Material uniformity improvements in a Gen II molecular beam epitaxy system Available to Purchase

It has been demonstrated how material uniformity can be improved in a Gen II molecular beam epitaxy system by increasing the incidence angle between the Group III cells and the substrate. The theoretical basis for this behavior has been verified experimentally by studying reflectivity maps from wafers grown without continuous azimuthal rotation. The incidence angle is most easily increased by tilting the substrate holder upward in the machine. For single-heater evaporation cells with conventional 60 cc crucibles (7° taper) the practical substrate tilt limit is approximately 13°. At this point the radius within which the center-normalized uniformity is larger than 99% is about 29 mm for cells in the lowest position. Uniformity dramatically worsens beyond 30 mm due to shadowing. By examining the geometry of the growth chamber it was found that this angle is the initial point where the beam cones from the lowest sources no longer cover the entire wafer surface. The shadowing can be reduced by using crucibles with a larger taper. Using a crucible with a 8.5° taper and a substrate tilt angle of 13° we were able to change the uniformity distribution from having a negative to a positive curvature. The maximum deviation from the center value under these conditions was about 0.5%. The switch of sign of the curvature demonstrates that the tilt angle can be further optimized and near perfect uniformity be obtained.