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.

Influence of oxide buffer layers on the growth of carbon nanotube arrays on carbon substrates

Physical and electrical processes involved in the lifecycle failure of metal contacts in MEMS RF cantilever beam contact switches are of a great interest to switch designers. The main failure of MEMS contact switches occurs at the metal contacts. This paper describes a specially designed nanoindenter based experimental setup for characterizing the physics and mechanics of MEMS scale electrical contacts when they are being cycled. The setup uses silicon cantilevers with contact bumps which are cycled mechanically to simulate the action of a MEMS contact switch. The cantilevers were sputter deposited with 300 nm of gold. Gold was investigated as the initial material of interest, as many current MEMS contact switches employ gold as a contact metal. The setup allows the physical evolution of the contact to be visually tracked and provides data on resistance, material property (e.g. strain hardening), and other changes in the contact behavior. Test data collected during a typical test includes contact adhesion force, contact stiffness change, resistance vs. cycles, and resistance vs. contact force relationships as a function of applied contact cycles. This paper provides details of this unique set-up and typical test results.

A Nanoindenter Based Method for Studying MEMS Contact Switch Microcontacts

Beta-phase gallium oxide (Ga 2 O 3 ) is a promising wide bandgap semiconductor possessing a larger bandgap (∼4.8 eV) and critical electric field strength (∼8 MV/cm) than GaN and SiC [1]. Early metal-oxide-semiconductor field-effect transistors (MOSFETs) have shown promising depletion-mode operation with high critical field strength [2], high-current density [3] and high breakdown voltage [4]. On native substrates, enhancement-mode operation has been achieved with a gated unintentional doped channel [5] and fin-channels [6], the latter achieving 600-V normally-off breakdown voltage. However, both are limited to ∼1 mA/mm or less. Here, we report a new enhancement-mode device achieved by gate recess with improved drain-current > 20 mA/mm and near 200-V breakdown for laterally scaled 3-μm source-drain distance (L sd ).

Gate-recessed, laterally-scaled β-Ga 2 O 3 MOSFETs with high-voltage enhancement-mode operation

Fatigue and crack growth in copper alloy–stainless steel bi-layer panels joined by hot isostatic pressing or explosive bonding were tested over the temperature range 25–225°C. Several material combinations under consideration for fusion reactor first wall applications were studied: solid plates of precipitation strengthened CuNiBe or dispersion strengthened CuAl 2 O 3 bonded to a 316L stainless steel plate and Cu–Al 2 O 3 in a powder form consolidated and bonded concurrently to a 316L stainless steel plate. Fatigue and crack growth were measured perpendicular to the interface through the copper alloy. Crack growth was also measured parallel to the interface along the bond line. The solid Cu–Al 2 O 3 –stainless steel laminate exhibited the best perpendicular crack growth resistance and fatigue performance of the bonded panels due primarily to the elongated Cu–Al 2 O 3 grains oriented perpendicular to the crack propagation direction. Before reaching the interface cracks propagating perpendicular to the interface caused delamination in most panels near the copper alloy–stainless steel bond line within the copper alloy. All bi-layer panels displayed minimal resistance to parallel crack growth.

Copper alloy–stainless steel bonded laminates for fusion reactor applications: crack growth and fatigue

A latching capacitive RF MEMS switch has been successfully designed, fabricated and tested. The switch uses a long thin metal cantilever which is electrostatically held to an upper electrode at the free end and to a lower electrode at the beam root end forming an S-shaped actuator. The upper electrode sits on top of a thin dielectric shell which also serves as part of the package for the device. Slight dielectric charging holds the cantilever in either the on-state or off-state between switching pulses. In the latched states with no bias, insertion loss is 0.2 dB at 10 GHz and isolation is >20 dB in a narrow band around 10 GHz. Repeatable switching has been demonstrated for actuation voltages below 20 V. Hot-switched power handling has been tested up to 6 W at 10 GHz without failure.

Kevin D. Leedy

Papers

Influence of oxide buffer layers on the growth of carbon nanotube arrays on carbon substrates

A Nanoindenter Based Method for Studying MEMS Contact Switch Microcontacts

Gate-recessed, laterally-scaled β-Ga 2 O 3 MOSFETs with high-voltage enhancement-mode operation

Copper alloy–stainless steel bonded laminates for fusion reactor applications: crack growth and fatigue

A Latching Capacitive RF MEMS Switch in a Thin Film Package