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Room-temperature ultraviolet lasing is demonstrated in mirrorless zinc oxide thin-film waveguides on (100) silicon substrate. Laser cavities, due to closed-loop optical scattering from the lateral facets of the irregular zinc oxide grains, are generated through the post-growth annealing of high-crystal-quality zinc oxide thin films obtained from the filtered cathodic vacuum arc technique. It is found that the lasing wavelength and linewidth of the zinc oxide random lasers under 355 nm optical excitation are around 390 nm and less than 0.4 nm, respectively. In addition, the lasing threshold characteristics are in good agreement with the random laser theory.

The integration of two-dimensional materials such as transition metal dichalcogenides with bulk semiconductors offer interesting opportunities for 2D/3D heterojunction-based device structures without any constraints of lattice matching. By exploiting the favorable band alignment at the GaN/MoS2 heterojunction, an Esaki interband tunnel diode is demonstrated by transferring large area Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current density of 446 A/cm2 with repeatable room temperature negative differential resistance, peak to valley current ratio of 1.2, and minimal hysteresis was measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of 1 kA/cm2 was measured in the Zener mode (reverse bias) at −1 V bias. The GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk semiconductor, and the electrostatics at the 2D/3D interface was found to be crucial in explaining the experimentally observed device characteristics.

Zinc oxide (ZnO) thin-film waveguides with ridge structures have been fabricated on n-type (100) silicon substrates. The deposition of high-crystal-quality ZnO thin films on the lattice-mismatched silicon substrate was achieved by using the filtered cathodic vacuum arc technique. A ridge structure is defined on the ZnO thin film by plasma etching. Room temperature amplified spontaneous emission with peak wavelength at 385 nm is observed under 355 nm optical excitation. The pump threshold is found to be around 0.45 MW/cm2. The maximum net optical gain of the ZnO waveguide is larger than 120 cm−1 at a pump intensity of 1.9 MW/cm2.

The etch rate of bulk ZnO in Cl2/Ar high density plasmas was found to be thermally activated with an activation energy of ∼0.31 eV at 200 °C).

We demonstrate experimentally significant improvement in the sensitivity of photoelectromotive-force (photo-EMF) laser vibrometers using pulsed-light sources. The vibrating surface is discretely sampled by individual laser pulses and recorded by the photo-EMF sensor via the generation of photocurrent pulses whose magnitudes are proportional to the instantaneous surface displacements. With a sufficiently high sampling rate, reconstruction of the vibration wave form can be achieved by conducting envelope (or peak) detection of the resultant series of photocurrent pulses. Significantly higher peak optical power levels of the probe laser pulses, which can be orders of magnitude greater than those of continuous-wave interrogation lasers with the same average power, lead to proportional enhancement in the photo-EMF response and remarkable improvement in detection sensitivity when the photodetection process is initially amplifier noise current limited.

We have investigated terahertz (THz) emission due to dynamical electron transport in wide miniband GaAs/Al0.3Ga0.7As superlattices by time-resolved THz emission spectroscopy. Quasiautocorrelation measurements were performed on the emitted THz electromagnetic waves by using a bolometer as a wideband detector. The bias-field dependence of the emitted THz radiation intensity clearly showed a crossover from the miniband transport to the formation of Wannier–Stark ladder. In the Wannier–Stark regime, a few cycle Bloch oscillations were observed even at 300 K in a superlattice whose miniband width is greater than the longitudinal optical phonon energy. Furthermore, it has been found that clear Bloch oscillations are visible only when the bottom of the miniband is photoexcited.

The investigation of As desorption from the (001) InP during metalorganic chemical vapor deposition was performed using surface photoabsorption (SPA). The behavior of the monochromatic SPA signal showed a clear existence of a metastable state after the AsH3 was turned off. SPA spectra at each stable surface were taken to confirm the interpretation. This result indicates that the previous criterion to measure the amount of As/P exchange reaction is not necessarily correct and that the As-desorption process should be understood as a two-step process.

