-insulating GaAs crystal points to a polarization mechanism different from that typically dominant, possibly driven by surface spin-dependent recombination.

High‐temperature electrical properties of CdTe:Bi single crystals

AbstractAt first the experimental results of detailed Hall effect measurements at high‐temperature defect equilibrium under Cd/Te vapor pressure in CdTe single crystals were obtained. They indicate that at these conditions Bi forms in CdTe crystals mainly BiCd donor centers. The analysis of the free electron density temperature dependencies allows to conclude that up to 800 K the electrons of dopant atoms define the sample's electrical properties. The native donor point defects begin to influence the electro‐neutrality condition above this temperature. The research of high‐temperature electrical properties of Bi‐doped CdTe samples shows that the real electrical active Bi content in the investigated crystals is ∼1017 at/cm3. The electron density dependencies vs PCd, modeled at constant temperatures (500‐900°C), in the assumption of deep Bi donor (EV+0.71 eV) are in good agreement with experiment. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical Interaction of CdTe and CdZnTe with Aqueous Solutions of H2O2-HI-Tartaric Acid

An iodine-based etching system, H2O2-HI-tartaric acid, was tested on CdTe samples with (110), (100), (111)A, and (111)B orientations. The etching rate of CdTe was shown to depend on the sample orientation, ratio of etchant composition components, temperature, and rotation speed of the disc. A study of the chemical composition and structure of the (211)B Cd1−xZnxTe surfaces etched under different conditions was carried out. X-ray photoelectron spectroscopy (XPS) measurements showed that a nearly stoichiometric surface was achieved after heating of the etched surface in vacuum. Reflection high-energy electron diffraction (RHEED) measurements revealed a very good single-crystalline surface layer in samples etched with HI-based solutions as compared with bromine-methanol treatment.

Sub-bandgap photoluminescence from as-grown and annealed layers of CdTe

Photoluminescence (PL) from the vapour deposited layers of CdTe have demonstrated a unique quality improvement after as-grown samples have been annealed at relatively low temperatures (500–600 ∘C). The sub-band gap emissions at 4.2 K have been centred at the 1.42 and 1.47 eV and they have different intensity-power dependencies. This is strong evidence that they originate from the recombination of various defects. Similar deep-level defect emissions in bulk CdTe arise in the region of the so-called A-centre PL band (1.42 eV), and extended defect emissions (1.47 eV). The latter emission demonstrates a super-linear excitation-power dependence that is a characteristic of the excitonic recombination processes. After a short annealing at 500 ∘C the extended defect emission practically disappeared making possible observation of the band-to-band recombination at 1.58 eV. As a result of this work optimal annealing conditions have been found for the as-grown samples of CdTe on various substrates (Si, sapphire).

The Effect on Thermoelectric Properties of Cd Substitution in PbTe

AbstractA recent theoretical study suggested that the substitution of Cd in PbTe can result in a distortion in the electronic density of states (DOS) near the bottom of the conduction band in PbTe. In this study we explored the effect of Cd doping on the thermoelectric properties of PbTe in an effort to test the theoretical prediction that DOS distortion can increase the Seebeck coefficient. We present detailed investigation of structural and spectroscopic data, transmission electron microscopy, as well as transport properties of samples of PbI2 doped PbTe-x% CdTe (x = 1, 3, 5, 7, 10). All samples follow the Pisarenko relationship and no enhancement of the Seebeck coefficient was observed due to DOS distortions. A low lattice thermal conductivity was achieved by nanostructuring observed via high resolution transmission electron microscopy. A maximum ZT of ˜1.2 at ˜720 K was achieved for the 1% CdTe sample.

