Growth of HgTe topological insulator structures, Weyl and Dirac semimetals
Project A04 is the central project for the growth of HgTe heterostructures within the SFB 1170. A continuously improving growth technique allows us to provide a large variety of samples with a high crystalline quality on demand. Apart from 2D and 3D topological insulators (TI) we are now able to extend the material class to Weyl-, Dirac-, and Kane-semimetals. Apart from the sample supply project A04 will focus on improvements of the II-VI on III-V expitaxy, band gap engineering, and as a new research aspect on the growth of nanowires with HgTe TI shell.
The growth of HgTe (II-VI) on doped GaAs (III-V) substrates has several advantages. First, it allows for back-gating and second it increases the number of samples per wafer significantly. One important aspect is to optimize the growth parameters in order to improve the carrier mobility and to minimize the surface roughness to allow for local gate applications and interfacing with superconducting contacts.
In an attempt to use substrates with different lattice constants for strain engineering we developed a growth sequence that is based on CdTe-Cd0.5Zn0.5 Te superlattices grown on GaAs wafers. Depending on the superlattice period lateral strain levels between +1.4 % (compressive) and -0.3 % (tensile) are achievable. The application of lateral strain affects the band structure for 2D and 3D HgTe layers differently. While tensile strain is required to turn bulk HgTe into a TI it closes the topological gap in a 2D quantum well system. Complementary, compressive strain increases the band gap in 2D layers while it turns a 3D TI into a Weyl semimetal.
Another way to access the topological phase transition between a normal and a TI state is to alloy HgTe with Cd. With a new high precision temperature control system, acquired with the financial sup- port of the SFB 1170, we were able to grow Hg1−xCdx Te layers with a high accuracy in composition (x) and a high crystalline quality by monitoring the substrate temperature during MBE growth. By varying the composition of the ternary compound it is possible to change the band gap from a nor- mal semiconductor (CdTe) into the regime of a semimetal with topological surface states (unstrained HgTe). At the transition point at x ≈ 0.14 (zero gap) a new type of Dirac system is established with is referred to as Kane-semimetal. Samples are now available for characterization.
A new challenge for project A04 is the growth of CdTe-HgTe core-shell nanowires which has been started successfully within the DFG priority program SPP 1666. Core-shell TI nanowires are interesting due to the quasi-1D character which imposes an additional quantization. The induced gap is controllable gap by a magnetic fields parallel to the wire axis. From the grow side we expect to optimize the shape, length and strain by pre-positioning the seed growth of the core wires. We are in- terested in exploring the transport properties especially in connection with superconducting contacts. The successful wire growth will expand the materials available for the research of topological states within the SFB 1170.