Doped CdTe Research Articles

Modification of the surface state and doping of CdTe and CdZnTe crystals by pulsed laser irradiation

The photoelectric and electrical properties of high-resistivity p-like CdTe and Cd 0.96Zn 0.04Te single crystals and barrier structures on their base before and after laser irradiation in different conditions are studied. Irradiation of samples with nanosecond ruby laser pulses was carried out in two different ways. In the first case, the Cd(Zn)Te crystals were subjected to laser action directly from the surface and irradiation within a certain range of intensities resulted in a decrease in the surface recombination rate and increase in the photoconductivity signal. The surface region with a wider bandgap in CdZnTe crystals was formed. In the second case, the samples were irradiated from the side pre-coated with a relatively thick In dopant film and it caused rectification in the I–V characteristics as a result of laser-induced doping of the thin Cd(Zn)Te surface region and formation of a built-in p-n junction. The application of the fabricated M- p- n structured In/Cd(Zn)Te/Au diodes for X-ray and γ-ray detectors is discussed.

Correlated structural reordering and dopant redistribution in annealed polycrystalline CdTe

Postdeposition heat treatments are found to strongly influence the structural and optical properties of chlorine doped polycrystalline CdTe films. From scanning electron microscopy images and diffraction measurements, annealing is shown to induce a major structural reordering. The strong texturation along the ⟨100⟩ orientation indicates that the overall crystalline quality has been considerably improved. Upon annealing, a bimodal distribution of grain sizes is also observed, which points out an abnormal grain growth mechanism. Additionally, deformation twins, which are detrimental for the transport properties, are replaced by thicker annealing twins. Through time-of-flight secondary ion mass spectroscopy and 4 K cathodoluminescence measurements, an overall decrease in the chlorine concentration and a spatial redistribution of chlorine atoms are evidenced and clearly correlated with the observed structural reordering: β acceptor complexes, which are commonly predominant in as-grown chlorine doped polycrystalline CdTe, are found to dissociate into chlorine donors and A centers upon annealing. This is attributed to the thermally activated diffusion of chlorine, which leads to its re-evaporation from the films. Furthermore, the low chlorine concentration in the annealed films allows one to infer a value of 135±3 meV for the ionization energy of the chlorine A centers, as deduced from the donor-acceptor pair position in the cathodoluminescence spectra.

The effect of impurities on the doping and VOC of CdTe/CdS thin film solar cells

The effect of introducing impurities in CdTe, namely antimony (Sb) and oxygen (O), on the net carrier concentration in CdS/CdTe solar cells and on their open-circuit voltage ( V OC) has been investigated. Oxygen was introduced in the CdTe films during the deposition of this layer by the close-spaced sublimation process. The total pressure was held constant at 1330 Pa (N 2 and O 2). The amount of oxygen was varied by varying its partial pressure. Antimony was introduced into CdTe using a post-deposition diffusion process. Following the deposition of CdTe a thin film (a few nm) of Sb was deposited onto the CdTe surface and subsequently heat-treated to cause in-diffusion of Sb. The temperature and time during the diffusion process were varied in the range of 300–525 °C and 20–160 min respectively. In both instances it was possible to vary (increase) the doping concentration in CdTe. The increase in doping was accompanied by an increase in V OC. However, in all instances the doping in CdTe reached a maximum value, beyond which further increases were not possible leading to saturation in V OC. The highest V OC measured was similar to state-of-the-art values in the range of 800–830 mV, and the highest doping concentration measured was in the 10 16 cm − 3 range.

Non-Exponential Photoionization of the DX Centers in Gallium Doped CdTe and Cd0.99Mn0.01Te

Non-Exponential Photoionization of the DX Centers in Gallium Doped CdTe and Cd0.99Mn0.01Te

Formation of DX-centers in indium doped CdTe

In CdTe, the achievable n-type doping is limited by the formation of DX-centers. A characteristic feature of DX-centers is the ‘persistent photoconductivity (PPC)’ which is created by illumination at low temperatures and caused by a metastable state of the DX-center. The DX-center and the PPC effect in n-type CdTe are theoretically explained by the ‘large lattice relaxation model’. PAC measurements on In doped CdTe using 111In/111Cd and, in addition, resistivity measurements on the same samples have been performed. Below 150 K, the samples showed a PPC effect that was accompanied by an increase of about 20% of the carrier concentration. This effect is not accompanied by any changes of the observed EFG. Possible explanations of the EFG observed, originally assigned to the DX-center, will be discussed. Finally, first reports on the investigation of DX-centers in CdTe using the radioactive isotope 117Cd decaying to 117In are presented.

