| 1. |
- Aggerstam, Thomas, et al.
(författare)
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Electron and hole capture cross-sections of Fe acceptors in GaN:Fe epitaxially grown on sapphire
- 2007
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Ingår i: Journal of Electronic Materials. - : Springer Science and Business Media LLC. - 0361-5235 .- 1543-186X. ; 36:12, s. 1621-1624
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Tidskriftsartikel (refereegranskat)abstract
- Carrier trapping of Fe (3+)/Fe2+ deep acceptors in epitaxially grown GaN:Fe on sapphire was studied by time-resolved photoluminescence. For the investigated Fe doping levels on the order of 10(18) cm(-3), the luminescence decay times are strongly dependent on the Fe concentration, indicating that Fe centers act as predominant nonradiative recombination channels. Linear dependence of the decay time on the iron concentration allows estimation of the electron capture cross-section for the Fe3+ ions, which is equal to 1.9 x 10(-15) cm(2). The upper bound for the cross-section of the hole capture of Fe2+ was evaluated as 10 x 10(-15) cm
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| 2. |
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| 3. |
- Akram, Nadeem, et al.
(författare)
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The effect of barrier composition on the vertical carrier transport and lasing properties of 1.55-mu m multiple quantum-well structures
- 2006
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Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 42:7, s. 713-714
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, the effect of barrier bandgap and composition on the optical performance of 1.55-mu m InGaAsP/In-GaAsP and InGaAsP/InGaAlAs multiple quantum-well structures and Fabry-Perot lasers is evaluated experimentally. Direct vertical carrier transport measurements were performed through strain-compensated multiple quantum-well (MQW) test structures using femto-second laser pulse excitation and time-resolved photoluminescence up-conversion method. MQW test structures were grown with different barrier composition (InGaAsP and InGaAlAs) and barrier bandgap (varied from lambda(g) = 1440 to 1260 nm) having different conduction band Delta E-c and valence band discontinuity Delta E-v, while keeping the same InGaAsP well composition for all the structures. The ambipolar carrier transport was found to be faster in the structures with lower valence band discontinuity Delta E-v. Regrown semi-insulating buried heterostructure Fabry-Perot (SIBH-FP) lasers were fabricated from similar QWs and their static light-current-voltage characteristics (including optical gain and chirp spectra below threshold) and thermal characteristics were measured. Lasers with InGaAlAs barrier showed improved high-temperature operation, higher optical gain, higher differential gain, and lower chirp, making them suitable candidates for high-bandwidth directly modulated uncooled laser applications.
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| 4. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Electrochemical etching of AlGaN for the realization of thin-film devices
- 2019
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 115:18, s. 182103-
-
Tidskriftsartikel (refereegranskat)abstract
- Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 µm x 1 µm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.
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| 5. |
- Berrier, Audrey, et al.
(författare)
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Accumulated sidewall damage in dry etched photonic crystals
- 2009
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Ingår i: Journal of Vacuum Science & Technology B. - : American Vacuum Society. - 1071-1023 .- 1520-8567. ; 27:4, s. 1969-1975
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Tidskriftsartikel (refereegranskat)abstract
- Evidence for accumulated damage is provided by investigating the effect of etch duration on the carrier lifetime of an InGaAsP quantum well (QW) inside the InP-based photonic crystal (PhC) structures. It is found that once the quantum well is etched through, additional etching reduces the carrier lifetimes from 800 to 70 ps. The surface recombination velocity (SRV) at the exposed hole sidewalls is determined from the measured carrier lifetimes of the PhC fields with different lattice parameters. The observed variation in the SRV with etch duration also confirms the presence of accumulated sidewall damage. It increases from 6x10(3) to 1.2x10(5) cm s(-1) as the etching time increases from 3 to 50 min. A geometric model based on sputtering theory and on the evolution of the hole shape is developed to explain the accumulation of sidewall damage. The model is used to estimate the number of impact events from sputtered species reaching the QW sidewalls, and the variation in the accumulated impact events with etch duration is shown to be qualitatively consistent with the experimental observations. Finally, the results suggest a new method for tailoring the carrier lifetimes in PhC membrane structures.
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| 6. |
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| 7. |
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| 8. |
- Berrier, Audrey, et al.
(författare)
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Impact of dry-etching induced damage in InP-based photonic crystals
- 2008
-
Ingår i: PHOTONIC CRYSTAL MATERIALS AND DEVICES VIII. - : SPIE. ; , s. U9890-U9890
-
Konferensbidrag (refereegranskat)abstract
- In this work variations of the carrier lifetime in a GaInAsP/InP quantum well in two-dimensional PhC structures etched by Ar/Cl-2 chemically assisted ion beam etching as a function of the processing parameters is investigated. It is shown that the deposition conditions of the SiO2 mask material and its coverage as well as other process steps such as annealing affect the carrier lifetimes. However the impact of patterning the semiconductor on the carrier lifetime is dominant, showing over an order of magnitude reduction. For given PhC lattice parameters, the sidewall damage is shown to be directly related to the measured carrier lifetimes. A simple qualitative model based on sputtering theory and assuming a conical hole-shape development during etching is used to explain the experimental results.
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| 9. |
- Bertulis, K., et al.
