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This article discusses challenges in designing optically transparent antennas and filters for smart city applications. The smart city concept seeks to alleviate urban challenges that include infrastructure and network capacity. The proposed frequencies for the next generation of wireless networks (5G) would result in shorter broadcast distances and network dead zones. Additional access points and signal repeaters embedded into the existing infrastructure, by inserting transparent antennas into windows via either meshed conductors or transparent conductive oxides (TCOs), would help to mitigate these issues. Glass-embedded frequency-selective surfaces (FSSs) can be used anywhere that requires microwave filtering and optical transparency. The cyberphysical system security in this forthcoming network also poses challenges to entities that require strict network security. This article focuses mainly on TCOs, particularly doped zinc oxide (ZnO), as promising material for transparent antennas and filters. To demonstrate the potential of ZnOs as highly transparent conductive materials for antenna and filter applications, we designed, fabricated, and tested a gallium-doped ZnO- (GZO) based optically transparent 2.4-GHz Wi-Fi antenna and two FSSs (bandpass and bandstop filters) operating in the 23-29-GHz frequency range. These results confirm the potential for ZnO-based TCOs as suitable alternatives to indium tin oxide (ITO) for microwave applications.

We report the direct current (DC) and microwave performance of BeMgZnO/ZnO heterostructure field effect transistors (HFETs) on sapphire substrates. The devices fabricated using Al2O3 as the gate dielectric with a gate length of 1.5 um and a gate width of 75 um exhibited a pinch-off voltage of −4.0 V and a maximum peak transconductance of 63 mS/mm. A current gain cutoff frequency fT of 5.0 GHz was achieved, highest among ZnO-based FETs. The corresponding electron velocity of above 1E7 cm/s estimated based on the gate transit time inches closer to the theoretical peak velocity in ZnO (3.5E7 cm/s). This value is significantly higher than the previously reported values in ZnO-based HFETs, which is attributed to the two-dimensional electron gas (2DEG) concentration at or near the resonance of longitudinal optical (LO)-phonon and plasmon frequencies as well as the improved quality of the heterostructure owing to optimized ZnO buffer growth and BeO and MgO alloying in the barrier. To probe the high-frequency response of the HFETs, extrinsic and intrinsic parameters of the small-signal equivalent circuit for the BeMgZnO/ZnO HFETs were investigated using the hybrid extraction method.

Previously we have conducted researches on the molecular beam epitaxy (MBE) of Zn-polar BeMgZnO/ZnO heterostructure field effect transistor (HFET) structures on GaN templates, which exhibit both high two-dimensional gas (2DEG) density and high electron mobility with relatively low Mg contents via the strain modulation of Be and Mg co-incorporation. In this contribution, we report on the growth of the HFET structures directly on sapphire substrate by employing a buffer consisting of a rock-salt structure MgO layer, a low-temperature (LT) ZnO layer and a high-temperature (HT) ZnO layer. Compared with growth of O-polar on sapphire, in which a thin and wurtzite MgO buffer is deposited at high temperatures above 700 °C, the MgO buffer for Zn-polar growth has to be reduced to 450 °C, in order to obtain smooth interface and surface for the BeMgZnO/ZnO HFETs. The residual electron sheet concentration in Zn-polar ZnO layers is ~2×1012 cm-2 on GaN while semi-insulating Zn-polar ZnO layers on sapphire have been obtained via controlling the buffer growth conditions, which is vital to the realization of HFET device structures.

Photoluminescence (PL) was used to estimate the concentration of carbon in GaN grown by hydride vapor phase epitaxy (HVPE). The PL data were compared with profiles of the impurities obtained from secondary ion mass spectrometry (SIMS) measurements. Comparison of PL and SIMS data has revealed that apparently high concentrations of C and O at depths up to 1 µm in SIMS profiles do not represent depth distributions of these species in the GaN matrix but are rather caused by post-growth surface contamination and knocking-in impurity species from the surface. In particular, PL analysis supplemented by reactive ion etching up to the depth of 400 nm indicates that the concentration of carbon in nitrogen sites is below 2–5 × 1015 cm−3 at any depth of GaN samples grown by HVPE. We demonstrate that PL is a very sensitive and reliable tool to determine the concentrations of impurities in the GaN matrix.

