  We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged
${T_1}$ energy relaxation times of up to ${270\,\mu s}$, corresponding to Q = 5
million, and a highest observed value of ${501\,\mu s}$. We use materials
analysis techniques and numerical simulations to investigate the dominant
sources of energy loss, and devise and demonstrate a strategy towards
mitigating them. The mitigation of loss is achieved by reducing the presence of
oxide, a known host of defects, near the substrate-metal interface, by growing
aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide
resonators shows that the improvement is owing to a reduction of dielectric
loss due to two-level system defects. We perform time-of-flight secondary ion
mass spectrometry and observe a reduced presence of oxygen at the
substrate-metal interface for the thicker films. The correlation between the
enhanced performance and the film thickness is due to the tendency of aluminum
to grow in columnar structures of parallel grain boundaries, where the size of
the grain depends on the film thickness: transmission electron microscopy
imaging shows that the thicker film has larger grains and consequently fewer
grain boundaries containing oxide near this interface. These conclusions are
supported by numerical simulations of the different loss contributions in the
device.