(författare)
-
GaBiAs : A material for optoelectronic terahertz devices
- 2006
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:20
-
Tidskriftsartikel (refereegranskat)abstract
- GaBiAs layers have been grown by molecular beam epitaxy at low (270-330 degrees C) temperatures and were characterized by several experimental techniques. It was shown that the spectral photosensitivity cutoff wavelength reaches similar to 1.4 mu m when the growth temperature is as low as 280 degrees C. Optical pump-terahertz probe measurements made on these layers have evidenced that the electron trapping time decreases with decreasing growth temperature from 20 to about 1 ps. GaBiAs layers were used for manufacturing photoconductive terahertz emitters and detectors, which, when excited with Ti:sapphire laser pulses, have demonstrated a signal bandwidth of 3 THz.
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| 10. |
- Butte, Raphael, et al.
(författare)
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Optical absorption edge broadening in thick InGaN layers : Random alloy atomic disorder and growth mode induced fluctuations
- 2018
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 112:3
-
Tidskriftsartikel (refereegranskat)abstract
- To assess the impact of random alloying on the optical properties of the InGaN alloy, high-quality InxGa1-xN (0 < x < 0.18) epilayers grown on c-plane free-standing GaN substrates are characterized both structurally and optically. The thickness (25-100 nm) was adjusted to keep these layers pseudomorphically strained over the whole range of explored indium content as checked by x-ray diffraction measurements. The evolution of the low temperature optical absorption (OA) edge line-width as a function of absorption energy, and hence the indium content, is analyzed in the framework of the random alloy model. The latter shows that the OA edge linewidth should not markedly increase above an indium content of 4%, varying from 17 meV to 30 meV for 20% indium. The experimental data initially follow the same trend with, however, a deviation from this model for indium contents exceeding only similar to 2%. Complementary room temperature near-field photoluminescence measurements carried out using a scanning near-field optical microscope combined with simultaneous surface morphology mappings reveal spatial disorder due to growth meandering. We conclude that for thick high-quality pseudomorphic InGaN layers, a deviation from pure random alloying occurs due to the interplay between indium incorporation and longer range fluctuations induced by the InGaN step-meandering growth mode.
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| 11. |
- Campi, Roberta, et al.
(författare)
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Studies on the carrier transport in InGaAlAdP/InGaAsP quantum well structures emitting at 1.3 μm
- 2000
-
Ingår i: Conference on Lasers and Electro-Optics Europe - Technical Digest. - 0780363191 ; , s. 141-
-
Konferensbidrag (refereegranskat)abstract
- A novel approach to improve high temperature performance in the IngaAsP lasers was examined by adding aluminum to the barrier,which allows to increase conduction band offset. To find optimal heterostructure parameters, different barrier material compositions were examined in structures with InGaAsP compressively strained wells and tensile strained InGaAlAsP barriers. The MQW structures were fabricated by low pressure MOVPE.
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| 12. |
- Carmody, C., et al.
(författare)
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Ion-implanted In0.53Ga0.47As for ultrafast optoelectronic applications
- 2003
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:22, s. 3913-3915
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Tidskriftsartikel (refereegranskat)abstract
- Undoped In0.53Ga0.47As epilayers were implanted with 2- MeV Fe+ ions at doses of 1x10(15) and 1x10(16) cm(-2) at room temperature and annealed at temperatures between 500 and 800 degreesC. Hall-effect measurements show that after annealing, layers with resistivities on the order of 10(5) Omega/square can be achieved. Carrier lifetimes as short as 300 fs are observed for samples annealed at 500 and 600 degreesC. For higher annealing temperatures, characteristic times of the optical response are on the order of a few picoseconds.
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| 13. |
- Carmody, C., et al.
(författare)
-
Ultrafast carrier trapping and recombination in highly resistive ion implanted InP
- 2003
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 94:2, s. 1074-1078
-
Tidskriftsartikel (refereegranskat)abstract
- MeV P+ implanted and annealed p-InP, and Fe+ implanted and annealed semi-insulating InP have both been shown to produce the high resistivity, good mobility, and ultrafast optical response desired for ultrafast photodetectors. Hall effect measurements and time resolved photoluminescence were used to analyze the electrical and optical features of such implanted materials. Low temperature annealing was found to yield the fastest response times-130 fs for Fe+ implanted and 400 fs for P+ implanted InP, as well as resistivities of the order similar to10(6) Omega/square. It was found that the electrical activation of Fe-related centers, useful for achieving high resistivities in Fe+ implanted semi-insulating InP, was not fully realized at the annealing temperatures chosen to produce the fastest optical response. Implanting p-InP in the dose regime where type conversion occurs, and subsequent annealing at 500degreesC, produces high resistivities and ultrafast carrier trapping times that are only marginally dose dependent.
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| 14. |
- Chen, Hui, et al.
(författare)
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Photon Walk in Transparent Wood: Scattering and Absorption in Hierarchically Structured Materials
- 2022
-
Ingår i: Advanced Optical Materials. - : Wiley. - 2162-7568 .- 2195-1071.
