Showing papers in "Applied Physics Express in 2008"

Enhanced Conversion Efficiencies of Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching Technique

Abstract: Cu2ZnSnS4 (CZTS) thin film solar cells have been fabricated by co-sputtering technique using three targets of Cu, SnS, and ZnS. CZTS-based thin film solar cells over 6.7% efficiency were obtained for the first time by soaking the CZTS layer on the Mo coated soda-lime glass substrate in deionized water (DIW) after forming the CZTS layer. It was found that DIW-soaking had the effect of preferential etching, which eliminated selectively metal oxide particles in the CZTS layer, by electron probe X-ray micro analysis.

Fabrication of a Nonvolatile Full Adder Based on Logic-in-Memory Architecture Using Magnetic Tunnel Junctions

Abstract: Nonvolatile logic-in-memory architecture, where nonvolatile memory elements are distributed over a logic-circuit plane, is expected to realize both ultra-low-power and reduced interconnection delay. We have fabricated a nonvolatile full adder based on logic-in-memory architecture using magnetic tunnel junctions (MTJs) in combination with metal oxide semiconductor (MOS) transistors. Magnesium oxide (MgO) barrier MTJs are used to take advantage of their high tunnel magneto-resistance (TMR) ratio and spin-injection write capability. The MOS transistors are fabricated using a 0.18 µm complementary metal oxide semiconductor (CMOS) process. The basic operation of the full adder is confirmed.

Vertical Solar-Blind Deep-Ultraviolet Schottky Photodetectors Based on β-Ga2O3 Substrates

Abstract: A vertical-type Schottky photodetector based on a (100)-oriented β-Ga2O3 substrate has been fabricated with simple processes of thermal annealing and vacuum evaporation. The photodetector exhibited a rectification ratio higher than 106 at ±3 V, and showed deep-ultraviolet-light detection at reverse bias. The spectral response showed solar-blind sensitivity with high photoresponsivities of 2.6–8.7 A/W at wavelengths of 200–260 nm. These values were 35–150 times higher than those derived assuming the internal quantum efficiency to be unity. This fact is attributed to the carrier multiplication occurring in the highly resistive surface region that is subject to a high internal electric field of about 1.0 MV/cm at the reverse bias of 10 V.

GaN-Based Trench Gate Metal Oxide Semiconductor Field-Effect Transistor Fabricated with Novel Wet Etching

Abstract: A novel method for fabricating trench structures on GaN was developed. A smooth non-polar (1100) plane was obtained by wet etching using tetramethylammonium hydroxide (TMAH) as the etchant. A U-shape trench with the (1100) plane side walls was formed with dry etching and the TMAH wet etching. A U-shape trench gate metal oxide semiconductor field-effect transistor (MOSFET) was also fabricated using the novel etching technology. This device has the excellent normally-off operation of drain current–gate voltage characteristics with the threshold voltage of 10 V. The drain breakdown voltage of 180 V was obtained. The results indicate that the trench gate structure can be applied to GaN-based transistors.

A Significant Shift of Schottky Barrier Heights at Strongly Pinned Metal/Germanium Interface by Inserting an Ultra-Thin Insulating Film

Abstract: At any metal/germanium (Ge) interfaces, Schottky junctions to n-Ge and ohmic ones to p-Ge are formed by the strong Fermi level pinning to the valence band edge of Ge. In this paper, we report that Schottky-ohmic characteristics are reversed by inserting an ultra-thin oxide film into the metal/Ge interface. A gradual change of Schottky barrier heights (SBHs) with increasing insulating film thickness has been found, which supports that the origin of Fermi level pinning at the metal/Ge junction is caused by the metal-induced gap states. Furthermore, the SBH change enables us to operate metal source/drain Ge n-channel metal–oxide–semiconductor field effect transistors (n-MOSFETs) without any impurity doping. We demonstrate the metal source/drain Ge n-MOSFET with a peak mobility of 270 cm2/(Vs).

Vertical GaN-Based Trench Gate Metal Oxide Semiconductor Field-Effect Transistors on GaN Bulk Substrates

Abstract: Completely vertical trench gate metal oxide semiconductor field-effect transistors (MOSFETs) have been produced using gallium nitride (GaN) for the first time. These MOSFETs exhibited enhancement-mode operation with a threshold voltage of 3.7 V and an on-resistance of 9.3 mΩcm2. The channel mobility was estimated to be 131 cm2/(Vs) when all the resistances except for that of the channel are considered. Such structures, which satisfy the key words "vertical", "trench gate", and "MOSFET", will enable us to fabricate practical GaN-based power switching devices.

