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.

A method is provided for fabricating a thin film transistor. A plurality of layers is deposited on a substrate. The plurality of layers includes a conductive gate contact layer, a gate insulator layer, an undoped channel layer, an etch-stop layer, and a conductive contact layer. The etch-stop layer is positioned between the conductive contact layer and the undoped channel layer. A portion of the conductive contact layer is selectively removed while removal of a portion of the undoped channel layer is prevented by the etch-stop layer during the selective removal. A portion of the etch-stop layer is selectively removed and an exposed portion of the etch-stop layer is converted from a conductor to an insulator by oxidizing the exposed portion of the etch-stop layer in air. A portion of remaining layers of the plurality of layers is selectively removed to form the thin film transistor.

Fabrication method for ZnO thin film transistors using etch-stop layer

We obtain room-temperature resistivities as low as ρ =1.4 x 10 -4 Ω-cm in transparent Ga-doped ZnO
grown on Al 2 O 3 by pulsed laser deposition (PLD) at 200 °C in 10 mTorr of pure Ar and then
annealed in a Zn enfivironment. Donor N D and acceptor N A concentrations are calculated
from a recently developed scattering theory that is valid for any degenerate semiconductor material
and requires only two input parameters, mobility μ and carrier concentration n measured at any
temperature in the range 5 - 300 K. By comparison with SIMS and positron annihilation
measurements, it has been shown that the donors in these samples are mostly Ga Zn , as expected, but
that the acceptors are point defects, Zn vacancies V Zn . PLD growth in Ar at 200 °C produces a high
concentration of donors [Ga Zn ] = 1.4 x 10 21 cm -3 , but V Zn acceptors are produced at the same time,
due to self-compensation. Fortunately, a large fraction of the V Zn can be eliminated by annealing in a Zn environment. The theory gives N D and N A , and thus [Ga Zn ] and [V Zn ], at each step of
the growth and annealing process. For convenience, the theory is presented graphically, as plots of μ
vs n at various values of compensation ratio K = N A /N D . From the value of K corresponding to the
experimental values of μ and n, it is possible to calculate N D = n/(1 - K) and N A = nK/(1 - K).

Making Highly Conductive ZnO: Creating Donors and Destroying Acceptors

This work investigates properties of surface plasmons on doped metal oxides in the 2-20 µm wavelength regime. By varying the stoichiometry in pulse laser deposited Ga and Al doped ZnO, the plasmonic properties can be controlled via a fluctuating free carrier concentration. This deterministic approach may enable one to develop the most appropriate stoichometry of ZnAlO and ZnGaO in regards to specific plasmonic applications for particular IR wavelengths. Presented are theoretical and experimental investigations pertaining to ZnAlO and ZnGaO as surface plasmon host materials. Samples are fabricated via pulsed laser deposition and characterized by infrared ellipsometry and Hall-effect measurements. Complex permittivity spectra are presented, as well as plasmon properties such as the field propagation lengths and penetration depths, in the infrared range of interest. Drude considerations are utilized to determine how the optical properties may change with doping. Finite element simulations verify these plasmonic properties. These materials not only offer potential use as IR plasmon hosts for sensor applications, but also offer new integrated device possibilities due to stoichiometric control of electrical and optical properties.

Mid- to long-wavelength infrared surface plasmon properties in doped zinc oxides

Experimental results pertaining to plasmon resonance tunneling through a highly conductive zinc oxide (ZnO) layer with subwavelength hole-arrays is investigated in the mid-infrared regime. Gallium-doped ZnO layers are pulsed-laser deposited on a silicon wafer. The ZnO has metallic optical properties with a bulk plasma frequency of 214 THz, which is equivalent to a free space wavelength of 1.4 μm. Hole arrays with different periods and hole shapes are fabricated via a standard photolithography process. Resonant mode tunneling characteristics are experimentally studied for different incident angles and compared with surface plasmon theoretical calculations and finite-difference time-domain simulations. Transmission peaks, higher than the baseline predicted by diffraction theory, are observed in each of the samples at wavelengths that correspond to the excitation of surface plasmon modes.

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Investigation of plasmon resonance tunneling through subwavelength hole arrays in highly doped conductive ZnO films

The effects of point and extended defects on concentration n and mobility μ are studied in thin films of Ga-doped ZnO (GZO) grown by pulsed laser deposition on quartz or ZnO itself. The most important defects in the bulk are point defects, mainly Zn vacancies, and their concentrations are similar in GZO/quartz and in GZO/ZnO. On the other hand, the dominant defects at the interface are extended defects, and their concentrations are much higher in GZO/quartz than in GZO/ZnO. Consequently, the mobility in GZO/quartz is lower and more thickness-dependent than that in GZO/ZnO. The effects of point and extended defects can be mitigated by annealing on Zn foil, and by use of buffer layers, respectively.

Kevin D. Leedy

Papers

Fabrication method for ZnO thin film transistors using etch-stop layer

Making Highly Conductive ZnO: Creating Donors and Destroying Acceptors

Mid- to long-wavelength infrared surface plasmon properties in doped zinc oxides

Investigation of plasmon resonance tunneling through subwavelength hole arrays in highly doped conductive ZnO films

Defects in highly conductive ZnO for transparent electrodes and plasmonics