Wurtzite AlGaAs Nanowires

Abstract

Semiconducting nanowires, unlike bulk, can be grown in both wurtzite and zincblende crystal phases. this unique feature allows for growth and investigation of technologically important and previously unexplored materials, such as wurtzite AlGaAs. Here we grow a series of wurtzite AlGaAs nanowires with Al content varying from 0.1 to 0.6, on silicon substrates and through a comparative structural and optical analysis we experimentally derive, for the first time, the formula for the bandgap of wurtzite AlGaAs. Moreover, bright emission and short lifetime of our nanowires suggest that wurtzite AlGaAs is a direct bandgap material.Polytypism 1 is an exceptional property of nanowires and a new degree of freedom which enables the engineering of the electronic structure without change of material. For example, today's atomically-precise control over the crystal-phase switching in nanowires 2,3 allows to grow strain-free polytypic formations along the growth axis 4,5 , even small enough to form quantum dots 6,7 . The wurtzite phase is not observable at ambient conditions in bulk of any A III B V materials except for nitrides, while it can be obtained in nanowires. For this property and its technological implications, a great deal of attention has been drawn, in recent years, to nanowires system from scientific community 8-10 . However, for designing of novel structures and devices, knowledge of bandgaps and band alignments of the different crystal phases of new materials is crucial.In particular, Al X Ga 1-X As nanowires provide a promising platform for fabrication of advanced devices. For example, adding the Al component to the widely studied GaAs 11,12 allows to tune the emission in a wide range of wavelengths while, AlGaAs, having higher energy than GaAs, allows the combination of these two materials to fabricate strain-free quantum devices 13 .However, the knowledge about wurtzite AlGaAs is limited in the literature [14][15][16][17] , and is mainly grown as a shell around wurtzite GaAs core 15,16 . Importantly, the bandgap of wurtzite AlGaAs was neither predicted theoretically nor measured experimentally.In this work, we grow wurtzite AlGaAs nanowires, in a wide range of Al content x, and we present a comparative optical and structural study, empirically revealing the trend for the bandgap of wurtzite Al X Ga 1-X As. We grow our samples by Au-catalyzed vapor-liquid-solid technique in a molecular beam epitaxy (MBE) reactor (see methods section for details) obtaining high crystalline quality structures with any chosen Al content.

Purcell Effect and Beaming of Emission in Hybrid AlGaAs Nanowires with GaAs Quantum Dots

“…According to our previous works, under the described growth conditions, AlGaA NWs are spontaneously formed by a core-shell structure [24]. Moreover, in the case of nominal Al 0.3 Ga 0.7 A composition, the real content of aluminum in the AlGaAs NW core is 16%, and 24% in the shell [25].…”

Checked patterned elemental distribution in AlGaAs nanowire branches via vapor–liquid–solid growth

“…Initiation of branches from adatom accumulation on the NW sidewalls has been reported in the past for other material systems, including GaAs/GaAsBi core/shell NWs 29 and GaSb NWs. 30 Besides, the unintentional formation of Al-rich shell is frequently observed in AlGaAs NWs, 24,[31][32][33][34] and can be established in the NW trunks of the current AlGaAs branched NWs via the radial EDX line scan, which is shown in section 4 of the ESI. † The high density of twins increases the roughness of the shell surface, leading to the formation of concave sites, where Ga preferentially accumulates.…”

Self-Catalyzed AlGaAs Nanowires and AlGaAs/GaAs Nanowire-Quantum Dots on Si Substrates

“…It has recently been reported that the WZ form of AlGaAs exhibits a significantly lower direct band gap than the ZB form. 57 With both forms present in a nanostructure, carriers may diffuse into the WZ segments prior to their recombination. The resultant lower PL emission energy would hence lead to a lower calculated effective Al composition, as the ZB band-gap values were used in the calculation.…”