| 1. |
- Marcinkevicius, Saulius, et al.
(författare)
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High spectral uniformity of AlGaN with a high Al content evidenced by scanning near-field photoluminescence spectroscopy
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:24, s. 241108-
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Tidskriftsartikel (refereegranskat)abstract
- Scanning near-field photoluminescence (PL) spectroscopy was applied to study spatial variations of emission spectra of AlxGa1-xN epilayers with 0.6 <= x <= 0.7. PL spectra were found to be spatially uniform with peak wavelength standard deviations of only similar to 2 meV and ratios between peak intensity standard deviations and average peak intensity values of 0.06. The observed absence of correlation between the PL peak wavelength and intensity shows that spatial distribution of nonradiative recombination centers is not related to band potential fluctuations. Our results demonstrate that the homogeneous broadening and the random cation distribution primarily determine PL line-widths for layers grown under optimized conditions.
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| 2. |
- Marcinkevičius, Saulius, et al.
(författare)
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Scanning near-field optical microscopy of AlGaN epitaxial layers
- 2016
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Ingår i: UV and Higher Energy Photonics. - : SPIE - International Society for Optical Engineering. - 9781510602434 - 9781510602441
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Konferensbidrag (refereegranskat)abstract
- Scanning near-field PL spectroscopy was applied to study spatial variations of the emission spectra of AlGaN epilayers with AlN molar fractions between 0.3 and 0.7. Experiments were performed at 300 K with 100 nm spatial resolution. In general, photoluminescence spectra were found to be highly uniform with the peak energy deviation of 2 to 6 meV for different alloy compositions. In the 30% and 42% Al layers, a slightly lower Al content and a higher point defect concentration at the boundaries of growth domains were detected. These features were attributed to the higher mobility of Ga adatoms during growth. The inhomogeneous broadening beyond the random alloy distribution was found negligible for the 30% and 42% Al samples, and about 40 - 50 meV for the layers with a larger Al content.
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| 3. |
- Liuolia, Vytautas, et al.
(författare)
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Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 95:9
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved transmission and photoluminescence measurements were performed on Al0.35Ga0.65N/Al0.49Ga0.51N quantum well structures with different well widths. Comparison of transmission and luminescence data shows that dynamics of electrons and holes excited into extended quantum well states are governed by nonradiative recombination. For excitation into potential minima formed by band gap fluctuations, localization of electrons was observed. Excitation energy dependence of the pump-probe transient shape allows estimating localization potential, which is about 80 meV independently of the well width, and is probably caused by fluctuations of AlN molar fraction.
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| 4. |
- Liuolia, Vytautas, et al.
(författare)
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Photoexcited carrier dynamics in AlInN/GaN heterostructures
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:24, s. 242104-
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Tidskriftsartikel (refereegranskat)abstract
- Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructures have been examined by time-resolved and scanning near-field photoluminescence (PL) spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited hole transfer from AlInN to GaN via sub-band-gap states. These states are attributed to extended defects and In clusters. Near-field PL scans show that diameter of the localization sites and the distance between them are below 100 nm. Spatial variations of the GaN PL wavelength have been assigned to the electric field inhomogeneities at the heterostructure interface.
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| 5. |
- Marcinkevicius, Saulius, et al.
(författare)
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Intrinsic electric fields in AlGaN quantum wells
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:8
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Tidskriftsartikel (refereegranskat)abstract
- Intrinsic electric fields in AlxGa1-xN/AlyGa1-yN quantum wells embedded into p-i-n structures are studied using photoluminescence experiments. Spectral shifts induced by external bias and screening by photoexcited carriers allow evaluating the intrinsic fields caused by piezoelectric and spontaneous polarizations. In quantum wells with low Al content, the field is about 1 MV/cm, which is in agreement with theoretical estimations. For high Al molar fractions (35% well, 50% barrier), the extracted intrinsic field is lower and, most importantly, has the opposite sign to that predicted by the theory.
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| 6. |
- Mickevicius, J., et al.
(författare)
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Photoluminescence dynamics of AlGaN quantum wells with built-in electric fields and localized states
- 2010
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Ingår i: Physica Status Solidi (A) Applications and Materials. - : Wiley. - 1862-6300. ; 207:2, s. 423-427
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Tidskriftsartikel (refereegranskat)abstract
- We report on photoluminescence (PL) studies of AlGaN/AlGaN multiple quantum well (MQW) structures with well widths spanning from 1.65 to 5.0 nm under various excitations and temperatures. The samples were fabricated by migration-enhanced metal-organic chemical vapor deposition technique (MEMOCVD (R)). Screening of the built-in electric field by photogenerated carriers reduced quantum-confined Stark effect (QCSE). This is confirmed by solving the Poisson and Schrodinger equations for AlGaN-based quantum wells (QWs) under study. Analysis of the PL internal quantum efficiency under different excitations in MQWs with different widths shows strong influence of the carrier localization on the radiative properties of MQWs.
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| 7. |
- Pinos, Andrea, et al.
(författare)
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Aging of AlGaN quantum well light emitting diode studied by scanning near-field optical spectroscopy
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 95:18
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Tidskriftsartikel (refereegranskat)abstract
- Emission from a 285 nm AlGaN quantum well light emitting diode has been studied by scanning near-field optical spectroscopy. The scans revealed micrometer-size domainlike areas emitting with a higher intensity and at a longer wavelength; presumably, because of a lower AlN molar fraction in these regions. Experiments performed on different days have shown that with time, intensity from these spots increases and emission wavelength shifts to the red, indicating a further change in the quantum well alloy composition. This has allowed distinguishing an aging mechanism that involves locally increased current, heating, and atom migration.
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| 8. |
- Pinos, Andrea, et al.
(författare)
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Carrier lifetimes in AlGaN quantum wells : electric field and excitonic effects
- 2008
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:15
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Tidskriftsartikel (refereegranskat)abstract
- Photoexcited carrier dynamics in a 280 nm AlGaN quantum well (QW) light emitting diode has been studied by time-resolved photoluminescence at forward and reverse bias. Long ( for AlGaN QWs with high Al content) room temperature carrier lifetimes of about 600 ps were measured with only a slight dependence on bias. These lifetimes are much longer than calculated free carrier tunnelling and thermionic emission times, pointing out the importance of excitonic effects for carrier dynamics in AlGaN QWs.
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| 9. |
- Pinos, Andrea, et al.
(författare)
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Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:11
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Tidskriftsartikel (refereegranskat)abstract
- Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25-40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination.
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| 10. |
- Pinos, Andrea, et al.
(författare)
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Optical studies of degradation of AlGaN quantum well based deep ultraviolet light emitting diodes
- 2010
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 108:9, s. 093113-
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Tidskriftsartikel (refereegranskat)abstract
- Aging under high current stress of AlGaN quantum well based light emitting diodes with high and low Al content in the wells emitting at 270 nm and 335 nm, respectively, has been studied by scanning near field optical spectroscopy and far field electroluminescence, photoluminescence and time-resolved photoluminescence. In the high Al content devices emission band related to optical transitions in the cladding involving nitrogen vacancies has been found. Evolution of this band during aging suggests that the role of N vacancies is crucial in the aging process by aiding defect generation and formation of high conductivity channels.
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