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Herein, the possibilities of nanoselective area growth (NSAG) of cubic gallium nitride on 3C‐SiC/Si (001) pseudo substrates are studied. Growth is masked by SiO2 patterned with hole arrays and groove structures. Nanosphere lithography and block‐copolymer lithography are employed to pattern holes with diameters of 130 and 17 nm, respectively. Electron beam lithography is used to pattern grooves. Patterns are transferred into SiO2 and 3C‐SiC by reactive ion etching with CHF3/Ar and SF6 chemistry, correspondingly. It is possible to demonstrate phase pure nanoselective nucleation of c‐GaN on <001> and <111> facets of 3C‐SiC. Phase pure nucleation of cubic gallium nitride is confirmed by transmission electron microscopy measurements on the nanoscopic scale. A hexagonal fraction of 17.6% is achieved on the macroscopic scale measured by high‐resolution X‐ray diffraction (HRXRD), employing V‐shaped grooves on 4° miscut substrates. Furthermore, the possibility of coalescence after NSAG is demonstrated with the dominant cubic phase. [ABSTRACT FROM AUTHOR]

Remote epitaxy is a relatively new area of research that offers several advantages, including the potential to reduce the influence of lattice mismatch and the ability to exfoliate films easily. This work is focused on adapting this growth method for the epitaxy of cubic gallium nitride (c‐GaN), a metastable phase. However, one faces challenges in enforcing the nucleation of the metastable cubic phase due to the weaker interactions between the substrate and the epitaxial layer compared to conventional epitaxy. Initially, only polycrystalline wurtzite gallium nitride could be grown. However, by optimizing the growth conditions and adding a cubic aluminum nitride buffer layer, predominantly cubic gallium nitride layers can be grown. High‐resolution X‐ray diffraction measurements confirm that the percentage of hexagonal inclusions is reduced from 80% to 23%. [ABSTRACT FROM AUTHOR]

Tunnel‐coupled pairs of optically active quantum dots—quantum dot molecules (QDMs)—offer the possibility to combine excellent optical properties such as strong light‐matter coupling with two‐spin singlet–triplet (S−T0$S-T_0$) qubits having extended coherence times. The S−T0$S-T_0$ basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter‐dot orbital couplings, operation of the S−T0$S-T_0$ qubits under optimal conditions remains challenging. Here, an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand is presented. A four‐phase optical and electric field control sequence facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter‐dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. One‐ and two‐hole charging efficiencies of (93.5 ± 0.8)% and (80.5 ± 1.3)% are achieved, respectively. Combining efficient charge state preparation and precise setting of inter‐dot coupling allows for the control of few‐spin qubits, as would be required for the on‐demand generation of 2D photonic cluster states or quantum transduction between microwaves and photons. [ABSTRACT FROM AUTHOR]

The size‐dependent strain relaxation in InAs quantum dots on the top face of GaAs(111)A nanopillars is studied experimentally by scanning transmission electron microscopy (STEM) and theoretically using molecular static simulations. In the experiment, a dislocation‐free, coherent state is observed for InAs dimensions below 10 nm in width and 7 nm in height, while 60° misfit dislocations occur for larger sizes. Experimental strain maps reveal the presence of a narrow‐strained region at the heterointerface with a 20–80% width of 1.2 nm in the coherent case and 0.4 nm in the dislocated state, in agreement with simulations. Moreover, an analysis of the amount of misfit relaxed by the observed dislocations shows that the transition between the purely elastic and plastic relaxation regimes appears to be gradual. Intensity profiles of STEM images reveal that the misfit dislocations are located directly at the GaAs/InAs heterointerface. [ABSTRACT FROM AUTHOR]

Self‐organized semiconductor quantum dots represent almost ideal two‐level systems, which have strong potential to applications in photonic quantum technologies. For instance, they can act as emitters in close‐to‐ideal quantum light sources. Coupled quantum dot systems with significantly increased functionality are potentially of even stronger interest since they can be used to host ultra‐stable singlet‐triplet spin qubits for efficient spin‐photon interfaces and for deterministic photonic 2D cluster‐state generation. An advanced quantum dot molecule (QDM) device is realized and excellent optical properties are demonstrated. The device includes electrically controllable QDMs based on stacked quantum dots in a pin‐diode structure. The QDMs are deterministically integrated into a photonic structure with a circular Bragg grating using in situ electron beam lithography. A photon extraction efficiency of up to (24 ± 4)% is measured in good agreement with numerical simulations. The coupling character of the QDMs is clearly demonstrated by bias voltage dependent spectroscopy that also controls the orbital couplings of the QDMs and their charge state in quantitative agreement with theory. The QDM devices show excellent single‐photon emission properties with a multi‐photon suppression of g(2)(0)=(3.9±0.5)×10−3. These metrics make the developed QDM devices attractive building blocks for use in future photonic quantum networks using advanced nanophotonic hardware. [ABSTRACT FROM AUTHOR]

