Contact Angles Between III–V Melts and Several Substrates

Abstract

The wetting behavior of several melts with different III-V compound semiconductors is studied. The characteristic parameter measured is the contact angle of a melt drop with a solid. It is evaluated by a geometrical measurement of the melt drop. The importance of clean surfaces of drop and solid is shown. Contact angles between 20 ~ and 60 ~ are found, depending on the type and composition of the melt and substrate. The temperature dependence amounts to about -4 ~ per 100 K increase. In addition, the contact angle varies with the substrate orientation. The (100) orientated substrates are wetted better than (111) A orientated ones.In liquid phase epitaxy (LPE) of a III-V compound semiconductor, a metallic melt is in contact with a solid substrate and a crucible, usually graphite. The wetting behavior of a III-V melt influences the carryover of melt, and by this the epitaxial quality with respect to unintentionally grown layers [e.g., (1)].The quality of the wetting may also affect the crystalline perfection of an epitaxial layer along the melt meniscus due to surface tension (2). When removing the melt from the semiconductor, it moves by a "stickslip" motion. This forms the well-known meniscus lines (3).The representative parameter for wetting is the contact angle at the melt meniscus between the melt surface and the substrate surface covered by the melt [e.g., (4)]. The better the wetting, the smaller the contact angle is. The literature data for Ga on GaAs with contact angles >90 ~ from Munir and co-workers (5, 6), do not agree with the phenomenological experience of capillary ascension at LPE of III-V compounds. When melt and substrate are brought into contact, one usually observes a creeping of the melt on the substrate under the melt chamber walls. Very recently, Small and Potemsl~i (7) reported on smaller angles for GaAs and Ga0.4A10.~As, when regarding a microscopical deformation of the solid around the periphery of the drop. We have evaluated contact angles in a macroscopical measure of melt and solid to find representative data for LPE. Several III-V compound systems and different substrate orientations have been studied in a hydrogen atmosphere. ExperimentalWe have used a horizontal LPE apparatus, consisting of a movable heat pipe furnace, a quartz reaction tube, a Pd diffusion cell, and a crucible of high purity graphite with fixed melt chambers, and a sliding substrate holder. A side-wall of one melt chamber was omitted in order to directly observe the melt contact angles. During a 3 hr heat cleaning at 1145 K in purified hydrogen, the melt drop, of about 5 mm diam, lies on the graphite slider within the chamber of 24 • 24 nuns separated from the substrate. The substrate is protected by a graphite cover. The sliding mechanism allows us to roll the melt drop on the graphite slider so that its surface can be exposed to reducing hydrogen, even its incipient contact area. Only after the heat cleaning is the substrate rapidly moved under the melt drop. The drop spreads on the substrate surface without touch...

Imaging Surface Topography using Lloyd’s Mirror in Photoemission Electron Microscopy

“…We are able to deduce the contact angle for the Ga droplet as t 22 . This result is significantly lower than the expected equilibrium contact angle (35 ) [20], but is in excellent agreement with the range of values observed for h110i oriented seg- ments using scanning electron microscopy of rapidly quenched samples. This low value is consistent with enhanced surface segregation of As during droplet spreading, which lowers LV [16,21] to restore local equilibrium via Young's equation, SV LV cos t SL cos b (Fig.…”

Controlling the formation of quantum dot pairs using nanohole templates

“…Given that the density of gallium droplets will correspond to the density of nanoholes, we can estimate the size of the gallium droplet in the following manner. We assume that the gallium droplets are spherical and have a contact angle of between 281 and 401 to the GaAs substrate depending on the amount of arsenic dissolved in the gallium droplet [25] and we estimate the density of gallium at 500 1C to be 5.78Â 10 3 kg/m 3 [26]. Since 1 ML of excess gallium corresponds to 6.2 Â 10 14 atoms/ cm 2 then, given that the total excess gallium on the surface is 2.85 ML and assuming that all this excess gallium is incorporated into droplets with density $ 1:1 Â 10 8 droplets=cm 2 , the average droplet radius can be estimated to be (80-90) nm, i.e.…”

Au-Seeded Growth of Vertical and in-Plane III–V Nanowires on Graphite Substrates

“…37 The γ GS is more difficult to estimate, because it is a solid− solid interface. Molten GaSb 45 and GaAs 46 show high contact angles (129 to 153°) on graphite, and it was observed that the molten GaSb completely dewets upon freezing. 45 Note that GaSb has been predicted to have a relatively good lattice matching with graphene.…”