Room-Temperature CW Lasing of a GaN-Based Vertical-Cavity Surface-Emitting Laser by Current Injection

Abstract: We report the demonstration of CW lasing at room temperature in a GaN-based vertical-cavity surface-emitting laser (VCSEL) by current injection. The active region of the VCSEL consisted of a two-pair InGaN/GaN quantum well active layer. The optical cavity consisted of a 7-λ-thick GaN semiconductor layer and an indium tin oxide layer for p-contact embedded between two SiO2/Nb2O5 dielectric distributed Bragg reflectors. The VCSEL was mounted on a Si substrate by wafer bonding and the sapphire substrate was removed by laser lift-off. Under CW operation for an 8-µm aperture device, the threshold current was 7.0 mA and the emission wavelength was approximately 414 nm.

High-Yield Separation of Metallic and Semiconducting Single-Wall Carbon Nanotubes by Agarose Gel Electrophoresis

Abstract: We have developed a novel separation method of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) using agarose gel electrophoresis. When the SWCNTs were isolated with sodium dodecyl sulfate (SDS) and embedded in agarose gel, only the metallic SWCNTs separated from the starting gel by an electric field. After 20 min, almost all SWCNTs applied to gel electrophoresis were separated into two fractions, containing ~95% semiconducting and ~70% metallic nanotubes. The difference in the response to the electric field between metallic and semiconducting SWCNTs can be explained by the higher affinity of semiconducting SWCNTs to agarose than to SDS.

Ti-mask Selective-Area Growth of GaN by RF-Plasma-Assisted Molecular-Beam Epitaxy for Fabricating Regularly Arranged InGaN/GaN Nanocolumns

Abstract: The Ti-mask selective-area growth (SAG) of GaN on Ti-nanohole-patterned GaN templates by rf-plasma-assisted molecular-beam epitaxy was employed to demonstrate the fabrication of regularly arranged InGaN/GaN nanocolumns. The SAG of GaN nanocolumns strongly depended on the growth temperature (Tg); at Tg below 900 °C, no SAG occurred, but above 900 °C, SAG occurred. However, an excessive increase in Tg to above 900 °C at a nitrogen flow rate (QN2) of 3.5 sccm brought about increased inhomogeneity in the nanocolumn shape. Upon reducing QN2 from 3.5 to 1 sccm, uniform nanocolumn arrays were successfully grown around the critical temperature of 900 °C.

Monolithic Polychromatic Light-Emitting Diodes Based on InGaN Microfacet Quantum Wells toward Tailor-Made Solid-State Lighting

Abstract: Monolithic polychromatic light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1100] direction and consists of (0001) and {1122} facets. The LEDs exhibit polychromatic emission, including white, due to the additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry easily controls the apparent emission color. Furthermore, simulations predict high light extraction efficiencies due to their three-dimensional structures. Those observations suggest that the proposed phosphor-free LEDs may lead to highly efficient solid-state lighting in which the color spectra of light sources are synthesized to satisfy specific requirements for illuminations.

AlGaN/GaN Heterostructure Field-Effect Transistors on 4H-SiC Substrates with Current-Gain Cutoff Frequency of 190 GHz

Abstract: We report on state-of-the-art AlGaN/GaN heterostructure field-effect transistor (HFET) technology in the scope of millimeter-wave applications. 60-nm-long-gate HFETs having 4- and 6-nm-thick Al0.4Ga0.6N barrier layers and SiN passivation layers formed by catalytic chemical vapor deposition (Cat-CVD) were fabricated on 4H-SiC substrates. Both structures had low sheet resistances of 200–220 Ω/sq that were due to not only high mobilities of 1900–2000 cm2/(Vs) but also high electron densities of (1.4-1.7)×1013 cm-2, which were provided by the high-Al-composition barrier layers and the Cat-CVD SiN. The devices with the 4- and 6-nm-thick barriers had maximum drain current densities of 1.4 and 1.6 A/mm and peak extrinsic transconductances of 448 and 424 mS/mm, respectively. Maximum fT and fmax reached 190 and 251 GHz, respectively.

