| 1. |
- Haratizadeh, Hamid
(författare)
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Optical Characterization of GaN/AIGaN Quantum Well Structures
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The III-nitride material family is particularly well suited for optoelectronic applications, e.g. as UV light emitters in future lighting solutions that are expected to replace the incandescent and fluorescent lamps in use today. Here a UV primary light emitting diode (LED) excites an efficient phosphor so that the entire LED package produces white light. The LED is expected to be built upon a technology involving multiple quantum wells (MQWs) in the AIGaN/GaN system as the active region. The AIGaN barriers will only have a moderate height, not to block the current flow in the LED, i.e. a low Al composition will be used.In order to optimize the device design, it is necessary to provide knowledge on the material properties and especially to understand the recombination mechanisms in nitride QWs. A peculiarity of the wurtzite nitrides is the significant macroscopic polarization with both a spontaneous and a piezoelectric component. These polarization charges create internal fields in the QWs that have a fundamental influence on the optical properties, strongly affecting the oscillator strengths of excitons as well as the spectral position of the corresponding photoluminescence (PL) peaks. The radiative recombination processes are strongly modified by these built-inelectric fields in the [0001] direction, which causes a substantial quantum confined stark effect (QCSE).Screening by doping and/or carrier injection strongly affects the polarization-induced properties. For instance modulation doping can be used to screen the macroscopic polarization field in nitride quantum wells. The screening of the field increases the overlap between confined electron and hole states, enhancing the radiative transition probability across the gap. This can explain the experimental findings of lower laser threshold current, reduced radiative lifetime, and increased emission intensity indoped MQWs.In this thesis, the influence of Si doping on the exciton localization in modulationdoped GaN/A10.07Ga0.93N multiple-quantum-well (MQW) structures and the radiative recombination mechanisms for the undoped and Si doped samples have been investigated. The thesis is mainly based on experimental investigations by using a combination of photoluminescence (PL), micro photoluminescence (μ-PL) and timeresolved photo-luminescence (TRPL) techniques.The results of the measurements reveal the hole localization even at highly modulation doped samples. Localization at interfaces leads to spectrally resolve well width fluctuations in the PL spectra for the undoped samples, while these appear to be screened for the case of high n-doping. The hole localization determines the residual PL linewidth in undoped samples, and the recombination kinetics in all samples at low temperatures. At elevated temperatures, the Si doping is found to improve the radiative efficiency.
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| 2. |
- Ivanov, SV, et al.
(författare)
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Plasma-assisted MBE growth and characterization of InN on sapphire
- 2004
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 269:1
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Tidskriftsartikel (refereegranskat)abstract
- We report on a close correlation between the growth conditions of InN/Al2O3 (0 0 0 1) epilayers grown by plasma-assisted molecular beam epitaxy (MBE), their optical properties in the IR range, and the In clustering in the layers. High-spatial resolution techniques, namely micro-cathodoluminesesence, back-scattered electron imaging and energy dispersive X-ray analysis, were used to establish this correlation. The In-rich growth conditions, achieved by increasing either the growth temperature or the effective In/N flux ratio, causes the In clustering in InN, responsible in our samples for the 0.7-0.8 eV luminescence and IR optical absorption. Growth under In/N = 1: 1 conditions slightly shifted to the N-rich side generally produces InN layers without visible In clusters, having an optical absorption edge around 1.4 eV. A possible mechanism of In cluster formation is suggested on the basis of thermodynamic considerations for InN MBE growth. (C) 2004 Elsevier B.V. All rights reserved.
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| 3. |
- Ratnikov, V, et al.
(författare)
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Bragg and Laue x-ray diffraction study of dislocations in thick hydride vapor phase epitaxy GaN films
- 2000
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Ingår i: Journal of Applied Physics. - 0021-8979 .- 1089-7550. ; 88:11, s. 6252-6259
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Tidskriftsartikel (refereegranskat)abstract
- The dislocation structure of hydride vapor phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tensor components. Symmetrical reflections (including reflections from planes forming a large angle with the basal plane) with two modes of scanning (theta and theta -2 theta) in two geometries (Bragg and Laue) are used to obtain the tensor components. The instant connections between the tensor components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaN films grown on sapphire with and without undoped and Si-doped metalorganic chemical vapor deposited templates. Transmission electron microscopy was used to confirm the x-ray results by direct visualization of defect rearrangements. (C) 2000 American Institute of Physics. [S0021-8979(00)06023-0].
