摘要:

A study of the mechanisms that determine the incorporation of nitrogen in Si3N4‐capped silicon by cw Ar+‐laser melting is presented. A theoretical description is given of the nitride capping layer erosion by the liquid silicon in the molten zone, the segregation at the slanted solidification interface, the diffusion in the molten zone of nitrogen expelled from the solid, and the solid‐state diffusion of nitrogen. This theoretical model is used to explain the shape and the dependency on laser melt conditions of the nitrogen profiles measured in Si3N4‐capped bulk silicon samples after laser melting. It is shown that the freshly frozen‐in nitrogen profile can be described by an exponentially decaying function of depth in the sample, which is modified by solid‐state diffusion and precipitation at the sample surface. It is suggested that the nitrogen concentration measured at the melt depth is indicative of the nitrogen concentration in the bulk of the molten zone. The results also apply to impurities other than nitrogen with a segregation coefficient smaller than 1 and to other continuous energy sources applied for superficial melting.

ISSN:0021-8979    

DOI:10.1063/1.354257

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The explosive crystallization of Si3N4layers deposited on silicon samples during laser melting of the silicon is investigated. The crystallization threshold is defined as the minimal laser‐beam power to start crystallization. The precipitate‐initiator hypothesis which states that the explosive crystallization is initiated by the formation of Si3N4precipitates in the molten zone is formulated. Based on this hypothesis the dependency of the crystallization threshold on the scan conditions and the sample structure can be completely and consistently explained. Methods to prevent crystallization are formulated.

ISSN:0021-8979    

DOI:10.1063/1.354258

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Calcium fluoride thin films have been deposited on various substrates by radio frequency sputtering of a hot‐pressed CaF2target in a pure argon plasma using load‐locked sputtering equipment. The sputtering power was less than 100 W to avoid crack formation and fracture of the target. As a result, the maximum deposition rate of films was only 2.1 nm/min. The films analyzed by Rutherford backscattering spectroscopy (RBS) were found to be fluorine deficient; depending on the deposition conditions, the content of oxygen atoms incorporated in the films can be as low as the RBS detection limit of 2 at. %. The crystallographic structure of the deposited material was identified to be a CaF2fluorite phase by x‐ray diffraction technique; this phase was preferentially oriented in the (111) direction for films of 0.4–0.5 &mgr;m in thickness. This preferred orientation vanished with increasing film thickness. Films of 2.5 &mgr;m in thickness were slightly (110) oriented. The sputter‐deposited CaFxfilms (withx≤1.85) exhibited compressive residual stresses. The apparent Knoop hardness of the films deposited on silicon substrates (harder than CaF2) and measured under a load of 10 and 20 g was about 3500 MPa, i.e., twice the hardness of the bulk CaF2compound. The surface and cross‐section morphology of CaFxfilms was examined by scanning electron microscopy. These films were resistant to oxidation in air at 500 °C. Tribological characteristics of CaFxfilms sputter‐deposited on cobalt‐based alloy disks were determined by pin‐on‐disk tests. The friction coefficient and wear resistance of these films sliding against Cr–C coated pin specimens in air at 500 °C were found to be encouraging. These films are potential candidates to serve as solid lubricant films for precision components (ball bearings) operating at high temperatures in oxidizing ambient.

ISSN:0021-8979    

DOI:10.1063/1.354259

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Initial stages of InAs deposition on EuBa2Cu3O7−y(EBCO) thin‐film superconductors with a SrF2interlayer are investigated by using synchrotron‐radiation photoelectron spectroscopy. Core‐level and valence‐band spectra can give imformation on both oxidation of InAs and reduction of the underlying EBCO with a very thin surface layer. These spectral evolutions demonstrate that SrF2does not affect the underlying EBCO, and that the two‐step growth procedure, comprising the initial InAs layer growth at room temperature and the top InAs layer growth at 200 °C, is effective in suppressing the interfacial redox reactions. Furthermore, cross‐sectional transmission electron microscopy and x‐ray‐diffraction studies comfirm that SrF2forms a uniform layer on the EBCO surface, and that the two‐step growth process can crystallize an InAs top layer, which is highly (111) oriented with a full width at half‐maximum of 0.25°.

ISSN:0021-8979    

DOI:10.1063/1.354260

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effect of electron cyclotron resonance plasma cleaning (100)Si surfaces with low energy Ne, Ar, and Xe ions is studied byinsitux‐ray photoelectron spectroscopy, high‐resolution cross‐sectional transmission electron microscopy, atomic force microscopy, and measurement of both the single‐surface reflectance and the modulated optical reflectance. Photoemission spectra show that complete removal of all surface contaminants with oxygen atoms knocked‐in from the original native Si oxide and noble gas atoms implanted into the Si substrate are a common characteristic of this plasma cleaning process. The oxygen concentration appears to decrease with ion energy for all three ions, whereas the noble gas concentration is inversely proportional to the ion mass and essentially independent of the ion energy. This low energy ion bombardment sputters the surface causing only point defects with Ne ions, formation of a continuous thin amorphous overlayer with either Ar or Xe ions, and in addition, occasional subsurface planar defects with Xe ions alone. The original smoothness of the monocrystalline Si surface is maintained with Ne ions, while an increase in surface roughness accompanies the amorphization caused by Ar and Xe ions. The magnitude of the roughness, which depends more on the energy than on the mass of either ion is insufficiently small to account for a change in specularity at visible wavelengths. An increase in both the single‐surface reflectance and the modulated optical reflectance is always observed after the plasma exposure and is shown to provide a sensitive measure of these changes in surface morphology with the latter technique being a somewhat more sensitive gauge of these effects.

