Electron and nuclear spin properties of the nanohole-filled GaAs/AlGaAs quantum dots

GaAs/AlGaAs quantum dots grown by in situ droplet etching and nanohole infilling offer a combination of strong charge confinement, optical efficiency, and spatial symmetry required for polarization entanglement and spin-photon interface. Here we study spin properties of such dots. We find nearly vanishing electron g-factor (ge < 0.05), providing a route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to 60% achieved. NMR spectroscopy reveals the structure of two types of quantum dots and yields the small magnitude of residual strain εb < 0.02% which nevertheless leads to long nuclear spin lifetimes exceeding 1000 s. The stability of the nuclear spin environment is advantageous for applications in quantum information processing. Central spin in semiconductor quantum dots is a prime candidate for applications in quantum information technologies.1,2 It is relatively isolated from the solid state effects and at the same time is accessible for coherent manipulation and can be interfaced optically. The coherence in this system is mainly limited by the hyperfine coupling with the nuclear spin bath.3,4 Single spin qubit manipulation in these structures, therefore, demands an auxiliary control over nuclear spin environment. Such control can be realized by maximizing polarization of 104− 105 nuclei in a single quantum dot,5–7 enabling the formation of well-defined nuclear spin states and in effect reducing the influence of the nuclear field fluctuations.8,9 Central spin manipulation in semiconductor quantum dot (QD) system using resonant ultrafast optical pulses10,11 has been demonstrated but scalability in such schemes is challenging. An alternative approach is to induce controlled spin rotation by manipulating the coupling to the external magnetic field.12 This can be achieved by electrical modulation of the g-factor. However, such scheme critically depends on the ability to change the sign of g, thus requiring quantum dots with close to zero electron or hole g-factor.13,14 Self-assembled InGaAs/GaAs QD has been the primary system of choice for spin studies over the last two decades, as quantum confinement in monolayer-fluctuation GaAs/AlGaAs dots is too weak. Only recently the potential of droplet epitaxial (DE) grown GaAs QDs has been identified.15–17 In particular nanohole-filled droplet epitaxial (NFDE) dots formed by in situ etching and nanohole infilling18 provide confinement and excellent optical efficiency, while on the other hand exhibiting high symmetry not achievable previously in self-assembled dots.19 Such unique com1 ar X iv :1 50 7. 06 55 3v 1 [ co nd -m at .m es -h al l] 2 3 Ju l 2 01 5 bination of properties make NFDE dots ideal candidates for polarization entanglement and spinphoton interface.20 This system has already exhibited an efficient interface between rubidium atoms and a quantum dot.21 However, the understanding of the spin properties in such quantum dots is still lacking. Here we use optical and nuclear magnetic resonance (NMR) spectroscopy to study the properties of the single charge spins and nuclear spin environment in NFDE grown GaAs/AlGaAs QDs. Magneto-photoluminescence measurements reveal close-to-zero electron g-factor, due to the electron wavefunction overlap with the AlGaAs barrier. We demonstrate efficient dynamic nuclear polarization (DNP) as large as 60 %. By measuring the excitation wavelength dependence we identify three mechanisms of DNP: (i) via optical excitation of the quantum well states, (ii) via resonant optical excitation of the dot ground or excited states, and (iii) via resonant excitation of the neighboring dot made possible by inter-dot charge tunneling. Radio frequency (rf) excitation is used to measure NMR spectra revealing the presence of small (< 0.02%) residual biaxial strain. Surprisingly, we observe two sub-ensembles of QDs one with compressive and another with tensile strain along the growth axis: this allows us to identify these two types of dots as formed in the nanoholes and at the rims of the nanoholes respectively. We show that small residual strain results in very stable nuclear spin bath with nuclear spin relaxation times > 1000 s, previously achievable only in selfassembled dots. The properties of the NFDE quantum dots revealed in this study make them a favorable system for electrical spin qubit manipulation with a potential for minimized decoherence effects from the nuclear spin bath. Single dot photoluminescence (PL) spectroscopy is performed with a confocal setup which collects PL at low temperature (T ≈ 4.2 K) from a ∼ 1 μm spot. Magnetic fields up to 10 T along sample growth axis (Faraday geometry) are employed in this study. The polarization degree of the nuclear spins is probed by measuring the hyperfine shifts in the Zeeman splitting of the quantum dot PL. Nuclear spin polarization and NMR spectroscopy studies are performed using the methods described in Reference.22 QD B QD A 1.56 1.58 1.60 1.62 1.64 1.66 1.68 1.70 Photon Energy (eV)