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This paper contributes to studying the bifurcations of closed invariant curves in piecewise-smooth maps. Specifically, we discuss a border collision bifurcation of a repelling resonant closed invariant curve (a repelling saddle-node connection) colliding with the border by a point of the repelling cycle. As a result, this cycle becomes attracting and the curve is destroyed, while a new repelling closed invariant curve appears (not in a neighborhood of the previously existing invariant curve), being associated with quasiperiodic dynamics. This leads to a global restructuring of the phase portrait since both curves mentioned above belong to basin boundaries of coexisting attractors.

The effects of miniaturization on theoretically predicted performance of dual-period guided mode resonance (GMR) transmission filters, which demonstrate immense potential for multispectral imaging in short wave infrared (SWIR), have been compared with experimental findings. With reducing filter size from 112 periods (90 µm) to 12 periods (10 µm), peak transmittance (Tpeak) of simulated and measured filters reduced gradually from 84% to 55% and from 76% to 65%, respectively, with a moderate change of 1 - 3 nm in full width at half maximum (FWHM). For 6 period filters (5 µm), simulations predict drastically reduced Tpeak = 14% accompanied by increase in FWHM by 12 nm. The Tpeak value is theoretically shown to increase to 46% with FWHM reduced by 7 nm upon placing metal reflectors at the optimum positions to increase the optical path length. Our findings indicate that four 5 µm × 5 µm size filters with metal reflectors designed for different resonance wavelengths can be used to form a single, 20 µm × 20 µm mosaic pixel for SWIR multispectral imaging.

Date of Conference: 1-6 February 2020 Conference Name: SPIE OPTO, 2020: Gallium Nitride Materials and Devices XV 2020 In this work, the effect of introducing a photonic crystal network of silicon nitride (SiN) micro-domes on the backside of silver coated gallium nitride (GaN) based light emitting diodes (LEDs) was studied. First, sapphire side of the top emitting LEDs, which is the bottom surface of the LEDs, is coated with silver (Ag). Light emitted towards the sapphire substrate is reflected upwards to the top surface and the amount of light extracted from the LED is expected to increase. In an alternative approach, SiN micro-domes forming a two dimensional photonic crystal, 2 μm in diameter and 80 nm in height in average, are deposited on the light emitting surface of the device with a period of 2 μm. Coating the backside with Ag has increased the efficiency of a top emitting LED by 11%. By introducing the SiN photonic crystal onto the Ag backside coated sample, total internal reflection is reduced via scattering and the amount of light emitted has been increased by 30% at 5·104 mA/cm2. Integration of SiN micro-domes with Ag coating has significantly impacted light extraction which has been shown to increase the efficiency of GaN based LEDs. Fabrication process and the results are discussed in detail.

In recent years, electrochemical biosensors based on semiconductor and metal nanostructures have attracted a great deal of attention as new instruments in the healthcare arsenal that could substant ...

Growth of nonpolar and semi-polar GaN and GaN-based structures offers the opportunity to reduce quantum confined Stark effect and possibly increase indium incorporation, as compared to polar structures, for enhanced performance in green and longer wavelength light emitters. However, the development of the nonpolar and semi-polar GaN growth is hampered by the lack of suitable substrates. Silicon, despite its large thermal-expansion and lattice mismatch with GaN, provides the advantages of the availability of large-size wafers with high crystalline quality at low cost, good electrical conductivity, and feasibility of its removal through chemical etching for better light extraction and heat transfer. In this article, we overview the recent progress in epitaxial growth of nonpolar and semi-polar GaN-based structures on patterned Si substrates. Also discussed are structural and optical properties of the resulting material.

WOS: 000430037600030, We investigate the effect of InGaN stress compensation layer on the properties of light emitting diodes based on InGaN/GaN multiple quantum well (MQW) structures with step graded electron injectors. Insertion of an InGaN stress compensation layer between n-GaN and the step graded electron injector provides, among others, strain reduction in the MQW region and as a result improves epitaxial quality that can be observed by 15-fold decrease of V-pit density. We observed more uniform distribution of In between quantum wells in MQW region from results of electro- and photoluminescence measurement. These structural improvements lead to increasing of radiant intensity by a factor of 1.7-2.0 and enhancement of LED efficiency by 40%. (C) 2018 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey [113G042]; TUBITAK, Financial support for this work is provided by the Scientific and Technological Research Council of Turkey (Grant No: 113G042). Ismail Altuntas acknowledges the Ph.D. Grant support from TUBITAK. We thank UNAM-National Nanotechnology Research Center at Bilkent University for access to fabrication and characterization equipment.

