Ján Dérer

InGaAs/GaAs metal-oxide-semiconductor heterostructure field-effect transistors with oxygen-plasma oxide and Al2O3 double-layer insulator

Surface condition before an insulator deposition is the key issue for the preparation of reliable GaAs-based metal-oxide-semiconductor (MOS) devices. This study presents the preparation and properties of InGaAs/GaAs MOS structures with a double-layer insulator consisting of an oxygen-plasma oxide covered by Al2O3. The structures were oxidized during 75 s and 150 s. Static measurements yielded a saturation drain current of ∼250 mA/mm at VG = 1 V. Capacitance measurements showed improved performance in the depletion region compared with the structures without the double-layer insulator. Trapping effects were investigated by conductance vs. frequency measurements. The trap state density was in order of 1011 cm−2·eV−1 with a continuous decrease with increased trap energy. The carrier mobility evaluation showed peak values of 3950 cm2/V·s for 75 s and 4570 cm2/V·s for 150 s oxidation times with the sheet charge density ≅2 × 1012 cm−2. The results demonstrate great potential of the procedure that was used to pr...

Scanning vector Hall probe microscope

We present a scanning vector Hall probe microscope for imaging the entire magnetic field vector in close proximity to magnetic and superconducting samples. The microscope combines a large scanned area and a high space resolution of the magnetic field vector measured. A special feature of the equipment is a vacuum-tight sample space connected with a moving system via a flexible metal bellows. The microscope is based on a vector Hall sensor that consists of three separate Hall probes of an active area 5×5 μm2, patterned on three sides of a GaAs pyramid. The top of the pyramid serves as a tunneling contact and helps to control the sensor–sample separation. The sensor and the sample are placed in a helium cryostat with a temperature control in the range 10–300 K. The sensor scans an area up to 5×5 mm2 in the whole temperature interval with a spatial resolution ∼5 μm.

Ferromagnetic resonance study of exchange and dipolar interactions in discontinuous multilayers

We studied interlayer and intergranular exchange and dipolar coupling in (Fe97Si3∕SiO2)5 discontinuous multilayer (DM) by means of ferromagnetic resonance (FMR). Due to a strong ferromagnetic exchange coupling (J∼−3ergs∕cm2), the precessional motions of magnetic moments of the granules are coupled, which results in acoustic and optical modes. Moreover, there is a notable FMR line splitting in the optical mode under the external field normal to the DM, which is explained by an interlayer dipolar coupling, only possible for the DMs. The detailed structural analysis and electrical measurements confirmed that the metal nanoparticles were separated by an insulating matrix, and the single-electron transport phenomena (Coulomb blockade and tunneling magnetoresistivity) were observed at low temperatures (77–100K).

Vertically stacked (Nb/Si)*10 Josephson junction

We report on “Josephson superlattice”—a stack of junctions consisting of a multilayer [Nb(7nm)/Si(2nm)]*10 deposited by dc sputtering. The silicon barriers are amorphous and Nb sublayers are polycrystalline. The stacked junctions exhibit both ac and dc Josephson effects. In the IC(B) diffraction pattern there is an extra periodicity of about 2–3 G in addition to larger period of about 21 G. The shape of IC(B) curve resembles the characteristics obtained on two JJs placed close to each other and connected in parallel. The possible explanation of this fine structure of the interference pattern is shortly discussed.

Resistive switching in nonplanar HfO2-based structures with variable series resistance

The authors have prepared μm-sized HfO2-based nonplanar resistive switching cells with TiN and Pt electrodes. The structures exhibited stable resistive switching loops with low resistance state current below 1 mA. The influence of series resistance of the TiN strip electrode on the switching characteristics was measured and simulated. It was shown that the series resistance reduces the low state resistance current as well as high to low resistance state ratio.

Low-temperature studies of magnetic Fe/FeSi multilayers

Abstract Magnetic coupling and magnetoresistance of Fe/FeSi multilayers were investigated in the range 4.2–300 K. From M(T,H) data and FMR measurements the magnetization arrangement of Fe sublayers is discussed. GMR contribution increases with decreasing temperature, but its value is very low.

Structural and electrical properties of superconducting Nb/Si multilayers

We have found the deposition technology at which the interface roughness of Nb/Si multilayer is strongly correlated and decreases with deposited layer number. On the basis of our Nb/Si multilayer we succeeded to prepare ten-fold stacked Josephson junction (JJ). Basic electric properties of stacked JJ are presented.