Dynamics of Droplet Consumption in Vapor–Liquid–Solid III–V Nanowire Growth

Abstract

We study experimentally and theoretically the consumption of the apical gallium droplet that mediates the self-catalyzed vapor-liquid-solid growth of GaP nanowires. Consumption is achieved after growth by providing only phosphorous and its progress is monitored ex situ in nanowire arrays fabricated by molecular beam epitaxy. We develop detailed calculations of the process, taking into account four channels of liquid gallium consumption. These include the formation of GaP using phosphorous delivered to the droplet by direct impingement or after re-emission from the substrate. We show that two other channels contribute significantly, namely the diffusion of phosphorous along the sidewalls and gallium back diffusion from the droplet. All currents are calculated analytically as a function of droplet geometry. Complementary experiments are performed to extract the two model parameters governing the diffusion currents. We then compute numerically the dynamics of the system exposed to a constant external phosphorous flux. Our quantitative model allows one to predict how droplet contact angle and radius change while operating blindly in a standard epitaxy chamber. Controlling these parameters is crucial for tailoring the crystal phase of III-V nanowires and fabricating quantum size structures.

Statistics of Nucleation and Growth of Single Monolayers in Nanowires: Towards a Deterministic Regime

“…Control of such structures at the ML level is actually necessary for promising applications. [44][45][46] Note that if group V sidewall diffusion contributed to droplet refill, [35] this would only require stems longer than the corresponding diffusion length, in addition to constant NW and droplet radii. [36,47] We showed that the reason for this quasideterministic growth regime is the compensation between the propagation time and preceding waiting time, expected to occur when there is never enough group V atoms in the catalyst particle to produce a full ML at nucleation.…”

“…At given temperature T, the state of the binary liquid is set by the atomic concentration x of As. The numbers of Ga and As atoms removed from the liquid to form any amount of the solid NW are equal, and we consider constant and equal rates of refilling of the droplet for Ga and As (by the direct and re-emitted beams [10,[33][34][35] and by surface diffusion, [36] which might even operate for group V species [35] ). We assume constant NW radius and droplet contact angle.…”

GaAs nanowires on Si nanopillars: towards large scale, phase-engineered arrays

“…The predeposition time, i.e. the time during which Ga flux is on, as well as the substrate temperature, 24 determine the droplet volume. This step is crucial as the volume of the droplet is one of the determining parameters for the contact angle, defined as β , discussed later in this section.…”

Лимитирующие Факторы Скорости Роста При Эпитаксии Полупроводниковых Соединений III-V

“…Ясно, однако, что диффузионная длина адатомов группы V на поверхности не может быть нулевой (в противном случае эти адатомы вообще не могли бы десорбироваться в составе димеров As 2 , P 2 или N 2 ). Недавно в работе [17] была рассмотрена диффузия адатомов P по боковой поверхности ННК GaP на расстояния до 400 nm. Вполне возможно также, что радиальный рост ННК [8,11,13,14] вне зависимости от типа катализатора или его отсутствия может контролироваться потоком элемента группы V. Целью настоящей работы является построение простой модели для скорости роста слоев III−V за счет поверхностной диффузии обоих элементов, а также установление условий, при которых рост действительно контролируется диффузией адатомов группы III.…”