| 1. |
- Leon, R., et al.
(författare)
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Defect states in red-emitting InxAl1-xAs quantum dots
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:8
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Tidskriftsartikel (refereegranskat)abstract
- Optical and transport measurements carried out in pn diodes and Schottky barriers containing multilayers of InAlAs quantum dots embedded in AlGaAs barriers show that while red emission from quantum dot (QD) states is obtained at similar to1.8 eV, defect states dominate the optical properties and transport in these quantum dots. These defects provide nonradiative recombination paths, which shortens the carrier lifetimes in QD's to tens of picoseconds (from similar to1 ns) and produce deep level transient spectroscopy (DLTS) peaks in both p and n type structures. DLTS experiments performed with short filling pulses and bias dependent measurements on InAlAs QD's on n-AlGaAs barriers showed that one of the peaks can be attributed to either QD/barrier interfacial defects or QD electron levels, while other peaks are attributed to defect states in both p and n type structures.
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| 2. |
- Tkachman, M.G., et al.
(författare)
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Phonon-assisted exciton luminescence in GaN layers grown by MBE and chloride-hydride VPE
- 2003
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Ingår i: Semiconductors (Woodbury, N.Y.). - : Pleiades Publishing Ltd. - 1063-7826 .- 1090-6479. ; 37:5, s. 532-536
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Tidskriftsartikel (refereegranskat)abstract
- Optical properties of GaN layers grown by the molecular-beam epitaxy (MBE) and chloride-hydride vapor-phase epitaxy (CHVPE) have been studied, and the quality of two types of samples has been compared. The photoluminescence spectra have an excitonic nature for both types of layers. To determine precisely the nature of exciton transitions, the reflectance spectra were studied. A key point was the investigation of phonon-assisted exciton luminescence, which provides information on the density distribution of the exciton states. Temperature dependences of the exciton transition energy and the ratio of intensities of one- and two-phpnon replicas were studied. The high quality of both types of layers has been confirmed, though the concentration of acceptors in MBE-grown samples is higher than in CHVPE samples. © 2003 MAIK "Nauka/Interperiodica".
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| 3. |
- Arnaudov, B, et al.
(författare)
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Hall effect data analysis of GaN n(+)n structures
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 872-876
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Tidskriftsartikel (refereegranskat)abstract
- We develop a model for analysis of Hall effect data of GaN structures composed of sublayers with different thicknesses and contacts placed on the top surface, We analysed the contributions of the conductivity of every sublayer of a planar sample taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from the upper layer. Correction factors, which reduce the contribution of the underlying layers to the measured whole sample conductivity, are obtained from the equations relevant to the respective equivalent circuit.
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| 4. |
- Arnaudov, B., et al.
(författare)
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Magnetic-field-induced localization of electrons in InGaN/GaN multiple quantum wells
- 2003
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 83:13, s. 2590-2592
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Tidskriftsartikel (refereegranskat)abstract
- A study was performed on the magnetic-field-induced localization of electrons in InGaN/GaN multiple quantum wells (MQW). A stepwise behavior of both the Hall coefficient and magnetoresistivity was observed. The peculiarities were explained by a magnetic-field-induced localization of electrons in a two-dimensional (2D) potential relief of the InGaN MQW.
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| 5. |
- Arnaudov, B, et al.
(författare)
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Modeling of the free-electron recombination band in emission spectra of highly conducting n-GaN
- 2001
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 64:4
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Tidskriftsartikel (refereegranskat)abstract
- We simulate the spectral distribution of the free-electron recombination band in optical emission spectra of GaN with a free-carrier concentration in the range of 5 x 10(17)-1 x 10(20) cm(-3). The influence of several factors, such as nonparabolicity, electron-electron interaction. and electron-impurity interaction on both the spectral and energy position and the effective gap narrowing are taken into account. The calculated properties of the free-electron-related emission bands are used to interpret the experimental photoluminescence and cathodoluminescence spectra of GaN epitaxial layers.
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| 6. |
- Arnaudov, B, et al.
