Growth of Atomically-flat Si/SiGe Heterostructures by Ultra-High-Vacuum Chemical Vapor Deposition

Abstract

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 the work in this thesis.

We successfully built an 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. By optimizing the Si regrowth interface preparation method, we are able to keep the contamination density at the regrowth interface below 3$\times$10$^{13}$ cm$^{-2}$. With a base pressure less than 5$\times$10$^{-9}$ torr, the O and C contamination inside the Si and SiGe layers are both below 5$\times$10$^{17}$ cm$^{-3}$ at growth temperatures of 575℃, which is 20 times better than layers grown by the old RT-CVD system.

We then focused on the morphology study of SiGe layers grown on relaxed SiGe buffer. Three types of SiGe roughening mechanisms were identified 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 flat relaxed Si$_{0.7}$Ge$_{0.3}$ layer grown on a polished graded relaxed SiGe buffer, flatter than previous work for a relaxed Si$_{0.7}$Ge$_{0.3}$ 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.