摘要:

A model of free exciton magnetic polarons (EMPs) in diluted magnetic semiconductors is proposed on the basis of the localization of the hole in the magnetic potential well created by its own exchange field. The corresponding calculations of the free EMP binding energies, as a function ofMn2+ion concentration and temperature, are performed. The results are in good agreement with recent experimental measurements. It is found that free EMPs are stable up to relatively high temperatures(T≲30K) over a wide range of Mn concentrationsxwithout any other primary localization, e.g., alloy potential fluctuations. The dependence of the critical temperature of free EMP formation on the Mn concentration is determined for various diluted magnetic semiconductor materials, and is in good agreement with the experimental results forCd1−xMnxTe. The suppression of free EMPs under the application of an external magnetic field is also considered. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364364

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Epitaxial Fe–Si(111) monolayer films and[Fe–Si(111)/Cr(111)]4multilayer films were grown on Si(111) substrates by dc facing-targets sputtering. The thickness dependence of the Fe–Si layer and the Cr layer on the crystallinity and the magnetic properties was studied. In the case of Fe-7.2 wt&percent; Si(111) monolayer films, excellent soft magnetic properties were observed in films thinner than 200 nm. The sudden deterioration of soft magnetic properties was observed at the thickness of 200 nm. Torque measurements of the films revealed that the increase of effective anisotropy energy caused the deterioration of soft magnetic properties. In the[Fe-7.2 wt&percent; Si(111)/Cr(111)]4multilayer films, permeability increased with increase of the Cr layer thickness, and excellent soft magnetic properties of multilayer films were obtained with a Cr layer thickness of 20 nm. Soft magnetic characteristics were improved in the multilayered structure compared with the monolayer film with identical total thickness of magnetic layers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364365

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A range of exchange-coupled two-phase nanocomposites composed of hard magneticSm2Fe17N3and soft magnetic &agr;-Fe was prepared by mechanical alloying with a view to optimizing the hysteresis loop shape. The main variables were the crystallization conditions, the nitriding treatment, and the chemical additives. A model of the diffusion of nitrogen in the two-phase nanocomposite is proposed that explains how the presence of Fe permits the nitrogenation of samples at lower temperatures than in single phaseSm2Fe17materials. Studies of samples both resin bonded and cold compacted measured in open and closed circuits revealed that the correct choice of demagnetizing factor used to correct demagnetizing fields depends critically on the sample density. Transmission electron microscopy (TEM) studies of the materials prepared revealed grain sizes in the range 10–50 nm. The shape of the magnetic hysteresis loop and resulting magnetic properties reflects the grain size of both phases. Image analysis of high resolution scanning electron microscopy micrographs of etched samples showed that in general two to three soft grains cluster together and are surrounded by hard grains, but the grain sizes of both phases were found to be the same. The crystallization of the hard phase from the mainly amorphous precursor is the primary factor determining grain size. Zr and Ta were the most successful additives in controlling the grain growth during crystallization, reducing the grain size from 20–30 to 10–20 nm. High resolution TEM indicated the presence of a grain boundary phase between the crystallites of the two phases. This phase was confirmed in Mo¨ssbauer studies of samples where it seems to constitute 15 vol &percent; of the samples and has a significant effect on the coupling between the two phases. Susceptibility measurements are an effective indicator of the degree of coupling between the hard and soft magnetic phases. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364389

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

PolycrystallineBaTiO3thin films with thickness ranging from 2100 to 20 000 Å were prepared on platinum substrates using off-axis radio-frequency magnetron sputtering. The variation in room temperature permittivity of the films was investigated with respect to thickness using x-ray diffraction and transmission electron microscopy. All films were ferroelectric and their room temperature permittivity, which was significantly higher than previously reported values, showed a strong dependence on film thickness. Higher permittivity was attributed primarily to the presence of ferroelectric domains. The room temperature permittivity of the thin films showed large variations with grain size, as in the case ofBaTiO3ceramics. The increase in permittivity with increasing film thickness was attributed to the decrease in defect concentration with grain growth. The 20 000 Å film showed an abrupt decrease in permittivity and the presence of an intergranular phase having titanium-excess composition; these phenomena are discussed in terms of domain boundary pinning and recrystallization. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364366

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Ce-dopedBaTiO3(BTO) films were grown by pulsed laser deposition. Highlyc-axis-orientedBTO:Ce/YBa2Cu3O7−xbilayer structures on aSrTiO3(100) substrate were fabricated for electrical property characterization. Typical hysteresis and butterfly typeC–Vplots were observed. The remanent polarization and coercive field are 3&mgr;C/cm2and 20 kV/cm, respectively. Highlya-axis-oriented Ce-doped and undoped BTO films were also deposited on MgO(100) substrates. Their linear and quadratic electro-optic effects were measured and compared. The Ce-doped films showed a larger linear electro-optic coefficient and obvious hysteresis in the quadratic electro-optic coefficient. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364367

