The electrical conductivity of diamond thin films produced by the hot-filament technique is found to increase when diborane is incorporated in the precursor gas mixture. The combination of well-defined bulk conductivity measurements with quantitative secondary-ion mass spectrometry and Raman spectroscopy shows that the conductivity increase is associated with atomic boron doping and rules out any significant role for a graphitic-type component.

Diamond thin films have been doped with nitrogen during growth by the hot-filament technique. For nitrogen concentrations in the films, determined by quantitative secondary ion-mass spectroscopy (SIMS) exceeding about 3×1018 atoms/cc, a decrease of several orders of magnitude is observed in the electrical conductivity for temperatures at or above room temperature. Qualitatively, this decrease is as expected, assuming compensation of existing acceptor states in nominally undoped diamond thin films by substitutional nitrogen which is known to introduce a deep-lying donor level.

Diamond films have been in-diffused with lithium in an effort to produce n-type diamond by interstitial doping. Although lithium incorporation was established, only small changes in electrical conductivity and no thermionic emission from donor levels, which should lie only a few tenths of an electron volt below the vacuum level, were observed. To account for these observations, studies of the spectral dependence of external photoemission of lithium-doped and undoped films were undertaken. These indicate that the lithium donors are compensated by high densities of acceptor states distributed over several electron volts. This first, direct observation of band-gap states in diamond films accounts for a number of reported properties including their relatively high electrical conductivity and small field effect.

Space-charge-limited hole currents in nominally undoped diamond thin films have been studied using thin, highly boron-doped (p+) diamond layers as injecting contacts. The results obtained from these p+-p-Si structures have been analyzed to determine, for the first time, the bulk distribution of localized states N(E) in polycrystalline diamond thin films. The values of N(E), covering an energy range of about 0.8–0.6 eV above the valence band, indicate that the density of states at 0.8 eV is about 1015 cm−3 eV−1 but rises rapidly, within the 0.2 eV, to about 1018 cm−3 eV−1.

No abstract