Table of contents

Semipolar (112) GaN was achieved by controlling anisotropic growth rates in a maskless r-plane patterned sapphire substrate. Upon optimizing the growth conditions, the growth rate of the GaN layer on etched c-plane-like sapphire was much higher than that on other planes such as the original r-plane sapphire. Singularly (112)-oriented GaN was confirmed when GaN was grown on only the c-plane-like sapphire sidewall. The control of the anisotropic growth rate is useful for growing nonpolar and semipolar layers using maskless patterned substrates.

Selective-area metalorganic vapor phase epitaxy of GaN has been investigated using the optimized growth conditions for the layer (Frank–van der Merwe) growth and GaN-template substrates with low dislocation density. The surface of a GaN hexagon with 16-µm diameter has a single wide terrace over almost the whole area (step-free surface), when there are no screw-type dislocations in the finite area. Step-free GaN hexagons grew in the two-dimensional nucleus growth mode and had approximately an eight times lower growth rate than that of a GaN film grown in the step-flow mode under the growth conditions used in this study.

The Ga incorporation behavior of AlGaN layers grown on nonpolar planes has been investigated comparing the (1120) and (1100) planes. AlGaN growth was performed on 4H-SiC(1120) and (1100) substrates by molecular-beam epitaxy under group-III-rich conditions. The Ga composition of the AlGaN layers was evaluated by energy-dispersive X-ray spectroscopy analysis as well as X-ray diffraction and cathodoluminescence measurements. The GaN mole fraction x of Al_1-xGa_xN layers grown on the (1100) plane is 0.12 for the Ga flux ratio [J_Ga/(J_Al+J_Ga)] of 0.32 at 750 °C, while it is negligibly small (x<0.01) for growth on the (1120) plane.

Polarized and time-resolved photoluminescence (PL) studies were carried out on the bulk GaN single crystals spontaneously nucleated by the Na-flux method. Simultaneously, structural studies as well as homoepitaxial growth were carried out. For the bulk crystals, typical full width at half maximum values of X-ray rocking curves were 37 arcsec for the (1010) diffraction and 31 arcsec for the (2021) diffraction. Their PL spectra exhibited polarized excitonic fine structures at 9 K. A proper effective PL lifetime being 212 ps and the absence of distinct deep emission bands at room-temperature imply sufficiently low point defect concentrations. GaN homoepitaxial films grown on naturally formed six m-planes exhibited a smooth surface morphology with monolayer atomic steps, of which tilt and twist mosaics were just inherited from the substrate crystal. The results prove that Na-flux GaN crystals can be used as a seed for growing strain-free thick GaN crystals.

By thermal oxidation of 4H-SiC at 1150–1300 °C, the Z_1/2 and EH_6/7 concentrations can be reduced to below 1×10¹¹ cm^-3. By the oxidation, however, a high concentration of HK0 center (E_V + 0.78 eV) is generated. Additional annealing in Ar at 1550 °C results in elimination of the HK0 center. Thus, all the major deep levels can be eliminated by the two-step thermal treatment. Based on the depth profiles of deep levels, a model for the defect generation and elimination is proposed. The carrier lifetime in 4H-SiC epilayers has been improved from 0.64 (as-grown) to 4.52 µs by this method.

The thermoelectric power factor of the narrow-gap semiconductor FeSb₂ is greatly enhanced in comparison to the isostructural homologues FeAs₂ and RuSb₂. Comparative studies of magnetic and thermodynamic properties provide evidence that the narrow and correlated bands as well as the associated enhanced thermoelectricity are only specific to FeSb₂. Our results point to the potential of FeSb₂ for practical thermoelectric application at cryogenic temperatures and stimulate the search for new correlated semiconductors along the same lines.

Novel Si₂Sb₂Te₆ phase change material for low-power chalcogenide random access memory is prepared by sputtering of Si₂Sb₂Te₆ alloy target. The motion of Te atom was confirmed from the X-ray diffraction patterns. Phase change random access memory device based on the Si₂Sb₂Te₆ alloy was successfully fabricated. Threshold current (I_th) of the device is only 1 µA. SET and RESET voltage pulse values are 1.2 and 2.4 V, respectively, with voltage pulse width of 150 ns. Up to 5×10⁵ cycles of endurance had been achieved with a resistance ratio of 100.

