Bistability of Contact Angle and Its Role in Achieving Quantum-Thin Self-Assisted GaAs nanowires

Abstract

Achieving quantum confinement by bottom-up growth of nanowires has so far been limited to the ability of obtaining stable metal droplets of radii around 10 nm or less. This is within reach for gold-assisted growth. Because of the necessity to maintain the group III droplets during growth, direct synthesis of quantum sized structures becomes much more challenging for self-assisted III−V nanowires. In this work, we elucidate and solve the challenges that involve the synthesis of galliumassisted quantum-sized GaAs nanowires. We demonstrate the existence of two stable contact angles for the gallium droplet on top of GaAs nanowires. Contact angle around 130°fosters a continuous increase in the nanowire radius, while 90°allows for the stable growth of ultrathin tops. The experimental results are fully consistent with our model that explains the observed morphological evolution under the two different scenarios. We provide a generalized theory of self-assisted III−V nanowires that describes simultaneously the droplet shape relaxation and the NW radius evolution. Bistability of the contact angle described here should be the general phenomenon that pertains for any vapor−liquid−solid nanowires and significantly refines our picture of how nanowires grow. Overall, our results suggest a new path for obtaining ultrathin one-dimensional III−V nanostructures for studying lateral confinement of carriers. KEYWORDS: III−V semiconductors on silicon, nanowires, nanoneedles, regular arrays, self-assisted growth, droplet size engineering, crystal structure, growth modeling F undamental physical properties and applications of semiconductor nanowires (NWs) may benefit from quantum confinement due to size-dependent modulation of the density of states as well as modification of the photon energy for the allowed optical transitions.1 In GaAs, quantum confinement occurs for a size below 25 nm.2,3 In addition to quantum confinement, the optical properties can be engineered by modifying the NW shape. For example, progressively tapered nanowires have exhibited adiabatic outcoupling in the emission of quantum dots, thereby enhancing their brightness. 4 A similar design may improve the light extraction or absorption in different optoelectronic devices including light emitting diodes, solar cells, and optical biosensors.5−11 Quantum confinement in the core of radial NW heterostructures allows for the fabrication of high quality NW-based quantum dots, which are a perfect platform for delicate quantum transport experiments. 12,13 It is admittedly challenging to obtain very thin (quantumconfined) III−V NWs directly by the vapor−liquid−solid (VLS) growth technique. There are several reasons for that (see ref 14 for a review), including the Gibbs−Thomson effect of elevation of chemical potential due to the curvature of the droplet surface and difficulties in obtaining very small droplets on the substrate surface. Thin tips of VLS GaAs NWs with a stable radius down to 5 nm have previously been grown by hydride vapor phase epitaxy 15,16 (a te...

Stabilization of the Morphology and Crystal Phase in Ensembles of Self‐Catalyzed GaAs Nanowires

“…At the critical contact angle , corresponding to the WZ‐to‐ZB phase transition, the truncated growth interface starts to develop on surface energetic grounds and suppresses the triple phase line nucleation in the WZ phase . According to the previous studies, in GaAs NWs, this is smaller, but very close to , as shown in Figure .…”

“…Very important results given in the previous study may help to resolve these issues and gain a much better control over the NW morphology and crystal phase purity. According to the data, supported by the surface energy modeling, gallium‐catalyzed GaAs NWs have two stable contact angles of the droplets seated on their tops. The small stable angle , around 90°, corresponds to thinning the NW top radius under high V/III flux ratios by introducing narrowing NW size facets.…”

“…Self‐catalyzed vapor–liquid–solid (VLS) growth of III–V nanowires (NWs) is a promising gold‐free method for synthesis of high crystal quality and size‐uniform ensembles of NWs. Most self‐catalyzed III–V NWs are grown by molecular beam epitaxy (MBE) in regular arrays on lithographically patterned SiO x /Si(111) substrates with gallium predeposition, or using other methods of gallium droplet organization . Recently, sub‐Poissonian narrowing of the length distributions has been demonstrated for gallium‐catalyzed GaAs NWs .…”

“…Surface energies of narrowing and widening facets relative to vertical (110) ZB facet of III–V NWs in the case of a) one stable contact angle or b) large separation between and , as in GaAs NWs . After the critical contact angle , the top part of the NW becomes truncated.…”

“…Of course, naming the NWs “thin” or “thick” has no absolute meaning as it relates to a particular length and V/III flux ratio. For example, self‐catalyzed GaAs NWs given in the previous study were first grown in the standard regime under gallium‐rich conditions, with radial growth in pure ZB phase, so they were all “thin” for this V/III flux ratio. When the arsenic flux was raised abruptly, the NWs first decreased their contact angles to and then started to taper, in predominantly WZ phase with some ZB insertions, so they became “thick” for a much higher V/III ratio.…”