Orders of Magnitude Reduction in Threading Dislocations in ZnO Grown on Facet-Controlled GaN
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Orders of Magnitude Reduction in Threading Dislocations in ZnO Grown on FacetControlled GaN Soo Jin Chua1,2, Hai Long Zhou3, Hui Pan3, and Thomas Osipowicz3 1 Institute of Materials Research and Engineering, 3 Research Link, Singapore, Singapore, 117602, Singapore 2 Center for Optoelectronics, National University of Singapore, 4 Engineering Drive 3, Singapore, Singapore, 117576, Singapore 3 Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore, Singapore, 117542, Singapore Introduction ZnO is great interest II-VI semiconductor for applications in opto- and nanoelectronics with a wide band gap of 3.37 eV. It has been reported that the blue-UV emission can be generated from ZnO thin-films[1], ZnO whiskers[2], p-type ZnO films[3], and thin-film diode structures[4] at room temperature. Thin layers of ZnO have been grown by various methods, including chemical vapor deposition (CVD) [5, 6], pulsed laser deposition (PLD) [7, 8], molecular beam epitaxy (MBE) [9,10], and dc- or rf- magnetron sputtering. [11, 12] In this work, we report the expitaxial overgrowth of ZnO films on facetcontrolled ELO GaN templates on sapphire (0001) using CVD with zinc vapor and oxygen as precursors. It is shown that that there is a two orders reduction of dislocations from the GaN substrate on which it is grown. Experimental details and results GaN is grown by Metal Organic Chemical Deposition (MOCVD) on oxide masked GaN with stripe window openings of 4 µm in width. Growth conditions were selected to achieve trapezoidal shaped cross section of GaN with {112 ¯ 2} sidewalls and (0001) top surface. ZnO is next epitaxially overgrown (EO) on these facets by Chemical Vapor Deposition (CVD). SiO2 is chosen as the masking material to prevent nucleation of ZnO on it. For different CVD growth parameters, the growth morphology of EO ZnO changes from long, needle-shaped nanorods to rectangular stripes with (0001) top facet and {112¯0} sidewalls and finally to triangular stripes with {112 ¯ 2} sidewalls. Fig.1 shows (a)
(b) ZnO
SiO2 2µm
ELO GaN 1µm
Figure 1.a) Typical SEM of as-grown ELO GaN on sapphire substrate. b) Crosssectional SEM image of ZnO grown on the ELO GaN template at 800 ºC
the cross-sectional SEM images of ZnO films that grown on the ELO GaN templates at 800 ºC. The ELO GaN grown along the GaN direction, and {112 ¯ 2} sidewall facets is clearly displayed. It can be seen that the original serrated ELO GaN stripe has a height of 5 µm and a width of 7 µm. After ZnO growth, the near rectangle shape is observed with a width of about 6.2 µm, indicating that the significant lateral growth of ZnO occurred on the ELO GaN. No growth was found on the SiO2 mask layer, which indicated that the ZnO top layer was selectively grown on the ELO GaN template. The microstructure of the top layer and the nature of the epitaxial growth of ZnO films grown at 800 ºC with an oxygen flow rate of 10sccm were further investigated by HRTEM. Fig. 2a shows a typical HRTEM image of the ZnO/ELO GaN interface. The ZnO
ZnO
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