WOS: 000425566100069, Enhancement of InGaN/GaN based light emitting diode performance with step graded electron injectors through a two-step passivation is reported. Perimeter passivation of LED dies with SiO2 immediately following ICP mesa etch in addition to conventional Si3N4 dielectric surface passivation leads to decrease in the reverse bias leakage current by a factor of two as well as a decrease in the shunt current under forward bias by an order of magnitude. Mitigation of the leakage currents owing to the two-step passivation leads to significant increase in the radiant intensity of LEDs by more than a factor of two compared to the conventional single step surface passivation. Further, micro-dome patterned surface of Si3N4 passivation layer allow enhanced light extraction from LEDs. (C) 2017 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey (TUBITAK) [113G042]; TUBITAK, Financial support for this project was provided by the Scientific and Technological Research Council of Turkey (TUBITAK) [Grant No: 113G042]. Ismail Altuntas, acknowledges the Ph.D. Grant support from TUBITAK. We thank to UNAM-National Nanotechnology Research Center at Bilkent University for the use of fabrication and characterization equipment.

The atomic composition with less than 1–2 atom% uncertainty was measured in ternary BeZnO and quaternary BeMgZnO alloys using a combination of nondestructive Rutherford backscattering spectrometry with 1 MeV He+ analyzing ion beam and non-Rutherford elastic backscattering experiments with 2.53 MeV energy protons. An enhancement factor of 60 in the cross-section of Be for protons has been achieved to monitor Be atomic concentrations. Usually the quantitative analysis of BeZnO and BeMgZnO systems is challenging due to difficulties with appropriate experimental tools for the detection of the light Be element with satisfactory accuracy. As it is shown, our applied ion beam technique, supported with the detailed simulation of ion stopping, backscattering, and detection processes allows of quantitative depth profiling and compositional analysis of wurtzite BeZnO/ZnO/sapphire and BeMgZnO/ZnO/sapphire layer structures with low uncertainty for both Be and Mg. In addition, the excitonic bandgaps of the layers were deduced from optical transmittance measurements. To augment the measured compositions and bandgaps of BeO and MgO co-alloyed ZnO layers, hybrid density functional bandgap calculations were performed with varying the Be and Mg contents. The theoretical vs. experimental bandgaps show linear correlation in the entire bandgap range studied from 3.26 eV to 4.62 eV. The analytical method employed should help facilitate bandgap engineering for potential applications, such as solar blind UV photodetectors and heterostructures for UV emitters and intersubband devices.

WOS: 000415768800128, The effect of a transparent ITO current spreading layer on electrical and light output properties of blue InGaN/GaN light emitting diodes (LEDs) is discussed. When finite conductivity of ITO is taken into account, unlike in previous models, the topology of LED die and contacts are shown to significantly affect current spreading and light output characteristics in top emitting devices. We propose an approach for calculating the current transfer length describing current spreading. We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of p-n junction equal to transfer length in the current spreading ITO layer allows one to increase the optical area of LED chip, as compared to the physical area of the die, light output power, and therefore, the LED efficiency for a given current density. A detailed study of unpassivated LEDs also shows that current transfer lengths longer than the distance between the contact pad and the edge of p-n junction leads to increasing surface leakage that can only be remedied with proper passivation. (C) 2017 Elsevier Ltd. All rights reserved., TUBITAK [113G042]; Scientific and Technological Research Council of Turkey (TUBITAK), This work has been performed with the support of TUBITAK (Grant no: 113G042). Ismail Altuntas, acknowledges the Ph.D. Grant support from the Scientific and Technological Research Council of Turkey (TUBITAK).

A wide range of optical bandgap modulation up to 5.1 eV was achieved for quaternary BeMgZnO thin films prepared using plasma assisted molecular beam epitaxy, enabling development of UV emitters and solar-blind photodetectors. The significantly improved structural quality and bandgap widening in BeMgZnO layers as compared to those of BeZnO and MgZnO ternaries indicate enhanced incorporation of both Be and Mg on Zn sites in the wurtzite lattice. Correlation of lattice parameters with optical bandgaps reveals that co-alloying MgO and BeO with ZnO helps overcome the impediment of limited bandgap extension offered by the corresponding ternary compounds due to phase separation and allows lattice-matched or nearly lattice-matched BeMgZnO/ZnO heterostructures. Optimization of growth conditions at higher Mg and/or Be fluxes is expected to provide bandgaps beyond 5.1 eV.

Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN.