The feasibility of using Cu(Mg) alloy film as a gate electrode for thin-film transistor (TFT) liquid crystal displays has been investigated. When pure Cu was used as a gate electrode, severe interdiffusion occurred between Cu and the gases SiH4, NH3, and CF4 during plasma-enhanced chemical vapor deposition of a gate dielectric, SiNx, and dry etching of the SiNx. On the other hand, the deposition of a Cu(Mg) alloy film gives rise to the formation of a MgO/Cu bilayer structure with low Cu resistivity, good adhesion to SiO2, higher leakage current density, and excellent passivation capability. A hydrogenated amorphous silicon TFT with a MgO encapsulated Cu gate exhibited a gate voltage swing of 0.91 V/dec. and a threshold voltage of 6.8 V, resulting in a reduction of process steps and better performance.

The optical properties of the gallium nitride (GaN) nanowires are examined by the transmission method in the ultraviolet-visible range (1–5 eV) and by the reflection method in the infrared range (500–4000 cm−1). The absorption edge of the GaN nanowires is blueshifted by 0.2 eV from the bulk edge. The temperature dependence of the energy gap is expressed by, Eg(T)=3.724−9.97×10−4/(861+T) eV. The plasma frequency and the free-carrier density of the GaN nanowires, deduced from the infrared reflectance minima, are estimated to be ωp=1100±120 cm−1 and nf=3.73×1017 cm−3, respectively. Obtaining the free-carrier density from the infrared reflectance spectra is especially useful in research on nanostructured semiconductors.

Photoluminescence and cross-sectional transmission electron microscopy, combined with x-ray compositional analysis, have been used to study quantum well intermixing in an InGaAsP quantum well laser structure. Quantum well intermixing is induced by capping the samples with a layer of InP grown at low temperature (300 °C) and subjecting them to rapid thermal anneal treatments in the temperature range 600–800 °C. The presence of the low temperature InP layer, which contains an abundance of nonequilibrium point defects, significantly enhances the intermixing on annealing, producing a large band-gap blueshift. The microscopy results show good broadening with smeared interfaces, and the compositional analysis suggests this can be attributed to the intermixing of group V atoms.

We investigated the transport properties in YBa2Cu3O7−x (YBCO) ramp-edge Josephson junctions before and after 1.3 GeV Pb-ion irradiation. The barrier of the junctions was formed by structural modification of the ramp-edge surface of the YBCO base electrode. After a dose of ∼2×1010 Pb ions/cm2, both critical current and normal resistance of the junctions increased by ∼30% and 10%, respectively, except near the transition temperature. We interpret such an enhancement of the critical current as a result of impeding Josephson vortex motion by pinned magnetic flux in the electrodes.

X-ray reflectivity has been used to measure nondestructively the density of thin, porous, silica xerogels used for interlayer dielectric applications. The critical angle, defined through total external reflection, was measured for multiple x-ray energies to correct for sample misalignment error in the determination of the density for the films. This density was used to extrapolate the percentage porosity, assuming a bulk SiO2 density standard. The results were compared to those obtained by Rutherford backscattering and ellipsometry techniques.

SiO2 layers were deposited on p-GaN (hole concentration 9×1017 cm−3) by inductively coupled plasma chemical vapor deposition using an 17O-enriched O2 precursor. The samples were then annealed at 500–900 °C and the SiO2 was removed. Secondary ion mass spectrometry profiling showed significant indiffusion of 17O into the GaN under these conditions, with an incorporation depth of ∼0.18 μm after the 900 °C anneal. The 17O diffusion profiles indicate that the high dislocation density in the GaN strongly affects the effective penetration depth. The GaN remained p type upon incorporation of the oxygen.

We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

The effect of interdiffusion on strained InGaAs/GaAs quantum-well infrared photodetectors is investigated. Photoluminescence measurements of the interband transition indicate that there is minimal deterioration of the annealed heterostructures, as it is also evident from both the transverse electric and transverse magnetic infrared intersubband optical transitions. The absorption peak wavelength is redshifted from the as-grown 10.2 μm to 10.5 and 11.2 μm for 5 and 10 s annealing, respectively, at 850 °C without appreciable degradation in absorption strength. The peak responsivity of the as-grown and annealed spectra is of comparable amplitude, whereas the annealed spectra become narrower in shape. The dark current of the annealed devices is about an order of magnitude higher than the as-grown one at 77 K.

