In Situ Synchrotron GISAXS Studies of InN Plasma-Enhanced Atomic Layer Deposition Nucleation and Growth Kinetics

The <i>in situ</i> characterization of atomic layer deposition (ALD) processes is challenged by the highly contaminating metal precursors, relatively high pressures, and harsh process environments which preclude the use of the powerful electron-based techniques commonly employed for ultrahigh vacuum thin film growth methods. An alternative approach is to utilize hard x-ray techniques which are compatible with arbitrary pressures and allow for the placement of both source and detector outside the reactor through the incorporation of x-ray transparent windows. Among such techniques, grazing incidence small-angle x-ray scattering (GISAXS) using synchrotron radiation is particularly well-suited to the study of ALD processes due to its exceptional surface sensitivity and ability to probe nanoscale structure as it evolves in real time. The application of GISAXS for the investigation of plasma-enhanced ALD (PEALD) processes is especially compelling, as even relatively simple plasmas may contain a broad range of species which influence the growth kinetics and resulting film properties.<br/><br/>In this work, the nucleation and early stage growth kinetics of InN PEALD processes are investigated using <i>in situ</i> GISAXS in a custom reactor. The InN films are grown on c-plane GaN using trimethylindium and N₂/Ar plasma as the metal precursor and reactant, respectively. Different regimes of plasma species generation, which are accessed by adjusting the relative flows of N₂ and Ar into the inductively coupled plasma (ICP) source, are explored, and the plasma properties are characterized by optical emission spectroscopy (OES) and Langmuir probe measurements. These plasma diagnostics are supported by modeling with the 2D Hybrid Plasma Equipment Model (HPEM), which is used to predict the fluxes of various reactive species produced in the plasma to the sample, including atomic N and metastable N₂. The growth mode is observed to be correlated to the concentration of atomic N in the plasma, with high concentrations promoting Volmer-Weber (i.e., island) growth and low concentrations promoting Stranski-Krastanov (i.e., layer-plus-island) growth. Under conditions of high atomic N production, both the mean island radius and critical thickness for island formation are found to increase with ion flux. The InN island center-to-center distance and areal density are found to change only during plasma exposure, and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. [1]<br/><br/>Building on these results, a similar series of InN films are grown on c-plane GaN using a commercial reactor and characterized by atomic force microscopy (AFM), high resolution x-ray diffraction (HRXRD), in-plane grazing incidence diffraction (IP-GID), synchrotron grazing incidence wide-angle x-ray scattering (GIWAXS), and transmission electron microscopy (TEM). Plasma diagnostics are used in order to confirm reasonable consistency in plasma properties between the commercial and custom reactors. The films are found to exhibit wurtzite phase and sixfold rotational symmetry with a clear epitaxial relationship to the GaN. Low concentrations of atomic N are found to promote larger domains, increased crystalline order, and smoother morphology compared to films grown with high atomic N concentrations, and a change in the dominant kinetic roughening mechanism from direct deposition on existing islands to diffusion to existing islands. For high atomic N concentrations, increasing the ion flux is found to promote a very rough morphology containing large cluster-like features and decreased in-plane crystalline order.<br/><br/>[1] J. M. Woodward <i>et al</i>., J. Vac. Sci. Technol. A <b>40</b>, 062405 (2022)