MBE Growth of Al/InAs and Nb/InAs Superconducting Hybrid Nanowire Structures

We report on \textit{in situ} growth of crystalline Al and Nb shells on InAs nanowires. The nanowires are grown on Si(111) substrates by molecular beam epitaxy (MBE) without foreign catalysts in the vapor-solid mode. The metal shells are deposited by electron-beam evaporation in a metal MBE. High quality supercondonductor/semiconductor hybrid structures such as Al/InAs and Nb/InAs are of interest for ongoing research in the fields of gateable Josephson junctions and quantum information related research. Systematic investigations of the deposition parameters suitable for metal shell growth are conducted. In case of Al, the substrate temperature, the growth rate and the shell thickness are considered. The substrate temperature as well as the angle of the impinging deposition flux are explored for Nb shells. The core-shell hybrid structures are characterized by electron microscopy and x-ray spectroscopy. Our results show that the substrate temperature is a crucial parameter in order to enable the deposition of smooth Al layers. Contrary, Nb films are less dependent on substrate temperature but strongly affected by the deposition angle. At a temperature of 200{\deg}C Nb reacts with InAs, dissolving the nanowire crystal. Our investigations result in smooth metal shells exhibiting an impurity and defect free, crystalline superconductor/InAs interface. Additionally, we find that the superconductor crystal structure is not affected by stacking faults present in the InAs nanowires.Comment: 8 pages, 10 figures, 1 tabl

Effect of gallium termination on InGaAs wetting layer properties in droplet epitaxy InGaAs quantum dots

Self-assembled quantum dots based on III-V semiconductors have excellent properties for applications in quantum optics. However, the presence of a 2D wetting layer which forms during the Stranski-Krastanov growth of quantum dots can limit their performance. Here, we investigate wetting layer formation during quantum dot growth by the alternative droplet epitaxy technique. We use a combination of photoluminescence excitation spectroscopy, lifetime measurements, and transmission electron microscopy to identify the presence of an InGaAs wetting layer in these droplet epitaxy quantum dots, even in the absence of distinguishable wetting layer photoluminescence. We observe that increasing the amount of Ga deposited on a GaAs (100) surface prior to the growth of InGaAs quantum dots leads to a significant reduction in the emission wavelength of the wetting layer to the point where it can no longer be distinguished from the GaAs acceptor peak emission in photoluminescence measurements

Phase-Pure Wurtzite GaAs Nanowires Grown by Self-Catalyzed Selective Area Molecular Beam Epitaxy for Advanced Laser Devices and Quantum Disks

The control of the crystal phase in self-catalyzed nanowires (NWs) is one of the central remaining open challenges in the research field of III/V semiconductor NWs. While several groups analyzed and revealed the growth dynamics, no experimental growth scheme has been verified yet, which reproducibly ensures the phase purity of binary self-catalyzed grown NWs. Here, we demonstrate the advanced control of self-catalyzed molecular beam epitaxy of GaAs NWs with up to a grade of 100% wurtzite (WZ) phase purity. The evolution of the most important properties during the growth, namely, the contact angle of the Ga droplet, the NW length, and the diameter is analyzed by scanning electron microscopy and transmission electron microscopy. Based on these results, we developed a comprehensive NW growth model for calculating the time-dependent evolution of the Ga droplet contact angle. Using this model, the Ga flux was dynamically modified during the growth to control and stabilize the contact angle in a certain range favoring the growth of phase-pure GaAs NWs. Although focusing on the self-catalyzed growth of WZ GaAs NWs, our model is also applicable to achieve phase-pure zinc blende (ZB) NWs and can be easily generalized to other III/V compounds. The self-catalyzed growth of such NWs may pave the way for substantial improvement of GaAs NW laser devices, the controlled growth of WZ/ZB quantum disks, and novel heterostructured core/multishell NW systems with a pristine crystalline order

Hard-Gap Spectroscopy in a Self-Defined Mesoscopic InAs/Al Nanowire Josephson Junction

Superconductor-semiconductor-nanowire hybrid structures can serve as versatile building blocks to realize Majorana circuits or superconducting qubits based on quantized levels such as Andreev qubits. For all these applications, it is essential that the superconductor-semiconductor interface is as clean as possible. Furthermore, the shape and dimensions of the superconducting electrodes need to be precisely controlled. We fabricated self-defined InAs/Al core-shell nanowire junctions by a fully in-situ approach, which meet all these criteria. Transmission electron microscopy measurements confirm the sharp and clean interface between the nanowire and the in-situ deposited Al electrodes that are formed by means of shadow evaporation. Furthermore, we report on tunnel spectroscopy, gate, and magnetic field-dependent transport measurements. The achievable short junction lengths, the observed hard gap, and the magnetic field robustness make this hybrid structure very attractive for applications that rely on a precise control of the number of subgap states, like Andreev qubits or topological systems