Final Report Summary - COSPINNANO (Coherent spin manipulation in hybrid nanostructures)
The rapid development of novel nanoelectronic devices reaching beyond the limitations of traditional semiconductor based technologies is a major challenge in nowadays nanosciences. A special emphasis is put on the fabrication and investigation of hybrid nanostructures exploiting the complementary benefits of metallic, semiconducting, magnetic as well as the recently explored, low dimensional carbon based systems (carbon nanotubes, graphene). In the framework of the project various hybrid nanostructures defined by atomic scale self-assembly as well as by electron beam lithography are designed, fabricated and studied in transport experiments. This is essential in order to explore electron charge and spin dynamics for multifunctional applications such as fast switching elements, combined logical and storage devices and quantum dot based semiconductor spin qbits, representing the fundamental building blocks of quantum computation schemes implemented in a solid state environment.
In pursuit of high speed switching and/or memory elements overriding the downscaling limitations of nowadays CMOS technology, resistive switching in Ag2S based memristive nanojunction devices has been studied. By suitable sample preparation reproducible resistive switching and readout were achieved, where both the ON and OFF states are of metallic nature enabling fast operation at room temperature as demonstrated by the nanosecond switching times. Point contact Andreev reflection spectroscopy has been introduced to determine the size and transmission probabilities of the active volume of the devices which revealed a small number of highly transmitting, nanometer scale conducting channels with reduced but not completely dissolved junction area also in the OFF state. Studying the dynamics of resistive switchings in such metallic junctions with a diameter of 2-5 nm revealed that the resistance change upon a switching bias voltage pulse exhibits a strongly non-exponential behaviour yielding markedly different response times at different bias levels ranging from seconds to nanosecond timescales. The observed switching threshold voltages along with the ON and OFF state resistances are quantitatively understood by taking the local overheating of the junction volume and the resulting structural phase transition of the Ag2S matrix into account. These findings demonstrate the merits of Ag2S nanojunctions as nanometer-scale non-volatile memory cells with stable switching ratios, high endurance as well as fast response to write/erase, and an outstanding stability against read operations at technologically optimal bias and current levels whereas the essential switching characteristics can also be routinely optimized by suitable sample preparation and biasing schemes.
Carbon and semiconductor based hybrid nanocircuits relying on the creation and manipulation of coherent spin states require highly transmitting connecting leads which are compatible with metallic, superconducting and ferromagnetic terminals and, at the same time, with routine nanofabrication techniques. Graphene nanoribbons designed for such purposes have been fabricated and tested in the wide temperature range of 1 – 300 K. The 30 – 600 nm wide ribbons were made by the alternative techniques of carbothermal etching, electron beam lithography followed by oxygen plasma etching and by the oriented cutting of the graphene flakes by the sharp tip of an atomic force microscope. The obtained mobility values reaching up to 5000 cm2/Vs compile to the state of the art of the field and, along with the demonstrated gate-tunability of the electrochemical potential in the ribbons, enable the successful utilization of graphene nanoribbons as the basic building blocks of multifunctional nanodevices.
The quantum mechanical aspects of phase coherent electron transmission in such narrow ribbons were studied in collaboration with the Department of Theoretical Physics at the Host Institute. An experimental method for the detection of magnetic field induced bound states around an antidot in a graphene nanoribbon is proposed via measuring the ballistic two-terminal conductance. The effect of bound states on the two-terminal conductance is investigated in details, demonstrating the emergence of Breit-Wigner-like resonances in the conductance as a function of the Fermi level close to the energies of the bound states.
Alternative substrate materials yielding to better mechanical properties at improved electron mobility in graphene based multifunctional nanocircuits are of great interest. As an example, the combination of low-stress dielectric layers with graphene gate electrodes are expected to significantly decrease stress induced gate degradation and thus improve the reliability of non-volatile flash memory devices. For this purpose we studied the structural and magnetotransport characteristics of graphene nanodevices exfoliated onto Si/SiO2/SiNx heterostructures. We found that, while exhibiting better mechanical and chemical stability, the effect of non-stoichiometric SiNx on the charge carrier mobility of graphene is comparable to that of SiO2, qualifying SiNx as an ideal material platform for graphene based nanoelectromechanical applications.
As a first step toward the exploration of non-equilibrium spin correlation effects in graphene, the magnetotransport properties of micrometer-scale graphene Hall-bars mounted with superconducting terminals were studied in the quantum Hall regime. For this purpose various Nb, NbN, NbTi and NbTiN superconducting compositions were tested and optimized for the deposition of Ohmic contacts on graphene as well as for tunable superconducting transition temperature and critical magnetic field values. Exceeding the critical magnetic field of the superconducting current injecting terminals, a corresponding transition in the graphene sheet’s quantum Hall plateau structure was found, indicating the influence of Cooper-pair conductance on the quantum Hall edge states.
