| 1. |
- Buyanova, Irina, 1960-, et al.
(author)
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Tunable laser spectroscopy of spin injection in ZnMnSe/ZnCdSe quantum structures
- 2002
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:12, s. 2196-
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Journal article (peer-reviewed)abstract
- Magneto-optical spectroscopy in combination with tunable laser excitation is employed to study exciton spin alignment and injection in ZnMnSe/ZnCdSe quantum structures. This approach enables us to selectively create preferred spin orientation and to separately monitor subsequent spin injection from individual spin states, thus shedding light on a possible source of spin loss. It is shown that the limited spin polarization in a nonmagnetic quantum well due to spin injection from a ZnMnSe-based diluted magnetic semiconductor (DMS) is not caused by a limited degree of spin alignment in the DMS, which is in fact complete, but rather occurs during subsequent processes.
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| 2. |
- Donchev, V., et al.
(author)
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High-temperature excitons in GaAs quantum wells embedded in AlAs/GaAs superlattices
- 2000
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In: Vacuum. - 0042-207X .- 1879-2715. ; 58:2, s. 478-484
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Journal article (peer-reviewed)abstract
- Photoluminescence (PL) spectra of GaAs quantum wells embedded in short-period AlAs/GaAs superlattices have been measured at 2 K and at room temperature. Two approaches have been applied in order to investigate the mechanisms of radiative recombination in these structures. In the first one, we studied the excitation density dependence of the PL intensity. In the second approach a line-shape analysis of the PL spectra is performed by means of a statistical model, which includes both free exciton, and free carrier recombinations. The fit based on this model reproduces with high accuracy the experimental spectra and allows to assess the relative contributions of excitons and free carriers to the radiative recombination process. The results of both approaches indicate the predominance of free excitons in the radiative recombination at room temperature.
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| 3. |
- Donchev, V., et al.
(author)
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Photoluminescene line-shape analysis in quantum wells embedded in superlattices
- 2001
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In: Materials science & engineering. C, biomimetic materials, sensors and systems. - 0928-4931 .- 1873-0191. ; 15:1-2, s. 75-77
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Journal article (peer-reviewed)abstract
- The temperature evolution of the main photoluminescence (PL) mechanisms, in GaAs quantum wells embedded in short-period AlAs/GaAs superlattices, is investigated. PL spectra are measured from 2 to 300 K. A detailed line-shape analysis of the PL peaks is performed by means of a statistical model, including both free exciton and free carrier recombination. The fits based on this model reproduce satisfactorily the experimental PL line shapes and allow to assess quantitatively the relative contributions of free excitons and free carriers to the radiative recombination at different temperatures. The results indicate the predominant role of free excitons in the radiative recombination up to room temperature, and are consistent with the mass action law. © 2001 Elsevier Science B.V. All rights reserved.
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| 4. |
- Wang, Xiangjun, et al.
(author)
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Growth and characterization of Na0.5K0.5NbO3 thin films on polycrystalline Pt80Ir20 substrates
- 2002
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In: Journal of Materials Research. - 0884-2914 .- 2044-5326. ; 17:5, s. 1183-1191
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Journal article (peer-reviewed)abstract
- Na0.5K0.5NbO3 thin films have been deposited onto textured polycrystalline Pt80Ir20 substrates using radio frequency magnetron sputtering. Films were grown in off- and on-axis positions relative to the target at growth temperatures of 500-700 degreesC and sputtering pressures of 1-7 Pa. The deposited films were found to be textured, displaying a mixture of two orientations (001) and (101). Films grown on-axis showed a prefered (001) orientation, while the off-axis films had a (101) orientation. Scanning electron microscopy showed that the morphology of the films was dependent on the substrate position and sputtering pressure. The low-frequency (10 kHz) dielectric constants of the films were found to be in the range of approximately 490-590. Hydrostatic piezoelectric measurements showed that the films were piezoelectric in the as-deposited form with a constant up to 14.5 pC/N.
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| 5. |
- Buyanova, Irina, 1960-, et al.
(author)
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Direct experimental evidence for unusual effects of hydrogen on the electronic and vibrational properties of GaNxP1−x alloys : a proof for a general property of dilute nitrides
- 2004
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In: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 70:24, s. 245215-245219
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Journal article (peer-reviewed)abstract
- Direct experimental evidence for dramatic effects of hydrogen incorporation on the electronic structure and lattice properties of GaNxP1−x alloys is presented. By employing photoluminescence excitation spectroscopy, postgrowth hydrogenation is shown to reopen the band gap of the GaNP alloys and to efficiently reduce the N-induced coupling between the conduction band states. By Raman spectroscopy, these effects are shown to be accompanied by hydrogen-induced breaking of the Ga-P bond in the alloy, evident from disappearance of the corresponding vibrational mode. According to the performed Raman and x-ray diffraction measurements, the hydrogenation is also found to cause a strong expansion of the GaNP lattice, which changes the sign of strain from tensile in the as-grown GaNP epilayers to compressive in the posthydrogenated structures, due to the formation of complexes between N and H.
