Thickness dependent density distribution of gap states in undoped CVD diamond films

Abstract

Density distribution of gap states in nominally undoped CVD diamond films has been studied as a function of film thickness. Co-deposited films with thickness of 3, 6, 9 and 12 μm were used for this study. Density of trap states around Fermi-level was resolved using space charge limited current (SCLC) method, from I – V characteristics of the films. Density of mid-gap states was explored using constant photocurrent method (CPM), from the photoconductivity excited by argon laser with photon energy of 2.41–3.53 eV. It is found that, as film thickness increases from 3 to 12 μm, the density of states at the Fermi-level N ( E f ) decreases from 3.1×10 16 to 9.8×10 15 cm −3 eV −1 , Fermi-level position moves from 0.99 to 0.90 eV above the top of the valence band, and the density of mid-gap states decreases up to two orders of magnitude. Sublinear dependence of photoconductivity on light intensity reveals variation of recombination mechanism from monomolecular one to bimolecular one with increasing film thickness. The dependence of the density of gap states on film thickness is supportive of improved crystal quality in undoped CVD diamond as film thickness increases.