PCSI2025 Session PCSI-TuM1: Oxide Semiconductor Materials I

Gallium oxide (Ga₂O₃) is rapidly developing as one of the most promising ultra-wide bandgap platforms for next-generation power electronics owing to properties like a high breakdown field, controllable (n-type) electrical conductivity, and commercially-available single-crystal substrates that can be grown via a number of industrially-scalable processes. Beyond exhibiting a number of polymorphs with similarly attractive properties, alloying with Al into Ga₂O₃ to form (Al_xGa_1-x)₂O₃ (AGO) alloys can lead to a significant increase of the band gap to potentially access higher power device figures of merit, analogous to the AlGaN system but spanning a much larger rage of ~4.8 eV-8.6 eV. Despite the progress with Ga₂O₃ and related alloys, a number of open questions remain on the nature of fundamental defects and the role of impurities and dopants in determining the observed optical and electrical properties of these materials. This is even more of an open question in the lesser studied alloys like AGO, particularly regarding the effectiveness of donor doping and how to overcome the possibility of compensation in the limit high Al-contents. In this work we survey the current understanding of point defects in Ga₂O₃, focusing on their potential optical and electrical consequences from insights gained through first-principles-based calculations employing hybrid functionals. We discuss what is known about donor and acceptor dopants, as well as their interactions with native defects and impurities incorporated through growth and processing steps. These results provide guidance for controlling defect populations and the electrical conductivity in Ga₂O₃ and related alloys and for facilitating next-generation power electronics based on this ultra-wide bandgap semiconductor family.

This work was partially performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory (LLNL) under contract DE-AC52-07NA27344 and partially supported by Laboratory Directed Research and Development funding under project number 22-SI-003 and by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. DOE EERE AMMTO.

We present the structural and electronic properties of Yb-doped SrTiO₃/Si(001) grown by molecular beam epitaxy. Other rare-earth dopants that result in n-type conductivity in the ABO₃ perovskite lattice typically substitute for Sr at the A-sites. In contrast, Yb is shown to substitute predominantly for Ti at the perovskite B-sites based on data from atomically resolved scanning transmission electron microscopy and energy dispersive spectroscopy, as well as extended x-ray absorption fine structure measurements. An atom beam flux (f) mismatch was present during film deposition because it was assumed that Yb would occupy A-sites. As a result, the fluxes were set such that f_Yb + f_Sr = f_Ti. The formation of Yb_Ti rather than Yb_Sr results in Sr vacancies and extraneous (i.e. non-lattice) Ti atoms in the films. Yb exhibits two distinct charge states as determined by x-ray absorption spectroscopy and associated theoretical modeling, +2.7 and +2.1. These aliovalent dopants are compensated by donor electrons from oxygen vacancies that form during epitaxial film growth. The defect complexes resulting from the flux mismatch, together with oxygen vacancies, lead to deep-level electron traps that were detected by resonant photoemission and predicted to be stable by ab initio theory, as well as much higher sheet resistance than that associated with, for instance, La-doped SrTiO₃ films. Ab initio calculations show that the preference for B-site occupancy is driven by low oxygen chemical potential at the growth front as required to successfully deposit epitaxial STO on Si without amorphous SiO₂ formation at the interface.

Zinc oxide (ZnO) and strontium titanate (SrTiO₃) have tunable optical constants that can be useful for optoelectronic applications. We study a third oxide, tin oxide (SnO₂), and investigate if its optical constants are also tunable. SnO₂ is a wide band gap semiconductor with attractive electrical, optical, etc properties. It is commonly used in gas sensors, solar cells, batteries, and energy-saving coatings. SnO₂ thin films were grown on a Si substrate using atomic layer deposition.

X-ray reflectivity (XRR) was performed for each SnO₂ thin film on Si. It was found that the SnO₂ thin film thicknesses are around 10-60 nm with an approximately 2 nm thick SiO₂ interfacial layer due to the wet cleaning before the deposition. We measured the ellipsometric angles ψ and Δ of SnO₂ thin films on Si at room temperature on a J. A. Woollam UV ellipsometer at 65° to 75° angles of incidence from 0.5 to 6.5 eV. We used a Tauc Lorentz oscillator to model the data for SnO₂ thin films. After obtaining a reasonable fit for SnO₂, we also performed a uniqueness fit for the film thickness. We compared the film thickness from all three techniques; XRR, Tauc Lorentz fit, and uniqueness fit. Finally, we extracted the dielectric function (ε) of the SnO₂ thin film from the obtained Tauc-Lorentz fit and compared it with all samples. The results show that the dielectric function (real ε₁ and imaginary ε₂ part) varies with thickness by no more than 15%. This observation is probably because of the weaker excitonic effects in SnO₂ or because the excitonic effects are not screened as for ZnO or SrTiO₃.

