Natural diamond has many attractive electronic properties such as high electron and hole mobility, high break-down voltage, negative electron affinity, and high resistivity. Much effort is made to apply these properties to insulating layer for silicon-on-insulator devices and conducting layer for diamond field effect transistors. But, unlike natural diamond, undoped CVD diamond films have relatively high electrical conductivity, which should be reduced to be employed as an insulating layer. And, the morphology and doping of diamond films should be also controlled for electrical applications. In this thesis, the conduction path and mechanism of undoped diamond films were investigated, and the electrical properties and morphological changes were observed with the addition of nitrogen and boron.
The electrical resistivity of natural diamond is about $10^{15}Ωcm$, and thermal conductivity is 20W/cmK which is the highest value by far of any solid at room temperature. Owing to this high electrical resistivity and thermal conductivity, diamond is regarded as the candidate of an insulating layer for silicon-on-insulator (SOI) structure. In order to apply diamond to SOI structure, diamond must be a thin film grown by CVD method. However, contrary to natural diamond, as-grown CVD diamond films have fairly low resistivity ($∼10^{6}Ωcm$) without any doping. Many models were proposed to explain the low resistivity of CVD diamond films. However, the origin of low resistivity is still not clear. In this study, the conduction path and mechanism of undoped diamond films were investigated.
It is believed that as-grown diamond films have a relatively high conductive layer near the surface, which can be removed by the oxidation of the surface using acid solutions. It is known that the current in undoped diamond films flows along film surface (surface conduction model). To investigate conduction path, the resistance of as-grown diamond films with different thickness and direction was measured. The thick films had higher resistance than thin ones. And, the resistance measured between two contacts on diamond surface was almost twice as high as the resistance measured between surface contact and silicon substrate. These results can not be explained by the surface conduction model. With Cu electroplating experiment, Cu was deposited on diamond surface. Cu was deposited only along the grain boundaries on the surface. No Cu was deposited on the surface, which was pre-cleaned with $H_2SO_4+HNO_3$ solution. From these results, we can say that the current in the undoped diamond films flows not along the surface, but through grainboundaries.
The resistivity of CVD diamond films changed after post-treatment. The resistivity of the as-grown CVD diamond films increased after oxygen ambient annealing, and the resistivity decreased by the subsequent hydrogen plasma treatment. The dangling-bond density of the post-treated diamond films was measured by ESR. The conducting films have lower dangling bond density, while the insulating films have higher value. The resistance of the as-grown and post-treated diamond films was measured as a function of temperature. All the films have the activation energy of 0.9eV above 200℃. But, below 200℃, the resistance of conducting diamond films increased slightly maybe due to the increase of mobility. Acceptor levels in undoped diamond films can be suggested to explain these results. One acceptor level is located at 0.9eV above valence band, and another is near the valence band. The high resistive undoped diamond films have only one acceptor level at 0.9eV, while the conducting diamond films have one more acceptor level near the valence band. The surface carbon bonding changes were also suggested with reported STM observation.
Low and unstable resistivity of undoped diamond films is not suitable for the insulating layer of SOI structure. It is reported that the electrical resistivity of nitrogen-incorporated diamond films increased several order of magnitudes compared with undoped diamond. In stead of undoped diamond films, nitrogen-incorporated diamond films which have a high electrical resistivity can be used. In this study, the resistance and morphology of nitrogen incorporated diamond films were investigated with the content of nitrogen and the heteroepitaxy diamond films were prepared. Nitrogen was incorporated into diamond films during diamond deposition in a microave plasma chemical vapor deposition system. Nitrogen gas was used as an impurity source, varying the nitrogen/methane ratio from 0 to 83000ppm. The substrate temperature was also changed from 800℃ to 920℃. The resistivity of the undoped diamond films was about $10^6∼10^7Ωcm$. The resistivity increased up to $10^{13}Ωcm$ as the $N_2$/$CH_6$ ratio increased and then slightly decreased with further increase of the ratio. The maximum resistivity of $10^{13}Ωcm$ was observed at 7000ppm nitrogen. And, the resistivity of nitrogen-incorporated diamond films was not changed after oxygen annealing and subsequent hydrogen plasma treatment. The morphology map with various particle shapes was constructed as a function of the substrate temperature and nitrogen concentration. With increasing nitrogen concentration and decreasing substrate temperature, the shape of diamond particle changed from cubo-octahedron to octahedron via truncated-octahedron. As the substrate temperature decreased, the shape transition occurred at lower nitrogen concentration. The nitrogen-incorporated heteroepitaxy diamond films were deposited by two step process; the first step was in the octahedron shape area and the second step was in the truncated octahedron shape area in the morphology map.
In order to use diamond as electronic active devices, several problems such as n-type and p-type doping and heteroepitaxial film growth should be solved. Doping is very important in endowing active electronic function to diamond. In spite of much efforts in finding adequate dopants, only p-type doping is possible at the present. However, it is possible to fabricate many devices such as field effect transistors and various types of sensors using only p-type doping. Boron is a p-type dopant, which is incorporated into diamond from gas phase during diamond deposition. We investigated the crystal habit changes of isolated diamond particles with various deposition temperatures and diborane concentrations. The electrical resistivities of films were measured and compared them with the morphology variation. The diamond particles and films were deposited using a microwave plasma assisted chemical vapor deposition method at 70 torr with various diborane concentrations from 0 to 4000 ppm in the hydrogen-2% methane gas. The shape of isolated particle changed from cubo-octahedron to octahedron via truncated-octahedron as the diborane concentration increased. The growth rate decreased rapidly and then maintained constant with further increasing diborane concentration. The changes of particle shape and growth rate were explained by the charge-polarity model of cursor gas, and by the relative growth rate of (100) and (111) facets, which were controlled by the boron concentration. The surface morphology of diamond films changed in accordance with the change of the shape of diamond particle. The resistivity of diamond film decreased due to the increase of carrier concentration as the diborane concentration increased to 500 ppm. The boron-doped heteroepitaxy diamond films were also deposited with two step process; the first step was in the octahedron shape area and the second step was in the truncated octahedron shape area in the morphology map.'
