| 1. |
- Adell, M, et al.
(author)
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Photoemission studies of the annealing-induced modifications of Ga0.95Mn0.05As
- 2004
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 70:12: 125204
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Journal article (peer-reviewed)abstract
- Using angle resolved photoemission we have investigated annealing-induced changes in Ga1-xMnxAs with x=0.05. We find that the position of the Fermi energy is a function of annealing time and temperature. It is also established that the Curie temperature is strongly correlated to the separation between the Fermi level and the valence band maximum. Valence band photoemission shows that the Mn3d spectrum is modified by the annealing treatments.
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| 2. |
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| 3. |
- Andresen, SE, et al.
(author)
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Spin-polarized electron tunneling across a Si delta-doped GaMnAs/n-GaAs interface
- 2003
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In: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 94:6, s. 3990-3994
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Journal article (peer-reviewed)abstract
- We study the spin-polarized tunneling of electrons from the valence band of GaMnAs into the conduction band of n-type GaAs with Si delta-doping at the interface. The injection of spin-polarized electrons is detected as circular polarized emission from a GaInAs/GaAs quantum well light emitting diode, corresponding to magneto-optical Kerr effect loops. The angular momentum selection rules are simplified by the strain-induced heavy-hole/light-hole splitting, allowing a direct relation between circular polarization and spin-polarization. Comparison with the influence of Zeeman splitting allow us to conclude a spin-injection from the majority spin-band.
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| 4. |
- Asklund, H, et al.
(author)
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Photoemission study of GaAs (100) grown at low temperature
- 2002
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 65:11
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Journal article (peer-reviewed)abstract
- GaAs(100) layers grown by low-temperature (LT) molecular beam epitaxy were studied by means of valence-band and core-level photoelectron spectroscopy. Small differences were found between valence-band spectra from the LT layers and corresponding layers grown at high temperature. In the Ga 3d spectra a new component was found in the LT-GaAs. The relative intensity of this component was found to be practically constant with varying probing depth. It is proposed that this component represents sites coordinated to the five-atom As clusters formed at As-Ga antisites. This interpretation implies a higher density of antisite defects in the near-surface region than typically found in the bulk.
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| 5. |
- Fedorych, OM, et al.
(author)
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Electrically detected magnetic resonance
- 2004
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In: Acta Physica Polonica. Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics. - 0587-4246. ; 105:6, s. 591-598
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Journal article (peer-reviewed)abstract
- We compare the results of electrically detected magnetic resonance in a 2D electron gas in Si/SiGe quantum wells with transport and magnetic resonance measurements on ferromagnetic Ga1-xMnxAs. The results lead us to the conclusion that observation of electrically detected magnetic resonance is possible only in the case of a slow spin relaxation, where the microwave resonant absorption leads to a noticeable change of spin magnetization.
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| 6. |
- Fedorych, OM, et al.
(author)
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Magnetic order in semiconducting, ferromagnetic Ga1-xMnxAs
- 2004
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In: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 19:4, s. 492-493
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Journal article (peer-reviewed)abstract
- Magnetic resonance studies allow us to distinguish paramagnetic, ferromagnetic and ferrimagnetic phases in Ga1-xMnxAs. The transition from ferromagnet to ferrimagnet is correlated with a metal to insulator transition. The analysis of spin wave resonance spectra, which occur in the ferrimagnetic phase, allows us to estimate the magnitude and the distance dependence of exchange coupling. The experimentally evaluated long range of exchange causes an effective averaging of the fluctuation of exchange interactions. As a consequence, in the semimetallic phase both spin subsystems coherently precess forming the ferrimagnetic structure. In the insulating phase, fluctuations of the local exchange field lead to a fast decoherence of the carrier spins and only the localized Mn spins form the ferromagnetic moment.
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| 7. |
- Figielski, T, et al.
(author)
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Magnetotransport through nanoconstriction in ferromagnetic semiconductor (Ga,Mn)As
- 2003
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In: Acta Physica Polonica A ( Proceedings of the XXXII International School on Physics of Semiconducting Compounds, Part 1). - 0587-4246. ; 103:6, s. 525-531
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Conference paper (peer-reviewed)abstract
- We studied narrow (submicron) constrictions in the layers of ferromagnetic semiconductor (Ga, Mn)As. We have demonstrated a contribution of the quantum localization effects to the magnetoresistance of the constricted samples. We have also found a negative contribution of a domain wall trapped in the constriction to the resistance, due presumably to the erasing of the localization effects by the domain wall.
