SwePub - search: WFRF:(Paskov Plamen 1959 )

Enumeration	Reference	Cover	Find
1.	Gällström, Andreas, 1978-, et al. (author) The Electronic Structure of the UD-4 defect in 4H, 6H and 15R SiC 2009 In: Materials Science Forum, Vols. 600-603. - : Trans Tech Publications. ; , s. 397-400 Conference paper (peer-reviewed)abstract The photoluminescence (PL) of the UD-4 defect is observed in semi-insulating bulk 4H, 6H and 15R SiC. In 4H and 6H SiC the UD-4 defect consists of two families of no-phonon (NP) lines, Ua and Ub, and in 15R SiC it consists of three families, Ua, Ub and U15R. The Ua family in 4H, 6H and 15R all show similar temperature behavior with higher energy NP lines becomming observable at higher temperatures. In the case of the Ub and U15R families, a luminescence line with lower energy than the prominent luminescence line appears at higher temperatures. The polarization and Zeeman measurements suggest that the defect has C3v symmetry.
2.	Monemar, Bo, 1942-, et al. (author) The 3.466 eV Bound Exciton in GaN 2001 In: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 489-492 Journal article (peer-reviewed)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.
3.	Arnaudov, B., et al. (author) On the nature of the near bandedge luminescence of InN epitaxial layers 2005 In: AIP Conference Proceedings. - 0094-243X .- 1551-7616. ; 772, s. 285-286 Journal article (peer-reviewed)
4.	Arnaudov, B., et al. (author) Radiative recombination mechanism in highly modulation doped GaN/AlGaN multiple quantum wells 2006 In: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 6, s. 1888-1891 Journal article (peer-reviewed)
5.	Arwin, Hans, 1950-, et al. (author) Assessment of phonon mode characteristics via infrared spectroscopic ellipsometry on a-plane GaN 2005 In: ICSN-6,2005. Conference paper (other academic/artistic)
6.	Blumenschein, N., et al. (author) Thermal conductivity of bulk and thin film β-Ga2O3 measured by the 3ω technique 2018 In: Oxide-based Materials and Devices IX. - : SPIE - International Society for Optical Engineering. - 9781510615519 ; , s. 105332G-1-105332G-8 Conference paper (peer-reviewed)
7.	Darakchieva, Vanya, 1971-, et al. (author) Anisotropic strain and phonon deformation potentials in GaN 2007 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195217- Journal article (peer-reviewed)abstract We report optical phonon frequency studies in anisotropically strained c -plane- and a -plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E1 (TO), E1 (LO), and E2 phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials c E1 (TO), c E1 (LO), and c E2 are determined. A distinct correlation between anisotropic strain and the A1 (TO) and E1 (LO) frequencies of a -plane GaN films reveals the a A1 (TO), b A1 (TO), a E1 (LO), and b E1 (LO) phonon deformation potentials. The a A1 (TO) and b A1 (TO) are found to be in very good agreement with previous results from Raman experiments. Our a A1 (TO) and a E1 (LO) phonon deformation potentials agree well with recently reported theoretical estimations, while b A1 (TO) and b E1 (LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented. © 2007 The American Physical Society.
8.	Darakchieva, Vanya, 1971-, et al. (author) Deformation potentials of the E1 (TO) and E2 modes of InN 2004 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:18, s. 3636-3638 Journal article (peer-reviewed)abstract The determination of deformation potentials of E1(TO) and E 2 modes of InN were discussed. The deformation potentials were evaluated for two sets of stiffness constants using x-ray diffraction, IR spectroscopic ellipsometry (IRSE), Raman scattering, and Grüneisen parameter values. The InN layer were grown on GaN buffer layers on (0001) sapphire by molecular beam epitaxy. It was found that the strain-free values of the InN E1(TO) mode was 477.9 cm-1 and 491.9 cm -1 for the E2 modes.
9.	Darakchieva, Vanya, 1971-, et al. (author) Phonon mode behavior in strained wurtzite AlN/GaN superlattices 2005 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:11, s. 115329- Journal article (peer-reviewed)abstract We have studied phonons in AlN/GaN superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified E1(TO), A 1(LO) and E2 localized, and E1(LO) and A 1(TO) delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the A1(TO) and E1(LO) phonons, as well as the free-carrier effect on the E1(LO) phonon are also discussed. ©2005 The American Physical Society.
