摘要:

This paper presents an analytical moderate inversion drain current model for polycrystalline silicon thin‐film transistors based on localized deep and tail states in the grain boundary regions. As verified by the published data, using the analytical model, that as compared to the subthreshold region in the bulk iliccon metal‐oxide‐silicon (MOS) devices, the less steep slope of the moderate inversion region has been explained as due to the lowering in the potential barrier height. In addition, the analytical model provides an accurate prediction that with a smaller average trap state density from the grain boundary regions, the polysilicon thin‐film transistor shows a sharper moderate inversion behavior. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361046

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The electrical characteristics of rapid thermal nitrided and re‐oxidized low‐pressure chemical‐vapor‐deposited (LPCVD) silicon dioxide metal–oxide–silicon (MOS) structures were investigated. Both nitridation temperature and time affect the properties of the MOS structures as revealed by capacitance–voltage characteristics. Nitridation at 1000 °C for 15 s followed by re‐oxidation for 60 s at 1000 °C in an oxygen/nitrogen ambient was found to be superior to the same nitridation for 60 s with no re‐oxidation. Typical values of fixed charge and interface state densities for devices subjected to nitridation and re‐oxidation in a mixture of oxygen and nitrogen were 4×1010cm−2and 7×1010eV−1 cm−2, respectively. Avalanche electron injection using electric fields of 3–3.5 MV/cm produced positive shifts in flatband voltage for devices nitrided at 1000 °C for 15 s followed by re‐oxidation, whereas samples nitrided at 1000 °C for 60 s without the re‐oxidation yielded negative shifts in flatband voltage. An electron barrier height of 2.4 eV was found for these nitrided samples. These results strongly suggest that device quality MOS dielectrics for high‐voltage power MOS field‐effect‐transistors can be realized by nitridation/re‐oxidation of LPCVD oxide. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361047

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Open‐circuit voltage characteristics of InP‐based multiple‐quantum‐well (MQW) solar cells are studied experimentally and theoretically. Experimental results are presented for spectral response and terminal characteristics of In0.53Ga0.47As/InP MQW cells and compared with results from a corresponding bulkp‐i‐ncontrol cell. Open‐circuit voltages measured for these cells, and for other InGaAs/InP and InAsP/InP MQW cells reported in the literature, are then analyzed using a simple ideal theory for MQW cells which attributes open‐circuit voltage reductions to increased radiative recombination in the quantum wells. The large (0.19–0.33 V) measured open‐circuit voltage reductions which accompany introduction of the quantum wells are shown to agree with predictions from the ideal theory to within ∼0.03 V on average. Finally, implications of this work for the design of efficient InP‐based MQW solar cells are discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361048

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report a comprehensive investigation of structural, magnetic, and transport properties of as‐quenched and annealed CoxCu1−x(0≤x≤0.20) granular alloys prepared by melt spinning. Using x‐ray diffraction, differential scanning calorimetry measurements, and magnetic characterization, we have uncovered a Co phase separation process which results in the variation of magnetic and transport properties of Co–Cu heterogeneous alloys. In the Co composition range (0≤x≤0.15), the maximum giant magnetoresistance (GMR) was observed for CoCu samples annealed at 450 °C for 30 min, where Co particle diameters are in the range of 3.5–4.5 nm. The variation of magnetic and transport properties with the concentration and size of precipitated Co clusters is discussed, and is consistent with the prediction of the two‐channel model, in which spin‐dependent scattering is dominated by the cluster‐matrix interfaces. The reduction of GMR in high Co concentration is attributed to the appearance of magnetic coupling among magnetic particles. In contrast, very small particles tend to behave superparamagnetically, resulting in the reduction of the ratio of spin‐dependent scattering to spin‐independent scattering potentials, and thus in the reduction of the GMR effect. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361049

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We analyze the effect of oxygen content and cation substitution on the anisotropy of high‐Tcsuperconductors. On the basis of the Josephson coupled block model, the relationship between the anisotropy parameter &ggr; and the oxygen doped (&dgr;, equivalent to doped oxygen) or ion substituted (x, substituted ion) in Y‐based systems is found to be &ggr;=&ggr;0/(1−x/xc), which gives a good explanation of the anisotropy variations of the superconducting samples YBa2Cu3O7−&dgr;, Y1−xPrxBa2Cu3O7−&dgr;, YBa2Cu3−xFexO7−&dgr;, and YBa2Cu3−xZnxO7−&dgr;; herexcis a fitting parameter. With the same phenomenological method, thex(or &dgr;) dependence of &ggr; of La2−xSrxCuO4−ycan be described as &ggr;=&ggr;0/x. It can be seen that the coupling between superconducting layers is closely related to the doping states. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361050