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| 8. |
- Figielski, T, et al.
(author)
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Microcircuit tailoring in ferromagnetic semiconductor (Ga,Mn)As
- 2003
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In: Physica Status Solidi. A, Applied Research. - : Wiley. - 0031-8965. ; 195:1, s. 228-231
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Journal article (peer-reviewed)abstract
- In order to search for novel giant-magnetoresistance systems, we fabricated and investigated narrow constrictions in the layers of the ferromagnetic semiconductor (Ga,Mn)As. We found that constrictions a few hundred nanometers wide, tailored by means of the electron-beam lithography and wet etching, were not conducting at liquid helium temperatures unless illuminated, probably due to the trapping action of surface states appearing on an extra surface area denuded by the etching. To avoid this, we used selective implantation of oxygen ions into the ferromagnetic layer to tailor the constrictions. We have shown that such an implantation inactivates Mn acceptors in the layer and destroys ferromagnetism. We propose an application of oxygen ion implantation as a method of fabricating microcircuits in future spin electronics based on Mn-containing III-V semiconductor compounds.
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| 9. |
- Hernandez, C, et al.
(author)
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Magnetic properties of GaMnAs single layers and GaInMnAs superlattices investigated at low temperature and high magnetic field
- 2003
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In: Acta Physica Polonica. Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics. - 0587-4246. ; 103:6, s. 613-619
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Journal article (peer-reviewed)abstract
- Magnetotransport properties of GaMnAs single layers and InGaMnAs/ InGaAs superlattice structures were investigated at temperatures from 4 K to 300 K and magnetic fields up to 23 T to study the influence of carriers confinement through different structures. Both single layers and superlattice structures show paramagnetic-to-ferromagnetic phase transition. In GaMnAs/InGaAs superlattice beside the Curie temperature (T-c approximate to 40 K), a new phase transition is observed close to 13 K.
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| 10. |
- Hernandez, C., et al.
(author)
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Magnetotransport properties of short period GaMnAs/InGaAs superlattices
- 2004
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In: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 272, s. 1429-1430
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Journal article (peer-reviewed)abstract
- Magnetotransport properties of GaMnAs/InGaAs superlattice structure and GaMnAs/GaAs single layers have been investigated from 4 to 300 K and magnetic fields up to 22.5 T. All these magnetic semiconductors show the so-called "paramagnetic-to-ferromagnetic phase transition''. However, in GaMnAs/InGaAs superlattice beside the Curie Wiess temperature (T-c similar to 40 K), a new phase transition is observed close to 13 K. (C) 2004 Elsevier B.V. All rights reserved.
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| 11. |
- Jiricek, P, et al.
(author)
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Electron mean free path for GaAs(100)-c(4x4) at very low energies
- 2004
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In: Proceedings of the 22nd European Conference on Surface Science (Surface Science). - : Elsevier BV. - 0039-6028. ; 566, s. 1196-1199
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Conference paper (peer-reviewed)abstract
- Electron mean free path (MFP) was determined by the angular resolved photoemission in 10-40 eV energy range for GaAs(100)-c(4 x 4) by the overlayer method. The investigation was based on the attenuation of the normal photoemission intensity of the Al 2p line from the molecular-beam-epitaxy grown GaAlAs layer buried four monolayers of GaAs below the surface. The energy dependence of the MFP shows a pronounced maximum at about 30 eV which is related to the corresponding section of the electron band structure of GaAs(100). (C) 2004 Elsevier B.V. All rights reserved.
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| 12. |
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| 13. |
- Kowalski, BJ, et al.