Spectroscopic ellipsometry is a prominent method for finding both the thickness and permittivity of unknown thin material films due to its sensitivity, flexibility, and self-referencing nature. For non-absorbing films, the thickness and permittivity can be readily retrieved due to an excess of data content, which produces a clearly defined best-fit for a series of test material parameters. However, in materials with absorption throughout the spectrum, there is often insufficient data content to uniquely characterize both film thickness and permittivity for the thin film material. This leads to a flat fit optimization curve that can produce apparently good fit results from a wide range of material parameters. To overcome this data content shortage, additional techniques are necessary to either increase the measured data content or reduce the unknown parameters and establish the unique material properties for the film. Here, we explore the use of spectroscopic ellipsometry combined with transmission intensity data and discuss the pitfalls of fitting such thin absorbing films. Specifically, we examine the case of titanium nitride, a rising refractory alternative plasmonic material and demonstrate that without proper ellipsometry fitting procedures, retrieved permittivity values can vary by a factor of three or more.

We have investigated the N- and Ga-polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi-ready after a chemical-mechanical polish (CMP). From the SPV measurements, the band bending in a sample having both surfaces treated with the CMP method was calculated to be about 0.83 and 0.70 eV for the Ga- and N-polar surfaces, respectively. The restoration of the SPV after ceasing the UV illumination showed that the SPV from CMP-treated surfaces behaved as predicted by a thermionic model, whereas the SPV from MP-treated surfaces restored with a much faster-than-predicted rate. This result can be interpreted by the hopping of charge carriers in the highly-defective near-surface layer of the MP-treated samples. Remarkably, removing the top 700 nm defective layer by dry etching restored the quality of the electrical and optical properties of GaN. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We perform a theoretical and experimental study of the optical properties of a $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ perovskite prepared by a vapor-assisted solution process, motivated in part by very high photovoltaic cell efficiencies. Several widespread theoretical approaches are used in an attempt to determine the most appropriate approach which would reproduce the experimental electronic structure and optical properties of the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ perovskite. We compare a semilocal approximation to the density functional theory with hybrid functionals and time-dependent hybrid functional calculations, evaluating the effects of exchange tuning and spin-orbit coupling. Using these methods we calculate the electronic structure, optical absorption spectrum, and frequency-dependent dielectric function of the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ perovskite. The results are compared to the experimentally obtained dielectric functions acquired from ellipsometry measurements. We demonstrate that inclusion of spin-orbit coupling in theoretical calculations is critical in describing the electronic and optical properties of the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ perovskite. Good agreement with experimental data is achieved when the optical spectra are computed using time-dependent hybrid density functional theory with spin-orbit coupling.

We report on the electrical and optical properties of InGaN based light emitting diodes (LEDs) utilizing Ga doped ZnO (GZO) grown by MBE as a highly transparent contact p -electrode (GZO-LEDs). For comparison, the LEDs with semi-transparent thin Ni/Au (5nm/5nm) current spreading layers were also fabricated on the same wafer (Ni/Au-LEDs). Pulsed electroluminescence measurements revealed that the relative external quantum efficiency (EQE) of the GZO-LED is 1.7-2 times higher than that of Ni/Au-LED at high current densities. Moreover, GZO-LEDs can withstand much higher current densities (4700 A/cm2) than the Ni/Au LEDs (3500 A/cm2) under pulsed mode. Unpackaged GZO-LEDs with 200 μm diameter showed negligible light output degradation for up to 30 mins under CW current of 100 mA (corresponding to a 318 Acm-2 current density), while the light output for Ni/Au-LEDs was reduced by 85% after only 5 mins of operation due to the severe current crowding effect (or current filamentation). (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

m -Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m -plane GaN films. The epitaxial relationship was found to be GaN(1 1 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1 1 2 0] || Si[1 1 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.

We report on the electrical and optical properties of InGaN based light emitting diodes (LEDs) utilizing Ga doped ZnO (GZO) grown by molecular beam epitaxy (MBE) as a highly transparent contact p-electrode (GZO-LEDs). For comparison, the LEDs with semi-transparent thin Ni/Au (5/5 nm) current spreading layers were also fabricated on the same wafer (Ni/Au-LEDs). Unpackaged GZO-LEDs with 200 μm diameter showed negligible light output degradation for up to 30 min under CW current of 100 mA (corresponding to a 318 A/cm 2 current density), while the light output for Ni/Au-LEDs was reduced by 85% after only 5 min of operation due to the severe current crowding effect (or current filamentation). Pulsed electroluminescence (EL) measurements revealed that the GZO-LEDs exhibit 50% higher EL intensity for the same current levels.

