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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 8K 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 295K 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 x 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. [ABSTRACT FROM AUTHOR]

The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells are examined in 8 K-300 K 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. [ABSTRACT FROM AUTHOR]

Formation of defects in semipolar ([formula])-oriented GaN layers grown by metal-organic chemical vapor deposition on patterned Si (001) substrates and their effects on optical properties were investigated by steady-state and time-resolved photoluminescence (PL) and spectrally and spatially resolved cathodoluminescence (CL). Near-band edge emission is found to be dominant in the c+-wings of semipolar ([formula])GaN, which are mainly free from defect-related emission lines, while the c- wings contain a large number of basal stacking faults. When the advancing c+ and c- fronts meet to coalesce into a continuous film, the existing stacking faults contained in c- wings continue to propagate in the direction perpendicular to the c-axis and, as a result, the region dominated by stacking fault emission is extended to the film surface. Additional stacking faults are observed within the c+ wings, where the growing c+ wings of GaN are in contact with the SiO2 masking layer. Out-diffusion of oxygen/silicon species and concentration of strain near the contact region are considered as possible causes of the stacking fault formation. CL linescans performed along the surface and across the thickness of the non-coalesced and coalesced layers revealed that, while most of the material in the near-surface region of the non-coalesced layers is relaxed, coalescence results in nonuniform strain distribution over the layer surface. Red-shifted near-band-edge emission from the near-surface region indicates tensile stress near the surface of a coalesced layer, reaching a value of 0.3 GPa. The regions near the GaN/AlN/Si(111) interface show slightly blue shifted, broadened near-band-edge emission, which is indicative of a high concentration of free carriers possibly due to incorporation of shallow-donor impurities (Si and/or O) from the substrate or SiO2 mask. Steady-state and time-resolved PL results indicate that semipolar ([formula])GaN on patterned Si exhibits optical properties (PL intensity and carrier lifetimes) approaching to those of the state-of-the-art c-plane GaN grown using in situ SiNx nanonetwork mask on c-plane sapphire. Long PL lifetimes (∼2 ns) for the ([formula])GaN layers show that the semipolar material holds promise for light emitting and detecting devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed. [ABSTRACT FROM AUTHOR]

We report on the mechanisms governing electron transport using a comprehensive set of ZnO layers heavily doped with Ga (GZO) grown by plasma-enhanced molecular-beam epitaxy on a-plane sapphire substrates with varying oxygen-to-metal ratios and Ga fluxes. The analyses were conducted by temperature dependent Hall measurements which were supported by microstructural investigations as well. Highly degenerate GZO layers with n > 5 × 1020 cm-3 grown under metal-rich conditions (reactive oxygen-to-metal ratio <1) show relatively larger grains (∼20-25 nm by x-ray diffraction) with low-angle boundaries parallel to the polar c-direction. For highly conductive GZO layers, ionized-impurity scattering with almost no compensation is the dominant mechanism limiting the mobility in the temperature range from 15 to 330 K and the grain-boundary scattering governed by quantum-mechanical tunnelling is negligible. However, due to the polar nature of ZnO having high crystalline quality, polar optical phonon scattering cannot be neglected for temperatures above 150 K, because it further reduces mobility although its effect is still substantially weaker than the ionized impurity scattering even at room temperature (RT). Analysis of transport measurements and sample microstructures by x-ray diffraction and transmission electron microscopy led to a correlation between the grain sizes in these layers and mobility even for samples with a carrier concentration in the upper 1020 cm-3 range. In contrast, electron transport in GZO layers grown under oxygen-rich conditions (reactive oxygen-to-metal ratio >1), which have inclined grain boundaries and relatively smaller grain sizes of 10-20 nm by x-ray diffraction, is mainly limited by compensation caused by acceptor-type point-defect complexes, presumably (GaZn-VZn), and scattering on grain boundaries. The GZO layers with n <1020 cm-3 grown under metal-rich conditions with reduced Ga fluxes show a clear signature of grain-boundary scattering governed by the thermionic effect in the temperature-dependent mobility but with much higher RT mobility values compared to the samples grown under oxygen-rich conditions [34 vs. 7.5 cm2/V·s]. Properties of GZO layers grown under different conditions clearly indicate that to achieve highly conductive GZO, metal-rich conditions instead of oxygen-rich conditions have to be used. [ABSTRACT FROM AUTHOR]

