摘要:

We have studied nonlinear refraction and absorption of band‐gap tuned hydrogenated amorphous silicon‐carbon alloys using 532‐nm picosecond degenerate four‐wave‐mixing, transient grating and excite/probe techniques. The highest induced absorption cross sections are obtained for low band‐gap samples. By comparison, grating diffraction efficiencies increase with band gap, indicating stronger refractive index contributions. The results from the four‐wave‐mixing and excite/probe experiments are combined to calculate refractive index cross sections &sgr;nand to derive the nonlinear coefficientsn2and &khgr;(3); these suggest useful strong optical nonlinearities in these amorphous alloys.

ISSN:0021-8979    

DOI:10.1063/1.345222

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Lithium‐doped ZnSe has been grown on (100) GaAs by molecular beam epitaxy. The epitaxial layers arep‐type with net acceptor concentrations (NA−ND) as high as 8×1016cm−3— the highest ever reported for molecular beam epitaxial ZnSe. Room temperature ac measurements show resistivities as low as 2.9 &OHgr; cm. Higher Li concentrations give rise to self‐compensation and a decrease inNA−ND. The details of the electrical and optical characterization of these layers are presented. Rudimentary blue light emittingpnjunction diodes have been fabricated. While these devices show dominant blue emission (463 nm) at room temperature, large turn‐on voltages indicate that thep‐ZnSe/p‐GaAs interface presents a large barrier to hole transport. Moreover, we find that difficulty in making device‐quality ohmic contacts top‐ZnSe is the next major obstacle to the fabrication of efficient blue light emitting diodes.

ISSN:0021-8979    

DOI:10.1063/1.345223

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Optical absorption and emission spectra have been measured for Nd‐doped Y3Al5O12ceramics obtained by a urea precipitation method. The optical properties of the ceramics are almost the same as those of single crystals grown by the Czochralski method and floating‐zone method, except for a higher background absorption of 2.5–3 cm−1. The energy‐level structure for Nd3+in the Y3Al5O12ceramics is determined for a 1‐at. % Nd concentration. The induced emission cross section for the 1‐at. %‐Nd‐doped ceramic is calculated to be 4.9×10−19cm2, which falls in the range reported for the Nd:Y3Al5O12single crystals. The Nd concentration dependence of the emission decay time at the 1064‐nm laser wavelength was also measured, and typical concentration quenching was observed. The decay time of the 1‐at. %‐Nd‐doped ceramic is 219 &mgr;s, which is slightly smaller than the reported values for single crystals.

ISSN:0021-8979    

DOI:10.1063/1.345224

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The near‐band emission mechanism of GaP crystals with very low concentration of nonstoichiometric deep levels has been studied at high temperatures from 77 K to room temperature. Free‐exciton recombination with emission and absorption of momentum conserving phonons at the site of shallow impurities are first found in the 77‐K photoluminescence spectra, which has not been observed in the low‐temperature luminescence. Also, excitons bound to ionized shallow acceptors Zn are identified. The free‐exciton recombination increases as the temperature increases and becomes the dominant emission mechanism at room temperature, with no observable effects of nonradiative centers.

ISSN:0021-8979    

DOI:10.1063/1.345225

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A theoretical model is presented to describe the time‐harmonic induction of eddy currents in a layered conductor containing a crack. The layered system consists of two parallel conducting plates separated by an insulating gap and it is assumed that the thicknesstSof the plate containing the (through‐the‐thickness) crack is small compared with the electromagnetic skin depth &dgr;S. Explicit expressions for the magnetic vector potential in the absence of the crack are derived for the particular case of induction by a circular air‐cored coil, and these results are used to calculate the change in coil impedance &Dgr;ZUdue to the induced currents in the uncracked system. The effect of a through‐the‐thickness crack is represented by an equivalent distribution of current sources along the line of the crack. The form of these generalized current vortex sources is examined and explicit expressions are presented for the current density and vector potential due to a single vortex. The vortex density, and hence the induced current distribution, are obtained by solving a one‐dimensional singular integral equation. This formalism is used to calculate the change in coil impedance &Dgr;ZCdue to a first‐ or second‐layer crack. Experimental measurements of &Dgr;Zwere carried out for several model systems and the theoretical calculations were found to agree with experiment to within 10% or better fortS/&dgr;Sup to 0.4. The implications for the detectability of cracks using eddy‐current nondestructive inspection is discussed.

