Haitao Ye

Spectroscopic impedance study of nanocrystalline diamond films

Nanocrystalline diamond films were synthesized by microwave plasma-enhanced chemical vapor deposition using Ar/H2/CH4 gas mixtures. A Fluke PM6306 RCL meter was used to study the electrical impedance of these diamond films in the frequency range 50 Hz to 1 MHz. The impedance dispersion measurement yields the real and imaginary parts in the form of a Cole-Cole plot in the complex plane. A single semicircular response of the impedance of nanocrystalline diamond films was observed at temperatures below 250u200a°C, with a second semicircular response appearing at low frequencies at temperatures above this. The semicircular responses were found to fit a double resistor-capacitor parallel circuit model. Physical mechanisms likely to be responsible for these observations are discussed in this paper.

Electrical conduction in polycrystalline CVD diamond: Temperature dependent impedance measurements

In this paper, we report the first measurement of impedance on freestanding diamond films from 0.1 Hz to 10 MHz up to 300 °C. A wide range of CVD materials have been investigated, but here we concentrate on ‘black’ diamond grown by MWPECVD. The Cole–Cole (Z′ via Z″) plots are well fitted to a RC parallel circuit model and the equivalent resistance and capacitance for the diamond films have been estimated using the Zview curve fitting. The results show only one single semicircle response at each temperature measured. It was found that the resistance decreases from 62 MΩ at room temperature to 4 kΩ at 300 °C, with an activation energy around 0.51 eV. The equivalent capacitance is maintained at the level of 102 pF up to 300 °C, suggesting that the diamond grain boundaries are dominating the conduction. At 400 °C, the impedance at low frequencies shows a linear tail, which can be explained that the ac polarization of diamond/Au interface occurs.

Imaging deep UV light with diamond-based systems

Diamond grown by chemical vapour deposition (CVD) techniques has shown great promise for the fabrication of high sensitivity, low dark current, fast and visible-blind deep UV photodetectors. In addition to the careful choice of substrate material, defect passivation treatments applied to the diamond after growth have been found to considerably enhance the detector characteristics achieved. In this paper, we report on the first purposefully designed one-dimensional CVD diamond imaging array for the detection of nanosecond 193-nm excimer laser pulses using this approach. It is shown to perform extremely well, giving less than 2% pixel-to-pixel variation in signal response, and is fast enough to avoid any sign of charge build-up during prolonged operation.

Dielectric properties of single crystal diamond

LCR measurements have been performed on type IIa and type Ib single crystal diamonds, in an attempt to identify any differences in their dielectric characteristics. It is found that the type IIa diamond, which contains little nitrogen, behaves as an ideal insulator, with the dielectric loss appearing to be frequency independent at low temperatures, whilst assuming a frequency dependence at high temperatures. In contrast, the type Ib diamond samples exhibited a dielectric loss peak in the temperature-dependent data, from which a thermal activated process with an Ea of 1.67 eV can be determined using a simple Debye law relationship. However, the data suggests that a dielectric relaxation constant (nitrogen-induced dipole) of 1017 Hz is evident in the Ib system, which is much higher than that predicted by the Debye model, as such it is unlikely due to the dipole moment itself. A possible physical mechanism responsible for this phenomena is simply the ionization of a nitrogen centre with a defect level lying at 1.7 eV below the conduction band.

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

Abstract The solid solution system Nd2−xCexTi2O7 has been investigated. The solubility limit of Ce in Nd2−xCexTi2O7 was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd2−xCexTi2O7 (xu200a=u200a0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point Tc of Nd2−xCexTi2O7 compounds. The results suggest that the Tc of Ce2Ti2O7 is <1445°C.

Diamond-based 1-D imaging arrays

Diamond has shown great promise for the fabrication of high sensitivity, low dark current, fast and visible-blind deep UV photodetectors. In addition to careful choice of substrate material, defect passivation treatments applied to the diamond after growth have been found to considerably enhance the detector characteristics achieved. In this paper we report on the first purposefully designed 1-D CVD diamond imaging array for the detection of nanosecond 193 nm excimer laser pulses using this approach. It is shown to perform extremely well, giving less than 2% pixel-to-pixel variation in signal response, and is fast enough to avoid any sign of charge build up during prolonged operation.