GaAs‐(111)‐nanostructures exhibiting second harmonic generation are new building blocks in nonlinear optics. Such structures can be fabricated through epitaxial lift‐off using selective etching of Al‐containing layers and subsequent transfer to glass substrates. Herein, the selective etching of (111)B‐oriented AlxGa1−xAs sacrificial layers (10–50 nm thick) with different aluminum concentrations (x = 0.5–1.0) in 10% hydrofluoric acid is investigated and compared with standard (100)‐oriented structures. The thinner the sacrificial layer and the lower the aluminum content, the lower the lateral etch rate. For both orientations, the lateral etch rates are in the same order of magnitude, but some quantitative differences exist. Furthermore, the epitaxial lift‐off, the transfer, and the nanopatterning of thin (111)B‐oriented GaAs membranes are demonstrated. Atomic force microscopy and high‐resolution X‐ray diffraction measurements reveal the high structural quality of the transferred GaAs‐(111) films. [ABSTRACT FROM AUTHOR]

In cubic (c‐)GaN Ge has emerged as a promising alternative to Si for n‐type doping, offering the advantage of slightly improved electrical properties. Herein, a study on Ge doping of the ternary alloy c‐AlxGa1−xN is presented. Ge‐doped c‐AlxGa1−xN layers are grown by plasma‐assisted molecular beam epitaxy. In two sample series, both the Al mole fraction x and the doping level are varied. The incorporation of Ge is verified by time‐of‐flight secondary ion mass spectrometry. Ge incorporation and donor concentrations rise exponentially with increasing Ge cell temperature. A maximum donor concentration of 1.4 × 1020 cm−3 is achieved. While the incorporation of Ge is almost independent of x, incorporation of O, which acts as an unintentional donor, increases for higher x. Dislocation densities start increasing when doping levels of around 3 × 1019 cm−3 are exceeded. Also photoluminescence intensities begin to drop at these high doping levels. Optical emission of layers with x > 0.25 is found to originate from a defect level 0.9 eV below the indirect bandgap, which is not related to Ge. In the investigated range 0 ≤ x ≤ 0.6, Ge is a suitable donor in c‐AlxGa1−xN up to the low 1019 cm−3 range. [ABSTRACT FROM AUTHOR]

We have investigated the optical properties of self‐assembled cubic GaN quantum dots (QDs) in a cubic AlN matrix grown in Stranski–Krastanov growth mode. Two different sample series are fabricated and optically characterized by photoluminescence measurements. Additionally, the experimental results are compared to theoretical calculations. Sample series A consists of one single QD layer with varying GaN amount. A red shift of emission energy of 140 meV is observed with increasing GaN deposition time. Simulations verify this behavior and can be explained by a decrease of the QD height. Sample series B has two layers of QDs on top of each other with different QD deposition amounts of three and six monolayers, respectively. The spacer layer thickness between the QD layers is 2 and 20 nm. A decreased emission intensity of the smaller QDs is measured for QD layers with smaller spacer layer thickness. This observation is a first hint of an electronic coupling between the two QD layers with thin spacer layer thickness resulting in tunneling of carriers from the smaller QDs to the larger QDs. Theoretical calculations further show that a strain induced structural coupling induces a vertical alignment of the QDs. [ABSTRACT FROM AUTHOR]

The carrier transfer via non‐resonant tunneling is of great significance for many devices like the quantum cascade laser. In this study time‐resolved photoluminescence is used for an investigation of this effect in asymmetric double quantum wells for low temperatures. The Al content in these asymmetric double quantum wells was determined by HRXRD to x = 0.64 ± 0.03. The growth of the asymmetric cubic GaN/AlxGa1−xN double quantum wells was performed by a radio‐frequency plasma‐assisted molecular beam epitaxy. As a substrate, 3C‐SiC (001) on top of Si (001) was used. Three samples with different barrier thickness d were analysed (1 nm, 3 nm, 15 nm). The two quantum wells are designed with the thicknesses 2.5 nm and 1.35 nm. Thus, three expected emission bands measured in luminescence can be resolved. The maximum intensities are 3.49 eV (wide well), 3.73 eV (narrow well) and 4.12 eV (AlGaN). A correlation between the carrier lifetimes of the quantum wells (QWs) and the barrier width is found. Exploiting rate equations, the intensity ratio of both QW emissions is calculated. The coupling of the two QWs starts below 3 nm barrier thickness, above this value there is no coupling. [ABSTRACT FROM AUTHOR]

