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The linear band dispersion of graphene's bands near the Fermi level gives rise to its unique electronic properties, such as a giant carrier mobility, and this has triggered extensive research in applications, such as graphene field-effect transistors (GFETs). However, GFETs generally exhibit a device performance much inferior compared to the expected one. This has been attributed to a strong dependence of the electronic properties of graphene on the surrounding interfaces. Here we study the interface between a graphene channel and SiO$_{2}$, and by means of photoelectron spectromicroscopy achieve a detailed determination of the course of band alignment at the interface. Our results show that the electronic properties of graphene are modulated by a hydrophilic SiO$_{2}$ surface, but not by a hydrophobic one. By combining photoelectron spectromicroscopy with GFET transport property characterization, we demonstrate that the presence of electrical dipoles in the interface, which reflects the SiO$_{2}$ surface electrochemistry, determines the GFET device performance. A hysteresis in the resistance vs. gate voltage as a function of polarity is ascribed to a reversal of the dipole layer by the gate voltage. These data pave the way for GFET device optimization., 29 pages, 7 figures

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

We have investigated the effects of silicon (Si) doping on the chemical bonding states and the electrical, optical, and mechanical properties of nitrogen-doped diamond-like carbon (N−DLC) films prepared via plasma-enhanced chemical vapor deposition using hydrogen as a dilution gas. N doping accelerated the formation of clustered sp2 C in N−DLC films, whereas the clustering was almost suppressed in Si and N doped DLC (Si−N−DLC) films. Sp3 C sites in the Si–N–DLC films were higher than those in the N–DLC films. For the Si−N−DLC films, sp3 C-N bonds were preferentially formed compared with the sp2 C=N bonds. The internal stress on the Si−N−DLC films was lower than that on the N−DLC films. The Si−N−DLC films exhibited higher optical bandgaps than the N−DLC films. As the N2 flow ratio increased, the optical bandgap of the Si−N−DLC films showed an increasing trend, whereas that of the N−DLC films showed an opposite trend. The current−voltage characteristics of Si−N−DLC/p-type Si heterojunctions were not rectifying; however, after postdeposition annealing, the heterojunctions exhibited rectifying characteristics.

In this paper, we report on the fabrication of multi-layer bismuth selenide (Bi2Se3) devices by lithography process and their electrical transport characteristics studied at room temperature. The observation of a non-linear current-voltage characteristic is analyzed with various current transport mechanisms such as Schottky barrier, space-charge limited conduction (SCLC), Fowler-Nordheim (F-N) tunneling and Poole-Frenkel (P-F) conduction. The F-N tunneling via field emission current induced in high-voltage was found to be the dominant conduction mechanism which could responsible for the nonlinear behavior as compared to SCLC and P-F conduction phenomena. The charge traps present in the Bi2Se3 bulk causes SCLC and P-F conduction. A carrier mobility in Bi2Se3 is extracted as ~ 1220 cm2 V−1 s−1. Our study further advances the understanding of the fundamental charge transport mechanisms appeared in Bi2Se3 which will be an essential parameter in the development of various electronic applications such as resistive memory switching and sensing devices.

We demonstrate optical couplers with a new multicore plastic optical fiber (MC-POF) that provide presettable splitting ratio and small variation in modal power distribution. They were constructed simply by mechanically removing all or a part of the second clad in the MC-POF. The MC-POF comprises polymethyl methacrylate-based 19 cores with two clad layers. It functions as an optical fiber normally and reliably, but each fiber can be separated individually as needed. The MC-POF couplers have shown remarkable stability in the splitting ratio and the modal power distribution compared with a conventional optical coupler.

The adsorption of CO2 molecules on monolayer epitaxial graphene on a SiC(0001) surface at 30 K was investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy. The desorption energy of CO2 on graphene was determined to be (30.1–25.1) ± 1.5 kJ/mol at low coverages and approached the sublimation energy of dry ice (27–25 kJ/mol) with increasing the coverage. The adsorption of CO2 on graphene was thus categorized into physisorption, which was further supported by the binding energies of CO2 in core-level spectra. The adsorption states of CO2 on graphene were theoretically examined by means of the van der Waals density functional (vdW-DF) method that includes nonlocal correlation. The experimental desorption energy was successfully reproduced with high accuracy using vdW-DF calculations; the optB86b-vdW functional was found to be most appropriate to reproduce the desorption energy in the present system.

