Structural properties of highly conductive ultra-nanocrystalline diamond films grown by hot-filament CVD

M Mertens,A Moeinian,D Manoharan,K Brühne,H J Fecht,I.-N Lin

doi:10.1063/1.4975226

Abstract

In this work we show the correlation of the electrical conductivity of ultra-nanocrystalline (UNCD) diamond films grown by hot filament chemical vapor deposition (HFCVD) with their structural properties. The substrate temperature, the methane to hydrogen ratio and the pressure are the main factor influencing the growth of conductive UNCD films, which extends from electrical resistive diamond films (&lt;10-4 S/cm) to highly conductive diamond films with a specific conductivity of 300 S/cm. High-resolution-transmission-electron-microscopy (HRTEM) and electron-energy-loss-spectroscopy (EELS) have been done on the highly conductive diamond films, to show the origin of the high electrical conductivity. The HRTEM results show random oriented diamond grains and a large amount of nano-graphite between the diamond crystals. EELS investigations are confirming these results. Raman measurements are correlated with the specific conductivity, which shows structural changes of sp2 carbons bonds as function of conductivity. Hall experiments complete the results, which lead to a model of an electron mobility based conductivity, which is influenced by the structural properties of the grain boundary regions in the ultra-nanocrystalline diamond films.

Highlights

Nanocrystalline diamond (NCD) is well known for the outstanding mechanical properties
N-type conductivity for the UNCD films was confirmed from the sign of the Hall coefficient, whereas the carrier concentration and mobility of the electrons in UNCD films can be extracted from the Hall coefficient
It is observed that the carrier concentration of 2.9x1019 ± 0,9x1019 was the same for the samples having a large difference in specific conductivity, whereas the mobility of the electrons is direct proportional to the specific conductivity that is shown in figure 1

Summary

Introduction

Nanocrystalline diamond (NCD) is well known for the outstanding mechanical properties. In combination with high electrical conductivity it becomes an interesting material system for different MEMS applications in sensitive and harsh environments.[1] The nanocrystalline diamond films shows a chemical flexible surface chemistry,[2] with chemical inert properties and biocompatibility, which is useful for electrical applications, especially when the films were rendered electrical conducting. The mechanism leading to electrical conductivity is described as grain boundary effect, because of sp[2] hybridized carbons with π bonds in the grain boundaries. Very small activation energies of single meV for the sp[2] hybridized carbons are explaining the room temperature conductivity in these films

Methods

Results

Conclusion