| 1. |
- Buyanova, Irina A., et al.
(författare)
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Optical properties of GaNAs/GaAs structures
- 2001
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Ingår i: Materials Science & Engineering. - 0921-5107 .- 1873-4944. ; 82:1-3, s. 143-147
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Tidskriftsartikel (refereegranskat)abstract
- We review our recent results on optical characterization of MBE-grown GaNAs/GaAs quantum structures with N content up to 4.5%, by employing photoluminescence (PL), PL excitation, and time-resolved PL spectroscopies. The dominant PL mechanism has been determined as recombination of excitons trapped by potential fluctuations of the band edge, due to composition disorder and strain nonuniformity of the alloy. The estimated value of the localization potential is around 60 meV for the low-temperature grown structures and can be reduced by increasing the growth temperature or using post-growth rapid thermal annealing (RTA). © 2001 Elsevier Science S.A.
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| 2. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Electronic Properties of Ga(In)NAs Alloys
- 2001
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Ingår i: MRS Internet Journal of Nitride Semiconductor Research. - 1092-5783. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- A brief review on the present knowledge of the electronic properties of the Ga(In)NAs ternary and quaternary alloys is given mainly from an experimental perspective. The discussion is focused on Ga(In)NAs with low N composition (< 10 %), where a large amount of experimental work has been done. Important fundamental electronic properties of the material system are analyzed with the emphasis on the nature of the giant band gap bowing in the alloy and nitrogen-induced modifications of the electronic structure of the conduction band. The current knowledge of the key material parameters, relevant for the device applications, such as electron effective mass, recombination processes and band alignment in Ga(In)NAs/GaAs heterostructures, is also reviewed.
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| 3. |
- Buyanova, Irina, 1960-, et al.
(författare)
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On the spin injection in ZnMnSe/ZnCdSe heterostructures
- 2002
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Konferensbidrag (refereegranskat)abstract
- We present results from a detailed study of spin injection in thin II-VI wide band gap semiconductor heterostructures by magnetooptical spectroscopy. It is shown that efficient spin alignment can be achieved in a diluted magnetic semiconductor barrier (a layer of ZnMnSe or ZnMnSe/CdSe superlattice) as thin as 10 nm. Rather efficient spin injection from such a thin spin aligner to a non-magnetic quantum well is demonstrated, even when the tunneling energy barrier is as thick as 10 nm. The effect of spin relaxation process on spin injection is also closely examined.
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| 4. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Photoluminescence characterization of GaNAs/GaAs structures grown by molecular beam epitaxy
- 2000
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Ingår i: Materials Science & Engineering. - 0921-5107 .- 1873-4944. ; 75:2-3, s. 166-169
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Tidskriftsartikel (refereegranskat)abstract
- A number of optical spectroscopies, including photoluminescence (PL), PL excitation and cathodoluminescence, are employed for characterization of GaNAs epilayers and GaAs/GaNxAs1-x quantum well structures grown by gas source molecular beam epitaxy at low temperature. The existence of strong potential fluctuations in the band edge of the GaNAs alloy is concluded, even for the samples with high optical quality, from a detailed analysis of the characteristic properties of the GaNAs-related PL emission. Based on the observed similarity in the PL properties between the GaNAs epilayers and the QW structures, the potential fluctuations are suggested to be mainly due to composition disorder and strain nonuniformity of the alloy. ⌐ 2000 Elsevier Science S.A. All rights reserved.
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| 5. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Resonant excitation spectroscopies of GaNAs/GaAs quantum structures
- 2000
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Konferensbidrag (refereegranskat)abstract
- We employ resonant optical excitation and Raman spectroscopies to study optical properties of GaNAs-based quantum structures grown by gas source molecular beam epitaxy (GS MBE). Under above band gap non-resonant excitation the PL spectra of GaNAs are shown to be dominated by the commonly observed featureless localised exciton emission. In contrast, when excitation energy is tuned close to the band edge of GaNAs alloy a series of additional narrow lines can be detected in the PL spectra. The peak positions of these lines are at about 10 meV (strongest), and at 20, 32, and 36 below the excitation energy. The dominant 10 meV line can only be excited within very narrow spectral range coinciding with the free exciton emission in GaNAs. Based on performed spectral, temperature dependent, and polarization studies the strongest 10 meV and the weaker 20 meV lines are tentatively attributed to disorder activated Raman scattering which is strongly enhanced close to the mobility edge of the GaNAs. The 32 and 36 meV lines are, on the other hand, caused by Raman scattering involving GaAs-like TO and LO phonons.
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| 6. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Tunable laser spectroscopy of spin injection in ZnMnSe/ZnCdSe quantum structures
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:12, s. 2196-
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Chen, Q., et al.
