A detailed mechanism for heteroepitaxial diamond nucleation under ion bombardment in a microwave plasma enhanced chemical vapour deposition setup on the single crystal surface of iridium is presented. The novel mechanism of Ion Bombardment Induced Buried Lateral Growth (IBI-BLG) is based on the ion bombardment induced formation and lateral spread of epitaxial diamond within a ~1 nm thick carbon layer. Starting from one single primary nucleation event the buried epitaxial island can expand laterally over distances of several microns. During this epitaxial lateral growth typically thousands of isolated secondary nuclei are generated continuously. The unique process is so far only observed on iridium surfaces. It is shown that a diamond single crystal with a diameter of ~90 mm and a weight of 155 carat can be grown from such a carbon film which initially consisted of 2 · 1013 individual grains.

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Ion bombardment induced buried lateral growth: the key mechanism for the synthesis of single crystal diamond wafers.

High growth rate homoepitaxial diamond deposition on off-axis substrates

Heteroepitaxial diamond films can be grown by bias enhanced nucleation on iridium buffer layers followed by an appropriate textured-growth step. Unlike epitaxial diamond films on silicon, the mosaicity reduction during textured growth includes tilt as well as twist. We conclude that different mechanisms causing the grain coarsening are working in the two cases. It is shown that the principle of evolutionary selection can be excluded as a decisive mechanism in the present films. Merging of neighboring grains by disclination formation yields an alternative explanation, that can convincingly substantiate the differences between the textured growth on iridium and silicon. From a Monte–Carlo type simulation describing the texture evolution due to merging of grains, a simple functional correlation between grain coarsening and mosaicity reduction is deduced. Comparison between simulation and experiment allows one to estimate the contributions of different processes. Finally, the general significance of the present findings for other materials is discussed.

Mosaicity reduction during growth of heteroepitaxial diamond films on iridium buffer layers: Experimental results and numerical simulations

This paper reviews some new developments in hard materials during the last decade: new approaches to diamond synthesis including chemical vapor deposition and the laser QQC method, tetrahedral carbons, deposition of cubic boron nitride, ternary boron-carbon-nitrogen and boron-carbon-silicon phases. The issue of ultimate hardness of diamond reappeared again because of pseudopotential calculation of cohesive energy of a hypothetical β-C 3 N 4 structure. The stability of carbon nitride phases is analyzed in connection with the authors study of synthesis and structure of carbon nitride by nitrogen microwave plasma. Only soft graphitic CN x , x phase, stable up to 900 °C, survived criticism.

Recent developments in hard materials

In this paper we report on the positron lifetime results obtained for brown and colourless natural diamond. Optical effects of the observed vacancy defects in brown, high pressure, high temperature (HPHT) treated colourless and naturally colourless type IIa diamond samples were studied by combining the positron measurement with monochromatic illumination. Brown diamond was found to contain optically active vacancy clusters (40–60 missing atoms) strongly correlated with the optical absorption spectra. The optical activity of these vacancy clusters is manifested by a photo-excitation induced change of charge from neutral to negative. The clusters gradually disappear during the HPHT treatments, and the samples treated at 2500 °C resemble colourless samples optically and show similar positron lifetimes. The results show that the brown colour originates from the vacancy clusters and that their removal by the HPHT treatment causes the loss of coloration.

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Properties of optically active vacancy clusters in type IIa diamond

Annihilation characteristics of positrons in various types of diamonds were studied by measurements of two‐dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra. From the measurements, it was found that there is a small amount of defects in type Ib synthesized diamond and the positron lifetime is 115 ps. On the other hand, it was found that positronium is formed in natural diamonds. Results of the double x‐ray measurements suggested that IIa, IIb, and Ia type natural diamonds also may have empty space between crystallites or grain boundaries.

Study of various types of diamonds by measurements of double crystal x‐ray diffraction and positron annihilation

Annihilation characteristics of positrons in vitreous silica (v-SiO2) were studied in the temperature range between 20 K and room temperature (RT). In the measured temperature range, almost all positrons are trapped by open spaces and annihilate mainly from positronium (Ps) states. Lifetimes of ortho-Ps increase with increasing temperature, where the distribution of the lifetimes at 20 K is shallower than that at RT. This fact was attributed to the thermal expansion of the open spaces and the broadening of their size distribution. In the temperature range between 170 K and 200 K, a certain structural transition of the open spaces was found. The transition temperature is decreased by electron irradiation or by the presence of hydroxyl groups. These results were attributed to the fact that an activation energy of the transition was lowered by the introduction of defects.

http://iopscience.iop.org/article/10.1088/0953-8984/7/26/019/pdf

Positron trapping by defects in vitreous silica at low temperature

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

Defects at Schottky contacts in GaAs devices were probed by a monoenergetic positron beam. Doppler broadening profiles of the annihilation radiation were measured as a function of incident positron energy for a Ti/GaAs specimen. From an analysis of the relationship between the S parameter and incident positron energy, vacancy-type defects were found to be present at the Ti/GaAs interface. Annihilation characteristics of positrons in an AuGeNi/GaAs specimen were also studied. In the alloyed layer, the diffusion length of positrons was decreased by scattering and/or trapping of positrons by precipitates and solute atoms, while no response indicating the presence of vacancy-type defects was observed in Doppler broadening profiles.

S. Tanigawa

Papers

Study of various types of diamonds by measurements of double crystal x‐ray diffraction and positron annihilation

Positron trapping by defects in vitreous silica at low temperature

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

Characterization of Metal/GaAs Interfaces by Monoenergetic Positron Beam

Vacancy-Oxygen Complexes in Si Probed by Positron Annihilation