Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

/pdf/two-dimensional-electron-gases-induced-by-spontaneous-and-11pht7aexp.pdf

Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

/pdf/luminescence-properties-of-defects-in-gan-4hjs241x7f.pdf

Luminescence properties of defects in GaN

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

Introduction: Classical particle accelerators offer proton pulses of some milliseconds duration. In contrast, the new technology of the high-intensity laser acceleration will produce ultimately shorter particle packages (up to one nanosecond) with substantially lower pulse frequency and higher pulse-dose achievement. Very little is known about the relative biological effectiveness (RBE) of this new beam quality, which could be a possible future application in radiation oncology. In our present study we investigate possible differences based on quantitative analysis of γ-H2AX fluorescence - a known marker of DNA double strand breaks (DSBs).

Differences in Gamma-H2AX Foci Formation after Irradiation with Continuous and Pulsed Proton Beams

In minibeam therapy, the sparing of deep-seated normal tissue is limited by transverse beam spread caused by small-angle scattering. Contrary to proton minibeams, helium or carbon minibeams experience less deflection, which potentially reduces side effects. To verify this potential, an irradiation facility for preclinical and clinical studies is needed. This manuscript presents a concept for a carbon minibeam irradiation facility based on a LINAC design for conventional carbon therapy. A quadrupole triplet focuses the LINAC beam to submillimeter minibeams. A scanning and a dosimetry unit are provided to move the minibeam over the target and monitor the applied dose. The beamline was optimized by TRAVEL simulations. The interaction between beam and these components and the resulting beam parameters at the focal plane is evaluated by TOPAS simulations. A transverse beamwidth of < 100 μm (sigma) and a peak-to-valley (energy) dose ratio of > 1000 results for carbon energies of 100 MeV/u and 430 MeV/u (∼ 3 cm and 30 cm range in water) whereby the average beam current is ∼ 30 nA. Therefore, the presented irradiation facility exceeds the requirements for hadron minibeam therapy.

A carbon minibeam irradiation facility concept

Erratum to: “Energy straggling of 60 MeV 58Niq+ ions in thin carbon foils and gases” [Nucl. Instr. and Meth. B 145 (1998) 261]

Assessment of the near-surface profiling capabilities of SIMS

In this work direct heavy ion mapping of Single Event Effect (SEE) and Single Event Burnout (SEB) of super-junction power MOSFETs utilizing a high energy (55MeV) carbon micro-beam is presented The resulting maps are sub-structurally resolved Effect location, signal level and possible connections to cosmic radiation events are discussed

Radiation Hardness Testing of Super-Junction Power Mosfets by Heavy Ion Induced SEE Mapping

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