Cu In CdTe Research Articles

Quantitative analysis of Cu XANES spectra using linear combination fitting of binary mixtures simulated by FEFF9

X-ray absorption near edge structure (XANES) is a powerful tool to probe the fingerprint of local structures, and when coupled with X-ray microscopy, the small spot size enables one to probe very specific regions of interest in a device or material, e.g. interfaces/bulk, different grains, good/bad electrical areas. In this work, we investigate the use of linear combination fitting (LCF) of XANES spectra for the particular case of Cu doping in CdTe. We show that the experimental data seem to be accurately represented by standards of Cu2Te and its substoichiometric counterpart, Cu1.43Te. We use Cu in CdTe as a case study to evaluate the accuracy of linear combination fitting using simulated standards, given that experimental standards for certain phases (e.g. Cu1.43Te) or defect structures (e.g. vacancies) cannot be readily obtained. We discuss how spectral features of the FEFF9-simulated standards, fitting ranges, and noise levels all dictate the accuracy of this type of analysis. We show that the greater the spectral difference between the two standards, the better the LCF is able to differentiate between the two structures and to tolerate experimental noise. Finally, we estimate the error of the fitted weights for different spectral features and noise levels and propose a framework to study local structures semi-quantitatively by using binary mixtures of FEFF9-simulated standards.

Cu diffusion in CdTe detected by nano-metal-plasmonic enhanced resonant Raman scattering

Copper is widely used as a p-dopant in CdTe thin film solar cells. However, Cu is not stable in CdTe and its diffusion is a big concern regarding the device stability when the CdTe solar cell is applied for long-term deployment. In this work, a new technique of nano-metal-plasmonic enhanced resonant Raman scattering (RRS) was demonstrated for quantitative study of Cu diffusion in CdTe along grain boundaries. The experimental setup was designed to directly detect Cu diffusion in CdTe. The experimental results showed that Cu diffusion in CdTe can be well described by the classical diffusion mechanism. The activation energy of Cu thermal diffusion was found to be 0.40 eV, which explains the easy diffusion of Cu in CdTe.

The Role of Intrinsic Defects for the Diffusion of Ag and Cu in CdTe

Ag and Cu diffused into CdTe exhibit unusual shapes of their concentration profiles, especially if the diffusion was performed under Cd vapor pressure. The shapes of the concentration profiles at low concentrations of Ag or Cu are well described by a model based on the interaction with intrinsic defects. In that model the characteristic features of the profiles can be independently reproduced by appropriate parameters. It turns out that the resulting profiles are determined by the diffusion of interstitial Cd atoms and reflect the actual distribution of the intrinsic defects in the crystal, i.e. the deviation from stoichiometry.

Diffusion and influence of Cu on properties of CdTe thin films and CdTe/CdS cells

The effective diffusion coefficients of Cu for thermal and photodiffusion in the CdTe films have been estimated from resistivity versus duration of thermal or photoannealing curves. In the temperature range 60–200°C the effective coefficient of thermal diffusion ( D t ) and photodiffusion ( D ph ) are described as D t =7.3×10 −7 exp(−0.33/ kT ) and D ph =4.7×10 −8 exp(−0.20/ kT ). It is found that the diffusion doping of CdTe thin films by Cu at 400°C results in a sharp decrease of resistivity up to 7 orders of magnitude of p-type material, depending on thickness of Cu film. The comparative study of performance of CdTe(Cu)/CdS and CdTe/CdS cells has been studied. It is shown that the diffusion doping of CdTe film by Cu increases efficiency of CdTe(Cu)/CdS cells from 0.9% to 6.8%. The degradation of photovoltaic parameters of CdTe(Cu)/CdS cell, during testing under forward and reverse bias at room temperature, proceeds at a larger rate than those of CdTe/CdS cell without Cu. The degradation of performance of CdTe(Cu)/CdS cells is tentatively assigned to electrodiffusion of Cu in CdTe, resulting in redistribution of concentration of Cu-related centers in CdTe film and heterojunction region.

