ALD/ALE 2021 Session AM4: ALD on Particles

Nanoparticle-based thin films are gaining interest in recent decades for catalysis, electronic or optical applications. However, their poor mechanical reliability and durability limit their industrial and commercial applications.

In this work, we report on a low temperature hybrid synthesis approach to insure the durability and encapsulation of photonic conversion films for its further integration in silicon solar cells. The proposed hybrid approach consists first on the synthesis of rare earths (Erbium and Ytterbium) codoped TiO₂ nanoparticles (size ~ 11 nm) fabricated by hydrothermal-assisted sol-gel method. Then, the nanoparticles are dispersed at the surface of silicon substrates using a spin-coating process (film thickness 750-900 nm). Finally, Er³⁺-Yb³⁺codoped TiO₂ upconversion nanoparticles in the powder phase are coated with an amorphous Al₂O₃ layer using atomic layer deposition technique (ALD) as a unique approach to reinforce mechanical properties of various photoluminescent nanoparticle porous thin films at a relatively low temperature without drastically changing their original structure.

The impact of different ALD-Al₂O₃ thicknesses and forming gas annealing step on the structural, compositional and mechanical properties ofEr³⁺-Yb³⁺ codoped TiO₂ nanoparticles assembled on n-type Si (100) substrate are investigated. For that purpose, various analysis approaches involving automated crystal phase and orientation mapping in TEM (ACOM-TEM), scanning transmission electron microscopy–energy-dispersive spectroscopy (STEM-EDS), scratch test and nanoindentation are performed to understand the links between the structural properties and the mechanical properties.

The results indicate a clear increase in the adhesion of nanoparticle thin films to the Si substrate with increasing ALD-Al₂O₃ thickness followed by forming gas annealing treatment. This behavior is a consequence of the infiltration of Al₂O₃ layer within the porous matrix, as a result, effective interparticle bonding and filling of the pores are accomplished.

ZnSe embedded chalcogenide glass (ChG) composites are optically active in the mid-infrared (IR) and are ideal for niche applications in medical diagnosis, military missile heat-sensing, and chemical identification. The embedded ZnSe crystals play a critical role in providing lasing activity while the ChG - As₂S_3-xS_ex matrix, acts as a host that can be drawn into fiber and provides the necessary mid-IR transparency. However, during synthesis, the ZnSe particles are prone to dissolution in the As₂S_3-xS_ex matrix due to a high temperature (650 C) melting and homogenization process. Here we demonstrate that a thin amorphous barrier layer of Al₂O₃ deposited using atomic layer deposition (ALD) on ZnSe microparticles 5-10 μm in average diameter is successful to prevent ZnSe dissolution in the As₂S_3-xS_ex matrix even after extended melting and homogenization at 650 C for 8 hours. This talk will highlight some of the key process innovations required to overcome challenges of ALD on powdered components for functionalizing and enhancing performance of multiphasic glasses for optical applications.

Amorphous aluminum oxide (Al₂O₃) was deposited using alternate pulses of trimethyl aluminum and H₂O on 1 gm batch size ZnSe using a customized rotary barrel reactor inside a viscous-flow ALD furnace attached to a quadrupole mass spectrometer (QMS). The temperature of deposition was kept at 180 C and the number of cycles were varied from 100 to 300 cycles. The ZnSe particles were sieved to an average size of 5-10 μm and a maximum size ≤ 20 μm. The coated particles were characterized by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The crystallinity of the ZnSe-As₂S_3-xS_ex glasses were evaluated using x-ray diffraction (XRD) and their composition stability spatially mapped across individual ZnSe particles embedded in As₂S_3-xS_ex matrix using Raman microspectroscopy.

Whereas 100 ALD cycles provide partial protection to dissolution for the ZnSe in As₂S_3-xS_ex, a 300 cycle ALD Al₂O₃ on ZnSe particles is sufficient to provide phase and composition stability to the ZnSe embedded As₂S_3-xS_ex glass. Through our investigation, we exemplify the efficacy and potential of ALD of ZnSe powder in improving the stability of optical materials while still achieving optimal mid-IR lasing and transparency.