Transfer of epitaxial, two-dimensional (2D) MoS2 on sapphire grown via synthetic approaches is a prerequisite for practical device applications. We report centimeter-scale, single crystal, synthesized MoS2 field effect transistors (FETs) transferred onto SiO2/Si substrates, with a field-effect mobility of 4.5 cm2 V−1 s−1, which is among the highest mobility values reported for the transferred large-area MoS2 transistors. We demonstrate simple and clean transfer of large-area MoS2 films using deionized water, which can effectively avoid chemical contamination. The transfer method reported here allows standard i-line stepper lithography process to realize multiple devices over the entire film area.

Electrical and optical characterization of two-dimensional/three-dimensional (2D/3D) p-molybdenum disulfide/n-gallium nitride (p-MoS2/n-GaN) heterojunction diodes are reported. Devices were fabricated on high-quality, large-area p-MoS2 grown by chemical vapor deposition (CVD) on hexagonal sapphire substrates. The processed devices were transferred onto GaN/sapphire substrates, and the transferred films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). On-axis XRD spectra and surface topology obtained from AFM scans were consistent with previously grown high-quality, continuous MoS2 films. Current-voltage measurements of these diodes exhibited excellent rectification, and capacitance-voltage measurements were used to extract a conduction band offset of approximately 0.2 eV for the transferred MoS2/GaN heterojunction. This conduction band offset was confirmed by internal photoemission (IPE) measurements. The energy band lineup of the MoS2/GaN heterojunction is proposed here. This work demonstrates the potential of 2D/3D heterojunctions for novel device applications., 11 pages, 6 figures

Strong photoluminescence is observed in strained GaInP quantum wells (QW) grown on GaP. Variable temperature photoluminescence indicates that the pseudomorphic quantum well consists of type‐I regions of ordered GaInP and type‐II regions of disordered GaInP. Observation of photoluminescence at room temperature suggests that this QW may be useful as an active layer for laser structures grown on GaP.

The intersubband infrared photodetector performance is theoretically analyzed for various stages of interdiffusion in AlGaAs/GaAs quantum well. The absorption strength and responsivity are enhanced for certain extents of interdiffusion and the peak detection wavelength red shifts continuously with a large tunable range from 7 to 38.4 μm. The dark current is at an acceptable value for small diffusion extent.

Electron cyclotron resonance plasma etch rates for GaN, InN, InAlN, AlN, and InGaN were measured for a new plasma chemistry, ICl/Ar. The effects of gas chemistry, microwave and rf power on the etch rates for these materials were examined. InN proved to be the most sensitive to the plasma composition and ion density. The GaN, InN, and InGaN etch rates reached ∼13 000, 11 500, and ∼7000 A/min, respectively, at 250 W rf (−275 V dc) and 1000 W microwave power. The etched surface of GaN was found to be smooth, with no significant preferential loss of N from the surface at low rf powers, and no significant residue on the surface after etching.

Reactively sputtered AlN is shown by electrical characterization of Pt/Au Schottky diodes to be an effect encapsulant for GaN annealed at 1100 °C. Schottky diodes formed on GaN encapsulated with AlN during the anneal had low reverse leakage currents with breakdown voltages in excess of 40 V. In contrast, samples annealed without the AlN layer had 3–4 orders‐of‐magnitude higher reverse leakage currents. Atomic force microscopy images of as‐grown and annealed samples also demonstrate an increase in surface roughness and a change in morphology of the uncapped samples following annealing. Auger electron spectroscopy supports the hypothesis that the AlN encapsulant is reducing N loss from the GaN substrate. N loss in the uncapped samples is expected to create an n+‐region at the surface that accounts for the high reverse leakage current and improved Ohmic behavior for the uncapped samples. The use of AlN encapsulation will enable the realization of all ion implanted GaN metal semiconductor field effect transis...

