Fermi Level Tuning by Low Temperature Atomic Layer Deposition/Epitaxy (ALD/ALE) of Ternary Chalcogenide Layers

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Atomic Layer Deposition(A LD) is the synthesis technique of choice for the deposition of telluride based ternary chalcogenide layers. Three chalcogenide systems,( Sb1-xBix)2Te3, (Ge Te)1-x(Sb2Te3)x and (GeTe)1-x(Bi2Te3)x) will be investigated for which the composition factor x will be optimized in order to level the Fermi energy in the Dirac cone, thus achieving a low carrier concentration and a zero Seebeck coefficient. For the latter physical quantity a Hall-Seebeck measurement platform with the additional capability for tuning the Fermi energy level by applying electric field will be utilized. The ternary layers will be synthesized by ALD processes on suitable amorphous and single-crystalline substrates at rather low temperatures (RT to 150'C). We will study the substrate influence (amorphous silica versus single-crystalline substrates like e .g. GaAs and BaF2) on the layer morphology band structure and topologic transport properties. For the suppression of antisite defects, the annealing under tellurium vapor is integrated in the synthesis procedure, although the ideal stoichiometric composition is achieved by the low-T A LD processes as well as the use of suitable (reactive) metal organic precursors. For achieving ideal ALD processes, the development and testing of specific volatile but thermally stable (storable) metal-organic precursors is one essential project goal, which will be addressed by the Schulz group from Essen for binary compounds such as Bi2Te3, Sb2Te3 and GeTe. Silyl- substituted telluranes of the general type (R3Si)2Te (R = Me, Et, i-Pr, Ph, …) are synthesized, which readily react with either antimony and bismuth alkoxides of the type M(OR')x and amides M(NR'2)x, (M = Sb, Bi; x = 3; M = Ge, x = 2; R' = Me, E t , n-Pr, n-Bu, i-Pr , i-Bu, t-Bu) with subsequent elimination of silylamides R3SiNR'2 or silylethers R3SiOR' due to the formation of thermodynamically stable Si-N or Si-O bonds. Variation of the organic substituents R and R' allows the fine-tuning of the thermal stability, hence the suitability for decomposition-free storage( R, R' = Me, Et, n-Pr, i-Pr). Moreover, the vapor and sublimation pressure (volatility) as well as reactivity of the compound significantly depends on the steric size of the organic group (kinetic stabilization). Based on the optimized ALD processes for binary semi-conductors the Nielsch group from Hamburg will optimize the topological properties of ternary systems by alternating applications of binary processes and carry out all major physical characterization (mentioned above).