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 ...

Accumulation of non-equilibrium hot longitudinal optical (LO) phonons limits the electron drift velocity for electronic devices operating under high electric field. Ultrafast decay of hot phonons can take place via plasmon-LO phonon resonance, which leads to fast electron energy relaxation and hence high electron drift velocity and optimum operation of the devices. This need motivates us to create heterostructures with 2DEG density close to the plasmon-LO phonon resonance region. Through incorporating a few percent of Be into the BeMgZnO barrier to switch the strain sign in the barrier from compressive to tensile, we have achieved 2DEG densities over a wide range in Zn-polar BeMgZnO/ZnO heterostructures with moderate Mg content (below 30%) grown by molecular beam epitaxy. We have obtained electron mobility of 250 cm2/Vs at room temperature (293 K) and 1800 cm2/Vs at 13 K in Be0.02Mg0.26ZnO/ZnO heterostructures. Via capacitance-voltage (CV) spectroscopy, we have explored the depth profiles of the apparent carrier density of samples grown under different conditions. The correlations between electrical properties and MBE growth parameters of Zn-polar BeMgZnO/ZnO heterostructures are discussed.

To integrate plasmonic devices into industry, it is essential to develop scalable and CMOS compatible plasmonic materials. In this work, we report high plasmonic quality titanium nitride (TiN) on c-plane sapphire grown by plasma-enhanced atomic layer deposition. TiN with low losses, high metallicity, and a plasma frequency below 500 nm was achieved at temperatures less than 500 °C by exploring the effects of chemisorption time, substrate temperature, and plasma exposure time on the material properties. A reduction in chemisorption time mitigates premature precursor decomposition at TS > 375 °C, and a trade-off between reduced impurity concentration and structural degradation caused by plasma bombardment is achieved for 25 s plasma exposure. 85 nm thick TiN films grown at a substrate temperature of 450 °C, compatible with CMOS processes, with 0.5 s chemisorption time and 25 s plasma exposure exhibited a high plasmonic figure of merit ( | e ′ / e ′ ′ |) of 2.8 and resistivity of 31 μ Ω cm. As a result of the improved quality, subwavelength apertures were fabricated in the TiN thin films and are shown to exhibit extraordinary transmission.

Growth of ZnO directly on c-plane sapphire, with a large in-plane lattice mismatch of 18%, is relatively well understood for O-polar variety. However, the two-dimensional (2D) growth of Zn-polar ZnO on c-sapphire, needed for 2D electron gas formation, with low background electron density is in its infancy. While Zn-polar ZnO can be grown on GaN with the resulting small lattice mismatch (1.8%), the parallel conduction through GaN (bulk and/or surface) in fabricated BeMgZnO/ZnO heterostructure field effect transistors (HFETs) limits its utility for the time being. In this contribution, the authors report on the growth of high-quality Zn-polar BeMgZnO/ZnO HFET structures directly on sapphire substrates, in an effort to avoid the aforementioned parallel conduction, by employing an MgO and low-temperature ZnO buffer stack. The residual 2D equivalent concentration in the HFET structure on GaN is ∼2 × 1012 cm−2, which is detrimental, while highly resistive (>100 Ω.cm) Zn-polar ZnO layers with smooth surfaces on c-sapphire have been obtained via controlling the buffer stack growth conditions, which is vital to the realization of HFET device structures. Compared with the highest room temperature electron mobility of ∼250 cm2/V s in BeMgZnO/ZnO HFETs on GaN templates, a slightly lower electron mobility of ∼220 cm2/V s was achieved on c-sapphire due to a somewhat lower overall crystalline quality.

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.

We investigate and find that the quality of titanium nitride grown at 350°C and 375°C using atomic layer deposition on silicon and sapphire substrates respectively are approaching the metallicity of sputtered films.