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| 4. |
- Ratnikov, VV, et al.
(författare)
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Determination of microdistortion components and their application to structural characterization of HVBE GaN epitaxial layers
- 2001
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Ingår i: Journal of Physics D. - 0022-3727 .- 1361-6463. ; 34:10A, s. A30-A34
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Tidskriftsartikel (refereegranskat)abstract
- The dislocation structure of hydride vapour phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tenser components. Symmetrical reflections (including reflections from planes forming large angles with the basal plane) with two modes of scanning (theta and theta -2 theta) in two geometries (Bragg and Lane) are used to obtain the tenser components. The instant connections between the tenser components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaN films grown on sapphire without and with undoped and Si-doped metal-organic chemical vapour deposited templates.
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| 5. |
- Scott, K., et al.
(författare)
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Non-stoichiometry and non-homogeneity in InN
- 2005
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Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 2, s. 2263-2266
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Tidskriftsartikel (refereegranskat)
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| 6. |
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| 7. |
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| 8. |
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| 9. |
- Shubina, Tatiana, et al.
(författare)
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Interface effects in type-II CdSe/BeTe quantum dots
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 229:1, s. 489-492
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Tidskriftsartikel (refereegranskat)abstract
- We report on optical and structural studies of the interface symmetry in CdSe/BeTe multiple-layer structures containing self-assembled quantum dots. Temperature and decay behavior of the broad photoluminescence (PL) band is consistent with the type-II transitions involving deeply localized electron states. Large linear in-plane polarization of the PL (up to 80%) is observed, implying the C-2v (or lower) symmetry of the individual places of the electron localization.
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| 10. |
- Shubina, Tatiana, et al.
(författare)
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Intrinsic electric fields in N-polarity GaN/AlxGa1-xN quantum wells with inversion domains
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:19
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Tidskriftsartikel (refereegranskat)abstract
- GaN/AlGaN quantum wells (QWs) of dominant N polarity with inversion domains (IDs), grown by molecular-beam epitaxy, have been studied. Two-band photoluminescence (PL), with the lower-energy band and an additional absorption edge related to the IDs, is observed in these QWs due to a difference in strain, electric field, and well width in the regions of different polarities. A time-resolved PL study reveals additionally strong inhomogeneity of the electric fields among the IDs. The intrinsic electric fields in the structures are relatively small-their maximal estimated value of 180 kV/cm is among the lowest ever reported. The low-scale electric fields indicate likely polarization deterioration in the N-polarity structures. These conditions are favorable for bright PL up to room temperature in 8-9-nm-wide wells.
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| 11. |
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| 12. |
- Shubina, Tatiana, et al.
(författare)
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Micro-photoluminescence spectroscopy of exciton-polaritons in GaN with the wave vector k normal to the c-axis
- 2001
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 481-484
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Tidskriftsartikel (refereegranskat)abstract
- We report on polarized micro-photoluminescence (mu -PL) and micro-reflectance (mu -R) studies of GaN layers grown by HVPE. A strong pi -polarized component in the vicinity of A exciton is observed in the mu -PL and attributed as a mixture of a bound B exciton, dominating at low temperature, and scattered A exciton-polariton states prevailing at higher temperatures. Temperature variation of exciton energies in the mu -R spectra reveals strain-induced difference between the top surface and the cleaved edges.
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| 13. |
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| 14. |
- Shubina, Tatiana, et al.
(författare)
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Nanometric-scale fluctuations of intrinsic electric fields in GaN/AlGaN quantum wells with inversion domains
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 919-923
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Tidskriftsartikel (refereegranskat)abstract
- Strain and electric field fluctuations in regions of different polarities in GaN/AlGaN quantum well (QW) structures of dominant N-polarity with inversion domains (IDs) split the photoluminescence (PL) emission into two bands. Micro-PL and time-resolved PL studies reveal strong inhomogeneity of the array of the IDs, where essential parameters, such as strain, electric fields, and sizes are fluctuating quantities. We demonstrate also that the ID formation decreases the intrinsic electric field magnitudes.
