Electron spin resonance investigation of electronic states in hydrogenated microcrystalline silicon

Abstract

Phosphorus-doped microcrystalline silicon with high-crystalline volume fraction was prepared by very highfrequency plasma enhanced chemical vapor deposition. The material is studied by electron spin resonance and transport measurements as a function of doping and temperature. In all samples a resonance at gϭ1.998 is found with spin densities very similar to the phosphorus dopant density and also the carrier density at high doping levels. This resonance is related to doping-induced excess electrons. Its spin density is largely temperature independent, and the corresponding electrons occupy dopant or conduction band tail states at low temperatures, while they are excited into the conduction band at high T. This gradual transition is accompanied by changes in linewidth, g value and spin-lattice relaxation time. Hyperfine interaction with P nuclei is only observed for intermediate doping levels and has very small intensity. From the value of the hyperfine splitting, the effective Bohr radius of the impurity wave function is estimated to 12 Å. Transport at low temperatures (TϽ20 K) proceeds via hopping between donor states and/or conduction-band tail states. A thermal activation energy of 3.5 meV and similar localization lengths as from the hyperfine data are found for this process. At temperatures above 20 K electronic transport is governed by a wide distribution of activation energies.

Stability of microcrystalline silicon for thin film solar cell applications

“…Factors supporting the picture of a change in occupation are: (i) The changes occur at g ¼ 2.0052, a line contribution which is found together with the superimposed signal at g ¼ 2.0043 in all our mc-Si:H material [15,22,23]. We consider that it originates from an intrinsic Si-DB defect in the material.…”

“…ESR was measured in the X-band at room temperature or at 40 K. Details of the experiment are published elsewhere [22,23].…”

“…(ii) Various findings, like the parallel existence of the DB (g ¼ 2.0043/52) and CE (g ¼ 1.996) states and the broad distribution in energy of the g ¼ 2.0052 upon shifting of the Fermi level by doping [23], suggest that the g ¼ 2.0052 states are located in the upper half of the mobility gap, possibly only in parts of the material.…”

Hole drift-mobility measurements in microcrystalline silicon

“…The bandtail multiple-trapping model appears to account well for the temporal and thermal variations of the hole displacement. Bandtail states have previously been proposed to account for electron paramagnetic resonance spectra, 14,15 for photocarrier recombination experiments, 16 and transient photoconductivity measurements 17 in c-Si. One paperreporting temperature-dependent ambipolar diffusion lengths 16 -estimates an exponential valence bandtail width of 26 meV, which seems reasonably consistent with the estimates of 31-32 meV in the present work.…”

Silicon-Based Third Generation Photovoltaics

“…As one can see on Fig. 9 (a) with increasing of B concentration, the PL intensity monotonously decreases (Fujii et al, 1998) (Müller et al, 1999) (Stegner et al, 2008). In P-doped Si nanocrystals, the situation is different.…”