Effects of island coalescence on the compensation mechanisms in chlorine doped polycrystalline CdTe

Combined structural, chemical, and spectroscopic analyses have been carried out on chlorine (Cl) doped polycrystalline CdTe before, upon, and after coalescence of the initial isolated islands in order to investigate the effects of the formation of grain boundaries on Cl incorporation and on the related compensation mechanisms. From 4 K cathodoluminescence measurements, coalescence is found to be associated with a change in the Cl related compensation mechanisms in particular with the appearance of shallow acceptor complexes noted β. Simultaneously, the donor acceptor pair transitions involving A centers shift toward higher energies, indicating a decrease of the donor to acceptor separation. These observations are tentatively assigned to the segregation of Cl atoms in the vicinity of grain boundaries, upon coalescence, as directly shown by time-of-flight secondary ion mass spectroscopy imaging. Furthermore, long after coalescence, on thick polycrystalline CdTe, the density of β acceptor complexes is even larger due to twinning deformation processes suggesting further segregation phenomena within grains.

Highly Arsenic Doped CdTe Layers for the Back Contacts of CdTe Solar Cells

AbstractIn an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 at·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.

Spectroscopic analysis of defects in chlorine doped polycrystalline CdTe

The effects of Cl doping of thick polycrystalline CdTe layers grown by close space sublimation on their crystalline structure and the compensation and passivation processes have been investigated. From an extensive low temperature photoluminescence study, it is shown that, in polycrystalline CdTe:Cl, the main compensation processes are similar to those occurring in monocrystalline CdTe:Cl. However, specific compensation processes are also evidenced in polycrystalline CdTe: Defect complexes can be formed between Cl atoms and impurities in the vicinity of extended defects such as dislocations, twin boundaries, and grain boundaries. Furthermore, using low temperature cathodoluminescence imaging, chlorine induced passivation processes are proposed for defects such as double positioning twin boundaries within the grains. On the structural side, a bimodal distribution of the grain size is found: While small grains (5μm in diameter) are predominant at low Cl doping (around 2×1017at.cm−3), larger grains (40μm in diameter) prevail at higher doping levels (around 6.2×1017at.cm−3), with the proportion of small versus large grain decreasing as the doping level increases. Interestingly, while the radiative emission of small grains is mainly attributed to compensating complexes implying chlorine and Cd vacancies, in larger grains, the luminescence comes from deeper levels most probably associated with compensating complexes implying residual impurities.

Photoelectrical and noise processes in CdTe crystals

AbstractTwo types of cadmium telluride single crystals were investigated: undoped CdTe grown by Bridgeman method and highly doped CdTe:Cl obtained by STHM. Photoconductivity properties of CdTe crystals were studied by the transient photoconductivity method. Persistent photo response due to the polarization effect was revealed in undoped samples. Noise spectra have been measured over a wide range of frequencies, from below 1 Hz up to 1 MHz, and for voltages applied the samples from 0 up to 200 V. Two type of noise dominates in the range of voltages and frequencies under investigation, namely excess 1/f noise and generation‐recombination (G‐R) noise. Two‐phase model were applied to explain noise experimental results, which consider the presence of precipitates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computational study of Ge and Sn doping of CdTe

The formation and ionization energies of substitutional Ge and Sn dopants in CdTe are calculated in a supercell model within the local-density approximation to density-functional theory. Doping on both the Cd and Te sublattices is considered, but the formation energy of both defects is predicted to be much lower on the Cd site under most growth conditions. The Ge and Sn on the Cd sites are predicted to be deep donors and hole traps with defect ionization levels near the midgap, with Ge slightly lower than Sn, in good agreement with experiments. The Ge and Sn on the Te sites are predicted to be shallow acceptors.