High-field electron transport in two-dimensional channels at ZnO/MgZnO heterointerfaces has been investigated experimentally. Pulsed current–voltage (I–V) and microwave noise measurements used voltage pulse widths down to 30 ns and electric fields up to 100 kV/cm. The samples investigated featured electron densities in the range of 4.2–6.5 × 1012 cm−2, and room temperature mobilities of 142–185 cm2/V s. The pulsed nature of the applied field ensured negligible, if any, change in the electron density, thereby allowing velocity extraction from current with confidence. The highest extracted electron drift velocity of ∼0.5 × 107 cm/s is somewhat smaller than that estimated for bulk ZnO; this difference is explained in the framework of longitudinal optical phonon accumulation (hot-phonon effect). The microwave noise data allowed us to rule out the effect of excess acoustic phonon temperature caused by Joule heating. Real-space transfer of hot electrons into the wider bandgap MgZnO layer was observed to be a limiting factor in samples with a high Mg content (48%), due to phase segregation and the associated local lowering of the potential barrier.High-field electron transport in two-dimensional channels at ZnO/MgZnO heterointerfaces has been investigated experimentally. Pulsed current–voltage (I–V) and microwave noise measurements used voltage pulse widths down to 30 ns and electric fields up to 100 kV/cm. The samples investigated featured electron densities in the range of 4.2–6.5 × 1012 cm−2, and room temperature mobilities of 142–185 cm2/V s. The pulsed nature of the applied field ensured negligible, if any, change in the electron density, thereby allowing velocity extraction from current with confidence. The highest extracted electron drift velocity of ∼0.5 × 107 cm/s is somewhat smaller than that estimated for bulk ZnO; this difference is explained in the framework of longitudinal optical phonon accumulation (hot-phonon effect). The microwave noise data allowed us to rule out the effect of excess acoustic phonon temperature caused by Joule heating. Real-space transfer of hot electrons into the wider bandgap MgZnO layer was observed to be a li...

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization energy of ∼0.9 eV for the YL1 center. In semi-insulating GaN, an abrupt and tunable quenching of the YL1 band is observed, where the apparent activation energy in the Arrhenius plot is not related to the ionization energy of the defect. In this case, the ionization energy can be found by analyzing the shift of the characteristic temperature of PL quenching with excitation intensity. We conclude that only one defect, namely, the YL1 center, is responsible for the yellow band in undoped and doped GaN samples grown by different techniques.

Single-crystal Pb(Zr x Ti 1− x )O 3 thin films have been grown on (0 0 1) SrTiO 3 and SrTiO 3 :Nb substrates by molecular beam epitaxy using metal-organic source of Zr and two different sources of reactive oxygen—RF plasma and hydrogen-peroxide sources. The same growth modes and comparable structural properties were observed for the films grown with both oxygen sources, while the plasma source allowed higher growth rates. The films with x up to 0.4 were single phase, while attempts to increase x beyond gave rise to the ZrO 2 second phase. The effects of growth conditions on growth modes, Zr incorporation, and phase composition of the Pb(Zr x Ti 1− x )O 3 films are discussed. Electrical and ferroelectric properties of the Pb(Zr x Ti 1− x )O 3 films of ~100 nm in thickness grown on SrTiO 3 :Nb were studied using current–voltage, capacitance–voltage, and polarization–field measurements. The single-phase films show low leakage currents and large breakdown fields, while the values of remanent polarization are low (around 5 μC/cm 2 ). It was found that, at high sweep fields, the contribution of the leakage current to the apparent values of remanent polarization can be large, even for the films with large electrical resistivity (∼10 8 –10 9 Ω cm at an electric filed of 1 MV/cm). The measured dielectric constant ranges from 410 to 260 for Pb(Zr 0.33 Ti 0.67 )O 3 and from 313 to 213 for Pb(Zr 0.2 Ti 0.8 )O 3 in the frequency range from 100 to 1 MHz.

We observed strong shifts of the blue and yellow luminescence bands with variation of excitation intensity in ZnO films grown on sapphire by MBE using hydrogen peroxide as a source of reactive oxygen. The blue band, having a maximum in the range from 2.85 to 3.15 eV in different samples and different excitation intensities at 10 K, is attributed to diagonal transitions from the conduction band (or shallow donors) to the valence band in realm of potential fluctuations caused by random distribution of charged point defects in a compensated semiconductor. The yellow band is related to an unidentified deep acceptor.

The effect of delta-doping of In0.06Ga0.94N barriers with Mg on the quantum efficiency of blue light-emitting-diodes (LEDs) with active regions composed of 6 (hex) 3-nm In0.15Ga0.85N is investigated. Compared to the reference sample, delta-doping of the first barrier on the n-side of the LED structure improves the peak external quantum efficiency (EQE) by 20%, owing to the increased hole concentration in the wells adjacent to the n-side, as confirmed by numerical simulations of carrier distributions across the active region. Doping the second barrier, in addition to the first one, did not further enhance the EQE, which likely indicates compensation of improved hole injection by degradation of the active region quality due to Mg doping. Both LEDs with Mg delta-doped barriers effectively suppress the drop of efficiency at high injection when compared to the reference sample, and the onset of EQE peak roll-off shifts from similar to 80 A/cm(2) in the reference LED to similar to 120 A/cm(2) in the LEDs with Mg delta-doped barriers. (C) 2015 AIP Publishing LLC.

