Properties of thin, highly oriented diamond films and their use in electronic applications

It is often required that the diamond films used in electronic devices such as transistors, sensors and UV and particle detectors be of high quality and have low surface roughness, low defect density and low concentrations of unwanted impurities. Unfortunately, large-area diamond films often show high defect densities caused by a high density of large-angle grain boundaries. Furthermore, the non-oriented arrangement of diamond crystals increases the surface roughness with increasing film thickness. Recently, we developed a repetitive pulse bias-enhanced nucleation (RP-BEN) process that enables the nucleation of highly oriented diamond seeds over an area of up to 30 cm(2) with only small radial variations. Using the RP-BEN process for nucleation, large-area, highly oriented diamond (HOD) films of 15 mu m-60 mu m thickness have been grown on 2" (001) silicon substrates by microwave plasma enhanced chemical vapor deposition (MPACVD). Using scanning electron microscopy, Xray diffraction analysis, Raman spectroscopy and Fourier-infrared spectroscopy, only minor radial changes in film properties could be observed. The films exhibit a surface roughness of less than 80 nm (peak to valley) with a thickness of 15 mu m, and full-width at half maximum (FWHM) values of the <111> diamond peak of 7.3 degrees for phi and 6.0 degrees for chi (as measured) are obtained for 60 mu m-thick films. Raman spectroscopy reveals a high phase purity and no macroscopic stress within the films. Laterally resolved film thickness measurements by Fourier-infrared spectroscopy show only +/-10% change over the entire deposition area. From periodic acoustic wave stimulations on free-standing, 15-mu m-thick diamond cantilever beams with integrated diamond piezoresistors, a Young's modulus of E = 825 GPa is obtained. This is approximately 80% of the value for single-crystal diamond. HOD films can be used in a variety of electronic applications including Shottky diodes, field effect transistors (FETs), UV detectors, mechanical microsensors, and thermal microactuators. Because of the large area. high radial homogeneity. low surface roughness and low internal stress of the HOD films, conventional semiconductor processing schemes can be implemented for large-scale fabrication. Fabricated surface channel FETs on HOD films demonstrate drain currents exceeding 20 mA/mm and, at a gate length L-g of 4 mu m, a breakdown voltage V-d of 60 V. Metal-Semiconductor-Metal (MSM) UV detectors show an ultrafast response of < 4 ns (FWHM), and an approximately 100% charge collection efficiency at high field strength. These are the first examples that give valuable feedback for further material refinement.