Optical and Conductive Characteristics of Metallic Single-Wall Carbon Nanotubes with Three Basic Colors; Cyan, Magenta, and Yellow

Abstract: We present protocols to prepare high-purity metallic single-wall carbon nanotubes (SWCNTs) with three basic colors, cyan, magenta, and yellow, through density gradient centrifugations. Addition of deoxycholate sodium salts as a co-surfactant could improve separation capability for metallic SWCNTs in centrifugations. We applied the improved separation protocols to the SWCNTs with different average diameters (1.34, 1.0, and 0.84 nm), and obtained the metallic SWCNTs with cyan, magenta, and yellow colors. Their optical/conductive characteristics were revealed, and conductive color films were formed from the metallic SWCNTs.

Self-organization of novel carbon composite structure: Graphene multi-layers combined perpendicularly with aligned carbon nanotubes

Abstract: A novel carbon composite structure consisiting of graphene multi-layers and aligned multi-walled carbon nanotubes (MWNTs) has been discovered. The composite structure, which was synthesized by chemical vapor deposition, has graphene multi-layers combined with the upper ends of vertically aligned MWNTs on a substrate. This microscopically-combined structure has been confirmed by transmission electron microscopy. The substrate with the new structure looks gray and shiny, which is completely different from the appearance of a substrate with the usual vertically-aligned MWNTs. The new composite structure is expected to have excellent electrical and thermal properties, and therefore is likely to find many applications in electronics.

High-Efficiency Continuous-Wave Operation of Blue-Green Laser Diodes Based on Nonpolar m-Plane Gallium Nitride

Abstract: We demonstrated nonpolar m-plane GaN-based blue-green laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 481 nm. A maximum output power of more than 20 mW was achieved, for which the threshold current and the corresponding threshold current density (Jth) were 61 mA and 6.1 kA/cm2, respectively. The value of Jth and the electroluminescence peak wavelength shift until lasing did not change with lasing wavelength in the range from 459 to 481 nm, though the reflectivities of the cavity facets were fixed for each LD. In addition, the slope efficiency increased with increasing lasing wavelength, from 0.37 W/A at 459 nm to 0.49 W/A at 481 nm. This is the remarkable advantages of nonpolar GaN-based material compared to c-plane material for the realization of green LDs.

Electronic Band Structure of Transparent Conductor: Nb-Doped Anatase TiO2

Abstract: We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations.

Superconductivity in Epitaxial Thin Films of Co-Doped SrFe2As2 with Bilayered FeAs Structures and their Magnetic Anisotropy

Abstract: Superconducting epitaxial films of Fe-based layered arsenide, Co-doped SrFe2As2, were grown at 700 °C on mixed perovskite (La,Sr)(Al,Ta)O3(001) single-crystal substrates by pulsed-laser deposition. Both the epitaxial film and an (001)-oriented film grown at 600 °C exhibited superconducting transitions at ~20 K. The zero-resistance states of the epitaxial film were sustained under a magnetic field (H) of 9 T at 9 K when H was parallel to the c-axis, while they were sustained at higher temperatures up to 10 K for H parallel to the a-axis. This is the first demonstration of superconducting thin films of FeAs-based new superconductors.

227 nm AlGaN Light-Emitting Diode with 0.15 mW Output Power Realized using a Thin Quantum Well and AlN Buffer with Reduced Threading Dislocation Density

Abstract: AlGaN multi-quantum-well (MQW) deep-ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on sapphire substrates with emission at 227 nm are demonstrated. A remarkable enhancement in the DUV LED output power was achieved by using a thin AlGaN quantum well only 1.3 nm in thickness, with atomically flat hetero-interfaces, together with an AlN buffer layer of reduced threading dislocation density. The AlGaN-MQW DUV LEDs exhibited single emission peaks. The output power was 0.15 mW with injection current of 30 mA and the maximum external quantum efficiency was 0.2%, under room temperature pulsed operation.

Piezoelectric Properties of (K,Na)NbO3 Films Deposited by RF Magnetron Sputtering

Abstract: High-piezoelectricity lead-free films of (K,Na)NbO3 (KNN) were successfully deposited on Pt/MgO and Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The KNN film was epitaxially grown on Pt/MgO with a high orientation in the pseudo-cubic perovskite structure. The KNN film on Pt/Ti/SiO2/Si was polycrystalline with a preferential orientation in the pseudo-cubic perovskite structure. The piezoelectric properties of the KNN films were determined from the tip displacement of KNN/Pt/MgO or KNN/Pt/Ti/SiO2/Si unimorph cantilevers. The transverse piezoelectric coefficients e31* (d31/s11E) of the KNN films on Pt/MgO and Pt/Ti/SiO2/Si were calculated to be -3.6 and -5.5 C/m2, respectively, which are amongst the highest values for KNN films ever reported.