Heteroepitaxial n-ZnO films have been grown on commercial p-type 6H-SiC substrates by plasma-assisted molecular-beam epitaxy, and n-ZnO/p-SiC heterojunction mesa structures have been fabricated and their photoresponse properties have been studied. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2 x 10-4 A/cm2 at -10 V, a breakdown voltage greater than 20 V, a forward turn on voltage of ∼5 V, and a forward current of ∼2 A/cm2 at 8 V. Photosensitivity of the diodes, when illuminated from ZnO side, was studied at room temperature and photoresponsivity of as high as 0.045 A/W at -7.5 V reverse bias was observed for photon energies higher than 3.0 eV.

We studied the structural and electronic properties of Zn1−xMnxO ( 0 x 0.5 ) layers grown by peroxide molecular beam epitaxy. Hall effect measurements showed that the layers were highly resistive, pointing to strong electrical compensation of the Zn1−xMnxO films by Mn incorporation. The lattice parameter c was found to increase linearly with increasing x for low Mn concentrations and tended to saturate for heavily doped films ( x ≥ 0.2 ) , suggesting a possible second-phase precipitation. Optical transmission measurements revealed an increase in the band gap of Zn1−xMnxO and an enhancement of the broad below-band-gap absorption associated with Mn ions with increasing Mn content. However, no blueshift in the near-band-edge emission was detected in the photoluminescence spectra.

We report on the fabrication of n-GaN/n-ZnO/p-GaN double heterostructure-based light-emitting diodes and their electroluminescence (EL) properties. The current–voltage characteristics of the fabricated diodes revealed rectifying behavior with a leakage current of 1.62 × 10 −5 A at −9 V, a forward current density of 1.56 × 10 −2 A at 5 V bias, and threshold and breakdown voltages of ∼3.2 and ∼11 V, respectively. Under forward bias an intense EL was observed, the spectrum of which depended on the injection current. From the comparison of the electroluminescence and photoluminescence spectra of the double heterostructure layers we concluded that the observed EL emission is most probably a superposition of emissions originating from ZnO, n-GaN, and p-GaN layers of the double heterostructure.

We report the attempt to prepare free-standing spherically bent Si crystals with radii of curvature as small as 30 cm and large uniform working areas up to several cm2. We also report on the use of these crystals in the Laue geometry to record two-dimensional high spatial resolution x-ray images in the hard x-ray region (λ∼0.5 A). We discuss how these bent crystals were made, and show how these spherically bent crystals were used in the Laue geometry to obtain hard x-ray images of test objects at the silver K-alpha wavelength at two different magnifications. The mean spatial resolution of this x-ray imaging scheme, determined from the recorded image traces, was found to be better than 10 μm over a field of view of 1.5×1.5 mm. Contributions of possible focusing mechanisms (dynamical and polychromatic) are discussed. Additional theoretical understanding of how such a scheme is working is needed.

We report on the evolution of AlGaN/GaN-based heterojunction field-effect transistor (HFET) operation under high-electric-field stress. Specifically, a 10 ~ 15 dB decrease in the flicker noise is observed after stress in contrast with what has been nominally observed and reported in the literature in the realm of direct-current characteristics. Gate lag measurements revealed a trap state with an activation energy of 0.20 eV in the pristine devices, which manifests itself as a generation-recombination peak in the flicker noise spectrum. This trap state becomes undetectable in gate lag and noise measurements after high-field stress. Analysis shows that the phenomena observed are consistent with the change of surface charge profile during high-electric-field stress.

International audience; Optical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-11) orientation on patterned 7°-off Si (001) substrates have been investigated. Studies performed at 8 K reveal the in-plane anisotropic behavior of the QW photoluminescence (PL) intensity for this semi-polar orientation. The time resolved PL measurements were carried out in the temperature range from 8 to 295 K to deduce the effective recombination decay times, with respective radiative and non-radiative contributions. The non-radiative component remains relatively weak with increasing temperature, indicative of high crystalline quality. The radiative decay time is a consequence of contribution from both localized and free excitons. We report an effective density of interfacial defects of 2.3 × 1012 cm−2 and a radiative recombination time of τloc = 355 ps for the localized excitons. This latter value is significantly larger than those reported for the non-polar structures, which we attribute to the presence of a weak residual electric field in the semi-polar QW layers.

