Transport properties of (Bi,Sb)₂Te₃ topological insulator crystals with lateral p-n junction

High-quality low-bulk-carrier-concentration 3D topological insulator crystals and films are required for the majority of their potential applications. Creating a p-n transition using composition grading is one of the ways to obtain compensated regions in the bulk of 3D topological insulator crystals. Eventual formation of a p-n junction in 3D topological insulator surface states is expected to enhance the surface-transport-related spin filtering and charge-to-spin conversion. Here we report a detailed study of the transport and surface electronic structure of composition-graded Bi2Te3 and Bi1.34Sb0.66Te3 single crystals with built-in lateral p-n transition. The defect compensation naturally achieved at the p-n interface results in a strong reduction of the bulk carrier concentrations in both crystals. In the Bi2Te3 crystal a high-mobility n-type conductivity region is formed with electron Hall mobility of 70 000 cm(2) V-1 s(-1) and Hall concentration of 2 x 10(18) cm(-3) at 4.2 K. In the Bi1.34Sb0.66Te3 crystal the region of intrinsic conductivity with the lowest observed hole Hall concentration of 6 x 10(17) cm(-3) and hole Hall mobility of 10 000 cm(2) V-1 s(-1) is formed in the vicinity of the p-n junction. The downward band bending was observed on the surface of the p-type conductivity region of the Bi2Te3 and Bi1.34Sb0.66Te3, providing an almost barrierless topological surface state electron channel with no topological p-n junction formed. The composition grading can be used as a reliable method of obtaining high-quality single crystals with relatively large areas of low bulk carrier concentrations and enhanced charge carrier mobility, which can be used in further nanoscale topological insulator device fabrication.