Control of domain wall position by electrical current in structured Co/Ni wire with perpendicular magnetic anisotropy

Abstract: We report the direct observation of the current-driven domain wall (DW) motion by magnetic force microscopy in a structured Co/Ni wire with perpendicular magnetic anisotropy. The wire has notches to define the DW position. It is demonstrated that single current pulses can precisely control the DW position from notch to notch with high DW velocity of 40 m/s.

Development of 4H-SiC Epitaxial Growth Technique Achieving High Growth Rate and Large-Area Uniformity

Abstract: A vertical hot-wall epi-reactor that makes it possible to simultaneously achieve a high growth rate and large-area uniformity has been developed. A maximum growth rate of 250 µm/h is achieved with a mirror-like morphology at 1650 °C. Under a modified epi-reactor setup, a thickness uniformity of 1.1% and a doping uniformity of 6.7% for a 65-mm-radius area are achieved while maintaining a high growth rate of 79 µm/h. A low doping concentration of ~1×1013 cm-3 is obtained for a 50-mm-radius area. The low-temperature photoluminescence (LTPL) spectrum shows the predominance of free exciton peaks with only few impurity-related peaks and the L1 peak below detection limit. The deep level transient spectroscopy (DLTS) measurement for an epilayer grown at 80 µm/h shows low trap concentrations of Z1/2: 1.2×1012 and EH6/7: 6.3×1011 cm-3. A 280-µm-thick epilayer with a RMS roughness of 0.2 nm and a carrier lifetime of ~1 µs is obtained.

Origin of Negative Differential Resistance Observed on Bipolar Resistance Switching Device with Ti/Pr0.7Ca0.3MnO3/Pt Structure

Abstract: Bipolar resistance switching was investigated on sputtered Pr0.7Ca0.3MnO3 (PCMO) sandwiched by Pt- and Ti-electrodes. Based on electrical conductivity measurements and a combination of electron energy loss spectroscopy analysis and transmission electron microscopy observation, we found that the negative differential resistance observed in the forming process originates from the motion of oxygen ions at the Ti/PCMO interface. We propose that the observed resistance switching is caused by an oxidation/reduction reaction at the interface.

Inter-Layer Screening Length to Electric Field in Thin Graphite Film

Abstract: Electric conduction in thin graphite film was tuned by two gate electrodes to clarify how the gate electric field induces electric carriers in thin graphite. The graphite was sandwiched between two gate electrodes arranged in a top and bottom gate configuration. A scan of the top gate voltage generates a resistance peak in ambiploar response. The ambipolar peak is shifted by the bottom gate voltage, where the shift rate depends on the graphite thickness. The thickness-dependent peak shift was clarified in terms of the inter-layer screening length to the electric field in the double-gated graphite film. The screening length of 1.2 nm was experimentally obtained.

Reduction of Optical Loss in Hydrogenated Amorphous Silicon/Crystalline Silicon Heterojunction Solar Cells by High-Mobility Hydrogen-Doped In2O3 Transparent Conductive Oxide

Abstract: Hydrogen-doped In2O3 (IO:H) films with high electron mobility and improved near-infrared transparency have been applied as a transparent conducting oxide (TCO) electrode in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon heterojunction solar cells. The incorporation of IO:H, instead of conventional Sn-doped In2O3, improved the short-circuit current density (Jsc) and the resulting conversion efficiency. Detailed optical analysis of the solar cells revealed that the improvement in Jsc is due to the reduction of reflection loss at the TCO/a-Si:H interface and less optical absorption in the TCO layer.

High-Temperature Degradation Mechanism of Cu(In,Ga)Se2-Based Thin Film Solar Cells

Abstract: The degradation mechanism of Cu(In,Ga)Se2 (CIGS) thin film solar cells under high-temperatures conditions has been investigated. CIGS thin film solar cells were heated at temperatures ranging from 100 to 450 °C in a vacuum for 30 min. It was found that the CIGS devices with chemical bath deposited (CBD)–CdS and CBD–ZnS(O,OH) buffer layers were stable below 320 and 350 °C, respectively. These results suggest that CIGS devices possess high heat-resistance enough for practical usage. Secondary ion mass spectrometer (SIMS) and electron beam induced current (EBIC) analyses revealed that the main cause for the degradation of CIGS devices at high temperatures is attributable to a shift of the space charge region (SCR) toward the Mo back contact due to excess diffusion of Cd into CIGS absorber layer.