The optical quality of semipolar (1 101)GaN layers was explored by time- and polarization-resolved photoluminescence spectroscopy. High intensity bandedge emission was observed in +c-wing regions of the stripes as a result of better structural quality, while -c-wing regions were found to be of poorer optical quality due to basal plane and prismatic stacking faults (BSFs and PSFs) in addition to a high density of TDs. The high optical quality region formed on the +cwings was evidenced also from the much slower biexponential PL decays (0.22 ns and 1.70 ns) and an order of magnitude smaller amplitude ratio of the fast decay (nonradiative origin) to the slow decay component (radiative origin) compared to the -c-wing regions. In regard to defect-related emission, decay times for the BSF and PSF emission lines at 25 K (~ 0.80 ns and ~ 3.5 ns, respectively) were independent of the excitation density within the range employed (5 – 420 W/cm2), and much longer than that for the donor bound excitons (0.13 ns at 5 W/cm2 and 0.22 ns at 420 W/cm2). It was also found that the emission from BSFs had lower polarization degree (0.22) than that from donor bound excitons (0.35). The diminution of the polarization degree when photogenerated carriers recombine within the BSFs is another indication of the negative effects of stacking faults on the optical quality of the semipolar (1101)GaN. In addition, spatial distribution of defects in semipolar (1101)-oriented InGaN active region layers grown on stripe patterned Si substrates was investigated using near-field scanning optical microscopy. The optical quality of -c- wing regions was found to be worse compared to +c-wing regions due to the presence of higher density of stacking faults and threading dislocations. The emission from the +c-wings was very bright and relatively uniform across the sample, which is indicative of a homogeneous In distribution.

Nonpolar m -plane GaN layers were grown on patterned Si (112) substrates by metal-organic chemical vapor deposition (MOCVD). A two-stage growth procedure involving a low-pressure (30 Torr) first stage to ensure formation of the m -plane facets and a high-pressure stage (200 Torr) for improvement of optical quality was employed. The layers grown in two steps show improvement of the optical quality: the near-bandedge photoluminescence (PL) intensity is about 3 times higher than that for the layers grown at low pressure, and deep emission is considerably weaker. However, emission intensity from m- GaN is still lower than that of polar and semipolar ( 1100 ) reference samples grown under the same conditions. To shed light on this problem, spatial distribution of optical emi ssion over the c+ and ci wings of the nonpolar GaN/Si was studied by spatially resolved cathodoluminescence and near-field scanning optical microscopy. Keywords: GaN, nonpolar, m-plane, Si substrate, MOCVD, time-resolved photoluminescence, near-field scanning optical microscopy

International audience; The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells are examined in 8K–300K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells. VC 2014 AIP Publishing LLC.

International audience; The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells grown on m-plane oriented substrate are studied in 8K–300K temperature range. The optical spectra reveal strong in-plane optical anisotropies as predicted by group theory. Polarized time resolved temperature-dependent photoluminescence experiments are performed providing access to the relative contributions of the non-radiative and radiative recombination processes. We deduce the variation of the radiative decay time with temperature in the two polarizations. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The effect of hot electrons on electroluminescence of m -plane double heterostructure light emitting diodes (with a single 6 nm active layer of In0.20Ga0.80N) is investigated. Diodes with an electron blocking layer (Al0.15Ga0.85N) demonstrate from 3 to 5 times higher electroluminescence efficiency than those without a blocking layer. The lower electroluminescence efficiency in devices without the blocking layer is ex- plained in terms of electron overflow caused by ballistic and quasi-ballistic transport of injected electrons across the InGaN active region. The same mechanism explains the decrease, observed at high current densities, of the electroluminescence efficiency (efficiency droop) in the In0.20Ga0.80N diodes with the Al0.15Ga0.85N blocking layer. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate a critical effect of a metal-to-oxygen ratio on the electrical, optical, and structural properties of ZnO films heavily doped with Ga (carrier concentration in the range of 1020–1021 cm–3) grown by plasma-assisted molecular beam epitaxy. The as-grown layers prepared under the metal-rich conditions exhibited resistivities below 3 × 10–4 Ω cm and an optical transparency exceeding 90% in the visible spectral range as well as a large blue shift of the transmission/absorption edge attributed to the Burstein–Moss shift of the Fermi level deep into the conduction band, indicating high donor concentration. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior properties. Furthermore, electrical measurements point to the nonuniform depth distribution of free carriers. An oxygen-pressure-dependent surface disordering is suggested to be responsible for the drastic effect of the metal-to-oxygen ratio on the film properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we compare electronic transport performance in HFETs based on single channel (SC) GaN/Al0.30GaN/AlN/GaN (2nm/20nm/1nm/3.5μm) and coupled channel (CC) GaN/Al0.285GaN/AlN/GaN/AlN/GaN (2nm/20nm/1nm/4nm/1nm/3.5μm) structures. The two structures have similar current gain cut-off frequencies (11.6 GHz for SC and 14 GHz for CC for ~ 1μm gate length) however, the maximum drain current, IDmax, is nearly doubled in the CC HFET (0.64 A/mm compared to 0.36 A/mm in SC). HFETs exhibit maximum transconductance (Gmmax) at a bias point close to where maximum f T occurs: VGS =-2.25 V and VDS =12 V and VGS = -2 V and VDS= 15 V for SC and CC HFETs, respectively. Since threshold voltage (Vth) is ~ -3.75 V for both SC and CC structures, devices are able to work at high frequencies with a high gm delivering higher ID. This is in contrast with device performance reported by others where f T is attained at VGS closer to Vth and therefore with lower ID/IDmax ratios and low Gm. Results are consistent in that CC HFET delivers higher IDmax because of the higher electron mobility (μ) and higher carrier density (n) in the channel. As the saturation drain current, IDsat, is attained at electric fields (~40KV/cm) lower than the critical electric field, Ecr , (~ 150KV/cm for GaN ) the higher f T in CC HFETs can be attributed, mainly, to a higher μ, which is in agreement with the Hall measurements. A higher μ in CC HFET is attributed to a shorter hot phonon lifetime.

