Doped CdS Research Articles

Metal Vacancies in Li and In Doped ZnSe and CdS

Metal Vacancies in Li and In Doped ZnSe and CdS

Optical properties of indium doped CdS thin films

Thin films of polycrystalline In doped CdS with a wide range of indium concentrations ( 10 17−10 21 cm −3 ) have been deposited by indium and CdS coevaporation. Their optical properties in the visible and near-infrared region have been determined by optical transmission measurements and correlated to the indium concentration and substrate temperature. The refractive index presents a maximum at an indium concentration in the range 10 19−10 20 cm −3 The energy dependence of the absorption edge on indium concentration is due to the Burnstein-Moss shift. The width of the exponential tail in the absorption coefficient increases with indium concentration and softens the abrupt abrorption edge of the CdS thin films.These dependences have been correlated with the incorporated quantity of indium into the CdS crystallites.

Electrical conduction in polycrystalline CdS films. II. Comparison of theory and experiment

For pt.I see ibid., vol.20, p.951-57 (1987). The electrical conduction in polycrystalline In doped CdS films has been studied over the temperature range 77-300 K using Hall effect and conductivity measurements. In the preceding paper a model for the electrical conduction in polycrystalline CdS has been developed. The comparison of the model to the experimental data gives a satisfactory interpretation of the transport parameters in the In doped CdS films. The conduction is limited by the energy barriers, even at high dopant densities, and tunnelling transport becomes predominant. The trap state density increases with increasing dopant density.

Morphology and electrical properties of In doped CdS thin films

The effect of In doping, used to reduce the high resistivities of CdS films deposited at substrate temperatures above 300 °C, on film structure and morphology was studied because of their importance as an appropriate n-type window layer in several types of heterojunction solar cells. Doping with In, even at the lowest doping levels utilized, resulted in the desired reduction in the resistivities from 105 to 10−2 Ω cm. The films were n type with electron concentration of 1018 to 1019 cm−3 and mobility of 14 to 75 cm2 V−1 s−1. At low doping levels, the CdS films remained compact, maintaining their columnar growth, with grain diameters ∼4 μm and the columns mostly along the c axis of the wurtzite phase of CdS being aligned normal to the substrate surface. With higher In incorporation, a slow transformation to much smaller elongated grains, with c axis along the substrate surface was observed. This has been attributed to the segregatino of In at the grain boundaries as CdIn2S4.

Luminescence of Rare-Earth-Activated Cadmium Sulfide

The narrow emission lines of Nd3+, Er3+, and Yb3+ have been observed from rare-earth doped CdS. These activators formed the principal radiative recombination centers in selected crystals. Decay-time measurements gave fluorescent lifetimes of ∼100 μsec for CdS:Yb, ∼300 μsec for CdS:Nd and ∼3000 μsec for CdS:Er. Both the emission spectra and lifetime measurements showed that the rare-earth ions occupied more than one type of site in the CdS lattice. No emission was observed from a rare-earth ion metastable level greater than 1.53 eV above the ground state, even though the CdS band gap was 2.43 to 2.52 eV in the temperature range of the experiments. This is taken as evidence for rare-earth-ion-acceptor-type defect pairing.

Low-field breakdown, non-ohmic conductivity, and photoconductivity of CdS at low temperatures

Low temperature electrical conductivity and photoconductivity effects were investigated in highly doped CdS samples having room temperature resistivities in the range 0.08–15 ohm-cm. These samples were found to have a low-field breakdown effect at electric fields which varied from 265 V cm to as high as 5500 V cm depending on the sample. At fields below the breakdown field, the conductivity was concluded to be of the impurity band conduction type since the Hall mobility at 4.2°K was found to be less than 0.03 cm 2 V-sec . At these fields the conductivity was found to increase markedly as the applied electric field was increased. These crystals were found to exhibit an impurity photoconductivity effect with a maximum response at 28μ and a long-wave cut-off at 32μ.