Polycrystalline CdTe Thin Films Research Articles

Electrochemical deposition and characterization of CdTe polycrystalline thin films

CdTe thin films have been prepared by potentiostatic electrodeposition from acidic solutions containing CdSO 4 and TeO 2, or CdCl 2 and TeO 2. The films were characterized by X-ray diffraction and scanning electron microscopy. The influence of the deposition potential and bath temperature on the presence of tellurium and crystallite size was studied. The effect of annealing treatment on the increase in grain size of the electrochemically deposited CdTe films has been investigated.

Study of the Defect Levels and Interface Properties of CdTe and CdS Polycrystalline Thin Films

ABSTRACTThe properties (electrical and structural) and the defect levels dominating cadmium telluride (CdTe) films prepared by radio frequency (rf) planar magnetron sputtering, and electrochemical deposition have been determined and compared. The properties of the deposited CdTe film and the behavior of its interface with cadmium sulfide (CdS) depend strongly on the method of depositing the CdTe film, and on postdeposition heat treatments. These treatments determine various parameters crucial to the device performance, including the type and concentration of the dominant defects, interface states, and deep trap levels. Photoluminescence (PL) emission from p-type CdTe polycrystalline thin films and single crystals is reported at low temperatures (9–50 K) as well as at room temperature. The room temperature PL peak at 1.58 eV due to band-to-band recombination was observed for the first time in polycrystalline thin films. The peak location of the exciton emission is indicative of the structure perfection of the film, which is also related to the deviation from stoichiometry. The as-grown polycrystalline films are composed of a close-packed array of preferentially oriented (the [100], [110], and [111] axes aligned perpendicular to the substrate) single-crystal grains of a size in the range of 0.25–2.0 μm for the electrochemically deposited films (E-film) and 0.3–4.0 μm for rf sputtered material (rf-film). The resistivity of the heat treated E-film was substantially lower than that of the rf-film (ρ = 1.0–5.0 and 300–500 Ωcm). The performance of the CdS/(E-CdTe) devices is limited by deep levels, while the performance of die rf-CdTe device is affected by a high density of interface states.

Electroreflectance, photoreflectance, and photoabsorption properties of polycrystalline CdTe thin films prepared by the gradient recrystallization and growth technique

In this work we report electroreflectance, photoreflectance, and photoabsorption measurements on CdTe polycrystalline thin films. These thin films were prepared by the gradient recrystallization and growth technique. The measurements were performed around the fundamental energy gap of the semiconductor at room temperature. The energy gap of this polycrystalline material coincides with that corresponding to the single-crystal material, and the phenomenological energy broadening parameter is larger than that corresponding to the single-crystal semiconductor by a factor of three, approximately. The photoabsorption signal is obtained by studying the origin of a low-energy peak which appears in the photoreflectance spectrum.

Hydrogenation effects in sputtered polycrystalline cadmium telluride

The hydrogenation of rf sputtered thin film polycrystalline CdTe is reported. Hydrogenation is achieved by reactive sputtering in an argon and hydrogen mixture. Results include an order of magnitude decrease in conductivity, an increase and sign change in band bending for Au-CdTe junctions, and a significant increase in the anomalous photovoltaic effect (APE). APE coplanar voltages increase from approximately 10 V/cm without hydrogen to a maximum of 350 V/cm with hydrogenation. The results are interpreted in terms of increased band bending at crystallite surfaces and interfaces due to hydrogenation. These results support a previously proposed mechanism for the APE due to surface photovoltages in inclined crystallites.

Electrical properties of Te-rich, Cd-rich and hydrogen-exposed polycrystalline CdTe thin films

Polycrystalline films of CdTe, stoichiometric, Te-rich, and Cd-rich, were grown using a vacuum evaporation technique on glass substrates kept at room temperature. The as-grown films were exposed to hydrogen gas at high pressure (200-500 PSI). Measurements of DC conductivity in the temperature range 77-300K, X-ray diffraction studies and electron probe microanalyses were made on these films. The conductivity data for the stoichiometric, Te-rich and Cd-rich films in the low temperature region have been analysed for variable range hopping conduction on the basis of Mott's model. The analysis of the high temperature conductivity data for Te-rich and hydrogen-exposed films has been based on Seto's model of thermionic emission. It is found that addition of Te and Cd improves the conductivity of the films by four and six orders of magnitude respectively. However, the density of trap states near the Fermi level, N(EF), also increases with increase of Cd and Te content. In the case of hydrogen-exposed films the conductivity decreases with increase of gas pressure.

Evaluation of CdTe thin films grown by quasi-rheotaxy for photovoltaic applications

Quasi-rheotaxial growth has been studied in p-type CdTe polycrystalline thin films grown by co-evaporation on Bi coated substrates. The thin film crystalline and electrical properties have been investigated as a function of growth temperature using SEM, X-ray and surface photovoltage techniques. Quantitative measurements of the grain size, orientation and minority carrier diffusion length indicate that material grown by this technique is suitable for photovoltaic applications provided the growth temperature and substrate preparation are well controlled. The results indicate that further improvement in film quality could be obtainable if quasi-rheotaxy could be utilised at higher growth temperatures.

Traps in polycrystalline CdTe thin films

The presence of localized states in polycrystalline CdTe thin films is investigated using measurements of thermally stimulated depolarization currents (TSDC) following photopolarization, of thermally stimulated currents (TSC), and of impurity-induced photoconductivity. In this way four groups of trapping levels situated at the depths of 0.12, 0.35, 0.47, and 0.73 eV are found. Die Anwesenheit von lokalisierten Zuständen in dünnen polykristallinen CdTe-Schichten wird durch folgende Messungen untersucht: thermisch stimulierte Depolarisationsströme (TSDC) nach Photopolarisation, thermisch stimulierte Ströme (TSC) und störstelleninduzierte Photoleitung. Auf diese Weise werden vier Gruppen von Haftstellen gefunden, die 0,12; 0,35; 0,47 und 0.73 eV tief liegen.