**7. Influence of nitrogen admixture on morphology**

An especially appealing field of application for UNCD is nitrogen doped semiconducting films. UNCD films are usually insulating, but n-doping is easily possible by admixture of nitrogen to the process gas. (Gruen, 2004)

To investigate the influence of the nitrogen admixture in the plasma on the film properties, more films were deposited at a pressure of 200 mbar with admixtures of nitrogen from 0 % to 7.5 %.

Fig. 5. High resolution SEM measurement of a UNCD film deposited with 2.5 % hydrogen and 2.5 % nitrogen admixture. The scale bar shown corresponds to 1 µm

High-resolution SEM pictures were taken to investigate the influence of hydrogen and nitrogen admixture on the morphology of the films. Fig. 5 shows a film deposited with 2.5 % hydrogen and 2.5 % nitrogen in the plasma. The diamond grains appear to be very fine. Increasing the nitrogen admixture to 7.5 % and keeping the hydrogen admixture at 2.5 % changes the shape of the diamond grains. They appear to be needle-shaped as shown in Fig. 6. These measurements show that the nitrogen admixture can influence the shape of the diamond grains.

Fig. 6. High resolution SEM measurement of a UNCD film deposited with 2.5 % hydrogen and 7.5 % nitrogen admixture. The scale bar shown corresponds to 200 nm

It is expected that the change in the crystal shape will have a strong influence on the propagation speed of sound in the material giving yet another degree of freedom when designing the material for specific applications.