-
Tidskriftsartikel (refereegranskat)abstract
- The optical response of hierarchical materials is convoluted, which hinders their direct study and property control. Transparent wood (TW) is an emerging biocomposite in this category, which adds optical function to the structural properties of wood. Nano- and microscale inhomogeneities in composition, structure and at interfaces strongly affect light transmission and haze. While interface manipulation can tailor TW properties, the realization of optically clear wood requires detailed understanding of light-TW interaction mechanisms. Here we show how material scattering and absorption coefficients can be extracted from a combination of experimental spectroscopic measurements and a photon diffusion model. Contributions from different length scales can thus be deciphered and quantified. It is shown that forward scattering dominates haze in TW, primarily caused by refractive index mismatch between the wood substrate and the polymer phase. Rayleigh scattering from the wood cell wall and absorption from residual lignin have minor effects on transmittance, but the former affects haze. Results provide guidance for material design of transparent hierarchical composites towards desired optical functionality; we demonstrate experimentally how transmittance and haze of TW can be controlled over a broad range.
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| 15. |
- Chen, Miaoxiang, 1962-, et al.
(författare)
-
1 micron wavelength photo- and electroluminescence from a conjugated polymer
- 2004
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:18, s. 3570-3572
-
Tidskriftsartikel (refereegranskat)abstract
- We report photo- and electroluminescence from an alternating conjugated polymer consisting of fluorene units and low-band gap donor-acceptor-donor (D-A-D) units. The D-A-D segment includes two electron-donating thiophene rings combined with a thiadiazolo-quinoxaline unit, which is electron withdrawing to its nature. The resulting polymer is conjugated and has a band gap of 1.27 eV. The corresponding electro- and photoluminescence spectra both peak at approximately 1 mum. Light-emitting diodes, based on a single layer of the polymer, demonstrated external quantum efficiencies from 0.03% to 0.05%.
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| 16. |
- Espenlaub, Andrew C., et al.
(författare)
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Evidence of trap-assisted Auger recombination in low radiative efficiency MBE-grown III-nitride LEDs
- 2019
-
Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 126:18
-
Tidskriftsartikel (refereegranskat)abstract
- By studying low radiative efficiency blue III-nitride light emitting diodes (LEDs), we find that the ABC model of recombination commonly used for understanding efficiency behavior in LEDs is insufficient and that additional effects should be taken into account. We propose a modification to the standard recombination model by incorporating a bimolecular nonradiative term. The modified model is shown to be in much better agreement with the radiative efficiency data and to be more consistent than the conventional model with very short carrier lifetimes measured by time-resolved photoluminescence in similar, low radiative efficiency material. We present experimental evidence that a hot carrier-generating process is occurring within these devices, in the form of measurements of forward photocurrent under forward bias. The forward photocurrent, due to hot carrier generation in the active region, is present despite the lack of any "efficiency droop"-the usual signature of band-to-band Auger recombination in high-quality III-nitride LEDs. Hot carrier generation in the absence of band-to-band Auger recombination implies that some other source of hot carriers exists within these low radiative efficiency devices, such as trap-assisted Auger recombination.
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| 17. |
- Gaarder, A., et al.
(författare)
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Dopant distribution in selectively regrown InP : Fe studied by time-resolved photoluminescence
- 2001
-
Ingår i: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 226:4, s. 451-457
-
Tidskriftsartikel (refereegranskat)abstract
- We apply time-resolved photoluminescence with 1 mum spatial resolution for the characterization of iron distribution in semi-insulating InP:Fe epitaxial layers regrown by hydride vapor-phase epitaxy around etched mesas. The InP:Fe regrowth was carried out on InP:S mesas etched both along the [110] and [110] crystallographic directions, as well as on InP/InGaAsP in-plane lasers. In all cases, the Fe concentration was found to be close to the target values and showed little variation along the regrown layers.
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| 18. |
- Gaarder, A., et al.
(författare)
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Time-resolved micro-photoluminescence studies of deep level distribution in selectively regrown GaInP : Fe and GaAs : Fe
- 2002
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 17:2, s. 129-134
-
Tidskriftsartikel (refereegranskat)abstract
- We apply time-resolved photoluminescence with 1-2 mum spatial resolution for the characterization of deep centre distributions in semi-insulating GaInP:Fe and GaAs:Fe epitaxial layers regrown by hydride vapour phase epitaxy around etched GaAs mesas and GaAs/AlGaAs quantum well laser structures. In InGaP, Fe ions act as the main carrier recombination centres, while in Fe-doped GaAs both the Fe ions and As antisite defects have to be considered. The distribution of Fe ions in InGaP was found to be rather uniform and close to the target value. For GaAs:Fe, the number of ionized Fe and EL2 centres showed a certain increase at the mesa interfaces. In both cases, the high trap concentration was maintained throughout the regrown layers indicating good semi-insulating material properties.
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| 19. |
- Gaarder, A., et al.
(författare)
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Time-resolved micro-photoluminescence studies of dopant distribution in selectively regrown GalnP : Fe around VCSELs
- 2002
-
Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; T101, s. 89-91
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Tidskriftsartikel (refereegranskat)abstract
- We apply time-resolved photoluminescence with 1.5 mum spatial resolution for characterization ofcarrier trap distribution in semi-insulating Ga0.51In0.49P:Fe layers regrown around GaAs/AlGaAs circular vertical cavity surface emitting laser mesas using hydride vapour phase epitaxy. The carrier trapping times are in the range from 10 to 15 ps and quite uniformly distributed throughout the burying GaInP:Fe layer, suggesting that the layer is semi-insulating everywhere. Simulations show that, in addition to the Fe dopants, the layer contains other, unintended carrier traps. The photoluminescence spectra reveal that the regrown GaInP:Fe material has several distinct regions with different band gaps. This is attributed to differences in the In/Ga composition and/or CuPt ordering of the GaInP.
