Ag In CdTe Research Articles

Internal drift effects on the diffusion of Ag in CdTe

Anomalous diffusion profiles of Ag in single crystalline CdTe were observed using the radiotracer 111Ag. The diffusion anneals were performed at 800 K under Cd or Te vapor and in vacuum with low Ag concentrations. The measured Ag profiles directly reflect the distribution of the self-interstitials and vacancies of the Cd sublattice and are the result of chemical self-diffusion which describes the variation of the deviation from stoichiometry of the binary crystal as a function of depth and time. It turned out that the spread of the Ag dopant essentially is determined by the drift of the charged defects within the electric field caused by the distribution of the extrinsic and intrinsic defects.

Thermal stability of substitutional Ag in CdTe

The thermal stability of substitutional Ag in CdTe was deduced from lattice location measurements at different temperatures. Substitutional Ag probe atoms were generated via transmutation doping from radioactive Cd isotopes. The lattice sites of Ag isotopes were determined by measuring the channeling effects of emitted conversion electrons. Below room temperature Ag occupies mainly substitutional lattice sites. With increasing measurement temperature the substitutional fraction of Ag atoms decreases and vanishes completely above 400 K. Simultaneously measured loss of Ag activity indicates that some of the Ag diffuses preferentially to the surface where it disappears. The remaining Ag may be trapped at extended defect complexes or precipitates. In Cd-rich CdTe we suspect an interstitial diffusion directly after the dissociation to the sinks, whereas in Te-rich CdTe this process is possibly retarded by retrapping at Cd vacancies.

Doping and compensation phenomena of Ag in CdTe

In this work the addition of Ag to p type CdTe is systematically studied by electrical characterisation combined with controlled ionimplantation. In one set of experiments radioactive Cd isotopes were implanted, acting during annealing as host atoms, but later transmuting according to the nuclear decay series into Ag acceptors. In this way a doping efficiency close to 100% is obtained. Furthermore Ag ions were implanted into the back side of CdTe wafers to investigate in situ the compensation process. The decrease in free hole density is caused by the formation of complexes involving acceptor impurities and interstitial Ag atoms.

Surface segregation of Ag in CdTe induced by photon absorption

The first observation of a new impurity segregation or gettering phenomenon has been demonstrated in CdTe using radioactive Ag. Noble metal dopant impurities have been found to segregate to the surface region of CdTe during exposure to ambient light. This phenomenon occurs while the sample is held at room temperature and is not induced by localized heating or surface chemical reactions. Experimental results suggest that a redistribution of excess carriers produced by photon absorption near the CdTe surface may be responsible for the behavior.

Complex behaviour of Ag in CdTe

Ag can be diffused into CdTe where it introduces an acceptor level at 108 meV above the valence band. A free hole concentration in the 10 16 cm -3 range is obtained. Samples are analysed by electrical measurements and low temperature PL studies. After diffusion, if the samples are stored at room temperature, they show a progressive degradation of their electrical and optical properties. After about two months, the carrier concentration has decreased to 2 × 10 15 cm -3 and the PL spectra show no memory of the Ag related lines. Simultaneously, the luminescence efficiency becomes very low and we observe the emergence of complex excitonic lines. On re-annealing these samples, their initial state is recovered. This degradation phenomenon is observed in all the Ag diffused samples, but not in the as-grown, annealed or Cu diffused samples.