We report a void defect in gallium nitride (GaN) and InGaN, revealed by aberration-corrected scanning transmission electron microscopy (STEM). The voids are pyramids with symmetric hexagonal {0001} base facets and {1011} side facets. Each pyramid void has a dislocation at the peak of the pyramid, which continues up along the [0001] growth direction to the surface. Some of the dislocations are hexagonal open core screw dislocations with {1010} side facets, varying lateral widths, and varying degrees of hexagonal symmetry. STEM electron energy loss spectroscopy spectrum imaging showed a large C concentration inside the void and on the void surfaces. There is also a larger C concentration in the GaN (or InGaN) below the void than above the void. We propose that inadvertent carbon deposition during metal organic chemical vapor deposition growth acts as a mask, stopping the GaN deposition locally, which in combination with lateral overgrowth, creates a void. Subsequent layers of GaN deposited around the C covered region create the overhanging {1011} facets, and the meeting of the six {1011} facets at the pyramid's peak is not perfect, resulting in a dislocation. [ABSTRACT FROM AUTHOR]

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-1020 cm−3 range with mobilities exceeding 100 cm2 V−1 s−1, 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 cm2 V−1 s−1). The first promising results have been reported for devices (light emitters and field effect transistors) entirely produced by sputtering. [ABSTRACT FROM AUTHOR]

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 properties, preparation methods accompanied by particulars of synthesis of thin films, devices based on them, and some projections into their future applications are discussed. [ABSTRACT FROM AUTHOR]

Cu-related green luminescence (GL) band in n-type ZnO layers grown by molecular beam epitaxy on sapphire and a bulk ZnO sample grown by hydrothermal method have been studied with above-bandgap excitation. Annealing of the samples in air at temperatures above 600 °C resulted in an increase in the concentration of CuZn acceptors, followed by a dramatic enhancement of the Cu-related GL band peaking at 2.45 eV, and a characteristic fine structure. The GL band quenched at temperatures between 250 and 500 K due to escape of holes from the excited state of the CuZn acceptor to the valence band. Energy position of this state in the bandgap of ZnO and its capture cross section for holes were estimated as 0.38 eV and 2×10-13 cm2, respectively. [ABSTRACT FROM AUTHOR]

There has been a resurgence of complex oxides of late owing to their ferroelectric and ferromagnetic properties. Although these properties had been recognized decades ago, the renewed interest stems from modern deposition techniques that can produce high quality materials and attractive proposed device concepts. In addition to their use on their own, the interest is building on the use of these materials in a stack also. Ferroelectrics are dielectric materials that have spontaneous polarization in certain temperature range and show nonlinear polarization-electric field dependence called a hysteresis loop. The outstanding properties of the ferroelectrics are due to non-centro-symmetric crystal structure resulting from slight distortion of the cubic perovskite structure. The ferroelectric materials are ferroelastic also in that a change in shape results in a change in the electric polarization (thus electric field) developed in the crystal and vice versa. Therefore they can be used to transform acoustic waves to electrical signal in sonar detectors and convert electric field into motion in actuators and mechanical scanners requiring fine control. In a broader sense the ferroelectric materials can be used for pyroelectric and piezoelectric sensors, voltage tunable capacitors, infrared detectors, surface acoustic wave (SAW) devices, microactuators, and nonvolatile random-access memories (NVRAMs), including the potential production of one transistor memory cells, and applications requiring nonlinear optic components. Another set of potential applications seeks to exploit the ferroelastic properties in stacked templates where they are juxtaposed to ferromagnetic materials. Doing so would allow the control of magnetic properties with electric field, which is novel. Such templates taking advantage two or more properties acquired a new name and now goes by multiferroics. After a brief historical development, this article discusses technological issues such as growth and processing, electrical and optical properties, piezo, pyro, and ferroelectric properties, degradation, measurements methods, and application of mainly lead-zirconate-titanate (PZT = PbZr1-xTixO3). The focus on PZT stems from its large electromechanical constant, large saturation polarization and large dielectric constant. [ABSTRACT FROM AUTHOR]

