Physical properties of semi-insulating polycrystalline silicon. I. Structure, electronic properties, and electrical conductivity

Abstract

Semi-insulating polycrystalline silicon (SIPOS) are thin SiOx films (0≤x≤2), deposited by means of low pressure chemical vapor deposition on suitable substrates (silicon or sapphire). Although SIPOS has important applications in the semiconductor and solar cell technology, its physical properties which depend strongly on the oxygen content x, are not well known. In the present contribution, SIPOS as deposited at 660°C in the range 0≤x≤1 is investigated by using different and complementary methods, namely nuclear reaction analysis, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and electrical conductivity measurements. On the basis of these experiments it is found that SIPOS consists of a nanometer-scale mixture containing Si, SiO2, and at least one suboxide (SiO1-Δ with Δ≊0.14). SIPOS with x≤0.034 is polycrystalline, while SIPOS with x≥0.4 is completely amorphous. In the range 0.034≤x≤0.4 there is a transition from crystalline Si grains to amorphous Si grains which is accompanied with a drastic decrease in grain size. The microstructure of SIPOS is consistent with a shell model, in which the Si grains are embedded in an amorphous oxide matrix consisting of SiO2 and SiO 1-Δ. The latter oxide is located at the grain boundaries of the Si grains, i.e., it forms the transition oxide between the Si grains and the SiO2 matrix. Within the shell model, conduction proceeds by tunneling of thermally activated carriers through the oxide barriers separating adjacent grains. The model is able to qualitatively reproduce the observed low-bias conductivity σ(x,T) in the high-temperature regime.