A novel method to fabricate uniform epitaxial graphene on C-face SiC substrates was investigated. Graphene was grown on the C-face 6H-SiC substrates with a sputtered SiC film by annealing temperatures ranging from 1400 to 1900 °C under an Ar ambient. The fractional area of the graphene having the layer number of two was about 95% in a 75×75 μm square by a Raman mapping and a low energy electron microscopy. Graphene on the C-face SiC fabricated by this method is quite uniform compared to that made by a conventional method without the sputtered SiC films and is thus suitable for high frequency analog devices.

InP-based high electron mobility transistors (InP-HEMTs) and graphene-channel FETs (G-FETs) are experimentally examined as photonic frequency converters for future broadband optical and wireless communication systems. Optoelectronic properties and three-terminal functionalities of the InP-HEMTs and G-FETs are exploited to perform single-chip photonic double-mixing operation over the 120 GHz wireless communication band. A 10 Gbit/s-class data signal on a 112.5 GHz carrier is mixed down to a 25 GHz IF band with an 87.5 GHz LO signal that is simultaneously self-generated from an optically injected photomixed beat note. The results suggest that the intrinsic channel of the G-FET can achieve a speed performance that is superior to that of an InP-HEMT having an equivalent device feature size. The reduction of the extrinsic parasitic resistances and the implementation of an efficient photo-absorption structure in the G-FET may allow a millimeter-wave and sub-THz photonic frequency conversion with a sufficiently high conversion gain for practical purposes.

This paper reviews recent advances in the terahertz (THz) graphene-based 2D-heterostructure lasers and amplifiers. The linear gapless graphene energy spectrum enables population inversion under optical and electrical pumping giving rise to the negative dynamic conductivity in a wide THz frequency range. We first theoretically discovered these phenomena and recently reported on the experimental observation of the amplified spontaneous THz emission and single-mode THz lasing at 100K in the current-injection pumped graphene-channel field-effect transistors (GFETs) with a distributedfeedback dual-gate structure. We also observed the light amplification of stimulated emission of THz radiation driven by graphene-plasmon instability in the asymmetric dual-grating gate (ADGG) GFETs by using a THz time-domain spectroscopy technique. Integrating the graphene surface plasmon polariton (SPP) oscillator into a current-injection graphene THz laser transistor is the most promising approach towards room-temperature intense THz lasing.

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

Detailed analysis on the planar defects in rotated 3C-SiC() films grown on Si(110) substrates have been conducted by using X-ray diffraction and cross-sectional transmission electron microscopy. While the film mostly consists of a rotated region, a portion was found in its nonrotated (3C-SiC(110)) configuration, forming domain boundaries in between. Inplane twins were also found to exist within both the rotated and nonrotated regions. Their origins and their impacts on the epitaxial graphene formation on the 3C-SiC film are discussed.

Eelectrically active defects in 3C–SiC are investigated by considering the structures and interactions of planar defects. An anti-phase boundary (APB) largely degrades the blocking property of semiconductor devices due to its semimetallic nature. Although APBs can be eliminated by orienting the specific polar face of 3C-SiC along a particular direction, stacking faults (SFs) cannot be eliminated due to Shockley-type partial dislocation glide. SFs with Shockley-type partial dislocations form a trapezoidal plate which expands the Si-terminated surface with increasing 3C-SiC thickness. Although the density of SFs can be reduced by counter termination, specific cross-junctions between a pair of counter SFs forms a forest dislocation, and this is regarded as an electrically active defect. This paper proposes an effective way to suppress the forest dislocations and APBs which nucleate during 3C-SiC growth.

The growth of graphene on 3C-SiC/Si heterostructure is a promising approach, which provides low production cost, high scalability and easiness of nanoelectromechanical system fabrication. However, the quality of graphene is still insufficient for device applications due to mediocre morphological and structural quality of the 3C-SiC epilayers compared to bulk SiC crystals and to excessive Si out-diffusion from the Si substrate. Here, we propose a solution of inserting a 4H-AlN layer between 3C-SiC and Si, which allows us to polish the 3C-SiC film without worrying about enhancement of the Si out-diffusion despite the thinning after the polishing. With this insertion, a considerable quality improvement is achieved in our graphene on silicon.

