Growth of Ni-Mn-Ga high-temperature shape memory alloy thin films by magnetron sputtering technique

Ni-Mn-Ga thin films have been fabricated by using magnetron sputtering technique under various substrate negative bias voltages. The effect of substrate negative bias voltage on the compositions and surface morphology of Ni-Mn-Ga thin films was systematically investigated by energy dispersive X-ray spectrum and atomic force microscopy, respectively. The results show that the Ni contents of the thin films increase with the increase of the substrate negative bias voltages, whereas the Mn contents and Ga contents decrease with the increase of substrate negative bias voltages. It was also found that the surface roughness and average particle size of the thin films remarkably decrease with the increase of substrate negative bias voltages. Based on the influence of bias voltages on film compositions, a Ni₅₆Mn₂₇Ga₁₇ thin film was obtained at the substrate negative bias voltage of 30 V. Further investigations indicate that the martensitic transformation start temperature of this film is up to 584 K, much higher than room temperature, and the film has a non-modulated tetragonal martensitic structure at room temperature. Transmission electron microscopy observations reveal that microstructure of the thin film exhibits an internally (1 1 1) type twinned substructure. The fabrication of Ni₅₆Mn₂₇Ga₁₇ high-temperature shape memory alloy thin film will contribute to the successful development of microactuators.     

Multiple Andreev reflections in ballistic Nb-InGaAs/InP-Nb junctions

The current–voltage characteristics of ballistic Nb-InGaAs/InP-Nb Josephson junctions have been investigated. At temperatures below 1 K a negative differential conductance, which usually leads to a hysteresis in the current–voltage characteristics, was resolved by connecting an additional external shunt resistor to the junction. The negative differential conductance is explained by heating and conductance enhancement due to multiple Andreev reflections. The structures observed in the differential resistance measurements as a function of the bias voltage are explained by self-detection of Josephson radiation at low bias voltages and subharmonic gap structures at higher bias voltages.     

Observation of light-induced anisotropy in ferric borate by acoustic resonance

A light-induced anisotropy of two-fold symmetry in the magnetically easy c-plane has been observed at 77 K in c-plane platelets of FeBO₃ using the method of acoustic resonance. The anisotropy field is shown to be negative (positive) in the direction of the net magnetization during illumination in those samples for which the susceptibility increases (decreases) with light.     

Deep level states and negative photoconductivity in n-ZnO/p-Si hetero-junction diodes

Negative photoconductivity (NPC) was observed in n-ZnO/p-Si heterojunction diode grown by ultra-high vacuum sputtering method under nitrogen ambient. Under the illumination of ultra-violet light, positive photoconductivity was observed at low bias voltages, whereas NPC was observed at high bias voltages. The defect states in the ZnO layers grown on Si were analyzed by photoluminescence and deep level transient spectroscopy measurements. Two deep levels were measured at E_c-0.51 eV and E_c-0.54 eV, which might be originated from oxygen vacancy and nitrogen atom related defects, respectively. Based on the simulation of band diagram, the defect states were located below Fermi level at zero bias voltage. However, as increasing the bias voltages, NPC was observed due to the increase of empty defect states. This analysis allowed us to consider the possibility that the NPC phenomenon in n-ZnO/p-Si heterojunction diode is originated dominantly from the defect states as a carrier recombination center in ZnO layer.     

Current noise in (111) n-channel Si-MOSFET's at T = 4.2 K

We have investigated the drain current-drain voltage characteristics and the spectral noise intensity of the drain current of (111) n-channel MOSFET's at T = 4.2 K. At T = 4.2 K the drain current-drain voltage characteristics showed a hysteresis which was not observed at T =77 K and at room temperature. A qualitative explanation of this hysteresis is given in terms of electron transfer from high mobility valleys to low mobility valleys due to hot electrons. In the spectra of the current noise three contributions could be distinguished: 1/ƒ-noise, white noise and generation-recombination noise. The 1/ƒ-noise is interpreted as number fluctuations noise. The effective trap density was found to be 2.3 × 10²² m^-3. At low drain voltages the white noise can be interpreted as diffusion noise. At higher drain voltages extra noise is observed over and above diffusion noise. This extra noise may be inter-valley noise. The generation-recombination noise was very sensitive to the gate voltage. A tentative explanation can be given if it is assumed that the traps which cause this noise have a non-uniform energy distribution.     

