A Schottky top‐gated two‐dimensional electron system in a nuclear spin free Si/SiGe heterostructure

Abstract

Quantum dots offer a promising two-level system for applications in solid state based quantum information processing [1]. Within these three-dimensionally confining structures, electrostatically defined quantum dots are a well studied system [2], mostly in III-V materials. A major source of decoherence in such devices is the interaction of the confined electron spin with the surrounding semiconductor host matrix, in particular with the nuclear spin bath [3]. Recently, single electron devices have been reported in materials systems like Si-Ge [4,5] or C [6] which contain a reduced amount of nuclear spins in their natural isotopic composition. As a next step, isotopical purification of the group-IV source materials Si, Ge and C can give access to virtually nuclear spin free materials. In this Letter, we report on the realization of two-dimensional electron systems (2DES) in a nuclear spin free environment. A 2DES forms in a strained 28 Si layer embedded into 28 Si 70 Ge. The ability to control the 2DES via top-gates is demonstrated by the implementation of split-gate structures which are able to locally deplete the 2DES. Suitable voltages allow the complete pinch-off of the narrow conducting channel.Samples are fabricated in solid source molecular beam epitaxy (MBE). The base pressure of the Riber Siva 45 MBE chamber is 1 × 10 -11 mbar. 28 Si and 70 Ge are evaporated from a custom made MBE-Komponenten electron beam source and effusion cell respectively. Figure 1a shows the typical layout of our isotopically engineered Si/SiGe heterostructures. A SiGe virtual substrate of natural isotopic composition is first deposited onto a (100) oriented Si substrate. The virtual substrate is realized by increasing the Ge content linearly by 8%/µm until the desired Ge content is reached. The graded layers are deposited at a substrate temperature of T s = 575 °C. The virtual substrate is fully relaxed within the experimental error of 10%. It typically displays a density of threading dislocations of about 1 × 10 6 cm -2 . The active part is thenWe report on the realization and top-gating of a two-dimensional electron system in a nuclear spin free environment using 28 Si and 70 Ge source material in molecular beam epitaxy. Electron spin decoherence is expected to be minimized in nuclear spin-free materials, making them promising hosts for solid-state based quantum information processing devices. The two-dimensional electron system exhibits a mobility of 18000 cm 2 /(V s) at a sheet carrier density of 4.6 × 10 11 cm -2 at low temperatures. Feasibility of reliable gating is demonstrated by transport through split-gate structures realized with palladium Schottky top-gates which effectively control the two-dimensional electron system underneath. Our work forms the basis for the realization of an electrostatically defined quantum dot in a nuclear spin free environment.

Isotope engineering of silicon and diamond for quantum computing and sensing applications

“…The Isonics 28 Si CVD epi-layers have the isotopic compositions 28 Si (∼99.924%), 29 Si (∼0.073%), and 30 Si (∼0.003%). Isotopically enriched, strained 28 Si thin layers were grown by solid-source molecular beam epitaxy (MBE) at the Technical University of Munich (TUM), Germany [42] and by CVD at Princeton University, USA. [43] Some electron doublequantum dots were fabricated using the strained silicon.…”

Theory of single electron spin relaxation in Si/SiGe lateral coupled quantum dots

“…[16][17][18][19] That is why Si based quantum dots have recently seen a revived interest. Although the quantum dot technology is not yet as mature as in GaAs, several perspective setups are being actively pursued.…”

“…(11), reads 55,57 where the tilted axes are such thatx is parallel to d. Since the first excited orbital state Γ A transforms likex, only the first term in Eq. (19) can lead to spin hot spots for moderate magnetic fields in the intermediate regime.…”

Equilibration in closed quantum systems: Application to spin qubits

“…the confinement, and the natural abundance of nuclear-spin carrying isotopes of the host material. In practice, this number can range between very few nuclear spins (in silicon-or carbon-based systems) [42][43][44][45][46][47][48] to millions (in GaAs-based QDs). [14][15][16] In recent years, the experimental control of spin qubits in QDs has developed to a state of perfection at the single-and two-qubit level.…”