InAs/(Al)GaAs quantum dots for intermediate band solar cells
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- Institutt for fysikk 
Intermediate band solar cells (IBSCs) have a theoretical conversion efficiency limit of 63.2%, compared to 40.7% for a single junction solar cell. The enhanced efficiency limit is due to an intermediate energy level positioned inside the semiconductor bandgap, which allows absorption of both low and high energy photons without severe loss of solar cell voltage. The intermediate band can be formed by introducing quantum dots (QDs) into the semiconductor. Self-assembled QDs can be grown by molecular beam epitaxy (MBE). A handful of groups world wide attempt to fabricate InAs/GaAs QDIBSCs, but no large efficiency improvements have yet been reported. There are several reasons for the lack of success. One of them is the choice of the InAs/GaAs material system, another is defect formation during QD growth. With optimized growth conditions, formation of defect free QD materials is possible. The challenge is to obtain defect free QD materials with the desired properties for QD-IBSCs. The main topic of this thesis has therefore been to optimize the growth conditions used during MBE growth in order to achieve a high density of small QDs with a narrow size distribution. At the same time it is important to avoid formation of defects deteriorating the optical material quality, and the performance of the solar cells. A systematic study of the influence of MBE growth parameters on the QD densities, size distributions and optical performance was performed. The substrate temperature during QD growth, the InAs growth rate, the In:As2 flux ratio, and the InAs thickness were varied systematically. The InAs QDs were grown either on GaAs or Al0.35G a0.65As. For growth on GaAs, the deposition method for InAs was also studied. Large ensembles of up to 12700 QDs for each sample where characterized by scanning electron microscopy (SEM), and atomic force microscopy (AFM). The extremely high QD densities (> 1011 cm−2) and the small QDs sizes (down to sub-nm heights, and < 10 nm in diameter) made the imaging challenging. Most of the samples showed bimodal growth, and a complete explanation of the formation process is given in the thesis. Photoluminescence (PL) characterization revealed a close connection between the density of large-sized QDs and the PL intensity. The optical material quality, quantified as the integrated intensity of the PL from the QDs, is reduced for QD densities above a certain limit. Three batches of QD-IBSCs were made and characterized as part of this thesis. We found a slightly enhanced short current density due to the presence of the QD layers. However, the main enhancement is most likely due to the two-dimensional wetting layer positioned below the QDs as it seems to be independent of QD density. The first batch of solar cells showed degradation of the n-base due to the thermal cycling when the QD stack was grown. In the second batch of QD-IBSCs, we removed the WL between the InAs QDs by capping the QDs with AlAs before the GaAs spacers were deposited. In the third batch of solar cells, we successfully fabricated the first InAs/AlGaAs QD-IBSC in the world. The EQE spectra showed that the IB is shifted deeper into the bandgap, as desired. The thesis hopefully sheds light on important aspects on how to, and how not to, fabricate QD-IBSCs in order to achieve enhanced conversion efficiencies in the future.