Optoelectronic transport mechanism from subband infrared absorption and tunneling regeneration

The main problems of conventional multi-quantum well infrared photodetectors (QWIPs) were discussed. In order to overcome the limitations of the conventional QWIPs, such as small photocurrent, high dark current and low response speed, novel QWIPs in which photocurrent increases with the number of well were proposed. The novel structure with several wells were calculated and analyzed in detail, and successfully fabricated. The dark current lower than conventional QWIPs by about one order of magnitude was obtained, well in agreement with theoretical value. I–V characteristics of the novel QWIPs with six wells has been presented, and six related negative differential resistance regions were observed at positive bias. The absorption photocurrents of the novel QWIPs at 77 K were found to increase with well numbers, confirming the mechanism of the new structure. Furthermore, the transportation of the optoelectronic and some other problems of the QWIPs were discussed.     

Scanning tunneling microscopy luminescence from nanoscale surface of GaAs(1 1 0)

Scanning tunneling microscopy luminescence (STML) was induced from the nanometer scale surfaces of cleaved n-type and p-type GaAs(1 1 0) wafers by using of an ITO-coated optical fiber probe in an ultrahigh-vacuum chamber. The STML from n-type GaAs(1 1 0) surface was induced under negative sample bias when the applied bias exceeds a threshold voltage around −1.5 V. Whereas the STML from p-type GaAs(1 1 0) surface was induced under positive sample bias when the applied bias exceeds a threshold voltage around +1.5 V. The excitation energies at the threshold voltages are consistent with the band gap of GaAs (1.42 eV) at 295 K. The typical quantum efficiencies for n-type and p-type GaAs are about 3 × 10⁻⁵ and 2 × 10⁻⁴ photons/electron, respectively. The observed STML from are attributed to a radiative recombination of electron-hole pairs generated by a hole injection for n-type GaAs under negative sample bias and an electron injection for p-type GaAs under positive sample bias, respectively.     

Characteristics of p-ZnO/n-GaN heterojunction photodetector

Thin film heterojuction of the type p-ZnO/n-GaN was prepared by spray pyrolysis and electron beam evaporation technique, respectively. Hall measurements demonstrate the firm p-type conductivity of the p-doped ZnO film. The structural and electrical properties of the p-ZnO/n-GaN heterojunction are investigated by X-ray diffraction (XRD) and current-voltage (I-V) measurements. The XRD shows that the p-ZnO/n-GaN heterojunction is highly crystalline in nature with preferred orientation along the [0001] direction. The current-voltage curve of the heterojunction demonstrates obvious rectifying diode behavior in the dark and under illumination conditions. The ideality factor of the detector was determined in case of forward bias at low voltages and it was found to be 13.35. The turn-on voltage appears at about 1V under forward-biased voltage, and the reverse breakdown voltage is about 4V. It was found that the current of the illumination increases with the increase of bias voltages.     

Effects of experimental parameters on composition of boron carbon nitride thin films deposited by magnetron sputtering

We have synthesized boron carbon nitride thin films by radio frequency magnetron sputtering. The films structure and composition were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicate that the three elements of B, C, N are chemically bonded with each other and atomic-level hybrids have been formed in the films. The boron carbon nitride films prepared in the present experiment possess a disordered structure. The influence of PN₂/PN₂+Ar, total pressure and substrate bias voltage on the composition of boron carbon nitride films is investigated. The atomic fraction of C atoms increases and the fractions of B, N decrease with the decrease of PN₂/PN₂+Ar from 75% to 0%. There is an optimum total pressure. That is to say, the atomic fractions of B, N reach a maximum and the fraction of C atoms reaches a minimum at the total pressure of 1.3 Pa. The boron carbon nitride films exhibit lower C content and higher B, N contents at lower bias voltages. And the boron carbon nitride films show higher C content and lower B, N contents at higher bias voltages.     

Studies of electron screening effects on the electron mobility in silicon surface inversion layers

Theoretical calculations of the effects of electron screening of the randomly distributed oxide charges on the electron mobility in surface inversion layers at 4.2 K show that screening substantially enhances the mobility even in weakly inverted surface layer. Surface conductances are measured at 4.2 and 77 K on n-channel MOS transistors with 4 × 10¹¹ ions/cm². Threshold voltages are obtained by matching the inversion layer conductance versus gate voltage data to the theory. The observed mobility magnitude at 4.2 K is about 2000 cm²/V-s which is in excellent agreement with the theory including screening but without adjustable parameters. A conductance tail of several volts wide below the theoretical threshold voltage and a large positive threshold voltage shift are observed at low temperatures which are attributed to an areal inhomogeneous distribution of fast surface states near the silicon conduction band edge.     

