Mitsubishi uses Laytec tools for crack-free GaN-on-Si HEMTs
Mitsubishi Electric Corporation has recently reported on the growth of crack-free low-bowing GaN-on-Si HEMTs. To improve the breakdown voltage and power-added efficiency, Atsushi Era and his team grew the GaN buffer layer doped with Fe and used LayTec's EpiCurve TT in-situ metrology tools to monitor surface roughness, growth rate and wafer bowing.
The image (above left) shows reflectance at 950nm and curvature measurements during the growth of wafers A and B. The reflectance of A has a clear slump during the GaN:Fe growth, which indicates a rough surface of the GaN:Fe layer. The compressive stress during the GaN growth of A is obviously insufficient to compensate the tensile stress dur- ing cooling down. The result is cracking over the whole area of Wafer A.
To suppress 3D island growth in the GaN:Fe, Wafer B is grown with a 100 nm thick undoped GaN interlayer (u- GaN IL) prior to GaN:Fe growth). The reflectance of B shows no slump, which indicates that the GaN:Fe layer grows nicely in the 2D mode. The compressive stress is well balanced, so that Wafer B is nearly flat after cooling-down. Its smooth, crack-free surface is confirmed by atomic force microscopy (AFM). Furthermore, the electron transfer characteristics of a device fabricated on Wafer B shows an ideal pinch-off behavior.
'Growth of crack-free GaN on Si HEMTs with Fe-doped GaN using un-doped GaN interlayer', by A. Era et al; ICSCRM proceedings (2015)
