Electronic states in hydrogenated microcrystalline silicon

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Abstract
Electronic states in the mixed-phase material microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition were investigated by continuous-wave and time-resolved electron spin resonance techniques in thermal equilibrium and under illumination. Samples prepared with various plasma-excitation frequencies v ex and various process gas mixtures (which leads to differences in the crystalline volume fractions and grain sizes) and samples with different p- and n-type doping levels were studied. Three main electron spin resonance contributions were found and attributed to dangling bonds in different structural environments in the material and to conduction electrons. The g values of the dangling bonds are shifted with respect to the g value of the dangling bond in amorphous silicon. The dangling-bond spin density remains largely unchanged over a wide range of plasma excitation frequencies but increases at the highest v ex and increases also at high silane gas concentrations when amorphous growth conditions are reached. Upon doping, giving a change in the dark conductivity of five orders of magnitude, the dangling-bond spin density varies by a factor of only four and decreases significantly only for the highest p doping levels. The intensity of the conduction-electron resonance is closely related to the dark conductivity of the material. From light-induced electron spin resonance, it is concluded that photoexcited charge-carrier pairs become separated into different regions of the material. This spatial separation results in very long recombination times. Pulsed electron spin resonance measurements show two distinct spin-lattice relaxation times T 1 in the material; the T 1 of dangling bonds is very similar to the corresponding relaxation time in amorphous silicon, and the T 1 of conduction electrons is several orders of magnitude less than the relaxation time of the dangling bonds. The electron spin resonance results are related to results from electronic conductivity and structural investigations. A qualitative band diagram is used for discussion.