HVPE Growth Approach
Cree Materials synthesizes GaN substrates using a hydride vapor phase epitaxy (HVPE)-based method. During the HVPE process, HCl reacts with molten Ga to form GaCl, which in turn reacts with NH3 to form GaN. The large growth rate enables the growth of self-supporting wafer thicknesses in a convenient time period.

HVPE Boule Approach
Cree Materials utilizes a seeded boule growth approach to improve crystalline quality and electrical purity. The HVPE growth of millimeters of GaN enables substrate slicing to make several substrates from one growth run. Dislocation density has been found to be reduced with boule length.

CMP and Wafer Finish
Cree Materials employs a patented-CMP process to remove damage introduced during previous wafer fabrication steps. The resulting surfaces have an RMS surface roughness less than 5 Â with a clearly visible atomic step structure.

Thermal Conductivity

The thermal conductivity of Cree Materials' substrates has been measured as a function of temperature by the laser flash method. At room temperature, the thermal conductivity of Cree Materials' wafers measured as high as 220 W/m-K.

Semi-Insulating GaN Wafers
Cree is developing semi-insulating GaN wafers by introducing iron to compensate for residual donors.

The resistivity of GaN:Fe is greater than 10^8 ohm-cm at room temperature, as shown in the COREMA resitivity map.  

Semi-insulating substrates are currently available in engineering quantities to select customers.