摘要:

The aim of this letter is to study, by using a time-reversal mirror (TRM), ways to overcome the distortions induced by multipaths in an acoustic transmission. A set of experiments is performed with a TRM made of 96 reversible transducers and results will be presented. One result is related to the high focusing property obtained with a TRM working in a bounded channel. It will be shown that the time-reversed beam is much thinner than the one measured in an unbounded medium. The second result concerns the time compression observed for the time-reversed acoustic pulse. The influence of the number of TRM elements on the time compression is discussed. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118730

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A plasma containing only silicon atoms and ions has been obtained by electron-beam evaporation of solid silicon through a helicon rf plasma source. The density of the silicon plasma in the diffusion chamber is3–5×1010cm−3,and the electron temperature 12 eV. These plasma conditions correspond to a predicted deposition rate from silicon ions of230±60nm/min, comparable to the deposition rate of 250 nm/min obtained using the same evaporation conditions, without generating a plasma. The large contribution of silicon ions, the high deposition rate, and the absence of other species such as hydrogen or argon, leads to novel conditions for plasma assisted deposition. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118699

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A nucleation model was proposed for a carbon nanotube enclosing a Ni bar which was grown by chemical vapor deposition (CVD) at 700 °C using round Ni particles. At an early stage of CVD, each round Ni particle with a diameter of about 30 nm is covered with graphite layers. The graphite-covered Ni particle is considered to be unstable because the graphite layers have a large curvature. This instability is thought to make the graphite-covered Ni particles transform into a Ni bar enclosed within a carbon nanotube. In order to verify this nucleation model, we show that the size of the round Ni particle is a decisive condition for carbon nanotube formation by CVD, and that an intermediate state of the transformation of the graphite-covered Ni particles to the carbon-nanotube-enclosed Ni bar was observed by transmission electron microscopy. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118700

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Generalized variable angle spectroscopic ellipsometry (gVASE) over the photon energy range from 1.5 to 3.5 eV has been used to study and distinguish the hexagonal and cubic phases of boron nitride in thin films (50–500 nm) deposited by magnetron sputtering onto (100) silicon. Furthermore, gVASE is used to characterize the anisotropic reflectivity of the hexagonal phase films with different microstructures. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118701

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

High-resistivity unintentionally-dopedIn0.49Ga0.51Plattice matched to GaAs has been grown via low-pressure metalorganic chemical vapor deposition at a reduced growth temperature. These layers have excellent surface quality and are single crystal. The resistivity increases exponentially as the growth temperature is decreased from 550 to 490 °C, resulting in a resistivity of∼109 &OHgr; cm for samples grown at 490 °C. In addition, the photoluminescence intensity decreases exponentially for growth temperatures below 550 °C, indicating an increase in nonradiative recombination related to an increasing trap concentration. For samples grown at 550 °C, constant capacitance deep level transient spectroscopy measurements show a strong broad peak at ∼200 °K with an ionization energy of 0.40±0.04 eV, verifying the presence of an electron trap. The gummel plot and I–V characteristics of an InGaP/GaAs heterojunction bipolar transistor (HBT) with a 2000-Å-thick InGaP buffer layer grown at 500 °C are identical to that of an HBT grown without the InGaP buffer layer, indicating that the semi-insulating InGaP layer is compatible with GaAs-based device epitaxy. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118702

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A binary Al–Co decagonal quasicrystal has formed after 100 keVCo+-ion implantation into pure Al with a dose of1.5×1017 ion/cm2,and the quasicrystal has shown a high thermal stability. The composition of stable decagonal quasicrystal is believed to beAl11Co4. Co+ion implantation into Al first produced a multiple layer surface structure. The first layer was an amorphous layer. The second layer consisted of a decagonal quasicrystal and a coexisting amorphous phase. The amorphous phase in the implanted region where composition is close toAl11Co4 was transformed into the decagonal quasicrystal during annealing in a temperature range between 550 and 600 °C. Possible transformation mechanisms for the experimental results are discussed. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118703

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report experimental and simulation results of capacitance of quantum well infrared photodetectors (QWIPs). We found that the QWIP capacitance displays unusual behavior as a function of voltage and frequency, deviatingfarfrom the constant geometric capacitance value. At high voltages, capacitance starts with a negative value at low frequencies, increases above zero with frequency, and eventually decays to the geometric capacitance value. The magnitude of negative capacitance exceeds the geometric capacitance by more than two orders of magnitude. Negative capacitance arises when the transient current in response to a voltage step is nonmonotonic with time. Simulation shows that this effect is due to nonequilibrium transient electron injection from the emitter resulting from the properties of the injection barrier and inertia of the QW recharging processes. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118704

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Ga self-diffusion in gallium phosphide (GaP) is measured directly in isotopically controlled GaP heterostructures. Secondary ion mass spectroscopy (SIMS) is used to monitor intermixing of69Gaand71Gabetween isotopically pure GaP epilayers which are grown by molecular beam epitaxy (MBE) on GaP substrates. The Ga self-diffusion coefficient in undoped GaP is determined to beD=2.0cm2 s−1exp(−4.5 eV/kBT)between 1000 and 1190 °C under phosphorus-rich condition. The self-diffusion entropy is found to be ∼4kB.©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118705

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The Monte Carlo method normally used for transport studies in semiconductors is extended and used to study free carrier absorption of subband gap radiation in semiconductors. The approach is applied ton-type silicon where we find very good agreement with experimental results and calculations based on quantum electrodynamics. This method also allows us to study free carrier absorption in semiconductors with a dc bias. With a dc bias, the absorption coefficient has a strong dependence on the polarization of the ac field. We show that the absorption coefficient can be directly related to the carrier temperature as well as the momentum and energy relaxation times. Thus important carrier dynamics information can be obtained from free carrier absorption measurements in the presence of a dc field. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118706

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A theoretical characterization of (001)-grown(Si2)m/(ZnS)2mmultiple quantum wells is presented. Confined states in the&Ggr;1cwell have strongX1cand light-hole admixtures, and there are two types of hole states in the valence band. Strong intersubband transitions can be exploited in the infrared for bothn- andp-doped devices. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118707

出版商:AIP    

年代:1997

数据来源: AIP