Atomic layer deposition: an overview.

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

Recent advances in ZnO nanostructures and thin films for biosensor applications: review.

Carbon nanotubes (CNTs) have been the focus of significant Nature article. CNTs have primarily been grown on flat substrates for a range of potential applications due to their superior mechanical, electrical and thermal properties. However, limited research has been accomplished on open cell foam structures. Initial attempts to grow CNTs onto as-received foam samples resulted in low CNTs growth. This then prompted surface treatment research to enhance CNTs growth onto the foam surface. For this study, the surface modification methods of interest included: nitric acid treatment, oxidation, coating with hexamethyldisilazane (HMDS), and atomic layer deposition (ALD) of oxides. Dense, aligned CNTs were grown on all surfaces of 3dimensional open cell reticulated carbon foam. The best results were achieved when a thin layer of Al2O3 was deposited on the foam via ALD prior to CNT growth. The foam samples were 1cm x 1 cm x 0.5cm, with CNTs found throughout the volume of the sample and not just the outer layers of the foam. Highly aligned CNTs were realized using floating catalyst chemical vapor deposition (FCT-CVD) with xylene and ferrocene as the carbon and catalyst sources, respectively. Growing CNTs on a 3-dimensional foam structure will dramatically increase the surface area available for heat dissipation, thus enhancing the thermal properties of the foam, as suggested by initial thermal testing. Preliminary results include scanning electron microscope images and thermal conductivity results.

https://briefs.techconnect.org/wp-content/volumes/Nanotech2013v1/pdf/542.pdf

The Effects of Surface Treatments on the Growth of CNTs on 3-Dimensional Carbon Foam Structures

Deep level defects and their instability in PLD-grown IGZO (In 2 Ga 2 Zn 5 O 11 ) thin films studied by thermally stimulated current spectroscopy

Novel Barium Strontium Titanate (BST) thin film varactors with parallel capacitor architecture were designed fabricated and tested. Varactors with 2–6 parallel capacitors were experimentally verified using large area processed bst thin films. The multicapacitor varactor shunt switches provided improved isolation at the expense of higher insertion loss. The multicapacitor varactors yielded reduced overall parasitic series inductance and resistances, resulting in higher quality factor Q. This is the first time such a device is reported.

Novel Multi-Capacitor Architecture for BST Thin Film Varactors

A method is provided for fabricating a thin film transistor. An insulating and a metal gate contact layer are deposited on a substrate with the insulating layer being positioned between the gate contact layer and the substrate. A portion of the gate contact layer is selectively removed utilizing reactive ion etching incorporating a gas that etches the gate contact layer but not the insulating layer. A plurality of layers is deposited over a remaining portion of the gate contact layer and insulating layer, which include a gate insulating layer, a channel layer, and a metal film. A portion of the metal film is selectively removed utilizing reactive ion etching incorporating the gas that etches the metal film but not the channel layer. The insulating layer includes a high resistivity insulator that can be deposited at temperatures less than 400° C. and the channel layer is comprised of a metal oxide semiconductor.

Metal oxide thin film transistor fabrication method

The optical properties of lanthanum nickelate (LNO) films grown by chemical solution decomposition on sapphire substrates were measured and extracted. The relatively low imaginary part of the dielectric function indicates that LNO may perform well in plasmonic applications at traditional telecommunication frequencies.

Kevin D. Leedy

Papers

The Effects of Surface Treatments on the Growth of CNTs on 3-Dimensional Carbon Foam Structures

Deep level defects and their instability in PLD-grown IGZO (In 2 Ga 2 Zn 5 O 11 ) thin films studied by thermally stimulated current spectroscopy

Novel Multi-Capacitor Architecture for BST Thin Film Varactors

Metal oxide thin film transistor fabrication method

Optical Transmittance and Reflectance of Lanthanum Nickelate at Telecommunication Frequencies