A Study of Bismuth Induced Levels in CZT Using TEES/TSC

AbstractConsidering the desirable effects of doping CdTe with heavy elements like Bi, we have grown a Cadmium Zinc Telluride (Zn=10%) ingot with Bi (doping levels ∼1014 to 1015 at/cm3) as the heavy element dopant for use as a room temperature radiation detector, using the Bridgman method. In-spite of a high bulk resitivity (∼1010?cm), and the ability to hold high electric field (>2000 V/cm), these lightly doped crystals had a poor spectral resolution for the Co-57 photo peaks and ??e measurements were so low that these measurement were not reliable. Thermo electric effect spectroscopy (TEES) and thermally stimulated current (TSC) experiments on samples C and F (single crystals close to the tip and the heel of the ingot respectively) have revealed various defect levels in the band gap. Among these defect levels, we have identified and characterized two Bi-related deep levels namely a deep donor level L5 (thermal ionization energy: 0.33[5] to 0.39[5] eV and trap cross-section: 7.1[5] × 10-17 to 2.54 [5] × 10-16 cm2), and a deep acceptor level L8 (thermal ionization energy of 0.82 [5] eV and trap cross-section of 2.59 [5] × 10-12 cm2). These levels were responsible for the observed high electrical resistivity (∼1010 ?*cm) in the CdZnTe samples. From a comparison to studies on Bi doped CdTe samples, level L8 was tentatively associated with the (0/-) transition of (BiCd- - OTe) complex, however is still under study. Since these defect levels also act as trapping centers for charge carriers, in spite of the semi-insulating behavior the samples are poor radiation detectors.

Bismuth-induced deep levels and carrier compensation in CdTe

First-principles calculations show that Bi on Cd site in CdTe can be either a donor ${\text{Bi}}_{\text{Cd}}^{+}$ or an acceptor ${\text{Bi}}_{\text{Cd}}^{\ensuremath{-}}$, depending on the Fermi level. The ${\text{Bi}}_{\text{Cd}}^{\ensuremath{-}}$ can bind a substitutional O $({\text{O}}_{\text{Te}})$ with large binding energy of 1.40 eV. The calculated $(0/\ensuremath{-})$ transition level for ${\text{Bi}}_{\text{Cd}}^{\ensuremath{-}}{\text{-O}}_{\text{Te}}$ complex is in good agreement with experimentally observed deep hole trapping level. Bi can also substitute Te to form an acceptor. The amphoteric nature of Bi in CdTe results in the pinning of the Fermi level and the high resistivity. The transition of the CdTe samples from semi-insulating to $p$ type at high Bi doping levels is explained by the formation of secondary phase that contains Bi and Te.

Ion implantation induced disorder in single‐crystal and sputter‐deposited polycrystalline CdTe characterized by ellipsometry and backscattering spectrometry

AbstractBulk single‐crystal CdTe, sputter‐deposited polycrystalline CdTe films of about 2.5 micron thickness, and single‐crystal Si (c‐Si) have been ion implanted using 350 keV Xe at fluences ranging from 1×1013 to 16×1013 cm–2 so as to create disorder in a controlled way from fully single‐crystalline to fully amorphous material. The general purpose of the investigations is to seek a parameterization of the critical point structures and establish a database for fitting the optical properties of CdTe films having different unknown grain sizes whereby the grain size will be described in terms of an effective defect density. The polycrystalline CdTe samples were magnetron sputtered onto c‐Si followed by CdCl2 and Br2‐methanol treatment to improve properties in terms of grain size and surface smoothness, respectively. The fluences for use in the ion implantation of CdTe were estimated using the SRIM (Stopping and Range of Ions in Matter) software, and cross‐checked by simultaneous implantation of bulk c‐Si samples. The optical properties were characterized by second derivative analysis and by a generalized critical point model. Although the damage created by 350 keV Xe in the simultaneously implanted c‐Si samples, as measured by both spectroscopic ellipsometry and Rutherford backscattering/ channeling spectrometry (RBS/C), agrees well with the expectations based on the SRIM simulation, the damage created in CdTe remains at a very low level even for doses several times higher than the amorphization level estimated by simulation. The character of the dechanneling of the RBS/C spectra indicates extended defects (presumably dislocation loops). This effect was similar in both single‐crystal and thin film polycrystalline CdTe, although less pronounced in thin film samples. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical Etching of CdTe in Aqueous Solutions of H2O2-HI-Citric Acid