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| 6. |
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| 7. |
- Donchev, V., et al.
(author)
-
Photoluminescence study of AlAs/GaAs superlattices containing enlarged wells
- 2000
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In: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 364:1, s. 224-227
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Journal article (peer-reviewed)abstract
- Photoluminescence (PL) spectra of MBE grown short-period AlAs/GaAs superlattices with one or two enlarged wells (5 and 12 nm) have been measured at 2 K. Sharp PL peaks corresponding to excitonic transitions between the lowest electron and heavy-hole states in the enlarged wells are observed. The excitonic transition energies are calculated by means of an envelope function based model, taking into account the exciton binding energies. The model incorporates a smooth potential at the interfaces, which is represented by a diffusion potential, the diffusion length being a parameter. The calculated and experimentally observed excitonic transition energies agree well if diffusion lengths of 3.5 and 4.5 monolayers are considered in the samples with and without a buffer layer, respectively. These values are consistent with the complicated nature of the growth kinetics and mechanisms of quantum heterostructures. The PL spectra reveal also complicated structures connected with the superlattice. Their qualitative discussion confirms the smooth potential model. Thus, an attempt is made to extend the analysis of complicated AlAs/GaAs heterostructures towards real interfaces, which is essential for advanced device fabrication.
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| 8. |
- Egilsson, T., et al.
(author)
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Excitation properties of hydrogen-related photoluminescence in 6H-SiC
- 2000
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:11, s. 7162-7168
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Journal article (peer-reviewed)abstract
- We have studied the excitation properties of a well-known hydrogen-related bound exciton (H-BE) photoluminescence (PL) in 6H-SiC. In the case of the so-called primary H-BE's, photoluminescence excitation (PLE) spectroscopy reveals several excited states that have not been reported previously. In order to explain these states we propose a pseudodonor model. The primary H-BE's are thus regarded as donors where strongly localized holes serve as the positive cores. From a comparison between the PLE spectra of the three different primary H-BE's corresponding to the three inequivalent substitutional lattice sites in 6H-SiC, we attempt to distinguish between the hexagonal and cubic lattice sites. We have also investigated the dependence of the optically induced quenching of the H-BE PL on the energy of the exciting light. We observe that the quenching of the H-BE PL is only efficient when the exciting light has energy above the threshold for phonon-assisted free-exciton (FE) formation or when its energy coincides with the energy needed for resonant absorption into the H-BE states. When creating FE's, we observe different types of behavior depending on the initial conditions. We argue that our results are best explained by the existence of two configurations of the same charge state of the H defect, namely a stable one: A (giving rise to the H-BE PL), and a metastable one: B (not revealed in the PL spectrum). The recombination of excitons bound at these two configurations can give rise to the transformations A?B and B?A. The existence of the B configuration is revealed through the effect of the B?A process on the temporal changes of the H-BE PL.
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| 9. |
- Egilsson, T., et al.
(author)
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Excitation spectra of nitrogen bound excitons in 4H- and 6H-SiC
- 2002
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 91:3, s. 2028-2032
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Journal article (peer-reviewed)abstract
- We report photoluminescence excitation spectra of the nitrogen (N) donor bound excitons (BE) in 4H- and 6H-SiC. The spectra reveal several excited states of the N-BEs. An attempt is made in the article to classify the N-BE states according to a simple shell model. © 2002 American Institute of Physics.
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| 10. |
- Egilsson, T, et al.
(author)
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Exciton and defect photoluminescence from SiC
- 2003
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In: Silicon carbide and related materials 2002. - 0878499202 - 9780878499205 ; , s. 81-120
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Book chapter (other academic/artistic)abstract
- Wide-bandgap semiconductors such as SiC, III-V nitrides and related compounds are attracting rapidly increasing attention due to their other, very interesting, physical properties which are often superior in many ways to those of conventional semiconductors. Steady improvements in crystal quality, and improved knowledge concerning their physical properties, are leading to rapid developments in high-power, high-temperature, high-frequency electronics and blue-light emitters. This book comprises the proceedings of the fourth European Conference on Silicon Carbide and Related Materials, held on the 1 to 5 September 2002 in Link3œping, Sweden. This conference series continued its tradition of being the main forum for presenting results, and discussing progress, among university and industry researchers who are active in the fields of SiC and related materials. These proceedings therefore document the latest experimental and theoretical understanding of the growth of bulk and epitaxial layers, the properties of the resultant material, the development of suitable processes and of electronic devices that can exploit and benefit best from the outstanding physical properties offered by wide-bandgap materials
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