Publications by authors named "P T Vuong"

145 Publications

Towards P-Type Conduction in Hexagonal Boron Nitride: Doping Study and Electrical Measurements Analysis of hBN/AlGaN Heterojunctions.

Nanomaterials (Basel) 2021 Jan 15;11(1). Epub 2021 Jan 15.

CNRS, UMI 2958, GT-CNRS, 2 rue Marconi, 57070 Metz, France.

Reliable p-doped hexagonal boron nitride (h-BN) could enable wide bandgap optoelectronic devices such as deep ultra-violet light emitting diodes (UV LEDs), solar blind photodiodes and neutron detectors. We report the study of Mg in h-BN layers as well as Mg h-BN/AlGaN heterostructures. Mg incorporation in h-BN was studied under different biscyclopentadienyl-magnesium (Cp2Mg) molar flow rates. 2θ-ω x-ray diffraction scans clearly evidence a single peak, corresponding to the (002) reflection plane of h-BN with a full-width half maximum increasing with Mg incorporation in h-BN. For a large range of Cp2Mg molar flow rates, the surface of Mg doped h-BN layers exhibited characteristic pleats, confirming that Mg doped h-BN remains layered. Secondary ion mass spectrometry analysis showed Mg incorporation, up to 4 × 10 /cm in h-BN. Electrical conductivity of Mg h-BN increased with increased Mg-doping. Heterostructures of Mg h-BN grown on n-type Al rich AlGaN (58% Al content) were made with the intent of forming a p-n heterojunction. The I-V characteristics revealed rectifying behavior for temperatures from 123 to 423 K. Under ultraviolet illumination, photocurrent was generated, as is typical for p-n diodes. C-V measurements evidence a built-in potential of 3.89 V. These encouraging results can indicate p-type behavior, opening a pathway for a new class of wide bandgap p-type layers.
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http://dx.doi.org/10.3390/nano11010211DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7829971PMC
January 2021

Effectiveness of selective area growth using van der Waals h-BN layer for crack-free transfer of large-size III-N devices onto arbitrary substrates.

Sci Rep 2020 Dec 10;10(1):21709. Epub 2020 Dec 10.

Georgia Institute of Technology (School of Electrical and Computer Engineering), UMI 2958, Georgia Tech-CNRS, Atlanta, GA, 30332-0250, USA.

Selective Area van der Waals Epitaxy (SAVWE) of III-Nitride device has been proposed recently by our group as an enabling solution for h-BN-based device transfer. By using a patterned dielectric mask with openings slightly larger than device sizes, pick-and-place of discrete LEDs onto flexible substrates was achieved. A more detailed study is needed to understand the effect of this selective area growth on material quality, device performance and device transfer. Here we present a study performed on two types of LEDs (those grown on h-BN on patterned and unpatterned sapphire) from the epitaxial growth to device performance and thermal dissipation measurements before and after transfer. Millimeter-size LEDs were transferred to aluminum tape and to silicon substrates by van der Waals liquid capillary bonding. It is shown that patterned samples lead to a better material quality as well as improved electrical and optical device performances. In addition, patterned structures allowed for a much better transfer yield to silicon substrates than unpatterned structures. We demonstrate that SAVWE, combined with either transfer processes to soft or rigid substrates, offers an efficient, robust and low-cost heterogenous integration capability of large-size devices to silicon for photonic and electronic applications.
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http://dx.doi.org/10.1038/s41598-020-77681-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7728776PMC
December 2020

Control of the Mechanical Adhesion of III-V Materials Grown on Layered h-BN.

ACS Appl Mater Interfaces 2020 Dec 25;12(49):55460-55466. Epub 2020 Nov 25.

Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France.

Hexagonal boron nitride (h-BN) can be used as a p-doped material in wide-bandgap optoelectronic heterostructures or as a release layer to allow lift-off of grown three-dimensional (3D) GaN-based devices. To date, there have been no studies of factors that lead to or prevent lift-off and/or spontaneous delamination of layers. Here, we report a unique approach of controlling the adhesion of this layered material, which can result in both desired lift-off layered h-BN and mechanically inseparable robust h-BN layers. This is accomplished by controlling the diffusion of Al atoms into h-BN from AlN buffers grown on h-BN/sapphire. We present evidence of Al diffusion into h-BN for AlN buffers grown at high temperatures compared to conventional-temperature AlN buffers. Further evidence that the Al content in BN controls lift-off is provided by comparison of two alloys, AlBN/sapphire and AlBN/sapphire. Moreover, we tested that management of Al diffusion controls the mechanical adhesion of high-electron-mobility transistor (HEMT) devices grown on AlN/h-BN/sapphire. The results extend the control of two-dimensional (2D)/3D hetero-epitaxy and bring h-BN closer to industrial application in optoelectronics.
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http://dx.doi.org/10.1021/acsami.0c16850DOI Listing
December 2020

Using Positive Spanning Sets to Achieve d-Stationarity with the Boosted DC Algorithm.

Vietnam J Math 2020 30;48(2):363-376. Epub 2020 Mar 30.

Department of Mathematics, University of Vienna, Vienna, Austria.

The Difference of Convex functions Algorithm (DCA) is widely used for minimizing the difference of two convex functions. A recently proposed accelerated version, termed BDCA for Boosted DC Algorithm, incorporates a line search step to achieve a larger decrease of the objective value at each iteration. Thanks to this step, BDCA usually converges much faster than DCA in practice. The solutions found by DCA are guaranteed to be critical points of the problem, but these may not be local minima. Although BDCA tends to improve the objective value of the solutions it finds, these are frequently just critical points as well. In this paper we combine BDCA with a simple Derivative-Free Optimization (DFO) algorithm to force the d-stationarity (lack of descent direction) at the point obtained. The potential of this approach is illustrated through some computational experiments on a Minimum-Sum-of-Squares clustering problem. Our numerical results demonstrate that the new method provides better solutions while still remains faster than DCA in the majority of test cases.
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http://dx.doi.org/10.1007/s10013-020-00400-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7357844PMC
March 2020

Gradient Methods on Strongly Convex Feasible Sets and Optimal Control of Affine Systems.

Appl Math Optim 2020 6;81(3):1021-1054. Epub 2018 Oct 6.

Institute of Statistics and Mathematical Methods in Economics, Vienna University of Technology, Vienna, Austria.

The paper presents new results about convergence of the gradient projection and the conditional gradient methods for abstract minimization problems on strongly convex sets. In particular, linear convergence is proved, although the objective functional does not need to be convex. Such problems arise, in particular, when a recently developed discretization technique is applied to optimal control problems which are affine with respect to the control. This discretization technique has the advantage to provide higher accuracy of discretization (compared with the known discretization schemes) and involves strongly convex constraints and possibly non-convex objective functional. The applicability of the abstract results is proved in the case of linear-quadratic affine optimal control problems. A numerical example is given, confirming the theoretical findings.
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http://dx.doi.org/10.1007/s00245-018-9528-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7319312PMC
October 2018