10.	Darakchieva, Vanya, 1971-, et al. (author) Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates 2005 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97:1, s. 013517- Journal article (peer-reviewed)abstract We have studied the lattice parameters of hydride vapor phase epitaxy (HVPE)-GaN quasisubstrates in relation to their structural properties. Layers grown on single-layer metalorganic vapor phase epitaxy (MOVPE) templates and on epitaxial lateral overgrown MOVPE templates are characterized by Raman scattering, high-resolution x-ray diffraction, and reciprocal space mapping. The strain relaxation in the films versus their thickness was found to proceed similarly in the GaN samples grown using the two types of templates but the strain saturates at different nonzero levels. The lattice parameters of relatively thin HVPE-GaN free-standing quasisubstrates indicate that no total strain relaxation is achieved after the sapphire removal. The lattice parameters of the thick quasisubstrates grown on different templates are not affected by the separation process and are found to have values very close to the reference strain-free lattice parameters of GaN powder. © 2005 American Institute of Physics.
11.	Delgado Carrascon, Rosalia, 1987- (author) Epitaxial strategies for defect reduction in GaN for vertical power devices 2022 Licentiate thesis (other academic/artistic)abstract Group-III nitride materials, gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) have direct band gaps with band gap energies ranging from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN) wavelengths and covering the entire spectral range from 0.7 eV to 6.2 eV upon alloying. The invention of the GaN-based blue LEDs, for which the Nobel prize in Physics was awarded in 2014, has opened up avenues for exploration of IIINitride material and device technologies and has inspired generations of researchers in the semiconductor field. Group-III nitrides have also been demonstrated to be among the most promising semiconductors for next generation of efficient high-power, high-temperature and high-frequency electronic devices.The need to build a sustainable and efficient energy system motivates the development of vertical GaN transistors and diodes for applications with power ratings of 50-150 kW, e.g., in electric vehicles and industrial inverters. The key is to grow GaN layers with low concentration of defects (impurities and dislocations), which enables an expansion in both voltage and current ratings and reduction of cost. Despite intense investigations and impressive advances in the field, defects are still a major problem hindering exploiting the full potential of GaN in power electronics. This Licentiate thesis focuses on the development of two different epitaxial approaches in MOCVD for reducing dislocation densities in GaN with controlled doping for power device applications: i) growth of planar GaN layers trough NWs reformation, which can be further exploited as templates for a subsequent growth of thick drift layers and ii) homoepitaxial GaN growth. Special attention is put on understanding homoepitaxial growth under different nucleation schemes and thermal stability of GaN. We have established conditions in homoepitaxy to deliver state-of-the-art GaN material with low impurity levels combined with a reasonable growth rate suitable for growth of thick drift layers.The results are summarized in two papers: In Paper I we investigate GaN layers with different thicknesses on reformed GaN NW templates and highlight this approach as an alternative to the expensive GaN HVPE substrates. The sapphire used as a substrate limits to some extent the reduction of threading dislocations, however, the resulting GaN material presents smooth surfaces and thermal conductivity close to the bulk value, which suggests the potential of this approach to be integrated in GaN development as an active material for power devices on various substrates. In Paper II extensive study of homoepitaxial GaN growth by hot-wall MOCVD is presented together with results on the thermal stability of GaN under typical conditions used in our growth reactor. Understanding the evolution of GaN surface under different gas compositions and temperatures allows us to predict optimum homoepitaxial conditions. Analysis in the framework of Ga supersaturation of epilayers simultaneously grown on GaN templates and on GaN HVPE substrates reveals that residual strain and screw dislocation densities affect GaN nucleation and growth and lead to distinctively different morphologies on GaN templates and native substrates, respectively. The established comprehensive picture provides guidance for designing strategies for growth conditions optimization in homoepitaxy. We demonstrate homoepitaxial GaN-on-GaN grown under optimum growth conditions with state-of-the-art smooth surface with an rms value of 0.021 nm and an average TDD of 1.4·106 cm-2 which provide good basis for augmenting power device structures.Future work will be focused on GaN NWs reformation on different substrates, p- and n-type doping of homoepitaxial GaN with impurity control and the fabrication of pn power diode device structures for further processing and assessment by C3NiT partners.
12.	Delgado Carrascon, Rosalia, 1987-, et al. (author) Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN : Understanding Nucleation and Design of Growth Strategies 2022 In: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 22:12, s. 7021-7030 Journal article (peer-reviewed)abstract Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 x 1015 cm-3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.