Vanadium dioxide (VO₂) is a popular thermochromic material owing to its potential applications in intelligent windows, sensors, and diverse electronic devices. VO₂ displays a distinctive metal-insulator transition (MIT) that occurs around 68°C temperature, which is accompanied by a structural transition and a dramatic change in the optical properties [1]. During this transition, the material shifts between its low-temperature insulating (transparent to near infrared rays) monoclinic phase (M1) and its high-temperature metallic (opaque to near infrared rays) rutile phase (R). From application perspective, it is highly imperative to stabilize VO₂ thin films with control on the phase transition properties. Ion-implantation is one of the effective route to tune the properties of VO₂ thin films [2, 3]. In the present work, we have studied B+ ion implanted VO₂ thin films.

Thin films of VO₂ were prepared by radio frequency (RF) magnetron sputtering technique. Apart from the pristine sample, three samples were implanted by B+ ions at different fluence ranging 5 × 10¹³ ions/cm² to 5 × 10¹⁴ ions/cm². B+ ions implanted VO₂ thin films revealed modified structural, electrical and optical properties. Structural phase transition and MIT temperatures were found to decrease with increasing implantation dose. Moreover, for ion-implanted VO₂ thin films, temperature dependent transmittance measurements depicted the shift in optical transition towards room temperature side. Such control of phase transition in VO₂ thin films is very crucial for device applications.

[1] Manish Kumar, S. Rani, J.P. Singh, K. H.Chae, Y. Kim, J.Park, and H. H. Lee, Appl. Surf. Sci. 521, 147093 (2020).

[2] Manish Kumar, S. Singh, W. C. Lim, K. H. Chae, H. H. Lee, Mater. Lett. 310, 131438 (2022).

[3] Qi Jia, Jörg Grenzer, Huabing He, Wolfgang Anwand, Yanda Ji, Ye Yuan, Kai Huang, Tiangui You, Wenjie Yu, Wei Ren, Xinzhong Chen, Mengkun Liu, Stefan Facsko, Xi Wang, Xin Ou Adv. Mater. Interfaces 5(8), 1701268 (2018).

Transparent conductive films (TCF) are essential for optoelectronic devices, such as transparent electrodes for light-emitting diodes and solar cells. Transparent thin films of indium tin oxide (ITO) and fluorine tin oxide (FTO) on glass substrates are the most widely used for these purposes [1]. These typical TCO thin films provide an electrical resistivity of 10⁻³ Ω·cm and a high transmittance of over 80% in the visible-light region. Carbon nanotubes (CNTs)/inorganic composites have received much attention due to their optical, mechanical, electrical, and thermal properties. However, the poor adhesion of CNTs onto the substrate becomes a problem during the usage. SiO₂ is a highly transparent insulating material in the UV, visible, and infrared regions. Therefore, if conductivity can be added to SiO₂, it will become a new transparent conductive film with high adhesion. The single-walled carbon nanotube (SWCNT)-silica composite thin film on a quartz glass was formed by ultraviolet irradiation (20–40 °C) onto a spin-coated precursor film. With 7.4 mass% SWCNTs, the electrical resistivity reached 7.7 × 10⁻³ Ω·cm after UV irradiation. The transmittance was >80% at 178–2600 nm and 79%–73% at 220–352 nm. Heat treatment increased the transparency and pencil hardness without affecting the low electrical resistivity. Raman spectroscopy and microscopic analyses revealed excellent film morphology with good SWCNT dispersal. The low refractive index (1.49) and haze value (<1.5%) are invaluable for transparent windows for novel optoelectronic devices. Herein, we also report a promising composite thin film as a transparent and conductive material on PET (polyethylene terephthalate) substrates for flexible transparent conductive films.