Semipolar (11macron01) GaN layers and GaN/InGaN LED structures were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. Optical properties of the semipolar samples were studied by steady-state and time-resolved photoluminescence (PL). Photon energies and intensities of emission lines from steady-state PL as well as carrier decay times from time-resolved PL were correlated with the distributions of extended defects studied by spatially resolved cathodoluminescence and nearfield scanning optical microscopy. Intensity of donor-bound exciton (DX) emission from both coalesced and non-coalesced semipolar layers is comparable to that of state-of-art c-plane GaN template. To gain insight into the contribution from near surface region and deeper portion of the layers to carrier dynamics in polar c-plane and semipolar (11macron01) GaN, time-resolved PL was measured with two different excitation wavelengths of 267 and 353 nm, which provide different excitation depths of about 50 nm and 100 nm, respectively. Time-resolve PL data indicate that the near-surface layer is relatively free from nonradiative centers (point and/or extended defects), while deeper region of the semipolar film (beyond of ~100 nm in depth) is more defective, giving rise to shorter decay times.

GaN-based vertical cavity structures containing bottom AlN/GaN DBRs with top dielectric DBRs on freestanding c-GaN and all dielectric DBRs on GaN on c-sapphire were investigated. Epitaxial lateral overgrowth (ELO) technique allowed the use of both top and bottom all dielectric reflector stacks without substrate removal and the fabrication of the active region containing InGaN multiple quantum wells entirely on the nearly defect-free laterally grown wing regions to avoid nonradiative centers caused by extended and point defects. Compared with the cavity containing hybrid-DBRs on freestanding GaN, the cavity with all dielectric DBRs exhibited quality factors up to 1200 at high optical excitation and an order of magnitude lower stimulated emission threshold density (nearly 5 μJ/cm 2 ). Vertical to lateral growth ratio for ELO could be enhanced up to 5 by increasing the V/III ratio and employment of NH3 modulation, which minimizes the use of dry etching to reduce the cavity thickness and therefore is promising for high quality vertical cavities with all dielectric DBRs.

For high efficiency at high current injection InGaN light emitting diodes (LEDs) necessitate active regions that can mitigate the aggravating electron overflow. Multi double-heterostructures (DHs), 3D active regions separated by low energy barriers, were investigated as optimum solutions for high efficiency as they can accommodate a larger number of states compared to multiple quantum wells (MQWs). However, the number of DH active regions is limited as the material degrades with increasing thickness; therefore, carrier cooling should be partially achieved before the active region using staircase electron injector (SEI) layers. Using electroluminescence (EL) efficiency measurements supported by simulations, active regions and electron injectors were optimized to minimize the electron overflow and the associated efficiency drop at high injection. For a single 3 nm DH LED, the electron overflow was nearly eliminated by increasing the two-step staircase electron injector layer thickness from 4+4 nm to 20+20 nm, whereas the change in SEI thickness had nearly no effect for the DH LEDs with thicker active region. Temperature and excitation density dependent photoluminescence (PL) spectroscopy allowed determination of the material quality and the internal quantum efficiency of device structures with varying active region and SEI thickness.