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.

An overview of the current status and prospects of thin-film Si photovoltaics, including both hydrogenated amorphous and microcrystalline Si as well their combination known as micromorph solar cells, with a major focus on the technological development is given. Although thin-film Si solar cells have been one of the first commercially successful photovoltaic devices, today they face a tremendous challenge from variety of bulk Si technologies (mono- and multicrystalline Si) and compound-semiconductor thin-film solar cells, both of which have managed to substantially reduce the production cost and improve power conversion efficiency. Thin-film Si photovoltaics benefiting from the mighty mainstream Si industry have demonstrated the ability to reduce the production cost and cost per peak power; however, the performance improvement of amorphous-Si panels has been only marginal and further progress is hard to expect. The power conversion efficiencies of micromorph solar cells have already exceeded those of amorphous-Si devices. However, to improve their market penetration and even to hold their current share, the micromorph Si solar modules need to improve their performance, which today is approaching 11%, along the path to potential 15%, while keeping the manufacturing cost low. Further progress will require the improvement of light-trapping technologies and contact performance.

This article provides an overview of recent development of sputtering method for high-quality III-nitride semiconductor materials and devices. Being a mature deposition technique widely employed in semiconductor industry, sputtering offers many advantages such as low cost, relatively simple equipment, non-toxic raw materials, low process temperatures, high deposition rates, sharp interfaces, and possibility of deposition on large-size substrates, including amorphous and flexible varieties. This review covers two major research directions: (1) ex situ sputtered AlN buffers to be used for subsequent growth of GaN-based structures by conventional techniques, such as metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy (MBE), and (2) deposition of the entire III-nitride layered stacks and device structures by sputtering. Replacing conventional in situ GaN or AlN buffer layers with ex situ sputtered AlN buffers for MOCVD, HVPE, or MBE growth of III-nitride films on sapphire and silicon substrates results in the improved crystal quality through reduction in dislocation density and residual strain. Extensive efforts in the field of sputter deposition of III-nitrides resulted in crystalline quality of sputtered III-nitride films compatible with that of MOCVD and MBE grown layers despite the lower temperatures used in sputtering. For example, sputtering techniques made it possible to achieve GaN layers heavily doped with Si and Ge to electron concentrations in mid-10alt;supagt;20alt;/supagt; cmalt;supagt;-3alt;/supagt; range with mobilities exceeding 100 cmalt;supagt;2alt;/supagt; Valt;supagt;-1alt;/supagt; salt;supagt;-1alt;/supagt;, resulting in conductivities as high as those of benchmark transparent conducting oxides such as indium tin oxide (ITO). For moderate levels of doping with Si, mobilities comparable to state-of-the-art MOCVD-grown material have been demonstrated (up to ~1000 cmalt;supagt;2alt;/supagt; Valt;supagt;-1alt;/supagt; salt;supagt;-1alt;/supagt;). The first promising results have been reported for devices (light emitters and field effect transistors) entirely produced by sputtering.

Compared with conventional display technologies, liquid crystal display (LCD), and organic light emitting diode (OLED), micro-LED displays possess potential advantages such as high contrast, fast response, and relatively wide color gamut, low power consumption, and long lifetime. Therefore, micro-LED displays are deemed as a promising technology that could replace LCD and OLED at least in some applications. While the prospects are bright, there are still some technological challenges that have not yet been fully resolved in order to realize the high volume commercialization, which include efficient and reliable assembly of individual LED dies into addressable arrays, full-color schemes, defect and yield management, repair technology and cost control. In this article, we review the recent technological developments of micro-LEDs from various aspects.

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 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.

Heteroepitaxial semipolar and nonpolar GaN layers often suffer from high densities of extended defects including basal plane stacking faults (BSFs). BSFs which are considered as inclusions of cubic zinc-blende phase in wurtzite matrix act as quantum wells strongly affecting device performance. Band alignment in BSFs has been discussed as type of band alignment at the wurtzite/zinc blende interface governs the response in differential transmission; fast decay after the pulse followed by slow recovery due to spatial splitting of electrons and heavy holes for type- II band alignment in contrast to decay with no recovery in case of type I band alignment. Based on the results, band alignment is demonstrated to be of type II in zinc-blende segments in wurtzite matrix as in BSFs.

