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1. Band alignment of sputtered and atomic layer deposited SiO2 and Al2O3 on ScAlN

   https://doi.org/10.1063/5.0131766  

   Xia, Xinyi; Li, Jian-Sian; Khan, Md Irfan; Khan, Kamruzzaman; Ahmadi, Elaheh; Hays, David C.; Ren, Fan; Pearton, S. J. (December 2022, Journal of Applied Physics)

   The band alignments of two candidate dielectrics for ScAlN, namely, SiO2 and Al2O2, were obtained by x-ray photoelectron spectroscopy. We compared the effect of deposition method on the valence band offsets of both sputtered and atomic layer deposition films of SiO2 and Al2O3 on Sc0.27Al0.73 N (bandgap 5.1 eV) films. The band alignments are type I (straddled gap) for SiO2 and type II (staggered gap) for Al2O3. The deposition methods make a large difference in relative valence band offsets, in the range 0.4–0.5 eV for both SiO2 and Al2O3. The absolute valence band offsets were 2.1 or 2.6 eV for SiO2 and 1.5 or 1.9 eV for Al2O3 on ScAlN. Conduction band offsets derived from these valence band offsets, and the measured bandgaps were then in the range 1.0–1.1 eV for SiO2 and 0.30–0.70 eV for Al2O3. These latter differences can be partially ascribed to changes in bandgap for the case of SiO2 deposited by the two different methods, but not for Al2O3, where the bandgap as independent of deposition method. Since both dielectrics can be selectively removed from ScAlN, they are promising as gate dielectrics for transistor structures. 

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2. Low-temperature growth of Al-doped ZnO by atomic layer deposition for plasmonics

   Dhruv Fomra, Kai Ding (March 2020, Proceedings Volume 11281, Oxide-based Materials and Devices XI)

   Transparent conducting oxides, such as Ga-doped ZnO (GZO) and Al-doped ZnO (AZO) are attractive materials for high-performance plasmonic devices operating at telecommunication wavelengths. In this contribution, we compare the growth of epsilon-near-zero GZO and AZO films on sapphire by two different deposition techniques: molecular beam epitaxy (MBE) and atomic layer deposition (ALD). For MBE of GZO, a multiple buffer consisted of a high-temperature MgO layer, a low-temperature ZnO, followed by a high-temperature ZnO layer is employed to assure the crystalline quality of the GZO film. By controlling the growth parameters, including Ga doping level, VI/II ratio, substrate temperature, we are able to produce GZO films at 350 °C with electron mobility between 30 and 50 cm2/V.s, electron concentration up to 7×1020 cm-3, and resistivity down to 2.5×10-4 Ω.cm. For ALD of AZO, without using any buffer, by reducing the Al pulse duration, we are able to grow the AZO films under a large ratio of Al to Zn pulses of 1:6, which improves the activation of Al as an effective dopant. Hence AZO films with electron concentration above 7×1020 cm-3, electron mobility between 10 and 20 cm2/V.s, and resistivity below 6×10-4 Ω.cm have been obtained at 250 °C. The corresponding epsilon-near-zero point in the ALD-grown material was tuned down to 1470 nm. Our data indicate that the ALD method provides a low-temperature route to plasmonic TCOs for telecommunication wavelength range. Effect of electron mobility on optical loss and, therefore, plasmonic figure of merit is discussed. 

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3. Al:ZnO as a platform for near-zero-index photonics: enhancing the doping efficiency of atomic layer deposition

   https://doi.org/10.1364/OME.409347  

   Fomra, D.; Ding, K.; Avrutin, V.; Özgür, Ü.; Kinsey, N. (November 2020, Optical Materials Express)

   Major technological breakthroughs are often driven by advancements in materials research, and optics is no different. Over the last few years, near-zero-index (NZI) materials have triggered significant interest owing to their exceptional tunability of optical properties and enhanced light-matter interaction, leading to several demonstrations of compact, energy-efficient, and dynamic nanophotonic devices. Many of these devices have relied on transparent conducting oxides (TCOs) as a dynamic layer, as these materials exhibit a near-zero-index at telecommunication wavelengths. Among a wide range of techniques employed for the deposition of TCOs, atomic layer deposition (ALD) offers advantages such as conformality, scalability, and low substrate temperature. However, the ALD process often results in films with poor optical quality, due to low doping efficiencies at high (>10²⁰cm⁻³) doping levels. In this work, we demonstrate a modified ALD process to deposit TCOs, taking Al:ZnO as an example, which results in an increase in doping efficiency from 13% to 54%. Moving away from surface saturation for the dopant (aluminum) precursor, the modified ALD process results in a more uniform distribution of dopants (Al) throughout the film, yielding highly conductive (2.8×10⁻⁴Ω-cm) AZO films with crossover wavelengths as low as 1320nm and 1370nm on sapphire and silicon substrates, respectively. 

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4. Band gaps of crystalline solids from Wannier-localization–based optimal tuning of a screened range-separated hybrid functional

   https://doi.org/10.1073/pnas.2104556118  

   Wing, Dahvyd; Ohad, Guy; Haber, Jonah B.; Filip, Marina R.; Gant, Stephen E.; Neaton, Jeffrey B.; Kronik, Leeor (August 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Accurate prediction of fundamental band gaps of crystalline solid-state systems entirely within density functional theory is a long-standing challenge. Here, we present a simple and inexpensive method that achieves this by means of nonempirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron from an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band-gap semiconductors to large band-gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 electronvolts (eV), and is found to yield quantitative accuracy across the board, with a mean absolute error of ∼0.1 eV and a maximal error of ∼0.2 eV. 

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5. Review Article: Atomic layer deposition of doped ZnO films

   https://doi.org/10.1116/1.5112777  

   Gao, Zhengning; Banerjee, Parag (August 2019, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films)

   This article reviews the process-structure-property relationship in doped ZnO thin films via atomic layer deposition (ALD). ALD is an important manufacturing-scalable, layer-by-layer, thin film deposition process that precisely controls dopant type and concentration at the nanoscale. ZnO is an important technological material, which can be doped to modulate structure and composition to tailor a wide variety of optical and electronic properties. ALD doped ZnO is viewed as a transparent conducting oxide for application in solar cells, flexible transparent electronics, and light-emitting diodes. To date, there are 22 elements that have been reported as dopants in ZnO via ALD. This article studies the underlying trends across dopants and establishes generalized relationships for (1) the role of ALD process parameters, (2) the impact of these parameters on the structure of the ZnO matrix, and (3) the impact of dopants on the optical and electrical properties. The article ends with a brief discussion on the limitations of the ALD-based doping scheme, knowledge gaps in the compositional maps, and a perspective on the future of ALD doped ZnO films. 

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