(författare)
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Multilayer model for Hall effect data analysis of semiconductor structures with step-changed conductivity
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:4
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Tidskriftsartikel (refereegranskat)abstract
- We present a multilayer model for analysis of Hall effect data of semiconductor structures composed of sublayers with different thicknesses and contacts placed on the top surface. Based on the circuit theory we analyze the contributions of the conductivity of every sublayer and derive general expressions for the conductivity and carrier mobility of a multilayer planar sample. The circuit analysis is performed taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from each upper layer. In order to solve the inverse problem of determining the electrical parameters of one of the sublayers, a procedure for analysis of the Hall effect data is proposed. The model is simplified for a structure composed of two layers with the same type of conductivity, and is used to determine the electrical parameters of GaN films grown on relatively thick GaN buffers.
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| 7. |
- Darakchieva, Vanya, 1971-, et al.
(författare)
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Strain-related structural and vibrational properties of thin epitaxial AIN layers
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:4, s. 045411-
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Tidskriftsartikel (refereegranskat)abstract
- The effect of film thickness on the strain and structural properties of thin epitaxial AIN films has been investigated by high resolution x-ray diffraction techniques and transmission electron microscopy. As a result a sublayer model of the degree of strain and related defects for all films is proposed. A sublayer with low defect density and a strain gradient is found to be present in all films and it reaches a maximum thickness of 65 nm. The films are compressively strained and the strain relaxation after a thickness of 65 nm is shown to be accompanied by misfit dislocation generation and increase of the mosaic tilt. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sublayer model has been successfully applied to the analysis of the ellipsometry data through model calculations of the infrared dielectric function which confirm the sublayer model. It is found that the strain gradient results in a gradient of the phonon mode frequencies and broadening parameter. The initial strain relaxation in the films leads to narrowing of the observable infrared modes, while further strain relaxation broadens the modes when substantial defect generation occurs.
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| 8. |
- Evtimova, S, et al.
(författare)
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Effect of carrier concentration on the microhardness of GaN layers
- 2003
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Ingår i: Journal of materials science. Materials in electronics. - 0957-4522 .- 1573-482X. ; 14:10-12, s. 771-772
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Tidskriftsartikel (refereegranskat)abstract
- The paper presents a microhardness study of thick crack-free hydride vapor phase epitaxial GaN layers (not intentionally doped), and of thin metal-organic vapor phase epitaxial (MOVPE) GaN layers (undoped and Si-doped), grown on sapphire. A Vickers indentation method was used to determine the microhardness under applied loads up to 2 N. An increase in the microhardness was observed with decreasing carrier concentration and increasing mobility. A dip at an indentation depth of about 0.75 mum is observed in the microhardness profile in the MOVPE films, and is correlated with peculiarities in the spatially resolved cathodoluminescence spectra. The relationship between the mechanical and electrophysical parameters is discussed. (C) 2003 Kluwer Academic Publishers.
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| 9. |
- Monemar, Bo, et al.
(författare)
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Optical characterization of InGaN/GaN MQW structures without in phase separation
- 2001
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:1, s. 157-160
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence and cathodoluminescence spectroscopies are used to investigate the properties of the band edge emission of InGaN/(In)GaN multiple quantum well (MQW) structures which do not show evidence of phase separation in high resolution electron microscopy. The data still show a clear low energy peak in the spectra. about 0.1 eV below the main exciton peak. Possible interpretations of this second peak are discussed.
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| 10. |
- Monemar, Bo, et al.
(författare)
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Optical investigation of AlGaN/GaN quantum wells and superlattices
- 2004
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2251-2258
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Tidskriftsartikel (refereegranskat)abstract
- We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 x 10(18) cm(-3) free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.
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| 11. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 161-166
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) with an In composition x in the QWs of about 0.1, and a small In composition y in the barrier of 0.01-0.02. The MOVPE growth procedure was optimized to allow growth without In segregation. The InyGa1-yN barriers had a Si doping of about 5 x 10(18) cm(-3) . The low temperature photoluminescence spectra show two sets of exciton-like spectra with quite different properties. The lower energy emission has a small thermal activation energy (about 5 meV), and thus disappears at elevated temperatures, it is not observed at room temperature. The higher energy exciton state has a decay time of about 5 ns, while the lower energy process is much slower. We have also done preliminary studies on samples where the MQW region is situated in a p-n junction field, with semi-transparent contacts, to study the effects of varying the bias across the MQW structure. The combination of optical data can e interpreted in terms of a substantial potential gradient across the MQW region for both samples. The conclusion is that probably only one QW is emitting at low T (and no bias), and the second lower energy PL peak originates from a shallow notch in the conduction band at the interface between the thick GaN buffer layer and the first Ga(In)N barrier.
