Cross-hatch strain effects on SiGe quantum dots for qubit variability estimation

TL;DR

This study investigates how residual cross-hatch strain in SiGe heterostructures affects interface quality and qubit properties, providing insights for optimizing growth conditions to improve qubit uniformity.

Contribution

It quantifies strain inhomogeneity effects on heterostructure interfaces and qubit variability, combining experimental measurements with modeling to suggest growth improvements.

Findings

Strain inhomogeneity can cause qubit detuning of 0.1 meV over 100 nm.

Interface roughness modestly reduces valley splitting from 77 to 70 μeV.

Thicker buffer layers and lower growth temperatures can mitigate roughening.

Abstract

SiGe heterostructures integrated with Si via virtual substrate (VS) growth are promising hosts for spin qubits. While VS growth targets plastic relaxation, residual cross-hatch strain inhomogeneity propagates into heterostructure overgrowth. To quantify strain inhomogeneity's influence on interface structure and qubit properties, we measure strained-silicon (s-Si)/Si$_{0.7}$Ge$_{0.3}$ heterostructures on 25 wafers processed via standard commercial chemical vapor deposition. Spatially-aligned images of strain (Raman microscopy) and interface structure (atomic force microscopy and cross-sectional scanning transmission electron microscopy) reveal strain-roughness interplay. A strain-driven surface diffusion model predicts the roughness and its temperature dependence. Measured strains suggest spurious double-dot qubit detunings of 0.1 meV over 100 nm distances may result. Modeling shows that interface roughness (atomic steps), when convolved with alloy disorder, only modestly reduces valley splitting (70$\pm$13 vs. 77$\pm$14 $\mu$eV on average). Our findings point to thicker VS buffer layers beneath heterostructures and lower-temperature growth (T $\le$ 700 $^{\circ}$C) to limit roughening.