| 1. |
- Chen, I. Ju, et al.
(författare)
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Conduction Band Offset and Polarization Effects in InAs Nanowire Polytype Junctions
- 2017
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:2, s. 902-908
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Tidskriftsartikel (refereegranskat)abstract
- Although zinc-blende (ZB) and wurtzite (WZ) structures differ only in the atomic stacking sequence, mixing of crystal phases can strongly affect the electronic properties, a problem particularly common to bottom up-grown nanostructures. A lack of understanding of the nature of electronic transport at crystal phase junctions thus severely limits our ability to develop functional nanowire devices. In this work we investigated electron transport in InAs nanowires with designed mixing of crystal structures, ZB/WZ/ZB, by temperature-dependent electrical measurements. The WZ inclusion gives rise to an energy barrier in the conduction band. Interpreting the experimental result in terms of thermionic emission and using a drift-diffusion model, we extracted values for the WZ/ZB band offset, 135 ± 10 meV, and interface sheet polarization charge density on the order of 10-3 C/m2. The extracted polarization charge density is 1-2 orders of magnitude smaller than previous experimental results, but in good agreement with first principle calculation of spontaneous polarization in WZ InAs. When the WZ length is reduced below 20 nm, an effective barrier lowering is observed, indicating the increasing importance of tunneling transport. Finally, we found that band-bending at ZB/WZ junctions can lead to bound electron states within an enclosed WZ segment of sufficient length, evidenced by our observation of Coulomb blockade at low temperature. These findings provide critical input for modeling and designing the electronic properties of novel functional devices, such as nanowire transistors, where crystal polytypes are commonly found.
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| 2. |
- Chen, Yang, et al.
(författare)
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One-dimensional electrical modeling of axial p-i-n junction InP nanowire array solar cells
- 2017
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Ingår i: 17th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2017. - 9781509053230 ; , s. 23-24
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Konferensbidrag (refereegranskat)abstract
- We demonstrate one-dimensional (1D) electrical modeling of InP nanowire array solar cells. This 1D modeling gives accurate description of the current voltage response even at high surface recombination velocity. The 1D electrical model decreases the simulation time by 3 orders of magnitude compared to a full three-dimensional (3D) model.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- Kivisaari, Pyry, et al.
(författare)
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Full optoelectronic simulation of nanowire LEDs : Effects of temperature
- 2017
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Ingår i: 17th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2017. - 9781509053230 ; , s. 109-110
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Konferensbidrag (refereegranskat)abstract
- Nanowires (NWs) have potential to enable new types of light-emitting diodes (LEDs), which also entail challenges for the development of improved full device simulation models. In this work we calculate the extraction efficiency and emission enhancement in NW array LEDs at different temperatures and couple the results to carrier transport simulations. Our results show that the extraction efficiency of NW LEDs can be strongly temperature-dependent, which complicates the measurement of the internal quantum efficiency of NW LEDs. Through this work we demonstrate the need for full optoelectronic modeling tools in studying and developing new NW-based LEDs.
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| 6. |
- 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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| 7. |
- 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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| 8. |
- Sadi, Toufik, et al.
(författare)
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Modeling of charge and photon transport in coupled intracavity light emitters
- 2017
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Ingår i: 17th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2017. - 9781509053230 ; , s. 201-202
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Konferensbidrag (refereegranskat)abstract
- To enable a more detailed analysis of our recent studies of intracavity double diode structures (DDSs), new simulation tools are needed. Such simulation models must account for both charge and photon transport in the studied structures, consisting of optically coupled AlGaAs/GaAs double heterojunction light emitting diode (LED) and GaAs p-n-homojunction photodiode (PD) structure, enclosed within a single semiconductor cavity. We apply the drift-diffusion formalism for charge transport and an optical model coupling the LED and the PD, with the aim of complementing our experimental work on the efficiency of these devices to understand better their suitability for electroluminescence cooling [1], and shedding further light on electroluminescence and optical energy transfer in the structures.
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| 9. |
- Tiira, Jonna, et al.
(författare)
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Intracavity double diode structures with GaInP barrier layers for thermophotonic cooling
- 2017
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Ingår i: Optical and Electronic Cooling of Solids II. - : SPIE. - 9781510606838 ; 10121
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Konferensbidrag (refereegranskat)abstract
- Optical cooling of semiconductors has recently been demonstrated both for optically pumped CdS nanobelts and for electrically injected GaInAsSb LEDs at very low powers. To enable cooling at larger power and to understand and overcome the main obstacles in optical cooling of conventional semiconductor structures, we study thermophotonic (TPX) heat transport in cavity coupled light emitters. Our structures consist of a double heterojunction (DHJ) LED with a GaAs active layer and a corresponding DHJ or a p-n-homojunction photodiode, enclosed within a single semiconductor cavity to eliminate the light extraction challenges. Our presently studied double diode structures (DDS) use GaInP barriers around the GaAs active layer instead of the AlGaAs barriers used in our previous structures. We characterize our updated double diode structures by four point probe IV-measurements and measure how the material modifications affect the recombination parameters and coupling quantum efficiencies in the structures. The coupling quantum efficiency of the new devices with InGaP barrier layers is found to be approximately 10 % larger than for the structures with AlGaAs barriers at the point of maximum efficiency.
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