Structural analysis of strained quantum dots using nuclear magnetic resonance

Abstract

Strained semiconductor nanostructures can be used to make single-photon sources, detectors and photovoltaic devices, and could potentially be used to create quantum logic devices. The development of such applications requires techniques capable of nanoscale structural analysis, but the microscopy methods typically used to analyse these materials are destructive. NMR techniques can provide non-invasive structural analysis, but have been restricted to strain-free semiconductor nanostructures because of the significant strain-induced quadrupole broadening of the NMR spectra. Here, we show that optically detected NMR spectroscopy can be used to analyse individual strained quantum dots. Our approach uses continuous-wave broadband radiofrequency excitation with a specially designed spectral pattern and can probe individual strained nanostructures containing only 1 × 10(5) quadrupole nuclear spins. With this technique, we are able to measure the strain distribution and chemical composition of quantum dots in the volume occupied by the single confined electron. The approach could also be used to address problems in quantum information processing such as the precise control of nuclear spins in the presence of strong quadrupole effects.

Element-sensitive measurement of the hole–nuclear spin interaction in quantum dots

“…For the studied InGaAs quantum dots where indium and gallium concentrations are estimated to be ρ In ≈ 20% and ρ Ga ≈ 80% (see ref. 16), we find negative γ ≈ −4%, whereas for more indium-rich InGaAs dots emitting at E PL ∼ 1.30 eV the value of γ ≈ −9% has been reported 8 . This suggests that for quantum dots with a particular indium concentration (ρ In ∼ 10%) one can expect close to zero (γ ≈ 0) total hole hyperfine shift induced by nuclear spin polarization along the Oz direction.…”

“…9) and InGaAs/GaAs (ref. 16) quantum-dot samples without electric gates (further details can be found in Supplementary Section S1). The photoluminescence spectra of neutral quantum dots placed at T = 4.2 K, in an external magnetic field B z normal to the sample surface, were measured using a double spectrometer and a CCD (charge-coupled device) camera.…”

“…The concept of the valence-band hyperfine constant measurement is based on detecting hole hyperfine shift E k h (equation (2)) as a function of electron hyperfine shift E k e (equation (1)) by varying the nuclear spin polarization I k z . Non-zero I k z is induced by optical nuclear spin pumping: circularly polarized light of the pump laser generates spin-polarized electrons that transfer their polarization to nuclei 16,17,29 through the hyperfine interaction. The magnitude of I k z is controlled by changing the degree of circular polarization 9 .…”

“…This is achieved by simultaneous and independent detection of the electron and hole Overhauser shifts using high-resolution photoluminescence spectroscopy of neutral quantum dots. In contrast to previous work 9 , we now also apply excitation with a radiofrequency oscillating magnetic field, which allows isotopeselective probing of the valence-band hole hyperfine interaction 16 . Using this technique we find that in all studied materials, cations (gallium, indium) have a negative hole hyperfine constant, whereas it is positive for anions (phosphorus, arsenic), a result attributed to the previously disregarded contribution of the cationic d-shells into the valence-band Bloch wavefunctions.…”

Near-band-gap photoinduced nuclear spin dynamics in semi-insulating GaAs: Hyperfine- and quadrupolar-driven relaxation

“…10,11 At the nanoscale, these same principles have been adapted to the nuclear bath within semiconductor quantum dots, where spin order has been investigated, e.g., as a route to mitigate nuclear-induced decoherence of the electron spin, 12,13 control a two-electron qubit, 14 tune the polarization of quantum-dotemitted photons, 15 or extract information on local strain. 16 Here we use light-assisted, high-field nuclear magnetic resonance (NMR) to probe the dynamics of nuclear spin relaxation and transport near crystal defects in semi-insulating (SI) GaAs. Our experiments capitalize on the unique conditions created at low illumination intensities, where competing mechanisms of nuclear spin relaxation coexist.…”