Ultraviolet light emitting diode with n-ZnO:Ga/i-ZnO/p-GaN:Mg heterojunction

n-ZnO:Ga/p-GaN:Mg heterojunction light emitting diodes (LEDs) were fabricated at different growth temperatures and carrier concentrations in the n-type region. The effects of growth temperature and carrier concentration on the electrical and emission properties were investigated. The I-V and EL results showed that the improved device performances such as lower turn-on voltage and true ultraviolet emission were achieved with the insertion of a thin intrinsic layer between n-ZnO:Ga and p-GaN:Mg. This observation was attributed to a lowering of energy barriers for the supply of electrons and holes into intrinsic ZnO and recombination in the intrinsic ZnO with the absence of deep-level emission.     

Linear m-gap-consecutive k-out-of-r-from-n:F systems

This paper proposes a new model that generalizes the linear consecutive k-out-of-r-from-n system. In this model the system consists of n linearly ordered statistically independent identical elements and fails if the gap between any pair of groups of r consecutive elements containing at least k failed elements is less than m elements. An algorithm for system reliability evaluation is suggested, which is based on an extended universal moment generating function. Examples of system reliability evaluation are presented.     

Electrical conduction through self-assembled monolayers in molecular junctions: Au/molecules/Au versus Au/molecule/PEDOT:PSS/Au

We fabricated and characterized a large number of octanedithiol (denoted as DC8) molecular devices as vertical metal–molecule–metal structure with or without using an intermediate conducting polymer layer of poly (3,4-ethylenedioxythiophene) stabilized with poly(4-styenesulfonic acid) (called as PEDOT:PSS). The electronic transport properties of DC8 molecular devices with and without PEDOT:PSS layer were statistically compared in terms of current density and device yield. The yields of the working molecular devices were found to be ~ 1.75% (84 out of 4800 devices) for Au/DC8/Au junctions and ~ 58% (74 out of 128 devices) for Au–DC8/PEDOT:PSS/Au junctions. The tunneling decay constants were obtained with the Simmons tunneling model and a multibarrier tunneling model for two kinds of molecular devices with and without PEDOT:PSS layer.     

Impact of a-Si:H hydrogen depth profiles on passivation properties in a-Si:H/c-Si heterojunctions

We explore the (near-)interface structure of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions as employed in high-efficiency heterojunction solar cells. We make use of secondary-ion-mass-spectroscopy profiles and minority carrier lifetime measurements taken on undoped deuterated amorphous silicon [(i)a-Si:D] layers deposited on c-Si from deuterated silane at identical conditions as the hydrogenated layers we have analyzed previously [T. F. Schulze et al., Appl. Phys. Lett. 96 (2010) 252102]. We briefly discuss the implications of the local interface structure for the c-Si surface passivation as well as for the heterojunction band offsets, and identify a route towards optimization of (i)a-Si:H layers as passivating buffers in a-Si:H/c-Si high-efficiency heterojunction solar cells.     

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.     

New wear-resistant material: Nano-TiN/TiC/TiNi composite

Near-eqiatomic TiNi alloy has been found to exhibit high resistance to wear, especially to erosion. The high wear resistance of the alloy may largely benefit from its pseudoelasticity. Recent studies demonstrate that the wear resistance of TiNi alloy can be considerably enhanced when hard particles such as TiC were added as a reinforcing phase. It was expected that the wear resistance of such a composite could be further improved if the TiNi matrix can be strengthened with retained pseudoelasticity. Attempt was made to develop such a tribo composite, using nano-TiN powder to strengthen the matrix of the TiC/TiNi composite. The composite was made using a vacuum sintering process. Sliding wear behavior of this material was evaluated. It was demonstrated that the nano-TiN/TiC/TiNi composite exhibited excellent wear resistance, superior to those of the TiC/TiNi composite and WC/NiCrBSi hardfacing overlay. In order to understand the role of the nano-TiN powder, localized mechanical behavior and micro-scale wear of the TiNi matrix with and without nano-TiN powder were investigated using a triboscope. Worn surfaces were examined using SEM to better understand the wear mechanism and to find out clues for further development.     