13.	Esmaeili, M., et al. (author) Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures : Influence of Al composition and Si doping 2007 In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 18:2 Journal article (peer-reviewed)abstract A detailed study of low-temperature photoluminescence (PL) in GaN/AlGaN multiple quantum well (MQW) nanostructures has been reported. We have investigated the effect of Si doping and Al content on PL spectra and PL decay time of these structures. The temperature dependence of radiative as well as non-radiative lifetimes have been evaluated between 2K and room temperature for different Si doping. We found that radiative recombination at higher temperatures even up to RT is stronger in the doped sample, compared to the undoped one. Hole localization in GaN/AlGaN MQWs with different compositions of Al is demonstrated via PL transient decay times and LO phonon coupling. It is found that there is an increasing of the decay time at the PL peak emission with increasing Al composition. For the undoped sample, a non-exponential PL decay behaviour at 2K is attributed to localized exciton recombination. A slight upshift in QWs PL peak with increasing Al composition is observed, which is counteracted by the expected rise of the internal QW electric field with increasing Al. The localization energies have been evaluated by studying the variation of the QW emission versus temperature and we found out that the localization energy increases with increasing Al composition. © IOP Publishing Ltd.
14.	Gil, B., et al. (author) Internal structure of the neutral donor-bound exciton complex in cubic zinc-blende and wurtzite semiconductors 2007 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:8 Journal article (peer-reviewed)abstract We calculate the fine structure splitting of the near band edge donor-bound excitons in major cubic semiconductors using an approach inspired by an earlier one that consists in replacing the Morse potential by a Kratzer one, in order to account for the repulsion between the donor and the hole. A regular trend is observed when plotting the computed results in terms of donor binding energies for all these semiconductors. Second, we extend the method to wurtzite semiconductors, namely CdS, GaN, and ZnO. The previously reported trend is found again, but enriched with the strong anisotropy of the dispersion relations in the valence band of these semiconductors. We end up in addressing a quantitative interpretation of the fine structure splitting of the donor bound exciton complex which includes the jj coupling between the valence band Bloch and the envelope nonrigid rotator hole states. © 2007 The American Physical Society.
15.	Gogova, Daniela, 1967-, et al. (author) Epitaxial growth of β-Ga2O3 by hot-wall MOCVD 2022 In: AIP Advances. - : AIP Publishing. - 2158-3226. ; 12:5 Journal article (peer-reviewed)abstract The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of beta-Ga2O3. Epitaxial beta-Ga2O3 layers at high growth rates (above 1 mu m/h), at low reagent flows, and at reduced growth temperatures (740 degrees C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial beta-Ga2O3 layers are demonstrated with a 201 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) beta-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of beta-Ga2O3.
16.	Harati Zadeh, Hamid, 1964-, et al. (author) Effect of n-type modulation doping on the photoluminescence of GaN/Al0.07Ga0.93N multiple quantum wells 2002 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:8, s. 1373- Journal article (peer-reviewed)
17.	Harati Zadeh, Hamid, 1964-, et al. (author) Optical observation of discrete well width fluctuations in wide band gap III-nitride quantum wells 2007 In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 244:5, s. 1727-1734 Journal article (peer-reviewed)abstract A detailed observation of discrete well width fluctuations via localized excitons in the photoluminescence (PL) spectra of MOCVD-grown undoped GaN/Al0.07Ga0.93 N multiple quantum wells (MQWs) has been reported. Doublet excitonic features with a distance varying between 10 and 25 meV for different well widths (1.5 to 4.5 nm) are observed in the PL spectra. They are explained in terms of discrete well width variations by one c-lattice parameter, i.e. two GaN monolayers. By mapping the PL measurements across the samples with different excitation spot size, it is shown that the extension of areas with a constant well width is less than 1 μm2. TEM pictures give evidence of interface roughness, although the contrast is weak at this low Al composition. In addition we observe a long-range variation of the PL peak position across the sample, interpreted as a fluctuation in Al composition in the barriers. The residual broadening of an excitonic peak (apart from the splitting related to well width fluctuations) is about 10 meV, somewhat larger for larger well widths, and is mainly ascribed to hole localisation potentials in the QWs. Additional broadening occurs in the MQWs due to inequivalent properties of each QW within the excitation spot. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.