(author)
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MnAs overlayer on GaN(000(1)under-bar)-(1 x 1) its growth, morphology and electronic structure
- 2004
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In: Acta Physica Polonica. Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics. - 0587-4246. ; 105:6, s. 645-650
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Journal article (peer-reviewed)abstract
- MnAs layer has been grown by means of MBE on the GaN(000 (1) under bar)-(1 x 1) surface. Spontaneous formation of MnAs grains with a diameter of 30-60 nm (as observed by atomic force microscopy) occurred for the layer thickness bigger than 7 ML. Ferromagnetic properties of the layer with Curie temperature higher than 330 K were detected by SQUID measurements. Electronic structure of the system was investigated in situ by resonant photoemission spectroscopy for MnAs layer thickness of 1, 2, and 8 ML. Density of the valence band states of MnAs and its changes due to the increase in the layer thickness were revealed.
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| 14. |
- Kowalski, BJ, et al.
(author)
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Surface states on GaN(000(1)over-bar)(1x1) - an angle-resolved photoemission study
- 2002
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In: Surface Science. - 0039-6028. ; 507, s. 186-191
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Journal article (peer-reviewed)abstract
- We present the results of angle-resolved photoemission studies on the (0 0 0 (1) over bar) (N-polar) surface of bulk gallium nitride crystals. The bulk and surface band Structure was investigated along Gamma-A and Gamma-K-M directions on the surface with (1 x 1) symmetry. The sets of spectra were obtained in normal and off-normal emission for the surface prepared by Ar+ ion sputtering and annealing and for the same surface enriched with Ga under molecular beam epitaxy conditions. The bulk and surface related features were identified by analysis of the acquired experimental data and by comparison with the results of band structure Calculations.
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| 15. |
- Laukkanen, P, et al.
(author)
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Electronic and structural analysis of Sb-induced GaAs(100)(2x4) and (2x8) surfaces
- 2004
-
In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 69:20
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Journal article (peer-reviewed)abstract
- Electronic and structural properties of Sb-induced GaAs(100)(2x4) and (2x8) surfaces are studied by means of core-level and valence-band photoelectron spectroscopy utilizing synchrotron radiation and scanning tunneling microscopy. Combining these results and showing good consistency among them, we demonstrate that the Sb/GaAs(100)(2x4) surface is well compatible with the delta structural model, which includes one Sb dimer in both the first and third atomic layers and two second-layer Ga dimers per unit cell (i.e., the Sb coverage of 0.5 ML), giving experimental support to generality of the delta-type model for III-V(100)(2x4) surfaces, proposed previously on the basis of ab-initio calculations. Deconvolution of the Sb 4d core-level spectrum from the (2x4) surface shows two components, which are tentatively connected to two inequivalent Sb-dimer sites in the delta unit cell. Angle-resolved valence-band photoelectron spectroscopy reveals Sb-induced surface-derived states at near 0.4 and 0.6 eV below the valence-band maximum (VBM) for the Sb/GaAs(100)(2x4) surface, which have not been found in earlier measurements. These two surface-derived features mapped along symmetry lines of the surface Brillouin zone are identified with previous electronic-structure calculations. The results are also compared to band-structure measurements of the As/GaAs(100)(2x4) surface found in the literature. For the Sb/GaAs(100)(2x8) surface, we propose a structural model which, in contrast to the recent model, obeys the electron counting rule and consists of Sb dimers in three atomic layers, showing the Sb coverage of 1.25 ML for the (2x8) surface. The Sb 4d core-level spectrum from this surface exhibits three components, which are discussed within the determined structural model. The valence-band measurements of the (2x8) surface propose a new Sb-induced surface state at near 0.5 eV below the VBM.
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| 16. |
- Mathieu, R, et al.
(author)
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Magnetization of ultrathin (Ga,Mn)As layers
- 2003
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 68:18: 184421
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Journal article (peer-reviewed)abstract
- Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin (Ga0.95Mn0.05)As layers. The thinner layers (below 250 A) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 A) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers.
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| 17. |
- Mickevicius, S, et al.
