A study of sputtered a-Si:H by thermal effusion and infrared spectroscopy

Hydrogenated amorphous silicon, a-Si:H, is an exciting new material with ideal properties for various types of optoelectronic devices including photovoltaic cells. It is well known that the quality of an a-Si:H alloy, for such an application, is closely related to its hydrogen content and to the nature of the silicon-hydrogen bonds. On the other hand, a-Si:H films often contain a significant amount of unintentionally incorporated impurities such as oxygen and nitrogen which significantly influence their optoelectronic properties. Hence, the main objective of this work is to study the various bonding configurations involving Si, H, 0 and N in the a-Si network. Infrared spectroscopy and thermal hydrogen effusion have been used to study the nature of the silicon-hydrogen, silicon-oxygen and silicon-nitrogen bonds in sputtered a-Si:H alloys. The samples were prepared by reactive radio frequency sputtering under different deposition conditions to produce hydrogen contents ranging from 12 to 35 at.%. Slightly contaminated samples were found to contain nitrogen and oxygen concentrations ranging from 0.0 to -1.5 and 0.0 to -3.0 at.% respectively. Samples were deposited onto crystalline silicon (100) substrates at a deposition rate of -4.0 nm/min and a substrate temperature of -120 °C with thicknesses ranging from -0.08 to 0.6 ^m. Some samples were annealed in vacuum or in a hydrogen atmosphere, for up to 60 minutes, at various temperatures in the 150-550 °C range. Infrared absorption spectra, from 370 to 5000 cm- i were obtained using a fourier transform infrared spectrometer. A computer program based on the simplex algorithm was then used to deconvolute the component peaks in the Si-H bending and stretching bands of these spectra. The thermal effusion spectra were obtained by heating the samples up to 800 °C at a rate of 60 - 100 °C/min., in a vacuum system with a base pressure of -10"9 Torr, and using a quadrupole mass spectrometer to monitor the hydrogen evolution. For the as-deposited samples: (i) The infrared results demonstrate that the oscillator strengths for the Si-H bending and stretching bands depend on the hydrogen, oxygen and nitrogen concentrations, (ii) The thermal effusion spectra show three peaks, two at -325 and 450 ° C due to weakly bonded hydrogen and the other at 725 °C due to tightly bonded hydrogen. The first low temperature peak is absent for samples with a multi-oxide layer on or near the surface region. Our results demonstrate that annealing produces a redistribution and transformation of different bonding configurations involving Si, H, 0 and N. Features observed in the thermal effusion and infrared spectra resulting from heat treatment have been compared and thus correlated with different groups of Si and H. The results have enabled us to propose a self consistent model of the behaviour of hydrogen, oxygen and nitrogen in amorphous silicon. It is concluded that this model in conjunction with some electrical and optical characterisations can lead us to better understanding the role of hydrogen in a-Si:H