The effect of compressive strain in buffer layer on strain relaxation and indium incorporation in InGaN multi-quantum wells (MQWs) is studied for two sets of samples grown side by side on both relaxed GaN layers and strained 10-pairs of AlN/GaN periodic multilayers. The 14-nm AlN layers were utilized in both multilayers, while GaN thickness was 4.5 and 2.5 nm in the first and the second set, respectively. The obtained results for the InGaN active layers on relaxed GaN and AlN/GaN periodic multilayers indicate enhanced indium incorporation for more relaxed InGaN active layers providing a variety of emission colors from purple to green.

Near-field scanning optical microscopy was applied to investigate the spatial variations of extended defects and their effects on the optical quality for semi-polar (1-101) and (11-22) InGaN light emitting diodes (LEDs). (1-101) and (11-22) oriented InGaN LEDs emitting at 450-470 nm were grown on patterned Si (001) 7° offcut substrates and m-sapphire substrates by means of nano-epitaxial lateral overgrowth (ELO), respectively. For (1-101) structures, the photoluminescence (PL) at 85 K from the near surface c+ wings was found to be relatively uniform and strong across the sample. However, emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations (TDs) and basal plane stacking faults (BSFs) as revealed from the local PL spectra. In case of (11-22) LED structures, near-field PL intensity correlated with the surface features and the striations along the direction parallel to the c-axis projection exposed facets where the Indium content was higher as deduced from shift in the PL peak energy.

Can, Nuri (Balikesir Author), Among the major obstacles for development of non-polar and semipolar GaN structures on foreign substrates are stacking faults which deteriorate the structural and optical quality of the material. In this work, an in-situ SiNx nano-network has been employed to achieve high quality heteroepitaxial semipolar (11 (2) over bar2) GaN on m-plane sapphire with reduced stacking fault density. This approach involves in-situ deposition of a porous SiNx interlayer on GaN that serves as a nano-mask for the subsequent growth, which starts in the nanometer-sized pores (window regions) and then progresses laterally as well, as in the case of conventional epitaxial lateral overgrowth (ELO). The inserted SiNx nano-mask effectively prevents the propagation of defects, such as dislocations and stacking faults, in the growth direction and thus reduces their density in the overgrown layers. The resulting semipolar (11 (2) over bar2) GaN layers exhibit relatively smooth surface morphology and improved optical properties (PL intensity enhanced by a factor of 5 and carrier lifetimes by 35% to 85% compared to the reference semipolar (11 (2) over bar2) GaN layer) which approach to those of the c-plane in-situ nano-ELO GaN reference and, therefore, holds promise for light emitting and detecting devices., Materials World Network grant from the National Science Foundation - DMR-1210282 German Research Foundation, DFG -FOR 957 Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK)

In the last decade, the photovoltaic industry grew at a rate exceeding 30% per year. Currently, solar-cell modules based on single-crystal and large-grain polycrystalline silicon wafers comprise more than 80% of the market. Bulk Si photovoltaics, which benefit from the highly advanced growth and fabrication processes developed for microelectronics industry, is a mature technology. The light-to-electric power conversion efficiency of the best modules offered on the market is over 20%. While there is still room for improvement, the device performance is approaching the thermodynamic limit of ∼28% for single-junction Si solar cells. The major challenge that the bulk Si solar cells face is, however, the cost reduction. The potential for price reduction of electrical power generated by wafer-based Si modules is limited by the cost of bulk Si wafers, making the electrical power cost substantially higher than that generated by combustion of fossil fuels. One major strategy to bring down the cost of electricity generated by photovoltaic modules is thin-film solar cells, whose production does not require expensive semiconductor substrates and very high temperatures and thus allows decreasing the cost per unit area while retaining a reasonable efficiency. Thin-film solar cells based on amorphous, microcrystalline, and polycrystalline Si as well as cadmium telluride and copper indium diselenide compound semiconductors have already proved their commercial viability and their market share is increasing rapidly. Another avenue to reduce the cost of photovoltaic electricity is to increase the cell efficiency beyond the Shockley–Queisser limit. A variety of concepts proposed along this avenue forms the basis of the so-called third generation photovoltaics technologies. Among these approaches, high-efficiency multi-junction solar cells based on III–V compound semiconductors, which initially found uses in space applications, are now being developed for terrestrial applications. In this article, we discuss the progress, outstanding problems, and environmental issues associated with bulk Si, thin-film, and high-efficiency multi-junction solar cells.