(author)
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Photoemission study of LT-GaAs
- 2004
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In: Proceedings of the European Materials Research Society Fall Meeting, Symposium B (Journal of Alloys and Compounds). - : Elsevier BV. - 0925-8388. ; 382:1-2, s. 234-238
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Conference paper (peer-reviewed)abstract
- The electronic structure of GaAs (100) grown by low-temperature molecular beam epitaxy was investigated by means of photoemission spectroscopy. Slight differences are found in the valence band spectra of GaAs layers grown at different As-2/Ga flux ratios. Analysis of As 3d core level spectra does not indicate qualitative differences in respect to high temperature grown GaAs (100) layers. The Ga 3d core level spectra include a new component in comparison to the spectra of high temperature grown GaAs. The origin of this component is attributed to high density of As antisites in low-temperature grown GaAs. (C) 2004 Elsevier B.V. All rights reserved.
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| 18. |
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| 19. |
- Mikkelsen, Anders, et al.
(author)
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Defect structure of Ga1-xMnxAs: A cross-sectional scanning tunneling microscopy study
- 2004
-
In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 70:8
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Journal article (peer-reviewed)abstract
- We have studied the atomic scale structure of molecular beam epitaxy grown Ga1-xMnxAs compounds with various Mn concentrations by cross-sectional scanning tunneling microscopy and first principles calculations. Only bright protrusions close to the top layer As atoms are directly correlated with the bulk Mn concentration. Atomically resolved filled and empty states images of this defect are compared to images derived from first principles calculations. We identify the Mn related defect as substitutional Mn in the second layer Ga site. Surprisingly no substitutional Mn is observed in the top-most Ga layer. The experimental results are consistent with the energetics of our first principles total energy calculations.
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| 20. |
- Mikkelsen, Anders, et al.
(author)
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Mn diffusion in Ga1-xMnxAs/GaAs superlattices
- 2004
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85:20, s. 4660-4662
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Journal article (peer-reviewed)abstract
- Ga1-xMnxAs/GaAs superlattices with Mn concentrations of 1% and 5% in the Ga1-xMnxAs layers and a GaAs spacer thickness of 4 and 60 GaAs monolayers have been studied by cross-sectional scanning tunneling microscopy. By achieving atomic resolution of the superlattices, we observe individual Mn atoms in the Ga1-xMnxAs layers and in the GaAs spacer. We find that about 20% of the total amount of Mn diffuses from the GaMnAs layers into the GaAs spacer layers. Our results can be related to previous measurements of the magnetic properties of short period Ga1-xMnxAs/GaAs superlattices.
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| 21. |
- Mikkelsen, Anders, et al.
(author)
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Structural properties of LT-GaAs(100) and GaMnAs(100) surfaces studied by scanning tunneling microscopy
- 2002
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In: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Conference paper (peer-reviewed)abstract
- GaAs and GaMnAs compounds grown by low temperature (LT) molecular beam epitaxy (MBE), are promising materials for fabricating novel devices. LT-GaAs has already been used in new devices, as it has subpicosecond carrier lifetimes, good carrier mobility and high dark resistivity. GaMnAs compounds are candidates for integrating ferromagnetic materials into semiconductor devices, as they can become ferromagnetic at around 110 K. We have studied the surface structure of As capped MBE grown LT-GaAs(100) and GaMnAs(100) surfaces as a function of annealing temperature by Scanning Tunneling Microscopy (STM). On LT-GaAs(100) we find an ordered 2×4 phase at 400 C and an ordered 4×6 phase at 600 C. These phases have some similarities with the phases found on ordinary GaAs (100). For GaMnAs (100) we identify a new 4×6 phase after annealing to 600 C, which is observed to be different in morphology and structure from the 4×6 ordered phases found on GaAs(100)
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| 22. |
- Sadowski, Janusz, et al.
(author)
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Influence of defects on the lattice constant of GaMnAs
- 2004
-
In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 69:7
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Journal article (peer-reviewed)abstract
- We study the influence of the major compensating defects As antisites and Mn interstitials known to occur in the GaMnAs ferromagnetic semiconductor on its structural properties. Our experimental results show that there is a balance between Mn interstitial and As antisite defects, leading to the reduced density of one type of defect upon increasing the density of the other defect. Significant differences in the lattice parameters of GaMnAs with different balances between these two types of defects were observed. The annealing-induced reduction of the GaMnAs lattice constant is inhibited in samples with a large density of As antisites.
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| 23. |
- Sorensen, BS, et al.
