Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2002-01-07
2004-08-10
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
Reexamination Certificate
active
06773505
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for the sublimation growth of at least one SiC single crystal, in which a stock of solid SiC is introduced into a storage area of a crucible, and at least one SiC seed crystal is introduced into at least one crystal area of the crucible. The crucible is brought to growth conditions during a starting phase. The SiC single crystal is grown during a growth phase, by the solid SiC of the stock being at least partially sublimed and converted into an SiC gas phase with SiC gas-phase components and the SiC gas-phase components being at least partially transported to the SiC seed crystal, where they are deposited on a growing SiC single crystal. A method of this type for the sublimation growth of an SiC single crystal is known, for example, from German Patent DE 32 30 727 C2.
In the known method, solid silicon carbide (SiC), which is situated in a storage area, is heated to a temperature of between 2000° C. and 2500° C. and is thereby sublimed. The SiC gas phase which forms through the sublimation contains as SiC gas-phase components, inter alia, pure silicon (Si) and carbide compounds Si
2
C, SiC
2
and also SiC. The gas mixture of the SiC gas phase diffuses through a porous graphite wall into a reaction or crystal area in which the SiC seed crystal is situated. Silicon carbide crystallizes out of the SiC gas phase on the seed crystal at a crystallization temperature of between 1900° C. and 2000° C. As well as the gas mixture of the SiC gas phase, there is also an inert gas, preferably argon (Ar), in the crystal area. A growth pressure of between 1 mbar and 5 mbar that is desired in the crystal area is set by suitable introduction of the argon gas. The overall pressure in the crystal area is composed of the vapor partial pressure of the SiC gas phase and the vapor partial pressure of the argon gas.
Before the actual growth phase commences, the crucible, and therefore also the crucible inner zone, are heated to a growth temperature. During the heat-up phase, a heat-up pressure, which is significantly higher than a growth pressure that is subsequently used during the growth phase, is established in the crucible by filling with the inert gas, for example with argon. After the growth temperature has been reached, the pressure is reduced by pumping gas out until a considerably lower growth pressure is reached.
European Patent EP 0 389 533 B1 also describes a method for the sublimation growth of the SiC single crystal, in which, prior to the growth phase, a crucible is heated to a growth temperature of the order of magnitude of 2200° C. A high heat-up pressure of approximately 530 mbar is established at the same time. After the growth temperature has been reached (it should be noted that this differs slightly in the storage area and in the crystal area, so that the temperature gradient required to transport the SiC gas-phase components is established), the pressure in the crucible is then set to a considerably lower growth pressure of approximately 13 mbar.
A further method for the sublimation growth of an SiC single crystal is known from the article Journal of Crystal Growth, Vol. 115, 1991, pages 733 to 739. In this method too, the crucible used for growth is heated up under a high argon gas pressure of approximately 1000 mbar (=atmospheric pressure). As soon as the desired growth temperature has been reached, the argon gas pressure is reduced to the growth pressure. In the sublimation method, the growth pressure may also be varied during the growth phase. The growth pressure may adopt values of between 1.33 mbar and 1000 mbar.
The article in Electronics and Communications in Japan, Part 2, Vol. 81, No. 6, 1998, pages 8-17 describes a method for the sublimation growth of an SiC single crystal in which the heating-up takes place under a high pressure of, for example, 800 mbar. The high pressure is required in order to avoid crystallization at a low temperature, i.e. at a temperature which is lower than the actual growth temperature. This is because otherwise crystallization at low temperature would lead to the formation of an undesirable polytype.
Therefore, all the known methods initially heat up the crucible to the desired growth temperature under a high heat-up pressure, before then reducing the pressure to a much lower growth pressure. However, it has been found that in these methods the crystal quality of the growing SiC single crystal may be impaired and, in particular, an undesirably high rate of defect formation may result.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for the sublimation growth of an SiC single crystal, involving heating under growth pressure which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which ensures improved seeding of the growing SiC single crystal on the SiC seed crystal and therefore allows a higher crystal quality of the growing SiC single crystal. In particular, it is intended to reliably avoid uncontrolled accumulation of SiC gas-phase components on a crystallization surface.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the sublimation growth of at least one SiC single crystal. The method includes the steps of introducing a stock of solid SiC into a storage area of a crucible, introducing at least one SiC seed crystal into at least one crystal area of the crucible, and bringing the crucible to growth conditions during a starting phase. The crucible is initially evacuated and then the crucible is filled with an inert gas, until a growth pressure is reached in the crucible, during the starting phase. Furthermore, during the starting phase, the crucible is initially heated to an intermediate temperature and then, in a heat-up phase, starting from the intermediate temperature, the crucible is heated to a growth temperature at a heating rate of at most 20° C./min. The SiC single crystal, during a growth phase, is grown by the stock of solid SiC being at least partially sublimed and converted into an SiC gas phase with SiC gas-phase components and the SiC gas-phase components being at least partially transported to the SiC seed crystal, where the SiC gas-phase components are deposited for growing the SiC single crystal.
The invention is based on the discovery that, in the method according to the prior art which has long been used, the SiC gas phase at the SiC seed crystal is not in thermodynamic equilibrium with the SiC seed crystal in particular during the reduction in pressure to the growth pressure. Consequently, uncontrolled accumulation of SiC gas-phase components at a crystallization surface of the SiC seed crystal may occur during the reduction in pressure. This may have an adverse effect on the seeding of the growing SiC single crystal on the SiC seed crystal that takes place at the beginning of the growth process.
In contrast, seeding of the SiC gas-phase components on a crystallization surface of the SiC seed crystal can be considerably improved by the heating-up of the crucible, of the SiC seed crystal and of the SiC stock to the growth temperature being carried out substantially already at the growth pressure which is subsequently also used during the growth phase. This eliminates the reduction in pressure, which is customary in the prior art, after the growth temperature has been reached and also the uncontrolled growth of the SiC gas-phase components on the crystallization surface of the SiC seed crystal that is caused by this drop in pressure. This is because slight and controlled seeding of the SiC gas-phase components on the crystallization surface of the SiC seed crystal takes place even during the heat-up phase, on account of slow heating-up at a heat-up rate of at most 20° C./min, preferably at a heat-up rate of at most 10° C./min, and a growth pressure in the crucible which has already been established during the heat-up phase. In the present context, the term growth pressure is always under
Kuhn Harald
Stein Rene
Voelkl Johannes
Anderson Matthew
Greenberg Laurence A.
Locher Ralph E.
Norton Nadine G.
Stemer Werner H.
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