An iodine-based etching system, H2O2-HI-citric acid, has been developed and tested on CdTe samples with orientations (111)A, (111)B, (110), and (100). The etching velocity of CdTe was shown to depend on sample orientation and other etching conditions. The surface roughness was comparable with that of the surfaces after Br-methanol treatment. A comparative study of the chemical composition of the (211)B CdZnTe surfaces etched under different conditions was performed. X-ray photoelectron spectroscopy (XPS) measurements showed that all treated surfaces of CdZnTe samples are enriched with Te. The HI-based treatment seems to be more acceptable than the Br-methanol treatment in terms of elimination of Te oxides from the surface, however.

A study of the optical absorption in CdTe by photoacoustic spectroscopy

We show that the Photo-Acoustic Spectroscopy (PAS) is an useful alternative method for the determination of the optical-absorption coefficient of CdTe thin films, in the spectral region near to the fundamental absorption edge, ranging from 1.0 eV to 2.4 eV, using an open cell in the transmission configuration. We applied this method to the optical characterization of CdTe layers for several values of their thickness. These CdTe samples were deposited by closed-space vapor transport (CSVT) technique under different growth conditions.

Frequency degenerate and nondegenerate two-photon absorption spectra of semiconductor quantum dots

The frequency degenerate and nondegenerate two-photon absorption (2PA) spectra of direct band gap semiconductor quantum dots are studied. Measuring the spectra for both cases in samples of CdSe and CdTe with different quantum dot sizes and size distributions, we observe that the 2PA spectra and the 2PA coefficient are size dependent, so that smaller dots have smaller 2PA even after taking into account the volume fraction. Theory considering the mixing of the hole bands, in a $\stackrel{P\vec}{k}∙\stackrel{P\vec}{p}$ model, explains the data quite well except for the smallest dots. A comparison with the parabolic band approximation is also shown.

Electron-Backscatter Diffraction of Photovoltaic Thin Films

AbstractIn electron-backscatter diffraction, crystalline orientation maps are formed while the electron beam of an SEM scans the sample surface. EBSD requires a flat sample to avoid shadowing of the electrons from the detector by surface features. In this work, we investigate the preparation of CdTe samples deposited by close-spaced sublimation for EBSD analysis. Untreated samples were rough, resulting in areas with no EBSD signal. We processed the samples by polishing and ion-beam milling. Polishing produced flat samples, but low-quality EBDS data, because the top surface of the samples had poor crystallinity. In contrast, ion-beam milling proved to be suitable for producing flat samples with minimal surface damage, yielding good EBSD data. We also analyzed the samples with atomic force microscopy, and correlated the quality of the EBSD data with sample roughness. The EBSD data showed that the CdTe films were randomly oriented and had columnar growth and a high density of <111> twin boundaries.

Transport Properties of ZnSe- ITO Hetero Junction

In this report, ITO(Indium Tin Oxide) was used on the glass substrates as the transparent electrode, and ZnSe layer was prepared by the vacuum deposition on this ITO. Then, the electrical characteristics of this sample were investigated by mans of the electric current transport analysis. The sample that ZnSe was prepared as 3.4 μm in case of ITO-ZnSe sample, has high density level at the junction surface. The ITO-ZnSe junction has two type of diffusion current. However, the ITO-ZnSe sample that ZnSe layer was prepared as 0.1 μm can be assumed as the ohmic contact, and ITO-ZnSe(0.1μm) -CdTe sample shows the avalanche breakdown, and it is considered that the avalanche breakdown occurs in CdTe layer. It is difficult to occur the avalanche breakdown, if ZnSe-CdTe junction has high-density level and CdTe layer has high-density defect. Hence, the ZnSe-CdTe sample that CdTe layer was prepared on ITO-ZnSe(0.1μm) substrate has not high-density level at the junction surface, and the CdTe layer with little lattice imperfection can be prepared. It found that ITO-ZnSe(0.1μm) substrate is available for the II-VI compounds semiconductor device through above analysis result.