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.

In both light emitting devices such as light emitting diodes (LEDs), and light absorbing devices such as solar cells (also photodetectors), which are gaining considerable interest for their energy saving and energy production capability, respectively, a compromise must be struck between the need to increase the light emitting/absorbing area/potential and the need for low series resistance of the metal contact grid. This undesirable compromise can be mitigated by using transparent conducting oxides (TCOs), which heretofore have been dominated by ITO (indium tin oxide—an In-rich alloy of indium oxide and tin oxide). Due to the expected scarcity of Indium used in ITO, efforts are underway to develop indium-free TCOs for the above-mentioned devices as well as flat panel displays. ZnO heavily doped with Ga or Al (GZO or AZO) is becoming a very attractive candidate for future generation TCOs. GZO and AZO as well as multilayer TCOs consisting of two TCO layers with a thin metal layer in between have been widely investigated for LEDs and solar cells to enhance device performance. This article succinctly reviews the latest developments in and properties of TCOs, particularly in relation to thin film transparent electrode applications for LEDs and solar cells. Pertinent critical issues and possible solutions are provided as well.

Both GaN- and ZnO-based diluted semiconductors (DMSs) have recently attracted considerable interest fueled by theoretical predictions of ferromagnetic ordering in these materials above room temperature, making them especially attractive for spintronics. The intense experimental research that followed has revealed, however, a great deal of controversy. The local structure and magnetic behavior of GaN- and ZnO-based DMSs were found to depend strongly on a preparation technique and growth conditions for the materials of the same nominal composition and the reported results varied considerably from group to group. This problem highlighted clearly the lack of theoretical understanding of physical mechanisms underlying ferromagnetisms in these materials and the inadequacy of standard characterization techniques used to probe structural and magnetic properties of the DMSs. In this paper, we report on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs and make special impact on critical discussion of experimental methods employed for investigation of their magnetic and optical properties.

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.

We review and critique the recent developments on multifunctional oxide materials, which are gaining a good deal of interest. Recongnizing that this is a vast area, the focus of this treatment is mainly on high-κ dielectric, ferroelectric, magnetic, and multiferroic materials. Also, we consider ferrimagnetic oxides in the context of the new, rapidly developing field of negative-index metamaterials. This review is motivated by the recent resurgence of interest in complex oxides owing to their coupling of electrical, magnetic, thermal, mechanical, and optical properties, which make them suitable for a wide variety of applications, including heat, motion, electric, and magnetic sensors; tunable and compact microwave passive components; surface acoustic wave devices; nonlinear optics; and nonvolatile memory, and pave the way for designing multifunctional devices and unique applications in spintronics and negative refraction-index media. For most of the materials treated here, structural and physical propertie...

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.

ZnO has gained interest in part because of its large exciton binding energy (60 meV), good for lasing with low threshold. The renewed interest in ZnO is fuelled by availability of high quality substrates, reports of p-type conductivity, and ferromagnetic behavior when doped with transitions metals. The field is also fuelled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. Despite great strides there are still many challenges, the chief among them is the attainment of reproducibly low resistivity p-type ZnO. While ZnO already has many industrial applications owing to its piezoelectric properties and bandgap in the near UV, its applications to optoelectronic devices utilizing electrical injection has not yet been put to practice mainly due to the lack of p-type epitaxial layers. This review provides a discussion of some of the growth issues of ZnO-based epitaxial and heteroepitaxial layers and their use in a variety of devices.

