摘要:

The effect of indium doping in the graded‐index regions of the InGaAs/GaAs/AlGaAs graded‐index separated confinement heterostructure quantum well laser has been studied. It was found that the threshold current density can be greatly reduced by a proper amount of In doping in the graded AlGaAs region. This result is attributed to an improvement in material quality due to a reduction in group III vacancies. The effect of thermal treatment on laser performance has also been studied. Although high temperature annealing can significantly improve non‐In‐doped lasers, it has little effect on In‐doped lasers.

ISSN:0021-8979    

DOI:10.1063/1.354319

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Surface acoustic wave (SAW) waveguide‐coupled resonators are of considerable interest for narrow‐band filter applications, though to date there has been very little published on the acoustic details of their operation. As in any resonator, one must fully understand its mode structure and herein we study the SAW mode profiles in these devices. Transverse mode profiles in the resonant cavity of the device were measured at various frequencies of interest using a knife‐edge laser probe. In addition we predict the mode profiles for the device structure by two independent methods. One is a stack‐matrix approach adapted from integrated optics and the other is a conventional analytical eigenmode analysis of the Helmholtz equation. Both modeling techniques are in good agreement with the measured results.

ISSN:0021-8979    

DOI:10.1063/1.354320

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Fluttering oscillations are sometimes observed in the falling sheets of water associated with dams and waterfall fountains, and a theoretical simulation of this phenomenon was recently reported. In this study approximate analytical solutions of the previous model are described and compared with the experimental and theoretical observations.

ISSN:0021-8979    

DOI:10.1063/1.354321

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A tuned and heated Langmuir probe has been used for combating the effect of radio frequency (rf) interference and contamination of the probe surface on the measurements in pure silane rf glow discharges. The measured probeI‐Vcharacteristic curves in the discharges were surprisingly reproducible over the period of 1 h. The non‐Maxwellian electron energy distribution functions in pure silane rf discharges have been successfully measured using the probe. The mean electron energy and electron density in the discharges have been inferred from the electron energy distribution functions. The experimental results have been used for selecting the optimum parameters of discharge conditions for depositing ana‐Si:H film with high quality at a high deposition rate of 1 nm/s.

ISSN:0021-8979    

DOI:10.1063/1.354322

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The potential structure, electric field, and sheath width for the ion‐matrix sheath in a cylindrical bore are calculated. The appropriate scaling length for this problem is found to be &sqrt;2 times larger than the planar ion matrix sheath width. If the radius of the bore is less than this scaling length, then the sheaths from ‘‘opposite’’ sides of the bore overlap, and the potential drop from the axis to the sidewalls decreases with the square of the bore radius. This limits the maximum energy of ions impacting the target. For bores larger than this scaling length, the asymptotic planar solutions are rapidly approached.  

ISSN:0021-8979    

DOI:10.1063/1.354323

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The frequency dependence of the sustaining voltage in a radio frequency discharge has been investigated under minimum sustaining and constant power conditions in Ar. In addition to the well‐known feature that the sustaining voltage is high at low frequency and low at high frequency, the difference between monoatomic and polyatomic gases is recognized. The phase shift between voltage and current at 1 Torr decreases with increasing frequency above 500 kHz. This results from the electron density modulation having a delay with respect to the applied voltage. The phase shift at high frequency increases with increasing applied voltage, due to the balance of drift and diffusion fluxes of electron. The spatiotemporal net excitation rate of Ar(3p5) was measured at 100 kHz, and 4 MHz, and 13.56 MHz. It is reconfirmed that the sustaining mechanism of the discharge at low frequency is ionization by secondary electrons from the electrode, while at high frequency it is ionization due to reflected electrons by the oscillating sheath. At middle frequency, 4 MHz, the sustaining mechanism drastically varies with input power density from the high‐frequency type to the low‐frequency type.  