Etching Behavior of Undoped and Doped CdTe in Aqueous H2O2-HBr-Ethylene Glycol Mixtures

The mechanism and kinetics of undoped and doped CdTe dissolution in aqueous H2O2-HBr-ethylene glycol mixtures are investigated. The results are used to construct the dissolution rate-etchant composition diagrams for the materials studied and to identify the rate-limiting step of the dissolution process. Dissolution in polishing etchants is shown to be diffusion-limited. Doping has a significant effect on the dissolution behavior of CdTe.

Density functional theory calculations establish the experimental evidence of the DX center atomic structure in CdTe.

The In DX center and the DX-like configuration of the Cd host atom in CdTe are investigated using density functional theory. The simultaneous calculation of the atomic structure and the electric field gradient (EFG) allows one to correlate the theoretically predicted structure of the DX center with an experimental observable, namely, the EFG obtained from radioactive 111In/111Cd probe atoms in In doped CdTe. In this way, the experimental identification of the DX center structure is established.

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.

P-type doping of II–VI heterostructures from surface states: Application to ferromagnetic Cd1−xMnxTe quantum wells

We present a study of p-type doping of CdTe and Cd1−xMnxTe quantum wells from surface states. We show that this method is as efficient as usual modulation doping with nitrogen acceptors, and leads to hole densities exceeding 2×1011 cm−2. Surface doping was applied to obtain samples with Cd1−xMnxTe quantum well with up to x=9.3% containing hole gas. We could also increase the growth temperature up to 280 °C, which results in sharper photoluminescence lines, when compared to the similar nitrogen doped samples. Carrier-induced ferromagnetism was observed in surface doped samples.

Back contact formation using CuTe as a Cu-doping source and as an electrode in CdTe solar cells

Cu2Te was utilized as a Cu source for p + doping in CdTe and as a primary back contact material in CdTe solar cells. A 60 nm-thick Cu2Te layer was deposited on CdTe film by evaporating Cu2Te and the samples were annealed at various temperatures. An amorphous layer was found at the Cu2Te/CdTe interface, while the Cu2Te has both orthorhombic and hexagonal phases. Annealing at 2001C completely crystallized the amorphous interlayer and enhanced the transformation of orthorhombic phase into hexagonal phase that has a coherent interface with CdTe. A good p + contact was formed at 1801C annealing, where the series

First-Principles Study of Doping Limits of CdTe

First-principles total energy and band structure calculations are performed to understand the factors that limit doping in CdTe. We calculated systematically the formation energies and transition energy levels of intrinsic and extrinsic defects and defect complexes. We find that n-type doping in CdTe is limited by the spontaneous formation of the intrinsic closed-shell cation vacancy VCd2— and/or DX centers. For p-type doping, it is limited by not having a dopant with both high solubility and shallow acceptor level.

Transport Properties and Viscosity of Liquid CdTe Doped with In, Ge, and Sn

Data are presented on the high-temperature electrical conductivity, thermoelectric power (up to 1825 K), and viscosity (up to 1400 K) of undoped and doped (In, Ge, and Sn) CdTe melts. All of the materials were found to retain semiconducting properties upon melting, with a gradual increase in the contribution of metallic bonding, especially pronounced for the CdTe + 2 mol % Sn melt. The results are interpreted in terms of the double-structured melt model, which considers the coexistence of densely packed metallic regions and crystal-like CdTe clusters. The transition to metallic behavior of conductivity is accounted for by a gradual increase in the volume fraction of the densely packed, metallic phase. The doping effects on the conductivity and thermoelectric power of liquid CdTe are interpreted in terms of s–p hybridization.

Electron correlation effects at variable-range hopping in doped GaAs, CdTe, Ge and Si

We report low-temperature electrical resistivity and magnetoresistivity measurements of doped semiconductors below the critical concentration of the metal-insulator transition. The series of sample...

Electron correlation effects at variable-range hopping in doped GaAs, CdTe, Ge and Si

Electron correlation effects at variable-range hopping in doped GaAs, CdTe, Ge and Si