Investigation of the effect of hydrogen on electrical and optical properties in chemical vapor deposited on homoepitaxial diamond films

Abstract

We have investigated electrical and optical properties of the high-conductivity layers formed in both undoped and B-doped diamond films prepared by chemical vapor deposition. It is found that both hydrogenated undoped and B-doped diamond films have high-concentration holes of ∼1018 cm−3 at 297 K. These films exhibit little temperature dependence of the hole concentration between 120 and 400 K, while that of the oxidized B-doped film has a strong temperature dependence with an activation energy 0.38 eV. The Hall mobility of all the hydrogenated films of ∼30 cm2/Vs at 297 K is one to two orders of magnitude smaller than that of the oxidized B-doped film and increases with increasing temperature. The I−V characteristics of Al–Schottky contacts to the hydrogenated undoped film show excellent rectification properties and the temperature dependence of their forward characteristics is well explained by a junction theory inclusive of the tunneling process, i.e., thermionic-field emission theory, indicating that the depletion layer becomes thin due to high-density space charge in the depletion layer. We have also found a broad cathodoluminescence peak at around 540 nm in the hydrogenated films which disappears with subsequent oxidation treatment, indicating the existence of hydrogen-related gap states in the subsurface region of as-deposited homoepitaxial diamond films. High density hydrogen is detected in the subsurface region of the hydrogenated films by secondary ion mass spectroscopy. These experimental results suggest the existence of hydrogen-induced shallow acceptors in the surface region of as-deposited (hydrogenated) diamond films and that the difference between the hydrogenated and the oxidized films observed in both electrical and optical properties originates from hydrogen incorporated in the subsurface region.