  Electron spins in silicon have long coherence times and are a promising qubit
platform. However, electric field noise in semiconductors poses a challenge for
most single- and multi-qubit operations in quantum-dot spin qubits. Here, we
investigate the dependence of low-frequency charge noise spectra on temperature
and aluminum-oxide gate dielectric thickness in Si/SiGe quantum dots with
overlapping gates. We find that charge noise increases with aluminum oxide
thickness. We also find strong dot-to-dot variations in the temperature
dependence of the noise magnitude and spectrum. These findings suggest that
each quantum dot experiences noise caused by a distinct ensemble of two-level
systems, each of which has a non-uniform distribution of thermal activation
energies. Taken together, our results suggest that charge noise in Si/SiGe
quantum dots originates at least in part from a non-uniform distribution of
two-level systems near the surface of the semiconductor.
