We are pleased to introduce the collection, 2022 Retrospective: Thin Solid Films. This collection aims to showcase original research and review-based papers on the most exciting developments of the past couple of years.The work presented here highlights the broad diversity of research performed across the section, and aims to put a spotlight on the main areas of interest. All research presented here seeks to display strong advances in theory, experiment, and methodology with applications to compelling problems.Authors are encouraged to showcase their most recent discoveries and to kickstart scientific discourse around them.

Highlighting research across multiple areas with compelling applications and latest discoveries.

. This research topic focuses on the development of novel methods for testing thin films and coatings, integrating metal oxides and GaAs semiconductors, and growing 2D InSe with an indium precursor layer. Highlights include: <ul> <li>Nanoindentation testing of thin films and coatings using Layer Compression Test (LCT) to emulate uniform uniaxial strain.</li> <li>Hold period and dwell time to correct for thermal drift in nanoindentation measurements.</li> <li>Thin layer of epitaxial TiO2 (110) on a GaAs (001) substrate with oxygen vacancies to inhibit diffusion of Ga and As atoms.</li> <li>Novel growth method for 2D InSe with an indium precursor layer by molecular beam epitaxy.</li> <li>Successful application of this method to a flexible substrate, such as aluminum foil.</li> </ul>

Frontiers in Materials

Thin Solid Films

2022 Retrospective: Thin Solid Films

Nanoindentation is the only way to test the local mechanical properties of thin films and coatings. Current analysis treats the measurement as a perturbation of a conventional half-space indentation, typically limiting testing to films with modulus within an order of magnitude of the supporting substrate and contact dimension much smaller than the film thickness. In the layer compression test (LCT), a flat punch is aligned and indented into the film with a contact size much greater than the film thickness. This produces a novel test condition emulating uniform uniaxial strain even with significant penetration into the film beyond plastic yield. In this work, we perform a finite element analysis to assess the quality of this approximation in the confined elastic regime of deformation up to the point of yield via a parametric study of punch radius to film thickness ratio and film to substrate modulus ratio for a simple elastic-plastic material. Our simulations were performed with a low E/Y ratio of 10, which is typical of polymers, biomaterials and other amorphous systems. We find that for substrates of sufficiently high stiffness relative to the film sample, once a simple substrate stiffness correction is performed the layer compression test load vs. displacement slope estimates the film confined modulus to within a few percent with only minor variation throughout the entire pre-yield strain region for a wide range of aspect ratios. We also present experimental layer compression test findings for a supported polymer film conducted over a contact aspect ratio range of 9–22 and discuss the trends observed relative to the simulations.

Nanoindentation is the only way to test the local mechanical properties of thin films and coatings. Current analysis treats the measurement as a perturbation of a conventional half-space indentation, typically limiting testing to films with modulus within an order of magnitude of the supporting substrate and contact dimension much smaller than the film thickness. In the layer compression test (LCT), a flat punch is aligned and indented into the film with a contact size much greater than the film thickness. This produces a novel test condition emulating uniform uniaxial strain even with significant penetration into the film beyond plastic yield. In this work, we perform a finite element analysis to assess the quality of this approximation in the confined elastic regime of deformation up to the point of yield via a parametric study of punch radius to film thickness ratio and film to substrate modulus ratio for a simple elastic-plastic material. Our simulations were performed with a low E/Y ratio of 10, which is typical of polymers, biomaterials and other amorphous systems. We find that for substrates of sufficiently high stiffness relative to the film sample, once a simple substrate stiffness correction is performed the layer compression test load vs. displacement slope estimates the film confined modulus to within a few percent with only minor variation throughout the entire pre-yield strain region for a wide range of aspect ratios. We also present experimental layer compression test findings for a supported polymer film conducted over a contact aspect ratio range of 9–22 and discuss the trends observed relative to the simulations.

The effect of contact aspect ratio and film to substrate elastic modulus ratio on stress vs. strain up to the point of yield during flat punch thin film indentation of an elastic-plastic film

Nanoindentation measurements frequently involve a hold period at the peak load to allow the material to reach a stabilized state. A dwell time is also commonly employed at a sufficiently low load after unloading, to quantify the thermal drift for correction of the raw load-displacement data. This communication presents numerical case studies of indentation loading on a viscoelastic thin film using the finite element method. We illustrate that a simple viscoelastic material can actually induce prominent drift even under a low indentation load. The drift is caused by the material itself, unrelated to the thermal drift of the instrument. The direction and magnitude of the viscoelastic drift are controlled by the prior loading/unloading history as well as the load-displacement response of the fully relaxed elastic state.

Nanoindentation measurements frequently involve a hold period at the peak load to allow the material to reach a stabilized state. A dwell time is also commonly employed at a sufficiently low load after unloading, to quantify the thermal drift for correction of the raw load-displacement data. This communication presents numerical case studies of indentation loading on a viscoelastic thin film using the finite element method. We illustrate that a simple viscoelastic material can actually induce prominent drift even under a low indentation load. The drift is caused by the material itself, unrelated to the thermal drift of the instrument. The direction and magnitude of the viscoelastic drift are controlled by the prior loading/unloading history as well as the load-displacement response of the fully relaxed elastic state.