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| 15. |
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| 16. |
- Shubina, Tatiana, et al.
(författare)
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Narrow-line excitonic photoluminescence in GaN/AlxGa1-xN quantum well structures with inversion domains
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:24, s. 241306-
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Tidskriftsartikel (refereegranskat)abstract
- Microphotoluminescence studies reveal strong and narrow lines of similar to1-meV minimal width in GaN/AlxGa1-xN quantum well (QW) structures having inversion domains (IDs). These narrow lines coexist in the spectra with broad (10-15 meV) peaks. The features of both kinds are characteristic for intersections of the IDs with QWs, which provide either three- or one-dimensional carrier confinement, depending on both the ID diameter and well width.
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| 17. |
- Shubina, Tatiana, et al.
(författare)
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Optical properties of GaN/AlGaN quantum wells with inversion domains
- 2003
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 195:3, s. 537-542
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Tidskriftsartikel (refereegranskat)abstract
- Two-band photoluminescence (PL) and respective absorption and reflection features are observed in GaN/AlGaN MBE-grown quantum well (QW) structures of dominant N polarity with inversion domains (IDs). The PL bands are related to transitions in the regions of different polarity, characterized by different strain and electric fields. A micro-PL study reveals sharp and narrow (1.5-2.5 meV) PL lines placed between the bands, which are tentatively attributed to recombination at localization sites associated with intersections of the QWs with the domains. Additionally, we demonstrate that the ID formation decreases the overall strength of the intrinsic electric fields in the QW structures.
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| 18. |
- Shubina, Tatiana, 1950-, et al.
(författare)
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Optical properties of InN related to surface plasmons
- 2005
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 202:14, s. 2633-2641
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Tidskriftsartikel (refereegranskat)abstract
- We report on the complex nature of infrared luminescence and absorption in InN films, which cannot be entirely explained by the concept of a conventional narrow-gap semiconductor. In particular, it concerns the detection of peaks near absorption edges by both thermally detected optical absorption and photoluminescence excitation spectroscopy and the observation of extraordinarily strong resonant enhancement of emission. To describe the experimental data a model is proposed, which takes into account surface plasmons in metal-like inclusions, modifying the optical properties of InN. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA,.
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| 19. |
- Shubina, Tatiana, et al.
(författare)
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Optical properties of InN with stoichoimetry violation and indium clustering
- 2005
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 202:3, s. 377-382
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that nonstoichiometry is one of the main reason of strong deviation of the InN optical gap in the 0.7-2 eV range, with N/In < 1 and N/In > 1 corresponding to the lower and higher energies, respectively. The phenomenon is discussed in terms of atomic orbital energies, which are strongly different for indium and nitrogen, therefore both excess atom incorporation and elimination could change the optical gap. We estimate such trends using the approximation of the empirical nearest-neighbor tight binding theory. It is also demonstrated that resonant absorption in In-enriched regions is an additional factor lowering an effective absorption edge.
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| 20. |
- Shubina, Tatiana, et al.
(författare)
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Peculiarities of exciton-polaritons in GaN at different polarizations studied by mu-photoluminescence spectroscopy
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 205-211
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Tidskriftsartikel (refereegranskat)abstract
- We report on polariton properties in high quality thick GaN grown by hydride vapor phase epitaxy on c-sapphire. A strong fine is observed in the vicinity of the A exciton in T-polarization (k perpendicular to c, E parallel to the c-axis) by micro-photoluminescence (mu-PL). Comparison of the mu-PL and mu-reflectance spectra confirms the internal origin of the polariton emission. In the samples with low density of residual donors the enhancement of the a-polarized component is induced mostly by interbranch scattering which occurs, possibly., due to the complex structure of the exciton-polariton branches at k perpendicular to c. The Gamma(1)-Gamma(5) exciton splitting in the C band is determined by the mu-reflectance as similar to1 meV.
