Field emission characteristics of thin-metal-coated nano-sheet carbon films

Nano-sheet carbon films (NSCFs) coated with very thin (≈5-nm-thick) metal layers were fabricated on Si wafer chips by means of quartz-tube-type microwave-plasma chemical-vapour-deposition method with hydrogen-methane gas mixture and an electron beam evaporation method. Field emission (FE) current densities obtained at a macroscopic average electric field, E, of ≈10 V/μm changed from 13 mA/cm² for NSCF with no coated metal to 1.7, 0.7 and 30 mA/cm² for Ti-, Al- and Au-coated NSCFs, respectively, while the threshold E varied from 4.4 V/μm for the former one to 5.3, 5.4 and 2.0 V/μm for the corresponding latter ones, respectively. As the FE currents of Au-coated NSCFs tended to be saturated in a higher E region, compared to those of NSCFs with no coated metal, no simple Fowler-Nordheim (F-N) model is applicable. A modified F-N model considering statistic effects of the FE tip structures and a space-charge-limited-current effect is successfully applied to an explanation for the FE data observed in the low and high E regions.     

Photorefractive polymer composites for the IR region based on carbon nanotubes

The photorefractive properties of polymer composites based on aromatic polyimide and single-wall carbon nanotubes are studied using radiation at a wavelength of 1064 nm. It is found that the nanotubes possess photoelectric sensitivity in this spectral region and that the kinetic photorefractive characteristics of the polymer composites are entirely determined by the photogeneration and charge transport characteristics of the layers. The two-beam gain coefficient of the signal beam measured for a composite consisting of aromatic polyimide and 0.25 wt % of single-wall carbon nanotubes in a constant electric field E₀ = 79 V/μm is equal to 84 cm^?1 and exceeds the optical absorption coefficient by 59 cm^?1. The refractive index modulation is equal to Δn = 0.004 at E₀ = 54 V/μm.     

Effect of excitation intensity and electric field on the photoconductivity relaxation in CdxHg1−x Te/GaAs polycrystalline layers

The photoconductivity relaxation and the stationary photoconductivity in the n-Cd_0.8Hg_0.2Te compensated polycrystalline layers at T=300 K have been investigated as a function of the light intensity and the strength of applied electric field E. It is demonstrated that, at low excitation intensities, the saturation of stationary photoconductivity and a decrease in the relaxation time with an increase in E are caused by the minority carrier extraction. The effect of minority carrier extraction is analyzed with due regard for the internal electric field of potential barriers in intergranular layers. It is assumed that the features of nonequilibrium-carrier recombination, which proceeds through several channels and depends on the excitation intensity and extraction electric field strength, can stem from the polycrystalline structure of the Cd_0.8Hg_0.2Te layers.     

Switching behaviour of ferroelectric liquid crystals probed by optical second-harmonic generation

Ferroelectric properties of thin (1.5–4 μm planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using electro-optic measurements, second-harmonic generation (SHG) and SHG interferometry. A switching behaviour of the FLC cells in external dc electric fields is observed. It is characterised by rotation of the polarisation plane of the transmitted light and by changes in the SHG intensity, phase and anisotropy dependences, which are attributed to a collective motion of the system as a ferroelectric uniform state with C₂ symmetry. Received: 16 October 2001 / Revised version: 18 March 2002 / Published online: 6 June 2002     

Long-term relaxation of the current in a naturally disordered Pb3O4 semiconductor

Charge transfer has been studied in metal-dielectric-metal structures based on polycrystalline layers of lead orthoplumbate Pb₃O₄ with a binder—cyanoethyl ester of polyvinyl alcohol, deposited on a glass substrate with a conducting ITO film. Time dependences of the current have been investigated in the temperature range T = 300–370 K and in the range of dc electric field strengths E = 2 × 10⁵?9 × 10⁵ V/m. Flowing of the relaxation polarization current leads to charge accumulation in the near-electrode region. Experimental dependences agree with the relay race mechanism of charge transfer with the participation of deep local levels. Microparameters of charge transfer are determined upon varying the experimental conditions.     