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| 20. |
- Gautier, S., et al.
(författare)
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AlGaN/AlN multiple quantum wells grown by MOVPE on AlN templates using nitrogen as a carrier gas
- 2008
-
Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 310:23, s. 4927-4931
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Tidskriftsartikel (refereegranskat)abstract
- AlxGa1-xN/AlN multiple quantum wells (MQWs) structures were grown by metalorganic vapour phase epitaxy (MOVPE) on pseudo AlN substrates using nitrogen as a carrier gas. Results of X-ray diffraction (XRD) and reciprocal space mapping (RSM) indicated no sign of strain relaxation in the quantum wells with respect to the AlN substrate. The MQW parameters such as thicknesses, growth rates and material compositions were extracted from XRD measurements and demonstrated an agreement with our growth conditions. No indication of parasitic reactions between ammonia and trimethyl-aluminium (TMAI) was detected in our growth process. Optical measurements revealed well-defined photoluminescence peaks at 288 and 280 nm, which are in a good agreement with the transmission experimental data. The piezoelectric field value in the Studied structures was estimated to be 900kV/cm.
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| 21. |
- Gelzinyte, Kristina, et al.
(författare)
-
High spatial uniformity of photoluminescence spectra in semipolar (20(2)over-bar1) plane InGaN/GaN quantum wells
- 2015
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 117:2, s. 023111-
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Tidskriftsartikel (refereegranskat)abstract
- Scanning near-field optical spectroscopy was applied to study spatial variations of emission spectra at room temperature in semipolar (20 (2) over bar1) InxGa(1-x)N/GaN single quantum wells (QWs) for 0:11 <= x <= 0:36. Photoluminescence (PL) was found to be highly uniform, with peak wavelength deviations and peak intensity deviations divided by average values in the range of 6-12 meV and 0.03-0.07, respectively. Near-field maps of PL parameters showed large, similar to 5 to 10 mu m size areas of similar values, as opposed to 100 nm scale variations, often reported for InGaN QWs. The near-field PL spectra were found to broaden with increasing InN molar fraction. In the low In content QWs, the broadening is primarily determined by the random cation distribution, while for larger InN molar fractions 10 nm scale localization sites with increasingly deeper band potentials are suggested as the linewidth broadening cause.
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| 22. |
- Guina, Mircea, et al.
(författare)
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Semiconductor saturable absorbers with recovery time controlled by lattice mismatch
- 2007
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Ingår i: Optical Components and Materials IV. - : SPIE. ; , s. 64690P-1-64690P-7
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Konferensbidrag (refereegranskat)abstract
- We propose and demonstrate a new method to reduce the absorption recovery time of semiconductor saturable absorber mirrors operating at the 1060-nm wavelength range. The method is based on controlling the amount of nonradiative recombination centers within the absorbing region by incorporating an InGaP epitaxial layer with a relative large lattice mismatch to GaAs (~2.2 %). The defect density within the absorbing region can be controlled by the thickness of a GaAs buffer layer grown between the InGaP lattice mismatched layer and the InGaAs/GaAs quantum-wells. For thickness of the GaAs buffer of ∼110 nm and∼570 nm the absorption recovery time was∼5 ps and ∼10 ps, respectively. It is important to note that the fast recovery time was achieved without degrading the nonlinear optical properties of the saturable absorber mirror. The practicality of the structures was proved by demonstrating a reliable self-starting operation of a mode-locked Yb-doped fiber laser.
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| 23. |
- Guina, Mircea, et al.
(författare)
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Semiconductor saturable absorbers with recovery time controlled through growth conditions
- 2007
-
Ingår i: Solid State Lasers XVI. - : SPIE. ; , s. 645113-1-645113-7
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Konferensbidrag (refereegranskat)abstract
- We discuss a new method to shape the temporal response of saturable absorption in semiconductors. In particular, we investigate the possibility to control independently the absorption recovery time of each quantum-well forming the semiconductor absorber. The recovery time is tailored by irradiation with nitrogen ions produced by an RF-plasma source. The irradiation is performed in-situ as one step of the epitaxial growth process; the quantum-wells are individually exposed to a flux of N-ions after they are grown. The amount of non-radiative recombination centers within the quantum-wells is strongly related to the time interval during which the N-ions flux is active and to the thickness of the semiconductor layer grown on top of each quantum-well before the irradiation is performed. We apply this method to fabricate fast semiconductor saturable absorbers operating in the 1-μm wavelength range. The absorption recovery time could be changed from 300 ps to 10 ps without degradation of the nonlinear optical response. The practicality of the design is finally proved by using the semiconductor saturable absorbers for mode-locking Yb-doped fiber lasers.
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| 24. |
- Ivanov, Ruslan, et al.
(författare)
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Impact of carrier localization on radiative recombination times in semipolar (2021) plane InGaN/GaN quantum wells
- 2015
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 107:21
-
Tidskriftsartikel (refereegranskat)abstract
- Semipolar (20 (2) over bar1) plane In,Ga1-xN quantum wells (QWs) of varying alloy composition were studied by time-resolved photoluminescence. A large difference in effective radiative lifetimes. from sub-ns for x = 0.11 to similar to 30 ns for x approximate to 0.35 was found. This effect is attributed to different properties of carrier localization. In low In content QWs. recombination at extended states with short recombination times is prevalent. In QWs with a high In content, the lifetimes are increased by localization of electrons and holes at separate sites. The zigzag shape of the QW interfaces and the resulting in-plane electric field are proposed as the cause for the separate electron and hole localization.
