Technique for growing silicon carbide monocrystals

Details Technique for growing silicon carbide monocrystals Technique for growing silicon carbide monocrystals

1999-07-20

2001-07-17

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

With decomposition of a precursor

C117S106000, C117S107000, C117S951000, C423S345000

Reexamination Certificate

active

06261363

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to generation of monocrystalline materials, in particular, silicon carbide, and it can be successfully used in the production of semiconductor devices based on silicon carbide. Silicon carbide (SiC) is a wide-band-gap semiconductor material suitable for the development of light sources, power diodes, field-effect transistors and photodiodes showing a high stability and capable of being operated at increased temperatures. The parameters of these devices are largely controlled by the purity of the material and its structural characteristics.
BACKGROUND OF THE INVENTION
The methods most commonly used in producing SiC single crystals are sublimation techniques based on the Lely method, which are realized by vapour-phase crystallization, as a result of evaporation of solid silicon carbide (U.S. Pat. No. 2,854,364; U.S. Pat. No. 4,866,005). As shown in (S. Yu. Karpov, Yu. N. Makarov, M. S. Ramm, R. A. Talalaev “Excess Phase Formation during Sublimation Growth of Silicon Carbide”. Presented at the 6th International Conference on Silicon Carbide, September, 1995, pp. 73-74, Kyoto, Japan), the SiC monocrystal growth out of vapour, without forming any secondary-phase inclusions, is only realized if the external silicon (Si) flux onto the growing surface exceeds the carbon (C) flux.
The required excess silicon flux is dependent on the temperature of the growing surface and determined, in the case of sublimation technique, by composition of the vapour adjacent said surface of growth. The conditions of the single-phase (i.e. without secondary phase inclusions) monocrystalline SiC growth are met with the vapour composition approximating the SiC—Si system (Spravochnyik po elektrotekhnicheskim materialam, Ed. by Yu. V. Koritsky, V. V. Pasynkov, B. M. Tareyev, Energoatomizdat/Leningrad/, 1988, p.449). It has been shown by experiment that non-observance of this condition results in a sharply increased density of dislocations, channels and micropipes in the growing crystal. Since it is the silicon molecules that have the maximum concentration in the gaseous phase, any drift of the substance from the growth zone will result in the vapour phase within the growth zone being depleted of silicon, and hence enriched with carbon, thus ultimately leading to graphitization of the source, degradation of the crystal quality, discontinuation of the growth process. The shift of the vapour composition, in the growth zone, towards the vapour phase corresponding to the SiC—Si system substantially improves the growth and contributes to a more perfect structure of the SiC single crystal grown. This is due to the fact that such system (SiC—Si) prevents the secondary phase of graphite from being generated, avoiding graphitization of the source and the growing surface of the seed crystal. It is also known, however, that excessive silicone in the growth zone may results both in the formation of defects on the growing surface of the SiC crystal, due to the precipitation thereon of excess silicon drops, and in generation of polytypes differing from the seed polytype.
In the patents U.S. Pat. No. 2,854,364; DE 3,230,727, it is proposed that SiC powder of a predetermined granularity, with a mass of more than three times the mass of the single crystal to be grown, be placed in a lump, in the growth zone, in order to maintain the required vapour phase composition for a certain time, the powder serving as the source of silicon carbide vapours. In this case, a relatively constant composition of the vapour phase within the growth zone is provided, because the drift of SiC vapours into the space outside the growth zone, which might have resulted in a continuous enrichment of the vapour phase with carbon atoms, is balanced out by an abundant generation of SiC vapours, since the vapour phase is substantially more enriched with silicon atoms than with carbon atoms. This enables a relative stability of the vapour phase composition in the vicinity of the growing surface of the SiC single crystal to be maintained for a certain period of time. The duration of the stable growth process, however, is limited, and so the SiC—C system comes to be realized, with time, in most of the volume of the growth zone, which has been previously shown as being detrimental to the growth process. The large amounts of the SiC powder consumed leads to an increased cost of the grown single.
In U.S. Pat. No. 4,866,005 a technique has been proposed which allows an essentially unlimited duration of the growth process by continuously feeding specified small portions of SiC powder of a given granularity into a predetermined temperature zone of the growth chamber. However, in this case, again, the mass of the material consumed in the SiC vapour source will be much in excess of that of the single crystal grown, due to the SiC vapours being removed to the space outside the growth zone, for the growth chamber communicates with the environment, making this method, like the methods of U.S. Pat. No. 2,854,364; DE 3,230,727, uneconomical. The loss of the SiC source material is also caused by the growth zone geometry, and particularly, by a relatively large separation (about 10 cm) of the evaporating surface of the SiC vapour source and the seed growing surface, which by far exceeds the Si, Si2C, SiC2 molecule track length at the working pressure in the growth zone.
The method most closely approximating that herein proposed is the sublimation technique of growing SiC single crystals, as disclosed in the patent U.S. Pat. No. 4,147,572 (the so-called “sandwich-method”). According to this disclosure, the evaporating surface of the SiC source and the growing surface of the SiC seed crystal are arranged in parallel, at the distance not exceeding 0.2 of the maximum linear dimension of the source, to form the growth zone. The single crystals are grown in a graphite crucible in an inert gas atmosphere, at temperatures of 1600 to 2000° C., with an axial thermal gradient of 5 to 200° C./cm, in the direction from the seed crystal to the source. With small gaps between the SiC source and the seed crystal, the loss of SiC vapours from the growth zone can be substantially reduced and their flow directed along the straight path from the source to the growing surface of the seed crystal. The growth zone is here screened from external impurity sources, resulting in a reduced concentration of impurities in the single crystal grown. In addition, the small gap between the evaporating surface of the source and the growing surface of the seed, in comparison to the size of the SiC source, allows a uniform temperature along their surfaces to be maintained and the temperature difference between them controlled.
This method suffers from a number of drawbacks. One of them is the small volume of the single crystals grown (less than 1 mm thick) due to a sharp drop in the growth rate, as the crystallization time increases, as a result of the silicon at the edge of the growth zone being volatilized beyond its bounds and consequently, excessive carbon released from the evaporating surface of the SiC source and the growing surface of the grown crystal, slowing down the growth process. In this case, the single crystals obtained by the above technique show defects such as secondary-phase inclusions (predominantly, graphite), micropipes with a density of more than 100 per cm
2
and dislocations numbering at least 10
−4
per cm
2
. They are also relatively inferior, as regards the concentration of residual impurities such as boron, oxigen, etc.
Known in the art is a favourable effect of tantalum (Ta) on the growth of monocrystalline SiC. In particular, theoretical investigations of the sublimation growth of monocrystalline SiC with a “sandwich-cell” in a tantalum container have shown that the vapour medium produced in the growth zone is close to the SiC—Si system with the partial silicon vapour pressure slightly exceeding the pressure in the SiC—Si system (D. Hofmann, S. Yu. Karpov, Yu. N. Makarov, E. N. Mokhov, M. S. Ramm, A. D. Roencov, Yu. A. Vodakov,

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