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Dependence of the remanent magnetization (Mr) anisotropy on the structure and shape of the superconductor and on the magnetizing procedure was studied for a number of high‐Tcsuperconductor ceramics and single crystals. The experiments were done by means of a vibrating‐sample magnetometer with a rotatable sample holder. It was found that the main contribution to the anisotropic behavior ofMris due to the surface screening currents, and therefore the anisotropy is subject to variation when sample shape is changed. The question is resolved, why the effective demagnetization factors for decoupled ceramic samples are different from those calculated in the inscribed‐ellipsoid approximation. Influence of inhomogeneous grain magnetization and global bulk currents on the angular dependencies ofMrin ceramic samples is investigated. The evolution of remanence in YBCO single crystals with an increase of the magnetizing field is described within an extended Bean model. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361051

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Physical vapor deposition technologies have been developed which allow the fabrication of multilayer structures consisting of two yttrium–barium–copper–oxide (YBa2Cu3O7−xor YBCO) layers separated by a thick (∼4 &mgr;m), low dielectric constant material. The YBCO is buffered from both the substrate and the other films with ion‐beam assisted deposited (IBAD) films of yttria‐stabilized zirconia (YSZ). The YSZ layer provides both the texture necessary to deposit high‐quality YBCO films and protection from the insulating layer material. Using these deposition processes, a variety of materials, such as Pyrex and Haynes alloy, may be used for the substrate. The critical temperature and current values obtained for the two YBCO layers of the completed structure were on the order of 85 K and 2×105A/cm2, respectively. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361052

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Meander line patterned infrared detectors, with values of resistanceRonsetat the onset temperatureTconsetof 3–5 k&OHgr;, were fabricated from YBa2Cu3O7−xsuperconducting material on MgO and SrTiO3crystalline substrates. Noise voltages from the samples were measured versus bias current, radiation modulation frequency, and temperature, in both the normal and superconducting states. Four major types of voltage noise were identified according to where they occurred in temperature relative toTconsetand the zero resistance temperatureTczero, and their dependence on frequency and bias current. They were also associated with the granularity of the superconducting film, which is related to the substrate material used. From these observations a specific cause for each type of noise is suggested. The results are as follows. (i) In the normal state with temperatureT≳Tconset, noise with a magnitude that is consistent with thermal (Johnson) noise is seen, but it depends linearly on bias current above a threshold value, at low frequencies.The suggested noise source is conductivity fluctuations due to Cooper pairs. (ii) Noise was found to occur belowTczeroin granular films. With increasing bias current its magnitude increases, and it shifts to a lower temperature range; however, the noise magnitude becomes constant as the current goes to zero. It is weakly dependent on frequency above 400 Hz. Suggested cause is voltage fluctuations in superconductor–normal–superconductor junctions at grain boundaries. (iii) This noise also occurs belowTczerowith peaks at various temperatures. With increasing bias current the peaked noise spreads to lower temperatures, but the noise goes to zero as the bias current goes to zero. Its suggested cause is magnetic flux tube motion. (iv) This noise occurs betweenTconsetandTczeroand is present in all samples, but lowest on samples prepared on SrTiO3substrates. Its suggested cause is fluctuations in the volume fraction of the superconducting phase along the current path. While the measured detectivityD* of our samples at a wavelength of 20 &mgr;m was only 106cm Hz1/2/W, engineering changes can be expected to raise the value to above 1010cm Hz1/2/W. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361053

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Structure and magnetic properties of the R2Fe17‐based quasiternary compounds (R=Sm, Er, Gd, Y, Pr, Nd; M=Al, Ga) and their nitrides have been investigated. The saturation magnetization of R2Fe17−xMxcompounds decreases with the increasing M content, while the M concentration dependence of the Curie temperature exhibits a maximum at aboutx=3.0 for the parent compounds. By nitrogenation the lattice parameters, the saturation magnetization, and the Curie temperature all have a corresponding increase compared with the parent alloys. The volume effects on the Curie temperature of these compounds have been studied. Thed ln Tc/d ln Vvalues were derived by comparing the change of the Curie temperature and volume before and after nitrogenation and show a linear dependence on the Curie temperatures for these systems investigated. This behavior can be quite well understood in terms of a combined model of localized and itinerant electrons suggesting more than 90%delectrons are localized, The same conclusion can be obtained from a analysis of the exchange fieldHexdependence of the iron sublattice magnetization in the R2Fe17compounds and their nitrides, and the iron proportionality constantVFebefore and after nitrogenation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361054

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Poly(vinylidene fluoride) (PVDF) was crystallized under high pressures on the basis of the pressure‐temperature (P‐T) phase diagram obtained recently, and piezoelectric and ferroelectric properties of the resulting films were evaluated. Crystallization in the metastable hexagonal phase appearing at high pressure and temperature below the triple point yielded a film comprising extended chain lamellar crystals of a mixture of &bgr; and &ggr; forms, the latter of which is transformed into &bgr; form by poling at an elevated temperature (120 °C). The poled film exhibits strong piezoelectric effect persistent up to the melting temperature (205 °C). The electromechanical coupling factorktfor the thickness extensional piezoelectric effect is 0.27 at 25 °C, the largest value ever found in PVDF. The results of studies on morphology, structure, and thermal properties are also described. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361055

出版商:AIP    

年代:1996

数据来源: AIP