摘要:

A model for electron transport and emission in CsI is proposed. It is based on theoretically calculated microscopic cross sections for electron interaction with the nuclear and the electronic components of the solid. A Monte Carlo program based on this model was developed to simulate secondary electron emission induced by x rays and electrons in the energy range of 1 to 10 keV. The calculated secondary emission yields agree with existing experimental data. The model provides all necessary characteristics for the design of radiation detectors based on secondary electron emission. It can be expanded to higher incident energies and other alkali halides.

ISSN:0021-8979    

DOI:10.1063/1.351984

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Homoepitaxial growth of single crystalline 6H‐SiC layers is performed by chemical vapor deposition (CVD). 6H‐SiC substrates are grown by a sublimation technique. They have vicinal surfaces inclined 1.5° to 2° from the (0001) plane towards the [11¯00] direction. We report CVD growth at 1600 °C in the hydrogen‐silane‐propane gas system with nitrogen as a dopant. High quality films are achieved with growth rates of about 1.8 &mgr;m per hour. The layers are examined by optical microscopy, infrared reflection, photoluminescence, and Rutherford backscattering. For electrical characterization capacitance‐voltage and Hall measurements are performed. Unintentionally doped layers have donor concentrations in the upper 1015cm−3range. Electron mobilities of 370 cm2/V s at room temperature and about 104cm2/V s at 45 K are observed. To the authors’ knowledge this is the highest mobility so far reported for 6H silicon carbide.

ISSN:0021-8979    

DOI:10.1063/1.351985

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Amorphous powders of a Nd15Fe77B8alloy were obtained by mechanical grinding of the alloy ingots. Mechanical grinding of Nd15Fe7B8alloy ingots reduced the average particle size to 1 &mgr;m within the relatively short period of milling times. Extended milling did not yield further reduction in particle size, but resulted in a change in the structures from Nd2Fe14B crystallites to an amorphous structure. It was found that the crystallization of &agr;‐iron from amorphous matrix took place in some stage of milling. The amorphous materials were stable up to 820 K at a heating rate of 0.33 K/s. Heat treatment of the amorphous materials above the crystallization temperature resulted in the crystallization of the Nd2Fe14B phase and the resultant materials showed a high coercivity of 11.9 kOe as in the case for the amorphous Nd‐Fe‐B materials produced by rapid quenching techniques.

ISSN:0021-8979    

DOI:10.1063/1.351986

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We simultaneously doped bromine and iodine (electron acceptors) into C60(fullerene) which is known to become superconductive by doping alkali metals (electron donors). The simultaneous doping of bromine and iodine into C60brought about a hysteresis in the magnetic moment versus magnetic field curves as well as a structural change when the molar ratio of IBr (the 1:1 compound of iodine and bromine) to C60is over about 2.5.

ISSN:0021-8979    

DOI:10.1063/1.351987

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Because the yields of GaAs metal‐semiconductor field‐effect transistor (MESFET) devices are negatively impacted if ranges of threshold voltages (Vth) occur for sets of devices fabricated over the total area of the substrate, it is desirable to pinpoint the physical origin of this problem before a solution may be implemented. The presence of high dislocation densities encountered in wafers produced from liquid‐encapsulated Czochralski‐grown boules of GaAs may be responsible for the phenomenon of wide distributions ofVth, although this view is presently controversial. Accordingly, we have implanted silicon as a dopant into commercially supplied semi‐insulating GaAs wafers, formed arrays of MESFETs, and attempted to correlate the dislocation densities determined by Lang x‐ray topography with the magnitude of the scattering ofVth, including the observation of spikes, across the wafers in the ⟨110⟩ direction. The present results indicate that there is no clear correlation between either the magnitudes or the spatial distributions of the threshold voltages and the presence of high densities of dislocations. An alternative explanation is presented based on the presence of point defects such as interstitial arsenic in the samples.

