Inner composition, defects and morphology of AlGaAs nanowires grown by Au-catalyzed MOVPE
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Inner composition, defects and morphology of AlGaAs nanowires grown by Au-catalyzed MOVPE P. Prete,1 B. Buick,2 E. Speiser,2 N. Lovergine3 and W. Richter2 1
IMM-CNR, Lecce Research Unit, Lecce, Italy
2
Physics Department, University of Rome ‘Tor Vergata’, Rome, Italy
3
Dept. of Innovation Engineering, University of Salento, Lecce, Italy
ABSTRACT The inner composition, defect content and morphology of AlGaAs nanowires (NWs) grown on (111)B-GaAs by Au-catalyzed MOVPE is reported. The NWs grow tapered with their [111] axis normal to the substrate. The Raman spectra of single AlGaAs NWs were measured in non-resonant conditions with sub-µ-meter spatial resolution, allowing determination of the Al content. NWs consist of GaAs for TG475°C, namely an AlxGa1-xAs core surrounded by an AlyGa1-yAs (yy) of a typical AlGaAs NW. The hatched area represents the portion of the NW grown by the Au-catalyst assisted mechanism. Right: Values of the Al-content within the core (S) and shell (U) portions of AlGaAs NWs as function of growth temperature (Arrhenius plot). The greyhatched area 475°C) in the vapor-grown (shell) portion of the NWs, their cross-section becomes triangular, the side facets of the NWs being oriented almost parallel to only three of the six { 211 } equivalent crystal planes, e.g. either { 211 }A or { 211 }B ones [Fig. 1(b)]; this indicates that the growth rates of AlGaAs on { 211 }A and { 211 }B planes greatly differ from each others. If AlGaAs is deposited from the vapor at high (>650°C) temperatures around a GaAs NW, the material uniformly grows along the six equivalent non-polar 110 directions normal to the NW axis [Fig. 1(d)], leading to the hexagonal cross-section observed for GaAs-AlGaAs coreshell NWs [9]. Thus, under these conditions the 110 planes are the slowest to grow. Upon increasing the temperature a gradual change between the two conditions is expected and indeed, this is the case for NWs grown at 550°C, which show a mixed cross-sectional shape [Fig. 1(c)].
CONCLUSIONS We reported on the inner composition, defect content and morphology of AlGaAs NWs grown on (111)B-GaAs by Au-catalyzed MOVPE. The NWs are highly tapered with their [111] axis normal to the substrate plane. The Raman spectra of single AlGaAs NWs were measured in non-resonant conditions with sub-µ-meter spatial resolution, and allowed precise determination of both Al content (xAl) and carrier concentration within single NWs. NWs grown at TG≤475°C are composed of GaAs (no Al); at higher growth temperatures they show a two-fold compositional structure, ascribed to a central AlxGa1-xAs core surrounded by a AlyGa1-yAs shell (with x>y), a result of the combined Au-catalyzed (axial) and conventional (sidewall) growth processes. The Al fraction xAl in both the core and shell portions of the NWs increases with temperature up to 550°C, where the core composition approaches that of the vapor phase. The cross-sectional shape of AlGaAs NWs changes from triangular (for TG=500÷525°C) to almost hexagonal one (for TG=550°C), as result of a gradual
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