| 1. |
- Dhaka, Veer, et al.
(författare)
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Aluminum-Induced Photoluminescence Red Shifts in Core-Shell GaAs/AlxGa1-xAs Nanowires
- 2013
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 13:8, s. 3581-3588
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Tidskriftsartikel (refereegranskat)abstract
- We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/AlxGa1-xAs nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All AlxGa1-xAs shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to similar to 66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.
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| 2. |
- Kivisaari, Pyry, et al.
(författare)
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Bipolar Monte Carlo simulation of hot carriers in III-N LEDs
- 2015
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Ingår i: 2015 International Conference on Simulation of Semiconductor Processes and Devices, SISPAD 2015. - 9781467378581 ; , s. 393-396
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Konferensbidrag (refereegranskat)abstract
- We carry out bipolar Monte Carlo (MC) simulations of electron and hole transport in a multi-quantum well light-emitting diode with an electron-blocking layer. The MC simulation accounts for the most important interband recombination and intraband scattering processes and solves self-consistently for the non-quasiequilibrium transport. The fully bipolar MC simulator results in better convergence than our previous Monte Carlo-drift-diffusion (MCDD) model and also shows clear signatures of hot holes. Accounting for both hot electron and hot hole effects increases the total current and decreases the efficiency especially at high bias voltages. We also present our in-house full band structure calculations for GaN to be coupled later with the MC simulation in order to enable even more detailed predictions of device operation.
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| 3. |
- Kivisaari, Pyry, et al.
(författare)
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Bipolar Monte Carlo simulation of hot carriers in III-N LEDs
- 2015
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Ingår i: 15th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2015. - 9781479983797 ; 2015-May, s. 11-12
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Konferensbidrag (refereegranskat)abstract
- We perform fully bipolar Monte Carlo simulations of electrons and holes in III-Nitride multi-quantum well light-emitting diodes (LEDs) to investigate the effects of hot carriers. Our results show how accounting for hot carriers affects the current-voltage characteristics and device efficiency. We also discuss the effects of bandstructure details on the simulation results. Further simulations with versatile QW and EBL configurations are needed to confirm the relationship between hot carrier effects and current-voltage characteristics.
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| 4. |
- Kivisaari, Pyry, et al.
(författare)
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Diffusion-driven current transport to near-surface nanostructures
- 2015
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Ingår i: 15th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2015. - 9781479983797 ; , s. 117-118
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Konferensbidrag (refereegranskat)abstract
- Diffusion-driven current transport (DDCT) has recently been proposed as a new way to organize the current injection in nanoscale optoelectronic devices. The very recent first proof-of-principle experiments have also shown that DDCT works as predicted theoretically. In this work we perform simulations on DDCT-based III-Nitride devices and demonstrate how the optimization of DDCT differs significantly from the optimization of conventional double heterostructure based devices.
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| 5. |
- Kivisaari, Pyry, et al.
(författare)
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Elimination of Lateral Resistance and Current Crowding in Large-Area LEDs by Composition Grading and Diffusion-Driven Charge Transport
- 2017
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Ingår i: Advanced Electronic Materials. - : Wiley. - 2199-160X. ; 3:6
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Tidskriftsartikel (refereegranskat)abstract
- Gallium nitride based light-emitting diodes (LEDs) are presently fundamentally transforming the lighting industry, but limitations in the materials and fabrication methods of LEDs introduce substantial challenges to their future development. Among the remaining key bottlenecks of GaN LEDs are the resistive losses and current crowding that strongly increase the heat generation at high powers. In this work the authors show how a new design paradigm based on diffusion-driven charge transport (DDCT) and selective-area growth (SAG) of GaN can be used to reduce the resistive losses of LEDs below the level achievable with presently available structures. The authors carry out full device simulations and demonstrate SAG of both n- and p-doped GaN on device templates with InGaN quantum wells that can be excited using DDCT. The results indicate that especially when combined with material composition grading, the new approach offers the possibility to substantially reduce the resistive heating in high-power LEDs.
