Structural properties and recombination processes in hydrogenated polymorphous silicon

¹ Département de Physique des Matériaux, UMR 5586 CNRS, Université Lyon I, 43 Bd. du 11 novembre 1918, 69622 Villeurbanne Cedex, France
² Laboratoire de Physico-Chimie des Matériaux Luminescents, UMR 5620 CNRS, Université Lyon I, 43 Bd. du 11 novembre 1918, 69622 Villeurbanne Cedex, France
³ Laboratoire de Physique des Interfaces et des Couches Minces, UMR 7647 CNRS, École Polytechnique, 91128 Palaiseau Cedex, France

Corresponding author: rmeaudre@dpm.univ-lyon1.fr

Received: 27 March 2002
Revised: 16 January 2003
Accepted: 23 January 2003
Published online: 14 May 2003

Abstract

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to powder formation, a new type of material, consisting of an amorphous matrix in which silicon nanocrystallites are embedded is obtained. This material, named hydrogenated polymorphous silicon (pm-Si:H), exhibits enhanced transport properties with respect to state-of-the-art hydrogenated amorphous silicon (a-Si:H). In order to understand the origin of such improved properties, we investigated structural characterization of pm-Si:H films. High resolution transmission electron microscopy (HRTEM) micrograph, micro-Raman and infrared spectra of the films are presented. The crystallite sizes deduced from the Raman spectra are supported by the HRTEM measurements. The infrared stretching modes of pm-Si:H films present a band at ~2030 cm⁻¹ attributed to hydrogen platelets. Two approaches are then given in order to explain the enhanced photoconductivity properties. The first one, qualitative, appeals to low density of states at the Fermi level and low capture cross-section of electrons due to the improved amorphous matrix. The second, more quantitative, suggests that recombination mainly occur at dangling bonds at the surface of the crystallites. Considering the dangling bond density at silicon surface passivated by hydrogen and the description of multiphonon carrier capture, both given in the literature, we derive  and values in good agreement with experiments.