Wurtzite phase control for self-assisted GaAs nanowires grown by molecular beam epitaxy

Abstract

The accurate control of the crystal phase in III–V semiconductor nanowires (NWs) is an important milestone for device applications. Although cubic zinc-blende (ZB) GaAs is a well-established material in microelectronics, the controlled growth of hexagonal wurtzite (WZ) GaAs has thus far not been achieved successfully. Specifically, the prospect of growing defect-free and gold catalyst-free wurtzite GaAs would pave the way towards integration on silicon substrate and new device applications. In this article, we present a method to select and maintain the WZ crystal phase in self-assisted NWs by molecular beam epitaxy. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the ZB or WZ phase is the V/III flux ratio. Using an analytical growth model, we show that the V/III flux ratio can be finely tuned by changing the As flux, thus driving the system toward a stationary regime where the wetting angle of the Ga droplet can be maintained in the range of values allowing the formation of pure WZ phase. The analysis of the in situ reflection high energy electron diffraction evolution, combined with high-resolution scanning transmission electron microscopy (TEM), dark field TEM, and photoluminescence all confirm the control of an extended pure WZ segment, more than a micrometer long, obtained by molecular beam epitaxy growth of self- assisted GaAs NWs with a V/III flux ratio of 4.0. This successful controlled growth of WZ GaAs suggests potential benefits for electronics and opto-electronics applications.

A Photoemission Analysis of Gold on Silicon Regarding the Initial Stages of Nanowire Metal-Catalyzed Vapor–Liquid–Solid Growth

“…This catalyst-activated growth mechanism was first reported by Wagner and Ellis in 1964 . Since then, it has been extensively studied, mainly regarding the growth parameters that affect the NWs morphology, structure, and/or repartition. − Despite the suggestion of various models, − fundamental aspects and mechanisms governing NWs nucleation and growth are still not fully understood due to the complexity of the involved dynamical processes, especially at the pregrowth stages, namely, the formation of the metal liquid droplets that catalyze the NWs crystalline growth on a substrate. For instance, depending on the preparation and composition of the thin metal film and/or of its substrate, other interrelated phenomena can take place such as Ostwald ripening, , alloying with the substrate, and self-propelled motion, , which influence the shape, crystallography, repartition, and growth direction of the NWs. ,− Gold is the most common catalyst to grow NWs by VLS: it has been used to grow IV–IV NWs, ,− III–V NWs, ,,− and oxide NWs as MgO or ZnO .…”

Insights into the Arsenic Shell Decapping Mechanisms in As/GaAs Nanowires by X-ray and Electron Microscopy

“…The Ga droplet at the NW tip is not visible due to its exposure to an As atmosphere, which induced the formation of a crystalline WZ GaAs segment of about 150 nm length. The origin of this ZB/WZ transition has been thoroughly discussed ,, and is thought to be resulting from the evolution of the Ga droplet contact angle during the growth. − …”

Hexagonal Ge Grown by Molecular Beam Epitaxy on Self-Assisted GaAs Nanowires

“…The occurrence of two crystal phases in GaAs NWs (WZ for Au catalyst and ZB for Ga catalyst) is closely related to the value of contact angle of the liquid catalyst during the VLS growth [16]- [18], and various strategies have been tested in order to control the synthesis of WZ in NW geometry [19], [20]. Recently, we demonstrated the growth of a long segment of WZ GaAs using the self-assisted method by an accurate control of the group III [21] or group V flux [22] to ultimately reach a stationary state of the catalyst contact angle in the range inducing the WZ phase [22]. The facets of obtained WZ GaAs NWs thus provide an ideal template for the epitaxial growth of h-Ge, in addition to their low lattice mismatch.…”

“…19,20 Recently, we demonstrated the growth of a long segment of WZ GaAs using the self-assisted method by an accurate control of the group III 21 or group V flux 22 to ultimately reach a stationary state of the catalyst contact angle in the range inducing the WZ phase. 22 The facets of obtained WZ GaAs NWs thus provide an ideal template for the epitaxial growth of h-Ge, in addition to their low lattice mismatch.…”

“…While Au is probably the most popular catalyst for the NW growth using the vapor–liquid–solid (VLS) method, Au is known to induce defects and can diffuse into the semiconductor. , An alternative solution consists in using the self-assisted method, where Au is replaced by Ga for the growth of GaAs NWs. , Although replacing Au by Ga is promising to avoid the formation of defects, it was observed that self-assisted GaAs NWs exhibit mostly the zinc-blende (ZB) phase, which is detrimental to the fabrication of h- Ge. The occurrence of two crystal phases in GaAs NWs (WZ for Au catalyst and ZB for Ga catalyst) is closely related to the value of contact angle of the liquid catalyst during the VLS growth, − and various strategies have been tested in order to control the synthesis of WZ in NW geometry. , Recently, we demonstrated the growth of a long segment of WZ GaAs using the self-assisted method by an accurate control of the group III or group V flux to ultimately reach a stationary state of the catalyst contact angle in the range inducing the WZ phase . The facets of obtained WZ GaAs NWs thus provide an ideal template for the epitaxial growth of h- Ge, in addition to their low lattice mismatch.…”