Investigation of thin films for fabrication of Nb/AlN/NbN tunnel junctions and microstrip lines of NbTiN-SiO2-Al.

The surface of thin films of Nb, Al, NbTiN, SiO2, Al2O3 is investigated in this work. These films are necessary for the fabrication of high-sensitive devices of THz range. The fabrication processes of such devices are described briefly. All films were fabricated using a Kurt J. Lesker magnetron sputtering system. The study of the film surface roughness was carried out using a Bruker Ikon atomic force microscope. The surface quality of films is determined not only deposition mode, but plasma etching process also. The best values of the root-mean-square deviation of the surface profile Rq = 2 nm were obtained for the used NbTiN film with a thickness of 325 nm. Thin Al-layers that is used for tunnel barrier formation is studied. It is shown than Al films with a thickness of more than 6 nm are already continuous. The surface roughness of the single-layer and multilayer films has been studied.

Strain-spectroscopy of strongly interacting defects in superconducting qubits

The proper functioning of some micro-fabricated novel quantum devices, such as superconducting resonators and qubits, is severely affected by the presence of parasitic structural material defects known as tunneling two-level-systems (TLS). Recent experiments have reported unambiguous evidence of the strong interaction between individual (coherent) TLS using strain-assisted spectroscopy. This work provides an alternative and simple theoretical insight that illustrates how to obtain the spectral response of such strongly interacting defects residing inside the amorphous tunnel barrier of a qubit's Josephson junction. Moreover, the corresponding spectral signatures obtained here may serve to quickly and efficiently elucidate the actual state of these interacting TLS in experiments based on strain- or electric-field spectroscopy.

Rare earth nitrides and their applications in magnetic tunnel junctions

<p>In this thesis, we investigate the rare earth nitrides, a family of materials containing many intrinsic ferromagnetic semiconductors, with a particular focus on GdN and SmN.We investigate the rare earth nitride formation reaction, explore some properties of GdN and SmN, and finally manufacture and measure magnetic tunnel junctions which incorporate rare earth nitrides. The investigations of the reaction and properties of the materials are used to improve and understand the magnetic tunnel junctions. All samples and devices are grown at room temperature, giving polycrystalline rare earth nitride films. We show that a rare earth surface can catalytically break theN2 molecule at ambient temperature and low pressures. We follow the nitrogen reacting with the rare earth to form a rare earth nitride in real time via conductance measurements. By comparing the N2 cracking, reaction, and diffusion at both a RE and a REN surface we propose a pressure range in which the nitrogen content in SmN can be manipulated and conclude that the nitrogen in the top monolayers in a SmN film is mobile. In the investigation of GdN and SmN, we find that the conductivity of SmN follows the same behaviour as GdN when changing the N2 pressure during deposition. We follow the conductance change in SmN during deposition and propose a minimum thickness for room temperature deposited SmN films for consistent conductivity measurements. We report structural and magnetic changes in GdN which has been exposed to N-ions. We also present data on materials making ohmic contact to both GdN and SmN. Finally, we report the manufacturing and investigation of magnetic tunnel junctions using GdN and SmN electrodes with a GaN tunnel barrier. A new pattern design produces 20 devices, in a single deposition, which show consistent behaviour and expands on previous work on this topic. The main focus of the investigation is the J-V characteristics of the magnetic tunnel junctions which shows clear non-linear behaviour arising from tunnelling through the GaN. A Simmons fit to the J-V characteristics yields a barrier height of 0:8 eV and barrier thicknesses close to experimentally determined thicknesses. The J-V characteristics are investigated with changing temperature and changing applied magnetic field to investigate the effect of the ferromagnetism of the GdN and SmN electrodes. The tunnel magnetoresistance (TMR) of the devices show two contributions, a low-temperature TMR contribution and a 50K TMR contribution, and the maximum TMR for all devices are between 100% to 600%. The devices can withstand current densities up to 4000A/cm² and voltages up to 5V which is promising for a wide range of future applications.</p>

