Investigation of boron and hydrogen concentrations in p-type diamond films by infrared spectroscopy

Abstract

Abstract The hydrogen related infrared absorption at 2900 cm −1 and the boron induced absorptions at 1300 cm −1 , 2460 cm −1 and 2835 cm −1 were studied to determine the nature and the concentrations of these impurities in diamond films. The diamond films were deposited in a microwave-CVD system, using a vapour gas source filled with trimethylborate diluted in methanol for p-type doping. The B-C ratio was varied by changing the TMB content or by changing the doping gas flow of the trimethylborate-methanol mixture. Total oxygen contents higher than 0.5% reduced the efficiency of boron doping because plasma reactions lead to the formation of boron oxide, thus reducing the boron concentration in the plasma. The hydrogen was found to be mainly incorporated as sp 3 CH 2 , which could be detected by its characteristic stretching absorptions at 2920 cm −1 and 2850 cm −1 . Depending on the gas mixture, the hydrogen content was found to vary from 500 ppm to 6000 ppm. In boron doped films, the electronic transitions of the boron acceptor at 2430 cm −1 and 2830 cm −1 and the phonon assisted transitions at 3700 cm −1 and 4100 cm −1 were found at the single crystal positions, but were significantly broadened due to the relatively high boron content and the polycrystalline nature of the diamond films. The strength of the one phonon absorption was found to be proportional to the boron concentration. This offers a simple method to evaluate the boron content of diamond films. The boron concentration was determined using the nuclear reaction 11 B ( p , α 1 ) 8 Be → 2 α 2 to be up to 700 ppm, depending on the gas mixture.