The introduction of dry etch damage into n‐type SiC has been measured by monitoring the sheet resistance after exposure to Ar plasmas under both reactive ion etching and electron cyclotron resonance conditions. The threshold rf powers for measurable resistance changes in 1 μm thick films are ∼250 W for reactive ion etching (RIE) conditions, and ∼150 W for electron cyclotron resonance (ECR) conditions. A major annealing stage occurs with an activation energy of ∼3.4 eV, but some damage remains even after 1050 °C annealing.

The influence of minority‐carrier injection on the reactivation of hydrogen passivated Mg in GaN at 175 °C has been investigated in p‐n junction diodes. The dissociation of the neutral MgH complexes is greatly enhanced in the presence of minority carrier and the reactivation process follows second‐order kinetics. Conventional annealing under zero‐bias conditions does not produce Mg‐H dissociation until temperatures ≥450 °C. These results provide an explanation for the e‐beam‐induced reactivation of Mg acceptors in hydrogenated GaN.

Exposure to a hydrogen plasma at 250 °C of p‐type GaN (Ca) prepared by either Ca+ or Ca+ plus P+ co‐implantation leads to a reduction in sheet carrier density of approximately an order of magnitude (1.6×1012 cm−2 to 1.8×1011 cm−2), and an accompanying increase in hole mobility (6 cm2/V s to 18 cm2/V s). The passivation process can be reversed by posthydrogenation annealing at 400–500 °C under a N2 ambient. This reactivation of the acceptors is characteristic of the formation of neutral (Ca–H) complexes in the GaN. The thermal stability of the passivation is similar to that of Mg–H complexes in material prepared in the same manner (implantation) with similar initial doping levels. Hydrogen passivation of acceptor dopants in GaN appears to be a ubiquitous phenomenon, as it is in other p‐type semiconductors.

A simple Cl2/Ar plasma chemistry without additional sample heating is found to produce etch rates above 1 μm/min for InP under high microwave power (1000 W) electron cyclotron resonance conditions. While the etch rate increases essentially linearly with Cl2 composition the root‐mean‐square (RMS) surface roughness measured by atomic force microscopy is strongly dependent on the Cl2‐to‐Ar ratio. Under optimized conditions (10Cl2/5Ar), RMS roughness of ∼2.6 nm is obtained on samples etched more than 1 μm, a typical unetched control sample displays a RMS value of 1.3–1.6 nm. The high ion current under ECR conditions appears to promote efficient sputter desorption of the InCl3 etch product and prevents buildup of the usual selvedge layer that requires elevated sample temperatures to desorb under more conventional reactive ion etching conditions. The result is a simplified plasma chemistry with avoidance of the polymer deposition and hydrogen passivation associated with CH4/H2 discharges.

Two types of ultraviolet (UV) light induced modification of the surface pretilt of alignment layers are reported. The UV modification allows the liquid crystal (LC) surface pretilt angle of a polyimide film to be selectively altered in a small area. Two device structures for fabricating two‐domain liquid crystal displays based on this selective alteration of the LC surface pretilt angle of polyimides are proposed. Results from 3D simulation show that each pixel splits into two domains for the proposed structures. Experimentally, two‐domain twisted nematic (TN) test panels and thin‐film‐transistor (TFT) addressed two‐domain TN panels were fabricated using this method. The UV modification described in this letter does not require conventional photoresist technology as do other methods of fabricating two domain liquid crystal displays (LCDs). This process is simple as it requires only one polyimide coating and one rubbing step for each substrate.

Changes in sheet resistance of n‐ and p‐type InGaP exposed to electron cyclotron resonance Ar plasmas have been used to measure the introduction of ion‐induced damage. p‐type material is much more resistant to change in its conductivity than n‐type InGaP, indicating that electron traps are the predominant entity produced by the ion bombardment. For short (∼1 min) plasma exposures the ion current is more important than ion energy in producing resistance changes. Annealing of damage in both conductivity types occurs with an activation energy of ∼3.4±0.5 eV.

