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- Buyanova, Irina A., et al.
(author)
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Effect of momentum relaxation on exciton spin dynamics in diluted magnetic semiconductor ZnMnSe CdSe superlattices
- 2005
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:16
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Journal article (peer-reviewed)abstract
- cw hot photoluminescence (PL) complemented by transient PL measurements is employed to evaluate momentum and spin relaxation of heavy hole (HH) excitons in ZnMnSe CdSe superlattices. The rate of acoustic-phonon assisted momentum relaxation is concluded to be comparable to the total rate of exciton decay processes, about (2-3) × 1010 s-1, independent of applied magnetic fields. In magnetic fields when the Zeeman splitting ? of the exciton states is below the energy of the longitudinal optical (LO) phonon (?LO), a surprisingly strong suppression of spin relaxation rate from the bottom of the upper spin band is observed, which becomes comparable to that of momentum scattering via acoustic phonons. On the other hand, dramatic acceleration of the spin relaxation process by more than one order of magnitude is found for the excitons with a high momentum K. The findings are interpreted as being due to electron and hole spin flip processes via exchange interaction with isolated Mn2+ ions. Experimental evidence for the efficient interaction between the hot excitons and Mn impurities is also provided by the observation of spin flip transitions within Mn2+ - Mn2+ pairs that accompany the momentum relaxation of the hot HH excitons. In higher magnetic fields ?= ?LO, abrupt shortening of the spin flip time is observed. It indicates involvement of a new and more efficient spin relaxation process and is attributed to direct LO-assisted exciton spin relaxation with a subpicosecond spin relaxation time. © 2005 The American Physical Society.
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- Buyanova, Irina, 1960-, et al.
(author)
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Control of spin functionality in ZnMnSe-based structures : Spin switching versus spin alignment
- 2003
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:11, s. 1700-
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Journal article (peer-reviewed)abstract
- The ability of attaining desired spin functionality by adjusting structural design is demonstrated in diluted magnetic semiconductor (DMS) quantum structures based on II-VI semiconductors. The following spin enabling functions are achieved by tuning the ratio between the rates of exciton spin relaxation within the DMS and exciton escape from it to an adjacent nonmagnetic spin detector. Spin switching is realized when using a thin layer of Zn0.95Mn0.05Se as a spin manipulator and is attributed to a fast exciton escape from the DMS preceding the spin relaxation. Spin alignment is accomplished in tunneling structures where the presence of an energy barrier inserted between a spin manipulator (a DMS-based superlattice) and a spin detector ensures a slow escape rate from the DMS layer.
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- Buyanova, Irina, 1960-, et al.
(author)
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On the Origin of Light Emission in GaNxP1-x
- 2002
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Conference paper (peer-reviewed)abstract
- Temperature dependent photoluminescence (PL) and absorption measurements are employed to clarify mechanism for light emission in GaNP alloys with low (< 4.1) nitrogen content. The PL emission in GaNP epilayers and GaNP/GaP multiple quantum well structures is shown to be dominated by optical transitions within deep states likely related to N clusters. With increasing N composition these states are shown to become resonant with conduction band of the alloy and thus optically inactive, leading to the apparent red shift of the PL maximum position. On the other hand, band-to-band recombination in the alloy remains predominantly non-radiative presumably due to the presence of a large number of competing recombination channels.
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- Buyanova, Irina, 1960-, et al.
(author)
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Radiative recombination mechanism in GaNxP1-x alloys
- 2002
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80, s. 1740-
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Journal article (peer-reviewed)abstract
- Based on the results of temperature-dependent photoluminescence (PL) and absorption measurements, the PL emission in GaNP epilayers and GaNP/GaP multiple quantum well structures with N composition up to 4% is shown to be dominated by optical transitions within deep states likely related to N clusters. With increasing N composition, these states are shown to become resonant with conduction band of the alloy and thus optically inactive, leading to the apparent redshift of the PL maximum position.
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- Buyanova, Irina, 1960-, et al.