Blue Laser Diodes Fabricated on m-Plane GaN Substrates

Abstract: Blue laser diodes (LDs) were fabricated on m-plane oriented GaN substrates by atmospheric-pressure metalorganic chemical vapor deposition. Typical threshold current for stimulated emission at a wavelength λ of 463 nm was 69 mA. Blueshift of the spontaneous emission peak with increasing injection current was examined in LDs fabricated on m- and c-plane GaN substrates. Blueshifts for the m-plane LD (λ=463 nm) and the c-plane LD (λ=454 nm) with an injection current density just below threshold were about 10 and 26 nm, respectively. These results confirm that the blueshift in quantum-wells fabricated on m-plane oriented substrates is smaller than on c-plane oriented substrates due to the absence of polarization-induced electric fields.

Device Model for Graphene Nanoribbon Phototransistor

Abstract: An analytical device model for a graphene nanoribbon phototransistor (GNR-PT) is presented. GNR-PT is based on an array of graphene nanoribbons with the side source and drain contacts, which is sandwiched between the highly conducting substrate and the top gate. Using the developed model, we derive the explicit analytical relationships for the source–drain current as a function of the intensity and frequency of the incident radiation and find the detector responsivity. It is shown that GNR-PTs can be rather effective photodetectors in infrared and terahertz ranges of spectrum.

Growth of Single-Walled Carbon Nanotubes from Ceramic Particles by Alcohol Chemical Vapor Deposition

Abstract: Al2O3 ceramic nano-particles, which were regarded as an inactive catalyst in the growth of carbon nanotubes in the past, have been prepared as the catalyst for single-walled carbon nanotube (SWCNT) growth using an alcohol chemical vapor deposition method. Dense SWCNTs have been successfully synthesized, indicating that Al2O3 serves as an efficient catalyst. Moreover, it was found that many SWCNTs were also grown from irregular large Al2O3 particles ranging from several tens of nanometers to hundreds of nanometers, which has never been observed in the case of metallic catalyst particles. These results give more insights into the role of catalyst in SWCNT growth.

Ferroelectric Polarization Reversal by a Magnetic Field in Multiferroic Y-type Hexaferrite Ba2Mg2Fe12O22

Abstract: Coexistence of the ferroelectric polarization and spontaneous magnetization has been found in Y-type hexaferrite Ba2Mg2Fe12O22. The reversal of magnetization by a small magnetic field below ~0.02 T accompanies an electric polarization reversal through the clamping of ferrimagnetic and ferroelectric domain walls. This behavior can be potentially used as a magnetically rewritable ferroelectric memory and an electrically rewritable magnetic memory.

Stimulated Emission at Blue-Green (480 nm) and Green (514 nm) Wavelengths from Nonpolar (m-plane) and Semipolar (1122) InGaN Multiple Quantum Well Laser Diode Structures

Abstract: Stimulated emission (SE) in the blue-green (480 nm) and green (514 nm) regime has been observed, at room temperature (RT) under optical pumping, from nonpolar m-plane (1010) and semipolar (1122) InGaN multi-quantum well (MQW) laser diode (LD) structures, respectively, grown on bulk GaN substrates. The emission intensity exhibited a clear threshold behavior with respect to the pump power. Optical anisotropy was also observed between the two perpendicular in-plane directions [1123] and [1010] for semipolar LD structures, with significantly lower pump thresholds for emission along [1123]. The SE wavelength, measured just above threshold, was blue-shifted with respect to the spontaneous emission wavelength measured just below threshold. These initial results indicate that semipolar (1122) GaN is a promising orientation for the realization of blue-green and green LDs.

L10 FePt-C nanogranular perpendicular anisotropy films with narrow size distribution

Abstract: We report the study of L10 FePt–C nanogranular perpendicular anisotropy films fabricated on oxidized Si substrates with a (100) textured MgO intermediate layer. The addition of a small amount of C (<12%) to a FePt (4 nm) film results in the formation of interconnected FePt particles, while a higher C addition leads to the formation of well-isolated L10 FePt nanoparticles with a c-axis texture. The FePt particle size can be reduced to 5.5 nm with a size distribution of 2.3 nm in variance by adjusting FePt thickness. Perpendicular coercivity is controllable between 8 and 15 kOe with high squareness. These results demonstrate that the FePt–C system can accomplish a nanogranular structure suitable for ultrahigh density perpendicular recording media.