The search for donors in diamond

One of the major issues in diamond electronics is the search for a useful n-type dopant. Nitrogen is a well-known, yet very deep, donor in diamond. Phosphor has been shown to have a rather deep donor level in diamond (E-a similar to 0.5 eV) and the electron mobility values achieved so far for CVD-grown P-containing diamond layers are still rather low. Interstitial Li, which has been predicted to act as a donor in diamond, has, so far, not yielded any convincing, useful electron conductivities. Sulfur has recently been reported to have a donor level in diamond; however, these results need further checks, due to some B contamination found in the S-doped samples. Attempts to dope diamond with P and with S by ion implantation have not yet yielded any significant donor signature. On the other hand, point defect-related electrical conductivity in diamond (as induced by ion or electron irradiation) was shown to act as compensating donors to B-doped diamond, and has been proven to yield rectifying junctions with p-type diamond. Ab initio computer simulations for diamond containing different dopant impurities show that substitutional S in diamond should have a rather shallow donor level (0.2 eV), with a formation energy lower then that of P. Hence sulfur could act as a useful shallow donor in diamond. This prediction has, however, not been experimentally confirmed as yet. In the present work we will review the experimental and theoretical status of n-type diamond, as well speculate on other possible donor states in diamond. (C) 2001 Elsevier Science B.V. All rights reserved.