(Invited) A Surface Science Toolbox for Understanding Atomic Layer Epitaxy
Authors: Eddy Charles R. , Rosenberg Samantha G , Woodward Jeffrey Michael , Anderson Virginia R , Johnson Scooter D. , Ludwig Karl F , Wagenbach Christa , Kozen Alex C , Walton Scott G , Boris David R. , Nepal Neeraj 
Published: February 2020

Atomic Layer Epitaxy (ALEp) is a promising subset of atomic layer processes (ALPs) which has the potential to open a new realm of non-equilibrium semiconductor growth. In ALEp, the objective is to grow crystalline, epitaxial layers on a crystalline substrate for active regions of electronic and optoelectronic devices. This requires expansion of the atomic layer deposition (ALD) processing space to higher temperatures and adds constraints of crystallinity and purity; electronic grade requires impurity concentrations less than ppm. While ALEp has been shown to maintain the self-limiting nature of ALD at temperatures up to 500°C, the additional materials quality criterion requires a more complete understanding of the ALEp process if it is to be fully successful. In this regard, it is essential to develop a set of surface science tools that can be employed either in situ or in vacuo to ensure that atmospheric exposure does not influence or interfere with observed process mechanisms.
In this work, we highlight the development and early application of a suite of in situ or in vacuo characterization techniques aimed at providing surface and near-surface structure assessments: low energy electron diffraction (LEED) and grazing incidence small angle x-ray scattering (GISAXS); as well as surface chemistry assessments: (x-ray photoelectron spectroscopy (XPS), resonant ion trap mass spectrometry (RIT-MS) and reflection-absorption infrared spectroscopy (RAIRS).
We present select results from these methods during efforts to develop plasma-assisted ALPs for GaN surface preparation for epitaxy and for early plasma-assisted ALEp growth of heteroepitaxial AlN, InN and AlInN films on GaN and Al2O3 substrates. In the former example, in situ GISAXS studies have been used to optimize an emulated gallium flash off (GFO) ALP of as-received GaN substrate surfaces as part of a preparation of such surfaces for epitaxy. A combination of an ex situ UV/O3 oxidation and concentrated HF etch followed by 10 cycles of an emulated GFO ALP result in the smoothest, cleanest surfaces – highly suitable for epitaxy [1,2]. In the latter application, in situ GISAXS studies are used to characterize the nature of the growth mode of AlN and InN binaries and select AlInN ternaries achieved through digital alloying on optimally prepared GaN substrate surfaces. For InN binary growth, which ultimately grows in a 3D mode, the duration of the plasma pulse is shown to influence the growth mode between a bimodal distribution of islands for short pulses to a single mode distribution for intermediate pulses to etching for the longest pulses [3]. Further, we found that the transition from early 2D to ultimate 3D growth and the shape and size of the islands that result are quite sensitive to growth temperature even inside the “ALEp window”[4]. We discovered that although pure InN and AlN grew in 3D and 2D modes, respectively, the InAlN growth mode did not follow a simple trend as the nominal In composition was tuned from InN to AlN. Instead, select compositions (50% and 83% In) showed more 3D growth while others (19% and 64% In) showed 2D growth. Changes in plasma chemistry are also found to affect growth mode and film quality and will be highlighted. These changes in film properties with plasma pulse variations are correlated to independent measurements of plasma properties in an effort to establish plasma process – film property relationships. A combination of RAIRS and RIT-MS will be presented to further illustrate the role of plasma chemistry, while combined in vacuo LEED and XPS will be presented to highlight early film growth evolution.
[1] S.G. Rosenberg, D.J. Pennachio, C. Wagenbach, S.D. Johnson, N. Nepal, A.C. Kozen, J.M. Woodward, Z.R. Robinson, H. Joress, K.F. Ludwig, C.J. Palmstrom and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 020908 (2019).
[2] S.G. Rosenberg, C. Wagenbach, V.R. Anderson, S.D. Johnson, N. Nepal, A.C. Kozen, J.M. Woodward, Z.R. Robinson, M. Munger, H. Joress, K.F. Ludwig and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 020928 (2019).
[3] N. Nepal, V. Anderson, S. D. Johnson, B. P. Downey, D. J. Meyer, Z. Robinson, J. M. Woodward, K. F. Ludwig, C. R. Eddy, Jr., Journal of Vacuum Science and Technology A 37, 020910 (2019).
[4] J.M. Woodward, S.G. Rosenberg, A.C. Kozen, N. Nepal, S.D. Johnson, C. Wagenbach, Z.R. Robinson, K.F. Ludwig, Jr. and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 030901 (2019).

Figure 1