Piezoelectric shear strain was measured for c-axis oriented epitaxial Pb(Zr,Ti)O₃ (PZT) thin films. The PZT films, with a composition near the morphotropic phase boundary (MPB), were epitaxially grown on (001) MgO substrates and then microfabricated into a rectangular shape by wet etching of the films. Lateral electrodes were deposited on both sides of the PZT films, to apply an external electric field perpendicular to the polarization. A sinusoidal input voltage of 100 kHz was applied between the lateral electrodes, and in-plane shear vibration was measured by a laser Doppler vibrometer. In-plane displacement due to shear mode piezoelectric vibration was clearly observed and increased proportionally with the voltage. Finite element method (FEM) analysis was conducted to determine the horizontal electric field in the PZT film, and the piezoelectric coefficient d₁₅ was calculated to be 440×10^-12 m/V. The d₁₅ of the PZT film represents the intrinsic shear piezoelectric effect, which is slightly smaller than that of bulk PZT, due to the absence of extrinsic effects such as longitudinal and transverse piezoelectric strain or domain rotation.

We have found that epitaxial MgO(111) thin films grow under a wide range of deposition conditions (substrate temperatures of 400–800 °C, oxygen partial pressures of 10^-4–10⁰ Pa) on α-Al₂O₃(0001) substrates by pulsed laser deposition (PLD), despite the strongly divergent electrostatic potential of MgO(111). The surfaces of the resulting thin films show step-and-terrace structures reflecting the surface of α-Al₂O₃(0001) with a small root-mean-square roughness of 0.62 nm even at a film thickness of 80 nm. These results present the possibility of fabricating various artificial oxide structures using flat MgO(111) films produced by a conventional PLD technique.

Self-assembled pi-conjugated nanowire stacks were imaged using a tuning-fork probe mounted in the qPlus configuration in frequency modulation mode under ultrahigh vacuum. High resolution topographic and dissipation images demonstrate the applicability of such probes to soft conjugated materials, with sub-molecular resolution achieved perpendicular to the stacking axis. A new insight is gained from the damping contrast into the local mechanical properties of edge-on pi-conjugated stacks.

Intense green cathodoluminescence (CL) was obtained from zinc oxide (ZnO) nanostructures prepared by a post-treatment of ZnO film deposited on a silicon (Si) substrate in a reducing gas ambient at a low temperature of 450 °C. The green emission peak was centered at around 500 nm with a CL luminance as high as 10580 Cd/m² at an excitation voltage of 10 kV. The strong green emission was ascribed to the increase in the oxygen vacancies on the surface of the fluted hexagonal cone nanostructures formed on the ZnO film during the annealing process.

Room-temperature continuous-wave operation of 520 nm InGaN-based green laser diodes on semi-polar {2021} GaN substrates was demonstrated. A threshold current of 95 mA corresponding to a threshold current density of 7.9 kA/cm² and a threshold voltage of 9.4 V were achieved by improving the quality of epitaxial layers on {2021} GaN substrates using lattice-matched quaternary InAlGaN cladding layers and also by adopting a ridge-waveguide laser structure.

Vertically conducting thin film high power deep ultraviolet (DUV) light emitting diodes with peak emission wavelength of 280 nm are reported. The light emitting diodes (LEDs) were fabricated using a laser assisted lift-off process. Single chip devices exhibited a record high output power of 5.5 mW at a continuous-wave (cw) current density of only 25 A/cm². Their lifetime is estimated to be well over 2000 h. We attribute the superior performance of the devices to reduced thermal impedance and the vertical current conduction device geometry.

We have successfully investigated degradation-induced variations in electronic band-gap states in fabricated organic light-emitting diodes (OLEDs) based on tris(8-hydroxyquinoline) aluminum (Alq₃) by using modified deep-level optical spectroscopy. The degraded OLED samples show a significant red-shift of deep-level traps and near-band-edge transitions in the emissive region of Alq₃ towards their corresponding bulk levels of an Alq₃ single layer. These variations in the interfacial electronic states are probably induced by the intrinsic degradation and indicate that initial molecular structures characteristic of the Alq₃ emissive zone are transformed into the bulk-like relaxed ones through the degradation.

Schottky diodes were fabricated on n-GaN films by coating them with an organic polyaniline layer as transparent conducting electrodes. These diodes have a high Schottky barrier height (1.28 eV) and a low reverse leakage current (2.7×10^-9 A/cm² at an applied bias of -1 V). The photovoltaic action of these diodes (V_OC = 0.67 V and external quantum efficiency ∼30%) was studied under the illumination of an Air Mass 1.5 solar simulator. The polyaniline/n-GaN Schottky contacts were found to be sensitive to shorter wavelengths, indicating their potential for use as solar cells.

Photocurrents in single-walled carbon nanotubes are theoretically investigated with focus on the excitonic effects and a new method to generate photocurrents due to E₁₁-exciton dissociation through the Auger recombination process is proposed. Those photocurrents whose carriers are dissociated E₁₁-excitons abruptly increase at the threshold laser-intensity, due to the efficient Auger recombination associated with exciton–exciton scattering. Our calculation predicts that by increasing the laser-intensity, the current originating from dissociated E₁₁-excitons significantly increases compared with that from electron–hole pairs of band-to-band excitations.