Chemical dissolution of CdTe and Cd1-x ZnxTe solid solution single crystals in bromine etchants of the H2O2-HBr-ethylene glycol system

Equal-etching-rate surfaces for single-crystalline CdTe and Cd1-xZnxTe solid solution samples in etching mixtures of the H2O2-HBr-ethylene glycol (EG) system are mapped under reproducible hydrodynamic conditions using simplex design. The dissolution rate laws for these materials are studied, and the concentration boundaries of existence for polishing and nonpolishing solutions are determined. The increasing zinc concentration in the Cd1-xZnxTe solid solution increases the etching rates, while the concentration boundaries of the polishing solutions in the diagram do not change significantly.

Luminescence investigation of ion milled CdTe

AbstractIon milling effects on CdTe single crystal and MOCVD p ‐CdxHg1–xTe/GaAS multi‐layer hetero‐structures with thick CdTe passivation layer the technology of such diodes are considered. It was revealed that CdTe (1 µm thick) passivation layer displayed the protective properties regarding ion milling action on active p ‐CdxHg1–xTe layer. Electrical measurements did not display the formation of any n ‐type layers in ion milled CdTe samples. Cathodoluminescence spectra of ion milled CdTe samples indicated some reconstruction of native defects in tellurium and cadmium sub‐lattices. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photo-induced current transient spectroscopy studies on polycrystalline CdTe

Abstract Polycrystalline CdTe samples were characterized using photo-induced current transient spectroscopy (PICTS). The spectra obtained show that the traps are not discrete ones, or localized bands in the gap like in monocrystalline samples, indeed the PICTS signal presents large peaks. With an improvement of the calculation method presented here, these spectra reveal two types of traps. The first ones are active at low temperature and have energies between 0.3 and 0.5 eV. The second ones are deep level traps located between 0.65 and 0.8 depending on the sample. Focusing on these deep level traps, their influence on the photoconductive response of the material is discussed.

Characterization of charge collection in CdTe and CZT using the transient current technique

The charge collection properties of Cadmium–Telluride (CdTe) and Cadmium–Zinc–Telluride (CZT) in comparison with Silicon (Si) are presented using the transient-current technique (TCT) where the current pulses are generated by α -particles emitted from an 241Am source. From the recorded current pulse shapes, the charge collection efficiency, the charge carrier mobility and the electric field distribution inside the detectors are extracted. In particular, the signals of the compound semiconductors CdTe and CZT are interpreted with respect to the build-up of space–charges in the sensor volume and the subsequent deformation of the electric field. As high-quality CdTe and CZT samples are now commercially available, the knowledge of these material characteristics is of outmost importance for the application of CdTe and CZT in X-ray imaging. In addition, the paper describes the influence of Ohmic and Schottky contacts on the current pulses in CdTe as well as the effects of polarization, i.e. the time-dependent degradation of the detector signals due to the accumulation of fixed charges within the sensor.

Finite pump-beam-size effects in optical pump-terahertz probe spectroscopy

Finite pump-beam size in optical pump-terahertz (THz) probe experiments introduces distortions in both the signal amplitude and the spectral form. We show both experimentally and analytically that the pump beam must be 3-4 times larger than the THz probe beam for a one-photon excitation process to achieve <10% spectral distortion. Our experiments use thin samples of single-crystal ZnTe or CdTe. Our model treats the pump-induced change in the THz probe field as radiation from a photoinduced current driven by the THz field. Interpretation of nonideal measurements and relaxation of the requirement on the pump-beam size by introduction of a metallic aperture are also discussed.