Department: Electrical & Computer Engineering
Faculty Advisor(s): Charles W. Tu

Primary Student
Name: Rui La
Email: rula@ucsd.edu
Phone: 352-871-4425
Grad Year: 2017

Dilute nitrides have been actively researched and developed in the last two decades. According to the band anti-crossing (BAC) model, the interaction between the host states and the N localized states results in two subband, E- and E+ bands. In the GaAsxP1-x host, the N level falls below the conduction band edge for x>0.3, resulting in a narrow E- band, which is suitable as the intermediate band for intermediate band solar cells (IBSCs). One of the advantages of III-V nanowires (NWs) grown on Si substrate is that the lattice-match constraint can be relaxed. By tuning the geometry of the array structure, the absorptivity of NWs can be increased compared with thin film configurations. Therefore, growing GaNAsP nanowires on Si (111) substrate will be of particular interest regarding to photovoltaic applications. GaAs/GaNAs, GaNP/GaNP and GaAs/GaAsSbN/GaAs core-shell NWs have been epitaxially grown on Si (111), but GaNAsP NWs have not been studied yet. In this work, we report epitaxial growth of dilute nitride GaNAsP NWs and GaAsP/GaNAsP core-shell NWs on Si (111) by a self-catalyzed method with gas-source molecular beam epitaxy. Different sizes of GaNAsP NWs were observed with various N contents. An increasing concentration of N consumes more Ga droplets and results in shorter nanorods with a larger diameter. Room-temperature photoluminescence (PL) is observed from the GaNAsP NWs. The optical emission from GaNAsP NWs indicates a decreasing bandgap with increasing N. Room-temperature cathodoluminescence (CL) measurements of single NWs shows a gradual shift of the emission spectra along tapered NWs at different locations, probably due to the shadowing effect. The spectral peak of temperature-dependent PL shows an S-shape, indicative of defect states in the bandgap. The room-temperature PL intensity is also increased after rapid thermal annealing (RTA). The growth of shells on core NWs is another crucial step to achieve radial composition and doping modulation in NW structures that will enable the design of complex solar cells. Following GaAsP core NW growth, the GaNAsP shell is grown at a lower substrate temperature. The shell layer thickness is measured as a function of growth time. A growth model explains the measured diameters of GaAsP/GaNAsP core-shell. By comparing the PL spectra from GaAsP/GaNAsP core-shell NWs with those of GaAsP NWs and GaNAsP NWs, we conclude that the PL emission from the GaAsP/GaNAsP core-shell NWs originates from the GaNAsP shell layer, which is expected from the band alignment.

Industry Application Area(s)
Electronics/Photonics | Materials | Semiconductor

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