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| 25. |
- Ivanov, Ruslan, 1990-
(författare)
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Impact of carrier localization on recombination in InGaN quantum wells with nonbasal crystallographic orientations
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The modern InGaN technology demonstrates high efficiencies only in the blue spectral region and low current operation modes. The growth of InGaN quantum wells (QWs) on nonbasal crystallographic planes (NBP) has potential to deliver high-power blue and green light emitting diodes and lasers. The emission properties of these QWs are largely determined by the localization of carriers in the minima of spatially inhomogeneous band potential, which affects the recombination dynamics, spectral characteristics of the emission, its optical polarization and carrier transport. Understanding it is crucial for increasing the efficiency of NBP structures to their theoretical limit.In this thesis, the influence of carrier localization on the critical aspects of light emission has been investigated in semipolar and nonpolar InGaN QWs. For this purpose, novel multimode scanning near-field optical microscopy configurations have been developed, allowing mapping of the spectrally-, time-, and polarization-resolved emission.In the nonpolar QW structures the sub-micrometer band gap fluctuations could be assigned to the selective incorporation of indium on different slopes of the undulations, while in the smoother semipolar QWs – to the nonuniformity of QW growth. The nanoscale band potential fluctuations and the carrier localization were found to increase with increasing indium percentage in the InGaN alloy. In spite to the large depth of the potential minima, the localized valence band states were found to retain properties of the corresponding bands. The reduced carrier transfer between localization sites has been suggested as a reason for the long recombination times in the green-emitting semipolar QWs. Sharp increase of the radiative lifetimes has been assigned to the effect of nanoscale electric fields resulting from nonplanar QW interfaces. Lastly, the ambipolar carrier diffusion has been measured, revealing ~100 nm diffusion length and high anisotropy.
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| 26. |
- Ivanov, Ruslan, et al.
(författare)
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Polarization-Resolved Near-Field Spectroscopy of Localized States in m -Plane InxGa1-x N/Ga N Quantum Wells
- 2017
-
Ingår i: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 7:6
-
Tidskriftsartikel (refereegranskat)abstract
- We present a polarization, spectrally, and spatially resolved near-field photoluminescence (PL) measurement technique and apply it to the study of wide m-plane InxGa1-xN/GaN quantum wells grown on on-axis and miscut GaN substrates. It is found that PL originates from localized states; nevertheless, its degree of linear polarization (DLP) is high with little spatial variation. This allows an unambiguous assignment of the localized states to InxGa1-xN composition-related band potential fluctuations. Spatial PL variations, occurring due to morphology features of the on-axis samples, play a secondary role compared to the variations of the alloy composition. The large PL peak wavelength difference for polarizations parallel and perpendicular to the c axis, the weak correlation between the peak PL wavelength and the DLP, and the temperature dependence of the DLP suggest that effective potential variations and the hole mass in the second valence-band level are considerably smaller than that for the first level. DLP maps for the long wavelength PL tails have revealed well-defined regions with a small DLP, which have been attributed to a partial strain relaxation around dislocations.
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| 27. |
- Ivanov, Ruslan, 1990-, et al.
(författare)
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Scanning near-field microscopy of carrier lifetimes in m-plane InGaN quantum wells
- 2017
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 110:3
-
Tidskriftsartikel (refereegranskat)abstract
- Time-resolved scanning near-field photoluminescence (PL) spectroscopy was applied to map carrier lifetimes in wide m-plane InGaN/GaN quantum wells grown on on-axis and miscut substrates. Both radiative and nonradiative lifetimes were found to be spatially nonuniform. Lifetime variations were smaller for quantum wells grown on miscut, as compared to on-axis substrates. Correlation with surface topography showed that largest deviations of recombination times occur at +c planes of pyramidal hillocks of the on-axis sample. Observed correlation between radiative lifetimes and PL peak wavelength was assigned to a partial electron localization.
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| 28. |
- Kivisto, Samuli, et al.
(författare)
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Pulse dynamics of a passively mode-locked Bi-doped fiber laser
- 2010
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Ingår i: Optics Express. - 1094-4087. ; 18:2, s. 1041-1048
-
Tidskriftsartikel (refereegranskat)abstract
- The pulse evolution in Bi-doped soliton fiber laser with slow and fast saturable absorber has been studied both experimentally and numerically. Semiconductor saturable absorbers with balanced slow and fast absorption recovery mechanisms exhibit a bi-temporal recovery dynamics which permits both reliable start-up of passive mode-locking and short pulse generation and stabilization. The pulse dynamics within the Bi fiber laser cavity have been investigated.
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| 29. |
- Krotkus, A., et al.
(författare)
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Be-doped low-temperature-grown GaAs material for optoelectronic switches
- 2002
-
Ingår i: IEE Proceedings - Optoelectronics. - : Institution of Engineering and Technology (IET). - 1350-2433 .- 1359-7078. ; 149:3, s. 111-115
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Tidskriftsartikel (refereegranskat)abstract
- Structural, electrical and recombination properties of Be-doped low-temperature MBE grown (LTG) GaAs have been investigated by using a number of different experimental techniques. These properties were analysed with respect to the applications of LTG GaAs in ultrafast optoelectronic devices. It has been found that a moderate Be-doping improves the structural quality of the layers and does not affect their semi-insulating behaviour. Electron and hole capture cross-sections, critical parameters for the design of optoelectronic devices from LTG GaAs, equal to sigma(n) = 1.1 x 10(-13) and sigma(p) = 1.8 x 10(-15) cm(2) were also determined.