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| 6. |
- Kivisaari, Pyry, et al.
(författare)
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Elimination of resistive losses in large-area LEDs by new diffusion-driven devices
- 2017
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Ingår i: Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XXI. - : SPIE. - 9781510606890 ; 10124
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Konferensbidrag (refereegranskat)abstract
- High-power operation of conventional GaN-based light-emitting diodes (LEDs) is severely limited by current crowding, which increases the bias voltage of the LED, concentrates light emission close to the p-type contact edge, and aggravates the efficiency droop. Fabricating LEDs on thick n-GaN substrates alleviates current crowding but requires the use of expensive bulk GaN substrates and fairly large n-contacts, which take away a large part of the active region (AR). In this work, we demonstrate through comparative simulations how the recently introduced diffusion-driven charge transport (DDCT) concept can be used to realize lateral heterojunction (LHJ) structures, which eliminate most of the lateral current crowding. Specifically in this work, we analyze how using a single-side graded AR can both facilitate electron and hole diffusion in DDCT and increase the effective AR thickness. Our simulations show that the increased effective AR thickness allows a substantial reduction in the efficiency droop at large currents, and that unlike conventional 2D LEDs, the LHJ structure shows practically no added efficiency loss or differential resistance due to current crowding. Furthermore, as both electrons and holes enter the AR from the same side without any notable potential barriers in the LHJ structure, the LHJ structure shows an additional wall-plug efficiency gain over the conventional structures under comparison. This injection from the same side is expected to be even more interesting in multiple quantum well structures, where carriers typically need to surpass several potential barriers in conventional LEDs before recombining. In addition to simulations, we also demonstrate selective-area growth of a finger structure suitable for operation as an LHJ device with 2μm distance between n- and p-GaN regions.
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| 7. |
- Kivisaari, Pyry, et al.
(författare)
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Monte Carlo study of non-quasiequilibrium carrier dynamics in III–N LEDs
- 2016
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Ingår i: Optical and Quantum Electronics. - : Springer Science and Business Media LLC. - 0306-8919 .- 1572-817X. ; 48:2
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Tidskriftsartikel (refereegranskat)abstract
- Hot carrier effects have been observed in recent measurements of III–Nitride (III–N) light-emitting diodes. In this paper we carry out bipolar Monte Carlo simulations for electrons and holes in a typical III–N multi-quantum well (MQW) LED. According to our simulations, significant non-quasiequilibrium carrier distributions exist in the barrier layers of the structure. This is observed as average carrier energies much larger than the 1.5kBT corresponding to quasi-equilibrium. Due to the small potential drop over the MQW being modest, the non-quasiequilibrium carriers can be predominantly ascribed to nnp and npp Auger processes taking place in the QWs. Further investigations are needed to determine the effects of hot carriers on the macroscopic device characteristics of real devices.
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| 8. |
- Kivisaari, Pyry, et al.
(författare)
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On the Monte Carlo Description of Hot Carrier Effects and Device Characteristics of III-N LEDs
- 2017
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Ingår i: Advanced Electronic Materials. - : Wiley. - 2199-160X. ; 3:6
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Tidskriftsartikel (refereegranskat)abstract
- Recent experiments have suggested that high energy charge carriers can have a significant effect on the operation of group III nitride (III-N) light-emitting diodes (LEDs), possibly playing an important role in the efficiency droop. As hot carriers are not accounted for by device simulation tools based on drift-diffusion (DD) and quasiequilibrium conditions, more advanced tools are needed. Here, fully self-consistent Monte Carlo (MC) simulations are developed to investigate the effects of hot carriers in device operation and to outline the shortcomings of the DD models in modeling multiquantum well (MQW) LEDs. The results show that hot carrier transport can lead to substantial electron overflow distributing the carriers more evenly in MQW structures, increasing the total recombination and leakage currents. Also Auger recombination is found to drive the distributions out of quasiequilibrium but, surprisingly, it does not contribute extensively to the leakage current. The simulations involve in-house ab-initio band structures as well as parameterized band structures, but qualitatively the results do not strongly depend on the band structure details. However, there is a clear discrepancy between the DD and MC simulations at bias voltages significantly exceeding the built-in potential when the LED consists of several deep quantum wells.
