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- Shang, Xiangjun, et al.
(författare)
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Effect of built-in electric field in photovoltaic InAs quantum dot embedded GaAs solar cell
- 2011
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 103:2, s. 335-341
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, three p-i-n GaAs solar cells were grown and characterized, one with InAs quantum dot (QD) layers embedded in the depletion region (sample A), one with QD layers embedded in the n (-) base region (B), and the third without QDs (control sample C). QD-embedded solar cells (samples A and B) show broad photoluminescence spectra due to QD multi-level emissions but have lower open-circuit voltages V (oc) and lower photovoltaic (PV) efficiencies than sample C. On the other hand, the short-circuit current density J (sc) in sample A is increased while it is decreased in sample B. Theoretical analysis shows that in sample B where the built-in electric field in QDs is zero, electrons tend to occupy QDs and strong potential variations exist around QDs which deteriorate the electron mobility in the n (-) base region so that J (sc) in sample B is decreased. Hole trapping and electron-hole recombination in QDs are also enhanced in sample B, resulting in a reduced V (oc) and thus a worse PV effect. In sample A, a strong built-in field exists in QD layers, which facilitates photo-carrier extraction from QDs and thus J (sc) is increased. However, QDs in the depletion region in sample A act also as recombination-generation centers so that the dark saturated current density is drastically increased, which reduces V (oc) and the total PV effect. In conclusion, a nonzero built-in electric field around QDs is vital for using QDs to increase the PV effect in conventional p-i-n GaAs solar cells.
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- Shang, Xiangjun, et al.
(författare)
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Effect of tunable dot charging on photoresponse spectra of GaAs p-i-n diode with InAs quantum dots
- 2015
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Ingår i: Journal of Applied Physics. - Melville, NY : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 118:24
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Tidskriftsartikel (refereegranskat)abstract
- Quantum dot (QD)-embedded photodiodes have demonstrated great potential for use as detectors. A modulation of QD charging opens intriguing possibilities for adaptive sensing with bias-tunable detector characteristics. Here, we report on a p-i-n GaAs photodiode with InAs QDs whose charging is tunable due to unintentional Be diffusion and trap-assisted tunneling of holes, from bias-and temperature (T)-dependent photocurrent spectroscopy. For the sub-bandgap spectra, the T-dependent relative intensities "QD-s/WL" and "WL/GaAs" (WL: wetting layer) indicate dominant tunneling under -0.9V (trap-assisted tunneling from the top QDs) and dominant thermal escape under -0.2 similar to 0.5V (from the bottom QDs since the top ones are charged and inactive for optical absorption) from the QD s-state, dominant tunneling from WL, and enhanced QD charging at >190K (related to trap level ionization). For the above-bandgap spectra, the degradation of the spectral profile (especially near the GaAs bandedge) as the bias and T tune (especially under -0.2 similar to 0.2V and at >190 K) can be explained well by the enhanced photoelectron capture in QDs with tunable charging. The dominant spectral profile with no degradation under 0.5V is due to a saturated electron capture in charged QDs (i.e., charging neutralization). QD level simulation and schematic bandstructures can help one understand these effects.
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- Shang, Xiangjun, 1984-
(författare)
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Study of quantum dots on solar energy applications
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis studies p-i-n GaAs solar cells with self-assembled InAs quantum dots (QDs) inserted. The values of this work lie in three aspects. First, by comparing the cell performance with QDs in the i-region and the n-region, the photocurrent (PC) production from QDs by thermal activation and/or intermediate band (IB) absorption is proved to be much lower in efficiency than tunneling. Second, the efficiency of PC production from QDs, characterized by PC spectrum, is helpful to design QD-based photodetectors. Third, closely spaced InAs QD layers allow a strong inter-layer tunneling, leading to an effective PC production from QD deep states, potential for solar cell application. Fourth, from the temperature-dependent PC spectra the minority photohole thermal escape is found to be dominant on PC production from QDs in the n-region. The thermal activation energy reflects the potential variations formed by electron filling in QDs. Apart from InAs QDs, this thesis also explores the blinking correlation between two colloidal CdSe QDs. For QD distance of 1 µm or less, there is a bunched correlation at delay τ = 0, meaning that the two QDs blink synchronously. Such correlation disappears gradually as QD distance increases. The correlation is possibly caused by the stimulated emission between the two nearby QDs.
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- Song, Yuxin, 1981, et al.
(författare)
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Metamorphic Quantum Well Lasers
- 2012
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Ingår i: Lattice Engineering: Technology and Applications. ; , s. 283-317
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- This chapter provides an overview of long wavelengthmetamorphic quantum well lasers. The idea of metamorphic growth is to compromise large lattice mismatch by utilizing a relaxed buffer layer. Structural design and growth optimization of metamorphic buffer layers are reviewed with emphasis on composition grading scheme and doping effects. Progress on long wavelength (>1.2 μm) metamorphic quantum well lasers is summarized.
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- Wang, Lijuan, et al.
(författare)
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Enhanced tunneling in the GaAs p(+)-n(+) junction by embedding InAs quantum dots
- 2012
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 27:11, s. 115010-
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Tidskriftsartikel (refereegranskat)abstract
- GaAs p(+)-n(+) junctions with and without a layer of InAs quantum dots (QDs) embedded at the interface are discussed in this article. The current density versus voltage (I-V) characteristics show that the junctions without QDs are weak degenerate due to the Beryllium(Be) atoms diffusion of nominal p(++)-GaAs; the junctions with QDs generate enhanced tunneling current at forward bias, because the QDs layer reduces the Be diffusion and enables a two-step tunneling process. At room temperature, the current density of the sample with QDs is enhanced to 122 A cm(-2) at a forward bias of +0.32 V, which is about 2 orders of magnitude higher than the reference sample without QDs.
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