SiGe/Si(111)/SiGe heterostructure for Si spin qubits with electrons confined in L valley of conduction band

TL;DR

This paper investigates SiGe/Si(111)/SiGe heterostructures under biaxial tensile strain to enable electrons in the L valley for silicon spin qubits, analyzing band structure shifts, strain effects, and structural feasibility.

Contribution

It provides a detailed theoretical analysis of strain-induced band structure modifications and the feasibility of fabricating SiGe/Si(111)/SiGe heterostructures for spin qubit applications.

Findings

Biaxial tensile strain shifts conduction band minimum to L valley.

Calculated critical thickness for strain relaxation in the heterostructure.

Identified strain and Ge concentration conditions for L valley qubit implementation.

Abstract

In Si(111) crystals, a strong biaxial tensile strain applied within the (111) plane is considered to shift the lowest energy point of the conduction band from the $\Delta$ valley to the L valley. Electrons confined in this L valley experience a splitting of their quadruply degenerate energy levels into an undegenerate single-level ground state (L1) and a triply degenerate excited state (L3). The energy of the single-level ground state is sufficiently low relative to the energies of the L3 valley and the $\Delta$ valley, making it optimal as a two-level system for a qubit. Using deformation potential theory and incorporating quantum effects from electron confinement in the SiGe/Si(111)/SiGe structure, we determine the value of the biaxial tensile strain causing the shift of the conduction band energy minimum from the $\Delta$ valley to the L valley, along with the corresponding Ge concentration. We also calculate the critical thickness for the plastic relaxation of the Si quantum well under this large biaxial tensile strain and examine the feasibility of realizing it as a SiGe/Si(111)/SiGe heterostructure.