Effect of stacked dielectric with high dielectric constant and surface modification on current enhancement in pentacene thin-film transistors

The effect of a stacked dielectric has been studied on pentacene thin-film transistors (TFTs) with respect to the current enhancement, the crystalline polymorph, and the structural change of the film. Here we show that the performance improvement of the device is successfully achieved by the dielectric effects of the high dielectric constant and the surface modification in hybrid dielectric configuration. The systematic analysis on the device feature governed by the interfacial property was carried out for a hybrid structured insulator system using SiO₂ and cross-linked (C-L) polyvinyl alcohol (PVA), including the surface modified layer of dilute polymethyl methacrylate (PMMA). Through thickness combinations of bilayer dielectrics with low-k SiO₂ and high-k PVA, the device also exhibits noticeable enhancement of the current drivability up to the current level of 94 μA at a practical gate bias of ?30 V. Moreover, we present the effect of a surface-modified layer with dilute PMMA. After the formation of ultra-thin PMMA layer in a bilayer insulator, the organic dielectric shows an effectively changed surface property into hydrophobicity even on a strong hydroxyl-rich dielectric surface, resulting in the distinct increase of structural order in the film due to the reduction of surface free energy.     

Optical spectroscopy of electronic surface states

In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.     

Time-resolved coherent spectroscopy of surface states

Received: 4 December 1998     

Diffusion constant and surface states of positrons in metals

Positron lifetime spectra and angular correlation curves for seven fine-grained powders of Fe, Co, Ni, and W are analyzed. From the lifetime data, the positron diffusion constant in metals atT=300°K was found to beD ₊=(1.0±0.5)×10^?2 cm² sec^?1. Evidence is presented that positrons are trapped in metal surface states.     

Levitating states of superconducting rings in the field of a fixed ring with constant current

We consider the possibility of designing a plasma trap with a magnetic system formed by super-conducting rings and coils levitating in the field of a fixed coaxial coil carrying constant current. An analytic dependence of the potential energy of such a system with one or two levitating superconducting rings having trapped preset magnetic fluxes on their coordinates in the uniform gravitational field is obtained in the thin ring approximation. Calculations performed in the Mathcad system show that equilibrium states of such a system exist for certain values of parameters. Levitating states of a single superconducting ring and two superconducting rings in the field of the coil with constant current are observed experimentally in positions corresponding to calculated values.     

Photoacoustic spectroscopy of surface defects states of semiconductor samples

This paper presents both theoretical and experimental issues connected with measurements and numerical analysis of the microphone amplitude and phase photoacoustic spectra of semiconductor samples exhibiting surface absorption connected with defects states located on their surfaces. The analytical model of surface absorption in semiconductors is described and the results of computations are compared with experimental amplitude and phase spectra for Zn_0.965Be_0.035Se crystal samples. This paper shows the importance of the phase spectra for the proper interpretation of the PA (photoacoustic) results.     

Electronic surface states investigated by means of photoemission spectroscopy

Ultraviolet photoemission spectroscopy (UPS) as an investigative method for surface states is presented. The measured energy distribution curve (EDC) furnishes information on the electronic density of states distribution in the valence band, nearest to the core levels and surface states. The measured angle resolved EDC's provide the possibility to determine the energy-momentum E(k) dependence of electrons in the bulk of the crystal and on the surface. Application of the synchrotron-storage ring system as a source of ultraviolet radiation in the energy range from 10 to 300 eV opens new avenues to investigate structure of electronic states. It features the following possibilities: (1) To distinguish the contribution of the surface and bulk states to the obtained EDC by measuring the change of the EDC when varying the exciting energy hv around the minimum of the escape depth (E = 80 eV) and outside of this region. (2) To discriminate in the valence band the contribution of the d-electrons from that of the s-p electrons due to a different change of the photoemission cross section of the d and s-p electrons with a change of hv. (3) To recognize the localized and delocalized contribution to the density of states in the valence band and to determine E(k) for these electrons by measurements of the angle resolved EDC. (4) To obtain information on the initial and final state distribution using the constant initial states (CIS) and/or constant final states (CFS) techniques. The potential of the photoemission technique will be illustrated by the results of the electronic structure investigation of some metals and semiconductors.     