ISSN:0021-8979    

DOI:10.1063/1.345227

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The transport process of sputtered Si atoms from the target to the substrate, crossing an argon‐hydrogen plasma, is modeled by using Monte Carlo techniques. The hydrogen and argon partial pressures, the dark zone voltage, and the target‐substrate distance are varied in the calculations. The effect of the above‐mentioned parameters upon the thermalization and energy distribution of atoms arriving at the substrate is calculated, allowing the determination of the growth conditions that minimize the damage produced on the films by highly energetic atoms, while maintaining a reasonable growth rate.

ISSN:0021-8979    

DOI:10.1063/1.345228

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The feasibility of aluminum nitride thin‐film deposition from chemical reactions of trimethyl aluminum (TMA) with active nitrogen downstream from a N2microwave plasma has been investigated. Chemiluminescence spectra of the downstream reaction showed the presence of CN and Al and the luminescence has been proposed to result from reactions between atomic nitrogen and methyl groups of the TMA. From nitrous oxide titration measurements of the atomic nitrogen concentration, the reaction rate constant has been estimated to bek=3×10−12cm3 s−1. Auger and infrared absorption spectroscopy measurements of a typical film indicated that AlN with carbon contamination in the form of CN had formed.

ISSN:0021-8979    

DOI:10.1063/1.345229

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Diamondlikea‐C:H films have been deposited by plasma decomposition of C2H2. 50 samples were prepared with a systematic variation of the deposition parameters: the substrate bias voltage was between −100 and −1400 V and the C2H2gas pressure was between 4×10−4and 2.6×10−1mbar. The following properties of the films were measured: the density by Rutherford backscattering, the total concentration of hydrogen by elastic recoil detection, the bonding ratiosp3/sp2by infrared spectroscopy, the internal stress by the bending beam method, and the hardness with a Knoop microhardness tester. It has been shown that the hardness and other mechanical properties cannot be correlated to the average carbon coordination numbermc. This is becausemcis calculated under the assumption of a homogenous single‐phase model, which does not seem to be justified. It is demonstrated that the mechanical properties can be explained by the application of a void model.

ISSN:0021-8979    

DOI:10.1063/1.345230

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A thermodynamic model is presented to account for the nongraphitizing behavior of small (<200 A˚) carbon black particles. Only pure carbon is considered, hence no compositional effects are included in these calculations. The ungraphitized carbon black particle is modeled as concentric turbostratic spheres of graphene layers. The partially graphitized particle is modeled to consist of a graphitic dodecahedral shell and a spherical turbostratic core. Free‐energy differences are calculated between a fully turbostratic particle and a partially graphitized particle. The temperature dependence of the free‐energy difference has not been included in these calculations. It is found that the effective activation barrier to graphitization increases with decreasing particle size. The concepts developed in this model are extended to explain the nongraphitizing behavior of all hard carbons.

ISSN:0021-8979    

DOI:10.1063/1.345231

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The slow wave radiation fields of a slotted line antenna are calculated inside the antenna region. For this, a realistic magnetic current distribution, which takes into account the wave‐slowing action of the antenna has been chosen. It is shown that the assumed distribution becomes accurate when the number of slots is large. The numerical results show that the radial component of the electric field is the dominant component, while the on‐axis value of the axial component of the magnetic field decreases with increasing antenna radius. Relevance of the results to high density plasma production using such structures is pointed out.

ISSN:0021-8979    

DOI:10.1063/1.345233

出版商:AIP    

年代:1990

数据来源: AIP