Thermodynamic magnetization of a strongly correlated two-dimensional electron system

We measure thermodynamic magnetization of a low-disordered, strongly correlated two-dimensional electron system in silicon. Pauli spin susceptibility is observed to grow critically at low electron densities—behavior that is characteristic of the existence of a phase transition. A new, parameter-free method is used to directly determine the spectrum characteristics (Landé g-factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the inter-level interactions between quasiparticles are avoided. It turns out that, unlike in the Stoner scenario, the critical growth of the spin susceptibility originates from the dramatic enhancement of the effective mass, while the enhancement of the g-factor is weak and practically independent of the electron density.     

Magnetic-Field-Induced Semimetal-Insulator-like Transition in Highly Oriented Pyrolitic Graphite

We report the extraordinarily large positive magnetoresistances （MR, 69400% at 4.5K under a magnetic field of 8.15 T）, de Hass-van Alphen oscillations effect at 10 K and the semimetal-Jnsulator-like transition in a wide range of temperature in highly oriented pyrolitic graphite （HOPG）. Besides a dominating ordinary MR （OMR） mechanism in the free-electron mode, it is realized from qualitative analysis that the Coulomb interacting quasiparticles within graphite layers play some roles. However it is difficult to associate the transition with the simple OMR theory. In order to investigate the possible origins of the transition, further analysis is carried out. It is revealed that the magnetic-field-induced behaviour is responsible for the semimetal-insulator-like transitions in HOPG.     

High Field Electrical Conduction in Pre-Formed A1-ZnS-AI Thin Films in Metal-Insulator-Metal Devices

The high field electrical conduction mechanism for the widely used ZnS thin films in the microelectronic industry is investigated. Experimental data on the dc conduction as a function of the applied bias for the A1-ZnS-A1 devices is carefully compared with the theoretical equations given by Schottky and Poole-Freukel. The resu/ts yield the value of the coefficient of the barrier lowering compatible with the Schottky theory rather than the Poole-Frenkel theory, which are also in agreement with the results reported earlier by Maekawa [Phys. Rev. Lett. 24 （1970） 1175]     

Inversion layer resistance in silicon inversion layer solar cells

The inversion layer resistance is very important for metal-insulator-semiconductor inversion layer (MIS/IL) solar cells, and usually it is the main part of the series resistance. It is found that the inversion layer resistance and the junction depth are determined by the operating voltage for an MIS/IL solar cell. On the basis of MIS theory, a general relationship between the operating voltage and the inversion layer resistance (and the junction depth) has been investigated. Practical computations have been done for MIS/IL solar cells with a silicon nitride insulator layer. It is found that the inversion layer resistance has a minimum value for operating voltage near 0.4 V, and the junction depth decreases monotonically with the increase of the operating voltage.     

The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.     

Influence of strain on the tunneling magnetoresistance in diluted magnetic semiconductor trilayer and double barrier structures

The influence of strain on hole tunneling in trilayer and double barrier structures made of two diluted magnetic semiconductors (DMS) (Ga, Mn)As, separated by a thin layer of non-magnetic AlAs is investigated theoretically. The strain is caused by lattice mismatch as the whole structure is grown on a (In_0.15Ga_0.85)As buffer layer. The tensile strain makes the easy axis of magnetization orient along the growth direction. We found that biaxial strain has a strong influence on the tunneling current because the spin splitting at is comparable to the Fermi energy E_F. Tensile strain decreases the tunneling magnetoresistance ratio.     

Effects of large cation size-disorder on the magnetic properties of the rare earth cobaltates, Ln0.5Ba0.5CoO3

Magnetic and electrical properties of Ln_0.5Ba_0.5CoO_3−δ with Ln=Dy and Er have been investigated to examine the effects of large cation size-disorder. While the Dy compound shows the small magnetic anomaly around 290 K just as the Gd derivative, the Er compound is essentially paramagnetic due to the large cation size-disorder. Compositions with the same average A-site cation radii as Dy_0.5Ba_0.5CoO_2.91 and Er_0.5Ba_0.5CoO_2.9, but with smaller size-disorder, show progressive evolution of ferromagnetism and metallic properties with decreasing disorder.     

Ground state magnetic properties of ultrathin fcc Fe and Co films on Cu(001) surfaces

The oxidation characteristics of silicon implanted with a low dose of nitrogen (1–3×10¹⁵cm^–2) have been studied for dry oxidation conditions at 1020°C. The wafers were subjected to a pre-oxidation annealing. Complete inhibition of the oxide growth occurs in the initial stage of oxidation, while the oxidation rate for prolonged oxidation is identical to that for pure silicon. The oxidation resistance increases with the implantation dose. The resistance is attributed to the formation of a nitrogen-rich surface film during annealing. This layer, which consists of only a few monolayers, is presumably composed of oxynitride. The electrical characteristics of MOS capacitors formed on implanted wafers show that the interface state density is not significantly increased by the low-dose N implantation.     

Surface acoustic wave propagation and inhomogeneities in low-density two-dimensional electron systems near the metal-insulator transition

We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime (<10¹⁰ cm⁻²) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES, which become very important at low density. This has implications for the putative metal-insulator transition in two dimensions.     

C-AFM-based thickness determination of thin and ultra-thin SiO2 films by use of different conductive-coated probe tips

The influence of the probe tip type on the electrical oxide thickness result was researched for four differently coated conductive tip types using SiO₂ (oxide) films with optical thickness of 1.7-8.3 nm. For this purpose, conductive atomic force microscopy (C-AFM) was used to measure more than 7200 current-voltage (IV) curves. The electrical oxide thickness was determined on a statistical basis from the IV-curves using a recently published tunnelling model for C-AFM application. The model includes parameters associated with the probe tip types used. The evolution of the tip parameters is described in detail. For the theoretical tip parameters, measured and calculated IV-curves showed excellent agreement and the electrical oxide thickness versus the optical oxide thickness showed congruent behaviour, independent of the tip type. However, differences in the electrical oxide thickness were observed for the different tip types. The theoretical parameters were modified experimentally in order to reduce these differences. Theoretical and experimental tip parameters were compared and their effect on the differences in the electrical oxide thickness is discussed for the different tip types. Overall, it is shown that the proposed model provides a comprehensive framework for determining the electrical oxide thickness using C-AFM, for a wide range of oxide thicknesses and for differently coated conductive tips.