18.	Harati Zadeh, Hamid, 1964-, et al. (author) Optical Studies of Wide Band Gap III-Nitride Semiconductor Quantum Wells and Superlattices 2006 In: European Materials Research Society E-MRS fall meeting 2006,2006. Conference paper (other academic/artistic)abstract
19.	Harati Zadeh, Hamid, 1964-, et al. (author) Photoluminescence study of Si-doped GaN/Al0.07Ga0.93N multiple quantum wells with different dopant position 2004 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:25, s. 5071-5073 Journal article (peer-reviewed)abstract The Si-doped GaN/Al0.07Ga0.93N multiple quantum wells (MQW) were investigated, using photoluminescence (PL) and time-resolved (PL) measurements. The influence of Si doping on the emission energy and recombination dynamics of the MWQs were also investigated, with different dopant position in the wells. It was observed that the redshifted emission of the MQWs was attributed to the self-energy shift of the electron states due to the correlated motion of the electrons exposed to the fluctuating potential of the donor ions. It was also observed that the PL decay time of the sample was ∼760 ps, at low temperature.
20.	Harati Zadeh, Hamid, 1964-, et al. (author) Photoluminescence study of Si doped GaN/AlGaN multi quantum wells 2003 In: ICPS 2002,2002. ; , s. D-109- Conference paper (peer-reviewed)
21.	Harati Zadeh, Hamid, 1964-, et al. (author) Photoluminescence study of Si doped GaN/GaN/Al0.07Ga0.93N multi quantum wells 2002 In: NANO-7/ECOSS-21,2002. ; , s. 13- Conference paper (other academic/artistic)
22.	Harati Zadeh, Hamid, 1964-, et al. (author) The influence of Si-donor doping on the exciton localization in modulation- doped GaN/AlGaN multi quantum wells 2002 In: 14th Indium Phosphide and Related Materials Conference IPRM 2002,2002. ; , s. 495- Conference paper (peer-reviewed)
23.	Hemmingsson, Carl, 1964-, et al. (author) Growth of bulk GaN in a vertical hydride vapour phase epitaxy reactor 2006 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 40:4-6 SPEC. ISS., s. 205-213 Journal article (peer-reviewed)abstract Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence. © 2006 Elsevier Ltd. All rights reserved.
24.	Honda, Y., et al. (author) DAP emission band in a carbon doped (1-101)GaN grown ob (001) Si substrate 2009 In: Phys. Stat. Sol. (c) Vol. 6. - : Wiley. ; , s. S772-S775 Conference paper (peer-reviewed)abstract Optical spectra of a C-doped (1-101) GaN are investigated via time resolved photoluminescence spectroscopy. Samples with different C-doping levels were prepared by metalorganic vapour phase epitaxy using C2H2 as the doping precursor. A carbon related emission peak is identified at 375 nm which shows typical behaviours for a donor-acceptor-pair emission band. The acceptor level is estimated to be 190 meV which is at 43 meV shallower than that in an Mg doped GaN. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
25.	Hsu, C-W., et al. (author) Luminescent Behaviors of GaN Nanowires with Different Surface Conditions : Toward Optical DNA Sensing 2007 In: International Conference on One-Dimensional Nanostructures ICON,2007. Conference paper (peer-reviewed)abstract
26.	Monemar, Bo, 1942-, et al. (author) Acceptors in Mg doped GaN, optical properties and metastability 2008 In: IWN 2008,2008. Conference paper (peer-reviewed)
27.	Monemar, Bo, 1942-, et al. (author) Dominant shallow acceptor related to oxygen and hydrogen in GaN 2006 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 376-377, s. 440-443 Journal article (peer-reviewed)abstract We present new photoluminescence (PL) data of deliberately O-doped and Mg-doped GaN layers grown by MOCVD. The combination of these data with positron annihilation spectroscopy (PAS) and SIMS results obtained on the same samples shows a clear correlation of the PL intensity of the acceptor related emissions at 3.466 and 3.27 eV (at 2 K) with O doping. The acceptor is stable upon annealing in N-2 in our highly resistive samples, while it is known be unstable in p-GaN. Our tentative conclusion is that this very commonly occurring acceptor is either a V-Ga-O-H complex or a second configuration of the Mg acceptor containing H.
