Conduction mechanisms in ion-implanted and annealed polycrystalline CVD diamond
Introduction
Irradiation damage of diamond has been a topic of investigation since Crookes first irradiated diamond in 1904 with α particles [2]. It is considered a useful technique for introducing dopant impurities into the diamond lattice for semiconductor applications. Also, ion implantation is valuable for characterising the nature of the defects introduced into the solid by studying their formation and disappearance under controlled annealing conditions.
The damage produced by ion implantation is of particular interest in the case of a diamond target, since the bonding configuration of diamond (sp3) is metastable with respect to graphite (sp2). For implantation doses which result in a density of defects below a certain critical value (1022 vacancies/cm3), it has been shown that the damaged diamond regrows upon annealing back to diamond, whereas for defect densities higher than this value, thermal treatment leads to graphitisation of the damaged diamond [3].
Above the critical density of 1022 vacancies/cm3, the electrical conduction occurs by means of variable-range hopping (VRH) [4], [5]. Below the graphitisation threshold, the layer can also become conductive [6] but this mechanism is not well understood and various conduction mechanisms have been evoked [1], [7], [8], [9], [10], [11].
In the present paper we apply these conduction mechanisms to ion-implanted polycrystalline chemical vapour deposited (poly-CVD) diamond and find consistency with the model developed by Baskin et al. [1]. Baskin's model shows that ion-damaged diamond, with a point-defect density smaller than a critical density of 1022 vacancies/cm3, exhibits defect-related conductivity. This model has previously been used to describe conduction in ion-damaged single-crystal natural diamond [12] and in this paper we show that this model can be extended to ion-damaged polycrystalline-CVD diamond. This result suggests that despite the presence of grain boundaries, the effect of ion induced damage is primarily to modify the in-grain conductivity. This is somewhat unexpected because one might have expected the grain boundaries to be more susceptible to ion beam damage and subsequent annealing than the grain themselves. However the similarity between the response of polycrystalline CVD and single crystal diamond suggests that this is not the case and that the conduction properties are dominated by in-grain effects rather than the grain boundaries.
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Experimental details
The studied samples were undoped free-standing polycrystalline thin films of diamond grown by microwave-assisted plasma CVD at the Naval Research Laboratory in Washington DC. The sample thickness was about 100 μm with an average grain size of 30 μm. The as-grown sample was implanted with 2 MeV He ions with a dose of 4.6 × 1015 ions/cm2 (which is just below the critical dose of 1022 vacancies/cm3). It should be noted that the depth profile of the damage within the crystal was non-uniform and was
Raman and photoluminescence spectroscopy
Room temperature Raman and photoluminescence (PL) spectroscopy was used to determine the quality of the sample before, and after implantation and subsequent isochronal annealing. Defect related Raman peaks are evident after implantation (Fig. 1). The defect related peaks are small in comparison to the diamond line, hence in the range 1400–1800 cm− 1 some of the spectra have been magnified. The spectra have been normalised to their respective diamond Raman peak to indicate the relative
Conclusion
In summary, the model for carrier transport in ion-damaged single crystal diamond, developed by Baskin et al. [1], was effective in accounting for the conduction in poly-CVD diamond implanted below the critical dose. The conduction properties of the implanted sample as a function of annealing temperature were consistent with carrier motion in a defect-related delocalised band formed by the neutral vacancy.
Acknowledgements
The implantation was performed by Paul Spizzirri at the University of Melbourne. We thank Jim Butler at the Navel Research Laboratory in Washington DC for the supply of the CVD diamond films. This work was supported in part by the Australian Research Council.
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