We have investigated the stacking of self‐assembled cubic GaN quantum dots (QDs) grown in Stranski–Krastanov (SK) growth mode. The number of stacked layers is varied to compare their optical properties. The growth is in situ controlled by reflection high energy electron diffraction to prove the SK QD growth. Atomic force and transmission electron microscopy show the existence of wetting layer and QDs with a diameter of about 10 nm and a height of about 2 nm. The QDs have a truncated pyramidal form and are vertically aligned in growth direction. Photoluminescence measurements show an increase of the intensity with increasing number of stacked QD layers. Furthermore, a systematic blue‐shift of 120 meV is observed with increasing number of stacked QD layers. This blueshift derives from a decrease in the QD height, because the QD height has also been the main confining dimension in our QDs. [ABSTRACT FROM AUTHOR]

We introduce germanium as an alternative to silicon for n-type doping of cubic gallium nitride. Layers with electron concentrations up to 3.7 × 1020 cm−3 were grown by plasma-assisted molecular beam epitaxy on 3C-SiC substrates. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements were performed to verify the incorporation of Ge into our layers. The incorporation of Ge is in good agreement with the trend of the Ge vapour pressure curve. For the highest doped sample a drop of the incorporation efficiency is observed. A reduction of the growth rate is noticed for high Ge fluxes. Additionally, a sample comprising an alternating pattern of Ge-doped and not intentionally doped interlayers was grown. In the recorded TOF-SIMS depth profile we observe that in doped regions the Ge concentration increases along the growth direction. A gradually decreasing amount of Ge is incorporated into each overlying not intentionally doped interlayer. We suppose these observations are due to segregation effects and a resulting accumulation of Ge at the sample surface during growth. [ABSTRACT FROM AUTHOR]

We successfully developed a process to fabricate freestanding cubic aluminium nitride (c-AlN) membranes containing cubic gallium nitride (c-GaN) quantum dots (QDs). The samples were grown by plasma assisted molecular beam epitaxy (MBE). To realize the photonic crystal (PhC) membrane we have chosen a triangular array of holes. The array was fabricated by electron beam lithography and several steps of reactive ion etching (RIE) with the help of a hard mask and an undercut of the active layer. The r/a-ratio of 0.35 was determined by numerical simulations to obtain a preferably wide photonic band gap. Micro-photoluminescence (µ-PL) meas urements of the photonic crystals, in particular of a H1 and a L3 cavity, and the emission of the QD ensemble were performed to characterize the samples. The PhCs show high quality factors of 4400 for the H1 cavity and about 5000/3000 for two different modes of the L3 cavity, respectively. The energy of the fundamental modes is in good agreement to the numerical simulations. [ABSTRACT FROM AUTHOR]

Low-dimensional transport in semiconductor meso- and nanostructures is a topical field of fundamental research with potential applications in future quantum devices. However, thermal non-equilibrium may destroy phase-coherence and remains to be explored experimentally. Here, we present effects of thermal non-equilibrium in various implementations of low-dimensional (non-interacting) electron systems, fabricated by etching AlGaAs/GaAs heterostructures. These include narrow quasi-two-dimensional (2D) channels, quasi-one-dimensional (1D) waveguide networks, quantum rings (QRs), and single 1D constrictions, such as quantum point contacts (QPCs). Thermal non-equilibrium is realized by current heating. The charge carrier temperature is determined by noise thermometry. The electrical conductance and the voltage-noise are measured with respect to bath temperatures, heating currents, thermal gradients, and electric fields. We determine and discuss heat transport processes, electron-energy loss rates, and electron-phonon interaction, and our results are consistent with the Wiedemann-Franz relation. Additionally, we show how non-thermal current fluctuations can be used to identify electric conductance anomalies due to charge states. [ABSTRACT FROM AUTHOR]

We analyse an InAs/GaAs-based electric field tunable single quantum dot diode with a thin tunnelling barrier between a buried n+-back contact and a quantum dot layer. In voltage-dependent photoluminescence measurements, we observe rich signatures from spatially direct and indirect transitions from the wetting layer and from a single quantum dot. By analysing the Stark effect, we show that the indirect transitions result from a recombination between confined holes in the wetting or quantum dot layer with electrons from the edge of the Fermi sea in the back contact. Using a 17 nm tunnel barrier which provides comparably weak tunnel coupling allowed us to observe clear signatures of direct and corresponding indirect lines for a series of neutral and positively charged quantum dot states. [ABSTRACT FROM AUTHOR]

In article number 2200049, Frederik Bopp, Jonathan J. Finley, and co-workers propose and demonstrate a measurement scheme allowing for a simultaneous optical charge state and electric coupling control of single QDMs. Front Cover: Quantum Dot Molecule Devices with Optical Control of Charge Status and Electronic Control of Coupling (Adv. The cover image shows a quantum dot molecule (QDM), in which the tunnel coupling is electrically controlled. [Extracted from the article]

