| 1. |
- Arnaudov, B, et al.
(författare)
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Energy position of near-band-edge emission spectra of InN epitaxial layers with different doping levels
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:11
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Tidskriftsartikel (refereegranskat)abstract
- We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the "free-to-bound" recombination model in degenerate semiconductors. For carrier concentrations above n>5x10(18) cm(-3) the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is E-g=692+/-2 meV for an effective mass at the conduction-band minimum m(n0)=0.042m(0).
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| 2. |
- Arnaudov, B., et al.
(författare)
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Free-to-bound radiative recombination in highly conducting InN epitaxial layers
- 2004
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 563-571
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7 × 1017-6 × 1018) cm-3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg = 692 meV for effective mass mn0 = 0.042m0 and Eg =710 meV for mn0 = 0.1m0. © 2004 Elsevier Ltd. All rights reserved.
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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. |
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| 8. |
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| 9. |
- 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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| 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. |
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| 12. |
- Paskova, Tanja, et al.
(författare)
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Donor-acceptor pair emission enhancement in mass-transport-grown GaN
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:3, s. 033508-
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Tidskriftsartikel (refereegranskat)abstract
- A dominating donor-acceptor pair (DAP) emission at about 3.27 eV is observed in the photoluminescence and cathodoluminescence spectra of intentionally undoped mass-transport (MT)-grown GaN, while only a weak presence of the DAP emission is recorded in the as-grown hydride vapor phase epitaxial GaN. A comparative study of impurity and native defect incorporation in the as-grown and MT GaN was performed, showing a significant increase of oxygen and empty clusters involving Ga vacancy and oxygen in the MT GaN. Based on the observed results as well as on doping analysis of the structure and kinetic analysis of the emission intensities, we propose an acceptorlike complex, creating a state as a semiclassical potential well near the valence-band top due to the local tensile strain caused by the empty clusters to be responsible for the dominating behavior of the DAP emission. © 2005 American Institute of Physics.
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| 13. |
- 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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| 14. |
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| 15. |
- Paskova, Tanja, et al.
(författare)
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Polar and nonpolar GaN grown by HVPE : Preferable substrates for nitride-based emitting devices
- 2004
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2265-2270
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Tidskriftsartikel (refereegranskat)abstract
- We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.
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| 16. |
- Paskova, Tanja, et al.
(författare)
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Properties of nonpolar a-plane GaN films grown by HVPE with AlN buffers
- 2005
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 281:1, s. 55-61
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Tidskriftsartikel (refereegranskat)abstract
- The influence of high temperature AlN buffer layers on the morphology, structural and optical characteristics of a-plane GaN grown by hydride vapour phase epitaxy on r-plane sapphire was investigated. While the morphology of the a-GaN was found to be significantly improved by using a-plane AlN buffer layer similarly to the effect observed in c-plane hydride vapour phase epitaxy GaN growth, the microstructure ensemble was revealed to be more complicated in comparison to that of the c-plane GaN. Higher dislocation density and prismatic stacking faults were observed. Moreover, in-plane anisotropic structural characteristics were revealed by high resolution X-ray diffraction employing azimuthal dependent and edge X-ray measurement symmetric geometry. In addition, the near band edge photo luminescence peaks, red-shifted with respect to that in c-plane GaN were observed. The latter were explained by the influence of the higher defect density and more complex strain distribution. (c) 2005 Elsevier B.V. All rights reserved.
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