Changes in conductivity of InN, In0.5Ga0.5N, and In0.5Al0.5N layers exposed to Ar plasmas under both electron cyclotron resonance and reactive ion etching conditions have been measured as a function of rf power, pressure, and exposure time. The combination of high microwave and high rf powers produces large increases (10–104 times) in sheet resistance of the nitrides, but conditions more typical of real etching processes (rf power

Material transformations occurring at the facets of optically ‘‘stressed’’ planar InGaAsP/InP diode lasers have been investigated by transmission electron microscopy and energy dispersive x‐ray spectroscopy. Catastrophic degradation lines (CDLs) which are characteristic of catastrophic optical damage are observed for optical power densities ∼107 W/cm2. Analysis of the microstructure reveals a series of 150 nm wide GaAs‐rich tracks and the formation of unique void/InGa‐rich precipitate pairs within the InGaAsP active layer. These observations suggest that the formation of local group III‐rich regions is the first stage in the formation of CDLs. Subsequently, the strong absorption of the impinging laser beam leads to propagation of an InGa‐rich melt, thereby producing the GaAs‐rich tracks through a process similar to liquid phase epitaxy. These results are discussed in the context of standard physical models for CDLs.

Low density microcellular foam from poly(4‐methyl‐1‐pentene) (PMP) has been metallized by laser‐induced chemical vapor deposition (LICVD). KrF excimer laser radiation (248 nm) was used to photolytically decompose molybdenum hexacarbonyl with and without a buffer gas. Metal oxycarbide deposits 100 μm in diameter were formed. The microstructure of the PMP foam was unaltered. The deposition was confirmed to be 100 μm using scanning electron microscopy and energy dispersive x‐ray spectroscopy. X‐ray fluorescence was used to determine that approximately 80 ng of material was deposited.

We calculate the three‐wave mixing susceptibility of an asymmetric AlGaAs quantum well for difference‐frequency generation of mid‐infrared radiation. The effects of carrier injection and input field strength on the interband mixing process are considered. A peak susceptibility more than two orders of magnitude larger than that of bulk GaAs is obtained. Based on these results, the possibility of difference‐frequency mixing within a semiconductor quantum well laser is discussed.

A new mechanism for the explanation of negative differential resistance(NDR) in the output characteristics of heterojunctionbipolar transistors is proposed. The traditional recourse to thermal effects is not required and modeled characteristics successfully reproduce the correct experimentally observed shape. The effective conduction band spike height variation with bias at the emitter‐base junction, together with the base narrowing effects, can account for NDR.

The growth and electrical characterization of the heterojunction formed between two-dimensional (2D) layered p-molybdenum disulfide (MoS2) and nitrogen-doped 4H silicon carbide (SiC) are reported. The integration of 2D semiconductors with the conventional three-dimensional (3D) substrates could enable semiconductor heterostructures with unprecedented properties. In this work, direct growth of p-type MoS2 films on SiC was demonstrated using chemical vapor deposition, and the MoS2 films were found to be high quality based on x-ray diffraction and Raman spectra. The resulting heterojunction was found to display rectification and current-voltage characteristics consistent with a diode for which forward conduction in the low-bias region is dominated by multi-step recombination tunneling. Capacitance-voltage measurements were used to determine the built-in voltage for the p-MoS2/n-SiC heterojunction diode, and we propose an energy band line up for the heterostructure based on these observations. The demonstration of heterogeneous material integration between MoS2 and SiC enables a promising new class of 2D/3D heterostructures.

We report on the vapor-solid growth of single crystalline few-layer MoS2 films on (0001)-oriented sapphire with excellent structural and electrical properties over centimeter length scale. High-resolution X-ray diffraction scans indicated that the films had good out-of-plane ordering and epitaxial registry. A carrier density of ∼2 × 1011 cm−2 and a room temperature mobility of 192 cm2/Vs were extracted from space-charge limited transport regime in the films. The electron mobility was found to exhibit in-plane anisotropy with a ratio of ∼1.8. Theoretical estimates of the temperature-dependent electron mobility including optical phonon, acoustic deformation potential, and remote ionized impurity scattering were found to satisfactorily match the measured data. The synthesis approach reported here demonstrates the feasibility of device quality few-layer MoS2 films with excellent uniformity and high quality.

