Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
1999-04-26
2001-08-07
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S097000, C117S101000, C117S106000, C117S913000, C117S923000, C117S951000, C423S345000, C428S627000, C428S641000
Reexamination Certificate
active
06270573
ABSTRACT:
TECHNICAL FIELD
This invention relates to a silicon carbide substrate having a hexagonal or cubic crystal structure, a silicon carbide substrate in which crystal layers each having a different crystal system are stacked, and a producing method of the silicon carbide substrate by utilizing homoepitaxial growth or heteroepitaxial growth, and a semiconductor device utilizing these silicon carbide substrates.
BACKGROUND ART
In recent years, silicon carbide crystals have drawn considerable attention in the industry because of their advantageous properties as a semiconductor material, such as a large band gap, high saturated electron drift velocity, and high thermal conductivity. Single crystalline substrates made of 6H or 4H hexagonal silicon carbide are commercially available and used. Among them, a cubic silicon carbide has been particularly expected for use in a semiconductor device with high speed and high power operation. However, the cubic silicon carbide is very difficult to be developed into a large single crystal usable as a substrate, and for this reason, a thin film heteroepitaxially grown on a single crystalline silicon or the like has been conventionally used.
Such silicon carbide crystals as above are usually formed by an atmospheric pressure CVD and the like method under a high temperature over 1300° C., normally around 1500° C., using a mixed gas of silane and propane as a source gas, and hydrogen as a carrier gas.
Nonetheless, the growth mechanism of a silicon carbide crystal has not yet been fully understood, and, therefore, an industrial technique for epitaxially growing a silicon carbide thin film with high reproducibility by controlling the relationship between an amount of a source gas to be supplied and a temperature of the substrate has not yet been established with full knowledge thereof. As a consequence, conventional production methods for a silicon carbide substrate have various drawbacks such as described below.
Firstly, since high temperatures as mentioned above are required for the crystal growth of silicon carbide, it has been difficult to grow a crystal in a selective area by using masking, and to carry out a nitrogen doping with high concentration. Specifically, since there is no suitable material for masking sufficiently resistant to such high temperatures, it is difficult to grow a crystal only in a predetermined region by patterning. In addition, silicon carbide is difficult to be subjected to a selective etching, and therefore it is difficult to form desired semiconductor devices and semiconductor circuits using silicon carbide. In addition, if a nitrogen doping is carried out under such a high temperature as above, the resulting film of the grown crystal is susceptible to roughness, and therefore a doping with a high concentration such as approximately more than 5×10
18
/cm
3
is difficult to attain. Furthermore, in a crystal growth at a high temperature, the decomposition, attaching to a surface of the substrate, re-evaporation and the like mechanisms of a supplied source gas are so complicated that it is rendered more difficult to, for example, epitaxially grow a silicon carbide thin film with a high reproducibility by controlling the relationship between the amount of the source gas to be supplied and the temperature of the substrate. It is noted that T. Kimoto et al. suggest on pp. 726-732 in the
Journal of Applied Physics
. Vol. 73, No. 2 (1993) a technique of forming a 6H silicon carbide crystal at a relatively low temperature by employing a silicon carbide substrate having an off-cut surface towards a [11{overscore (2)}0] direction of a {0001} face by using a step-flow growth and the like. However, even with this technique, it is required that the substrate be heated at approximately 1200° C.
Secondly, a single crystal thin film composed of hexagonal or cubic silicon carbide with a good crystallinity is difficult to be epitaxially grown. In particular, when a cubic silicon carbide crystal is formed on a silicon substrate, a large lattice mismatch occurs and therefore a good crystallinity is difficult to obtain. In addition, when a cubic silicon carbide crystal is formed on a 6H hexagonal silicon carbide crystal, the resulting cubic silicon carbide crystal is apt to contain double positioning boundaries caused by the occurrence of twin. It is noted that the present inventors have disclosed in Japanese Unexamined Patent Publication No. 07-172997 a method for producing a cubic silicon carbide thin film having a (001) face by having silicon atoms present in excess of carbon atoms on the growth surface of a silicon carbide crystal, and a method for producing a cubic silicon carbide thin film having (111) surface or a hexagonal carbon silicon thin film having (0001) surface by having carbon atoms present in excess of silicon atoms on the growth surface of a silicon carbide crystal. However, even with these methods, it has been difficult to suppress the occurrence of twin certainly or drastically although a relatively good crystallinity can be obtained thereby.
Moreover, it has not yet been made possible to heteroepitaxially grow on a silicon carbide substrate a silicon carbide crystal having a different crystal system from a crystal system of the substrate. For example, in the case of a step-flow growth of a silicon carbide crystal on a hexagonal silicon carbide substrate, the resulting silicon carbide crystal is apt to retain a hexagonal structure because the crystal structure of the substrate tends to restrict the resulting crystal, and it is therefore difficult to heteroepitaxially grow a cubic carbon carbide on such a substrate.
In view of the above problems, it is an object of the present invention to provide a method for producing a silicon carbide substrate that it is capable of epitaxially growing a silicon carbide having a good crystallinity at a relatively low temperature, that a crystal growth in a selective region by masking and a high concentration nitrogen doping are easily carried out, and that silicon carbides each having a different crystal system are heteroepitaxially grown and stacked with a good interface therebetween.
It is another object of the present invention to provide a silicon carbide substrate produced by the above method.
It is further another object of the present invention to provide a semiconductor device capable of high-speed operation utilizing the above silicon carbide substrate.
DISCLOSURE OF THE INVENTION
The present inventors have found that, by controlling an existence ratio of carbon and silicon on a growth surface of a silicon carbide crystal so that silicon atoms are in excess of carbon atoms, a smooth surface can be obtained in a good reproducibility even at a relatively low temperature, and that a high quality epitaxial thin film can be thereby obtained. The present invention has been completed based on these findings. The more detailed description now follows.
As shown in
FIG. 1
, on a silicon carbide crystal growth surface
1
a
of a substrate
1
, an excessive silicon atom
2
bonded with the substrate
1
by a Si—Si bond
3
tends to be actively diffused on the growth surface
1
a
. This is because the Si—Si bond 3 is a weaker bond than a C—Si bond and a C—C bond of carbon atoms. Therefore, when carbon atoms are supplied under the condition in which silicon atoms are in excess of carbon, silicon atoms tend to be bonded with the substrate
1
and carbon atoms at an accurate lattice position, and thereby a good crystallinity with an excellent uniformity and flatness can be easily obtained even at a relatively low temperature. It should be noted here that, as previously described, the present inventors disclosed in Japanese Unexamined Patent Publication No. 07-172997 a technique in which silicon atoms are supplied in excess in the case of a cubic silicon carbide having (001) face and carbon atoms are supplied in excess in the case of a hexagonal silicon carbide having (0001) face. However, the present inventors have discovered that, if silicon atoms are supplied slight
Kitabatake Makoto
Takahashi Kunimasa
Uchida Masao
Kunemund Robert
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel LLP
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