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

/pdf/ultrawide-bandgap-semiconductors-research-opportunities-and-49ev1zu2ym.pdf

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

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

A resonant circuit combining a 3D inductor with the barium strontium titanate thin film varactor, is presented in this work. The filter was fabricated using a 3D inductor fabrication process. The modeling of the filter was examined using the Advanced Design System (ADS). The measurement results showed that the resonant frequencies were around 10 GHz and correlated to different number of turns of 3D inductor. The inductances extracted from the equivalent circuit varied from 0.28 nH to 0.37 nH.

A resonant circuit realization using a 3D inductor in combination with thin film varactor technology

Nanocrystalline ZnO (nc-ZnO) thin film transistors (TFT) are being developed for applications that require much higher performance than TFTs available from other all thin film technologies. Because of their unique closely packed nanocolumnar structures, uniform quality nc-ZnO films can be fabricated over large non-conformal surfaces. Excellent performance characteristics were demonstrated with devices fabricated on GaAs1 and Si2 substrates. Devices with 2µm gate lengths had field effect mobilities in excess of 100 cm2/V.s, drain current on/off ratios of better than 1012, transconductance of 80mS/mm and current densities higher than 400mA/mm. High frequency cut-off frequency values of f T =2.45GHz and f max =7.45GHz were demonstrated with 1.2µm gate length devices.

Growth temperature influence on nanocrystalline ZnO thin film FET performance

High-Speed and Transparent Nanocrystalline ZnO Thin Film Transistors

/pdf/a-reconfigurable-cpw-bow-tie-antenna-using-an-integrated-2g3fr3b3i5.pdf

A reconfigurable CPW bow-tie antenna using an integrated ferroelectric thin film varactor

ZnO thin film electronics have received much attention due to the relatively high electron mobility of ZnO thin films in comparison to amorphous silicon (a-Si) and organic thin films. There is significant interest in using ZnO thin film transistors (TFTs), or similar oxides such as InGaZnO and zinc tin oxide, to replace a-Si TFTs in large area display technologies such as active matrix liquid crystal display (AMLCD) devices and active matrix organic light-emitting diode (AMOLED) displays where transparency in the visible range and high carrier mobilities are significant advantages. In addition, the integration of high dielectric constant (high-k) dielectrics in ZnO TFTs has demonstrated performance advantages including reduced operating voltage, increased I on /I off ratios, and larger transconductance. HfO 2 has emerged as a high-k dielectric of choice for both silicon microelectronics and thin film electronics due to the high dielectric constant (e r ∼ 25e 0 ), low leakage current, and low synthesis temperature.

Kevin D. Leedy

Papers

A resonant circuit realization using a 3D inductor in combination with thin film varactor technology

Growth temperature influence on nanocrystalline ZnO thin film FET performance

High-Speed and Transparent Nanocrystalline ZnO Thin Film Transistors

A reconfigurable CPW bow-tie antenna using an integrated ferroelectric thin film varactor

Illumination instability analysis of ZnO thin film transistors with HfO 2 gate dielectrics