We review on time and spatially resolved two-color pump-probe magneto-optical Kerr effect (MOKE) microscopy studies of electron spins in bulk n-GaAs and GaAs (110) quantum wells (QWs) at low lattice temperatures. The influence of photocarrier heating by above-bandgap optical spin excitation on the spatially resolved magneto-optical spin detection is considered in detail. We demonstrate that a continuous-wave (cw) measurement of the local Kerr rotation at a fixed arbitrary probe wavelength in general does not correctly reveal the local spin polarization when hot electrons are present. For bulk GaAs we determine the true lateral electron spin polarization profile from cw measurements of the spatial dependence of the full excitonic Kerr rotation spectrum. For the (110) QWs, we directly obtain the electron spin diffusion coefficient from picosecond real space imaging of the time evolution of an optically excited electron spin packet, which we observe with a spectrally broad probe pulse. [ABSTRACT FROM AUTHOR]

Semiconductor nanostructures are of interest in the field of spintronics, because they allow us to employ special features of low-dimensional transport for spin polarization, manipulation, and detection. Proposals for spintronic devices are based on coherent transport and interference effects in extended electron waveguide (EWG) structures. We present the state-of-the-art of low-dimensional EWGs and Aharonov-Bohm (AB) interferometers, fabricated in GaAs/AlGaAs heterostructures. Low-temperature measurements in etched quantum point contacts (QPCs) show large subband spacing and, in the presence of a source-drain DC bias, the differential conductance is resonantly enhanced due to a many-body modification of the barrier potential because of spin fluctuations. Multiterminal asymmetric quantum wire rings (QRs) have been fabricated and AB conductance oscillations are used to detect electrostatically induced continuous phase shifts and [ABSTRACT FROM AUTHOR]

In this paper, we discuss the magnetic properties of Gd-doped GaN. This diluted magnetic semiconductor shows hysteretic magnetization behavior at room temperature, which is attributed to ferromagnetism with a Curie temperature well above 300 K. However, the experimental results regarding the magnetic properties are not completely consistent and the microscopic origin for the reported magnetic properties is still unclear. We discuss the role of the growth method of the GaN comparing molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) including GaN doped during the growth process with Gd as well as Gd-implanted material. It seems that in general it is easier to obtain hysteretic magnetization behavior for MBE-grown material probably due to the higher oxygen and lower hydrogen content. An exception is Gd-implanted GaN MBE-grown on Si(111) where we observe no ferromagnetism. We will present experiments where by oxygen implantation and annealing the impurity concentration was manipulated. The role of native defects is addressed and new experiments where additional defects have been introduced by nitrogen implantation in MBE-grown GaN:Gd are discussed. We present our results on anomalous Hall effect observed in a Gd-implanted GaN/AlGaN heterostructure. [ABSTRACT FROM AUTHOR]

We analyse the spin relaxation length during vertical electron transport in spin light-emitting diode devices at room temperature. We obtain a spin relaxation length of 27 nm in remanence. When a magnetic field is applied, spin relaxation is significantly reduced during transport to the active region of the device. This results in a nearly doubled spin relaxation length at 2T magnetic field strength. [ABSTRACT FROM AUTHOR]

The effects of confinement on biexciton renormalization in self-assembled InAs and interfacial GaAs quantum dot (QD) ensembles are studied using two-dimensional Fourier-transform spectroscopy. We find that in thermally annealed InAs QDs, changes in the biexciton transition energy are strongly correlated with those of the exciton and that the biexciton binding energy is similar for all QDs in the ensemble. These results are in contrast to those from GaAs QDs formed from interfacial fluctuations of a narrow quantum well (QW). In both the GaAs QW and QDs, correlation is reduced and the biexciton binding exhibits a strong dependence on localization. Comparison with simulations reveals how confinement and Coulomb interactions modify biexciton renormalization. [ABSTRACT FROM AUTHOR]

Grazing-incidence X-ray diffraction measurements on single GaAs nanowires (NWs) grown on a (111)-oriented GaAs substrate by molecular beam epitaxy are reported. The positions of the NWs are intentionally determined by a direct implantation of Au with focused ion beams. This controlled arrangement in combination with a nanofocused X-ray beam allows the in-plane lattice parameter of single NWs to be probed, which is not possible for randomly grown NWs. Reciprocal space maps were collected at different heights along the NW to investigate the crystal structure. Simultaneously, substrate areas with different distances from the Au-implantation spots below the NWs were probed. Around the NWs, the data revealed a 0.4% decrease in the lattice spacing in the substrate compared with the expected unstrained value. This suggests the presence of a compressed region due to Au implantation. [ABSTRACT FROM AUTHOR]