Optical constants of silicone-like (Si:Ox:Cy:Hz) thin films deposited on quartz using hexamethyldisiloxane in a remote RF hollow cathode discharge plasma

Deposition of amorphous silicone-like (Si:O_x:C_y:H_z) thin films in a remote RF hollow cathode discharge plasma using hexamethyldisoloxane as monomer and Ar as feed gas has been investigated for films optical constants and plasma diagnostic as a function of RF power (100-300 W) and precursor flow rate (1-10 sccm). Plasma diagnostic has been performed using Optical Emission Spectroscopy (OES). The optical constants (refractive index, extinction coefficient and dielectric constant) have been obtained by reflection/transmission measurements in the range 300-700 nm. It is found that the refractive index increases from 1.92 to 1.97 with increasing power from 100 to 300 W, and from 1.70 to 1.92 with increasing precursor flow rate from 1 to 10 sccm. The optical energy band gap E_g and the optical-absorption tail ΔE have been estimated from optical absorption spectra, it is found that E_g decreases from 3.28 to 3.14 eV with power increase from 100 to 300 W, and from 3.54 to 3.28 eV with precursor flow rate increase from 1 to 10 sccm. ΔE is found to increase with applied RF power and precursor flow rate increase. The dependence of optical constants on deposition parameters has been correlated to plasma OES.     

Formation of a-Si:H and a-Si1-xCx :H nanowires by Ag-assisted electroless etching in aqueous HF/AgNO3 solution

The formation of hydrogenated amorphous silicon-carbon alloy (a-Si_1-xC_x:H) nanowires is studied for different carbon concentrations (0-7%) by using Ag-assisted electroless etching of the thin a-Si_1-xC_x:H films deposited by plasma-enhanced chemical vapour deposition from silane/methane gas mixtures. The nanowires morphologies (length, density, …), studied by scanning electron microscopy, strongly depend on the concentration of the etchant (aqueous solution of hydrofluoric acid and silver nitrate), the etching time, and the carbon concentration of the deposited layer.     

Evolution of nanodomains in the uniaxial relaxor Sr/sub 0.61/Ba/sub 0.39/Nb/sub 2/O/sub 6/:Ce

The evolution of the nanodomain pattern of the uniaxial relaxor ferroelectric strontium barium niobate doped with cerium was studied by piezoresponse force microscopy (PFM). The fractal-like nanodomains observed at room temperature decay on heating. At temperatures up to about 15 K above the Curie temperature, T_c = 320 K, areas of correlated polarization are still visible. On cooling from the paraelectric state to below T_c, a slow isothermal growth of nanodomain was found. The mean domain size increases according to a logarithmic law as predicted for the three-dimensional random field Ising model     

Copper oxycarbonates and mercury-based cuprates: Promising high-Tc superconductors

A short overview of two recently discovered series of high-T _c superconductors is given. The first one deals with copper oxycarbonates which exhibit _c 's up to 82 K. Their structural principle is based on the fact that rows of CO₃ groups can assure the connection between either octahedral or pyramidal copper layers forming new intergrowths withT _c 's superior to those of the mother structures. The second family concerns the mercury-based cuprates, characterized byT _c 's ranging from 27 to 132 K. The structure of the mercury compounds is closely related to those of the thallium cuprates TlBa₂Ca_m–1Cu _m O_2m+3 which exhibit thallium monolayers; they differ from the Tl cuprates by a high oxygen deficiency at the level of the mercury layers due to the preference of this cation for two-fold coordination.     

SiNx/a-SiCx:H passivation layers for p- and n-type crystalline silicon wafers

In this work we present a detailed investigation of Si surface passivation obtained by a PECVD double dielectric layer, composed of intrinsic hydrogenated amorphous silicon-carbon (a-SiC_x:H), followed by a silicon nitride (SiN_x). The double layers have been deposited on p- and n-type of mono- and multi-crystalline silicon wafers. IR spectra have been carried out to evaluate the structure of a-SiC_x:H layers on monocrystalline wafers. The passivation effects have been studied performing the following measurements: the photoconductance decay, to measure contactlessly the effective lifetime of passived mono and multi Si wafers; the capacitance voltage profile of Al/SiN_x/Si, Al/a-SiC_x:H/Si and Al/SiN_x/a-SiC_x:H/Si MIS structures, to estimate the field effect at the dielectric/silicon interface and individuate the passivation mechanism on silicon surfaces. It has been found that the mechanism of the surface passivation depends on the doping type of the silicon wafer. Indeed from C-V measurements it has been realized that the great amount of positive charge within the SiN_x is able to promote an inversion layer if it is deposited on a-SiC_x:H/Si p-type and an accumulation if it is grown on a-SiC_x:H/Si n-type.     

