NAMBE 2023 Session ST-WeM1: Advancement in MBE Growth Approaches

Thermal laser epitaxy (TLE) uses quasi-CW lasers to heat the substrate and the sources in a deposition geometry similar to MBE. The high energy densities of the laser beams enable the evaporation or sublimation of any element in the periodic table in any combination and substrate temperatures exceeding the melting point of sapphire at 2040 °C. This leads to a dramatic expansion of the parameter space available to MBE-type growth modes such as, e.g., the adsorption-controlled growth of oxides.

In addition, TLE offers new opportunities in terms of the available gas pressures and environments during growth. A TLE growth chamber is conceptually simple, besides the laser entrance windows it basically contains only mechanical mounts for substrates and sources. This means that background gases at any pressure and with any reactivity are possible, as long as these parts do not get damaged, and the laser beam is not absorbed in the gas phase. This is the case for most reactive gases such as oxygen, ozone, plasma-excited nitrogen or ammonia.

As an example, we study the evaporation of Al in a molecular oxygen atmosphere. At high laser power densities, the flux of the source is independent of the oxygen pressure up to 10^-1 hPa. At lower power densities, the source flux depends on the oxygen pressure and the area of exposed metallic surface on the source. This is consistent with volatile suboxide molecules proportional to the oxygen flux being generated on the exposed surface. For the current maximum laser power, the source finally gets passivated by a stable oxide at pressures above 10^-1 hPa, and no longer produces a stable flux. The deposition rate on the substrate starts dropping at 10^-2 hPa for the given working distance of 80 mm, as the mean free path drops below this distance and the transport transitions from the ballistic to the diffusive regime. Irrespective of the reactive flux generation and deposition, the more than five orders of magnitude dynamical range of the deposition flux and high maximum growth rates are maintained.

This implies that TLE is able to cover the entire range of growth conditions traditionally employed in MBE, GS-MBE, MOMBE, MOCVD, CBE and CVD. TLE thereby enables the evaporation or sublimation of practically any combination of elements, together with reactive gas sources, at low and also very high growth rates. CVD growth modes, e.g. for thick buffer layers, may be combined with kinetically limited growth, e.g. for active layers at low temperatures, in a single growth to produce devices with very dissimilar layers and growth conditions in a single growth run.

Results of TLE with nitrogen and ammonia will be presented and discussed.

The interest in sapphire as an electronic materialis due to itslow cost, superior properties over silicon, high quality wafer availability, and the possible integration with silicon.Indeed, its rapidly growing market share suggests sapphire to be the substrate materialof the future.[1] A cornerstone for further establishing sapphire in a wide range of (electronic) applications is the ability to grow high quality homoepitaxial sapphire films. Key applications that can benefit from this are, e.g, diodes and high-power electronics based on nitrides [1] and (ultra) wide band gap semiconductors, respectively [2,3]. However, epitaxial films of c-plane oriented sapphire, one of the most common cuts of sapphire, have so far been out of reach due to a preferred formation of the Al₂O₃γ-phase.[3]

I will present how thermal laser epitaxy (TLE) overcomes this issue thanks to the availability of a parameter space that far exceeds the reach of other deposition methods.[4] The substrate laser heating system allows to easily heat sapphire substrates even beyond their melting point. The high accessible temperatures enable a precise and smooth sapphire substrate preparation – the first step for successful homoepitaxy.[5] I further present how the crystal quality and surface smoothness of homoepitaxial sapphire increase with increasing substrate temperature in case the growth is performed in the adsorption-controlled mode. Even at the respective high temperatures growth rates far exceeding 1 μm/h can be realized. Films were investigated by scanning transmission electron microscopy, atomic force microscopy, and x-ray diffraction. At a growth temperature of 1600 °C, the films were found to be practically undistinguishable from the underlying substrate.

References

[1] M.S. Akselrod, F.J. Bruni, “Modern trends in crystal growth and new applications of sapphire,” J. Cryst. Growth, 360, pp. 134-145, 2012.

[2] H. Okumura, “Sn and Si doping of α-Al₂O₃ (10-10) layers grown by plasma-assisted molecular beam epitaxy”, JJAP, 61, 125505, 2022.

[3] R. Jinno et al., “Crystal orientation dictated epitaxy of ultrawide-bandgap 5.4- to 8.6-eV α-(AlGa)₂O₃ on m-plane sapphire”, Sci. Adv., 7, eabd5891, 2021

[4] W. Braun and J. Mannhart, “Film deposition by thermal laser evaporation,” AIP Advances, 9, 085310, 2019.

Materials are the key components of nearly all advanced technologies, including quantum information systems, microelectronics, catalysis, and energy conversion and storage. Modern synthesis methods enable the fabrication of an ever-expanding array of novel, non-equilibrium, and/or metastable materials and composites that may possess unique and desirable functionality. Thin film deposition by molecular beam epitaxy (MBE) can produce atomically precise (or nearly so) materials with a wide range of functional electronic, magnetic, ferroelectric/multiferroic, optical, and/or ion-conducting properties. The current state of the art in precision design of functional materials is to manually explore the “growth phase space” of the deposition techniqueto optimize the film properties of interest. Limitations of time and resources often result in incomplete exploration of the growth phase space and resulting properties. Faced with this lack of complete information, materials design and synthesis decisions are made based in part on intuition and luck, slowing both materials optimization and materials discovery.This current synthesis paradigm can be disrupted by employing artificial intelligence (AI)-accelerated analysis of in situ and ex situ data streams that will enable targeted synthesis of novel materials with desired structure, chemical stability, and functional properties.Here we present a preliminary implementation of such an AI-controlled MBE.We are integrating the control of key synthesis parameters (temperatures, gas flow rates, shutters) with AI-guided computer control.Guidance will be based on near-real-time analysis of reflection high energy electron diffraction (RHEED) patterns using sparse data analytics, with low-latency feedback to the control software.As an initial demonstration, we will control the morphology and phase purity of epitaxial anatase TiO₂ thin films.

Light decreases dislocation mobility in bulk ZnS, as was recently shown through photo-indentation measurements. Here we investigate the impact of above band gap optical excitation on ZnS epilayers grown by molecular beam epitaxy (MBE) on (001) GaP. For example, optical excitation during MBE could potentially increase the critical thickness for misfit dislocation (MD) nucleation at the ZnS/GaP heterovalent interface. GaP (001) homoepitaxial buffer layers are prepared on GaP wafers, As-capped, and transferred to the chalcogenide system where ZnS is grown by compound source MBE at 150C on the As-desorbed GaP surface. Using electron channeling contrast imaging (ECCI), the MD ensembles are measured at various film thicknesses to determine the onset of MD formation. Although HRXRD shows negligible strain-relaxation in films up to 50nm, ECCI reveals that the MD nucleation processes begins at ZnS thicknesses of between 15 and 20nm, far lower than previously reported critical thicknesses. Additionally, high densities of dipole-like features appear at large densities, which appear to be dislocation loops nucleated at the surface. Image analysis is used to quantify the MD density, length, as well as the density of dipole features as a function of film thickness. We will discuss changes in the MD content with and without above band gap excitation. Additionally, post-growth strain biasing is used to increase the MD content taking advantage of the thermal mismatch via temperature cycling. Photoluminescence spectra are acquired revealing the emergence of a sub-band gap peak at 3.1 eV, which increases in intensity within ZnS epilayers upon repeated temperature cycles. We will discuss changes in the MD content and PL spectra for samples thermally cycled with and without optical excitation.