Studies of the Effects of Annealing, Partial Degassing and Light Soaking on Sputtered A-Si:H Films Through Analysis of Infrared and Thermal Effusion Results

Abstract

Hydrogenated amorphous silicon, a-Si:H, is a promising material with ideal properties for various types of optoelectronic devices. At the same time, the material suffers from some serious disadvantages including Staebler wronski effect (SWE), in which the efficiency of a solar cell decreases gradually with prolong exposure to light. It is believed that the quality of a-Si:H alloy is closely related to its hydrogen content and to the nature of silicon-hydrogen bonds. On the otherhand, hydrogen has indirect influence on the occurrence of SWE. a-Si:H often contains significant amount of impurities such as oxygen and nitrogen which have sufficient influence on their optoelectronic properties. Hence, the various bonding configurations involving Si, H, O and N has been studied to investigate their role in the light induced structural changes in a-Si network. Infrared spectroscopy (IR) and thermal hydrogen effusion (TE) have been used to characterize the sputtered a-Si:H samples. The results have been analyzed to study the nature of the silicon-hydrogen, silicon-oxygen and silicon-nitrogen bonds and to identify the effects of partial degassing, annealing and light soaking on these bonds. The samples were prepared by reactive radio frequency sputtering under different deposition conditions to produce hydrogen contents ranging from 10 to 35 at.%. Slightly contaminated samples were found to contain oxygen and nitrogen concentrations up to ~3 and 1.5 at.% respectively. Samples were deposited, generally, onto crystalline silicon (100) with few exceptions onto stainless steel substrates at a deposition rate of -4 nm/min. The substrate temperature was controlled at ~120 °C and the thickness of the films were from -0.08 to 6 um. Different sets of samples were subjected to partial degassing (in vacuum), annealing (in hydrogen atmosphere) and light soaking (under sun light AM1). Infrared absorption spectra, from 370 to 5000 cm', were obtained using a fourier transform infrared spectrometer. A quadrupole mass spectrometer was used to monitor the evolution of hydrogen. Several computer softwares were used to analyze the resulting data. Uncontaminated samples show three hydrogen-related distinct features at 650, 850-895 and 2090 cm1 due to wagging, bending and stretching vibrations, respectively, of the various silicon-hydrogen bonds. Nitrogen containing samples show, in addition, a features at 790 cm1. Samples containing oxygen exhibit the distinct features at 940-980 and 1000-1045 cm1 regions due to Si-O-Si and H/Si-SiO2-O-SIO2-Si/H configurations respectively. The evolution of weakly bonded hydrogen occured at different temperatures; more contaminated samples, having evolutions peaks at higher temperatures, indicate better stability to thermal degradation. Incorporation and/or movement of oxygen atoms are also a consequence of partial degassing. Higher hydrides are related to 850-895 cm' doublet and have lesser contribution to feature at 650 cm. The peak at 2000 cm1 has contributions from (Si-H2), and clustered Si-H. The 2090 cm'' peak is a result of Si-H2, (Si-H2), Si-H3 and Si-H groups. In annealed samples, some IR inactive hydrogen become IR active and there is evidence of the movement of oxygen atoms from the bulk of the film towards the film- substrate interface. The peaks at 2000 and 2090 cm moves towards higher wavenumber with increasing annealing temperatures. Oxygen atoms become mobile as a result of prolonged illumination and seem to move through a-Si network. Also the oxygen atoms in the crystalline Si-O form new configurations and /or diffuse into the bulk of a-Si:H film. No evidence of the movement of the nitrogen atoms through a-Si network was observed. Some weak Si-H bonds are broken and the hydrogen atoms become mobile in the a-Si network. Thus it appears that hydrogen atoms move through an a-Si:H network as a result of the breaking of Si-H bonds due to the trapping of free-charge carrier following illumination. This creates dangling bonds in the interior of the material. After prolonged illumination, the overall number of new dangling bonds becomes large enough to cause permanent structural damage or photodegradation in the interior of the film.