摘要:

We have studied the soft laser sputtering of (100)GaAs with 337 nm photons, starting from the threshold for particle emission (a few tens of mJ/cm2) to some 300 mJ/cm2fluences. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured, and their time of flight determined using laser resonant ionization mass spectrometry. The surface after laser irradiation is examined by scanning electron microscopy and electron microprobe analysis.One observes a significant preferential emission of arsenic in the form of As2. This leads to the formation of perturbed Ga‐rich surface structure which appears even at low fluence and after a few tens of laser shots on the same spot. This initial transformation seems to determine the further evolution of the irradiated surface. First, Ga atoms aggregate to form Ga islands on the surface; after a sufficient number of shots, micrometric structures are produced which finally behave as pure Ga metal. This evolution of the surface state after multipulse irradiation appears practically the same for low and medium laser fluences, the only difference being in the number of shots required to obtain the same microscopic structure. The velocity distribution of Ga atoms and As2molecules is well fitted by half‐space Maxwellian distributions. The kinetic temperatures are in broad agreement with the results obtained from a model of laser heating of the surface. The gross features of the experimental results can be interpreted from the particular thermodynamics properties of GaAs which exhibits very large As2pressure above the solid as soon as the temperature exceeds 950 K. After a few laser shots, corresponding to particle emission from defect sites, the thermodynamics of GaAs appears to govern the further evolution of the laser‐sputtered surface. Two sputtering regimes are evidenced: In the low‐fluence regime (from threshold to 90 mJ/cm2) sputtering appears to be dominated by surface defect emission, whereas for higher fluences emission is more characteristic of thermal process accompanied by preferential sputtering of arsenide. According to these experimental results, a simple analytical model was developed which relates the quantitative surface to the quantitative sputtered cloud compositions. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359970

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Multilayer polyimide structures may be used as an etch stop mechanism in laser ablation processing. By changing the optical properties of the polymer it is possible to better control the depth of the patterns produced by the lasers. We present the effect of the repetition rate in the ablation of multilayer structures. The results are obtained from a photothermal model that includes the light absorption by the decomposed fragments, which shield the polymer from the incoming laser beam. The model also includes an intermediate zone in which the polymer suffers a phase transition. The evolution of the temperature profiles is simulated during each pulse taking into account the pulse shape; however, a simple diffusion model is used between pulses. The results of the simulations indicate the range of values for which the multilayer structure may be used as an effective etch stop mechanism. We found that the effectiveness of the multilayer structure deteriorates for increasing repetition rates. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359971

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Filling of the pore network of porous silicon layers with Ge and Si has been demonstrated using a chemical vapor deposition (CVD) technique. It is shown that at low growth rate the species are deposited throughout the whole layer which can be completely filled. At high growth rate, the deposition takes place mainly on the top surface leading to pore mouth bridging. Investigation of the experimental data through a model shows that pore filling is a powerful tool for the study of the mechanisms involved in CVD processes at low temperature. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359972

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Dynamic processes of Vickers indentation on glass surfaces were observed with a newly developed apparatus. It enables us to take successive picture images of a growing indentation from behind a transparent specimen with a CCD camera through an optical microscope with a minimum time interval of 16.7 ms. Growing rates of indentations were measured under several conditions of loading weight and collision velocity of the indenter with the sample surface. An indentation rapidly grows at the initial stage, followed by decrease in growing rate, finally approaching a value specified with the loading weight and the hardness of glass. This behavior can be described by a Voigt model, indicating that ‘‘visco‐elastic’’ nature in a microscopic sense is involved in the indentation process of glass. It is also found that the initial growing rate highly depends on the collision velocity; that is, the larger the collision velocity is, the higher the initial growing rate is. The result suggests that local temperature rise in the vicinity of the indentation due to friction or adiabatic compression reduces the viscosity of indented material. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359973

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Mechanical milling of the orderedL12‐Ni3Al was performed in a high‐energy ball mill. The milling process was monitored by x‐ray diffraction, high‐field magnetization, ac magnetic susceptibility, and differential scanning calorimetry (DSC). It was found that antisite disorder is generated in the early stages of milling and a phase transformation from the disorderedL12compound to the disordered fcc solid solution of Al in Ni was observed after milling for long periods. Partial amorphization occurs after prolonged periods of milling. The long‐range‐order parameter decreases monotonously with time in the early stages of milling and attains a zero value after relatively short periods of milling. The lattice parameter and the relative strain increase with milling time. Magnetic properties of ball‐milled Ni3Al at 4.2 K differ markedly from those of the ordered state. Based on the magnetic data, it is concluded that short‐range disordering could be the third source of energy storage during milling of Ni3Al. Exothermic heat effects resulting from atomic reordering and phase restoration and/or crystallization are evident in the DSC scans of ball‐milled Ni3Al. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359974

