Mary A. Machonkin

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.

On the thermal dissociation of hydrogen

The important role atomic hydrogen plays in the low‐temperature and pressure deposition of diamond has renewed interest in the dissociation processes of hydrogen. Following a method originally developed by Langmuir and co‐workers, the voltage‐current characteristics of refractory filaments in vacuum and reduced‐pressure gaseous environments are analyzed. Using hydrogen, deuterium, and helium, it is concluded that the difference in the power consumption by the filament in hydrogen and in vacuum is a good measure for the rate of hydrogen dissociation. This rate is shown by experiments and mathematical modeling to depend on the geometry of the refractory heater element. Relatively high dissociation rates, normalized per heater area, are obtained for small‐diameter wires, and it is argued that this is indicative of a nonequilibrium dissociation process.

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.

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 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.

Charge transport in boron-doped diamond thin films

Abstract Detailed studies are reported for the temperature dependence of nominally undoped and boron-doped diamond thin films over the temperature range 400 to 150K; boron doping concentrations in the films range from 20 to 968 parts per million. In all samples, no single-valued activation energy is observed although the dependence on temperature systematically decreases with increasing boron con-centration. This suggests that the acceptor states introduced by the boron are themselves energetically situated in a relatively high density of impurity or defect states extending from the valence band edge. It is believed that charge transport in these films involves a combination of extended-state conduction and hopping within these distributions of localized states and evidence of impurity band formation is observed.

Charge-transfer processes in Buckminsterfullerene films

Abstract The spectral dependence of the photogeneration efficiency. η in sublimed undoped C60-C70 films has been determined from photoinduced xerographic discharge curves. The dependence of η on the exciting light wavelength from 7250 to 3500A and electric field are reported. At an applied field of 2–7 × 105 V cm−1 η is essentially wavelength independent with a value of about 5 × 10−3 from 7250 to 5500A. At shorter wavelengths, η is essentially proportional to the absorption coefficient, rising by more than an order of magnitude to about 6 × 10−2 at 3500A. The difference between the charge-transfer efficiencies of these two regions is discussed in terms of an Onsager surface-mediated dissociation of singlet excitons.

The nature of extrinsic photoconductivity in diamond thin films

Abstract While extrinsic photoeffects in chemically vapour-deposited thin films of diamond can originate from impurity-controlled photogeneration and photoinjection of carriers from contiguous metal electrodes, it is important, and yet difficult, to differentiate between these two distinctive processes. Detailed studies and results are described, involving the use of non-absorbing water-based electrodes which allow a clear distinction between these two mechanisms to be made. In addition, the near-infrared-visible photoeffect in diamond thin films, as in single-crystal diamond, is associated with the photoionization of unionized acceptors based on its observed quenching in nitrogen-doped compensated films.