Impulse breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF6

This paper presents the impulse pre-breakdown and breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF₆ gas. The breakdown voltage–time (V–t) characteristics and the breakdown voltage–gas pressure (V–p) characteristics of a highly non-uniform SF₆ gas gap under positive and negative lightning impulse voltages are investigated in the pressure range between 0.1 and 0.5 MPa. The pre-breakdown developments are examined by the corona current and light emission measurements with high time resolution. As a result, the dielectric strengths versus time-to-breakdown of SF₆ gas gap under positive lightning impulse voltages were nearly independent of the gas pressure. The first streamer corona was initiated at the tip of the needle electrode, and the streamer corona pulses developed with a stepwise propagation. The discharge paths were zigzag, and the branches of the discharge channel for positive polarity were created. On the other hand, the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.     

Controlling charge current through a DNA based molecular transistor

Molecular electronics is complementary to silicon-based electronics and may induce electronic functions which are difficult to obtain with conventional technology. We have considered a DNA based molecular transistor and study its transport properties. The appropriate DNA sequence as a central chain in molecular transistor and the functional interval for applied voltages is obtained. I–V characteristic diagram shows the rectifier behavior as well as the negative differential resistance phenomenon of DNA transistor. We have observed the nearly periodic behavior in the current flowing through DNA. It is reported that there is a critical gate voltage for each applied bias which above it, the electrical current is always positive.     

Fabrication and electrical characterization of thermally deposited amorphous Se0.82In0.18 on n-Si (100)

Amorphous (a-) Se_0.82In_0.18 thin films have been deposited onto n-type silicon (n-Si) single crystal, using the three-temperature technique, in the fabricated configuration of Au/a-Se_0.82In_0.18/n-Si/Al. The current density-voltage (J–V) characteristics have been measured at different isotherms in the range of 198–313 K, thus inspecting the conduction mechanisms comparing with Au/a-Se/n-Si/Al heterojunctions. The analysis proved that the forward bias is characterized by two parts: current increasing exponentially with the applied voltage (low voltage bias region, V<0.2 V), and non-exponentially in the higher voltage region (V>0.2 V). At the low bias region, the current was dominated by a multi-tunneling capture-emission process with a rather temperature-independent effect in the temperature range investigated. However, at the high voltage region, the effect of temperature becomes more pronounced with an ohmic character in the range of 198 to 273 K. For temperatures higher than 273 K, and below the glass transition temperature of a-Se_0.82In_0.18 (T _g～330 K), the high voltage region could be subdivided into two parts: an ohmic conduction range that limited at bias voltage of 0.20 V<V<0.46 V, and a space charge limited current region for bias voltage of V>0.46 V. The reverse J–V characteristics showed a deviation from that of the ideal diode behavior, analogous to that of pure a-Se/n-Si heterojunctions.     

Abnormal current-voltage characteristics and metal-insulator transition of amorphous carbon film/silicon heterojunction

The amorphous carbon film/n-Si (a-C/n-Si) heterojunctions have been fabricated by direct current magnetron sputtering at room temperature, and their current-voltage characteristics have been investigated. The results show that these junctions have good rectifying properties in the temperature range 80-300 K. The interesting result is that the current-voltage curve changes dramatically with increasing applied voltage and temperature. For the forward bias voltages, the junction shows Ohmic mechanism characteristic in the temperature range 240-300 K. However, the conduction mechanism changes from Ohmic for the low bias voltages to space charge limited current for the high bias voltages in the temperature range 80-240 K. While for the reverse bias voltages, it changes from Schottky emission to breakdown with increasing voltage. Another important phenomenon is that the temperature dependence of the junction resistance shows a metal-insulator transition, whose transition temperature can be controlled by the bias voltage.     

Mechanism of charge transfer in injection photodiodes based on the In-n +-CdS-n-CdS x Te1 − x -p-Zn x Cd1 − x Te-Mo structure

Photosensitive In-n ⁺-CdS-n-CdS _x Te_{1 ? x} -p-Zn _x Cd_{1 ? x} Te-Mo film structures based on II–VI semiconductors and operating in the wavelength range λ = 0.490–0.855 μm have been fabricated. These structures in the forward current direction at high bias voltages operate as injection photodiodes and exhibit a high integrated sensitivity S _int ≈ 700 A/lm (14500 A/W) at room temperature. It has been found that, in the fabricated structures at low illuminance levels and low forward bias voltages (0.05–0.50 V), the diffusion and drift fluxes of nonequilibrium charge carriers are directed toward each other. This effect leads to the sign reversal of the photocurrent, which makes it possible on the basis of these structures to create selective photodetectors with injection properties. In the reverse direction of the photocurrent, these structures also operate in the mode of internal amplification of the primary photocurrent, but the integrated sensitivity in this mode is considerably less than that in the forward current direction.     