Atomic force microscopic studies of thin oriented films of ferroelectric liquid crystalline elastomers with varying network architecture

Cross-linked ferroelectric liquid crystalline (FLC) polymers were studied by atomic force microscopy (AFM). Polysiloxane copolymers were synthesized with mesogenic and photo-cross-linkable side groups, the latter connected either directly to the backbone via a short spacer or as a terminal group on a part of the mesogens. These elastomers were prepared as thin, freely suspended films in homeotropic orientation. Topographic measurements depict plateaus separated by steps of characteristic height corresponding to the surfaces and edges of smectic layers. From the temperature behavior, as well as from the reaction to mechanical deformation, a model for the network architectures is proposed. In the first case (“intralayer cross-linking”), a predominantly two-dimensional network is formed within the backbone layers separating the smectic layers; in the second case (“interlayer cross-linking”), a three-dimensional network is established that is dependent on the mesophase present during cross-linking.     

Electrical conduction in a semicrystalline polyethylene terephthalate in high electric field

Measurements of conduction currents were carried out on polyethylene terephthalate semi crystalline PET in an electrical field region varying from 40 to 260 V/μm. Various mechanisms of conduction (Richardson-Schottky, Fowler-Nordheim, Poole-Frenkel, space charge limited current SCLC, Hopping), can cause non-linear characteristics. To highlight the mechanism responsible for conduction in the PET, we used the slope of the experimental curve Log(J)=f(E) in the region 150<E<260 V/μm, to calculate the value of the dielectric permittivity. The characteristic curve Log(J)=f(1/T) shows that the Poole-Frenkel mechanism of conduction is prevalent in the semi crystalline PET. The characteristic J=f(E) for various thicknesses shows that the field threshold increases with the thickness.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

Novel high-voltage power lateral MOSFET with adaptive buried electrodes

A new high-voltage and low-specific on-resistance (R on,sp ) adaptive buried electrode (ABE) silicon-on-insulator (SOI) power lateral MOSFET and its analytical model of the electric fields are proposed. The MOSFET features are that the electrodes are in the buried oxide (BOX) layer, the negative drain voltage V d is divided into many partial voltages and the output to the electrodes is in the buried oxide layer and the potentials on the electrodes change linearly from the drain to the source. Because the interface silicon layer potentials are lower than the neighboring electrode potentials, the electronic potential wells are formed above the electrode regions, and the hole potential wells are formed in the spacing of two neighbouring electrode regions. The interface hole concentration is much higher than the electron concentration through designing the buried layer electrode potentials. Based on the interface charge enhanced dielectric layer field theory, the electric field strength in the buried layer is enhanced. The vertical electric field E I and the breakdown voltage (BV) of ABE SOI are 545 V/μm and -587 V in the 50 μm long drift region and the 1 μm thick dielectric layer, and a low R on,sp is obtained. Furthermore, the structure also alleviates the self-heating effect (SHE). The analytical model matches the simulation results.     

Suppression of relaxation modes in dye dispersed SmC* phase

We report the results of dielectric and electro-optical properties of ferroelectric liquid crystal (FLC), Felix 17/100, exhibiting chiral smectic C phase and dye dispersed FLCs. The polarization measurement on pristine and dye dispersed FLC mixture shows decrease in the value of polarization, indicating the distribution of dye dipole in a direction opposite to the orientation of FLC molecule. The rotational viscosity also decreases accordingly as shown by the measurement of response time. Dielectric measurement shows existence of two relaxation modes both in pure FLC and dye dispersed FLC. The relaxation strength of Goldstone mode decreases with the dispersion of dye and the relaxation frequency of this mode shifts towards the high-frequency side. The second relaxation mode arises due to the formation of domains at the surface interface. The dispersion of dye into FLC suppresses the domains.     

