scispace - formally typeset
Search or ask a question
Author

Marko J. Tadjer

Bio: Marko J. Tadjer is an academic researcher from United States Naval Research Laboratory. The author has contributed to research in topics: Diamond & Breakdown voltage. The author has an hindex of 32, co-authored 189 publications receiving 4808 citations. Previous affiliations of Marko J. Tadjer include Technical University of Madrid & University of Maryland, College Park.


Papers
More filters
Journal ArticleDOI
TL;DR: 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 in this article.
Abstract: 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.

1,535 citations

Journal ArticleDOI
TL;DR: The UWBG semiconductor materials, such as high Al‐content AlGaN, diamond and Ga2O3, advanced in maturity to the point where realizing some of their tantalizing advantages is a relatively near‐term possibility.
Abstract: 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

785 citations

Journal ArticleDOI
TL;DR: In this article, the performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors on Ga2O3 has been evaluated and shown to benefit from the larger critical electric field relative to either SiC or GaN.
Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.

348 citations

Journal ArticleDOI
TL;DR: The dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO(2), GaN and Al(2)O(3) substrates using a thermal release tape is demonstrated to enable EG films amenable for use in device fabrication on arbitrary substrates and films that are deemed most beneficial to carrier transport.
Abstract: In order to make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO2, GaN and Al2O3 substrates using a thermal release tape. We further report on the impact of this process on the electrical properties of the EG films. This process enables EG films to be used in flexible electronic devices or as optically transparent contacts.

201 citations

Journal ArticleDOI
23 Feb 2010-ACS Nano
TL;DR: In this article, the authors demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO(2), GaN and Al(2)O(3) substrates using a thermal release tape.
Abstract: To make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO(2), GaN and Al(2)O(3) substrates using a thermal release tape. Subsequent Hall effect measurements illustrated that minimal degradation in the carrier mobility was induced following the transfer process in lithographically patterned devices. Correspondingly, a large drop in the carrier concentration was observed following the transfer process, supporting the notion that a gradient in the carrier density is present in C-face EG, with lower values being observed in layers further removed from the SiC interface. X-ray photoemission spectra collected from EG films attached to the transfer tape revealed the presence of atomic Si within the EG layers, which may indicate the identity of the unknown intrinsic dopant in EG. Finally, this transfer process is shown to enable EG films amenable for use in device fabrication on arbitrary substrates and films that are deemed most beneficial to carrier transport, as flexible electronic devices or optically transparent contacts.

181 citations


Cited by
More filters
01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

29,323 citations

Journal ArticleDOI
TL;DR: The roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates are reported, showing high quality and sheet resistances superior to commercial transparent electrodes such as indium tin oxides.
Abstract: The outstanding electrical, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as approximately 125 ohms square(-1) with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as approximately 30 ohms square(-1) at approximately 90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.

7,709 citations

Journal ArticleDOI
22 Jul 2010-Nature
TL;DR: Cai et al. as discussed by the authors used a surface-assisted coupling of the precursors into linear polyphenylenes and their subsequent cyclodehydrogenation to produce GNRs of different topologies and widths.
Abstract: Graphene nanoribbons, narrow straight-edged strips of the single-atom-thick sheet form of carbon, are predicted to exhibit remarkable properties, making them suitable for future electronic applications. Before this potential can be realized, more chemically precise methods of production will be required. Cai et al. report a step towards that goal with the development of a bottom-up fabrication method that produces atomically precise graphene nanoribbons of different topologies and widths. The process involves the deposition of precursor monomers with structures that 'encode' the topology and width of the desired ribbon end-product onto a metal surface. Surface-assisted coupling of the precursors into linear polyphenylenes is then followed by cyclodehydrogenation. Given the method's versatility and precision, it could even provide a route to more unusual graphene nanoribbon structures with tuned chemical and electronic properties. Graphene nanoribbons (GNRs) have structure-dependent electronic properties that make them attractive for the fabrication of nanoscale electronic devices, but exploiting this potential has been hindered by the lack of precise production methods. Here the authors demonstrate how to reliably produce different GNRs, using precursor monomers that encode the structure of the targeted nanoribbon and are converted into GNRs by means of surface-assisted coupling. Graphene nanoribbons—narrow and straight-edged stripes of graphene, or single-layer graphite—are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices1,2,3. In particular, although the two-dimensional parent material graphene4,5 exhibits semimetallic behaviour, quantum confinement and edge effects2,6 should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical7,8,9, sonochemical10 and lithographic11,12 methods as well as through the unzipping of carbon nanotubes13,14,15,16, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling17,18 of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation19,20. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots21, superlattice structures22 and magnetic devices based on specific graphene nanoribbon edge states3.

2,905 citations

01 Jan 2010
TL;DR: This work reports a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation.
Abstract: Graphene nanoribbons—narrow and straight-edged stripes of graphene, or single-layer graphite—are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices. In particular, although the two-dimensional parent material graphene exhibits semimetallic behaviour, quantum confinement and edge effects should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical, sonochemical and lithographic methods as well as through the unzipping of carbon nanotubes, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots, superlattice structures and magnetic devices based on specific graphene nanoribbon edge states.

2,355 citations