We report the fabrication, chemical, optical, and photoluminescence characterization of amorphous silicon-rich oxynitride (SiOZNY :H) thin films by plasma-enhanced chemical-vapor deposition. The film compositions were followed by changes in the refractive index. X-ray photoelectron and Fourier transform infrared spectroscopy indicate that the chemical composition is dominated by silicon suboxide bonding with N present as a significant impurity. A broad tunable photoluminescence (PL) emission is visible at room temperature with a quantum efficiency of 0.011% at peak energies to 3.15 eV. The radiative lifetimes are less than 10 ns, and there is nearly no temperature dependence of the PL intensity down to 80 K. Ex situ annealing at temperatures above 850 "C results in an increase in PL efficiency by nearly three orders of magnitude, and the PL intensity is independent of the annealing ambient. The PL results are remarkably similar to literature results in oxidized porous silicon and oxidized nanocrystalline Si thin films, and suggest that the radiative center is due to the defect structure in the silicon suboxide moiety. 0 1995 American institute qf Physics.
We report on the development of Mo/CdTe/CdS/indium-tin-oxide, thin-film solar cells grown by radio-frequency magnetron sputtering. This is an inverted configuration compared to the conventional glass/tin-oxide/CdS/CdTe/metal cells. Molybdenum was chosen as a substrate because its thermal expansion coefficient and the work function are close to those of CdTe. We have achieved AM1.5 conversion efficiencies of 7.8 percent on 0.05 cm2 area devices. Our best cells had a nitrogen-doped ZnTe layer between the molybdenum and the CdTe for a somewhat improved back contact. However, we observe a significant rollover in the IV curve in forward current that indicates a back-diode effect. This implies the need for improvement of the electronic properties of the molybdenum - CdTe and possibly CdS - ITO interfaces.
We have studied the photoluminescence ͑PL͒ mechanism of photo-and electroluminescent amorphous silicon oxynitride films grown by plasma enhanced chemical vapor deposition. The composition of the films was determined by Rutherford backscattering spectrometry and monitored by the index of refraction with single-wavelength ellipsometry. Two sets of samples were grown, each with different reactant gas residence times in the deposition chamber. For samples grown with a residence time of about 5 s, the energy of the PL peak for 2.54 eV excitation is 2.3 eV for stoichiometric films and redshifts with increasing silicon content to 1.7 eV for the most silicon-rich films. The energy of the PL peak for 3.8 eV excitation is 2.8 eV for stoichiometric films and 2.1 eV for the most silicon-rich films. For stoichiometric films, the PL intensity is independent of temperature between 80 and 300 K using 2.54 eV excitation, but the PL intensity decreases by a factor of two over the same temperature range for 3.8 eV excitation. The authors interpret these aspects of the PL as consistent with tail-state recombination. Other results imply the PL is due to a specific luminescence center related to Si-Si or Si-H bonding. A 450 °C anneal reduces the paramagnetic defect density in the films, as detected by electron paramagnetic resonance, by an order of magnitude, but does not increase the PL intensity, while a 950 °C anneal increases both the defect density and the PL intensity. In addition, films in a second set of samples, grown with a residence time of 1.8 s, display very different PL behavior than samples in the first set with the same composition. Samples near stoichiometry in the second set have a PL peak at 2.06 eV and are 20 times less intense than stoichiometric samples in the first set. Optical absorption measurements indicate both types of samples contain Si-Si bonds, with the second set containing many more Si-Si bonds than the first. Fourier-transform infrared measurements indicate the presence of a Si-H bond that is stable at temperatures of 950 °C in the first set, but not in the second set. Thus, the study as a whole suggests a complete picture of luminescence in our silicon oxynitride films must incorporate elements of both tail-state and luminescence center models. The relation of the results to other PL studies in silicon alloys and porous silicon is discussed.
We report electroluminescence ͑EL͒ from 50 nm silicon oxynitride films on p-type crystalline silicon substrates in a Au/silicon oxynitride/Si structure. The EL intensity has a peak below 2.45 eV, and is consistent with radiative recombination of injected carriers. The EL is present only in annealed samples, and the emission is similar to the photoluminescence from the same samples. The current-voltage behavior is indicative of space charge-limited current. No polarity or field dependence of the EL peak energy is observed. This phenomenon is attributed to the relaxation of carriers down the band tails before recombination.
An essential processing step in CdTe/CdS polycrystalline solar cells is heat treatment in CdCl 2 . We present photoluminescence results from single crystals of CdTe that have been exposed to CdCl 2 treatments at 387 C similar to those used in actual cell fabrication. Using sub band gap excitation from a tunable diode laser, we probe states in the interior of the crystal. We show that high-purity (99.998 percent) CdCl 2 treatment results in the appearance of a 1.45 eV donor-acceptor transition that is likely due to a Cl-Cu center. Low purity (99.7 percent) CdCl 2 treatment results in the appearance of the 1.45 eV line and a 1.555 eV Cu-related emission. These results indicate that the CdCl 2 treatment has an effect on the interior of CdTe grains, in addition to its already well established effect on grain boundaries in polycrystalline CdS/CdTe devices. They also imply that CdCl 2 treatment may result in the incorporation of Cu into the CdTe grains. The results will be related to the effects of CdCl 2 on polycrystalline CdS/CdTe devices that have been observed by other groups. This work is supported by NREL
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