(författare)
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Calculation of Debye temperature for crystalline structures - A case study on Ti, Zr, and Hf
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:6, s. 947-961
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Tidskriftsartikel (refereegranskat)abstract
- The methods to calculate the Debye temperature from elastic moduli have been reviewed. The approximation approach due to Moruzzi e al was critically examined by considering experimental elastic constant data for all the cubic elements. It was found that many cubic elements are exceptions with regard to the assumed constant scaling factor for the expression of the average sound velocity in terms of the bulk modulus, and consequently the Debye temperature of a cubic element must be calculated from the knowledge of all the elastic constants of the system. On the other hand, a fairly constant scaling factor has been found to exist fur the hexagonal elements. Through the study of experimental data, some empirical relationships have been observed between the high temperature entropy-Debye temperature theta (D)(0) and the low temperature limit of the Debye temperature theta (D)(-3). For those structures that are dynamically unstable at low temperatures, we proposed a way to obtain their theta (D)(0) from the calculated isotropic bulk moduli. The methods have been applied to calculate the Debye temperatures of hcp, bcc, and fee Ti, Zr, and Hf from their elastic moduli derived from ah initio calculations. The calculated results agree very well with the experimental data.
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| 8. |
- Chen, Q., et al.
(författare)
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Computation of partial equilibrium solidification with complete interstitial and negligible substitutional solute back diffusion
- 2002
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Ingår i: Materials transactions. - : Japan Institute of Metals. - 1345-9678 .- 1347-5320. ; 43:3, s. 551-559
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Tidskriftsartikel (refereegranskat)abstract
- A simple numerical scheme is presented to simulate partial equilibrium solidification with complete interstitial and negligible substitutional solute back diffusion in multi-component and multi-phase systems. Based on this scheme, a computing tool capable of using Thermo-Calc databases directly has been developed for the estimation of solidification behavior of steels and other interstitial-containing alloys. Agreements between calculated and experimental as well as DICTRA results have been obtained on the microsegregation, fraction of eutectic, and freezing range of several steels, This suggests that the partial equilibrium assumption and proposed numerical scheme are reasonable and satisfactory, and confirms that the carbon back diffusion plays a very important role in the solidification of steels.
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| 9. |
- Chen, Weimin, 1959-, et al.
(författare)
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Magneto-optical spectroscopy of defects in wide bandgap semiconductors : GaN and SiC
- 2000
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Ingår i: Proceedings Conference on Optoelectronic and Microelectronic Materials and Devices. - : IEEE. - 0780366980 ; , s. 497-502
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Konferensbidrag (refereegranskat)abstract
- We review recent progress in our understanding of intrinsic defects in GaN and SiC, gained from magneto-optical studies by Zeeman measurements and optically detected magnetic resonance. The two best-known intrinsic defects in these two wide bandgap semiconductors, i.e. the Ga interstitial in GaN and the silicon vacancy in SiC, are discussed in detail. The Ga interstitial is the first and only intrinsic defect in GaN that has so far been unambiguously identified, either in the presumably isolated form or in a family of up to three complexes. The silicon vacancy is among the most studied intrinsic defect in SiC, at least in two charge states, and yet still remains controversial.
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| 10. |
- Chen, Weimin, 1959-, et al.
(författare)
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Optical Detection of Cyclotron Resonance (ODCR) in GaNAs/GaAs Quantum Well Structures
- 2000
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Konferensbidrag (refereegranskat)abstract
- ODCR has been employed to study effective masses and carrier recombination in GaNAs/GaAs multi-quantum well (MQW) structures, prepared by MBE with the nitrogen composition up to 4.5 above GaAs bandgap excitation consists of the excitonic recombination within the GaNAs MQW, the band edge PL emissions from GaAs and a broad 0.8-eV PL of unknown origin. When monitoring these emissions under the above GaAs excitation, the ODCR spectrum is dominated by the electron and hole CR in GaAs, with effective mass values 0.07m0 and 0.5m_0, respectively. The ODCR mechanism is discussed in terms of hot carrier effects, resulting in a reduced carrier recombination in GaAs and an enhanced carrier trapping in the GaNAs MQW. Under resonant excitation of the GaNAs MQW only a broad ODCR signal can be observed corresponding to an effective mass value 0.1m_0, attributed to the electron CR in the GaNAs MQW, where a higher electron effective mass value and a much lower mobility are expected.
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| 11. |
- Hai, P. N., et al.
(författare)
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Ga-related defect in as-grown Zn-doped GaN : An optically detected magnetic resonance study
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:16, s. R10607-R10609
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Tidskriftsartikel (refereegranskat)abstract
- A detailed study of as-grown Zn-doped GaN employing optically detected magnetic resonance (ODMR) spectroscopy is presented. Besides the well-known ODMR spectra of an effective-mass-like donor and Zn acceptor, a positive ODMR signal of an S=1 / 2 paramagnetic center was observed when monitoring the dominating blue luminescence band peaking at 2.8 eV. The involvement of a single Ga nucleus in the defect center is revealed from the rather well-resolved hyperfine interactions involving the isotopes 71Ga (39.9%) and 69Ga (60.1%), both with nuclear spin I=3 / 2. The C3v symmetry, the hyperfine interaction and the defect formation suggest a Ga-related complex nature of this center.