ISSN:0021-8979    

DOI:10.1063/1.351988

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Recognizing the potential importance of diamond thin film growth from combustion environments, a computational investigation of diamond synthesis in low pressure premixed flames has been conducted. The model employed solves the two‐dimensional continuity, momentum, global energy, and species conservation equations in stagnation point flow geometry, and accounts for gas phase and surface reaction kinetics. The heterogeneous mechanism employed to describe diamond growth assumes that the methyl radical is the primary growth precursor. The gas phase mechanism includes elementary reaction pathways which generate methyl radicals from acetylene and in addition, includes a mechanism for cyclization (the formation of benzene) via acetylene and ethylene precursors. In this way, the pathway towards soot formation, which is believed to be a consequence of the formation of fused polycyclic aromatics, is shown to be a possible explanation for an eventual decrease in diamond growth rates at increasing fuel to oxygen flow ratios. A competition between oxidative pyrolysis of post flame hydrocarbons and cyclization establishes a criterion for optimum growth conditions.

ISSN:0021-8979    

DOI:10.1063/1.351989

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Aluminum oxide films were etched using low energy argon ions generated by a microwave electron cyclotron resonance (ECR) source argon plasma. The argon ion energies were controlled by biasing substrates placed on a 13.56 MHz capacitively coupled electrode. Reactively sputtered aluminum oxide films were used to study the relationship between the dc bias applied to these substrates and the etch rate of their films.Insitux‐ray photoemission spectra of the Al 2pand O 1stransitions showed that the ECR plasma was effective in completely removing native aluminum oxide and adventitious hydrocarbon in 1 min at ion energies as low as 100 eV. This preclean technology did not change the dielectric breakdown distribution of antenna structures with 12‐nm‐thick gate oxide capacitors.

ISSN:0021-8979    

DOI:10.1063/1.351990

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have shown that silicon homoepitaxial growth can be achieved by conventional rf diode sputtering at a low growth temperature (Ts∼400 °C). Spectroscopic ellipsometry and the electron channeling pattern are presented to show that the film deposited on a HF‐treated substrate is a single‐crystalline film while the film deposited without HF treatment is a polycrystalline film. Ar‐discharge cleaning of the substrate is also found to greatly improve the surface morphology of the deposited films.

ISSN:0021-8979    

DOI:10.1063/1.351991

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Hydrogenated amorphous silicon nitride (a‐SiNx:H) films have been prepared by plasma‐enchanced chemical‐vapor deposition with a mixture of SiH4and NH3at a substrate temperature of 250 and 300 °C. The properties of these films have been investigated using x‐ray diffraction, infrared absorption, photoluminescence, and electron probe microanalysis. From the x‐ray‐diffraction measurement, it is found that the short‐range order ofa‐SiNx:H with a basic unit of 4.31 A˚ appears when the nitrogen‐to‐silicon ratio in the film exceeds 0.75 at a substrate temperature of 300 °C. In the infrared‐absorption measurement, the sample was annealed repeatedly at various temperatures (300–800 °C) to identify the molecular unit responsible for each absorption peak. It is found that part of the infrared‐absorption bands between 870 and 1100 cm−1are caused not by the absorption but by the reflection of infrared radiation due to reststrahlen effect. The absorption peak at 840 cm−1is assigned to the isolated N in the Si‐host network, whereas the peak at 885 cm−1is assigned to a local bonding arrangement involving a terminal N‐H group attached to the amorphous Si network, Si‐NH‐Si. The transverse optical phonon of crystalline Si3N4is found to peak at 870 cm−1. Two peaks are observed in photoluminescence experiments when the mole fraction of ammonia in gas phase is smaller than 0.8 indicating the inhomogeneity of thea‐SiNx:H films. The higher‐energy peak is visible in the red.

ISSN:0021-8979    

DOI:10.1063/1.351992

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The surface recombination current at the emitter‐base heterojunction of AlGaAs/GaAs heterojunction bipolar transistors prepared either by liquid phase or molecular‐beam epitaxy have been studied. An ideality factor close to 1.5 was observed for those transistors with an undoped AlxGa1−xAs (x=0,0.1,0.2) spacer layer being employed in between the emitter‐base heterojunction. Evidence is provided indicating that the rate‐limiting process for surface current transport is the electron injection through the surface channel near the emitter‐spacer junction surface. It was also found that the emitter edge‐thinning design will be most effective when the emitter‐base junction is graded and the exposed base surface is a wide band‐gap AlGaAs layer. By using compositional grading of the emitter‐base junction and emitter edge‐thinning design, high gain (4200)npnAlGaAs/GaAs heterojunction bipolar transistors prepared by molecular‐beam epitaxy can be obtained.

ISSN:0021-8979    

DOI:10.1063/1.351941

出版商:AIP    

年代:1992

数据来源: AIP