Partial-SOI high voltage P-channel LDMOS with interface accumulation holes

A new partial SOI (silion-on-insulator) (PSOI) high voltage P-channel LDMOS (lateral double-diffused metal-oxide semiconductor) with an interface hole islands (HI) layer is proposed and its breakdown characteristics are investigated theoretically. A high concentration of charges accumulate on the interface, whose density changes with the negative drain voltage, which increase the electric field (E_I) in the dielectric buried oxide layer (BOX) and modulate the electric field in drift region . This results in the enhancement of the breakdown voltage (BV). The values of E_I and BV of an HI PSOI with a 2-μm thick SOI layer over a 1-μm thick buried layer are 580V/μm and -582 V, respectively, compared with 81.5 V/μm and -123 V of a conventional PSOI. Furthermore, the Si window also alleviates the self-heating effect (SHE). Moreover, in comparison with the conventional device, the proposed device exhibits low on-resistance.     

The enhanced field-emission properties of screen-printed single-wall carbon-nanotube film by electrostatic field

In this work, we improved the field-emission properties of a screen-printed single-wall carbon-nanotube (SWCNT) film by applying a strong electrostatic field during the drying process after the printing. By applying the strong field, more tips of SWCNTs could emerge from the screen-printed film and turn somewhat toward the erecting direction because of the repulsive force among the SWCNTs. The field-emission properties of the film were thus improved obviously. The improved field emitters sample has low electron emission turn-on field (E_to = 1.22 V/μm), low electron emission threshold field (E_th = 2.32 V/μm) and high brightness with good uniformity and stability. The lowest operating field of the improved sample is below 1.0 V/μm and its optimum current density exceeds 3.5 mA/cm².     

A new analytical model of high voltage silicon on insulator （SOI） thin film devices

A new analytical model of high voltage silicon on insulator (SOI) thin film devices is proposed, and a formula of silicon critical electric field is derived as a function of silicon film thickness by solving a 2D Poisson equation from an effective ionization rate, with a threshold energy taken into account for electron multiplying. Unlike a conventional silicon critical electric field that is constant and independent of silicon film thickness, the proposed silicon critical electric field increases sharply with silicon film thickness decreasing especially in the case of thin films, and can come to 141V/μm at a film thickness of 0.1μm which is much larger than the normal value of about 30V/μm. From the proposed formula of silicon critical electric field, the expressions of dielectric layer electric field and vertical breakdown voltage (V_B,V) are obtained. Based on the model, an ultra thin film can be used to enhance dielectric layer electric field and so increase vertical breakdown voltage for SOI devices because of its high silicon critical electric field, and with a dielectric layer thickness of 2μm the vertical breakdown voltages reach 852 and 300V for the silicon film thicknesses of 0.1 and 5μm, respectively. In addition, a relation between dielectric layer thickness and silicon film thickness is obtained, indicating a minimum vertical breakdown voltage that should be avoided when an SOI device is designed. 2D simulated results and some experimental results are in good agreement with analytical results.     

Dynamic response of converse magnetoelectric effect in a PMN-PT-based multiferroic heterostructure

A multiferroic heterostructure, consisting of a 25 μm thick Metglas® ribbon affixed to a lead magnesium niobate–lead titanate (PMN-PT) crystal, was systemically studied to investigate the time response of converse magnetoelectric coupling under the application of electric fields at low frequencies (0.05<f<10 Hz). This multiferroic heterostructure exhibits a considerably strong converse magnetoelectric effect, CME=?80%, where CME=[M(E)?M(0)]/M(0), and a converse ME coupling constant, A=22.5 Oe-cm/kV, at frequencies below 1 Hz and near saturation electric polarization. A switching time (t _s), representing the response time of the CME coupling, is measured to be 0.6 seconds for this heterostructure under the application of instantaneous electric fields. The switching time results in significant influences on the magnetoelectric effect especially at frequencies higher than 2 Hz. The dynamic response of CME coupling is predominantly determined by ferroelectric relaxation within the PMN-PT crystal, as opposed to the magnetic relaxation of the Metglas® ribbon. A model was used to describe the dynamic behavior of CME coupling in disordered systems such as PMN-PT.     

