摘要:

The luminescence properties of lead ions and their sensitizing effect on cerium activators in strontium sulfide thin‐film electroluminescence (TFEL) devices have been investigated. Polycrystalline SrS:Pb and SrS:Ce,Pb thin active layers for such TFEL devices have been prepared by multi‐source deposition. Emission spectroscopy under application of a high electric field (EL), undere‐beam excitation [cathodoluminescence (CL)], and under CL conditions with additional EL drive has been carried out. It has been found that the luminescence of SrS:Pb under EL drive is very weak, while the CL signal is considerably higher. This CL signal has shown strong quenching effects upon application of an additional ac voltage. The Pb2+emission dropped to some 5% at an applied voltage of 100 V0p, being roughly 40 V below EL threshold. More than half of this drop occurred between 0 and 30 V0p. A remarkably lower quenching effect on the Ce3+activator luminescence has been observed. Such luminescence quenching is attributed to the field induced ionization of the luminescent ions. From the obtained results it is concluded that the optical energy transfer from Pb2+sensitizers to Ce2+activators in SrS TFEL devices is low. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363225

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The electroluminescence and photosensitive surface effects from metal/porous silicon/Si sandwich structures based onn‐type ultraviolet‐porous silicon were investigated in this study. Contacts were made by bonding to semitransparent coatings of Au (deposited by a novel technique of thermal evaporation) via Al electrodes. In the case of electroluminescence, studied by applying a current across the electrodes, orange light is emitted beneath the semitransparent gold film, and a reversible avalanche breakdown was observed at the interface at a reverse bias of around 22 V. The photosensitivity of the surface, which was coated with a thin gold film having an initial transmission of less than 60%, was observed to get worse when the above process was repeated several times. Furthermore, the avalanche breakdown voltage shifted slightly to higher values, possibly because of both structural fluctuations in the porous networks and/or Schottky conductive changes in between the metal‐porous silicon interface resulting from the high current passed at breakdown and causing excess heat inside the device. Photovoltaic effects at the Schottky metal/porous Si barrier were also observed under a number of different types of illumination in the range from UV to visible, and under white light of different powers. The current–voltage characteristics of various sandwich structures operating as solar cells were measured, along with the power efficiency of the solar cells under a calibrated tungsten bulb, this being over 0.35% under an illumination power of 12 mW/cm2. The power efficiency (=−a+bPin) is found to increase linearly with increasing illumination power up to a saturation value that depends simply on preparation conditions. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363226

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Stress generated on GaAs‐SiO2interface during annealing and the effect of this stress on tin diffusion in GaAs have been investigated using laser Raman spectroscopy. It was found that compressive stress exists on the surface of GaAs after annealing, which increases with increase of the SiO2cap layer thickness. The compressive stress on GaAs is generated during annealing and is due to the difference of the thermal expansion coefficients between GaAs and SiO2. The increase of compressive stress on the surface of GaAs decreases the diffusion coefficient of tin in GaAs. This occurs due to the reduction of Ga vacancy in a compressively stressed sample from the equilibrium Ga vacancy of an unstressed sample. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363271

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have tracked down one of the major causes for nonuniformities in film thickness in large‐area deposition of hydrogenated amorphous silicon,a‐Si:H. To simulate improper substrate‐to‐electrode attachment we deliberately introduced a gap behind the substrate. The rf‐excitation‐frequency dependence of the influence of this gap on the deposition rate is presented. We show that a local small gap behind the glass has a detrimental effect on the local deposition rate, and therefore on the uniformity of the films. For example, at a frequency of 60 MHz typically the reduction of the deposition rate amounts to 25% when a gap of 1 mm is present. To explain the observed effects the plasma‐sheath dynamics are considered. The relations between the dc self‐bias voltage, the amplitude of the applied rf voltage, and the deposition rate are determined experimentally. A theoretical model that explains the reduction of the deposition rate is presented. We conclude from the model that the ion density in the sheath is independent of the excitation frequency. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363227

