Device and method to producing single crystals by vapour deposition

Abstract

A method and a device to grow from the vapor phase, a single crystal of either SiC, a group III-nitride, or alloys thereof, at a growth rate and for a period of time sufficient to produce a crystal of preferably several centimeters length. The diameter of the growing crystal may be controlled. To prevent the formation of undesirable polycrystalline deposits on surfaces in the downstream vicinity of the single crystal growth area, the local supersaturation of at least one component of the material grown is lowered by introducing a separate gas flow comprising at least one halogen element or a combination of said halogen and hydrogen species.

Description

FIELD OF THE INVENTION[0001]The present invention describes a device and a method to grow single crystals by high temperature deposition from a vapor phase. In particular the device can be used to produce large and high quality bulk crystals of a) silicon carbide, b) a group III-nitride, for example GaN or AlN, or c) an alloy of SiC and a group III-nitride.BACKGROUND AND PRIOR ART[0002]Wide band-gap semiconducting crystals such as silicon carbide (SiC), group III-nitrides e.g. gallium nitride (GaN) and aluminum nitride (AlN), offer several attractive electrical and physical properties for fast switching power devices and optoelectronic devices. These wide band gap semiconductors and their alloys also differentiate themselves from other important semiconductors, such as silicon and gallium arsenide, by the fact that they cannot at present be directly grown from a melt or a liquid solution under practically and economically interesting conditions. Instead, ingots of SiC, GaN or AlN ar...

AI Technical Summary

Benefits of technology

[0018]In particular, it is an object of preferred embodiments of the invention to slow down or eliminate the formation of polycrystalline and other solid deposits downstream of the single crystal crystallization area to avoid a partial or complete obstruction of a susceptor exhaust path by a gas mixture fed to the crystallization area. A correlated purpose of preferred embodiments of the invention is to control the diameter of the growing single crystal and prevent growth of polycrystalline material around it, thereby preventing structural defects generation during either the high temperature growth phase or the subsequent cooling phase.

[0019]A further object of preferred embodiments of the invention is to decrease the concentration of unwanted metallic impurities in the growing single crystal by removing from the vapor phase active metallic elements released by parts heated downstream of the crystallization area.

Problems solved by technology

Despite these achievements, there are however some challenges and limits in the sublimation technique.

For example, as long as no continuous feeding mechanism can be devised, the initial mass of the feedstock limits the duration of a continuous crystal growth process and thus the crystal length.

One difficulty may, for example, be the need to control a changing sublimation rate and a drift of the sublimed species stoichiometry during growth.

Instabilities in the source material supply and drifts of the temperature distribution in the source feedstock, for example, cause drifts of the growth rate and of the incorporation of doping species.

If not properly controlled, such drifts tend to adversely affect the yield of the crystal growth process.

Further downstream, as the temperatures decreases and the supersaturation increases, the less dense polycrystalline deposits grow even faster, eventually obstructing the gases outlet path within 2 to 4 hours.

If the pressure differential is allowed to reach a few mbars, a rapid deterioration of the polytype and the structural quality of the single crystal occurs.

The thermal properties of the insulating material are then rapidly deteriorating due to reaction with silicon, which forces the growth to be interrupted.

The parasitic deposition of polycrystalline solid phases thus leads to a catastrophic runaway of the system, forcing to terminate the growth process before a crystal of a desired length is produced.

It has been found that this solution, as presented in or derived from these documents, does not solve the problem described above to an extent sufficient to grow SiC or other crystals of a length more than a few mm.

Experiments using an inert blanketing gas, such as helium or argon, showed that too rapid polycrystalline deposition still occurred on the downstream regions of the single crystal growth front.

Under typical process conditions leading to a single crystal growth rate of 0.5 to 1 mm / h, it is however observed that such a design only leads to a further downstream location of the uncontrollable polycrystalline SiC deposition.

This small improvement of the blocking time is not sufficient to continuously grow several cm long crystals.

As a decreasing temperature distribution is required in or next to the seed holder to promote the growth of the single SiC crystal, it is believed that such passageways will inevitably by subject to a catastrophic blocking by either polycrystalline SiC, pyrolytic graphite or polycrystalline Si deposition.

Despite a beneficial cleaning action of polycrystalline deposits induced by the rotation of the seed holder, such a mechanical cleaning induces stresses in either the rotation mechanism or the seed holder and the elements coming in contact with it.

This can lead to mechanical failure of any of the above mentioned parts.

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