  SiGe heteroepitaxial growth yields pristine host material for quantum dot
qubits, but residual interface disorder can lead to qubit-to-qubit variability
that might pose an obstacle to reliable SiGe-based quantum computing. We
demonstrate a technique to reconstruct 3D interfacial atomic structure spanning
multiqubit areas by combining data from two verifiably atomic-resolution
microscopy techniques. Utilizing scanning tunneling microscopy (STM) to track
molecular beam epitaxy (MBE) growth, we image surface atomic structure
following deposition of each heterostructure layer revealing nanosized SiGe
undulations, disordered strained-Si atomic steps, and nonconformal uncorrelated
roughness between interfaces. Since phenomena such as atomic intermixing during
subsequent overgrowth inevitably modify interfaces, we measure post-growth
structure via cross-sectional high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM). Features such as nanosized
roughness remain intact, but atomic step structure is indiscernible in $1.0\pm
0.4$~nm-wide intermixing at interfaces. Convolving STM and HAADF-STEM data
yields 3D structures capturing interface roughness and intermixing. We utilize
the structures in an atomistic multivalley effective mass theory to quantify
qubit spectral variability. The results indicate (1) appreciable valley
splitting (VS) variability of roughly $\pm$ $50\%$ owing to alloy disorder, and
(2) roughness-induced double-dot detuning bias energy variability of order
$1-10$ meV depending on well thickness. For measured intermixing, atomic steps
have negligible influence on VS, and uncorrelated roughness causes spatially
fluctuating energy biases in double-dot detunings potentially incorrectly
attributed to charge disorder.