The potential for next-generation electronic applications has expanded significantly with the development of energy-efficient, high-performance broadband photodetectors utilizing cost-effective amorphous metal oxide semiconductors. Current commercially available photodetectors use various semiconductors to detect light across different wavelengths, from ultraviolet (UV) to near-infrared (NIR). However, their versatility is limited by the need for specific materials to target different wavelengths. This study explores the use of indium gallium zinc oxide (IGZO) as a metal oxide semiconductor, eliminating the need for additional external photo absorption layers. To enhance charge carrier generation and create subgap states in the IGZO film, hydrogen (H2) plasma treatment was applied, enabling wide-spectrum detection from UV to NIR without extra layers. Furthermore, a ferroelectric and high-k dielectric was introduced as the gate dielectric to induce a strong electric field in the channel, resulting in low-power operation. The H2 plasma-treated IGZO phototransistors demonstrated ultra-high photoresponsivity and detectivity over a broad range of wavelengths (400 to 1000 nm), making them a promising candidate for next-generation optoelectronic devices. This study presents a favorable approach for advancing energy-efficient, cost-effective, and high-performance broadband photodetectors.

The vulnerability of solution-processed metal-oxide semiconductors is significantly influenced by organic chemical-induced defects such as voids, holes, and organic residues. Moreover, solution-processed Oxide TFTs require a high temperature process to vaporize various organic compounds. Hydroxyl radicals (OH•), known as strong oxidants, are effective in eliminating and decomposing organic compounds.

In this study, we introduced UV-DI with hydroxyl radicals into the IGZO Sol-Gel mixture to produce lower boiling points components and deposit IGZO active layers with fewer defects. Hydroxyl radicals were generated in deionized (DI) water through an O₃/UV process, confirmed via potassium iodide (KI) and ultraviolet-visible (UV-vis) spectroscopy analysis. The intensity of the absorbance peaks at wavelengths of 290 nm and 350 nm increased with longer UV irradiation times. Additionally, we found that the density of hydroxyl radicals in DI water could be modified by adjusting the initial volume of DI water while maintaining the same UV irradiation duration. A DI water volume of 4.5 ml resulted in a higher concentration of hydroxyl radicals compared to 1.5 ml of DI water. Thermogravimetric and differential scanning calorimetry (TG-DSC) analysis revealed that the organic materials in the IGZO solution mixture with UV-DI treatment began to decompose at a lower temperature (121.6℃) than those in the pristine IGZO mixture (144.5℃). An abrupt weight loss was also observed in the IGZO solution with UV-DI compared to the pristine IGZO solution.The field-effect mobility and sub-threshold slope of the a-IGZO TFTs made with ozonated water were improved compared to the conventional process, increasing from 1.37 to 1.44 cm^2/V·s and decreasing from 0.24 to 0.20 V/dec, respectively. In addition, threshold voltage shifts over 7000 seconds under negative-bias stress conditions (NBS) were -7.25 V and -5.67 V for pristine IGZO and IGZO with UV-DI, respectively.

These results suggest that the addition of ozonated water to the sol-gel mixture is a simple method to achieve high-performance TFTs by reducing organic chemical-induced defects through low-temperature processing.

Indium gallium zinc oxide (IGZO) is regarded as a highly promising material in semiconductor applications such as thin-film transistors (TFTs) and memory devices, due to its excellent properties, including low leakage current and high electron mobility [1,2]. However, there are still unresolved issues with IGZO, one of which is the high contact resistance between IGZO and source/drain electrodes, which significantly affects the performance of miniaturized transistor devices. Therefore, understanding and improving the contact resistance of IGZO is essential [3]. Various factors influence the contact resistance between IGZO and metals, but the metal's work function and the interaction between the metal and IGZO are particularly critical [4,5].

In this study, various thin metal films with a thickness of less than 5 nm were inserted between the electrodes and IGZO in transmission line model (TLM) devices to investigate the effect of metal oxide formation on contact resistance (fig.1). Based on these results, we analyzed how the metal's work function, oxidation tendency, and the band structure of metal oxides influence the contact properties with IGZO, and proposed strategies to effectively reduce contact resistance. These findings are expected to serve as valuable foundational data for improving the performance of various IGZO-based semiconductor devices.

[1] K. Ide, et al., Phys. Status Solidi A 216, 1800372 (2019).

[2] A. Belmonte, et al.,IEEE Int. Electron Devices Meeting (IEDM) (2020).

[3] D. Ha, et al., IEEE Int. Memory Workshop (IMW) (2024).

[4] T. T. Trinh, et al., Mater. Sci. Semicond. Process. 38, 50-56 (2015).

[5] D.-H. Lee, et al., ACS Appl. Electron. Mater. 4, 6215-6228 (2022).

⁺Author for correspondence: mjs1748@g.skku.edu