  A new record-high room temperature electron Hall mobility (${\mu}_{RT} =
194\space cm^{2}/V\space s$ at $n\sim 8\times 10^{15}\space cm^{-3}$) for
${\beta}$-Ga2O3 is demonstrated in the unintentionally doped thin film grown on
(010) semi-insulating substrate via metalorganic chemical vapor deposition
(MOCVD). A peak electron mobility of $\sim 9500\space cm^{2}/V\space s$ is
achieved at 45 K. Further investigation on the transport properties indicate
the existence of sheet charges near the epi-layer/substrate interface. Si is
identified as the primary contributor to the background carrier in both the
epi-layer and the interface, originated from both surface contamination as well
as growth environment. Pre-growth hydrofluoric acid cleaning of the substrate
lead to an obvious decrease of Si impurity both at interface and in epi-layer.
In addition, the effect of MOCVD growth condition, particularly the chamber
pressure, on the Si impurity incorporation is studied. A positive correlation
between the background charge concentration and the MOCVD growth pressure is
confirmed. It is noteworthy that in a ${\beta}$-Ga2O3 film with very low bulk
charge concentration, even a reduced sheet charge density can play an important
role in the charge transport properties.