Pseudomorphic Si[sub 0.76]Ge[sub 0.24]/Si heterostructures grown by molecular-beam epitaxy were irradiated with 350-keV Ge[sup +] ions at a temperature of 400°C so that the peak of the ions’ energy losses was located within the silicon substrate (deeper than the SiGe–Si interface). The effect of ion implantation on the relaxation of elastic stresses and the defect structure formed as a result of postimplantation annealing is studied. It is found that annealing at a temperature even as low as 600°C makes it possible to ensure a very high degree of relaxation of elastic stresses in the heterostructure and a comparatively low density of threading dislocations in the SiGe layer (<10[sup 5] cm[sup –2]). The results obtained make it possible to suggest a method for the formation of thin SiGe/Si layers that feature a high degree of relaxation, low density of threading dislocations, and a good surface morphology. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

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 Å). 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. [ABSTRACT FROM AUTHOR]

Single-crystal, single-phase Pb(ZrxTi1-x)O3 films (x=0–0.4) were grown on (001) SrTiO3 and SrTiO3:Nb substrates by molecular beam epitaxy. Layer-by-layer growth of the Pb(Zr,Ti)O3 films was achieved by using PbTiO3 buffer layers between the SrTiO3 substrates and the Pb(Zr,Ti)O3 films. The layers with low Zr content showed high crystallinity with full width at half maximum of ω-rocking curves as low as 4 arc min, whereas increase in Zr concentration led to pronounced angular broadening. The PbZr0.07Ti0.93O3 films exhibited remanent polarization as high as 83 μC/cm2, but local areas suffered from nonuniform leakage current. [ABSTRACT FROM AUTHOR]

The need for diluted magnetic semiconductors has stimulated research on Mn-doped ZnO. However, the type of magnetic coupling (ferro/para) in ZnMnO remains an issue of debate. We have investigated the magnetic properties of Mn-doped ZnO layers grown by molecular beam epitaxy. Some samples showed a hysteresis with remnant magnetization on the order of 10-5 emu, thus eventually suggesting ferromagnetism. We observed that the critical influence of the substrate substantially affects magnetic property measurements. This has to be taken into account in order to clearly confirm ferromagnetism. In our case, after subtraction of the substrate effect, there is no evidence of a ferromagnetic behavior for the ZnMnO samples. [ABSTRACT FROM AUTHOR]

Electrical behavior of Sb in ZnO:Sb layers doped in a wide concentration range was studied using temperature dependent Hall effect measurements. The layers were grown by plasma-enhanced molecular beam epitaxy, and the Sb concentration was changed by varying the Sb flux, resulting in electron concentrations in the range of 1016 to nearly 1020 cm-3. Upon annealing, the electron concentration increased slightly and more notable was that the electron mobility significantly improved, reaching a room-temperature value of 110 cm2/V s and a low-temperature value of 145 cm2/V s, close to the maximum of ∼155 cm2/V s set by ionized impurity scattering. Hall data and structural data suggest that Sb predominantly occupies Zn sublattice positions and acts as a shallow donor in the whole concentration range studied. In the layers with high Sb content (∼1 at. %), acceptor-type compensating defects (possibly Sb on oxygen sites and/or point-defect complexes involving SbO) are formed. The increase of electron concentration with increasing oxygen pressure and the increase in ZnO:Sb lattice parameter at high Sb concentrations suggest that acceptors involving SbO rather than SbZn-2VZn complexes are responsible for the compensation of the donors. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012. [ABSTRACT FROM PUBLISHER]

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011. [ABSTRACT FROM PUBLISHER]

Spatial distribution of extended defects in semipolar [formula]-oriented GaN layers grown on patterned (001) Si substrates with striped grooves of varying width was investigated by optical means only using near-field scanning optical microscopy (NSOM) and cathodoluminescence (CL). A high density of basal and prismatic stacking faults was observed in the c- wings, and the threading dislocations in c+ wings, which appear as dark patterns in the NSOM and CL images, were found to bend toward the surface during the initial stages of growth. In the case when growing c+ front of GaN made contact with the SiO2 masking layer during growth, stacking faults were found to form also in the c+ wings. These additional stacking faults effectively blocked propagation of dislocations along the c+ direction, resulting in high quality stripes virtually free of defects. As revealed by optical means only without the need for any structural investigation, such control over the threading dislocation density using select growth geometries is potentially advantageous for improving semipolar [formula]GaN. [ABSTRACT FROM AUTHOR]