ALD2019 Session AS2-TuA2: Late Breaking Abstracts

Indium aluminum nitride (InAlN) is an attractive material for power electronic applications. However, conventional methods of epitaxial growth of InAlN are challenged by a large miscibility gap and the significant differences in optimal growth conditions for the constituent aluminum nitride (AlN) and indium nitride (InN) binary compounds. Despite these challenges, the epitaxial growth of InAlN alloys throughout the entire compositional range has been demonstrated using plasma-assisted atomic layer epitaxy (ALEp)¹, a variant of atomic layer deposition in which relatively higher temperatures are utilized. In the ALEp growth of InAlN, the desired alloy compositions are achieved by forming ultra-short period superlattices of alternating InN and AlN layers, referred to as digital alloys (DA). In order to further advance these empirical efforts, significant research is needed to better understand the nucleation and growth kinetics of ALEp DA growth. To this end, we employ in situ grazing incidence small angle X-ray scattering (GISAXS) for the real-time study of the evolving ternary InAlN surfaces as has been done previously for binary InN² and AlN³.

Here we present in situ GISAXS studies of ALEp growth of InN, AlN, and a range of InAlN DAs on GaN (0001) substrates, which were performed at Brookhaven National Laboratory’s NSLS-II using a custom reactor. The InAlN DAs studied include In_0.19Al_0.81N (3 AlN cycles and 2 InN cycles per supercycle), In_0.5Al_0.5N (1 AlN cycle and 3 InN cycles per supercycle), In_0.64Al_0.36N (1 AlN cycle and 5 InN cycles per supercycle) and In_0.83Al_0.17N (1 AlN cycle and 14 InN cycles per supercycle). Preliminary analysis of the data suggests that while the pure InN and AlN grew in 3D and 2D modes, respectively, the InAlN growth mode did not follow a simple trend as the nominal composition was tuned from InN to AlN. Instead, select compositions (50% and 83% In) exhibited predominantly 3D growth, while others (19% and 64% In) exhibited 2D growth. Ex situ X-ray reflectivity data show evidence of phase separation in the 83% In DA.

¹ N. Nepal, V.R. Anderson, J.K. Hite, and C.R. Eddy, Thin Solid Films 589, 47 (2015)

² J.M. Woodward, S.G. Rosenberg, A.C. Kozen, N. Nepal, S.D. Johnson, C. Wagenbach, A.H. Rowley, Z.R. Robinson, H. Joress, K.F. Ludwig Jr, C.R. Eddy Jr, J. Vac. Sci. Technol. A 37, 030901 (2019)

³ V.R. Anderson, N. Nepal, S.D. Johnson, Z.R. Robinson, A. Nath, A.C. Kozen, S.B. Qadri, A. DeMasi, J.K. Hite, K.F. Ludwig, and C.R. Eddy, J. Vac. Sci. Technol. A 35, 031508 (2017)

Over the past decade, researchers have developed a deeper understanding of how the chemistry of ALD can be applied to polymeric substrates. Polymers are fundamentally unlike traditional ALD substrates in that precursors diffuse through the polymer and associate with functional groups that are distributed throughout the volume of polymer, rather than on a two-dimensional surface.

In one implementation of ALD in polymers called sequential infiltration synthesis (SIS), very long static exposures of precursors are used to enable diffusion into the polymer bulk. The published SIS materials library is quite limited compared to the broad ALD materials library. Aluminum oxide (Al₂O₃), as synthesized via SIS with trimethylaluminum (TMAl) and water, is by far the most studied system and has been utilized in applications ranging from lithography to oil sorption. Prior attempts to expand this library build upon on a primary SIS cycle of Al₂O₃ to act as a scaffold for subsequent synthesis of other materials. However, the insulating Al₂O₃ is detrimental to the properties of optoelectronically interesting materials. A detailed understanding of the physical and chemical processes of solvation, diffusion, and complexation between ALD precursors and polymer functionalities is required to engineer hybrid polymer-metal oxide devices and to develop a broader library of SIS materials.

Through a combined experimental and first principles computational study we have developed primary SIS processes for two previously unreported materials – indium oxide (In₂O₃) and gallium oxide (Ga₂O₃) – using congeners to TMAl (TMIn, TMGa) and water. Through Fourier transform infrared spectroscopy and density functional theory, we elucidate the binding configuration and affinity of TMAl, TMIn, and TMGa for the carbonyl functional groups in polymethylmethacrylate (PMMA). We find that the three form isostructural adducts, though TMAl binds three times more strongly, and observe that the kinetics of adduct formation/dissociation for TMAl are more than ten times slower than for TMIn or TMGa.