Deep Levels In CdTe Research Articles

Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

This work is aimed at studying defect level distributions in the bandgap of CdTe thin films, used for solar cell development. In particular, the effects of CdCl2 treatment on the defect levels are the main objectives of this research. Four different nearly optimised CdTe thin films were electroplated using three different Cd-precursors (CdSO4, Cd(NO3)2 and CdCl2), and bulk CdTe wafers purchased from industry were studied using low temperature photoluminescence. The finger prints of defects, 0.55 eV below the conduction band down to the valence band edge were investigated. In all of the CdTe layers, four electron trap levels were observed with varying intensities but at very similar energy positions, indicating that the origin of these defects are mainly from native defects. CdCl2 treatment and annealing eliminates two defect levels completely and the mid-gap recombination centres are reduced drastically by this processing step. The optical bandgap of all four as-deposited CdTe layers is ~1.50 eV, and reduces to ~1.47 eV after CdCl2 treatment. The material grown using the CdCl2 precursor seems to produce CdTe material with the cleanest bandgap, most probably due to the built-in CdCl2 treatment while growing the material.

Electrical Characterization of Cu Composition Effects in CdS/CdTe Thin-Film Solar Cells With a ZnTe:Cu Back Contact

We study the effects of Cu composition on the CdTe/ZnTe:Cu back contact and the bulk CdTe. For the back contact, its potential barrier decreases with Cu concentration while its saturation current density increases. For the bulk CdTe, the hole density increases with Cu concentration. We identify a Cu-related deep level at ∼0.55 eV whose concentration is significant when the Cu concentration is high. The device performance, which initially improves with Cu concentration then decreases, reflects the interplay between the positive influences (reducing the back contact potential barrier while increasing the saturation current density of the back contact and hole density in CdTe bulk) and negative influences (increasing deep levels in CdTe) of Cu.

Investigation of the origin of deep levels in CdTe doped with Bi

Combining optical (low temperature photoluminescence), electrical (thermoelectric effect spectroscopy), and structural (synchrotron X-ray powder diffraction) methods, the defect structure of CdTe doped with Bi was studied in crystals with dopant concentration in the range of 1017–1019at.∕cm3. The semi-insulating state observed in crystals with low Bi concentration is assigned to the formation of a shallow donor level and a deep donor recombination center. Studying the evolution of lattice parameter with temperature, we postulate that the deep center is formed by a Te–Te dimer and their formation is explained by a tetrahedral to octahedral distortion, due to the introduction of Bi in the CdTe lattice. We also shows that this model agrees with the electrical, optical, and transport charge properties of the samples.

Deep-level photoluminescence in semi-insulating CdTe(In) and CdTe(Sn)

Photoluminescence (PL) of two groups of samples was investigated. One group was doped with Sn having a deep level near the middle of the forbidden gap. The second group was doped with In, which acts as a shallow donor. The nature of the deep levels in CdTe:In is not well known so far. A very intensive PL at deep levels is observed in case of CdTe:Sn samples. The intensity is approximately two orders of magnitude higher than in In-doped samples. Comparison of the radiative recombination activity at deep levels (0.3–1.2 eV) with the total PL signal (0.3–1.6 eV) shows that ∼0.2–1% of radiative recombination occurs at deep levels in case of CdTe:In samples and 7–15% in case of CdTe:Sn. A qualitative correlation between the mobility-lifetime product of electrons and the integral PL intensity at deep levels was observed. A correlation between integral PL intensity of A-centers and deep levels in CdTe:In samples was found. This supports the idea that the deep levels in CdTe:In samples are primarily complexes formed together with A-centers. The integral PL in the spectral range of deep levels can therefore serve as an indicator of sample quality from the point of view of its suitability for fabrication of detectors of gamma and X-ray radiation.

Cathodoluminescence characterization of Ge-doped CdTe crystals

Cathodoluminescence (CL) microscopic techniques have been used to study the spatial distribution of structural defects and the deep levels in CdTe:Ge bulk crystals. The effect of Ge doping with concentrations of 1017 and 1019 cm−3 on the compensation of VCd in CdTe has been investigated. Dependence of the intensity distribution of CL emission bands on the dopant concentration has been studied. Ge doping causes a substantial reduction of the generally referred to 1.40 eV luminescence, which is often present in undoped CdTe crystals, and enhances the 0.91 and 0.81 eV emissions.

Deep levels in semi-insulating CdTe

A general review on deep levels in CdTe created by doping of group IV elements and of 3d transition metal impurities is presented. We compare these data with some of our experimental results on Fe- and V-doped CdTe crystals. The problem of compensation is discussed.

Investigation of Deep Levels In X-Ray Detector Material With Photo Induced Current Transient Spectroscopy (Picts)

ABSTRACTDeep levels have a great influence on the recombination behavior of the free carriers in semiconductors. For several years PICTS has been used to investigate the deep levels in high resistivity material such as GaAs or CdTe used in detector applications. An important feature of the PICTS measurements is the analysis of the current transients after pulse excitation. We propose using a new method based on Tikhonov regularization. This method was implemented in the program FTIKREG (Fast Tikhonov Regularization) by one of the authors. The superior resolution of the regularization method in comparison to conventional techniques is shown using simulated data. Moreover, the method is applied to investigate deep levels in CdTe:Cl, SI-GaAs and GaAs:Cr samples used for room temperature radiation detectors. A relation between deep level properties and detector performance is proposed.

Characterization of deep levels in CdTe by photo-EPR and related techniques

In photo-EPR, the EPR signature of microscopically identifiable defects is used to monitor light-induced electronic transitions. In this paper we summarize photo-EPR investigations of group IV donors, 3d transition metal impurities and complexes in CdTe. Energy levels and Huang-Rhys factors are derived for the 3d impurities by fitting a model calculation to the photo-ionization cross-section which is derived from constant photo-EPR. Comparison with results for other II–VI compounds confirm host-independent relative zero-phonon level positions for different 3d metal impurities and large lattice relaxation energies.

Deep levels in CdTe

We have used a variety of complementary techniques to study electronic trapping levels that are far from either band edge in CdTe. These deep levels play an important role in determining the electrical properties of the material. We have studied and attempted to identify levels due to specific defects and those due to either intentional or unintentional dopants. These findings have been correlated with information from the literature. The methods used included both DC and AC photoconductivity, photocurrent induced transient spectroscopy (PITS), and deep level transient spectroscopy (DLTS). CdTe samples doped with aluminum, indium, tin, cesium, and phosphorus, as well as nominally undoped sampled have been examined. Fifteen different levels have been identified as related to these impurities.