Results of investigations of the SiC/Si growth from monomethylsilane are reported. Growth conditions favoring the rotated epitaxy of 3C-SiC(111) films on Si(110) are determined experimentally. Surface energies of clean and hydrogen covered 3C-SiC(110) and 3C-SiC(111) surfaces are calculated with the density functional theory approach. It is shown that the change of the 3C-SiC film orientation with decreasing surface temperature and increasing monomethylsilane pressure may be induced by a reduction of the surface energy anisotropy due to the surface passivation by hydrogen.

We propose an ultrafast synthesis method to obtain quasi-free-standing bilayer epitaxial-graphene (QFSEG) on SiC(0001). By applying a microwave annealing (MWA) in air to a monolayer epitaxial graphene (EG) grown on SiC(0001), the EG's (6√3 × 6√3) R30° reconstructed buffer layer is decoupled from the SiC substrate and becomes the second graphene layer. Oxidation of the SiC surface is suggested as the most likely mechanism of the decoupling, and the electrical properties of this MWA-QFSEG are actually quite similar to those of the QFSEGs formed by conventional annealing in air. The process time has however been reduced by more than one order of magnitude, which will surely contribute to the betterment of the productivity of the EG-based devices.

3C-SiC is the only polytype that grows heteroepitaxially on Si substrates and, therefore, it is of high interest for various potentail applications. However, the large (~20 %) lattice mismatch of SiC with the Si substrate causes a serious problem. In this respect, rotated epitaxy of 3C-SiC(111) on the Si(110) substrate is highly promising because it allows reduction of the lattice mismatch down to a few percent. We have systematically searched the growth conditions for the onset of this rotated epitaxy, and have found that the rotaed epitaxy occurrs at higher growth temperatures and at lower source-gas pressures. This result indicates that the rotated epitaxy occurs under growth conditions that are close to the equilibrium and is thefore thermodynamically, rather than kinetically, driven.

In the formation of epitaxial graphene on Si substrates, the growth of high-quality 3C-SiC thin films on Si substrates is a key to success. As a solution to the large mismatch between the Si substrate and the 3C-SiC film, rotated epitaxy in which 3C-SiC(111) films are grown on Si(110) substrates is quite attractive. In some applications, on the other hand, a certatin thickness (~100 nm or more) is required for this 3C-SiC films as well. A two-step growth method has been thus developed to realize a high-rate, qualified rotated epitaxy. A qualified graphene is found to be formed on this rotated epi-film, as typified by the increase of the grain size by a factor of 1.6 from the non-rotated epitaxy.

Optical and/or injection pumping of graphene can enable negative-dynamic conductivity in the terahertz (THz) spectral range, which may lead to new types of THz lasers [1,2]. In the graphene structures with p-i-n junctions, the injected electrons and holes have relatively low energies compared with those in optical pumping, so that the effect of carrier cooling can be rather pronounced, providing a significant advantage of the injection pumping in realization of graphene THz lasers [3,4]. We implement a forward-biased graphene structure with a lateral p-i-n junction in a distributed-feedback dual-gate graphene-channel field-effect transistor (DFB-DG-GFET) and experimentally observe a single mode emission at 5.2 THz at 100K. The device exhibits a nonlinear threshold-like behavior.

The gapless energy spectra and linear dispersion relations of electrons and holes in graphene lead to nontrivial features such as a high carrier mobility and a flat, broadband optical response. This article reviews recent advances in graphene-based materials and devices for terahertz science and technology. After an introduction to the fundamental basis of the optoelectronic properties of graphene, the synthesis and crystallographic characterization of graphene materials are described, with a particular focus on the authors’ original heteroepitaxial graphene-on-silicon technology. The nonequilibrium dynamics of carrier relaxation and recombination in optically or electrically pumped graphene is discussed to introduce the possibility of negative dynamic conductivity over a wide terahertz range. Recent theoretical advances toward the creation of current-injection graphene terahertz lasers are described, followed by the unique terahertz dynamics of two-dimensional plasmons in graphene. Finally, the advantages of graphene materials and devices for terahertz applications are summarized.