Dynamics of domain-wall motion in ferroelectric liquid crystals in an electric field

The director reorientation in smectic liquid crystals with ferroelectric properties has been considered in the case where the interaction of liquid-crystal molecules with the surface leads to a partial unwinding of the helical structure of the liquid crystal and the reorientation occurs as a result of the domain-wall motion. The dependences of the velocity of domain-wall motion on the electric field strength, electric field variation frequency, boundary conditions, spontaneous polarization, and viscosity of the liquid crystal have been determined. It has been demonstrated that an increase in the electric field variation frequency or the polar part of the anchoring energy and the spontaneous polarization of the liquid crystal at a constant field frequency results in an increase of the velocity of domain-wall motion. As a consequence, the time of the electro-optic response of the liquid crystal in weak electric fields (from 0.4 to 2.0 V/μm) decreases by a factor of more than three.     

Integrated Electro-Optic Switches Based on Ferroelectric Liquid Crystal Waveguides

Two integrated electro-optic switches based on smectic C * ferroelectric liquid crystal (FLC) planar waveguides oriented in the bookshelf geometry have been designed and realized. In order to test the first switch, a particular waveguide structure consisting of a three-stage device, having a thin FLC film middle stage and two glass waveguides as other stages, has been designed and realized. The second one is realized by a planar waveguide with a smectic C * ferroelectric liquid crystal overlayer. In this article the design, the realization, and the experimental characterization of electro-optical switches are reported. The electro-optical behavior and the response time of both devices have been studied for different configurations for both TE and TM polarization. Our main aim was to demonstrate the feasibility of an integrated electro-optic device based on a FLC waveguide, and our experimental results provide a number of interesting indications about device optimization and practical applications.     

Electroclinic effect in free-standing smectic elastomer films

The coupling of the mesogenic tilt in smectic liquid crystals to external electric fields in the layer plane (electroclinic effect) provides an opportunity to control the smectic layer thickness with electric fields. In ordered ferroelectric smectic elastomers it is possible to achieve a macroscopic electromechanical response. The effect is particularly pronounced near the smectic A-to-smectic C^* phase-transition temperature. In this work, the electrostriction of weakly cross-linked smectic elastomers is studied by means of optical interferometry of thin films. Its observed magnitude corresponds to an induced tilt of approximately 7.7°, yielding a layer contraction of 1%, in a 3 MV/m electric field. This value is in agreement with earlier measurements of tilt susceptibility in a structurally similar polymer, but is considerably smaller than previously reported data on elastomers with a comparable chemical structure. PACS 61.30.Vx; 83.80.Va; 77.65.Bn     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also discussed.     

Field emission property of copper nitride thin film deposited by reactive magnetron sputtering

Copper nitride (Cu₃N) thin film was deposited on silicon (Si) substrate by reactive magnetron sputtering method. X-ray diffraction measurement showed that the film was composed of Cu₃N crystallites with anti-ReO₃ structure and exhibited preferential orientation of [1 0 0] direction. The field emission (FE) result showed that Cu₃N film had a turn-on electric field of about 3 V/μm at a current density of 1 μA/cm² and a current density of 700 μA/cm² was obtained at the electric field of 24 V/μm. The emission mechanism inferred by Fowler-Nordheim (FN) plot is shown as following: thermal electron emission at low field region and tunneling electron emission at high field region.     

Structure and optical analysis of Ta2O5 deposited on infrasil substrate

Electron beam gun technique was used to prepare Ta₂O₅ thin films onto infrasil substrates of thicknesses 333 and 666 nm. The structure characterization was investigated using X-ray diffraction patterns. Transmittance measurements in the wavelength range (240-2000 nm) were used to calculate the refractive index n and the absorption index k depending on Swanepole's method. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transparent region. The analysis of the optical absorption data revealed that the optical band gap E_g was indirect transition. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple-DiDomenico model, from which the dispersion parameters (E_o and E_d) and the high frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration to the effective mass ratio were estimated according to the model of Spitzer and Fan. Graphical representation of the relaxation time as a function of photon energy was also presented.