Growth dynamics of InAs/InP nanowire heterostructures by Au-assisted chemical beam epitaxy

Self-Consistent Model for the Compositional Profiles in Vapor-Liquid-Solid III-V Nanowire Heterostructures Based on Group V Interchange

Due to the very efficient relaxation of elastic stress on strain-free sidewalls, III-V nanowires offer almost unlimited possibilities for bandgap engineering in nanowire heterostructures by using material combinations that are attainable in epilayers. However, axial nanowire heterostructures grown using the vapor-liquid-solid method often suffer from the reservoir effect in a catalyst droplet. Control over the interfacial abruptness in nanowire heterostructures based on the group V interchange is more difficult than for group-III-based materials, because the low concentrations of highly volatile group V atoms cannot be measured after or during growth. Here, we develop a self-consistent model for calculations of the coordinate-dependent compositional profiles in the solid and liquid phases during the vapor-liquid-solid growth of the axial nanowire heterostructure Ax0B1-x0C/Ax1B1-x1C with any stationary compositions x0 and x1. The only assumption of the model is that the growth rates of both binaries AC and BC are proportional to the concentrations of group V atoms A and B in a catalyst droplet, requiring high enough supersaturations in liquid phase. The model contains a minimum number of parameters and fits quite well the data on the interfacial abruptness across double heterostructures in GaP/GaAsxP1-x/GaP nanowires. It can be used for any axial III-V nanowire heterostructures obtained through the vapor-liquid-solid method. It forms a basis for further developments in modeling the complex growth process and suppression of the interfacial broadening caused by the reservoir effect.

Quantum-Dot Single-Electron Transistors as Thermoelectric Quantum Detectors at Terahertz Frequencies

Low-dimensional nanosystems are promising candidates for manipulating, controlling, and capturing photons with large sensitivities and low noise. If quantum engineered to tailor the energy of the localized electrons across the desired frequency range, they can allow devising of efficient quantum sensors across any frequency domain. Here, we exploit the rich few-electron physics to develop millimeter-wave nanodetectors employing as a sensing element an InAs/InAs_0.3P_0.7 quantum-dot nanowire, embedded in a single-electron transistor. Once irradiated with light, the deeply localized quantum element exhibits an extra electromotive force driven by the photothermoelectric effect, which is exploited to efficiently sense radiation at 0.6 THz with a noise equivalent power <8 pWHz^-1/2 and almost zero dark current. The achieved results open intriguing perspectives for quantum key distributions, quantum communications, and quantum cryptography at terahertz frequencies.

Heat Driven Transport in Serial Double Quantum Dot Devices

Studies of thermally induced transport in nanostructures provide access to an exciting regime where fluctuations are relevant, enabling the investigation of fundamental thermodynamic concepts and the realization of thermal energy harvesters. We study a serial double quantum dot formed in an InAs/InP nanowire coupled to two electron reservoirs. By means of a specially designed local metallic joule-heater, the temperature of the phonon bath in the vicinity of the double quantum dot can be enhanced. This results in phonon-assisted transport, enabling the conversion of local heat into electrical power in a nanosized heat engine. Simultaneously, the electron temperatures of the reservoirs are affected, resulting in conventional thermoelectric transport. By detailed modeling and experimentally tuning the interdot coupling, we disentangle both effects. Furthermore, we show that phonon-assisted transport is sensitive to excited states. Our findings demonstrate the versatility of our design to study fluctuations and fundamental nanothermodynamics.

Self-Catalyzed InSb/InAs Quantum Dot Nanowires

The nanowire platform offers great opportunities for improving the quality and range of applications of semiconductor quantum wells and dots. Here, we present the self-catalyzed growth of InAs/InSb/InAs axial heterostructured nanowires with a single defect-free InSb quantum dot, on Si substrates, by chemical beam epitaxy. A systematic variation of the growth parameters for the InAs top segment has been investigated and the resulting nanowire morphology analyzed. We found that the growth temperature strongly influences the axial and radial growth rates of the top InAs segment. As a consequence, we can reduce the InAs shell thickness around the InSb quantum dot by increasing the InAs growth temperature. Moreover, we observed that both axial and radial growth rates are enhanced by the As line pressure as long as the In droplet on the top of the nanowire is preserved. Finally, the time evolution of the diameter along the entire length of the nanowires allowed us to understand that there are two In diffusion paths contributing to the radial InAs growth and that the interplay of these two mechanisms together with the total length of the nanowires determine the final shape of the nanowires. This study provides insights in understanding the growth mechanisms of self-catalyzed InSb/InAs quantum dot nanowires, and our results can be extended also to the growth of other self-catalyzed heterostructured nanowires, providing useful guidelines for the realization of quantum structures with the desired morphology and properties.