This paper reviews recent advances in spectroscopic study on ultrafast carrier dynamics and terahertz (THz) stimulated emission in optically pumped graphene. The gapless and linear energy spectra of electrons and holes in graphene can lead to nontrivial features such as negative dynamic conductivity in the THz spectral range, which may lead to the development of new types of THz lasers. First, the non-equilibrium carrier relaxation/recombination dynamics is formulated to show how photoexcited carriers equilibrate their energy and temperature via carrier-carrier and carrier-phonon scatterings and in what photon energies and in what time duration the dynamic conductivity can take negative values as functions of temperature, pumping photon energy/intensity, and carrier relaxation rates. Second, we conduct time-domain spectroscopic studies using an optical pump and a terahertz probe with an optical probe technique at room temperature and show that graphene sheets amplify an incoming terahertz field. Two different types of samples are prepared for the measurement; one is an exfoliated monolayer graphene on SiO2/Si substrate and the other is a heteroepitaxially grown non-Bernal stacked multilayer graphene on a 3C-SiC/Si epi-wafer.

A carbonaceous field effect transistor, designated as a ‘DLC-GFET’, consisting of a graphene channel and a diamondlike carbon (DLC) top-gate dielectric film was fabricated. A DLC film was formed ‘directly’ onto the graphene channel using our original photoemission-assisted plasma-enhanced chemical vapor deposition (PA-CVD) without forming complex passivation interlayers. The DLC-GFET exhibits clear ambipolar characteristics with a slightly positive shift of the neutral points (Dirac voltages). The Raman analysis suggests that the electrical characteristics are due to unintentional hole doping from the DLC film into the graphene channel. The maximum transconductances per unit device width in the n-channel and the p-channel modes (gmn,max and gmp,max) are 14.6 mS/mm and 8.8 mS/mm, respectively, with a gate thickness of 48 nm and a gate length of 5 μm. Formation of the DLC top-gate does not cause obvious damage to the graphene channel. Hence, the PA-CVD method is suitable to form gate dielectrics onto graphene.

By conducting a heteroepitaxy of a 3C-SiC film on a Si substrate and by annealing its surface in a UHV ambient, epitaxial graphene can be formed on such 3C-SiC virtual substrates. While the growth on the Si-terminated 3C-SiC(111)/Si (111) surface is known to proceed in a similar manner as on the Si-terminated 6H-SiC(0001) surface, successful growth of graphene on 3C-SiC(100)/Si (100) and 3C-SiC(110)/Si (110) surfaces remains puzzling. We have carried out detailed cross-sectional transmission-electron-microscopy observations on these systems to find out that (111)-facets may play crucial roles in the initiation of graphene on these surfaces. This observation also accounts for the absence of the interface layer at the graphene/SiC in these orientations.

A method to realize a novel hybrid orientations of Si surfaces, Si(111) on Si(110), has been developed by use of a Si(111)/3C-SiC(111)/Si(110) trilayer structure. This technology allows us to use the Si(111) portion for the n-type and the Si(110) portion for the p-type channels, providing a solution to the current drive imbalance between the two channels confronted in Si(100)-based complementary metal oxide semiconductor (CMOS) technology. The central idea is to use a rotated heteroepitaxy of 3C-SiC(111) on Si(110) substrate, which occurs when a 3C-SiC film is grown under certain growth conditions. Monomethylsilane (SiH 3 –CH 3 ) gas-source molecular beam epitaxy (GSMBE) is used for this 3C-SiC interlayer formation while disilane (Si 2 H 6 ) is used for the top Si(111) layer formation. Though the film quality of the Si epilayer leaves a lot of room for betterment, the present results may suffice to prove its potential as a new technology to be used in the next generation CMOS devices.

We report on the fabrication and transport characteristics of nanoscale stacked-junctions of thin graphite flake. The stacked-junctions were fabricated using a three-dimensional focused-ion-beam milling. By varying the effective in-plane area down to submicron scale, the stacked-junctions with in-plane area A (from 2 down to 0.25 μm 2 ) and stack height–length (from 300 to 100 nm) along c -axis were fabricated. The nano-stack shows perfect c -axis transport characteristics in which we observed a semiconducting behavior for T > 65 K and metallic behavior for T c -axis electrical conduction mechanism. The stack with in-plane area A of 0.25 μm 2 showed nonlinear concave-like I – V characteristics even at 300 K; however the stack with A ⩾ 1 μm 2 were shown an ohmic-like I – V characteristic at 300 K for both low and high-current biasing. It turned into nonlinear characteristics when the temperature goes down. The observation of this anomalous transport characteristics were discussed in detail with stack capacitance calculations. The nonlinear characteristics observed at 300 K for the stack with A of 0.25 μm 2 were shown best fit with Fowler–Nordheim tunneling model.

