Epitaxial growth and improved electronic properties of (Bi_1-xSb_x)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 (Bi_1-xSb_x)₂Te₃ 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 (Bi_1-xSb_x)₂Te₃ 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 (Bi_1-xSb_x)₂Te₃ 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 (Bi_1-xSb_x)₂Te₃ 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 (Bi_1-xSb_x)₂Te₃ films is achieved upon annealing at 580 K for 1 h. The highest power factor in p-type Bi_0.6Sb_1.4Te₃ and Sb₂Te₃ films is obtained at around 150 K and it increases from 0.2 mWm^-1 K^-2 (Bi_0.6Sb_1.4Te₃) 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 (Bi_1-xSb_x)₂Te₃ films grown by MBE assures excellent carrier mobility and is a contributing factor to their outstanding power factors.