Development Of Manufacturing Methods For Growing Large Beta Silicon Carbide Single Crystals

Beta-SiC is a promising, wide bandgap material for high power electronic devices capable of operation at high temperatures. Its high saturation velocity, high breakdown electric field, and high thermal conductivity point to superior performance for high frequency applications. The successful fabrication of Beta-SiC devices requires high quality films to be epitaxially grown on lattice-matched substrate materials. Single crystals of Beta-SiC offer the optimum substrate material for lattice matching. The major problem to be overcome in the growth of large single crystals of Beta-SiC is polytype alpha-SiC formation. Cubic Beta-SiC crystallizes only below 2000 deg C. Above this temperature, SiC undergoes a phase transformation from the Beta-to the alpha-phase. In Phase I, we investigated two crystal growth techniques: sublimation and gas-vapor transport. We were able to grow small 3C-SiC crystals by both methods.

From the dissociation curve (P vs. T), an equation for the rate of raising the pressure P of the binary vapor for obtaining the required linear growth rate of the crystal of c centimeters per hour is derived. It is shown that the rate is nearly proportional to P. Modifications of the furnace since the last report (Mat. Res. Bull. 4, S85, 1969) are described. Justification for the use of helium as the inert ambient gas is given. Two techniques are used: (1) growing with constant temperature of the crucible point and (2) growing with constant pressure of the sublimation bottle. To date, only polycrystalline boules consisting of large grains have been obtained. It is believed, however, that with certain technological improvements the methods that are developed here will ultimately yield single crystal boules. As a by-product, small cubic crystals, about one mm in the largest dimension, with good quality faces (cube and octahedron) have been obtained.

Changes in crystal size and morphology were obtained by modifying the process for growing crystals of betasilicon carbide from solution in carbon-saturated melts. Well developed laths and plates were grown under conditions favoring dendritic growth mechanisms (low thermal gradient with moderate stirring). Polyhedral crystals grew under high thermal gradient conditions when high velocity stirring was employed. Three-dimensionaltype growth was also produced from a 73 wt percent ironsilicon alloy under conditions that normally produce lath-type growth. Uncorrected electron mobilities of 700 to 1000 sq cm/v-sec were measured at room temperature, and a few preliminary Hall measurements were made over the temperature range from 77 to 300 K. (Author).

Equipment and growth procedures for growing beta-silicon carbide epitaxially on beta-silicon carbide substrates from methyltrichlorosilane in a carrier gas of hydrogen is described. Hall and resistivity measurements and electron spin resonance measurements are discussed. The results show that the quality of the epitaxially grown material is comparable with that of the best solution grown crystals. Processing steps such as oxidation, masking, and etching have been performed and simple electroluminescent diodes have been fabricated. (Author).

A high-pressure, high-temperature furnace system is described for crystal growth experiments using crucibles up to 13 cm in diameter and 26 cm high. The vertical temperature gradient is electronically controlled during growth such that the ends of the crucible can be maintained at temperatures above or below the crucible center. Temperatures up to 2800C can be maintained at pressures up to 50 atmospheres. A vacuum capability up to .000001 torr at 1800C has been incorporated into the system. Single crystals of alpha silicon carbide grown in this system at 2600C are described to illustrate its use. (Author).