The unusual influence of intrinsic defects on the diffusion of Ag and Cu in CdTe

The diffusion of Ag and Cu in CdTe was investigated using the radiotracers 111Ag and 67Cu. The diffusion properties are similar for both dopants and are essentially determined by diffusion of intrinsic defects in the crystal. Both dopants show up interstitial fractions which are incorporated substitutionally if Cd vacancies are present. The resulting unusual diffusion profiles map the actual distribution of the intrinsic defects and indicate the chemical self diffusion in CdTe.

Cu K-edge XAFS in CdTe before and after treatment with CdCl2

AbstractWe have used the fine structure in the Cu K-edge x-ray absorption spectrum to help elucidate the lattice location of Cu in polycrystalline, thin-film CdTe solar cells. In particular, we have studied how the typical CdCl2 vapor treatment in dry air changes the local environment of the Cu in CdTe. We find the Cu absorption spectrum to be similar to that of Cu2Te in the as-deposited CdTe film but to convert to a spectrum similar to Cu2O environment after the vapor CdCl2 treatment.

Room-temperature detection of mobile impurities in compound semiconductors by transient ion drift

We show that the transient ion drift (TID) method, which is based on recording junction capacitance under constant reverse bias [A. Zamouche, T. Heiser, and A. Mesli, Appl. Phys. Lett. 66, 631 (1995)], can be used not only for measurements of the diffusion coefficient of mobile impurities, but also to estimate the concentration of mobile species as part of the total dopant density. This is illustrated for CdTe, contaminated by Cu, and intentionally doped by Li or Ag and for CuInSe2. We show also that, with some restrictions, the TID method can be used if the mobile ions are major dopants. This is demonstrated using Schottky barriers on CdTe, and p–n junction devices in (Hg,Cd)Te, and CuInSe2. The values that we obtain for the diffusion coefficients (for Li, Ag, and Cu in CdTe and for Cu in CuInSe2) agree well with measured or extrapolated values, obtained by other methods, as reported in the literature. Furthermore, we could distinguish between diffusion and chemical reactions of dopants, as demonstrated for the case of Cu in CdTe and Ag-doped (Hg,Cd)Te. In the former case this allows us to separate copper-free from contaminated CdTe samples.

Structural and electrical properties of RF sputtered Cu/Te/CdTe contact

The Cu/Te/CdTe contact has been investigated using electrical and structural measurements. The contact resistance rc exhibits peaks depending on the annealing temperature. These peaks are attributed to successive phase transformations. Structural investigations show that two phase transformation stake place: at low temperature, the diffusivity of Cu in CdTe is the most important leading to the phase transformation: Cu2−xTe → Cu7Te5, while at high temperatures, the diffusivity of Te is greater and the reverse phase transformation takes place.

A comparison of the diffusion of copper, gallium and cadmium in cadmium telluride

AbstractThe rates of diffusion of both Ga and Cu in CdTe have been measured over the temperature ranges 350–820 and 200–400°C respectively and the results are compared with the self‐diffusion of Cd (350–920°C).In the case of the Cd self‐diffusion the Arrhenius graph showed two active diffusion mechanisms, one dominating above 500°C and the other below. It suggested that in the temperature range 500–800°C the main defect responsible for diffusion was Cd, with a significant contribution at lower temperatures from the associated defect of the form (CdiVcd)x Below 500°C another mechanism predominated which was possibly due to residual impurities.The results for the diffusions were in close agreement with those obtained by other workers. The Ga diffusions showed a complex behaviour which suggested that two mechanisms were active simultaneously, one independent of Cd partial pressure and the other decreasing with increasing Cd partial pressure.It was concluded that CdTe would be suitable as a diffusion barrier to protect HgxCd1 − xTe devices from Ga contamination from GaAs substrates but would not be efficient at reducing Cu contamination from the substrate in the device.

Some Diffusion and Solubility Measurements of Cu in CdTe

The diffusivity and solubility of Cu in CdTe has been measured between 97° and 300°C using radiotracer techniques. The system studied involved chemically formed Cu2Te films on undoped zone-purified CdTe single crystals. The solubility was found to vary from approximately 3×1016 to 2×1018 cm−3 in the temperature range studied. The diffusivity corresponding to these solubilities is given by D(cm2·sec−1) =3.7× 10−4 exp (−0.67 eV/kT). A much faster diffusing ``tail'' involving about 1015 cm−3 Cu is also observed.