J. Mort

Boron doping of diamond thin films

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.

Compensation effects in nitrogen‐doped diamond thin films

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.

LITHIUM DOPING AND PHOTOEMISSION OF DIAMOND THIN FILMS

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.

Temperature dependence of photoconductivity in buckminsterfullerene films

We report the temperature dependence from 330 to 230 K of the photoconductivity measured in sublimed films of buckminsterfullerene, specifically, C60/70, up to 3 μm thick. In contrast to the temperature dependence of the dark conductivity, where reversible structure, associated with the onset of a phase transition to orientational ordering below 250 K, is observed, the photoconductivity shows an essentially monotonic decrease to lower temperatures. This provides additional evidence that the thermal generation rate increases as the C60/70 becomes orientationally ordered. It also suggests that the increased order below the transition temperature does not lead to significantly increased carrier mobilities.

Density of states distribution in diamond thin films

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.

Geminate recombination in a‐Si:H

Combined xerographic measurements and a delayed‐collection field technique have been applied to a‐Si:H. to determine whether geminate recombination controls the photogeneration process in a‐Si:H. The delayed‐collection field method allows the photogeneration efficiency at low applied fields to be measured. The results show that, depending on sample preparation, the zero‐field quantum efficiency ranges from 0.44 to 0.55. The measured field dependence of the quantum efficiency give best fits to the Onsager theory with r0  ∼45 to 80 A for Φ0=1.

Electronic carrier transport and photogeneration in buckminsterfullerene films

Information concerning electronic carrier transport and photogeneration in films of undoped C60/70 has been obtained from photoinduced xerographic discharge curves in sublimed films up to 40 μm in thickness. The results, using strongly absorbed light, unambiguously reveal that the electron range, μτ, where μ is the electron drift mobility and τ the deep trapping lifetime, is ∼10−7 cm2/V. By contrast, a barely detectable discharge is observed to be associated with holes. The photogeneration efficiency for electrons is field dependent with a value at 4000 A of 5×10−2 at a field of 2×105 V/cm. These results are discussed in a context where both charge photogeneration and transport in undoped C60/70 involve localized electronic states.

Infrared sensitive organic photoconductor

Photoconductivity and basic electrophotographic photoreceptor parameters are reported on a composite photoreceptor structure in which the sensitizing photoconductive layer consists of vanadyl phthalocyanine. The results indicate useful electrophotographic performance with a photogeneration efficiency of ∼0.2 at 45 V/mm extending from 4000 to 9000 A. This covers the spectral emission range of GaAs solid‐state lasers and is an excellent match to the solar spectrum. E;81 72.40.+w 72.20.Jv, 72.80.Le

Photosensitization of diamond thin films

Photosensitization of diamond thin films, prepared by the hot‐filament technique, has been achieved with thin overcoatings of hydrogenated amorphous silicon. It is observed that injection of electrons, photogenerated in the amorphous silicon, proceeds with efficiencies approaching unity. To reconcile this with the reported electron energy structures of these two materials, the presence of localized, acceptor‐like states 2 eV above the valence band of diamond must be invoked. In addition their density must be sufficiently high to account for the inferred lower limit of 10−8 cm2 /V for the electron range.

Infrared absorption in boron‐doped diamond thin films

Detailed studies of infrared absorption in nominally undoped and boron‐doped, free‐standing diamond thin films are reported. Difference measurements reveal absorption at 1300 cm−1 (0.16 eV) due to boron‐induced single phonon, vibronic excitations. A relatively sharp peak at about 2420 cm−1 (0.30 eV), a stronger, broader band centered at 3060 cm−1 (0.38 eV), and a weak, broad peak at 4200 cm−1 (0.52 eV), are identified as electronic transitions, with or without phonon assistance, of the boron acceptor. These results provide important confirmation of the hitherto presumed substitutional nature of boron doping and recent suggestions concerning electronic transport mechanisms in diamond thin films.