A refined plan-view specimen preparation technique for high-quality electron microscopy studies of epitaxially grown atomically thin 2D layers.

The structural studies of two-dimensional (2D) van der Waals heterostructures and understanding of their relationship with the orientation of crystalline substrates using transmission electron microscopy (TEM) presents a challenge in developing an easy-to-use plan-view specimen preparation technique. In this report, we introduce a simple approach for high-quality plan-view specimen preparation utilizing a dual beam system comprising focused ion beam and scanning electron microscopy. To protect the atomically thin 2D heterostructure during the preparation process, we employ an epoxy layer. This layer serves as a protective barrier and enables the creation of a TEM specimen comprising a thin substrate fragment with an overgrown 2D structure covered by a thin, electron-transparent epoxy layer. The coexistence of both 2D layers and substrate is essential for investigating the relative crystallographic orientations between the grown 2D structures and the substrates. The thickness of the specimen is monitored using low-voltage scanning electron microscopy. We apply this technique to prepare plan-view specimens of 2D germanium-antimony-telluride (GST) on Si and hexagonal boron nitride (h-BN)/epitaxial graphene (EG) heterostructures grown on 6H-SiC substrates. The grain-like atomic structure observed in the 2.2 nm thick GST layer on Si substrate provides evidence of the mosaicity of GST during the early stages of epitaxial growth. H-BN/EG on 6H-SiC structural studies indicate a rotation of h-BN/EG around the 6H-SiC[0001] axis by an angle of 30°. The observed BN particles with sizes in the nanometer range on top of the sample have the wurtzite lattice type and random orientation. The developed specimen preparation technique offers a powerful tool for TEM studies of atomically thin layers on crystals. Its simplicity and ability to provide valuable insights into the in-plane relationships between 2D structures and crystalline substrates make it a promising complement to grazing incident X-ray diffraction.

Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots.

Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid-state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC (ref. ) reaches 99% and beyond, it has been limited to 50-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent 'dark' nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed-the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time.

Far-Infrared and Raman Spectroscopy Investigation of Phonon Modes in Amorphous and Crystalline Epitaxial GeTe-Sb2Te3 Alloys.

A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the Fm3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.

Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry.

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

Nanomembrane quantum-light-emitting diodes integrated onto piezoelectric actuators.

We integrate resonant-cavity light-emitting diodes containing quantum dots onto substrates with giant piezoelectric response. Via strain, the energy of the photons emitted by the diode can be precisely controlled during electrical injection over a spectral range larger than 20 meV. Simultaneously, the exciton fine-structure-splitting and the biexciton binding energy can be tuned to the values required for entangled photon generation.

Comparative study of low temperature growth of InAs and InMnAs quantum dots.

The evolution of InAs and In(0.85)Mn(0.15)As quantum dots grown at 270 °C is studied as a function of coverage. We show that, in contrast to what occurs at high temperature, the two-dimensional to three-dimensional transition is not abrupt but rather slow. This is due to the finding that part of the deposited material also contributes to the wetting layer growth after quantum dot formation. This aspect is particularly accentuated in In(0.85)Mn(0.15)As deposition. The Voronoi area analysis reveals a significant spatial correlation between islands.