We propose and substantiate the concept of terahertz (THz) laser based on the optically pumped graphene layers and the resonant cavity of the Fabri–Perot type. The pumping scheme which corresponds to the optical interband excitation of graphene followed by the emission of an optical phonons cascade provides the population inversion for the interband transitions in a relatively wide range of THz frequencies. We demonstrate that the THz lasing in the device under consideration at room temperatures is feasible if its structure is optimized. The frequency and output power of the generated THz radiation can be tuned by varying the distance between the mirrors.

Lasing has been achieved in clusters of fluorescent polymer microbeads in an aqueous environment. In contrast to single microbeads, the fluorescence emission spectra of clusters operated above lasing threshold exhibit a characteristic spectral fingerprint, which allows us to identify different clusters. The fingerprint originates in the superposition of the lasing modes of the individual microbeads constituting the cluster and thus is a unique feature of each cluster due to the finite size dispersion inherent to colloidal suspensions. The method promises to be useful for optical biosensing because of its full compatibility with state-of-the-art spotting techniques.

We propose and demonstrate a simple and unique method to fabricate the multiple-wavelength vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm by grading only the first low-index spacer layer of SiO₂/Ta₂O₅ dielectric mirror. A multi step exposure to UV lithography followed by selective wet chemical etching has been applied to create spacer layer gradients. We have successfully realized equally spaced four-channel VCSELs with wavelength span exceeding 30.0 nm by grading a 230.0 nm spacer layer and achieved single mode lasing with a side mode suppression ratio in excess of 30.0 dB.

The atomic ordering of Co₂MnSi (CMS) full-Heusler film and the interface structure of CMS/MgO/CMS magnetic tunnel junctions (MTJs) were investigated by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). We observed the atomic ordering of L2₁ and B2 structures of CMS from the atomic number (Z) contrast STEM images. We also confirmed that the interface structure consists of the layer next to the Co layer terminating in the CMS to MgO layer from the layer periodicity along the [001] direction, however, site-disorder exists between two atomic layers at the termination of CMS, including locally L2₁-ordered MnSi terminated structure.

Epitaxial films of NdFeAsO were grown on GaAs substrates by molecular beam epitaxy (MBE). All elements including oxygen were supplied from solid sources using Knudsen cells. The X-ray diffraction pattern of the film prepared with the optimum growth condition showed no indication of impurity phases. Only (00l) peaks were observed, indicating that NdFeAsO was grown with the c-axis perpendicular to the substrate. The window of optimum growth condition was very narrow, but the NdFeAsO phase was grown with a very good reproducibility. Despite the absence of any appreciable secondary phase, the resistivity showed an increase with decreasing temperature.

The memory properties of a nanodot-type floating gate memory with Co bio-nanodots (Co-BNDs) embedded in HfO₂ were investigated. High-density and uniform Co-BNDs were adsorbed on the HfO₂ tunnel oxide using ferritin. The fabricated metal oxide semiconductor (MOS) capacitor exhibited a capacitance–voltage (C–V) curve with large hysteresis. The memory window size was 30 times higher than that of the MOS capacitor with a SiO₂ gate oxide. Not only a large memory window but also excellent charge retention and reliability characteristics were obtained for a MOS field-effect transistor (MOSFET). This research confirmed that the proposed memory is promising for use in next-generation memory devices.

We report on electron transport measurements of a lithographically-defined silicon double quantum dot (DQD) coupled in series with a top gate and side gates. The structure of the top gate coupled uniformly to the DQD is suitable for realizing a few-electron regime. The obtained small DQD enables us to observe a clear honeycomb-like charge stability diagram at a temperature of 4.5 K. The validity of the DQD structure is confirmed by theoretical calculations. Furthermore, we demonstrate successful modulation of the inter-dot electrostatic coupling by the side gate. Externally tunable coupling is essential for practical implementation of spin-based quantum information devices.

We present a theoretical scheme that seamlessly handles the crossover from fully ballistic to diffusive thermal transport regimes and apply it to carbon nanotubes. At room temperature, micrometer-length nanotubes belong to the intermediate regime in which ballistic and diffusive phonons coexist. According to our scheme, the thermal conductance of these nanotubes exhibit anomalous nonlinear dependence of tube length due to this coexistence. This result is in excellent agreement with molecular-dynamics simulation results showing the nonlinear thermal conductance. Additionally, we clarify the mechanism of crossover in terms of the length-dependent characteristic frequency.

In lithography, the normalized image log slope (NILS) is an important metric that describes the quality of an aerial image of incident photons. The chemical gradient is also an important metric that describes the quality of a latent image in terms of line edge roughness. The relationship between NILS and the chemical gradient has been theorized in studies on photolithography. In extreme ultraviolet (EUV) resists, however, secondary electrons contribute to the image formation in contrast to the case of photoresists. In this study, we proposed a NILS in which secondary electron migration is taken into account.