I–V characterization of a quantum well infrared photodetector with stepped and graded barriers

I–V characterization of an n-type quantum well infrared photodetector which consists of stepped and graded barriers has been done under dark at temperatures between 20–300 K. Different current transport mechanisms and transition between them have been observed at temperature around 47 K. Activation energies of the electrons at various bias voltages have been obtained from the temperature dependent I–V measurements. Activation energy at zero bias has been calculated by extrapolating the bias dependence of the activation energies. Ground state energies and barrier heights of the four different quantum wells have been calculated by using an iterative technique, which depends on experimentally obtained activation energy. Ground state energies also have been calculated with transfer matrix technique and compared with iteration results. Incorporating the effect of high electron density induced electron exchange interaction on ground state energies; more consistent results with theoretical transfer matrix calculations have been obtained.     

Modulating microstructure and optical properties of hydrogenated nanocrystalline silicon photovoltaic materials prepared under hydrogen diluted silane PECVD by various DC bias

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were fabricated by plasma enhanced chemical vapor deposition under the various negative substrate bias voltages with hydrogen as a diluent of silane. The microstructure and optical properties of nc-Si:H thin films were studied by Raman scattering spectroscopy, X-ray diffraction (XRD), transmission electron microscopy, and optical transmission spectroscopy. Raman spectra and XRD pattern reveal that applying negative bias voltages at the moderate level favors the enhancement of crystalline volume fraction, increase of crystallite sizes and decrease of residual stress. We also demonstrated that the negative direct current bias can be used to modulate the volume fraction of voids, refractive index, absorption coefficient, compactness and ordered degree of nc-Si:H films. It is found that the film deposited at −80 V shows not only high crystallinity, size of crystallite, and static index n₀ but also low residual stress and volume fraction of voids. Furthermore, the microstructural evolution mechanism of nc-Si:H thin films prepared at different bias voltages is tentatively explored.     

Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions

We present the results of low-temperature transport measurements on chains of superconductor-normal constriction-superconductor (SNS) junctions fabricated on the basis of superconducting PtSi film. A comparative study of the properties of the chains, consisting of 3 and 20 SNS junctions in series, and single SNS junctions reveals essential distinctions in the behavior of the current-voltage characteristics of the systems: (i) a gradual decrease of the effective suppression voltage for the excess conductivity observed at zero bias as the quantity of the SNS junctions increases; (ii) a rich fine structure on the dependences dV/dI-V at dc bias voltages higher than the superconducting gap and corresponding to some multiples of 2Δ/e. A model explaining this above-energy-gap structure based on the energy relaxation of electrons via Cooper-pair-breaking in the superconducting island connecting normal metal electrodes is proposed.     

Deposition of NiFe(200) and NiFe(111) textured films onto Si/SiO<Subscript>2</Subscript> substrates by DC magnetron sputtering

The effect of substrate temperature T_sub and bias voltage U_bias on the texture of NiFe films with thickness d ～ 30–340 nm deposited by DC magnetron sputtering onto Si(111)/SiO₂ substrates under working gas pressure ～ 0.2 Pa has been investigated. It has been demonstrated that films grown at room substrate temperature have the (111) texture that is refined under a negative bias voltage. The deposition of films onto a grounded (U_bias ～ 0) substrate heated to T_sub ～ 440–640 K results in the formation of textured NiFe(200) films.     

Pulsed excitation of luminescence in dihydrated oxalic acid

Luminescence has been excited by an a.c. electric field in oxalic acid dihydrate at temperatures 273, 283 and 293 K. The voltage and frequency dependence of the emitted light flux has been studied. The brightness (B) has been observed to increase with voltage (V_rms) and frequency (v). The relation b = b₀ exp(-b/V^{$\text{1}\text{2}$_rms)} describes the voltage dependence of brightness quite accurately at frequencies greater than 500 Hz. The brightness wave forms at different voltages and frequencies of the applied field have also been studied. One primary and one secondary peak have been observed in each half cycle of the applied sinusoidal voltage.