We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited MoS2. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films. For a hole density of 3.1 × 1020 cm−3, Hall mobility of 8.5 cm2 V−1 s−1 was determined, which matches well with the theoretically expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane crystalline quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered crystalline nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides.

Both current‐voltage and photoemission measurements of the conduction‐band discontinuity of the same InGaP/GaAs p‐n heterojunction have been carried out. Interpretation of the current‐voltage results using thermionic emission theory applied to a heterojunction bipolar transistor have resulted in a conduction‐band offset value of 0.21 eV in the case of a compositionally abrupt junction. This figure has been confirmed by performing independent photoemission measurements on the same junction.

We report, for the first time, the direct write laser patterning of highly conductive Al from a liquid precursor, triisobutylaluminum (TIBA). Al wires were written on Si with a scanned Ar+ laser from liquid TIBA at speeds of up to several mm/s. Wires 3 μm wide by 1 μm high with a resistivity of 5.6 μΩ cm were routinely achievable.

Microstructure and magnetic hysteresis have been compared for two samples of the YBa2Cu3O7−δ (δ∼0) high‐temperature superconductor, one unirradiated, and one irradiated with fast neutrons (E≳0.1 MeV) to a fluence of 3×1018 n/cm2. Notable changes in the microstructure include strain‐induced contrast from regions 2–7 nm in size. An intrinsic critical current density (Jc) of 4.6×106 A/cm2 in zero field at 4 K has been determined from magnetic hysteresis measurements for the irradiated sample while 1.2×106 A/cm2 is noted for the unirradiated sample. We propose that the observed defect structure in the irradiated material is responsible for increased pinning and consequently higher Jc’s.

We developed a gas-filled capillary with a tapered density for laser wakefield acceleration, of which the tapering was realized by employing gas feed-lines with different cross-sections. Plasma diagnostics show that the capillary plasma has a significant longitudinal density tapering and a transverse parabolic profile. By using the tapered capillary plasma, high transmission (over 90%) of laser beams, meaning good optical guiding, was observed. These results demonstrate the potential of the tapered plasma source for high-energy laser wakefield acceleration, where the dephasing problem is minimized.

We demonstrate an interferometric implementation of nonlinear optical infrared microscopy in which a probe beam interrogates the refractive index change of samples that is induced by molecular vibrational absorption of an infrared pump beam. To compensate for the phase noise caused by the ambient airflow and external vibration, we configure two coupled Mach-Zehnder interferometers sharing a single microscope setup in the sample arm. Proper control over the phase between the two interference channels allows two simultaneous quadrature mode outputs. The validity of our method is proven by imaging polystyrene bead aggregates and comparing the images with the conventional single-channel results.

Bi2Te3 is a well-known thermoelectric material for room-temperature operations because it has a high Seebeck coefficient, and the charge carrier type is easily tunable. However, the carrier type of Bi2Se3 is not tunable, which is one of the weaknesses for the thermoelectric applications, in spite of its high Seebeck coefficient. Here, we report the tuning of charge carriers in Bi2Te3 from n type to p type by doping Mn into the Bi sites. The carrier type is n type up to the Mn doping level of 5% and changes to p type above 5% Mn doping. The temperature-dependent resistivity of Bi2−xMnxSe3 shows a metallic behavior for x

We have demonstrated fluidic chip self-assembly on Si wafers for fabricating three-dimensional integrated circuits. In this self-assembly technique, small droplets of hydrofluoric acid were employed to simultaneously align many millimeter-scale chips and directly bond them to the hydrophilic bonding areas formed on the host wafers by oxide–oxide bonding. The liquid surface tension enables many Si chips to be self-assembled with the highest alignment accuracy of 50 nm. In addition, many chips were tightly bonded to the hydrophilic bonding areas without applying a mechanical force after the liquid was evaporated at room temperature.

