  We have studied the magnetotransport properties of the metallic, p-type
Sb2Te2Se which is a topological insulator. Magnetoresistance shows Shubnikov de
Haas oscillations in fields above B=15 T. The maxima/minima positions of
oscillations measured at different tilt angles with respect to the B direction
align with the normal component of field Bcosine, implying the existence of a
2D Fermi surface in Sb2Te2Se. The value of the Berry phase determined from a
Landau level fan diagram is very close to 0.5, further suggesting that the
oscillations result from topological surface states. From Lifshitz-Kosevich
analyses, the position of the Fermi level is found to be EF =250 meV, above the
Dirac point. This value of EF is almost 3 times as large as that in our
previous study on the Bi2Se2:1Te0:9 topological insulator; however, it still
touches the tip of the bulk valence band. This explains the metallic behavior
and hole-like bulk charge carriers in the Sb2Te2Se compound.