ISSN:0021-8979    

DOI:10.1063/1.354324

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The influence of plasma shielding on the coupling of laser energy to a target surface during picosecond pulsed laser–material interactions is demonstrated using a He and Ar gas atmosphere. An inductively coupled plasma‐atomic emission spectrometer (ICP‐AES) is used to monitor the quantity of copper material removed during picosecond and nanosecond pulsed‐laser sampling. The intensity of Cu iemission from the ICP‐AES was found to be 16.4 times larger with He as the gas medium compared to Ar during picosecond laser sampling. It was also observed that depth of craters in the copper targets decreased as the gas pressure was increased beyond 10 Torr in Ar and 100 Torr in He. Possible mechanisms of shock waves, multiphoton ionization, and plasma shielding to explain these observations are discussed. For plasma shielding to occur in the picosecond time regime, the existence of high‐energy photoelectrons emitted from a Cu sample during the leading edge of laser pulse is postulated. These electrons form a plasma in the gas above the target via an inverse bremsstrahlung process and the plasma absorbs part of laser energy. The electron density versus pressure was calculated from a simple model and found to have similar behavior as the crater‐depth data.

ISSN:0021-8979    

DOI:10.1063/1.354325

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electrical characteristics of rf discharges in SF6and in its mixtures with N2were experimentally investigated. In addition space‐ and time‐resolved emission spectroscopy was used to gain a better understanding of kinetics of processes leading to various observed characteristics. A complicated dependence of minimum sustaining voltage on frequency was observed with a peak at 3 MHz. It was explained as the result of transition from the conditions where discharge is sustained by ionization in the bulk and the double‐layer region to the conditions where secondary electron yield makes a large contribution. Another possible explanation is the one invoking transition from the conditions where at high‐frequency double layer is formed by electron modulation to the condition where a double layer is formed by positive and negative ions. In voltage‐current dependencies at 13 MHz two distinct regions were observed similar to the &agr; to &ggr; transitions observed for electropositive gases. Ionization by secondary electrons, however, is not supported by spatiotemporal emission measurements, thus explanation may be sought in different processes such as development of double layers and increased field in the bulk. The current‐to‐voltage phase is much smaller than in electropositive gases, thus the discharge appears more resistive. For a range of operating conditions the phase even becomes inductive due to negative ion inertia.

ISSN:0021-8979    

DOI:10.1063/1.354326

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The heat capacity ofa‐Si thin film prepared by electron beam evaporation method was measured from 360 to 820 K by a differential scanning calorimeter. For the as‐prepareda‐Si specimen, two novel irreversible endothermic processes and one irreversible broad exothermic reaction were found. The origins of the endothermic reactions were not known. It is suggested that they may be caused by a change in the number and distribution of voids that occurs at approximately 465 K and the creation of dangling bonds at the higher temperature regime (≳620 K). The exothermic reaction is attributed to heat release during structural relaxation. When measuring the heat capacity ofa‐Si,Ca‐Sip, these irreversible reactions were first eliminated by annealing the specimens at high temperatures. The heat capacity of crystalline Si,Cc‐Sip, was also measured and the difference, &Dgr;CSip=Ca‐Sip−Cc‐Sip, was used to evaluate the thermodynamic melting temperature of thea‐Si,Tal, which is determined to be 1400 K.

ISSN:0021-8979    

DOI:10.1063/1.354329

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The electrical and optical properties of Er‐implanted Si are shown to be critically dependent on the presence of impurities and defects. A large enhancement in the electrical activation of Er (up to three orders of magnitude) is obtained by coimplanting Er with O or C at 300 °C. The use of C also allows one to obtain a good quality crystal after implantation and annealing. This is shown to be crucial in the photoluminescence process. In fact, in spite of the large amount of active Er atoms, photoluminescence is inhibited in the presence of the high concentration of precipitates and crystallographic defects which are left after annealing of the Er and O coimplants. The photoluminescence intensity is, on the other hand, enhanced by the high concentration of active Er atoms in the defect‐free crystal which is left after annealing of the Er and C coimplants. Moreover, a clear shift in the main photoluminescence peaks is observed in Er‐ and C‐coimplanted samples as a result of the different surroundings experienced by the Er atoms.

ISSN:0021-8979    

DOI:10.1063/1.354330

出版商:AIP    

年代:1993

数据来源: AIP