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| 12. |
- Paskova, Tanja, 1961-, et al.
(författare)
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Characterization of mass-transport grown GaN by hydride vapour-phase epitaxy
- 2004
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 273:1-2, s. 118-128
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive study of the morphological, optical and microstructural properties of mass-transport (MT) and conventionally grown GaN by hydride vapour-phase epitaxy is presented. Spatially resolved techniques have been utilized to reveal in a comparative way, the characteristics of the material grown either in predominant vertical or lateral growth modes. A strong donor-acceptor pair (DAP) emission is observed from the MT regions with a distinctive intensity contrast between the exciton and DAP emission bands from MT and nontransport regions. Secondary ion mass spectroscopy and imaging were employed to investigate the impurity incorporation into different regions. An increase of residual oxygen and aluminium impurity concentrations was found in the MT areas. In addition, positron annihilation spectroscopy showed a strong signal of Ga vacancy clusters in the MT grown material. The increase of the point defect concentrations of both Ga vacancy and oxygen impurity, most likely forming defect complexes, is related to the enhancement of the DAP emission.
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| 13. |
- Paskova, Tanja, et al.
(författare)
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Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer
- 2001
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Ingår i: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 230:3-4, s. 381-386
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Tidskriftsartikel (refereegranskat)abstract
- The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy. © 2001 Elsevier Science B.V. All rights reserved.
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| 14. |
- Paskova, Tanja, et al.
(författare)
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Hydride vapour phase homoepitaxial growth of GaN on MOCVD-grown 'templates'
- 2000
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Konferensbidrag (refereegranskat)abstract
- We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN 'template' layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN 'template'. Additionally the effect of Si doping of the GaN buffer layers on the HVPE-GaN properties was analysed.
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| 15. |
- Tanja, S, et al.
(författare)
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Air temperature triggers the recovery of evergreen boreal forest photosynthesis in spring
- 2003
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Ingår i: Global Change Biology. - : Wiley. - 1354-1013. ; 9:10, s. 1410-1426
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Tidskriftsartikel (refereegranskat)abstract
- The timing of the commencement of photosynthesis (P-*) in spring is an important determinant of growing-season length and thus of the productivity of boreal forests. Although controlled experiments have shed light on environmental mechanisms triggering release from photoinhibition after winter, quantitative research for trees growing naturally in the field is scarce. In this study, we investigated the environmental cues initiating the spring recovery of boreal coniferous forest ecosystems under field conditions. We used meteorological data and above-canopy eddy covariance measurements of the net ecosystem CO2 exchange (NEE) from five field stations located in northern and southern Finland, northern and southern Sweden, and central Siberia. The within- and intersite variability for P-* was large, 30-60 days. Of the different climate variables examined, air temperature emerged as the best predictor for P-* in spring. We also found that 'soil thaw', defined as the time when near-surface soil temperature rapidly increases above 0degreesC, is not a useful criterion for P-*. In one case, photosynthesis commenced 1.5 months before soil temperatures increased significantly above 0degreesC. At most sites, we were able to determine a threshold for air-temperature-related variables, the exceeding of which was required for P-*. A 5-day running-average temperature (T-5) produced the best predictions, but a developmental-stage model (S) utilizing a modified temperature sum concept also worked well. But for both T-5 and S, the threshold values varied from site to site, perhaps reflecting genetic differences among the stands or climate-induced differences in the physiological state of trees in late winter/early spring. Only at the warmest site, in southern Sweden, could we obtain no threshold values for T-5 or S that could predict P-* reliably. This suggests that although air temperature appears to be a good predictor for P-* at high latitudes, there may be no unifying ecophysiological relationship applicable across the entire boreal zone.
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