1. Field of the Invention
The present invention relates to highly-oriented diamond films of which crystal grains are highly oriented, methods for manufacturing the same, and electronic devices having highly-oriented diamond films. Specifically, the present invention relates to highly-oriented diamond films suitable for electronic devices such as transistors and diodes, methods for manufacturing the same, and electronic devices having the highly-oriented diamond films.
2. Description of the Related Art
A highly-oriented diamond film is a polycrystalline film in a broad sense, but a growth direction and an in-plane direction of the crystal grains are each oriented in one direction and the surface is in a characteristic state of a series of flat (001) facets. Therefore, the highly-oriented diamond film is known that the crystal defect density near the surface is lower than that of general polycrystalline films and that the carrier mobility near the surface is about two figures higher than that of general polycrystalline films. Because of this, it is thought that the highly-oriented diamond film is suitable for electronic devices such as a field-effect transistor utilizing the carrier mobility in the lateral direction.
Such a highly-oriented diamond film can be formed by, for example, subjecting a silicon substrate to microwave radiation while applying a negative bias voltage in a gas phase containing methane gas (U.S. Pat. No. 5,523,160). Furthermore, a method for preparing a heteroepitaxial diamond film having a flat surface is suggested in B. Dischler, C. Wild, eds. Low-Pressure Synthetic Diamond, 1998:153-158 (referred to as non-patent document, hereinafter). In the method, after the application of a bias voltage to a substrate, predominant [001] growth of diamond (a first growth) is performed at a rate of 3.9 to 4.5 μm/hr for improving a degree of orientation of crystalgrains, and then surface-planarizing growth (a second growth) of diamond is performed at a rate of 2 μm/hr or less for planarizing the surface.
The above-mentioned conventional technologies have some problems as follows: In an electronic device such as a transistor having a diamond film, properties of the device are improved with an increase in the diamond crystal grain size. For example, in the application to a transistor, since a gate length is generally 30 to 100 μm, the highly-oriented diamond film must have a crystal grain size of at least 30 μm, preferably 100 μm or more. When the highly-oriented diamond film is prepared according to the method disclosed in the non-patent document, diamond crystals grow in a columnar form in the first growth to gradually increase the grain sizes. However, since the increasing rate is small, a film having a large thickness is required for ensuring a crystal grain size of 30 μm or more; which is a problem.
Furthermore, a diamond film having a flat surface is desirable. However, in the method described in the non-patent document, the diamond in the second growth is synthesized under such conditions that a growth rate in [111] orientation is higher than that in [001] orientation. Therefore, even if protruding non-oriented crystals are slight, the non-oriented crystals predominantly grow. When a highly-oriented diamond film containing non-oriented crystals is applied to an electronic device as a substrate, the surface is planarized by polishing after a film is epitaxially deposited on the diamond film for preventing inner damage caused by the polishing and for doping. This causes changes in growth sectors at the non-oriented crystal portions. Since distribution coefficients of an impurity differ in the growth sectors, the non-oriented crystal portions and the other portions each have an impurity content being different from that of each other when the epitaxially grown film is doped with the impurity. This is undesirable for device movement. Therefore, when the method disclosed in the non-patent document is conducted, the first growth must be performed until the non-oriented crystals do not protrude. Consequently, a thin film, for example, having a thickness of 50 μm or less cannot be formed by this method; which is a problem.
The non-patent document discloses that a smooth diamond film having a film thickness is 5 μm or less can be formed by using a favorable SiC film as a buffer layer. However, in this case, a ratio of the average crystal grain size to the film thickness (average crystal grain size/film thickness) is still ½ or lower, so a large film thickness is required for yielding an average crystal grain size of 30 μm or more. This problem has not been solved yet.