Richard B. Jackman

n-type conductivity in ultrananocrystalline diamond films

Hall effect measurements have been carried out to determine the carrier density and mobilities in ultrananocrystalline diamond films grown with added nitrogen. The results show clear n-type conductivity with very low thermal activation energy. Mobility values of 1.5cm2V−1s−1 are found for a sheet carrier concentration of 2×1017cm−2. These measurements indicate that ultrananocrystalline films grown with high nitrogen levels in the growth gas mixture can have bulk carrier concentrations of up to 1021, which is very high for diamond films. The n-type nature of this material was also confirmed by Seebeck effect measurements.

An X-ray photoelectron spectroscopic investigation of the oxidation of Manganese

Abstract X-ray photoelectron spectroscopy has been used to investigate a range of manganese oxide compounds and the initial stages of oxidation of manganese metal. It is shown that the Mn 2p3/2 binding energy, the extent of multiplet splitting of the 3s level, and the O Is : Mn 2p3/2 signal intensity ratio vary systematically between the different oxides. This information is used to identify oxide species present on the surface of manganese films during oxidation. A reaction sequence Mn→MnO→Mn2O3→MnO2 is demonstrated.

The use of nanodiamond monolayer coatings to promote the formation of functional neuronal networks

Nanostructured materials provide a new dimension of interaction with biological systems that takes place on a sub-cellular level with a high degree of specificity. In the field of neuroscience the nanoscale corresponds to the size of synapses; the specific connections between brain cells. In this context, diamond-based materials have attracted much attention due to their extreme mechanical and electrical properties and their chemical inertness. Here the suitability of nanodiamond (ND) monolayers to act as a platform for neuronal growth is investigated. Neurons cultured on various ND-coated substrates perform remarkably well, and similar to those grown on standard protein-coated materials with respect to their initial cell attachment, sustained neurite outgrowth, cell-autonomous neuronal excitability and functionality of the resulting electrical networks. ND layering provides an excellent growth substrate on various materials for functional neuronal networks and bypasses the necessity of protein coating, which promises great potential for chronic medical implants.

High carrier mobility in polycrystalline thin film diamond

Polycrystalline diamond films have been found to display p-type surface conductivity. No bulk impurity is added to the films; the p-type characteristics of the undoped diamond are thought to be due to a surface or near surface hydrogenated layer. Carrier concentrations within the range 1017–1019 cm−3 have been measured; control over the carrier concentration can be achieved by annealing the “as-grown” films in air. For a given annealing temperature a stable carrier concentration arises. The Hall carrier mobility has been explored and a value of >70 cm2/Vs has been measured for a film with a carrier concentration of ∼5×1017 cm−3, the highest reported for polycrystalline thin film diamond and equivalent to boron doped single crystal diamond.

Polycrystalline diamond photoconductive device with high UV‐visible discrimination

Planar metal–diamond–metal photoconductive devices have been fabricated from free standing large grain (20–30 μm) polycrystalline thin film diamond. An interdigitated electrode design with spacings of 20 μm was used to produce effective UV photodetecting devices at bias values in the range 0.1–10 V. A methane‐air treatment has been used to modify the structures such that unprecedented performance characteristics have been recorded (106 higher response to 200 nm than visible wavelengths, <0.1 nA dark currents); spectral features similar to those observed in natural diamond crystals have been observed indicating that the treatment used led to near ideal electronic characteristics from polycrystalline material.

Chapter 6 Diamond-based radiation and photon detectors

Publisher Summary This chapter discusses different forms of detector that can be realised from diamond for photodetection and ionising radiation detection applications. Progress in this field is encouraging, diamond detectors are already commercially available, and it is likely that diamond detectors will become the first choice within several niche market sectors. As with other electronic applications for diamond, described elsewhere in this book, progress in diamond detector technology has relied upon the dramatic improvements in the quality of chemical vapor deposition material over the last few years. However, it is also apparent that careful device design is required if the full potential of diamond is to be achieved. Effective doping of diamond films is also required for many device structures, and this remains a difficult area. Boron and phosphorus can be used to generate p-and n-type character, respectively, but both dopants form relatively deep states within the band gap. The generation of doped-diamond films with useful carrier concentrations at room temperature and good carrier mobility values would greatly assist future developments in diamond detector fabrication. Another challenge that must be faced is the generation of device structures that can be used at high temperatures. Diamonds semiconductor properties are ideally suited for high temperature device operation, but the surface of diamond can be unstable at elevated temperatures and passivation processes will be required to prevent this becoming a problem.

Diamond UV photodetectors: Sensitivity and speed for visible blind applications

Abstract UV photodetectors displaying visible blind characteristics (> 10 6 discrimination between 200 nm light and wavelengths longer than 230 nm), low dark currents and high sensitivity have been fabricated. Interdigitated planar devices have been formed on CVD diamond films with differing grain sizes; the highest yet reported photoconductive gain levels (10 6 ) have been measured, although these devices are moderately slow to turn off. Post-fabrication gas treatments have enabled a good sensitivity (gain ∼ 50) to be achieved alongside switching speeds that are fast enough to be used with conventional display technology enabling imaging applications to be considered.

Chemical vapour deposition on silicon: In situ surface studies

Abstract The decomposition of Fe(CO) 5 to produce Fe films on Si(100) has been examined in situ using Auger electron spectroscopy. Processes occurring during pyrolytic, UV photolytic and electron beam deposition are determined. The nature of the surface films produced by these different methods is compared and contrasted.

AN INSIGHT INTO THE MECHANISM OF SURFACE CONDUCTIVITY IN THIN FILM DIAMOND

Abstract Diamond containing hydrogen at or near the surface displays p-type conductivity. The origin of this effect has been controversial. We have used I–V, Hall effect, SIMS, Raman, UPS and XPS to study hydrogenated polycrystalline CVD diamond films. The direct formation of acceptor states by hydrogen, which resides within the top 20 nm of the film, is the origin of the carriers present rather than surface band bending. Up to 1019 holes cm−3 can be measured and mobilities as high as 70 cm2 Vs−1 recorded. H-termination of the surface is important for the formation of high quality metal-diamond interfaces.

Hydrogen-induced transport properties of holes in diamond surface layers

Three hydrogen-terminated diamonds with different surface roughness and morphologies have been investigated by conductivity and Hall experiments in the temperature regime 0.34–350 K. The sheet hole densities are weakly temperature dependent above a critical temperature Tc (20 K⩽Tc⩽70 K), below Tc carriers freeze out. The mobilities of holes show a minimum at Tc increasing towards higher and even stronger towards lower temperatures significantly up to 400 cm2/V s. A transport model is introduced where holes propagate in the valence band where a disorder-induced tail of localized states is present.