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| 12. |
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| 13. |
- Hai, P. N., et al.
(författare)
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Properties of GaAsN/GaAs quantum wells studied by optical detection of cyclotron resonance
- 2001
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Ingår i: Materials Science & Engineering. - 0921-5107 .- 1873-4944. ; 82:1-3, s. 218-220
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Tidskriftsartikel (refereegranskat)abstract
- Effective masses and carrier recombination in GaNAs/GaAs quantum wells (QWs) with nitrogen composition up to 4.5%, have been studied by optical detection of cyclotron resonance (ODCR). When monitoring the PL emissions under the conditions of above-GaAs barrier excitation, the ODCR spectrum is dominated by the electron CR in GaAs with an effective mass value 0.066m0. The ODCR mechanism is discussed in terms of hot carrier effects, resulting in a reduced carrier recombination in GaAs and an enhanced carrier trapping in the GaNAs QW. Under resonant excitation of the GaNAs QWs, only a broad ODCR signal can be observed, corresponding to an effective mass value of 0.12m0 and 0.19m0 when the N composition is about 1.2 and 2%, respectively. This is attributed to the electron CR in the GaNAs QW with a lower electron mobility. This sizeable increase in the electron effective mass is in agreement with earlier theoretical predictions. ⌐ 2001 Elsevier Science B.V.
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| 14. |
- Monemar, Bo, et al.
(författare)
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Optical characterization of III-nitrides
- 2002
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Konferensbidrag (refereegranskat)abstract
- Recent developments in material properties of GaN and related heterostructure combinations are reviewed, with emphasis on optical data. We discuss recent polarized photoluminescence (PL) data on the free excitons in GaN, obtained from thick HVPE grown layers. The exchange splitting constant is found to be about 0.6 meV, a more accurate value than previous suggestions. The PL signatures of shallow donors and acceptors, i.e. the bound excitons, are discussed and tentatively identified. Intrinsic point defects are discussed in terms of stability and experimental signatures. Quantum well structures in the InGaN/GaN and GaN/AlGaN systems are briefly discussed, with emphasis on localization of carriers and excitons. © 2002 Published by Elsevier Science B.V.
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| 15. |
- Monemar, Bo, 1942-, et al.
(författare)
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The 3.466 eV Bound Exciton in GaN
- 2001
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 489-492
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the available optical data for the 3.466 eV bound exciton in GaN, which has been a controversial issue in the recent literature. We conclude that the experimental results are only consistent with the identification as an exciton bound at a neutral acceptor with a spin-like bound hole. The chemical identity is still not clear.
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| 16. |
- Nguyen, Son Tien, 1953-, et al.
(författare)
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Hole effective masses in 4H SiC
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:16
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Tidskriftsartikel (refereegranskat)abstract
- Optically detected cyclotron resonance (ODCR) at X-band frequency (~9.23 GHz) was used to study hole effective masses in 4H SiC. In addition to the known ODCR signal related to the cyclotron resonance (CR) of electrons we have observed an ODCR peak at a higher magnetic field, which is attributed to the CR of holes. In the vicinity of the maximum of the uppermost valence band, the constant energy surface can be considered as an ellipsoid with the principal axis along the c axis and the effective masses of the holes were determined as mh? = (0.66▒0.02)m0 and mh?=(1.75▒0.02)m0. The influence of the polaron coupling effect on the effective mass values in 4H SiC is discussed. ⌐ 2000 The American Physical Society.
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| 17. |
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| 18. |
- Nguyen, Son Tien, 1953-, et al.
(författare)
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The Carbon Vacancy Pair in 4H and 6H SiC
- 2000
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Ingår i: Materials Science Forum, Vol. 338 - 342. - : Trans Tech Publications. ; , s. 821-824
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Konferensbidrag (refereegranskat)abstract
- Electron paramagnetic resonance (EPR) was used to study defects in p-type 4H and 6H SiC irradiated with 2.5 MeV electrons at elevated temperatures (400 °C). After irradiation, an anisotropic EPR spectrum, labeled EI4, having monoclinic symmetry (C1h) and an effective spin S = 1 was observed in both 4H and 6H SiC. The same g-tensor with the principal values, gx = 2.0051, gy = 2.0038 and gz = 2.0029 was determined for the spectra in both polytypes. Here the z- and x-axis lie in the (1 1 2̄ 0) plane and the y-axis perpendicular to this plane. The angle between the principal z-axis and the c-axis is 54°. The fine structure parameters were determined as D = 3.28×10-2 cm-1 and 3.44×10-2 cm-1 for 6H and 4H SiC, respectively; E = 0.67×10-2 cm-1 for both polytypes. From the obtained 29Si hyperfine structure and spin Hamiltionian parameters, the defect can be identified as the pair of two carbon vacancies in the (1 1 2̄ 0) or equivalent planes.
