Optimizing the Carrier Density and Thermoelectric Properties of Sb2Te3 Films by Using the Growth Temperature

Designer Topological Insulator with Enhanced Gap and Suppressed Bulk Conduction in Bi2Se3/Sb2Te3 Ultrashort-Period Superlattices

“…In addition, the HR-TEM image clearly displays the quintuple layer structure of the individual layers, as well as the high crystallinity of the structure. Although atomically well ordered, the interfaces show some interface steps, or interface roughness, which appears more pronounced on the surfaces of the Sb2Te3 layers, consistent with observations by ourselves and others, that MBE grown Sb2Te3 has a higher degree of roughness than Bi2Se3 11,29 .…”

“…More than an order of magnitude reduction in carrier density was obtained between the samples with larger periods (~12 nm) and those with smaller periods (~5 nm). The low carrier densities of our smaller period TI/TI SLs are comparable to the best values reported for the constituent TI materials grown by MBE directly on sapphire substrates 11,12,31,32 . We believe that these values can be significantly improved, as these results were achieved only by our first efforts.…”

“…The two data points for samples with a period above 14 nm do not to follow this trend, and have been excluded from our discussion since, at sufficiently large layer thickness, a superlattice behavior may no longer apply. The low carrier densities of our smaller-period TI/TI SLs are comparable to the best values reported for the constituent TI materials grown by MBE directly on sapphire substrates. ,,, We believe that these values can be significantly improved, as these results were achieved only by our first efforts. We propose that mastering the properties of TI/TI SLs provides a promising and yet-to-be-explored platform for building truly insulating topological materials.…”

“…Despite their attractive properties, the still relatively small bandgap of these materials of a few hundred meV, and the ease of formation of electrically active defects 9 result in high bulk conductivities, masking the features of their exotic surface states. In particular, when grown by molecular beam epitaxy (MBE), Bi2Se3 has n-type conductivity in the bulk 10 and Sb2Te3 has ptype conductivity 11 . Many attempts to reduce the bulk carrier density in TIs have been previously reported, including modification of the growth conditions and the substrates used 11,12 , impurity compensation doping [13][14][15] , and the growth of mixed alloys 16 .…”

“…In particular, when grown by molecular beam epitaxy (MBE), bulk Bi 2 Se 3 is n -type and Sb 2 Te 3 is p -type . Many attempts to reduce the bulk carrier density in TIs have been reported, including modification of the growth conditions and the substrates used, , impurity compensation doping, − and the growth of mixed alloys . Despite observed reduction of bulk conductivity through these methods, it is evident that significant improvement toward truly insulating bulk TIs requires the invention of fundamentally new protocols.…”