Epitaxial growth and improved electronic properties of (Bi1-xSbx)2Te3 thin films grown on sapphire (0001) substrates: The influence of Sb content and the annealing

In this research, we report on the epitaxial growth of basal plane-oriented (Bi1-xSbx)(2)Te-3 films (0 <= x <= 1) on sapphire (0001) substrates through Molecular Beam Epitaxy (MBE) and demonstrate the influence of composition, crystal orientation and post-annealing process on their electronic properties. The as-grown (Bi1-xSbx)(2)Te-3 films change gradually from a strong n-type to a strong p-type conduction when the Sb content increases from 0 to 1, which is attributed to the charge carrier compensation between the n-type Te-Bi and p-type Sb-Te antisite defects. The crossover between the n- and p-type conduction is found for x between 0.6 and 0.7. We also find that post-annealing (at 580 K) is beneficial for the electronic properties of the p-type (Bi1-xSbx)(2)Te-3 films: they attain improved carrier mobility and significantly increased hole density. However, annealing plays a negative role in the electronic properties of the n-type structures leading to an enhanced resistivity as well as a reduced Seebeck coefficient. The most plausible explanation for such annealing effects is an introduction of p-type defects in both the n-type and p-type (Bi1-xSbx)(2)Te-3 films. The as-grown Bi2Te3 film possesses the largest thermoelectric power factor among all n-type films, reaching 4.1 and 2.5 mWm(-1) K-2 at 122 and 300 K respectively, due to the high carrier mobility and proper carrier doping. In contrast, a remarkably improved power factor in p-type (Bi1-xSbx)(2)Te-3 films is achieved upon annealing at 580 K for 1 h. The highest power factor in p-type Bi0.6Sb1.4Te3 and Sb2Te3 films is obtained at around 150 K and it increases from 0.2 mWm(-1) K-2 (Bi0.6Sb1.4Te3) and 1.8 mWm(-1) K-2 (Sb2Te3) for the as-grown films to 3.5 mWm(-1) K-2, respectively 3.7 mWm(-1) K-2 for the annealed films. The highly crystalline nature of (Bi1-xSbx)(2)Te-3 films grown by MBE assures excellent carrier mobility and is a contributing factor to their outstanding power factors. (C) 2015 Elsevier B.V. All rights reserved.