Hydrogen-terminated Diamond Films Research Articles

Electrostatic force microscopy mapping of electrical conductivity of hydrogen-terminated diamond films

A method for mapping the surface conductivity of hydrogen-terminated (H-terminated) diamond on a sub-100nm scale is presented. The measuring technique relies on electrostatic force microscopy imaging of the voltage distribution of a current-carrying H-terminated diamond film. The uniform linear voltage drop in highly conductive H-terminated diamond surface layers indicates that the layers behave as homogeneous, diffusive conductors with a well-defined value of the sheet resistance. On the other hand, we observe conductive as well as insulating regions that coexist for not perfectly H-terminated diamond surfaces with poor electric conductivity.

Experimental study of cavitation damage on hydrogen-terminated and oxygen-terminated diamond film surfaces

To investigate the effect of hydrophobic surface on cavitation damage, hydrogen-terminated diamond film and oxygen-terminated diamond film are prepared for the cavitation damage experiment. Because the film surfaces have the same roughness and hardness, the hydrophobic property effect is focused on. Obvious damage pits appear on the hydrogen-terminated surface after 4 h cavitation experiment, while no such pits are found on oxygen-terminated surface. Surface testing results also show that the surface roughness of hydrogen-terminated surface is higher than that of oxygen-terminated surface after the experiment. Such results indicate that the collapse of bubbles growing from the hydrophobic wall to be damaged plays important role in the generation of cavitation damage.

Effect of electronic structures on electrochemical behaviors of surface-terminated boron-doped diamond film electrodes

In order to analyze the electrochemical behaviors of hydrogen-terminated and oxygen-terminated boron-doped diamond film electrodes, experiments of the cyclic voltammetry and AC impedance spectroscopy have been performed. For the purpose of clarifying the detailed electronic structures of these diamond films, current–voltage spectroscopy curves and XPS spectra have been investigated by scanning probe microscopy and X-ray photoelectron spectroscopy, respectively. For the hydrogen-terminated boron-doped diamond film electrode, its potential window is narrower than that of the oxygen-terminated boron-doped diamond. The impedance results indicate that the diamond film resistances and capacitances corresponding to different surface-terminated boron-doped diamond electrodes vary significantly. The surface band gap of hydrogen-terminated diamond film is narrow with empty surface states in it. In contrast, the surface band gap of oxygen-terminated diamond film is wide and clean. Compared with hydrogen-terminated diamond film, the surface energy bands of oxygen-terminated diamond film bend downwards. Based on their electronic structures, the electrochemical behaviors of the two boron-doped diamond film electrodes have been discussed.

Intrinsic hydrogen-terminated diamond as ion-sensitive field effect transistor

Ion-sensitive field-effect transistors (ISFET) are fabricated using intrinsic hydrogen-terminated mono-crystalline diamond films. Transistor properties are realized by a surface conductive channel on hydrogen-terminated diamond. The gating is realized by immersing the diamond surface into electrolyte solution which is contacted by a platinum electrode. Hydrogen termination of diamond surface acts as a gate insulation without any additional oxide layer. The response of gate potential to pH is about − 56 mV/pH. The results are discussed in terms of transfer doping mechanism, Nernst equation, and electrochemical properties of diamond surfaces. They are also compared with ISFETs which employ ion-sensitive gate oxides.

Gold-nanoparticles-induced pattern metallization on high-roughness diamond film surfaces

In this letter, we report an approach toward the pattern metallization of high roughness diamond film surfaces under mild experimental conditions. First, hydrogen-terminated diamond film surfaces were activated through bonding with allylamine molecules under 254nm UV light irradiation. Then, amino prepatterns on the diamond surfaces were constructed by a photolithography process. Subsequently, gold nanoparticle (AuNP) patterns were obtained through the immobilization of 15±3nm AuNPs on the amino prepatterns and the remnant photoresist was removed by rinsing with acetone. Finally, the AuNP patterns were enhanced through silver deposition and intact silver-gold micropatterns were constructed on diamond surfaces.

Protein-modified nanocrystalline diamond thin films for biosensor applications.

Diamond exhibits several special properties, for example good biocompatibility and a large electrochemical potential window, that make it particularly suitable for biofunctionalization and biosensing. Here we show that proteins can be attached covalently to nanocrystalline diamond thin films. Moreover, we show that, although the biomolecules are immobilized at the surface, they are still fully functional and active. Hydrogen-terminated nanocrystalline diamond films were modified by using a photochemical process to generate a surface layer of amino groups, to which proteins were covalently attached. We used green fluorescent protein to reveal the successful coupling directly. After functionalization of nanocrystalline diamond electrodes with the enzyme catalase, a direct electron transfer between the enzyme's redox centre and the diamond electrode was detected. Moreover, the modified electrode was found to be sensitive to hydrogen peroxide. Because of its dual role as a substrate for biofunctionalization and as an electrode, nanocrystalline diamond is a very promising candidate for future biosensor applications.

2D-hole accumulation layer in hydrogen terminated diamond

Hydrogen-terminated diamond films have been investigated by Contact Potential Difference Measurements (CPD or Kelvin force), Hall-effect experiments and theoretical calculations where the Schrodinger and Poisson equations have been solved to calculate the density of state (DOS) distribution at the surface of hydrogen terminated diamond. From CPD experiments we detect the Fermi-energy to be about 700 meV deep in the valence band. This is in agreement with results from numerical calculations, where Fermi-energies in the range 240 to 880 meV below the valence-band maximum at the surface are deduced. The calculations show that a two-dimensional (2D) density-of-state is present at the surface. Temperature dependent hole sheet-densities and mobilities are used to discuss the properties of the 2D-DOS. From this data we conclude that the 2D-DOS is distorted by disorder arising from non-perfect hydrogen termination of the surface, by surface roughness and/or by ionic molecules which are part of the adsorbate layer.