28.	Monemar, Bo, 1942-, et al. (author) Growth of thick GaN layers by hydride vapor phase epitaxy 2005 In: Journal of Ceramic Processing Research. - 1229-9162. ; 6:2, s. 153-162 Journal article (peer-reviewed)abstract In this paper we describe recent experimental work on the growth of thick GaN layers (up to >300 μm) on sapphire with hydride vapour phase epitaxy (HVPE), the removal of the sapphire substrate by the laser liftoff technique, and the properties of these thick GaN layers. Two different growth setups were used, one horizontal and one vertical system. Specific conditions in the growth procedure, like gas flow pattern, growth rate and the use of buffer layers, strongly influence the properties of the grown layers. Important defect problems are cracking (both during and after growth), and the generation of dislocations and surface pits. A large bowing is also observed for thick layers, depending very much on the properties of the initially grown material. For growth of thick layers excessive parasitic growth of GaN upstream of the substrate has to be avoided. Laser liftoff is demonstrated to be a feasible process to remove the sapphire substrate, causing the GaN surface bowing to decrease and revert from convex to concave. The threading dislocation density of 300 μm thick GaN layers is found to be about 107 cm-2, rather independent of the type of buffer layer employed. It reduces further in thicker layers. The pit density varies with growth conditions, it can be reduced if the parasitic growth is avoided. The bowing is a serious problem, since the layers have to be polished to make the surface epi-ready. The XRD rocking curve widths measured seem to correlate with the bowing of the layers, a reduction by about a factor two is often observed when the substrate is removed. Optical characterisation like photoluminescence (PL) and ir spectroscopic ellipsometry (IRSE) is very useful to monitor strain in the layers, as well as impurities and point defects. Residual shallow donors are related to O and Si, shallow acceptors are mainly of intrinsic origin, i.e. complexes with the Ga vacancy.
29.	Monemar, Bo, 1942-, et al. (author) Influence of polarization fields and depletion fields on photoluminescence of AlGaN/GaN multiple quantum well structures 2003 In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 237:1, s. 353-364 Journal article (peer-reviewed)abstract We report on a detailed study of low temperature photoluminescence (PL) in Al0.07Ga0.93N/GaN multiple quantum wells (MQWs). The structures were grown on sapphire with the conventional low temperature AlN nucleation layer and thick GaN buffer layer. Several sets of 5 QW MQW samples were studied, one set with Si doping in the barriers up to or above the metallic limit. Nominally undoped MQW samples were also studied. The spectral behaviour of the doped samples was strongly affected by the near surface depletion field, causing overlap of different spectra from non-equivalent QWs. The QWs closest to the surface are presumably inactive in some samples, due to a very high depletion field. For the case of undoped samples, on the other hand, the near surface QWs are active and most prominent in the PL spectra. The structure from discrete well width variations is here resolved in the PL spectra. The results demonstrate that for structures with no additional capping layer both the depletion field and the polarisation fields need to be considered in the interpretation of experimental data. The theoretically estimated fields in this work are consistent with the experimental spectra. The presence of localisation even in the case of metallic samples, as observed by a constant PL decay time independent of doping, is discussed in terms of penetration of the hole wave functions into the AlGaN barriers. This localisation is also manifested in a sizeable LO phonon coupling strength in all samples studied.
30.	Monemar, Bo, et al. (author) Mg related acceptors in GaN 2010 In: Phys. Status Solidi C 7. ; , s. 1850- Conference paper (peer-reviewed)
31.	Monemar, Bo, 1942-, et al. (author) Optical properties and acceptor states in Mg doped GaN 2008 In: 8^th Akasaki Symposium,2008. Conference paper (other academic/artistic)
32.	Monemar, Bo, 1942-, et al. (author) Optical properties of InN - The bandgap question 2005 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 38:1, s. 38-56 Journal article (peer-reviewed)abstract The bandgap value of InN was analyzed, with reference to optical data on single crystalline thin films samples grown on sapphire. It was found that the line shape of the photoluminescence spectra could be quite well reproduced in a model for the optical transitions from the conduction band states to localized states above the valence band. It was also found that the conduction band transition requires the presence of deep donor at a concentration close to 1020 cm∓3. The results show that the bandgap of pure InN should be about 0.69 eV at 2 K.