Low voltage substrate current： a monitor for interface states generation in ultra-thin oxide n-MOSFETs under constant voltage stresses

The low voltage substrate current （Ib） has been studied based on generation kinetics and used as a monitor of interface states （Nit） generation for ultra-thin oxide n-MOSFETs under constant voltage stress. It is found that the low voltage Ib is formed by electrons tunnelling through interface states, and the variations of Ib（△Ib） are proportional to variations of Nit （△Nit）. The Nit energy distributions were determined by differentiating Nit（Vg）. The results have been compared with that measured by using gate diode technique.[第一段]     

The determination of the FIR dielectric constant in a reststrahlenband by surface wave laser spectroscopy

The application of an optically pumped FIR laser to surface wave spectroscopy is described. The surface phonon polariton on CsBr was excited with a prism coupler. The optical constants of the prism material polyethylene and the sample CsBr are given for and 85.30 cm^–1.     

Transient current trap spectroscopy (TCTS) in GaAs/GaAlAs heterojunctions

In this paper, we have shown that it is possible to do a full study of deep centers in double heterojunction LED using spectral analysis of transient current. Traps are characterized by their apparent physical parameters (activation energy, concentration, capture cross section).     

Parameter optimization of a silicon pressure-sensitive element based on dual-drain MIS transistors

The object of investigation was an integrated pressure transducer with a differential pressure-sensitive element based on two parallel-connected p-channel dual-drain MIS transistors. The transistors were made on the planar side of a silicon membrane with the rigid central region near the edges of membrane’s thin part (valley). The optimum design of the transducer and its basic characteristics were determined.     

Transient surface photoconductivity of GaAs emitter studied by terahertz pump-emission spectroscopy

This paper investigates the ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs in detail by using a terahertz pump-emission technique. Based on theoretical modelling, it finds that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse. Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed after the excitation of pump pulse and we attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperature-dependent measurements are also performed to verify this trapping model.     

Josephson current through a quantum dot molecule with a Majorana zero mode and Andreev bound states

Based on the Green's function method, we investigate the interplay between Majorana zero mode (MZM) and Andreev bound states (ABSs) in a quantum dot molecule side coupled to a topological superconducting nanowire with a pair of MZMs forming a Josephson junction. Since the strong electron–hole asymmetry induced by the nanowire with a topologically non-trivial phase, the MZM suppress the ABSs. The suppression induced by the MZM is robust against the Coulomb repulsion. The interplay between the MZM and the ABSs in Josephson junction presents a feasible experimental means for distinguish between the presence of MZM and ABSs.     

Behaviour of surface states on a GaAs/AlGaAs heterostructure investigated by capacitance spectroscopy

Low temperature depletion capacitance-voltage data, taken from a novel open design of surface superlattice fabricated on a GaAs/AlGaAs heterostructure, are compared with two theoretical models. These assume either that the Fermi level on the free semiconductor surface is pinned, or that the surface charge state is unaltered (frozen) when the superlattices are biased. The data are in much better agreement with the frozen surface model.     

Investigation of interfacial states in MIS structures by a single-frequency admittance method

Two models of tunneling charge exchange on interfacial states buried in the insulator are examined. In one model the spatial distribution of these states is assumed to be uniform and in the other model the volume density of traps decreases exponentially with increasing depth. Analytical expressions are obtained for the width and position of the peak in the normalizedconductance curves in both models, and the accuracy and limitations of these expressions are indicated. A new procedure for investigating interfacial states by the admittance method is proposed, based on the use of G-V characteristics of metal-insulator-semiconductor (MIS) structures measured at a fixed frequency. Zh. Tekh. Fiz. 67, 55–59 (October 1997)     

Investigation of the spectrum of surface states in bismuth by scanning tunneling spectroscopy

Scanning tunneling spectroscopy of trigonal (0001) and “quasitrigonal” surfaces of a twin interlayer on a cleaved face of bismuth is performed. It is found that both surfaces are characterized by surface electron states with spectra exhibiting clearly defined singularities, namely, relatively narrow maxima and minima of the density of states in the energy range of ?1 eV. An analysis of the behavior of the current-voltage characteristics at low (of the order of tens of millivolts) voltages has revealed the existence on the bismuth surface of a two-dimensional layer, in which the density of states of electrons, unlike its anomalously small value in the bulk of bismuth, is of the order of magnitude typical of metals.     

On a FEN-theory for surface states in III-V compounds

In this paper we discuss the existence and the position of the surface energy bands with respect to the bulk energy scheme, applying the FEN-theory respectively to the case of a clean (111)-boundary plane of a sphalerite crystal model and also to the case that the (111) surface is completely covered by a layer of adsorbed foreign atoms. The surface bands are determined by transcendental equations, which are solved numerically and discussed.