摘要:

X-ray absorption above theCr Kedge has been used to study two Cr-doped olivine-structure materials. ForMg2SiO4,the extended fine structure shows that Cr resides in octahedral and tetrahedral sites with Cr–O distances of1.98±0.03and1.68±0.03 Å,while the near edge structure indicates that Cr is in3+and4+oxidation states. ForCa2GeO4,site-size constraints limit Cr to the tetrahedral site with a Cr–O distance of1.745±0.02 Å,and this Cr is only in the4+oxidation state. This study illustrates the utility of x-ray absorption for probing and relating the oxidation states and sites of constituent elements. In this specific case of Cr in olivine hosts, such understanding is critical to the development of these materials as tunable infrared solid-state lasers: Cr in the4+state provides the desired emission centered near 1.3 &mgr;m. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119615

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report the deliberately controlled growth of epitaxial metallic oxideSrRuO3thin films in three distinctly different growth modes. Scanning tunneling microscopy and x-ray diffraction indicate that the growth mechanism for films on exact (001)SrTiO3substrates is two-dimensional nucleation, which results in a two domain in-plane structure. As the miscut angle of vicinal (001)SrTiO3substrates is increased, the growth mechanism changes to step flow which leads to single domain thin films. Films on (001)LaAlO3substrates have an incoherent three-dimensional island growth due to the large lattice mismatch, resulting in a bulk-like lattice. The vast difference in the growth mechanisms of these films leads to a corresponding difference in their electrical transport and magnetic behavior. Such nanoscale control of growth mechanism, surface morphology, and domain structure can be very important in the fabrication of novel perovskite oxide devices. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119616

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A model of the ordering mechanism in SiGe films is developed to explain the occurrence of two types of ordered structures. We investigate the stability of ordered structures by strain energy calculation. It is suggested that atomic diffusion, which is enhanced by strain in a film, influences the formation of ordered structures. The process of atomic exchange that forms the ordered structure at a reconstructed surface during growth is also discussed. ©1997 American Institute of Physics. 

ISSN:0003-6951    

DOI:10.1063/1.119617

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A physical effect of ultrasound induced lubricity is reported. We studied the dynamic friction dependence on out-of-plane ultrasonic vibration of a sample using friction force microscopy and a scanning probe technique, the ultrasonic force microscope, which can probe the dynamics of the tip–sample elastic contact at a submicrosecond scale. The results show that friction vanishes when the tip–surface contact breaks for part of the out-of-plane vibration cycle. Moreover, the friction force reduces well before such a break, and this reduction does not depend on the normal load. This suggests the presence on the surface of a layer with viscoelastic behavior. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120417

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Experimental evidence is introduced for circularly birefringent and dichroic behavior in aMgF2thin-film helicoidal bianisotropic medium (TFHBM), which is a rotationally inhomogeneous and anisotropic material. Optical rotation (OR) of an incident linearly polarized monochromatic plane wave is observed upon transmission through theMgF2/glassTFHBM/substrate system, and it is accompanied by an ellipticity in the polarization of the transmitted plane wave. Both OR and ellipticity spectra have remarkable features within a narrow wavelength zone not unlike the Cotton effect in isotropic chiral media. The features in the experimental spectra are in accord with theoretical predictions. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119618

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report on infrared spontaneous emission between subbands in GaAs quantum wells excited by an interband optical pumping. The active region consists of 15 periods of four coupled quantum wells which are embedded in a mid-infrared optical waveguide. Electrons are selectively injected from the valence band in the excited subbands of the quantum wells using interband optical pumping. Intersubband spontaneous emission is observed at low temperature between theE5andE4subbands of the quantum wells(E5−E4≈162 meV≈7.7 &mgr;m).The intersubband luminescence vanishes when theE5subband is not selectively populated. The emission is polarized along the growth axis of the quantum wells as expected for intersubband transitions. The collected infrared power exhibits a linear dependence with the interband optical power with a slope≈560 pW/W.©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119619

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Thermal admittance spectroscopy has been used to determine the ground-state energies of the boron impurity in6H-SiC. The background doping,NA−ND,of the samples used in this study ranged from3×1016to1×1018 cm−3.From electron spin resonance studies, it is known that boron substitutes for silicon in the silicon carbide lattice occupying three inequivalent sites. Using admittance spectroscopy the ground state energies ofEv+0.27 eV,Ev+0.31 eV,andEv+0.38 eVwere determined for the shallow boron acceptor in6H-SiC. The free carrier concentration does not appear to be the only determining factor for which the boron acceptor level is observed. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119620

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The electronic structure of hydrogenated amorphous silicon layers is calculated within the empirical tight binding approximation. We predict an important blueshift due to the confinement for layer thickness below 3 nm, and we compare with crystalline silicon layers. The radiative recombination rate is enhanced by the disorder and the confinement but remains much weaker than that in direct band gap semiconductors. The comparison of our results with experimental data shows that the density of defects and localized states in the studied samples is quite small. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119621

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

ZnSe molecular beam epitaxial growth on GaAs(110) substrates has been studied using reflection high-energy electron diffraction (RHEED), atomic force microscopy, and transmission electron microscopy. An atomically flat and low defect homoepitaxial buffer GaAs(110) was grown with high V/III ratio(⩾150)and at low growth temperature(∼430 °C).At the beginning of ZnSe growth on a GaAs(110) buffer epitaxial layer, RHEED oscillation was observed and no facet was seen on a pseudomorphic ZnSe(110) surface. Low defect ZnSe films (defect density⩽105 cm−2) were also obtained without the Zn preexposure process necessary for low defect ZnSe(001) growth. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119622

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

SiO2/Genanocrystal/SiO2structures have been fabricated by deposition of Ge film on aSiO2layer and subsequent oxidation of the structure at a temperature between 800 °C and 1000 °C. Secondary ion mass spectrometry results indicate that the Ge precipitates into the bulkSiO2at a density of1×1012 cm−2.Raman spectra show a sharp peak at300 cm−1for the nanocrystallized Ge. The nanocrystal diameter is determined to be 5 nm on average. In the metal–insulator–silicon structure, electron storage occurs in theSiO2/Ge/SiO2potential well via electron tunneling into the oxide film. Capacitance-voltage measurements indicate that flatband voltage(VFB)shifts to 0.91 V after the electron injection. TheVFBshift is attributed to the charge storing for a single electron per potential well.©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119623

出版商:AIP    

年代:1997

数据来源: AIP