We have deposited unhydrogenated and hydrogenated Si-incorporated DLC (Si-DLC) films by pulsed laser deposition using KrF excimer laser, and systematically examined the structure and the mechanical and tribological properties of the films. Hydrogenated Si-DLC films were prepared by atomic-hydrogen irradiation during deposition. The Si/(Si+C) ratio in DLC films increased by atomic-hydrogen irradiation during deposition, indicating that the hydrogen etching is more effective for C atoms compared with Si atoms. The formation of Si–C bonds in the films and silicon oxides only at the surfaces was confirmed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was found that the atomic-hydrogen irradiation led to the formation of Si–H bonds to prevent the surface oxidation of the Si-DLC films. The scratch tests revealed that the critical loads of the films deposited with hydrogen were higher than those of the films deposited without hydrogen. We found that the moderately hydrogen-irradiated Si-DLC films tended to have higher wear resistance than the unhydrogenated Si-DLC films.

Kinetics of the step flow growth on the (16 × 2) reconstructed Si(110) surface has been studied experimentally and with computer simulations. It is shown that during Si growth under DC heating vicinal steps on the (16 × 2) reconstructed Si(110) surfaces undergo a kinetic step bunching and develop extended segments preferentially oriented along the (16 × 2) reconstruction domains. The final step configuration depends crucially on the direction of the applied electric field. In particular, when DC is applied in the [ 1 1 ¯ 2 ] direction, an array of straight multisteps parallel to the current direction and rotated in respect to the original orientation of the vicinal steps can be fabricated. Surprisingly, the observed step transformations are not affected by the polarity of the applied electrical field. Using a simple model of the Si/Si(110)-(16 × 2) growth and kinetic Monte Carlo simulations we show that the step bunching and step rotation on Si(110)-(16 × 2) might be induced by an incoherent matching of the (16 × 2) reconstruction domains across the vicinal steps on the surface.

We have deposited unhydrogenated diamond-like carbon (DLC) films on Si substrate by pulsed laser deposition using KrF excimer laser, and investigated the effects of atomic-hydrogen exposure on the structure and chemical bonding of the DLC films by photoelectron spectroscopy (PES) using synchrotron radiation and Raman spectroscopy. The fraction of sp 3 bonds at the film surface, as evaluated from C1s spectra, increased at a substrate temperature of 400 °C by atomic-hydrogen exposure, whereas the sp 3 fraction decreased at 700 °C with increasing exposure time. It was found that the sp 3 fraction was higher at the surfaces than the subsurfaces of the films exposed to atomic hydrogen at both the temperatures. The Raman spectrum of the film exposed to atomic hydrogen at 400 °C showed that the clustering of sp 2 carbon atoms progressed inside the film near the surface even at such a low temperature as 400 °C.

Since the pioneering work by Novoselov and Geim in 2004 [1], graphene has attracted skyrocketing attention as a new electronic material. Behind this enthusiasm are the fundamental limitations that silicon is facing, which impede further scaling of Si devices down into sub-10 nm regions. In this respect, the expectation for graphene is two-fold: medium term and long term. As for the former, the expectation as a new channel material in field-effect transistors (FET) is at its center (More Moore strategy). As for the latter, graphene’s novel quantum properties are expected to bring about new paradigm of information processing (Beyond CMOS strategy). The fact that many quantum phenomena, such as quantized Hall effect [2] and spin-polization of the current [3], can be observed at room temperatures in graphene, which is not usual in other materials, fuels the trend.

This paper reports preparation of top-gate epitaxial graphene FETs (EGFETs) on silicon substrates. Epitaxial graphene is obtained from a thermal decomposition at the surface of a 3C–SiC layer grown on Si substrates. We provide the first demonstration and comparison of the EGFETs fabricated on Si(1 1 0) and Si(1 1 1) substrates. For all EGFETs, an n-type transistor operation is observed by the gate voltage modulation. We find that the Si(1 1 1) substrates give better flatness at the surface of the 3C–SiC layer. As a result, the EGFETs fabricated on Si(1 1 1) substrates exhibit higher channel currents than those on Si(1 1 0) substrates.