Rare earth nitrides and their applications in magnetic tunnel junctions

<p>In this thesis, we investigate the rare earth nitrides, a family of materials containing many intrinsic ferromagnetic semiconductors, with a particular focus on GdN and SmN.We investigate the rare earth nitride formation reaction, explore some properties of GdN and SmN, and finally manufacture and measure magnetic tunnel junctions which incorporate rare earth nitrides. The investigations of the reaction and properties of the materials are used to improve and understand the magnetic tunnel junctions. All samples and devices are grown at room temperature, giving polycrystalline rare earth nitride films. We show that a rare earth surface can catalytically break theN2 molecule at ambient temperature and low pressures. We follow the nitrogen reacting with the rare earth to form a rare earth nitride in real time via conductance measurements. By comparing the N2 cracking, reaction, and diffusion at both a RE and a REN surface we propose a pressure range in which the nitrogen content in SmN can be manipulated and conclude that the nitrogen in the top monolayers in a SmN film is mobile. In the investigation of GdN and SmN, we find that the conductivity of SmN follows the same behaviour as GdN when changing the N2 pressure during deposition. We follow the conductance change in SmN during deposition and propose a minimum thickness for room temperature deposited SmN films for consistent conductivity measurements. We report structural and magnetic changes in GdN which has been exposed to N-ions. We also present data on materials making ohmic contact to both GdN and SmN. Finally, we report the manufacturing and investigation of magnetic tunnel junctions using GdN and SmN electrodes with a GaN tunnel barrier. A new pattern design produces 20 devices, in a single deposition, which show consistent behaviour and expands on previous work on this topic. The main focus of the investigation is the J-V characteristics of the magnetic tunnel junctions which shows clear non-linear behaviour arising from tunnelling through the GaN. A Simmons fit to the J-V characteristics yields a barrier height of 0:8 eV and barrier thicknesses close to experimentally determined thicknesses. The J-V characteristics are investigated with changing temperature and changing applied magnetic field to investigate the effect of the ferromagnetism of the GdN and SmN electrodes. The tunnel magnetoresistance (TMR) of the devices show two contributions, a low-temperature TMR contribution and a 50K TMR contribution, and the maximum TMR for all devices are between 100% to 600%. The devices can withstand current densities up to 4000A/cm² and voltages up to 5V which is promising for a wide range of future applications.</p>

Local Electric Property Modification of Ferroelectric Tunnel Junctions Induced by Variation of Polarization Charge Screening Conditions under Measurement with Scanning Probe Techniques

Scanning tunneling spectroscopy in ultrahigh vacuum conditions and conductive atomic-force microscopy in ambient conditions were used to study local electroresistive properties of ferroelectric tunnel junctions SrTiO3/La0.7Sr0.3MnO3/BaTiO3. Interestingly, experimental current-voltage characteristics appear to strongly depend on the measurement technique applied. It was found that screening conditions of the polarization charges at the interface with a top electrode differ for two scanning probe techniques. As a result, asymmetry of the tunnel barrier height for the opposite ferroelectric polarization orientations may be influenced by the method applied to study the local tunnel electroresistance. Our observations are well described by the theory of electroresistance in ferroelectric tunnel junctions. Based on this, we reveal the main factors that influence the polarization-driven local resistive properties of the device under study. Additionally, we propose an approach to enhance asymmetry of ferroelectric tunnel junctions during measurement. While keeping the high locality of scanning probe techniques, it helps to increase the difference in the value of tunnel electroresistance for the opposite polarization orientations.

Indium-contacted van der Waals gap tunneling spectroscopy for van der Waals layered materials

The electrical phase transition in van der Waals (vdW) layered materials such as transition-metal dichalcogenides and Bi2Sr2CaCu2O8+x (Bi-2212) high-temperature superconductor has been explored using various techniques, including scanning tunneling and photoemission spectroscopies, and measurements of electrical resistance as a function of temperature. In this study, we develop one useful method to elucidate the electrical phases in vdW layered materials: indium (In)-contacted vdW tunneling spectroscopy for 1T-TaS2, Bi-2212 and 2H-MoS2. We utilized the vdW gap formed at an In/vdW material interface as a tunnel barrier for tunneling spectroscopy. For strongly correlated electron systems such as 1T-TaS2 and Bi-2212, pronounced gap features corresponding to the Mott and superconducting gaps were respectively observed at T = 4 K. We observed a gate dependence of the amplitude of the superconducting gap, which has potential applications in a gate-tunable superconducting device with a SiO2/Si substrate. For In/10 nm-thick 2H-MoS2 devices, differential conductance shoulders at bias voltages of approximately ± 0.45 V were observed, which were attributed to the semiconducting gap. These results show that In-contacted vdW gap tunneling spectroscopy in a fashion of field-effect transistor provides feasible and reliable ways to investigate electronic structures of vdW materials.