33.	Monemar, Bo, 1942-, et al. (author) Optical signatures of dopants in GaN 2006 In: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:1-3, s. 168-174 Journal article (peer-reviewed)abstract The characteristic optical spectra for shallow donors and acceptors in GaN are discussed. The most accurate photoluminescence (PL) data are obtained from samples grown on freestanding GaN substrates, where strain shifts are absent and a low spectroscopic line width is obtained. Recent PL data for excitons bound to the O and Si donors are discussed in some detail, giving accurate values for the binding energies and excited bound donor states. The Mg-acceptor is the most important one for p-doping, but the related optical spectra are controversial. We show that there are two acceptors present in Mg-doped GaN, with two different acceptor bound exciton peaks, and also two corresponding lower energy donor-acceptor pair spectra. We give tentative evidence for their interpretation. © 2006 Elsevier Ltd. All rights reserved.
34.	Monemar, Bo, 1942-, et al. (author) Oxygen related shallow acceptor in GaN 2005 In: MRS Fall Meeting,2004. - : Materials Research Society. ; , s. E5.10.11-E5.10.11 Conference paper (other academic/artistic)
35.	Monemar, Bo, 1942-, et al. (author) Properties of dopants and defects in GaN from bound exciton spectra 2008 In: Meijo International Symposium on Nitride Semiconductors 2008,2008. Conference paper (other academic/artistic)
36.	Monemar, Bo, 1942-, et al. (author) Recombination of free and bound excitons in GaN 2008 In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 245:9, s. 1723-1740 Journal article (peer-reviewed)abstract We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <1016 cm-3 in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 meV and 33.2 meV for the O donor. The PL transients of the DBEs reveal a non-exponential decay for the NP lines. The DBE NP transient lineshape is assumed to be influenced by optical dispersion and scattering in the vicinity of exciton resonances, as well as by surface effects. The DBE decay time can most properly be deduced from the PL decay of the respective TETs and LO replicas, leading to values in the range of 1.1-1.8 ns. These values differ significantly from previous theoretical predictions, where values about two orders of magnitude shorter were obtained. A tentative discussion of the main observed features of acceptor bound excitons (ABEs), which are much less studied in GaN, is given. A decay time of about 0.9 ns for the shallowest 3.466 eV ABE is estimated, i.e. shorter than that for the shallow donor BEs. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
37.	Monemar, Bo, et al. (author) Transient photoluminescence of shallow donor bound excitons in GaN 2010 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 82, s. 235202- Journal article (peer-reviewed)abstract We present a detailed study of photoluminescence transients for neutral donor bound excitons (DBEs) in GaN, notably the ON donor DBE at 3.4714 eV and the SiGa DBE at 3.4723 eV. The studied samples are thick strain free nominally undoped bulk GaN samples, with a spectroscopic linewidth <0.5 meV at 2 K. The photoluminescence (PL) decay curves for these no-phonon (NP) lines are strongly nonexponential, and do not allow a proper assessment of the characteristic BE decay time. The decay of the LO-phonon replicas as well as the so-called two-electron transitions (TETs) at lower energies show a nicely exponential behavior, and allow extraction of DBE decay times of about 1.1 ns for the Si DBE and 1.8 ns for the O DBE, respectively. The initial nonexponential decay behavior of the NP lines has been studied in both the common front surface excitation-detection mode and with detection in transmission through the sample. This initial decay is explained as related to scattering processes in the near surface region, involving the DBEs and free excitons (FEs). Light scattering processes may also contribute to this complex decay shape. The DBE-LO-phonon decay does not discriminate between the O and Si DBEs because of spectral overlap involving different LO modes. The TET decays at 2 K are very different for transitions related to the DBE ground state and DBE excited states (going to p-like donor final states), for T>10 K thermalization between the DBE ground state and DBE excited states produces a common decay time. Thermalization between free and bound excitons appears to occur above about 20 K, when the DBE decay follows the FE decay. A simple two-level modeling of exciton capture and recombination for the PL decay curves of the FE and the DBEs, as commonly used in the literature, is shown to be generally inadequate. A broad PL background in the TET spectral region is suggested to be related to a radiative Auger process, where the DBEs recombine while leaving the donors ionized.
38.	Ochalski, T.J., et al. (author) Optical study of AlGaN/GaN based HEMT structures 2005 In: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 2, s. 2791-2794 Journal article (peer-reviewed)
39.	Paskov, Plamen, 1959-, et al. (author) Anisotropy of the free exciton emission in GaN grown on a-plane sapphire 2002 In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 75-79 Journal article (peer-reviewed)abstract The influence of the anisotropic in-plane strain on the optical response of GaN layers grown on a-plane sapphire is investigated. A splitting of the G5 states of the A and B free excitons into two components, polarized parallel and perpendicular to the strain direction, is observed. The experimental results are discussed in the framework of the effective exciton Hamiltonian formalism.