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| 19. |
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| 20. |
- Terentev, Ya. V., et al.
(författare)
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Semimagnetic ZnMnSe/CdSe Fractional Monolayer Superlattice as an Injector of Spin-Polarized Carriers
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 229:2, s. 765-768
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Tidskriftsartikel (refereegranskat)abstract
- We report on effective injection of spin-oriented carriers from a semimagnetic semiconductor structure to a non-magnetic ZnCdSe/ZnSe quantum well (QW). A short-period ZnMnSe/CdSe fractional monolayer superlattice was used as a spin injector. The carriers are photo-generated in the semimagnetic region, spin-polarized via the effect of giant Zeeman splitting and then injected into the non-magnetic QW. The spin injection efficiency was estimated by circular polarization measurements of recombination emission from the QW. The maximal detected degree of the spin polarization is about 25%.
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| 21. |
- Toropov, A. A., et al.
(författare)
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Exciton magnetic polarons in a type II ZnMnSe/ZnSSe superlattice
- 2004
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Ingår i: Physica status solidi (c)Special Issue: 11th International Conference on II–VI Compounds (II–VI 2003)Volume 1, Issue 4. - physica status solidi (c), Vol. 1, Issue 4 : WileyVCH. ; , s. 847-850
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Konferensbidrag (refereegranskat)abstract
- We present a time-resolved experimental study of excitonic magnetic polaron (EMP) dynamics in type II quantum wells ZnMnSe/ZnSSe. Two photoluminescence peaks were observed, which reflects coexistence of localized excitons and EMPs in the same sample. This was possible due to the competition of two localization mechanisms - magnetic localization of heavy holes in the ZnMnSe layers and non-magnetic localization of electrons in the ZnSSe layers.
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| 22. |
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| 23. |
- Toropov, A. A., et al.
(författare)
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Magneto-photoluminescence studies of Cd(Mn)Se/Zn(Mn)Se diluted magnetic nanostructures
- 2001
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - 1386-9477 .- 1873-1759. ; 10:1-3, s. 362-367
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Tidskriftsartikel (refereegranskat)abstract
- We report on cw and time-resolved photoluminescence (PL) studies of Cd(Mn)Se/Zn(Mn)Se diluted magnetic semiconductor nanostructures grown by molecular beam epitaxy. Excitonic PL intensity, decay time and Zeeman splitting have been studied systematically as a function of Cd(Mn)Se nominal thickness, Mn concentration and sample design. Wave function mapping has been performed, evidencing the formation of semi-magnetic quantum disk islands in the samples with thick enough Cd(Mn)Se insertions. ⌐ 2001 Elsevier Science B.V.
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| 24. |
- Toropov, A.A., et al.
(författare)
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Magneto-photoluminescence studies of diluted magnetic semiconductor type-II quantum wells ZnMnSe/ZnSSe
- 2003
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Ingår i: Proceedings of the International Conference on Superlattices, Nano-structures and Nano-devices ICSNN 2002 o-structures and Nano-devices ICSNN 2002. ; , s. 352-354
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Konferensbidrag (refereegranskat)abstract
- Magneto-photoluminescence (PL) has been studied in type II Zn1-xMnxSe/ZnSySe1-y multiple quantum wells (QWs) with the Mn content x between 0.03 and 0.15. Two PL lines are observed in the samples with x equal to 0.05 and 0.15. The lines are attributed to the excitons of two different types, coexisting in the QWs. At least one of them is associated with the formation of an exciton magnetic polaron. © 2002 Elsevier Science B.V. All rights reserved.
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| 25. |
- Toropov, A. A., et al.
(författare)
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Magnetooptical studies of CdSe/(Zn,Mn)Se semimagnetic nanostructures
- 2000
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Ingår i: 8th International Symposium Nanostructures: Physics Technology,2000. - : Ft. Belvoir Defense Technical Information Center. ; , s. 440-443
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Konferensbidrag (refereegranskat)abstract
- In conclusion the performed magneto-PL measurements exhibit fonrmation of 0D excitons in CdSe/(Zn,Mn)Se nanostructures. The excitonic states forming the inhomogeneously broadened emission band differ both in their energy (due to fluctuations in the sizes and compositions among the localization sites) and in magnetic properties (most probably, due to the effect of Mn ions clustering in the nearby regions). The structure potential for spin-related optoelectronic applications is demonstrated, owing to almost complete circular polarization of the emitted light within the wide spectral range (about 1OO meV) at moderate magnetic fields.
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