(author)
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Dependence of Curie temperature on the thickness of epitaxial (Ga,Mn)As film
- 2002
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 66:23
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Journal article (peer-reviewed)abstract
- We present the magnetotransport properties of very thin (5-15 nm) single (Ga,Mn)As layers grown by low-temperature molecular beam epitaxy. A lower (Ga,Mn)As thickness limit of 5 nm for the ferromagnetic phase and the dependence of the Curie temperature on (Ga,Mn)As thickness are determined from electrical-transport measurements. The Curie temperature is determined to be 97 K for the thinnest ferromagnetic sample and is found to decrease for increasing layer thickness. A carrier density of 7.1x10(20) cm(-3) for the 5 nm thick (Ga,Mn)As layer is determined from Hall measurements. Differences between magnetotransport properties of thick and thin (Ga,Mn)As layers are observed and discussed.
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| 24. |
- Sorensen, BS, et al.
(author)
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Effect of annealing on carrier density and Curie temperature in epitaxial (Ga,Mn)As thin films
- 2003
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:14, s. 2287-2289
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Journal article (peer-reviewed)abstract
- We report a clear correspondence between changes in the Curie temperature and carrier density upon annealing in epitaxially grown (Ga,Mn)As layers with thicknesses in the range between 5 and 20 nm. The changes are dependent on the layer thickness, indicating that the (Ga,Mn)As-GaAs interface has importance for the physical properties of the (Ga,Mn)As layer. The magnetoresistance shows additional features when compared to thick (Ga,Mn)As layers, that are at present of unknown origin. (C) 2003 American Institute of Physics.
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| 25. |
- Szuszkiewicz, W, et al.
(author)
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Short-period GaMnAs/GaAs superlattices: Optical and magnetic characterization
- 2003
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In: Journal of Superconductivity (Proceedings of the PASPS Conference held July 2002 in Würzburg, Germany). - 1572-9605 .- 0896-1107. ; 16:1, s. 209-212
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Conference paper (peer-reviewed)abstract
- Superlattices with magnetic layers containing from 8 to 16 GaMnAs monolayers corresponding to the mixed crystal composition between 5 and 6% of Mn and from 4 to 10 GaAs monolayers were grown by the low temperature MBE technique and characterized by Raman scattering. Folded acoustic phonons were observed for all superlattices in the Raman scattering spectra. The interlayer exchange coupling, found previously by the wide-angle neutron diffraction in selected superlattices was investigated by the spin-polarized neutron reflectometry. It was always of ferromagnetic type, no trace of antiferromagnetic coupling was found.
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| 26. |
- Tuomisto, F, et al.
(author)
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Ga sublattice defects in (Ga,Mn)As: Thermodynamical and kinetic trends
- 2004
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In: Physical Review Letters. - 1079-7114. ; 93:5
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Journal article (peer-reviewed)abstract
- We have used positron annihilation spectroscopy and infrared absorption measurements to study the Ga sublattice defects in epitaxial Ga1-xMnxAs with Mn content varying from 0% to 5%. We show that the Ga vacancy concentration decreases and As antisite concentration increases with increasing Mn content. This is in agreement with thermodynamical considerations for the electronic part of the formation energy of the Ga sublattice point defects. However, the absolute defect concentrations imply that they are determined rather by the growth kinetics than by the thermodynamical equilibrium. The As antisite concentrations in the samples are large enough to be important for compensation and magnetic properites. In addition, the Ga vacancies are likely to be involved in the diffusion and clustering of Mn at low annealing temperatures.
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| 27. |
- Tuomisto, F, et al.
(author)
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Observation of vacancies in Ga1-xMnxAs with positron annihilation spectroscopy
- 2003
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In: Acta Physica Polonica. Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics. - 0587-4246. ; 103:6, s. 601-606
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Journal article (peer-reviewed)abstract
- Positron annihilation spectroscopy can be used to determine the role of vacancy defects in semiconductors, by identification and quantification of the vacancies and their chemical surroundings. We have studied 0.5-0.8 mum thick low temperature MBE GaMnAs layers with Mn content 0.5-5% and different As-2 partial pressures at growth. The Doppler broadening results show that the Ga vacancy concentration in the layers decreases with increasing Mn content and decreasing As-2 partial pressure.
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