Self-assembled Ge nanostructures


Silicon-germanium (SiGe) nanostructures have emerged as a promising material for the realization of spin qubits [1]. In the past several years intense research has been devoted in studying quantum dots (QDs) defined in SiGe two dimensional electron gases [2] and P donors in Si [3]. Spin relaxation times of a few seconds and coherence times of close to a second have been reported [4]. On the other hand there are much less studies which have focused on holes and in particular confined in SiGe. In 2012, spin relaxation times in Ge/Si nanowire QDs were reported to be in the ms range [5] and in 2014 the dephasing time was reported to be more than one order of magnitude larger than that of III-V materials [6]. All these studies point out the importance of group-IV elements in the field of spin qubits.
In our group we study holes confined in Ge self-assembled nanostructures. Ge self-assembled nanostructures are created by means of lattice-mismatched heteroepitaxial growth, i.e. when a material with a larger lattice constant (Ge) is deposited on a smaller lattice constant substrate (Si). In the so called Stranski-Krastanow (SK) growth mode, the elastic strain stored in the growing film is relaxed by the formation of three-dimensional (3D) islands on top of a thin, pseudomorphic wetting layer. Islands of different sizes and geometries [hut clusters, pyramids, domes] can be created [7].
In 2010 the first realization of single-hole transistors based on these individual SiGe islands was reported [8]. Transport spectroscopy revealed largely anisotropic hole g-factors. By changing the number of holes localized within the SiGe QDs, a clear modulation of the g-factor was observed, indicating that the g-factors are linked to the corresponding orbital wavefunctions. Dual gate devices demonstrated that the g-factor of the same orbital wavefunction can be changed by more than 300% when changing the value of a perpendicular electric field while keeping a constant number of holes [9]. This result indicated that this material system might be interesting for performing spin manipulation by means of g-tensor modulation.
However in order to move towards the realization of spin qubits one needs to move away from single quantum dot devices; charge sensors and double quantum dots need to be realized. For achieving these two building blocks, we are aiming to use two different types of structures: a) self-organized Ge nanostructures, i.e. Ge islands grown on prepatterned Si [10]. This approach might allow us to nucleate two islands next to each other, into a double dot configuration. b) one dimensional Ge nanowires, so called hut-wires (HWs) [11,12]. As stated above, self-assembled hut clusters emerge after the deposition of a few monolayers of Ge. Such hut clusters elongate and form nanowires with lengths of up to 1 micrometer and above, after annealing them for a few hours. A particularly interesting feature of the HWs is that they are solely oriented along [100] and [010] [11,12], whereas the wire height and width remain constant below 2 and 20 nm, respectively, leading thus to very strong confinement.

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[10] E. Lausecker et al., Appl. Phys. Lett 98 (2011) 143101
[11] Z. Z. Zhang et al., Phys. Rev. Lett. 109, 085502 (2012)
[12] H. Watzinger et al., APL Materials 2, 076102 (2014).