A ferroelectric liquid crystal, S-2-methylbutyl 4-n-octanoyloxybiphenyl-4′-carboxylate, undergoing a characteristic polarization inversion, has been studied by H1 nuclear magnetic resonance (NMR). In addition to the discontinuities at the first-order isotropic and recrystallization phase transitions, the nuclear magnetic relaxation revealed a weak anomaly attributed to the soft mode at the Sm-A−Sm-C∗ liquid crystalline phase transition. Besides, the polarization inversion in the Sm-C∗ phase was manifested in the nuclear magnetic relaxation as well as in the NMR lineshape and in the temperature-dependent relative peak positions of the NMR line components, indicating that the polarization inversion is associated with a conformational transition.

Microscopic environments and dynamics in LiCoO2 systems were probed by L7i nuclear magnetic relaxation measurements with regard to the structural defects as revealed by x-ray diffraction and electron spin resonance measurements. Thus, the structural defects of differing degrees, associated with Li vacancies, were well accounted for in the temperature-dependent spin-lattice relaxation rate analysis, and in the structural and dynamical inhomogeneity as derived from the relaxation patterns.

We report on the enhanced quantum efficiency of GaN-based light-emitting diodes (LEDs) fabricated on inverted hexagonal pyramid dielectric mask (IHPDM)-embedded structure. The ray-tracing calculation showed that the extraction efficiency of LEDs fabricated on IHPDM-embedded structure could be enhanced up to 56%. Compared to the reference, the n-GaN template grown on IHPDM-embedded structure also showed a reduction in the dislocation density by 57%, leading to an increase in photoluminescence intensity by 82%. The LED fabricated on IHPDM-embedded structure exhibited a reduction in the forward leakage current by one order of magnitude (

Nonradiative energy transfer from the ZnO nanorods (donor) to fluorescent dye-doped polymer (acceptor) was investigated in heterostructures of ZnO nanorods/dye-doped polymer films, with the fluorescent dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, being doped into the host material, poly(9-vinylcarbazole). Introduction of the ZnO nanorods layer to the dye-doped polymer film devices turned out to give much enhanced and blueshifted photoluminescence.

The growth mechanism of In-doped ZnO (ZnO:In) nanorods grown on AuGe/Si(111) substrates has been investigated. Unlike to general vapor-liquid-solid mechanism, the adatom kinetics on the surface determines the length-diameter relationship [L(D)], initial nuclei size (D∗), and spatial distribution of impurity atoms. The incorporated In-concentration was estimated to be within solid solubility limit of ZnO in terms of x-ray diffraction analysis, but the evolution of photoluminescence peak position and intensity revealed an evidence of extrinsic carrier increment. Hall-effect measurement was used to evaluate the carrier concentration of ZnO:In nanorods.

The effect of edge and screw dislocations on the electrical and optical properties of n-type Al0.34Ga0.66N is investigated. It is found that edge dislocations strongly affect the electrical properties of n-type Al0.34Ga0.66N. Both free carrier concentration and mobility decrease with increasing edge dislocation density. Edge dislocations also enhance nonradiative recombination, which is indicated by decreasing near-band-edge UV as well as parasitic blue photoluminescence. The UV/blue ratio is found to be independent of the edge dislocation density but strongly depends on the Si doping concentration.

The growth of wurtzite ZnO on cubic MgO (001) substrates by molecular beam epitaxy is reported. ZnO epilayers exhibit the nonpolar m-plane orientation based on x-ray diffraction and transmission electron microscopy analysis. Comparative studies of ZnO grown by pulsed laser deposition on MgO (001) result in preferred growth in the polar c-plane orientation. Investigation by high resolution transmission electron microscopy confirms an abrupt ZnO∕MgO interface and monolayer spacing consistent with m-plane growth of ZnO. X-ray diffraction pole figures indicate two preferred rotational domains in-plane with a twist angle of 30°.