Electronic properties of the 2D-hole accumulation layer on hydrogen terminated diamond

Hydrogen-terminated diamond films have been investigated by Contact Potential Difference Measurements (CPD or Kelvin force), Hall effect experiments and theoretical calculations where the Schrödinger and Poisson equations have been solved to calculate the density of state (DOS) distribution at the surface of hydrogen-terminated diamond. From CPD experiments, we detect the Fermi energy to be about 700-meV deep in the valence band. This is in agreement with results from numerical calculations, where Fermi energies in the range 240–880 meV below the valence band maximum at the surface are deduced. The calculations show that a two-dimensional (2D) density of state is present at the surface. Temperature-dependent hole sheet densities and mobilities are used to discuss the properties of the 2D-DOS. From this data, we conclude that the 2D-DOS is distorted by disorder arising from nonperfect hydrogen termination of the surface, by surface roughness, and/or by ionic molecules, which are part of the adsorbate layer.

Diamond growth on hot-filament chemically vapour-deposited diamond for surface conductive device applications

Hydrogen-terminated diamond films are capable of displaying p-type character in the near-surface region. This effect can be used to fabricate surface unipolar electronic devices such as diodes and field effect transistors. However, the presence of contaminants within the diamond surface can reduce the carrier concentration and mobility values achieved within this layer to the point where devices can no longer be made. This paper reports on the formation of thin over-layers on relatively inexpensive contaminated ‘black’ polycrystalline diamond grown by hot-filament chemical vapour deposition. It has been found that such a layer is sufficient to enable sheet carrier concentrations approaching 10 14 cm −2 to be generated, with mobility values greater than 20 cm 2 V −1 s −1, comparable with values obtained on high-quality single crystals at these carrier concentrations. Effective Schottky diodes have been produced.

Interaction of thermally activated and molecular oxygen with hydrogenated polycrystalline diamond surfaces studied by synchrotron radiation techniques

In the present work we study the interactions between thermally activated and molecular (unactivated) oxygen gas with a hydrogen-terminated polycrystalline diamond film surface. As revealed by H + photodesorption measurements using excitation energies in carbon and oxygen K-edges, and X-ray photoelectron spectroscopy, molecular oxygen does not adsorb onto the hydrogenated diamond surface. However, thermally activated oxygen does adsorb onto it. A dominant reaction path leads to the formation of surface C–O–H bonds, although it is likely that some abstraction of chemisorbed hydrogen with formation of carbonyl bonds happens as well. The effect of oxygen on the electron emission properties of the films is studied by photon induced secondary electron emission (SEE) spectroscopy. The hydrogen-terminated, oxygen-free diamond surface exhibits negative electron affinity and high SEE intensity. Adsorption of thermally activated oxygen results in a slightly positive electron affinity (∼0.4 eV) and a decrease in the intensity of SEE, whereas electron emission properties of this sample following exposure to molecular oxygen seem to be unaffected.

Effect of average grain size on the work function of diamond films

The work function of hydrogen-terminated polycrystalline diamond films deposited by electrophoresis on molybdenum was studied using ultraviolet photoelectron spectroscopy with 21.2 eV photons for average grain sizes ranging from 0.32 to 108 μm. The work function has a maximum of about 5.1 eV at 0.32 μm, then decreases with increasing grain size to a minimum of about 3.2 eV at an average grain size of about 4 μm and then increases to a value of about 4.8 eV at a grain size of 108 μm. The results are consistent with a model in which the work function is controlled by the work function of single crystal diamond (111) at the larger grain sizes, graphitic carbon at the smaller grain sizes, and by a negative electron affinity that increases with decreasing grain size due to defects near diamond (111) crystallite edges for the intervening grain sizes. The large change in work function (almost a factor of 2) could be useful to make conductors with different work functions for microelectronic gate structures.

Hypothesis on the conductivity mechanism in hydrogen terminated diamond films

Abstract An electrical model of the surface conductive layer in diamond induced by hydrogen plasma treatment is proposed. The model is discussed by analysing the characteristics of Schottky diodes and resistors built on the hydrogen terminated diamond. It is proposed that the hydrogen induced acceptors in diamond are separated from the surface by a separation layer characterised by dielectric constant e∼50 and thickness of about 30–50 nm. This layer allows the complete depletion of the hydrogen induced acceptors by a 0.5–1 eV Schottky barrier, but prevents the tunneling through the barrier at forward bias. Also, this model allows charges on the surface of the proposed separation layer. The charging/discharging of surface states can explain the degradation of the device performance during testing.

Properties of photochemically modified diamond films

Abstract Photochemical surface modification of polycrystalline diamond films was achieved. Clean, hydrogen-terminated diamond films were chlorinated by UV irradiation in Cl 2 (g). Amine-terminated diamond films were produced by irradiating the chlorinated diamond films in NH 3 (g). The modified surfaces were characterized by X-ray photoelectron spectroscopy and diffuse-reflectance Fourier transform infrared spectroscopy. The results of current-voltage ( I-V ) measurements indicated that surface modification by chlorine affected the ohmic nature of the gold-diamond contact, the contact resistance and the diamond surface conductivity.