On the Viscoelastic Drift Behavior During Nanoindentation

The integration of metal oxides and GaAs semiconductors is quite attractive for its potential applications, but interfacial diffusion and lattice mismatch usually cause huge challenges toward achieving high-performance electronic devices. In this article, we reported a thin layer of epitaxial TiO2 (110) on a GaAs (001) substrate with significant compressive strain, lattice distortion, and oxygen vacancies, where the oxygen vacancies proved to be the critical factor to induce the compressive strain and lattice distortion. In this case, the lattice mismatches between this compressed TiO2 (110) and GaAs (001) surface were calculated to be as small as 1.3 and 0.24% along the [110] and [001] orientations of TiO2, respectively. Further, no Ga-oxides or As-oxides were found at the interface, indicating that the TiO2 layer inhibited the diffusion of Ga and As atoms effectively. In summary, TiO2 film can be grown epitaxially on GaAs (001) substrates with non-negligible compressive strain, lattice distortion, oxygen vacancies, and a high-quality interface. This study also provides an approach to integrate different functional oxides on TiO2-buffered GaAs for various GaAs-based electronic devices with higher reliability and performance.

The integration of metal oxides and GaAs semiconductors is quite attractive for its potential applications, but interfacial diffusion and lattice mismatch usually cause huge challenges toward achieving high-performance electronic devices. In this article, we reported a thin layer of epitaxial TiO2 (110) on a GaAs (001) substrate with significant compressive strain, lattice distortion, and oxygen vacancies, where the oxygen vacancies proved to be the critical factor to induce the compressive strain and lattice distortion. In this case, the lattice mismatches between this compressed TiO2 (110) and GaAs (001) surface were calculated to be as small as 1.3 and 0.24% along the [110] and [001] orientations of TiO2, respectively. Further, no Ga-oxides or As-oxides were found at the interface, indicating that the TiO2 layer inhibited the diffusion of Ga and As atoms effectively. In summary, TiO2 film can be grown epitaxially on GaAs (001) substrates with non-negligible compressive strain, lattice distortion, oxygen vacancies, and a high-quality interface. This study also provides an approach to integrate different functional oxides on TiO2-buffered GaAs for various GaAs-based electronic devices with higher reliability and performance.

Formation and Effect of Deposited Thin TiO2 Layer With Compressive Strain and Oxygen Vacancies on GaAs (001) Substrate

A two-dimensional (2D) material known as indium selenide (InSe) is widely considered a promising layered semiconductor with potential applications in electronics and optoelectronics. However, the single phase of InSe is still a challenge due to the close formation energy of InSe and In2Se3. In this study, we demonstrate a novel growth method for 2D InSe with an indium precursor layer by molecular beam epitaxy. Indium pre-deposited on substrate at room temperature followed by growth of InSe at 550°C can overcome the problem of stoichiometry control and can be applied on amorphous substrate with high quality. According to Raman scattering spectra, X-ray diffraction, and high-resolution transmission electron microscopy results, we find that 2D InSe phase can be facile formed under both indium-rich and -poor conditions. The pre-deposited indium precursor effectively induces replacement with subsequent Se and In atoms to form the InSe phase while suppressing the In2Se3 phase. Additionally, this single phase InSe is stable in the atmosphere, exhibiting superior electronic properties even after over 100 days exposure. Recently, this method has been successfully applied to a flexible substrate, such as aluminum foil, resulting in reliable InSe quality. Our results demonstrate an innovative and forward-looking approach to developing 2D InSe material.

A two-dimensional (2D) material known as indium selenide (InSe) is widely considered a promising layered semiconductor with potential applications in electronics and optoelectronics. However, the single phase of InSe is still a challenge due to the close formation energy of InSe and In2Se3. In this study, we demonstrate a novel growth method for 2D InSe with an indium precursor layer by molecular beam epitaxy. Indium pre-deposited on substrate at room temperature followed by growth of InSe at 550°C can overcome the problem of stoichiometry control and can be applied on amorphous substrate with high quality. According to Raman scattering spectra, X-ray diffraction, and high-resolution transmission electron microscopy results, we find that 2D InSe phase can be facile formed under both indium-rich and -poor conditions. The pre-deposited indium precursor effectively induces replacement with subsequent Se and In atoms to form the InSe phase while suppressing the In2Se3 phase. Additionally, this single phase InSe is stable in the atmosphere, exhibiting superior electronic properties even after over 100&nbsp;days exposure. Recently, this method has been successfully applied to a flexible substrate, such as aluminum foil, resulting in reliable InSe quality. Our results demonstrate an innovative and forward-looking approach to developing 2D InSe material.