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| 9. |
- Kivisaari, Pyry, et al.
(författare)
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Optimization of Current Injection in AlGaInP Core−Shell Nanowire Light-Emitting Diodes
- 2017
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 17:6, s. 3599-3606
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Tidskriftsartikel (refereegranskat)abstract
- Core–shell nanowires offer great potential to enhance the efficiency of light-emitting diodes (LEDs) and expand the attainable wavelength range of LEDs over the whole visible spectrum. Additionally, nanowire (NW) LEDs can offer both improved light extraction and emission enhancement if the diameter of the wires is not larger than half the emission wavelength (λ/2). However, AlGaInP nanowire LEDs have so far failed to match the high efficiencies of traditional planar technologies, and the parameters limiting the efficiency remain unidentified. In this work, we show by experimental and theoretical studies that the small nanowire dimensions required for efficient light extraction and emission enhancement facilitate significant loss currents, which result in a low efficiency in radial NW LEDs in particular. To this end, we fabricate AlGaInP core–shell nanowire LEDs where the nanowire diameter is roughly equal to λ/2, and we find that both a large loss current and a large contact resistance are present in the samples. To investigate the significant loss current observed in the experiments in more detail, we carry out device simulations accounting for the full 3D nanowire geometry. According to the simulations, the low efficiency of radial AlGaInP nanowire LEDs can be explained by a substantial hole leakage to the outer barrier layer due to the small layer thicknesses and the close proximity of the shell contact. Using further simulations, we propose modifications to the epitaxial structure to eliminate such leakage currents and to increase the efficiency to near unity without sacrificing the λ/2 upper limit of the nanowire diameter. To gain a better insight of the device physics, we introduce an optical output measurement technique to estimate an ideality factor that is only dependent on the quasi-Fermi level separation in the LED. The results show ideality factors in the range of 1–2 around the maximum LED efficiency even in the presence of a very large voltage loss, indicating that the technique is especially attractive for measuring nanowire LEDs at an early stage of development before electrical contacts have been optimized. The presented results and characterization techniques form a basis of how to simultaneously optimize the electrical and optical efficiency of core–shell nanowire LEDs, paving the way to nanowire light emitters that make true use of larger-than-unity Purcell factors and the consequently enhanced spontaneous emission.
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| 10. |
- Sadi, Toufik, et al.
(författare)
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Electroluminescent cooling in intracavity light emitters : modeling and experiments
- 2018
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Ingår i: Optical and Quantum Electronics. - : Springer Science and Business Media LLC. - 0306-8919 .- 1572-817X. ; 50:1
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Tidskriftsartikel (refereegranskat)abstract
- We develop a coupled electronic charge and photon transport simulation model to allow for deeper analysis of our recent experimental studies of intracavity double diode structures (DDSs). The studied structures consist of optically coupled AlGaAs/GaAs double heterojunction light emitting diode (LED) and GaAs p–n-homojunction photodiode (PD) structure, integrated as a single semiconductor device. The drift–diffusion formalism for charge transport and an optical model, coupling the LED and the PD, are self-consistently applied to complement our experimental work on the evaluation of the efficiency of these DDSs. This is to understand better their suitability for electroluminescent cooling (ELC) demonstration, and shed further light on electroluminescence and optical energy transfer in the structures. The presented results emphasize the adverse effect of non-radiative recombination on device efficiency, which is the main obstacle for achieving ELC in III-V semiconductors.
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