Spin-dependent transport phenomena in low-dimensional semiconductor systems have been studied in carbon doped p-GaAs/AlGaAs based quantum point contacts exhibiting strong spin-orbit interactions. The experiments contributed to the better understanding of the role of the confinement potential in the emergence of preferred spin orientations in the adjacent one-dimensional electronic sub-bands in the presence of a magnetic field. The effects of the random potential imperfections on the commonly observed anomalies in the one-dimensional quantized conductance have also been clarified.
In pursuit of high speed switching and/or memory elements overriding the downscaling limitations of nowadays CMOS technology, resistive switching in Ag2S based memristive nanojunction devices has been studied. By suitable sample preparation reproducible resistive switching and readout were achieved, where both the ON and OFF states are of metallic nature enabling fast operation at room temperature as demonstrated by the nanosecond switching times. Point contact Andreev reflection spectroscopy has been introduced to determine the size and transmission probabilities of the active volume of the devices which revealed a small number of highly transmitting, nanometer scale conducting channels with reduced but not completely dissolved junction area also in the OFF state. Studying the dynamics of resistive switchings in such metallic junctions with a diameter of 2-5 nm revealed that the resistance change upon a switching bias voltage pulse exhibits a strongly non-exponential behaviour yielding markedly different response times at different bias levels ranging from seconds to nanosecond timescales. The observed switching threshold voltages along with the ON and OFF state resistances are quantitatively understood by taking the local overheating of the junction volume and the resulting structural phase transition of the Ag2S matrix into account. These findings demonstrate the merits of Ag2S nanojunctions as nanometer-scale non-volatile memory cells with stable switching ratios, high endurance as well as fast response to write/erase, and an outstanding stability against read operations at technologically optimal bias and current levels whereas the essential switching characteristics can also be routinely optimized by suitable sample preparation and biasing schemes.
Carbon and semiconductor based hybrid nanocircuits relying on the creation and manipulation of coherent spin states require highly transmitting connecting leads which are compatible with metallic, superconducting and ferromagnetic terminals and, at the same time, with routine nanofabrication techniques. Graphene nanoribbons designed for such purposes have been fabricated and tested in the wide temperature range of 1 – 300 K. The 30 – 600 nm wide ribbons were made by the alternative techniques of carbothermal etching, electron beam lithography followed by oxygen plasma etching and by the oriented cutting of the graphene flakes by the sharp tip of an atomic force microscope. The obtained mobility values reaching up to 5000 cm2/Vs compile to the state of the art of the field and, along with the demonstrated gate-tunability of the electrochemical potential in the ribbons, enable the successful utilization of graphene nanoribbons as the basic building blocks of multifunctional nanodevices.
The quantum mechanical aspects of phase coherent electron transmission in such narrow ribbons were studied in collaboration with the Department of Theoretical Physics at the Host Institute. An experimental method for the detection of magnetic field induced bound states around an antidot in a graphene nanoribbon is proposed via measuring the ballistic two-terminal conductance. The effect of bound states on the two-terminal conductance is investigated in details, demonstrating the emergence of Breit-Wigner-like resonances in the conductance as a function of the Fermi level close to the energies of the bound states.
Alternative substrate materials yielding to better mechanical properties at improved electron mobility in graphene based multifunctional nanocircuits are of great interest. As an example, the combination of low-stress dielectric layers with graphene gate electrodes are expected to significantly decrease stress induced gate degradation and thus improve the reliability of non-volatile flash memory devices. For this purpose we studied the structural and magnetotransport characteristics of graphene nanodevices exfoliated onto Si/SiO2/SiNx heterostructures. We found that, while exhibiting better mechanical and chemical stability, the effect of non-stoichiometric SiNx on the charge carrier mobility of graphene is comparable to that of SiO2, qualifying SiNx as an ideal material platform for graphene based nanoelectromechanical applications.
As a first step toward the exploration of non-equilibrium spin correlation effects in graphene, the magnetotransport properties of micrometer-scale graphene Hall-bars mounted with superconducting terminals were studied in the quantum Hall regime. For this purpose various Nb, NbN, NbTi and NbTiN superconducting compositions were tested and optimized for the deposition of Ohmic contacts on graphene as well as for tunable superconducting transition temperature and critical magnetic field values. Exceeding the critical magnetic field of the superconducting current injecting terminals, a corresponding transition in the graphene sheet’s quantum Hall plateau structure was found, indicating the influence of Cooper-pair conductance on the quantum Hall edge states.
Spin-dependent transport phenomena in low-dimensional semiconductor systems have been studied in carbon doped p-GaAs/AlGaAs based quantum point contacts exhibiting strong spin-orbit interactions. The experiments contributed to the better understanding of the role of the confinement potential in the emergence of preferred spin orientations in the adjacent one-dimensional electronic sub-bands in the presence of a magnetic field. The effects of the random potential imperfections on the commonly observed anomalies in the one-dimensional quantized conductance have also been clarified.