Molecular Beam Epitaxy of GaAsSbN and Its Applications
Date Issued
2010
Date
2010
Author(s)
Ma, Ta-Chun
Abstract
In this dissertation, we use a RF-plasma-assisted molecular beam epitaxy system to investigate the growth of GaAsSbN mixed-group V alloys. Our research topics include the characterization of plasma source, the incorporation behavior of the N in GaAsSbN, optical properties of lattice-matched GaAsSbN and the characteristic of GaAs/GaAsSbN heterojunctions on GaAs substrate.
Due to the complexity of the plasma source, we use optical emission spectroscopy and quadruple mass spectroscopy to identify the relation between the operation parameters and N-radical species. We placed a shutter in front of the plasma aperture, and obtained atomic N-radical source for GaAsSbN growth. We found that N incorporation in GaAsSbN grown with atomic N-radicals decreases with the increase of Sb flux, which is very different from that of samples grown with meta-stable molecular N-radicals. Through the study of plasma characterization, we also found that the condition with low RF power, low N2 flow rate, and without the blocking of shutter can provide a source dominated by meta-stable molecules. When meta-stable molecule source is used, the N incorporation in GaAsSbN grown with meta-stable molecule source increases with the increase of Sb flux, which is consistent with the reports in literature.
Based on the results of plasma characterization, we have successfully grown lattice-match GaAsSbN epilayers on GaAs substrates. The lowest absorption edge achieved in this study is as low as 0.8 eV. In addition, according to the results of PL measurement, we found that the samples grown at 420 oC have the strongest PL intensity resulting from better homogeneity, as compared with samples grown at other temperatures. Finally, we fabricated and characterized n+-GaAs/p-GaAsSbN/p+-GaAs heterojunctions. We found that when GaAsSbN exceeds a certain thickness, inhomogeneity related defects begin to be generated in the epilayer. After thermal annealing, the optimum GaAsSbN thickness is 500 nm and the conversion efficiency under AM1.5G solar simulator is 3.6%. The high forward-biased current of the heterojunctions results in low open-circuited voltage. After the junction passivation with SiN and SiO2, the open-circuited voltage increases by 50 % and the conversion efficiency increases by 60% to 5.7%.
Subjects
molecular beam epitaxy
GaAsSbN
RF plasma
heterojunction
solar cell
Type
thesis
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