Strongly confined excitons in self-assembled InGaAs quantum dot clusters produced by a hybrid growth method

We present a few electron double quantum dot device defined in an isotopically purified 28 Si quantum well (QW). An electron mobility of 5.5 · 10 4 cm 2 (Vs) −1 is observed in the QW which is the highest mobility ever reported for a two-dimensional electron system in 28 Si. The residual concentration of 29 Si nuclei in the 28 Si QW is lower than 10 3 ppm, at the verge where the hyperfine interaction is theoretically no longer expected to dominantly limit the T 2 spin dephasing time. We also demonstrate a complete suppression of hysteretic gate behavior and charge noise using a negatively biased global top gate.PACS numbers: 73.63. Kv, 73.23.Hk, 73.21.Fg, 28.60.+s, 72.70.+m, 73.50.Td Semiconductor quantum dots (QD) are among the candidates for a scalable implementation of electron spin based qubits in solid state systems. Silicon (Si) has been widely recognized as a well suited material system for decoupling electron spin qubits from their volatile solid state environment owing to the weak spin-orbit and weak hyperfine interaction. Very long spin relaxation times (T 1 ) on the order of seconds have been reported for Si on the basis of electrostatically defined QDs 1 , single phosphorous donors 2 or triplet-singlet relaxation times in double QDs 3 . Recently, also a spin dephasing time of T * 2 = 360 ns has been observed in a time ensemble measurement in a Si double QD 4 . These milestones highlight the great potential for quantum information processing in Si.The adverse impact from nuclear spins on electron spin coherence 5 can be further reduced in the Si material system by means of isotopic enrichment of the 28 Si isotope which has zero nuclear spin. Recent technological advances have enabled the fabrication of highly enriched 28 Si crystals 6 with isotopic fractions of the nuclear spin carrying 29 Si isotope smaller than 4 · 10 2 ppm. In such ultra-clean 28 Si bulk samples, the spin coherence time T 2 for donor-bound electrons 7 achieves unprecedentedly long values of T 2 = 10 s. This offers a promising perspective for qubit applications with electrostatically defined QDs in 28 Si heterostructures. However, the integration of isotopically purified material with low impurity concentrations into molecular beam epitaxy (MBE) or chemical vapor deposition growth processes is still a challenge. Hence, no QD devices have been demonstrated so far for two-dimensional electron systems (2DES) in 28 Si.In this letter, we report on the fabrication and characterization of an electrostatically defined few electron double QD within a high mobility 2DES in a MBEgrown 28 Si/SiGe heterostructure. We find a concentration of residual 29 Si nuclei in the quantum well (QW) smaller than 10 3 ppm and achieve a peak mobility of 5.5 · 10 4 cm 2 (Vs) −1 at a 2DES density of 3 · 10 11 cm −2 . We combine our double QD with a global top gate (TG) and demonstrate a strong suppression of hysteretic gate behavior and charge noise as a negative voltage is applied to the global TG.Our heterostructures are grown in a solid source MBE system equ...

Highly asymmetric dynamic nuclear spin pumping is observed in a single self-assembled InGaAs quantum dot subject to resonant optical excitation of the neutral exciton transition. A large maximum polarization of 54% is observed and the effect is found to be much stronger upon pumping of the higher energy Zeeman level. Time-resolved measurements allow us to directly monitor the buildup of the nuclear spin polarization in real time and to quantitatively study the dynamics of the process. A strong dependence of the observed dynamic nuclear polarization on the applied magnetic field is found, with resonances in the pumping efficiency observed for particular magnetic fields. We develop a model that accounts for the observed behavior, where the pumping of the nuclear spin system is due to hyperfine-mediated spin-flip transitions between the states of the neutral exciton manifold.