ISSN:0021-8979    

DOI:10.1063/1.354261

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The reverse‐biased performance of a molecular‐beam‐epitaxy‐grown high‐power optothyristor has been systematically characterized for pulsed power‐switching applications. The device has aP+N‐SI‐PN+thyristor‐like structure with the bipolar junctions formed by AlGaAs. The semi‐insulating (SI) GaAs used is liquid‐encapsulated‐Czochralski grown, undoped, and 650 &mgr;m in thickness. It is found that the reverse‐biased optothyristor can be triggered by a light‐emitting diode operated at 10−5W, and miniature semiconductor lasers can trigger the switch with 132 A current using only a 1‐mm‐diam optical aperture. The reverse switchingdi/dtand the maximum peak current are reported as a function of blocking voltage. The effects of bipolar junctions on both sides of the SI‐GaAs are also reported by comparing the bulk photoconductive current with the optothyristor switched current. It is shown that a laser beam of 0.05 &mgr;J can be used to trigger on and switch about the same current as a 0.3 &mgr;J laser beam, suggesting the possibility of integrating miniature semiconductor lasers and the optothyristors on the same chip to form a portable, compact, high‐power solid‐state pulser.

ISSN:0021-8979    

DOI:10.1063/1.354262

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We report accurate two‐dimensional simulations of switch‐on speeds in hydrogenated amorphous silicon thin‐film transistors. The trap charge density along, or transverse to, the direction of semiconductor channel is highly nonuniform and the trap filling time dominates the switching time as compared to the transit time, which is about four orders of magnitude smaller. Near both contacts, direction of the transverse current is always upwards toward the insulator‐semiconductor interface due to the strong electric fields. However, at the central region of the channel, the transient current is quite complex and is discussed here. When the channel length varies from 2 to 10 &mgr;m, the switching‐on time is of the order of 10−3s. The occupation function everywhere displays a partial filling of higher‐energy trap states during the switch‐on. This is in contrast to results presented by other investigators. Finally, the relationship between the transit time and the switch‐on time with respect to the amount of trap states is discussed.

ISSN:0021-8979    

DOI:10.1063/1.354263

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The impedance and noise of a resistive superconductive quantum interference device (R‐SQUID) have been measured as a function of the dc and rf currents applied to it. The Josephson junction was adjustable so that data were also taken for several values of the junction critical current. The results were compared with the predictions of a resistively shunted junction (RSJ) model which takes into account the influence of the Josephson junction on the impedance and noise. The agreement was found to be quite good and demonstrates that the noise in the circuit is well understood. Use of the R‐SQUID as a noise thermometer below 1 K is assessed in terms of corrections due to the RSJ model. It is demonstrated how the dc and rf currents may be adjusted so that the total noise of the R‐SQUID is reduced to within 0.1% of the Johnson noise generated by the resistor alone. Under these conditions, the R‐SQUID may be used as a noise thermometer to determine thermodynamic temperature to this inaccuracy from 6 to 700 mK.

ISSN:0021-8979    

DOI:10.1063/1.354264

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An account of the operation of a step‐tunable, low‐power, continuous‐wave gyrotron, GYROTRON V, at the fundamental and second harmonic of the electron cyclotron frequency is presented. The design of this gyrotron was especially optimized for the second harmonic. The gyrotron operating frequency at the fundamental can be tuned from 150 to 300 GHz and extended by second‐harmonic operation to above 600 GHz. Under normal operating conditions where the accelerating voltage is 10 kV and the beam current is 50 mA, power levels in excess of 20 W are obtained at the fundamental and hundreds of milliwatts at the second harmonic. The paper includes observations of single mode operation at the second harmonic and mode competition between second harmonic and fundamental.

ISSN:0021-8979    

DOI:10.1063/1.354265

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We observed new transport properties in an InGaAs/InAlAs field‐effect transistor with source‐drain separation of ∼40 nm. Through measurements of real‐space transfer current to the gate we obtained clear evidence of ballistic electron transport in the two‐dimensional channel of the device. Another important new feature is the drain current saturation at low drain biases. This effect is explained by a poor electron exchange at the interface of a two‐dimensional gas with bulk at the device contacts.

ISSN:0021-8979    

DOI:10.1063/1.354266

出版商:AIP    

年代:1993

数据来源: AIP