We have fabricated and characterized the field effect transistors having an epitaxial-graphene channel grown on Si substrates. Epitaxial graphene is usually formed on SiC substrates by ultrahigh-vacuum (UHV) annealing. We used an approach to grow 3C–SiC layer on Si substrates and subsequently to anneal them in UHV to make few layers of graphene on the sample surface. Backgate transistors were able to be formed by using the SiC layer as a gate insulator. Although the gate-leakage current is not negligible, the drain current modulation by means of the gate voltage is confirmed by extracting the channel current from the total drain current. A new evaluation method of the area contact resistance between graphene and metal is also proposed.

Surface chemistry involved in the epitaxy of graphene by sublimating Si atoms from the surface of epitaxial 3C-SiC(111) thin films on Si(111) has been studied. The change in the surface composition during graphene epitaxy is monitored by in situ temperature-programmed desorption spectroscopy using deuterium as a probe (D2-TPD) and complementarily by ex situ Raman and C1s core-level spectroscopies. The surface of the 3C-SiC(111)/Si(111) is Si-terminated before the graphitization, and it becomes C-terminated via the formation of C-rich (6√3 × 6√3)R30° reconstruction as the graphitization proceeds, in a similar manner as the epitaxy of graphene on Si-terminated 6H-SiC(0001) proceeds.

This article reviews recent advances in emission of terahertz radiation from two-dimensional (2D) electron systems in semiconductor nano-heterostructures. 2D plasmon resonance is first presented to demonstrate intense broadband terahertz emission from InGaP/InGaAs/GaAs material systems. The device structure is based on a high-electron mobility transistor and incorporates the author's original interdigitated dual-grating gates. The second topic focuses on graphene, a monolayer carbon-atomic honeycomb lattice crystal, exhibiting unique carrier transport and optical properties owing to the massless and gapless energy spectrum. Coherent stimulated terahertz emission from femtosecond infrared-laser pumped epitaxial graphene is experimentally observed, reflecting the occurrence of negative dynamic conductivity and population inversion.

We experimentally observed the fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in a heteroepitaxial graphene-on-Si material under pumping with a 1550-nm, 80-fs pulsed fiber laser and probing with the corresponding terahertz beam generated by and synchronized with the pumping laser. The time-resolved electric-nearfield intensity originating from the coherent terahertz photon emission is electrooptically sampled in total-reflection geometry. The Fourier spectrum fairly agrees the product of the negative dynamic conductivity and the expected THz photon spectrum reflecting the pumping photon spectrum. This phenomenon is interpreted as an amplified stimulated terahertz emission.

We have deposited boron- and/or nitrogen-incorporated DLC films by radio-frequency magnetron sputtering, and systematically investigated the structure and the mechanical and tribological properties. The N content in DLC films increased with increasing N2 flow ratio [N2/(Ar + N2)], and it tended to be saturated at higher N2 flow ratios. The N content further increased with an increase in the B content of the targets. The B/C ratios of the films were almost the same as those of the B-containing targets regardless of the N content. Scratch tests revealed that the adhesion strength of N-incorporated DLC films decreased with increasing N2 flow ratio and the critical loads of B-incorporated films were lower than that of an unincorporated film. It was found that for B, N-coincorporated films there was an optimum N2 flow ratio at which the critical load became a maximum value, which was higher than that of the unincorporated film. The optimum N2 flow ratio increased with an increase in the B composition of the targets. The N-incorporated films peeled off during ball-on-plate friction tests. On the other hand, the B, N-coincorporated films showed good wear-resistant properties that the specific wear rates were lower than those of the unincorporated and B-incorporated films.

Temperature dependence of the growth rate of 3C-SiC(001) films on Si(001) substrates during ultralow-pressure (ULP: ~10-1 Pa) CVD using monomethylsilane has been investigated in detail by using pyrometric interferometry. A novel behavior, i.e. a sharp division of the growth mode into two regimes depending on the growth temperature, has been found to exist. Based on this finding, we have developed a two-step process, which realizes a low-temperature (900 °C), high-rate growth of single-crystalline 3C-SiC film on Si substrates, whose rate of 3 m/h is extremely high for this ULP process.