Koji Okumura

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.

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.

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.

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.

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.

Photovoltaic effects at the interface between amorphous selenium and organic polymers

Band bending in the surface layer of amorphous selenium induced by contact with an insulating organic polymer was detected by photoelectric signals generated at the interface under no external bias. The magnitudes and polarities of the signal varied among samples involving different polymers, and also among samples involving the same polymer with and without doping by an electropositive organic impurity. Details of this behavior, along with the observation that the polarity of the signal was reversed when the method of making the contact between selenium and polymer was altered, indicate that the preferred orientation of polymer molecules containing dipole moments on the selenium surface is the principal cause of the surface potential gradient in the selenium.

Electronic transport and density of states distribution in diamond thin films

Abstract The results of measurements which shed light on the important question of the density and distribution of localized states within the bandgap of hot-filament grown diamond thin films are discussed. These include temperature dependence studies of undoped and boron-doped diamond film, over the temperature range 400-150 K, with a range of doping concentrations. In all samples, no single-valued activation energy is observed although the dependence on temperature systematically decreases with increasing boron concentration. This suggests that states due to boron overlie a relatively high density, N(E) , of localized acceptor states already present in nominally undoped diamond thin films. Using space-charge limited hole currents, N(E) , 0.8 eV above the valence band, is ∼ 10 15 cm −3 eV −1 but rises rapidly, within 0.2 eV, to ∼ 10 18 cm −3 eV −1 . These states have also been detected by the observation of electrical compensation in nitrogen-doped films. Charge transport in these diamond films range from extended state to impurity hopping and band conduction. The implications of these results for field-effect phenomena and diamond thin film transistors are discussed.