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| 30. |
- Leon, R., et al.
(författare)
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Defect states in red-emitting InxAl1-xAs quantum dots
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:8
-
Tidskriftsartikel (refereegranskat)abstract
- Optical and transport measurements carried out in pn diodes and Schottky barriers containing multilayers of InAlAs quantum dots embedded in AlGaAs barriers show that while red emission from quantum dot (QD) states is obtained at similar to1.8 eV, defect states dominate the optical properties and transport in these quantum dots. These defects provide nonradiative recombination paths, which shortens the carrier lifetimes in QD's to tens of picoseconds (from similar to1 ns) and produce deep level transient spectroscopy (DLTS) peaks in both p and n type structures. DLTS experiments performed with short filling pulses and bias dependent measurements on InAlAs QD's on n-AlGaAs barriers showed that one of the peaks can be attributed to either QD/barrier interfacial defects or QD electron levels, while other peaks are attributed to defect states in both p and n type structures.
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| 31. |
- Leon, Rosa, et al.
(författare)
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Dislocation-induced spatial ordering of InAs quantum dots : Effects on optical properties
- 2002
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 91:9, s. 5826-5830
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Tidskriftsartikel (refereegranskat)abstract
- Misfit dislocations were used to modify the surface morphology and to attain spatial ordering of quantum dots (QDs) by molecular beam epitaxy. Effects of anneal time and temperature on strain-relaxed InxGa1-xAs/GaAs layers and subsequent spatial ordering of InAs QDs were investigated. Photoluminescence (PL) and time-resolved PL was used to study the effects of increased QD positional ordering, increased QD uniformity, and their proximity to dislocation arrays on their optical properties. Narrower inhomogeneous PL broadening from the QDs ordered on dislocation arrays were observed, and differences in PL dynamics were found.
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| 32. |
- Leon, R., et al.
(författare)
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Effects of proton irradiation on luminescence emission and carrier dynamics of self-assembled III-V quantum dots
- 2002
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Ingår i: IEEE Transactions on Nuclear Science. - 0018-9499 .- 1558-1578. ; 49:6, s. 2844-2851
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Tidskriftsartikel (refereegranskat)abstract
- The effects of proton irradiation (1.5 MeV) on photoluminescence intensities and carrier dynamics were compared between III-V quantum dots and similar quantum well structures. A significant enhancement in radiation tolerance is seen with three-dimensional (3-D) quantum confinement.. Measurements were carried out in different quantum dot (QD) structures, varying in material (InGaAs/GaAs and InAlAS/AlGaAS), QD surface density (4 x 10(8) to 3 x 10(10) cm(-2)), and substrate orientation [(100) and (311) B]. Similar trends were observed for all QD samples. A slight increase in PL emission after low to intermediate proton doses, are also observed in InGaAs/GaAs (100) QD structures. The latter is explained in terms of more efficient carrier transfer from the wetting layer via radiation-induced defects.
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| 33. |
- Lindberg, H., et al.
(författare)
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Mode locking a 1550 nm semiconductor disk laser by using a GaInNAs saturable absorber
- 2005
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Ingår i: Optics Letters. - 0146-9592 .- 1539-4794. ; 30:20, s. 2793-2795
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Tidskriftsartikel (refereegranskat)abstract
- Passive mode locking of an optically pumped, InP-based, 1550 nm semiconductor disk laser by using a GaInNAs saturable absorber mirror is demonstrated. To reduce material heating and enable high-power operation, a 50 mu m thick diamond heat spreader is bonded to the surface of the gain chip. The laser operates at a repetition frequency of 2.97 GHz and emits near-transform-limited 3.2 ps pulses with an average output power of 120 mW.
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| 34. |
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| 35. |
- Liuolia, Vytautas, et al.
(författare)
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Carrier localization in m-plane InGaN/GaN quantum wells probed by scanning near field optical spectroscopy
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 97:15, s. 151106-
-
Tidskriftsartikel (refereegranskat)abstract
- Scanning near field optical microscopy (SNOM) was applied to study the carrier localization in single InGaN/GaN quantum well structures grown on nonpolar m-plane GaN substrates. Dual localization potential consisting of hundreds of nanometers- to micrometer-size areas as well as smaller localization centers were identified from the SNOM scans and near field photoluminescence spectral widths. The localization areas were found to align along the [0001] direction, which was attributed to partial strain relaxation at the monolayer steps. (C) 2010 American Institute of Physics. [doi:10.1063/1.3502482]
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| 36. |
- 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
-
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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| 37. |
- Liuolia, Vytautas, et al.