We overview fundamental properties, preparation techniques, and potential device applications of single- and few-monolayer-thick molybdenum disulfide MoS2 belonging to a new emerging class of materials: 2-dimensional semiconductors. To a large extent, the interest in the 2-dimensional materials is fueled by the quest for alternatives to graphene, which is hardly suitable for electronic devices because of the lack of a band gap. A unique combination of physical properties, including flexibility, high electron mobility, and optical transparency combined with a large band gap tunable from indirect 1.2 eV for bulk to direct 1.9 eV for a monolayer, make MoS2 attractive for a variety of electronic and optoelectronic devices. The first device demonstrations are very encouraging: field-effect transistors with high current ON/OFF ratios, high-sensitivity phototransistors, logic circuits, and amplifiers based on monolayer-MoS2 have been demonstrated. The layered structure of MoS2 with other transition-metal dichalcogenides (WS2, WSe2, MoSe2, etc.) makes it possible to grow ultrathin, so-called van der Waals heterostructures with very abrupt interfaces and low defect density and paves the way for fabricating multilayered materials for novel electronic and optical devices. However, further progress towards potential commercialization of monolayer-MoS2-based devices requires serious efforts in the development of scalable fabrication techniques capable of producing large-area electronic-quality material.

Reduced electric field in semipolar (1122) GaN/InGaN heterostructures makes this orientation attractive for high efficiency light emitting diodes. In this work, we investigated indium incorporation in semipolar (1122) GaN grown by metal-organic chemical vapor deposition on planar m-plane sapphire substrates. Indium content in the semipolar material was compared with that in polar c-plane samples grown under the same conditions simultaneously side by side on the same holder. The investigated samples incorporated dual GaN/InGaN/GaN double heterostructures with 3nm wide wells. In order to improve optical quality, both polar and semipolar templates were grown using an in-situ epitaxial lateral overgrowth (ELO) technique. Indium incorporation efficiency was derived from the comparison of PL spectra measured on the semipolar and polar structures at the highest excitation density, which allowed us to minimize the effect of quantum confined Stark effect on the emission wavelength. Our data suggests increased indium content in the semipolar material by up to 3.0%, from 15% In in c- GaN to 18% In in (1122) GaN.

Enhancement of optical and structural quality of semipolar (11‾22) GaN grown by metal-organic chemical vapor deposition on planar m-sapphire substrates was achieved by using an in-situ epitaxial lateral overgrowth (ELO) technique with nanoporous SiNx layers employed as masks. In order to optimize the procedure, the effect of SiNx deposition time was studied by steady-state photoluminescence (PL), and X-ray diffraction. The intensity of room temperature PL for the (11‾22) GaN layers grown under optimized conditions was about three times higher compared to those for the reference samples having the same thickness but no SiNx interlayers. This finding is attributed to the blockage of extended defect propagation toward the surface by the SiNx interlayers as evidenced from the suppression of emissions associated with basal-plane and prismatic stacking faults with regard to the intensity of donor bound excitons (D 0 X) in lowtemperature PL spectra. In agreement with the optical data, full width at half maximum values of (11‾22) X-ray rocking curves measured for two different in-plane rotational orientations of [1‾100] and [11‾23] reduced from 0.33o and 0.26o for the reference samples to 0.2o and 0.16o for the nano-ELO structures grown under optimized conditions, respectively.