40.	Paskov, Plamen, 1959-, et al. (author) Anisotropy of the in-plane strain in GaN grown on A-plane sapphire 2002 In: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 892-896 Journal article (peer-reviewed)abstract A comparative study of GaN layer grown by hydride vapour phase epitaxy on A-plane sapphire before and after removal of the substrate is presented. A large anisotropy of the in-plane strain in the as-grown sample is revealed by X-ray diffraction measurements and polarized photoluminescence. The strain anisotropy is found to modify the selection rules for the transitions leading to a splitting of the optically active states of the A and B excitons. Almost complete strain relaxation and recovery of the optical isotropy in the (0001) plane is observed in the free-standing layer.
41.	Paskov, Plamen, 1959-, et al. (author) Internal structure of free excitons in GaN 2001 In: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 467-470 Journal article (peer-reviewed)abstract Polarized photoluminescence is used to study the fine structure of free excitons in thick GaN layers grown on differently oriented sapphire substrates. The singlet-triplet splitting of the A exciton is measured and the exchange interaction constant in GaN is determined. For the samples grown on the a-plane sapphire, splitting of the A and B excitons induced by the uniaxial in-plane stress is also observed.
42.	Paskov, Plamen, 1959-, et al. (author) Luminescence of GaN layers grown in nonpolar directions 2008. - 1 In: Nitrides with nonpolar surfaces: growth, properties, and devices. - Weinheim : Wiley. - 9783527407682 ; , s. 185-217 Book chapter (other academic/artistic)abstract This is the first monograph to discuss in detail the current stage of development of nonpolar nitrides, with specific emphasis on the three main topics of crystal growth, properties and device studies. World-class researchers summarize their own recent achievements in their respective fields of expertise, covering both nonpolar and semipolar nitride materials. The bulk of the discussion in each chapter is related to the physical properties of the material obtained by the respective technique, in particular, defect density and properties of the defects in nonpolar nitrides. In addiiton, the optical and vibrational properties are also addressed in several chapters, as is progress in heterostructures, quantum wells and dots based on the AlGaN/GaN and the InGaN/GaN systems. Finally, an outlook of the application areas of the differently grown materials is presented in most chapters, together with the capabilities and limitations of the respective growth approaches used.
43.	Paskov, Plamen, 1959-, et al. (author) Nonpolar GaN layers grown by sidewall epitaxial lateral overgrowth : optical evidences for a reduced stacking fault density 2008 In: Proc. of the 7th International Conference on Nitride Semiconductors,2007. - Linköping : Department of Physics, Chemistry and Biology, Linköping University. ; , s. 1768-1770 Conference paper (peer-reviewed)abstract Nonpolar a -plane and m -plane GaN layers grown by MOCVD employing sidewall epitaxial lateral overgrowth (SELO) are studied by photoluminescence (PL) and spatially resolved micro-PL. The effects of the groove orientations and the groove/terrace width ratio on the emission spectra, particularly on the stacking fault (SF) related emission bands in the 3.29–3.42 eV spectral region, are examined. The PL spectra of both types of nonpolar layers reveal a significant reduction of the defect related emissions when the grooves are oriented perpendicular to the c-axis of GaN and the groove/terrace width ratio is smaller than one. The suppression of SF formation in the areas where a lateral overgrowth along the [0001] GaN direction occurs is confirmed by micro-PL measurements showing no SF related emissions over the terrace regions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
44.	Paskov, Plamen, 1959-, et al. (author) Optical characterization of bulk GaN substrates with c-, a-, and m-plane surfaces 2008 Conference paper (peer-reviewed)
45.	Paskov, Plamen, 1959-, et al. (author) Optical properties of nonpolar a-plane GaN layers 2006 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 40:4-6 SPEC. ISS., s. 253-261 Journal article (peer-reviewed)abstract We have studied optical properties of nonpolar a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition and hydride vapor phase epitaxy using different nucleation schemes. Several emission bands, which are not typical for c-plane GaN, are observed in the photoluminescence spectra and their excitation-intensity, temperature, and polarization dependencies are examined. In addition, the spatial distribution of the emissions was examined by cathodoluminescence imaging and relations of the different emissions with particular structural features in the layers are revealed. The results are discussed with emphasis on the origin of the emission line and particular recombination mechanisms. © 2006 Elsevier Ltd. All rights reserved.