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| 21. |
- Shubina, Tatiana, et al.
(författare)
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Recombination dynamics and lasing in ZnO/ZnMgO single quantum well structures
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:20
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Tidskriftsartikel (refereegranskat)abstract
- We report a time-resolved study of the recombination dynamics in molecular beam epitaxy grown ZnOZnMgO single quantum wells (SQWs) of 1.0-4.5 nm width. The SQWs exhibit different emission properties, depending on both the well width and defect density. Stimulated emission has been achieved at room temperature in a separate confinement double heterostructure having a 3 nm wide SQW as an active region. It has been found that a critical parameter for the lasing is the inhomogeneous broadening of both QW and barrier emission bands. © 2007 American Institute of Physics.
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| 22. |
- Shubina, Tatiana, et al.
(författare)
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Resonant light delay in GaN with ballistic and diffusive propagation
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100:8
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Tidskriftsartikel (refereegranskat)abstract
- We report on a strong delay in light propagation through bulk GaN, detected by time-of-flight spectroscopy. The delay increases resonantly as the photon energy approaches the energy of a neutral-donor bound exciton (BX), resulting in a velocity of light as low as 2100 km/s. In the close vicinity of the BX resonance, the transmitted light contains both ballistic and diffusive components. This phenomenon is quantitatively explained in terms of optical dispersion in a medium where resonant light scattering by the BX resonance takes place in addition to the polariton propagation.
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| 23. |
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| 24. |
- Shubina, Tatiana, et al.
(författare)
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Surface-plasmon resonances in indium nitride with metal-enriched nano-particles
- 2006
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 288:2, s. 230-235
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Tidskriftsartikel (refereegranskat)abstract
- Plasmonic resonances in In-enriched nano-particles, spontaneously formed during growth, can dramatically modify optical properties of InN. Experimental support for this is provided from detailed studies of absorption and infrared emission in InN. In particular, thermally detected optical absorption and photoluminescence excitation spectroscopy reveal a peak below the region of strong absorption in InN. A higher-energy part of the infrared emission having a noticeable p-polarization is markedly enhanced with excitation along the surface. These peculiarities are discussed in terms of the Mie resonances, arising in metallic spheroids with different aspect ratio, and their coupling with recombining states, whose strength depends on energy separation between the states and the resonances. © 2005 Elsevier B.V. All rights reserved.
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| 25. |
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| 26. |
- Toropov, A.A., et al.
(författare)
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Polarization-resolved phonon-assisted optical transitions of bound excitons in wurtzite GaN
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:19
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Tidskriftsartikel (refereegranskat)abstract
- The selection rules of phonon-assisted optical transitions of bound excitons in bulk wurtzite GaN are studied both experimentally and theoretically. The linearly polarized photoluminescence is detected within the phonon replicas of the lines of impurity bound excitons in the geometry, when the light wave vector is normal to the hexagonal axis, and the electric field vector is either perpendicular or parallel to it. The degree and even sign of the linear polarization is found to depend on the symmetry of the involved optical phonon. To explain these data, a group-theory approach is applied to derive the selection rules for zero-phonon and phonon-assisted transitions involving excitons bound to either substitutional (C3v symmetry) or interstitial (C3v, Cs, or C1 symmetries) impurities. The obtained theoretical selection rules are in an agreement with the experimental results, provided the exciton is bound to the impurities with the C3v symmetry. © 2008 The American Physical Society.
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| 27. |
- Toropov, AA, et al.
(författare)
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Temperature-dependent polarized luminescence of exciton polaritons in a ZnO film
- 2005
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 202:3, s. 392-395
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Tidskriftsartikel (refereegranskat)abstract
- We report on the studies of linearly polarized photoluminescence (PL) in a (0001) oriented ZnO epitaxial film, grown by metal organic chemical vapor deposition on a GaN template. The emission of mixed longitudinal-transverse exciton polariton modes was observed up to 130 K that evidences polaritonic nature of the excitonic spectrum up to this elevated temperature.
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