Why choose liquid phase epitaxy method?

The unique properties of SiC make it challenging to grow single crystals. The conventional growth methods used in the semiconductor industry, such as the straight pulling method and the descending crucible method, cannot be applied due to the absence of a Si:C=1:1 liquid phase at atmospheric pressure. The growth process requires a pressure greater than 105 atm and a temperature higher than 3200°C to achieve a stoichiometric ratio of Si:C=1:1 in the solution, as per theoretical calculations.

Compared with the PVT method, the liquid phase method for growing SiC has the following advantages:

1. low dislocation density. the problem of dislocations in SiC substrates has been the key to constrain the performance of SiC devices. Penetrating dislocations and microtubules in the substrate are transferred to the epitaxial growth, increasing the leakage current of the device and reducing the blocking voltage and breakdown electric field. On the one hand, the liquid-phase growth method can significantly reduce the growth temperature, reduce the dislocations caused by thermal stress during cooling down from the high-temperature state, and effectively inhibit the generation of dislocations during the growth process. On the other hand, the liquid-phase growth process can realize the conversion between different dislocations, the Threading Screw Dislocation (TSD) or Threading Edge Dislocation (TED) is transformed into stacking fault (SF) during the growth process, changing the propagation direction, and finally discharged into the layer fault. The propagation direction is changed and finally discharged to the outside of the crystal, realizing the decrease of dislocation density in the growing crystal. Thus, high-quality SiC crystals with no microtubules and low dislocation density can be obtained to improve the performance of SiC-based devices.

2. It is easy to realize larger-size substrate. PVT method, due to the transverse temperature is difficult to control, at the same time, the gas phase state in the cross-section is difficult to form a stable temperature distribution, the larger the diameter, the longer the molding time, the more difficult to control, the cost as well as the time consumption is large. The liquid-phase method allows for relatively simple diameter expansion through the shoulder release technique, which helps obtain larger substrates quickly.

3. P-type crystals can be prepared. Liquid-phase method due to the high growth pressure, the temperature is relatively low, and under the conditions of Al is not easy to volatilize and lose, liquid-phase method using flux solution with the addition of Al can be easier to obtain a high carrier concentration of P-type SiC crystals. PVT method is high in temperature, the P-type parameter is easy to volatilize.

Similarly, the liquid-phase method also faces some difficult problems, such as sublimation of flux at high temperatures, control of impurity concentration in the growing crystal, flux wrapping, floating crystal formation, residual metal ions in the co-solvent, and the ratio of C: Si has to be strictly controlled at 1:1, and other difficulties.