Orbital Tuning of Tunnel Coupling in InAs/InP Nanowire Quantum Dots

We report results on the control of barrier transparency in InAs/InP nanowire quantum dots via the electrostatic control of the device electron states. Recent works demonstrated that barrier transparency in this class of devices displays a general trend just depending on the total orbital energy of the trapped electrons. We show that a qualitatively different regime is observed at relatively low filling numbers, where tunneling rates are rather controlled by the axial configuration of the electron orbital. Transmission rates versus filling are further modified by acting on the radial configuration of the orbitals by means of electrostatic gating, and the barrier transparency for the various orbitals is found to evolve as expected from numerical simulations. The possibility to exploit this mechanism to achieve a controlled continuous tuning of the tunneling rate of an individual Coulomb blockade resonance is discussed.

Microwave-Assisted Tunneling in Hard-Wall InAs/InP Nanowire Quantum Dots

With downscaling of electronic circuits, components based on semiconductor quantum dots are assuming increasing relevance for future technologies. Their response under external stimuli intrinsically depend on their quantum properties. Here we investigate single-electron tunneling in hard-wall InAs/InP nanowires in the presence of an off-resonant microwave drive. Our heterostructured nanowires include InAs quantum dots (QDs) and exhibit different tunnel-current regimes. In particular, for source-drain bias up to few mV Coulomb diamonds spread with increasing contrast as a function of microwave power and present multiple current polarity reversals. This behavior can be modelled in terms of voltage fluctuations induced by the microwave field and presents features that depend on the interplay of the discrete energy levels that contribute to the tunneling process.

Highly symmetric and tunable tunnel couplings in InAs/InP nanowire heterostructure quantum dots

We present a comprehensive electrical characterization of an InAs/InP nanowire (NW) heterostructure, comprising of two InP barriers forming a quantum dot (QD), two adjacent lead segments and two metallic contacts. We demonstrate how to extract valuable quantitative information of the QD. The QD shows very regular Coulomb blockade resonances over a large gate voltage range. By analyzing the resonance line shapes, we map the evolution of the tunnel couplings from the few to the many electron regime, with electrically tunable tunnel couplings from <1 μeV to >600 μeV, and a transition from the temperature to the lifetime broadened regime. The InP segments form tunnel barriers with almost fully symmetric tunnel couplings and a barrier height of ∼350 meV. All of these findings can be understood in great detail based on the deterministic material composition and geometry. Our results demonstrate that integrated InAs/InP QDs provide a promising platform for electron tunneling spectroscopy in InAs NWs, which can readily be contacted by a variety of superconducting materials to investigate subgap states in proximitized NW regions, or be used to characterize thermoelectric nanoscale devices in the quantum regime.

Thermoelectric Conversion at 30 K in InAs/InP Nanowire Quantum Dots

We demonstrate high-temperature thermoelectric conversion in InAs/InP nanowire quantum dots by taking advantage of their strong electronic confinement. The electrical conductance G and the thermopower S are obtained from charge transport measurements and accurately reproduced with a theoretical model accounting for the multilevel structure of the quantum dot. Notably, our analysis does not rely on the estimate of cotunnelling contributions, since electronic thermal transport is dominated by multilevel heat transport. By taking into account two spin-degenerate energy levels we are able to evaluate the electronic thermal conductance K and investigate the evolution of the electronic figure of merit ZT as a function of the quantum dot configuration and demonstrate ZT ≈ 35 at 30 K, corresponding to an electronic efficiency at maximum power close to the Curzon-Ahlborn limit.