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Reflection high‐energy electron diffraction (RHEED) technique, with the possibility to shift, rotate, and tilt the sample with respect to the electron beam, has been used to investigate the structure of the transition layers formed at the interface between titanium substrate and diamond films. The diamond films were deposited on partially masked titanium sheets by means of the chemical vapor deposition technique, using CH4/H2mixtures activated by hot filament. Deposition experiments have been performed at 650 and 730 °C. The RHEED analysis has been carried out in selected area conditions on the boundary between coated and uncoated regions; it enabled us to detect and characterise the inhomogeneous and complex structural configuration of the diamond/titanium interface. For the samples deposited at 650 °C we identified, starting from the titanium surface: a mixed phase constituted by titanium hydride and carbide precipitates, graphitic clusters embedded into a titanium hydride/carbide phase, TiC layers, and finally diamond films. At 730 °C both growth location and formation sequence of the various structures were found to be similar to those detected at 650 °C, resulting, however, in the complete absence of the Ti hydride phase. A first indication about the time scale of the process has been determined from structural investigations of deposits grown by runs lasting from 3 up to 20 min. A schematic model is presented which describes the growth sequence of the various species inside the intermediate reaction layers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363228

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Laser ablation of graphite has been carried out using 1.06 &mgr;m radiation from aQ‐switched Nd:YAG laser and the time of flight distribution of molecular C2present in the resultant plasma is investigated in terms of distance from the target as well as laser fluences employing time resolved spectroscopic technique. At low laser fluences the intensities of the emission lines from C2exhibit only single peak structure while beyond a threshold laser fluence, emission from C2shows a twin peak distribution in time. The occurrence of the faster velocity component at higher laser fluences is explained as due to species generated from recombination processes while the delayed peak is attributed to dissociation of higher carbon clusters resulting in the generation of C2molecule. Analysis of measured data provides a fairly complete picture of the evolution and dynamics of C2species in the laser induced plasma from graphite. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363229

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We measured the spatial distribution of microwave fields and dissipative surface currents in various passive and active superconducting thin‐film devices during their operation at a temperature of about 4 K. We used low‐temperature scanning electron microscopy which offers a spatial resolution of about 1 &mgr;m and we present sample response models for interpreting the imaging results. Such investigations may have important consequences for the improvement of the design and preparation of superconducting thin‐film microwave devices. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363230

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Thepassivatedemitter,rearlocallydiffused(PERL) cells, fabricated in our laboratory, reach an efficiency of 24.0%, the highest value for any silicon‐based solar cell under terrestrial illumination. In an attempt to improve the rear surface passivation, which is usually obtained by a thermally grown oxide, we add a floating (i.e., noncontacted)p–njunction at the rear surface, resulting in thepassivatedemitter,rearfloatingp–njunction (PERF) cell design. Although these cells exhibit record 1‐sun open‐circuit voltages of up to 720 mV, their efficiency is degraded by nonlinearities (‘‘shoulders’’) in the logarithmicI–Vcurves. In order to understand and manipulate such nonlinearities, this paper presents a detailed investigation of the internal operation of PERF cells by means of numerical modelling based on experimentally determined device parameters. From the model, we derive design rules for optimum cell performance and develop a generalized argumentation that is suitable to compare the passivation properties of different surface structures. For example, the oxidized rear surface of the PERL cell is treated as an electrostatically induced floating junction in this approach and analogies to the diffused floatingp–njunction are drawn. Our simulations indicate that optimum rear surface passivation can be obtained in three different ways. (i) The floating junction of the PERF cell should be very lightly doped, resulting in a sheet resistivity of 5000 &OHgr;/&laplac;, and losses due to shunt leaking paths between thep–njunction and the rear metal contacts must be avoided. (ii) The rear surface of the PERL cell should be passivated by chemical vapor deposition of a silicon nitride film containing a larger positive interface charge density than exists in thermally grown oxides. (iii) An external gate can be added at the rear with low leakage currents and gate voltages of around 15 V. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363231

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The fluorescence from a thin ruby film, formed by epitaxial growth on a sapphire substrate, is shown to be a sensitive monitor of both the irradiation dose and the strain produced by irradiation of argon ions having an end of range exceeding the thickness of the ruby film. Decreases in fluorescence intensity are detectable for doses in excess of 1012cm2, whereas no damage is detectable by Rutherford backscattering spectrometry/channeling until doses almost two orders of magnitude larger. Using the systematic shift in fluorescence frequency observed with irradiation, it is concluded that lattice strain accumulates rapidly for doses in excess of 1014cm2. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363232

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A simple experiment involving only the measurement of dark currentIdarkand 1.1 &mgr;m photocurrentIPCin semi‐insulating (SI) GaAs allows an accurate determination of the electron capture cross section &sgr;nfor the important defect EL2 in GaAs. For 45 SI GaAs samples, from 12 different boules, grown by three different techniques, we find thatIPC/Idark=1.96±0.05 at 300 K. This relationship gives &sgr;n=1.4±0.4×10−16cm2, which is compared to previously estimated values. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363233

出版商:AIP    

年代:1996

数据来源: AIP