Owing to its suitable material properties, such as large direct bandgap (3.37 eV) and large exciton binding energy (60 meV), and ease of production compared to other compound semiconductors, ZnO has been a focus of momentous interest for potential applications in optical devices, transparent ohmic contacts, transparent thin film and heterostructure transistors, chemical/bio sensors, and piezotransducers. However, lack of reproducible and sufficient p-type conductivity in ZnO has so far prevented its widespread introduction to the commercial applications. Like for other semiconductor systems, advanced deposition methods such as molecular beam epitaxy (MBE) has allowed invaluable insight into the growth dynamics and fundamental properties of ZnO thanks to its fine control over growth parameters. This chapter discusses MBE growth of ZnO and related materials on various substrates and the recent progress in the development of high quality epitaxial layers and ZnO-based devices after reviewing the growth-related material properties such as crystal structure, lattice parameters, and thermal properties. In addition, doping and heterostructure devices, mainly optical emitters and transistors based on ZnO are discussed. Although ZnO is facing very stiff competition from GaN in terms of devices, it could address niche applications such as transparent contacts and thin film transistors and polariton lasers. Moreover recent results on p-type conductivity in ZnO achieved by codoping using MBE are very promising and further improvements might in fact pave the way for high efficiency ZnO light emitting devices.

BexMgyZn1−x−yO semiconductor solid solutions are attractive for UV optoelectronics and electronic devices owing to their wide bandgap and capability of lattice-matching to ZnO. In this work, a combined experimental and theoretical study of lattice parameters, bandgaps, and underlying electronic properties, such as changes in band edge wavefunctions in BexMgyZn1−x−yO thin films, is carried out. Theoretical ab initio calculations predicting structural and electronic properties for the whole compositional range of materials are compared with experimental measurements from samples grown by plasma assisted molecular beam epitaxy on (0001) sapphire substrates. The measured a and c lattice parameters for the quaternary alloys BexMgyZn1−x with x = 0−0.19 and y = 0–0.52 are within 1%–2% of those calculated using generalized gradient approximation to the density functional theory. Additionally, composition independent ternary BeZnO and MgZnO bowing parameters were determined for a and c lattice parameters and the b...

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic GaMnAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the GaMnAs/GaAs interface. While high resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the GaMnAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance-voltage profiling. The gradient in Curie temperature seems to be a general feature of GaMnAs layers grown at low-temperature. Possible explanations are discussed.

High resolution transmission electron microscopy and aberration-corrected scanning transmission electron microscopy (STEM) reveal a new void defect in GaN, Si-doped GaN, and InGaN. The voids are pyramid shaped with symmetric hexagonal {0001} base facets and {10-11} side facets. The pyramid void has a closed or open core dislocation at the peak of the pyramid, which continues up along the [0001] growth direction. The closed dislocations have a 1/3 11-20 edge dislocation Burgers vector component, consistent with known threading dislocations. The open core dislocations are hexagonal shaped with pure screw character, {10-10} side facets, varying lateral widths, and varying degrees of hexagonal symmetry. STEM electron energy loss spectroscopy spectrum imaging revealed a larger C concentration inside the void and below the void than above the void. We propose that carbon deposition during metal organic chemical vapor deposition growth acts as a mask, stopping the GaN deposition locally. Subsequent layers of GaN deposited around the C covered region create the overhanging {10-11} facets, and the meeting of the six {10-11} facets at the pyramid's peak is not perfect, resulting in a dislocation.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The effect of active layer design on the efficiency of InGaN light emitting diodes (LEDs) with the light emission in blue (~420 nm) ha s been studied. Correlation between the internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) and salient features of st ructures on c-plane InGaN LEDs which contain multiple quantum wells (MQWs) of different barrier height ( either In 0.01 Ga 0.99 N or In 0.06 Ga 0.94 N barriers ) and thicknes s (3 nm and 12 nm) as well as different double heterostructure (DH) designs (3 nm , dual 3 n m , 6 nm , dual 6 nm , 9 nm and 11 nm) with inserted 3 nm In 0.06 Ga 0.94 N barrier . Pulsed electroluminescence (EL) and optical excitation power -dependent photo luminescence (PL) measurements indic ated that the thinner and lower In 0.06 Ga 0.94 N barriers bode well for high EQE and IQE. Furthermore , increase of the effective active region thickness by multiple InGaN DH structures (dual, quad and hex) separated by 3 nm In

By using aberration corrected scanning transmission electron microscopy we have found no small scale lateral In composition fluctuations exist in the In0.15Ga0.85N active region of a light emitting diode. Images were acquired at 2% of the electron dose known to create electron beam damage, so the acquired images reflect the intrinsic structure of the InGaN active region. Position averaged convergent beam electron diffraction reveals the local sample thickness where images were acquired is 4.8 nm, eliminating the possibility that the absence of composition variation was observed due to projection through a thick sample. In addition, 2-3 atomic layer steps were observed in the top surface of In0.08Ga0.92N layers and the In0.15Ga0.85N active layers, providing a possible mechanism for lateral carrier confinement.