(författare)
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Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells
- 2010
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 108:2, s. 023101-
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Tidskriftsartikel (refereegranskat)abstract
- Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick m-plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. CharacteristiC time of the exciton transfer is 80-100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200-500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain. (C) 2010 American Institute of Physics. [doi:10.1063/1.3460278]
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| 38. |
- Liuolia, Vytautas, 1984-
(författare)
-
Localization effects in ternary nitride semiconductors
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- InGaN based blue and near-ultraviolet light emitting diodes and laser diodes have been successfully commercialized for many applications such as general lighting, display backlighting and high density optical storage devices. Despite having a comparably high defect density, these devices are known for their efficient operation, which is attributed to localization in potential fluctuations preventing carriers from reaching the centers of nonradiative recombination. Nitride research is currently headed towards improving deep ultraviolet AlGaN and green InGaN emitters with higher Al and In molar fractions. The efficiency of these devices trails behind the blue counterparts as the carrier localization does not seem to aid in supressing nonradiative losses. In addition, the operation of ternary nitride heterostructure based devices is further complicated by the presence of large built-in electric fields. Although the problem can be ameliorated by growing structures in nonpolar or semipolar directions, the step from research to production still awaits.In this thesis, carrier dynamics and localization effects have been studied in three different nitride ternary compounds: AlGaN epitaxial layers and quantum wells with high Al content, nonpolar m-plane InGaN/GaN quantum wells and lattice matched AlInN/GaN heterostructures. The experimental methods of this work mainly consist of spectroscopy techniques such as time-resolved photoluminescence and differential transmission pump-probe measurements as well as spatial photoluminescence mapping by means of scanning near-field microscopy.The comparison of luminescence and differential transmission measurements has allowed estimating the localization depth in AlGaN quantum wells. Additionally, it has been demonstrated that the polarization degree of luminescence from m-InGaN quantum wells decreases as carriers diffuse to localization centers.What is more, dual-scale localization potential has been evidenced by near-field measurements in both AlGaN and m-InGaN. Larger scale potential fluctuation have been observed directly and the depth of nanoscopic localization has been estimated theoretically from the recorded linewidth of the near-field spectra. Lastly, efficient carrier transport has been observed through AlInN layer despite large alloy inhomogeneities evidenced by broad luminescence spectra and the huge Stokes shift. Inhomogeneous luminescence from the underlying GaN layer has been linked to the fluctuations of the built-in electric field at the AlInN/GaN interface.
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| 39. |
- Liuolia, Vytautas, et al.
(författare)
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Near- and far-field optical characterization of InGaN photonic crystal light emitting diodes
- 2012
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Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley. - 1610-1634 .- 1610-1642. ; 9:7, s. 1664-1666
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Tidskriftsartikel (refereegranskat)abstract
- Light extraction from InGaN photonic crystal (PhC) light emitting diodes (LEDs) has been studied by scanning near-field optical microscopy and time-resolved photoluminescence. The observed patterns of the near-field intensity are complex and do not correspond to the physical structure of the photonic crystal. Comparison with conventional LEDs without the PhC showed that PhC not only improves the light extraction, but also makes the LED emission more homogeneous.
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| 40. |
- Liuolia, Vytautas, et al.
(författare)
-
Photoexcited carrier dynamics in AlInN/GaN heterostructures
- 2012
-
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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| 41. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Carrier capture and relaxation in modulation doped InAs quantum dots
- 2005
-
Konferensbidrag (refereegranskat)abstract
- Carrier dynamics in n and p-doped and undoped InAs/GaAs quantum dots were studied by time-resolved photoluminescence with selective excitation. Ultrafast carrier relaxation in doped dots is attributed to efficient carrier-carrier scattering.
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| 42. |
- Marcinkevicius, Saulius, et al.
(författare)
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Carrier capture and relaxation in quantum dot structures with different dot densities
- 2000
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Ingår i: Microelectronic Engineering. - 0167-9317 .- 1873-5568. ; feb-51, s. 79-83
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Tidskriftsartikel (refereegranskat)abstract
- Carrier dynamics has been measured by time-resolved photoluminescence in self-assembled InGaAs/GaAs quantum dot structures with dot density of the order of 10(8) to 10(10) cm(2). The time of carrier transfer into a dot, which is in the region from 2 to 20 PS, has been found to decrease with increasing quantum dot density. The temperature and photoexcited carrier density dependencies of the carrier transfer times suggest that potential barriers at the barrier, wetting layer and quantum dot interfaces hinder carrier capture in low-density quantum dot structures.
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| 43. |
- Marcinkevicius, Saulius, et al.
(författare)
-
Carrier dynamics and localization in AlInN/GaN heterostructures
- 2013
-
Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley. - 1610-1634 .- 1610-1642. ; 10:5, s. 853-856
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Tidskriftsartikel (refereegranskat)abstract
- Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructure have been examined by time-resolved photoluminescence (PL), transient photoreflectance and scanning near-field optical spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited carrier transfer from AlInN to GaN via sub-band edge states. Near-field PL scans and photoreflectance data show that the diameter of the localization sites and the distance between them are well below 100 nm. Majority of these states is assigned to In clusters, in which the valence band has a higher energy than the valence band in a uniform AlInN alloy. It is likely that the carrier transport through the sub-band edge states proceeds via high conductivity channels involving extended defects.
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| 44. |
- Marcinkevicius, Saulius, et al.
(författare)
-
Carrier redistribution between different potential sites in semipolar (20(2)over-bar1) InGaN quantum wells studied by near-field photoluminescence
- 2014
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:11, s. 111108-
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Tidskriftsartikel (refereegranskat)abstract
- Scanning near-field photoluminescence (PL) spectroscopy at different excitation powers was applied to study nanoscale properties of carrier localization and recombination in semipolar (20 (2) over bar1) InGaN quantum wells (QWs) emitting in violet, blue, and green-yellow spectral regions. With increased excitation power, an untypical PL peak energy shift to lower energies was observed. The shift was attributed to carrier density dependent carrier redistribution between nm-scale sites of different potentials, Near-field PL scans showed that in (20 (2) over bar1) QWs the in-plane carrier diffusion is modest, and the recombination properties arc uniform, which is advantageous for photonic applications.
