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Introduction

There exist different methods to introduce impurities into diamond. The diffusion technique has been successfully implemented in various semiconductors, like silicon for example, and could therefore be a promising method to be applied also to diamond. However, in diamond, it was found that the solubility of species like phosphorus, or lithium, is very low. Therefore, even at high temperature, the diffusion of impurities is impossible in diamond, and this technique is inapplicable in this case. Another way to introduce impurities is during the chemical vapor deposition (CVD) of diamond. In this process, the potential dopant is present in the environment and would bond to the rest of the film when a very specific temperature is applied. In practice the concentration of impurities found inside the film is very low, and relatively high on grain boundaries. The doping by this method is therefore inefficient, although the number of defects created during the process is low, in contrast to ion implantation.

The doping technique widely used to introduce impurities is by ion implantation, which appears to be the most promising doping technique in the case of diamond. In this method, the species are accelerated toward the target and shot into the diamond solid. During this process, bonds are broken and vacancy and interstitial defects are created along the ion track. The stability of these defects will determine the final state of the sample. When applied to the diamond crystal, which is electrically insulating, ion implantation may give rise to a very particular structure since the stable bonding configuration of carbon (at NTP) is not diamond but graphite. In graphite the carbon atoms are sp2 bonded, leaving delocalized $\pi$electrons available for electrical conduction. Thus, the structure of the damaged regions induced by ion implantation or irradiation of diamond, is expected to contain a mixture of sp2 (graphitic) and sp3 (diamond-like) bonds. The relative percentage of sp2 bonds created during the implantation will determine the final phase obtained after post-implantation annealing (see below), and the electrical behavior of the sample.

The parameters that may influence the final configuration of a diamond crystal bombarded by atoms are the energy of these atoms, the dose and the flux of the beam, and the temperature of the target [4]. The energy loss in the collision of the atoms of the incident beam with the lattice atoms can be roughly approximated by a nuclear energy loss (nuclear stopping) referred to interaction between the moving ions and the target atoms, and an electronic energy loss (electronic stopping) associated with excitation of electrons in the target atoms. The former dominates at low energy irradiation in diamond (< keV) while the latter is preponderant from few tens of keV (for beams of carbon atoms for example). The main difference between these processes is that in the nuclear stopping, many atoms can be dislodged from their lattice site, leading to an extensive damage, whereas the electronic stopping can induces only bond breaking and structural rearrangement [4].

It is obvious that the higher the number of incident atoms on the target (per unit area), the larger the amount of damage in the crystal. Thus, high dose implantation creates much more damage than low dose implantation does. Furthermore, the damage cascade induced by the implanted ions is accompanied by a very fast relaxation of the damaged crystal that relieves internal strain. Thus if the implantation is made at low rate, a relaxation can take place before further implantation occurs, leading to a different kind of structure than in the case of high flux. Another parameter that may influence the crystal relaxation is the temperature of the target during or after the implantation. Indeed, high temperature (hot) implantation induces substantial annealing of damage, leading to either graphitization or diamond reconstruction (according to the implantation dose [5]), and implantation at low temperature will freeze in the defects created during the damage cascade, leaving mainly clusters of point defects.


next up previous contents
Next: Doping of diamond by Up: Ion implantation Previous: Ion implantation
David Saada
2000-06-22