| 1. |
- Chen, Yang, et al.
(författare)
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Optimization of the short-circuit current in an InP nanowire array solar cell through opto-electronic modeling
- 2016
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:43
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Tidskriftsartikel (refereegranskat)abstract
- InP nanowire arrays with axial p-i-n junctions are promising devices for next-generation photovoltaics, with a demonstrated efficiency of 13.8%. However, the short-circuit current in such arrays does not match their absorption performance. Here, through combined optical and electrical modeling, we study how the absorption of photons and separation of the resulting photogenerated electron-hole pairs define and limit the short-circuit current in the nanowires. We identify how photogenerated minority carriers in the top n segment (i.e. holes) diffuse to the ohmic top contact where they recombine without contributing to the short-circuit current. In our modeling, such contact recombination can lead to a 60% drop in the short-circuit current. To hinder such hole diffusion, we include a gradient doping profile in the n segment to create a front surface barrier. This approach leads to a modest 5% increase in the short-circuit current, limited by Auger recombination with increased doping. A more efficient approach is to switch the n segment to a material with a higher band gap, like GaP. Then, a much smaller number of holes is photogenerated in the n segment, strongly limiting the amount that can diffuse and disappear into the top contact. For a 500 nm long top segment, the GaP approach leads to a 50% higher short-circuit current than with an InP top segment. Such a long top segment could facilitate the fabrication and contacting of nanowire array solar cells. Such design schemes for managing minority carriers could open the door to higher performance in single- and multi-junction nanowire-based solar cells.
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| 2. |
- 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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| 3. |
- Sintonen, Sakari, et al.
(författare)
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Incorporation and effects of impurities in different growth zones within basic ammonothermal GaN
- 2016
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 456, s. 43-50
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Tidskriftsartikel (refereegranskat)abstract
- The ammonothermal method is one of the most promising candidates for large-scale bulk GaN growth due to its scalability and high crystalline quality. However, emphasis needs to be put on understanding the incorporation and effects of impurities during growth. This article discusses how impurities are incorporated in different growth zones in basic ammonothermal GaN, and how they affect the structural, electrical and optical properties of the grown crystal. The influence of growth time on the impurity incorporation is also studied. We measure the oxygen, silicon, and carbon impurity concentrations using secondary ion mass spectrometry, and measure their effect on the lattice constant by high resolution x-ray diffraction (HR-XRD). We determine the resulting free carrier concentration by spatially resolved Fourier transform infrared spectroscopy and study the optical properties by spatially resolved low-temperature photoluminescence. We find that oxygen is incorporated preferentially in different growth regions and its incorporation efficiency depends on the growth direction. The oxygen concentration varies from 6.3×1020 cm−3 for growth on the {112¯2} planes to 2.2×1019 cm−3 for growth on the (0001) planes, while silicon and carbon concentration variation is negligible. This results in a large variation in impurity concentration over a small length scale, which causes significant differences in the strain within the boule, as determined by HR-XRD on selected areas. The impurity concentration variation induces large differences in the free carrier concentration, and directly affects the photoluminescence intensity.
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