Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions

Abstract

Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type-precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance.

Selective area epitaxy of magnesium oxide thin films on gallium nitride surfaces

“…During growth, this Cl ML must remain, at least partially, on the MgO surface, similar to the behavior of other film growth "surfactants." 25,55 The use of Cl as a surfactant for (111) MgO surfaces is consistent with prior observations. Koranyi et al have FIG.…”

“…We see a similar need to provide a constant H 2 O surfactant flux when growing smooth, -OH terminated (111) MgO epilayers on GaN. 25 If the H 2 O flux is removed at any point during film growth, the surfactant effect is lost and a rapid transition from 2D to 3D growth mode occurs. Indeed, the concentrations of Cl are noticeably lower on the surface of the thick MgO films [ Fig.…”

“…This reduced surface resonance time likely precludes the dissociation of molecular O 2 , which is known to be a rate limiting step in reactive MgO film growth. 45,46 E. Retainment of Cl adatoms on the surface of MgO epilayers Our prior work 25,26 using hydroxyls as surfactants to energetically stabilize the (111) surface of MgO epilayers during film growth made us curious as to whether the Cl adatoms could show similar surfactant behavior. Growth of atomically smooth (111) MgO epilayers is essential for good insulating performance and so has great technological relevance.…”

“…Growth of atomically smooth (111) MgO epilayers is essential for good insulating performance and so has great technological relevance. 25 Several parallels could be made between the hydroxide and chloride behaviors including the observation that hydroxylated (111) MgO epilayers have a slower growth rate than nonhydroxylated epilayers. In this study, we used high Mg and O 2 fluxes to force growth on the "HCl treated" surfaces.…”

First-principles calculations of surfactant-assisted growth of polar CaO(111) oxide film: The case of water-based surfactant

“…The surfactant hydrogen changes its position and remains on the surface during the growth process, providing a dynamical solution for the polarity constraints throughout the growth of MgO(111), by effectively healing (quenching) the surface dipole moment and thus lowering the surface energy. At the same time, in the presence of a water-based surfactant, Paisley et al [11] have demonstrated the surfactant-assisted growth of atomically flat polar CaO(111) oxide film with arbitrary thickness by MBE growth of CaO(111) on GaN(0001). In addition, they have used ab initio thermodynamic calculations to predict that in the presence of the water surfactant, flat CaO(111) surfaces are stabilized by hydroxylation during surfactant-assisted growth at temperatures below 253 o C. However, the water-based surfactant-assisted growth mechanism remains unknown.…”