Chapter 7 Optical properties of CVD diamond

Abstract

This chapter discusses the use of photocurrent techniques for monitoring optical transitions in nominally undoped, B- and P-doped chemical vapor deposition diamond layers. Photocurrent setups in three main configurations have been used: the constant photocurrent method, photothermal ionization spectroscopy and phonon-assisted Oscillatory Photoconductivity measurement, and the novel Fourier transform photocurrent spectroscopy. All these techniques allow to measure very low defect and impurity concentrations in chemical vapor deposition (CVD) diamond thin films. All undoped CVD diamond films show very specific characteristics in subgap photocurrent spectra. The photocurrent has ionization onset at about 0.9 eV. This characteristic photocurrent is induced by a defect absorption, which is called the characteristic defect (Dx), rising up to the gap and found in all measured CVD diamond films. By photo-hall measurements, we could prove that the level Dx is acceptor-like. Based on the Dx level and deep N-impurity, we could explain the origin of photosensitivity changes for hydrogenated, oxidized, and annealed CVD diamond samples. The photosensitivity is highest for hydrogenated samples with the Fermi level pinned by transfer doping by adsorbants at the hydrogenated diamond surface. Enhanced photoconductivity can be also reached by B-doping.