剥离型环氧树脂/粘土纳米复合材料热/力学性能

以质子化的2,4,6-三(二甲氨基甲基)苯酚(DMP-30)作为有机修饰剂对原始粘土进行改性,采用"粘土-丙酮淤浆复合法"合成了环氧树脂/粘土纳米复合材料。透射电镜观察显示粘土片层以3~5片为聚集单元均匀、无规地分布在整个环氧树脂的基体中,呈现高度、无规剥离形态。差示扫描量热测试表明粘土质量分数为3%时的环氧树脂/粘土纳米复合材料的Tg提高了10℃,这归咎于无规剥离结构及粘土片层与环氧树脂基体间较强的界面作用力。弯曲试验揭示,室温条件下纳米复合材料的弯曲强度随着粘土含量的增加而呈缓慢下降趋势;而在80℃时,弯曲强度则随粘土的加入而显著增加。     

Properties of PEDOT:PEG/ZnO/p-Si heterojunction diode

The zinc oxide (ZnO) and poly(3,4-ethylenedioxythiophene) bis-poly(ethyleneglycol) (PEDOT:PEG) films were deposited on p-Si substrate by sputter and spin coating methods, respectively. An organic/inorganic heterojunction diode having PEDOT:PEG/ZnO on p-Si substrate was fabricated. The barrier height (BH) and the ideality factor values for the device were found to be 0.82 ± 0.01 eV and 1.9 ± 0.01, respectively. It has been seen that the value of BH is significantly larger than those of conventional Au/p-Si metal–semiconductor contacts. The PEDOT:PEG/ZnO/p-Si heterostructure exhibits a non-ideal I–V behavior with the ideality factor greater than unity that could be ascribed to the interfacial layer, interface states and series resistance. The modified Norde's function combined with conventional forward I–V method was used to extract the parameters including the barrier height and series resistance. At the same time, the physical properties of ZnO and PEDOT:PEG films deposited by sputter and spin coating technique, respectively, were investigated at room temperature. The obtained results indicate that the electrical parameters of the diode are affected by structural properties of ZnO film and PEDOT:PEG organic film.     

Un nouveau fluorure de platine : PtIIPtIVF6

From a temperature dependent study of the platinum-flourine system, a new fluoride Pt^IIPt^IVF₆ has been isolated, which contains platinum in an oxidation state lower than that found in the already known platinum fluorides PtF₄, PtF₅ and PtF₆. The material has been characterized by various analytical methods. The fluoride Pt^IIPt^IVF₆ crystallizes in the rhombohedral LiSbF₆ - type structure (R3) with the lattice parameters $\text{a}\text{= 5.56}{}_5\text{A}\text{?}$, α = 53.85°. Magnetic studies on polycristalline samples have shown that Pt^IIPt^IVF₆ orders ferromagnetically below 16 K. This is the first example of platinum (+II) in a fluorine environment and with a high spin structure in an octahedral surrounding.     

XPS study of the a-C:H/Ti and a-C:H/a-Si interfaces

In this study ultrathin hydrogenated amorphous carbon (a-C:H) films have been grown onto the titanium and amorphous silicon (a-Si) overlayers by direct ion beam deposition using acetylene gas as a hydrocarbon source. X-ray photoelectron spectroscopy (XPS) was used for study of the DLC-Ti and DLC-Si interfaces. It was revealed that a-Si is a good interlayer for improvement of adhesion in the case of diamond-like carbon film deposition onto the steel substrate at room temperature. a-C:H film growth without substantial intermixing occurred on the a-Si. On the other hand, adhesion between the Ti interlayer and the diamond like carbon film was very sensitive to the deposition conditions (presence of the pump oil) as well as structure and stress level of the Ti film. It was explained by strong intermixing between the growing carbon film and Ti. Bad adhesion between the growing DLC film and Ti interlayer was observed despite formation of the TiC. At the same time, formation of the TiO_x was not an obstacle for good adhesion. It is shown that composition of the used hydrocarbon gas, structure of the Ti thin film and mechanical stress in it had greater influence on adhesion with a-C:H film than elemental composition of the Ti interlayer surface.