The spin of electrons in silicon quantum dots has been a promising candidate for qubits for quantum computing applications in recent years, demonstrating long coherence time due to its weak spin-orbit coupling and the existence of stable zero nuclear spin isotopes. However, a fundamental challenge is the degeneracy of the conduction band minima, which is a decoherence source. The realization of atomically flat Si/SiGe heterostructures which can potentially solve the small valley splitting issue in Si/SiGe quantum dots applications motivated this work.
We successfully built a Ultrahigh Vacuum Chemical Vapor Deposition (UHV-CVD) system to overcome the limitations of a previous Rapid Thermal CVD system to grow Si/SiGe heterostructures. The within-wafer uniformity is better than 3% and the wafer-to-wafer uniformity is better than 5%, after improving the heating configuration. With a base pressure less than 5×10-9 torr, the O and C contamination inside SiGe layers are both 20 times better than layers grown by the old RT-CVD system at growth temperatures of 575℃.
We then focused on the morphology study of SiGe layers grown on relaxed SiGe buffer. Three types of SiGe roughening mechanisms were identied and investigated: low-temperature roughening, high-temperature roughening, and initial interface effects. By introducing a thin Si buffer layer on top of the polished SiGe relaxed buffer, we demonstrated a nearly-atomically at relaxed Si0.7Ge0.3 layer grown on a polished graded relaxed SiGe buffer, flatter than previous work for a relaxed Si0.7Ge0.3 layer ready for subsequent epitaxy by roughly a factor of four. We attributed the smoothing effect of the silicon to high ad-atom surface mobility during silicon growth. We further demonstrated that on the scale of silicon quantum dots (~100 nm), the RMS roughness was only 0.08 nm, about half of an atomic step height. This result may enable the subsequent growth of a tensile-Si channel with a large valley splitting.
Zoom link: https://princeton.zoom.us/j/2577056981