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| 45. |
- Marcinkevicius, Saulius, et al.
(författare)
-
Carrier spin dynamics in modulation-doped InAs/GaAs quantum dots
- 2006
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 100:5, s. 054310-
-
Tidskriftsartikel (refereegranskat)abstract
- Photoexcited electron and hole spin relaxation was studied in modulation-doped and undoped InAs/GaAs quantum dots by means of time-resolved photoluminescence. After excitation into the barriers or the wetting layer, the electron spin polarization is preserved during the capture and relaxation in the dots, especially in the p-doped structures, and decays with a characteristic time of about 100 ps. Spin state admixture in combination with electron interaction with acoustic phonons is suggested as the spin relaxation mechanism. Rapid spin polarization decay during carrier relaxation in undoped quantum dots is attributed to electron-optical phonon interaction. For carrier excitation directly into the dots, no significant spin polarization was observed, which points to the mixed nature of hole levels in quantum dots. The hole spin polarization randomizes on a much shorter time scale and is not detected in the experiment.
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| 46. |
- Marcinkevicius, Saulius, et al.
(författare)
-
Changes in carrier dynamics induced by proton irradiation in quantum dots
- 2002
-
Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 314:04-jan, s. 203-206
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of proton irradiation on carrier dynamics were investigated by time-resolved photoluminescence on different InGaAs/GaAs quantum-dot (QD) structures varying in QD surface density and substrate orientation, as well as thin InGaAs quantum wells. The carrier lifetimes in the dots are much less affected by proton irradiation than in the wells. Decrease in lifetimes of only 40 percent at the highest proton dose are observed in some of the QDs, whereas an similar to20 to similar to40-fold decrease is observed in the wells. Similar trends were observed for all quantum dot samples.
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| 47. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Changes in luminescence intensities and carrier dynamics induced by proton irradiation in In_xGa_1-xAs/GaAs quantum dots
- 2002
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:23, s. 235314-
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of proton irradiation (1.5 MeV) on photoluminescence intensities and carrier dynamics were compared between InGaAs/GaAs quantum dots and similar quantum well structures. A significant enhancement in radiation tolerance is seen with three-dimensional quantum confinement. Measurements were carried out in different quantum dot structures varying in dot surface density (4x10(8)-3x10(10) cm(-2)) and substrate orientation [(100) and (311)B]. Similar trends were observed for all quantum dot samples. A slight increase in photoluminescence emission intensity after low to intermediate proton doses is observed in InGaAs/GaAs (100) quantum dot structures. The latter is explained in terms of more efficient carrier transfer from the wetting layer via radiation-induced defects.
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| 48. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Dynamics of carrier injection through V-defects in long wavelength GaN LEDs
- 2024
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 124:18
-
Tidskriftsartikel (refereegranskat)abstract
- The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs.
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| 49. |
- Marcinkevicius, Saulius
(författare)
-
Dynamics of carrier transfer into In(Ga)As self-assembled quantum dots
- 2008. - 1
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Ingår i: Self-Assembled Quantum Dots. - New York, NY : Springer. - 9780387741901 ; , s. 129-164
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Bokkapitel (refereegranskat)abstract
- The chapter reviews ultrafast dynamics of carrier transfer into InAs/GaAs and InGaAs/GaAs quantum dots of different doping, densities and interlevel energies. Results from theoretical modeling and time-resolved optical studies are discussed. Considered effects include carrier transport in the barriers, capture into the dots and relaxation in the dots. Several carrier capture and relaxation mechanisms, such as longitudinal optical (LO) phonon emission, carrier-carrier scattering, capture through defect states and relaxation through barrier/wetting layer continuum states are analyzed. LO phonon-assisted capture appears to be a rather universal capture mechanism. For the carrier relaxation in doped quantum dots and/or at high carrier densities, the carrier-carrier scattering-assisted relaxation dominates. In undoped dots and low excitation levels, relaxation through LO phonon emission is found to be the relevant process. Whatever the mechanism, the carrier transfer in QD structures is, as a rule, fast, occurring in 1 to 20 ps time range.
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| 50. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Effect of Mg doping on carrier recombination in GaN
- 2023
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 134:8
-
Tidskriftsartikel (refereegranskat)abstract
- Time-resolved photoluminescence measurements have been performed on Mg-doped GaN for Mg concentrations in the low- to mid-1019 cm−3. As-grown and annealed (600-675 °C) samples were studied. In the as-grown samples, the nonradiative carrier lifetime was found to be about 200 ps and nearly independent of the Mg concentration. Upon annealing, the carrier lifetimes shorten to ∼150 ps but, again, show little dependence on the annealing temperature. The analysis of possible Shockley-Read-Hall recombination centers and their behavior during doping and annealing suggests that the main nonradiative recombination center is the Mg-nitrogen vacancy complex. The weak dependence of the PL decay times on temperature indicates that carrier capture into this center has a very low potential barrier, and the nonradiative recombination dominates even at low temperatures.
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