46.	Paskov, Plamen, 1959-, et al. (author) Optical properties of undoped, n-doped and p-doped GaN/AlN superlattices 2007 In: Proc. of the MRS Fall Meeting,2006. - Materials Research Society Symposium Proceedings, vol. 955E : The Materials Research Society. ; , s. I12.3- Conference paper (peer-reviewed)
47.	Paskov, Plamen, 1959-, et al. (author) Optical up-conversion processes in InAs quantum dots 2000 In: the 25th International Conference on the Physics of Semiconductors ICPS-25,2000. ; , s. 2080-2083 Conference paper (peer-reviewed)abstract Up-converted photoluminescence (UPL) in InAs/GaAs self-assembled quantum dots (QDs) is reported. With a resonant excitation of QDs we observe an efficient emission from the GaAs barrier. The excitation spectrum of the UPL shows that the signal disappears if the excitation energy is tuned below the absorption band of the QDs. The intensity of UPL exhibits an almost quadratic dependence on the excitation intensity. The observed effect is explained by a two-step two-photon absorption process involving quantum dot states as intermediate states.
48.	Paskov, Plamen, 1959-, et al. (author) Optical up-conversion processes in InAs quantum dots 2001 In: Japanese Journal of Applied Physics. - 0021-4922 .- 1347-4065. ; 40:3 B, s. 2080-2083 Journal article (peer-reviewed)abstract Up-converted photoluminescence (UPL) in InAs/GaAs self-assembled quantum dots (QDs) is reported. With a resonant excitation of QDs we observe an efficient emission from the GaAs barrier. The excitation spectrum of the UPL shows that the signal disappears if the excitation energy is tuned below the absorption band of the QDs. The intensity of UPL exhibits an almost quadratic dependence on the excitation intensity. The observed effect is explained by a two-step two-photon absorption process involving quantum dot states as intermediate states. ⌐ 2001 The Japan Society of Applied Physics.
49.	Paskov, Plamen P., 1959-, et al. (author) Optical Properties of III-Nitride Semiconductors 2017. - 1 In: Handbook of GaN Semiconductor Materials and Devices. - Boca Raton : CRC Press. - 9781498747134 - 9781498747141 ; , s. 87-116 Book chapter (peer-reviewed)abstract The optical properties of the group-III-nitride materials are obviously of direct relevance for optoelectronic applications, but experiments measuring optical properties also give information on a range of electronic properties. There is already a wealth of data in the literature on the optical properties of III-nitrides [1–4], and here we will concentrate on some of the most recent additions to the scientific knowledge. The focus, looking at the present situation concerning technical applications of these materials, has been on GaN, InGaN, and AlGaN in recent decades. AlGaN materials are important for ultraviolet (UV) emitters and high electron mobility transistor (HEMT) structures and AlGaN optical properties have accordingly been studied over the entire Al composition range. InGaN materials (with In content <50%) have also been studied extensively, and the light-emitting diode (LED) applications based on InGaN/GaN quantum structures have already been awarded a Nobel Prize in 2014. However, the applications of InN are lagging behind. The development of growth procedures for InN and In-rich InGaN has been difficult, and their optical properties were consequently much less studied in the past.
50.	Paskov, Plamen P., 1959-, et al. (author) Point Defects in group-III nitrides 2018. - 1 In: Defects in Advanced Electronic Materials and Novel Low Dimensional Structures. - : Woodhead Publishing Limited. - 9780081020531 - 9780081020548 ; , s. 27-61 Book chapter (peer-reviewed)abstract Point defects in semiconductors play a fundamental role for the material properties. Dopants like impurities forming shallow donors and acceptors provide the means of controlling the electrical conductivity of the material, which is the basis of many applications in devices. Native defects like vacancies and interstitial atoms, and their combination with impurities introduce, mostly unwanted deep levels in the bandgap, and thus may serve as traps or recombination centers for the carriers. Some of these defects are introduced during the growth of the material, others by the processing steps necessary in the device production. In this chapter, we present current knowledge about point defects in the III-nitrides based on recent works, both experimental and theoretical, in the field. Materials discussed are AlN, GaN and InN and the ternary alloys between them.

Skapa referenser, mejla, bekava och länka

Permalink

Träfflista för sökning "WFRF:(Paskov Plamen 1959 ) "

Refine your search

Year