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Defect structure and electron field-emission properties of boron-doped diamond films Substrate materials used for growing diamond films were observed to modify thin films' electron field emission properties significantly. Using heavily doped silicon ͑LR-Si͒ as a substrate lowered the turn-on field from (E 0 ) Si ϭ14.4 V/m to (E 0 ) LR-Si ϭ9.7 V/m and increased the emission current density from (J e ) Si ϭ4 A/cm 2 to (J e ) LR-Si ϭ40 A/cm 2 ͑at 16 V/m͒. However, electron field emission properties can be further improved only by using Au precoatings to modify the characteristics of interfacial layer. The turn-on field was lowered further to (E 0 ) Au-Si ϭ8.7 V/m and emission current density was increased further to (J e ) Au-Si ϭ400 A/cm 2 ͑at 16 V/m͒. Secondary ion mass spectroscopic examination indicated that the main interaction is the outward diffusion of Au species into amorphous carbon layer, lowering the resistivity of this interfacial layer. The electrons can therefore be transported easily from Si substrate across the interfacial layer to the diamonds and subsequently field emitted.

Diamonds were successfully nucleated on SiO 2 -coated silicon substrates using a high power microwave plasma enhanced chemical vapor deposition process. Nucleation rates on SiO 2 surfaces ͑i.e., 0.5ϫ10 10 cm Ϫ2 ͒ were, however, still smaller than those on Si surfaces ͑i.e., 1.0ϫ10 10 cm Ϫ2 ͒. The major advantage in using high power microwaves was revealed by optical emission spectroscopy to be that the atomic C and H species produced are more abundant and energetic. Therefore, the negative bias effect is enabled and the formation of sp 3 bonds is enhanced. The nucleation of diamonds on SiO 2 surface is thus made possible. The growth of diamonds behaved similarly on the prenucleated surface, regardless of the nature of the substrates. Diamonds were of single grain columnar structure with random orientation when deposited without bias and were of multi-grain columnar structure with ͓111͔ or ͓001͔ preferred orientation when deposited under Ϫ100 V dc bias. Multi-grain columnar structure was ascribed to the induction of secondary nucleation at the presence of bias voltage.

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