Tem Study of Growth Defects in CVD Diamond Films
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ABSTRACT Transmission electron microscopy (TEM) has been used to examine defects in diamond films grown by the microwave plasma-enhanced chemical vapor deposition (CVD) method. Graphite was used as the sole carbon source during the CVD process with silicon substrates. Growth defects including twins, stacking faults, dislocations and second-phase precipitates were observed in the films. In plan-view TEM, defect clusters at the centers of diamond grains were observed, where the film is also the thickest. Cross-sectional TEM was carried out to show that the defect clusters fan out from a single nucleation site in each grain, at the diamond-silicon interface. Possible growth mechanisms of the defect clusters in diamond grains are considered. INTRODUCTION Diamond has a number of unique and potentially useful properties such as high hardness, high thermal conductivity and wide bandgap, but it is difficult to make defect-free in thin-film form. CVD diamond usually contains high density of defects. These defects are obstacles to the utilization of CVD diamond in electronic applications, although several diamond electronic devices have been developed. It is therefore of great importance to understand the nucleation and growth mechanisms of CVD diamond. Planar defects (twins and stacking faults) and line defects (dislocations) are commonly observed in CVD diamond using TEM [1-6]. Also, several workers have studied the substrate-film interfaces using cross-sectional TEM [ 1,2]. A high density of dislocations emanating from a single nucleation site at the film-substrate interface is commonly observed, supporting the general conclusion that the CVD diamond grows in a 3-D rather than 2-D mode [1,2]. In this study, diamond films were grown by a novel technique [7,8] using graphite as the carbon source instead of using the more usual hydrocarbon gas solution. Carbon is delivered to the growing film by reacting graphite with a hydrogen plasma, and transporting hydrocarbon complexes to the substrate. In this study, we investigate the structures of films produced using this method. EXPERIMENTAL The diamond films were prepared in a Vactronic PECVD-60-M microwave plasma-enhanced CVD system as described elsewhere [9]. The microwave plasma reactor, modified by introduction of a parallel-plate graphite as the carbon source, was configured to confine an intense hydrogen plasma between the graphite plates. High quality diamond films were subsequently deposited on silicon substrates either suspended from the top plate, placed on the bottom one, or both. The (100) silicon substrates were pre-treated by scratching with 1 pim diamond grit. Gas pressures of 45, 55, 65, 75, 85 and 95 Torr were used in a series of depositions on different substrates. The process time for each sample was 24 hours. Continuous diamond films of thickness about 3 Pm were obtained. The diamond film used in this TEM study was grown at 75 Torr. Both plan-view and cross-sectional TEM specimen were prepared. For plan-view TEM specimens, 3 mm diameter disks were cut from a d
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