Growth and Electrical Characterization of Hybrid Core/Shell InAs/CdSe Nanowires

Core-only InAs nanowires (NWs) remain of continuing interest for application in modern optical and electrical devices. In this paper, we utilize the II-VI semiconductor CdSe as a shell for III-V InAs NWs to protect the electron transport channel in the InAs core from surface effects. This unique material configuration offers both a small lattice mismatch between InAs and CdSe and a pronounced electronic confinement in the core with type-I band alignment at the interface between both materials. Under optimized growth conditions, a smooth interface between the core and shell is obtained. Atom probe tomography (APT) measurements confirm substantial diffusion of In into the shell, forming a remote n-type doping of CdSe. Moreover, field-effect transistors (FETs) are fabricated, and the electron transport characteristics in these devices is investigated. Finally, band structure simulations are performed and confirm the presence of an electron transport channel in the InAs core that, at higher gate voltages, extends into the CdSe shell region. These results provide a promising basis toward the application of hybrid III-V/II-VI core/shell nanowires in modern electronics.

Correlations between Cascaded Photons from Spatially Localized Biexcitons in ZnSe

Radiative cascades emit correlated photon pairs, providing a pathway for the generation of entangled photons. The realization of a radiative cascade with impurity atoms in semiconductors, a leading platform for the generation of quantum light, would therefore provide a new avenue for the development of entangled photon pair sources. Here we demonstrate a radiative cascade from the decay of a biexciton at an impurity-atom complex in a ZnSe quantum well. The emitted photons show clear temporal correlations revealing the time-ordering of the cascade. Our result establishes impurity atoms in ZnSe as a potential platform for photonic quantum technologies using radiative cascades.

Efficient Single-Photon Sources Based on Chlorine-Doped ZnSe Nanopillars with Growth Controlled Emission Energy

Isolated impurity states in epitaxially grown semiconductor systems possess important radiative features such as distinct wavelength emission with a very short radiative lifetime and low inhomogeneous broadening, which make them promising for the generation of indistinguishable single photons. In this study, we investigate chlorine-doped ZnSe/ZnMgSe quantum well (QW) nanopillar (NP) structures as a highly efficient solid-state single-photon source operating at cryogenic temperatures. We show that single photons are generated due to the radiative recombination of excitons bound to neutral Cl atoms in ZnSe QW and the energy of the emitted photon can be tuned from about 2.85 down to 2.82 eV with ZnSe well width increase from 2.7 to 4.7 nm. Following the developed advanced technology, we fabricate NPs with a diameter of about 250 nm using a combination of dry and wet-chemical etching of epitaxially grown ZnSe/ZnMgSe QW structures. The remaining resist mask serves as a spherical- or cylindrical-shaped solid immersion lens on top of NPs and leads to the emission intensity enhancement by up to an order of magnitude in comparison to the pillars without any lenses. NPs with spherical-shaped lenses show the highest emission intensity values. The clear photon-antibunching effect is confirmed by the measured value of the second-order correlation function at a zero time delay of 0.14. The developed single-photon sources are suitable for integration into scalable photonic circuits.

Discretization of the total magnetic field by the nuclear spin bath in fluorine-doped ZnSe

The coherent spin dynamics of fluorine donor-bound electrons in ZnSe induced by pulsed optical excitation is studied in a perpendicular applied magnetic field. The Larmor precession frequency serves as a measure for the total magnetic field exerted onto the electron spins and, surprisingly, does not increase linearly with the applied field, but shows a step-like behavior with pronounced plateaus, given by multiples of the laser repetition rate. This discretization occurs by a feedback mechanism in which the electron spins polarize the nuclear spins, which in turn generate a local Overhauser field adjusting the total magnetic field accordingly. Varying the optical excitation power, we can control the plateaus, in agreement with our theoretical model. From this model, we trace the observed discretization to the optically induced Stark field, which causes the dynamic nuclear polarization.

Understanding the electron and nuclear spin interactions is essential to the application of quantum information devices. Here the authors report a step-like electron Larmor frequency versus external magnetic field due to the discretization of the total magnetic field by the nuclear spin bath in ZnSe:F.

Strain Compensation in Single ZnSe/CdSe Quantum Wells: Analytical Model and Experimental Evidence

The lattice mismatch between CdSe and ZnSe is known to limit the thickness of ZnSe/CdSe quantum wells on GaAs (001) substrates to about 2-3 monolayers. We demonstrate that this thickness can be enhanced significantly by using In_0.12Ga_0.88As pseudo substrates, which generate alternating tensile and compressive strains in the ZnSe/CdSe/ZnSe layers resulting in an efficient strain compensation. This method enables to design CdSe/ZnSe quantum wells with CdSe thicknesses ranging from 1 to 6 monolayers, covering the whole visible spectrum. The strain compensation effect is investigated by high resolution transmission electron microscopy and supported by molecular statics simulations. The model approach with the supporting experimental measurements is sufficiently general to be also applied to other highly mismatched material combinations for the design of advanced strained heterostructures.

Optical properties of strain-compensated CdSe/ZnSe/(Zn,Mg)Se quantum well microdisks

Strain-compensated CdSe/ZnSe/(Zn,Mg)Se quantum well structures that were grown on (In,Ga)As allow for efficient room-temperature photoluminescence and spectral tuning over the whole visible range. We fabricated microdisk cavities from these samples by making use of a challenging chemical structuring technique for selective and homogeneous removal of the (In,Ga)As sacrificial layer below the quantum structure. The observed whispering gallery modes in our microdisks are mainly visible up to photon energies of ~ 2.3 eV due to strong self-absorption. As extinction coefficients and effective refractive indices are dominated by the quantum well material CdSe, thick quantum wells (> 3 monolayer) are necessary to observe resonances in the corresponding quantum well emission.

Optical pumping of a single electron spin bound to a fluorine donor in a ZnSe nanostructure

Here we demonstrate optical pumping of a single electron within a semiconductor nanostructure comprised of a single fluorine donor located within a ZnSe/ZnMgSe quantum well. Experiments were performed to detect optical pumping behavior by observing single photons emitted from the nanostructure when the electron changes spin state. These results demonstrate initialization and read-out of the electron spin qubit and open the door for coherent optical manipulation of a spin by taking advantage of an unconventional nanostructure.

Entangling single photons from independently tuned semiconductor nanoemitters

Quantum communication systems based on nanoscale semiconductor devices is challenged by inhomogeneities from device to device. We address this challenge using ZnMgSe/ZnSe quantum-well nanostructures with local laser-based heating to tune the emission of single impurity-bound exciton emitters in two separate devices. The matched emission in combination with photon bunching enables quantum interference from the devices and allows the postselection of polarization-entangled single photons. The ability to entangle single photons emitted from nanometer-sized sources separated by macroscopic distances provides an essential step for a solid-state realization of a large-scale quantum optical network. This paves the way toward measurement-based entanglement generation between remote electron spins localized at macroscopically separated fluorine impurities.

Indistinguishable photons from independent semiconductor nanostructures

We demonstrate quantum interference between photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum-well nanostructures. The ability to generate single photons from these devices is confirmed by autocorrelation experiments, and the indistinguishability of photons emitted from two independent nanostructures is confirmed via a Hong-Ou-Mandel dip. These results indicate that donor impurities in appropriately engineered semiconductor structures can portray atomlike homogeneity and coherence properties, potentially enabling scalable technologies for future large-scale optical quantum computers and quantum communication networks.