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Transparent glass ceramics containing &bgr;‐PbF2:Tm3+crystallites have been prepared by heat treatment of glasses in GeO2‐PbO‐PbF2‐TmF3system. Average crystallite size of &bgr;‐PbF2evaluated from the full width at half maximum of x‐ray diffraction lines varies from about 16 to 20 nm as the heat treatment temperature increases from 340 to 400 °C when 50GeO2⋅40PbO⋅10PbF2⋅1TmF3glass is heat treated. The glass ceramics containing &bgr;‐PbF2:Tm3+of 16 nm is transparent in the visible region. The incorporation of Tm3+into &bgr;‐PbF2crystalline phase has been demonstrated from the linewidth and peak position of optical absorption spectrum. Frequency upconversion fluorescence has been observed for the transparent glass ceramics at room temperature. The upconversion fluorescence intensities due to the1D2→3H6and1D2→3F4transitions are higher for the transparent glass ceramics than for the glass. This phenomenon is explainable by assuming that the multiphonon relaxation rate of the3H4state is smaller in the glass ceramics than in the glass. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359975

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experiments are presented on the plasma‐assisted decomposition of dilute concentrations of methanol and trichloroethylene in atmospheric pressure air streams by electrical discharge processing. This investigation used two types of discharge reactors, a dielectric‐barrier and a pulsed corona discharge reactor, to study the effects of gas temperature and electrical energy input on the decomposition chemistry and byproduct formation. Our experimental data on both methanol and trichloroethylene show that, under identical gas conditions, the type of electrical discharge reactor does not affect the energy requirements for decomposition or byproduct formation. Our experiments on methanol show that discharge processing converts methanol to COxwith an energy yield that increases with temperature. In contrast to the results from methanol, COxis only a minor product in the decomposition of trichloroethylene. In addition, higher temperatures decrease the energy yield for trichloroethylene. This effect may be due to increased competition from decomposition of the byproducts dichloroacetyl chloride and phosgene. In all cases plasma processing using an electrical discharge device produces CO preferentially over CO2.

ISSN:0021-8979    

DOI:10.1063/1.359976

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Conventional and electromagnetically casted multicrystalline silicon solar cells are fabricated following different passivation schemes. Thin layers (∼100 A˚) of thermal dry and plasma‐enhanced chemical‐vapor‐deposition (PECVD) SiO2are implemented for surface oxide passivation of multicrystalline silicon solar cells and compared. It is found that growing thin layers of thermal dry oxide results in efficient surface passivation. However, for thin PECVD SiO2layers it is necessary to perform low‐temperature forming gas anneal, postdeposition, in order to observe the surface passivation effect. In addition, hydrogen plasma passivation has been optimized for achieving deep penetration of atomic hydrogen in the material (≳30 &mgr;m) and as a consequence very effective bulk passivation of multicrystalline silicon solar cells. By combining front and back thermal dry SiO2passivation with hydrogen remote plasma treatment, a cell efficiency of 17% (independently confirmed) on 4 cm2area and 180 &mgr;m thickness is realized without any Al gettering. On the other hand, the cell efficiencies obtained using thin layers of PECVD SiO2are found to be very comparable to the efficiency of the cells fabricated with thermal dry SiO2layers provided that PECVD Si3N4/SiO2are used as a double‐layer antireflection coating. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359977

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This paper presents results of a microscopic photoluminescence (&mgr;‐PL) characterization of strained InAs0.5P0.5/InGaAsP multiquantum well (MQW) crystals with a series of well numbers (n=2–14). This method is useful in detecting misfit dislocations in the crystals, especially at the initial stage of their generation. The &mgr;‐PL intensity profile measurements of a laser diode (LD) cavity shows the intensity, being mainly influenced by the [01¯1]‐directed misfit dislocations, is closely correlated to the threshold current of the LD. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359978

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An experimental method, the thermally stimulated bias stress (TSBS) method, is presented to evaluate the stability of amorphous silicon thin film transistors (TFTs). TSBS experiment monitors the drain currents under constant gate bias while increasing the temperature at a constant rate. The changes of the drain current during TSBS experiment are analyzed by numerical calculation, adopting the conventional stretched exponential formula which describes the degradation behavior of TFT under bias stress at constant temperature. The TFT stability is characterized by stretched exponential parametersE&tgr;andT0, which are related to the effective barrier height and the width of the distribution of barriers for bias induced degradation. For +20 V gate bias stress,E&tgr;=0.88 eV andT0=850 K are obtained. Higher bias stress lowers the effective barrier heightE&tgr;. For three different cases of the initial state of a TFT (annealed, light soaked, and bias stressed), stabilities are investigated by TSBS method. For the initially light soaked case, parametersE&tgr;andT0are about the same as those of the annealed case. But for the initially bias stressed case,E&tgr;increases andT0decreases. We discuss the effects of gate bias and initial treatment on the bias induced instabilities in terms of charge trapping and defect creation models. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360750

出版商:AIP    

年代:1995

数据来源: AIP