Diffusion of hydrogen in undoped, p-type and n-type doped diamonds

Abstract

Hydrogen is a key impurity in diamond since it is unintentionally incorporated in all chemical vapor deposition (CVD) grown diamond layers. Its presence in the material can grossly affect its electrical and optical properties. Theoretically, hydrogen has been predicted to be present in diamond in one of the three charge states, H +, H 0 and H −. Moreover it may form complexes with impurities, native defects or with other hydrogen atoms. This paper is comprised of two parts: (a) a review of previous results of studies investigating different aspects of the diffusion of hydrogen (deuterium) in various kinds of diamonds. The diamonds studied are: undoped type IIa diamonds, undoped CVD diamond layers containing growth defects only, p-type B-doped homoepitaxially CVD grown diamond layers or B ion implanted type IIa diamonds and n-type P doped homoepitaxially CVD grown diamond or N-doped type Ib natural diamonds. Hydrogen is introduced in diamond by exposing the diamond surface to hydrogen plasma or by using hydrogen ion implantation. The following issues are discussed: (1) the influence of the interaction between H and the dopants and defects on the hydrogen diffusion. (2) The kinetic of (B, H), (P, H) and (N, H) pair formation and dissociation. (3) The modification of the optical and electrical properties as a result of hydrogen incorporation and annealing. It is found that, under certain conditions, H diffuses into the B containing layer and it passivates B acceptors. In contrast, no H diffusion could be observed in n-type diamonds, up to 1000 °C. (b) Recent results of our group regarding other aspects related to the diffusion of H in diamond are presented. These include results on: (i) the influence of ion implantation related defects on the diffusion of deuterium. For this study type IIa samples implanted with B or non-dopant ions are used. (ii) The determination of the charge state of H or H/defects complex as a function of diamond type. For that, annealing under bias is applied to deuterated diamond layers. We show that the presence of implantation defects retards the deuterium diffusion in a B-implantation doped diamond, demonstrating that D strongly interacts with defects, thus inhibiting diffusion. The new-formed complexes deteriorate the electrical properties of the diamonds and are very stable up to high temperatures. We confirm that, as expected, in highly B-doped CVD diamond layers, H diffuses as a positive ion. In lightly B-doped homoepitaxial layers, however, D is incorporated in complexes which seem to be negatively charged.