The effects of stacking faults (SFs) on optical processes in epitaxially grown semipolar (1122) GaN on m-sapphire substrate have been investigated in detail using steady-state photoluminescence (PL) and time- and polarization-resolved PL. We demonstrate that the carrier recombination dynamics are substantially influenced due to strong carrier localization in the stacking faults. In addition to nonradiative recombination, carrier trapping/detrapping and carrier transfer between the stacking faults and donors are also found to be among the mechanisms affecting the recombination dynamics at different temperatures. PL decay times of both I 1 -type BSF and 3.31 eV SF (E-type BSF or prismatic stacking fault) do not show temperature dependence up to 80 K while 3.31 eV SF exhibits longer PL decay times (~3 ns) at low temperatures as compared to I 1 -type BSF (~1 ns), indicative of lower efficiency for radiative recombination. After 80 K, PL decay times decreased by power of ~-1 and ~-2 for 3.31 eV SF and I 1 -type BSF, respectively. It is obtained from radiative decay times with respect to temperature that the carrier localization becomes higher in I 1 -type BSF compared to 3.31 eV SF increasing the temperature. I 1 -type BSF also shows higher PL intensity, which is attributed to larger density, and therefore, larger contribution to recombination dynamics as compared to other type of stacking faults. Polarization-resolved PL measurements also revealed that the degree of polarization for the I 1 -type BSF (0.30) was twice that for the 3.31 eV SF.

We studied the temperature dependence and the recombination dynamics of the photoluminescence of (1-101)-oriented semi-polar Al0.2Ga0.8N/GaN multiple quantum wells (MQW). The polarized low-temperature PL measurements reveal that radiative recombination exhibit an anisotropic behavior. The PL intensity at room temperature is reduced by one order of magnitude with respect to low temperature. The radiative decay time exhibits a mixed behavior: it is roughly constant between 8K to ranging near 140-150K and then rapidly increases with a slope of 10 ps.K-1. This behavior is indicative of coexistence of localized excitons and free excitons which relative proportion are statistically computed.

Boron nitride occurs in several polymorphs, among which cubic and hexagonal varieties have been extensively studied over many years and have found numerous practical applications. Although various forms of BN have been used as lubricant and abrasive material for many years, the unique electronic properties of BN have not been yet utilized. Due to their wide optical bandgaps (the widest among nitride semiconductors), cubic and hexagonal polymorphs of BN are promising candidates for optoelectronic devices operating in the deep-ultraviolet range and for high-power electronic devices for switching and radio-frequency applications. The layered structure of hexagonal BN allows preparation of ultrathin nanosheets with very smooth surfaces and low defect density, which are proved to be useful as supporting substrates and gate dielectric layers in graphene-based structures. The first device demonstrations are very encouraging. In this contribution, the recent progress in properties, fabrication technologies, and emerging applications of BN as a wide-gap semiconductor material is reviewed.

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.

Aberration-corrected scanning transmission electron microscopy images of the In0.15Ga0.85N active region of a blue light-emitting diode, acquired at ~0.1% of the electron dose known to cause electron beam damage, show no lateral compositional fluctuations, but do exhibit one to four atomic plane steps in the active layer’s upper boundary. The area imaged was measured to be 2.9 nm thick using position averaged convergent beam electron diffraction, ensuring the sample was thin enough to capture compositional variation if it was present. A focused ion beam prepared sample with a very large thin area provides the possibility to directly observe large fluctuations in the active layer thickness that constrict the active layer at an average lateral length scale of 430 nm.

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.

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)

The dislocation structure and photoluminescence of partially relaxed Si1-xGex layers on Si(001) substrates were studied to reveal the contribution from dislocations localized in different regions of the heterostructure (SiGe layer, SiGe/Si interface, Si substrate) to the dislocation-related PL. The D1 and D2 lines were ascribed to products of dislocation reactions in intersection sites. The known dependence of the D4 line spectral position on the Ge content is not observed, which is explained by the effect of elastic strain in the SiGe/Si heterostructure.

Experimental data and Monte-Carlo modeling of hot-electron fluctuations are used for investigation of hot-electron energy relaxation in donor-doped bulk wurtzite ZnO and GaN subjected to a pulsed dc. The extracted electron energy relaxation time depends on the electron density. The experimental value of 0.84 ps is obtained at 5.5×1017 cm-3 in a reasonably good agreement with the results reported from time-resolved optical measurements. The relaxation becomes faster at a lower electron density where the optical data are not available. The density dependence is discussed in the frame of hot-phonon effect.