The formation of p–n junction in double-walled carbon nanotubes (DWNTs) is successfully investigated for the first time through encapsulating heteromolecules/atoms via a plasma–ion irradiation method. DWNTs filled with both electron donor atoms and electron acceptor molecules are synthesized during the plasma ion-irradiation process. It is found that the electrical transport properties of DWNTs after encapsulating either Cs–C₆₀ or Cs–I are significantly different from those of unipolar p- or n-type DWNTs encapsulating one kind of molecules or atoms. The p–n junctions with excellent rectifying characteristics are successfully realized in many of DWNT-based devices, suggesting a new way in making functional DWNTs.

The potentiality of optical absorption spectroscopy (OAS) for the estimation of mean diameter of single-wall carbon nanotubes (SWCNTs) from electronic transition energies has been explored. The observed dependence of electronic transition energies of both metallic and semiconducting SWCNTs on their mean diameters clearly showed that transition energies scale inversely with the tube diameter. In the present study, the applicability of this estimation method has been experimentally confirmed for the diameter range of 1–2 nm and is expected to be useful for the characterization of wide range of diameters of SWNCTs.

We demonstrated molecular resolution imaging of biological samples such as bacteriorhodopsin protein molecules in purple membrane and isolated chaperonin (GroEL) protein molecules, both adsorbed on mica using frequency modulation atomic force microscope (FM-AFM) in liquid. We also showed that the frequency noise of FM-AFM in liquid can be greatly reduced by the reduction of the noise-equivalent deflection of an optical beam deflection sensor.

We fabricated a highly stable and sensitive gas sensor based on single-walled carbon nanotubes (SWNTs) protected by metal-oxide coating layer. SWNTs were deliberately decorated with 2–5 nm of oxygen-deficient SiO_x and AlO_x by pulsed laser deposition, followed by annealing under Ar/H₂ ambient. Surpassing the as-grown SWNTs, the metal-oxide coated SWNTs showed an excellent sensing stability with a variation of sensor response less than 7–8%. The obtained sensitivity to NO₂ was also improved. Moreover, the relationship between NO₂ concentration and sensor response can be described by the Frumkin–Temkin isotherm.

Early stages of film growth for Sn-doped In₂O₃ (ITO) deposited on amorphous carbon (a-C) films by dc magnetron sputtering were investigated by transmission electron microscopy (TEM). ITO islands appeared as dark regions in the TEM images, and their density and size increased with increasing deposition time. Furthermore, electron diffraction analyses revealed that the intensity of the ring pattern for In₂O₃(222) gradually increased with increasing deposition time. This indicated that crystallized ITO islands were formed on the a-C surface in the early stages of the film growth.

Combinatorial plasma etching, which can be used to acquire large databases and to establish process-science map, is proposed for developing a process for etching low-k dielectric films. This process promises to save the cost and time spent on accumulating scientific databases for optimizing etching processes since it can acquire many results in a single operation. We interpreted the etching characteristics by using internal parameters such as the ion and the radical densities. By doing this, it is possible to eliminate the dependence of the etching characteristics on the etching system.

We describe a method for estimating the quality factors of lateral comb drive resonators, especially with trench structures in free-molecule regime, and compare their calculated and experimental values. Although, in past, many studies on air damping of resonators have been carried out, but air damping model for comb drive resonators in the free-molecule regime has not been addressed. We propose mechanism of the energy loss which occurs in the gap between movable plates (known as trench structure), and between a movable plate and a fixed wall. It was shown that the trench structures have almost no effect on the decrease of the quality factor.

This study investigates the effect of Cu alloying on electromigration behavior of Ag metallization. Electromigration tests are performed on pure Ag and Ag (1.5% Cu) samples deposited by e-beam evaporation. The experiments show that Ag (Cu) alloy interconnect has superior elctromigration resistance compared to pure Ag interconnect. X-ray diffraction, four point probe measurements and electron microscopy were used to investigate the test structures and corresponding thin film samples. The Cu improves the lifetime of interconnect test structures by hindering Ag diffusion and increasing 111 texture of Ag. Also, Cu addition seemingly reduces the agglomeration in Ag.

A mechanical sensor is used to probe the stress evolution in deep sub-micron damascene Cu interconnects. Stress increases with post-plating anneal temperature. However, a reduction in volume average stress can occur when dominated by stress relaxation in other axes. The stress was shown to increase following extended anneals due to restricted grain growth in the bamboo like interconnects. This indicates the post-plaiting anneal can play an important role in defining interconnect stress. Furthermore, this work demonstrates that the current technique can be used to monitor stress in back-end-of-line metallization and meet future nano-scale characterization requirements.