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We calculated the elastic strain relaxation in(GaN)n-(AlN)n,(GaN)n(AlxGa1−xN)nand(GaN)n(InxGa1−xN)nsuperlattices wherenis the number of layers in the superlattice cell. This calculation and a similar calculation for a semiconductor–insulator–semiconductor structure allowed us to determine the lower and upper bounds for the elastic strain relaxation in(GaN)m(AlN)nsuperlattices with arbitraryn/mratios, i.e., we determine a full range of the critical thicknesses forGaNm(AlN)nsuperlattices. The obtained theoretical results can also be applied to other superlattices based on III nitrides and their solid solutions. Our theory agrees with the experimental data for GaN-AlN superlattices. Also, we show that the piezoelectric effect may cause a large shift of the absorption edge in defect-freeGaNm(AlxGa1−xN)nsuperlattices. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364368

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this paper we present the Raman scattering of self-assembled InSb dots grown on (001) oriented InP substrates. The samples were grown by pulsed molecular beam epitaxy mode. Two types of samples have been investigated. In one type the InSb dots were capped with 200 monolayers of InP; in the other type no capping was deposited after the InSb dot formation. We observe two peaks in the Raman spectra of the uncapped dot, while only one peak is observed in the Raman spectra of the capped dots. In the case of the uncapped dots the peaks are attributed to LO-like and TO-like vibration of completely relaxed InSb dots, in agreement with high resolution transmission electron microscopy photographs. The Raman spectra of the capped dot suggest a different strain state in the dot due to the capping layer. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365169

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Efficient Er-related photo-, cathodo-, and electroluminescence at 1539 nm was detected from Er and O co-implantedn-type GaN on sapphire substrates. Several combinations of Er and O implants and postimplant annealing conditions were studied. The Er doses were in the range(0.01–5)×1015ions/cm2and O doses(0.1–1)×1016ions/cm2. GaN films implanted with2×1015Er2+/cm2at 350 keV and co-implanted with1016O+/cm2at 80 keV yielded the strongest photoluminescence intensity at 1539 nm. The annealing condition yielding the strongest Er-related photoluminescence intensity was a single anneal at 800 °C (45 min) or at 900 °C (30 min) in flowingNH3. The optimum O:Er ratio was found to be between 5:1 and 10:1. Co-implanting the GaN:Er films with F was also found to optically activate the Er, with slightly (20&percent;) less photoluminescence intensity at 1539 nm compared to equivalent GaN:Er,O films. The Er-related luminescence lifetime at 1539 nm was found to depend on the excitation mechanism. Luminescence lifetimes as long as 2.95±0.15 ms were measured at 77 K under direct excitation with an InGaAs laser diode at 983 nm. At room temperature the luminescence lifetimes were 2.35±0.12, 2.15±0.11, and 1.74±0.08 ms using below-band-gap excitation, above-band-gap excitation, and impact excitation (reverse biased light emitting diode), respectively. The cross sections for Er in GaN were estimated to be4.8×10−21cm2for direct optical excitation at 983 nm and4.8×10−16cm2for impact excitation. The cross-section values are believed to be within a factor of 2–4. ©1997 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.364369

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The fluorescence properties of Sm3+and Sm2+ions incorporated in silicate glasses were measured using steady-state and laser-induced line narrowing techniques and the effect of Al3+ions to the local structure around the Sm2+ion was investigated. Sm3+ion-containing glasses prepared by a sol–gel method were reacted with H2gas to form the Sm2+ions. The energies of the three Stark levels of the5D0→7F1transition were analyzed usingC2vsymmetry. The addition of Al3+ions facilitates dispersion of the Sm2+ions and increases the coordination number of the Sm2+ion due to the long distance between the Sm2+and O2−ions, resulting in the increased fluorescence intensity. The fluorescence intensity of the Sm2+and Sm3+ions considerably increases in glasses containing more than 5 mol &percent; Al2O3. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364370

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The optical absorption edge and the near-absorption edge characteristics of undoped ZnO films grown by laser ablation on various substrates have been investigated. The band edge of films onC[(0001)] andR-plane [(1102)] sapphire, 3.29 and 3.32 eV, respectively, are found to be very close to the single crystal value of ZnO (3.3 eV) with the differences being accounted for in terms of the thermal mismatch strain using the known deformation potentials of ZnO. In contrast, films grown on fused silica consistently exhibit a band edge∼0.1 eVlower than that predicted using the known deformation potential and the thermal mismatch strains. This behavior is attributed to the small grain size (50 nm) realized in these films and the effect of electrostatic potentials that exist at the grain boundaries. Additionally, the spread in the tail(E0)of the band edge for the different films is found to be very sensitive to the defect structure in the films. For films grown on sapphire substrates, values ofE0as low as 30 meV can be achieved on annealing in air, whereas films on fused silica always show a value>100 meV.We attribute this difference to the substantially higher density of high-angle grain boundaries in the films on fused silica. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364393

出版商:AIP    

年代:1997

数据来源: AIP