(author)
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Resonant suppression of exciton spin relaxation in Zn0.96Mn0.04Se/CdSe superlattices
- 2003
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 93:10, s. 7352-
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Journal article (peer-reviewed)abstract
- Spin relaxation processes in strained Zn0.96Mn0.04Se/CdSe superlattices are studied in detail by using hot photoluminescence combined with tunable excitation spectroscopy. A drastic enhancement in occupation of the upper-lying |+1/2,-3/2> state of the heavy-hole excitons is observed when excitation photon energy is resonantly tuned near an integer number of the LO phonon energy above the |+1/2,-3/2> state. Assuming the Boltzmann distribution between the excitonic states, the spin temperature of the excitons is deduced to be as high as 85 K, well above the lattice temperature of 2 K. The observed behavior provides experimental evidence for a surprisingly strong suppression of spin relaxation from the upper spin-split excitonic branch for small values of wave vector.
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- Izadifard, Morteza
(author)
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Optical characterization of dilute nitride semiconductors and related quantum structures
- 2006
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Doctoral thesis (other academic/artistic)abstract
- Dilute nitrides (i.e. nitrogen-containing, anion-mixed III-V ternary and quaternary alloys) have recently attracted much attention due to their unusual fundamental properties promising for device applications in optoelectronics and photonics, such as highly efficient and low cost near infrared lasers, efficient visible light emitting diodes (LEDs), multi-junction solar cells, as well as heterojunction bipolar transistors (HBTs). In order to fully explore the potential of these new materials, detailed knowledge on their fundamental and material-related properties is required.The work presented in this thesis focuses on optical studies of the electronic structure, radiative recombination processes and also characterization of material properties of several dilute nitrides systems that are relevant to device applications, such as GaNP/GaP, GaNP/Si, GaNAs/GaAs and GaInNP/GaAs. The thesis is divided into two parts. The first part includes five chapters that give a general introduction to the research field and also describes experimental methods utilized in the research work. The second part contains seven original scientific papers.Papers I and II report detailed studies of effects of post-growth hydrogen incorporation on the electronic structure of GaNAs and GaNP alloys by using photoluminescence (PL), PL excitation (PLE), and Raman spectroscopies, as well as high resolution X-ray diffraction (HRXRD) measurements. Introduction of hydrogen in the alloys was found to cause passivation of N-related localized states. Additionally, profound and rather astonishing changes in the band structure upon H incorporation were observed, such as a recovery of the bandgap energies of the parental GaAs and GaP, i.e. deactivation of the N-induced bandgap bowing. In GaNP, this was accompanied by a reduction in the N-induced coupling between the conduction band states. Raman spectroscopy has showed that these effects are related to hydrogeninduced breaking of the Ga-N bond. Raman and HRXRD measurements have also shown that the hydrogenation caused a strong expansion of the GaNP lattice, which changes the sign of strain from tensile strain in the as-grown GaNP epilayers to compressive strain after hydrogenation, due to formation of complexes between N and H.Paper III-IV discuss optical quality and defect properties of GaNP/Si and GaNP /GaP alloys, as well as effects of rapid thermal annealing (RTA). By employing a variety of optical characterization techniques including cathodoluminescence (CL), cw- and time-resolved PL, PLE, and optically detected magnetic resonance (ODMR), high optical quality of the GaN0.018P0.982 epilayers lattice matched to Si substrates was demonstrated and was shown to be comparable to that of the “state-of-the-art" GaNP alloys grown on GaP substrates. The growth of GaNP on Si is, however, found to facilitate the formation of several point defects, including complexes involving Ga interstitials (Gai). A reduction and removal of competing non-radiative point defects by RTA has been concluded to be responsible for a substantial increase in radiative efficiency of the GaNP epilayers subjected to the post-growth annealing, evident from reduced thermal quenching of the PL intensity as well as from a substantial increase in carrier lifetime at room temperature.Papers V-VII are devoted to detailed studies of Ga0.46In0.54NxP1-x alloys lattice matched to GaAs, by using cw- and time-resolved PL, PLE, and optically detected cyclotron resonance (ODCR) measurements. The type-II band alignment at the Ga0.46In0.54NxP1-x/GaAs interface was concluded with x ≥ 0.5% based on (i) highly efficient photoluminescence upconversion (PLU) observed in the N containing alloys and (ii) appearance of a near-infrared PL emission attributed to the spatially indirect type-II transitions. Compositional dependence of the conduction band offset at the GayIn1-yNxP1-x/GaAs interface was also estimated. Origin of the PLU process was determined as being due to two-step two-photon absorption (TS-TPA). Different from other direct band gap dilute nitrides, the low temperature PL emission was shown to largely arise from radiative transitions involving spatially separated localized electronhole pairs. The observed charge separation was tentatively attributed to the long range CuPt ordering promoted by the presence of nitrogen.
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