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Frequency-dependent carrier dynamics in dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT)- based organic field-effect transistors and metal-insulator-semiconductor capacitors were studied by using admittance measurements. Both the real (conductance) and imaginary (capacitance) components of admittance, which have a mutually complementary relationship, were analyzed in the same way. On the basis of an experimental study of devices with different contact geometries, peaks observed in the conductance spectra corresponding to slopes in the capacitance spectra were classified into two groups. The high-frequency peak was attributed to the carrier injection from the top contact to the DNTT/insulator interface just underneath the contact while the low-frequency peaks were attributed to the drift current spreading all over the interface. The model calculation of carrier diffusion reproduces the low-frequency peaks very well. It is shown that the field-effect mobility from the accumulation region to the subthreshold region can be estimated by the fitting. [ABSTRACT FROM AUTHOR]

Multisubband transport of the two-dimensional electron gas (2DEG) in modulation-doped Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. It is found that the mobility of the 2DEG in the first subband in a triangular quantum well at the heterointerface decreases significantly, while the mobility of the 2DEG in the second subband increases, when the Al[sub 0.22]Ga[sub 0.78]N barrier is partially relaxed. Such behavior of the 2DEG mobility is explained by the nonuniformity of the piezoelectric polarization field at the heterointerface induced by the Al[sub 0.22]Ga[sub 0.78]N relaxation and strong interface scattering. Meanwhile, it is concluded that the scattering from the remote ionized donors is the main mechanism contributing to the quantum scattering time and responsible for the intersubband scattering in the quantum well at the heterointerface. [ABSTRACT FROM AUTHOR]

We fabricate semi-insulating InGaAs/GaAs multiple quantum wells and observe the excitonic enhancement of the photorefractivity in the Franz-Keldysh geometry at wavelengths of 0.92-0.94 μm. A maximum two-wave mixing gain of 138 cm[sup -1] and a maximum diffraction efficiency of 1.5x10[sup -4] are obtained. The saturation intensity and the spatial resolution are also measured by four-wave mixing. The diffraction efficiency is saturated at a high external electric field. The dominant cause of this saturation is the deviation of the excitonic electroabsorption from its quadratic law. © 2001 American Institute of Physics. We fabricate semi-insulating InGaAs/GaAs multiple quantum wells and observe the excitonic enhancement of the photorefractivity in the Franz-Keldysh geometry at wavelengths of 0.92-0.94 μm. A maximum two-wave mixing gain of 138 cm[sup -1] and a maximum diffraction efficiency of 1.5x10[sup -4] are obtained. The saturation intensity and the spatial ! resolution are also measured by four-wave mixing. The diffraction efficiency is saturated at a high external electric field. The dominant cause of this saturation is the deviation of the excitonic electroabsorption from its quadratic law. © 2001 American Institute of Physics. We fabricate semi-insulating InGaAs/GaAs multiple quantum wells and observe the excitonic enhancement of the photorefractivity in the Franz-Keldysh geometry at wavelengths of 0.92-0.94 μm. A maximum two-wave mixing gain of 138 cm[sup -1] and a maximum diffraction efficiency of 1.5x10[sup -4] are obtained. The saturation intensity and the spatial resolution are also measured by four-wave mixing. The diffraction efficiency is saturated at a high external electric field. The dominant cause of this saturation is the deviation of the excitonic electroabsorption from its quadratic law. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Carrier dynamics through a heterointerface of a Dirac material and a semiconductor was studied by the measurement of the full-range time-resolved core-level photoemission spectroscopy using synchrotron radiation. The electron-hole recombination process during relaxation of the surface photovoltage effect was delayed in a graphene layer due to the bottleneck effect of Dirac cones. When an intermediate buffer layer exists, the recombination mainly takes place at the interfacial Si dangling-bond sites and the relaxation time shortens by one-order of magnitude. The present result demonstrates unusual carrier dynamics at a semiconductor surface, terminated by a layer of the Dirac material. [ABSTRACT FROM AUTHOR]

Magnetointersubband scattering (MIS) oscillations of two-dimensional electron gas (2DEG) in Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures have been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Double periodic Shubnikov–de Haas oscillations modulated by MIS oscillations have been observed due to the intersubband scattering of the 2DEG at the two lowest subbands in the triangular quantum well at the heterointerface. By using the fast Fourier transform analysis, it is found that the MIS oscillations become slightly weaker with an increase in temperature. From the MIS frequency, the energy separation between the first and the second subbands is determined to be 80 meV. The observation of the MIS effect indicates that the effective masses of the electrons in the first and second subbands are the same in Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultrafast magnetization reversal of a ferrimagnetic metallic alloy GdFeCo was investigated by time-resolved resonant magneto-optical Kerr effect measurements using a seeded free electron laser. The GdFeCo alloy was pumped by a linearly polarized optical laser pulse, and the following temporal evolution of the magnetization of Fe in GdFeCo was element-selectively traced by a probe free electron laser pulse with a photon energy tuned to the Fe M-edge. The results have been measured using rotating analyzer ellipsometry method and confirmed magnetization switching caused by ultrafast heating. [ABSTRACT FROM AUTHOR]

Hot carrier dynamics in the Dirac band of n-type epitaxial graphene on a SiC substrate were traced in real time using femtosecond-time-resolved photoemission spectroscopy. The spectral evolution directly reflects the energetically linear density of states superimposed with a Fermi-Dirac distribution. The relaxation time is governed by the internal energy dissipation of electron-electron scattering, and the observed electronic temperature indicates cascade carrier multiplication. [ABSTRACT FROM AUTHOR]

Nonuniformity of heavy-ion-beam (HIB) illumination is one of key issues in the HIB inertial confinement fusion (ICF): implosion symmetry should be less than a few percent in order to compress a fuel sufficiently and release fusion energy effectively. In this paper a new HIB illumination scheme is presented in order to realize a robust illumination scheme against a displacement of a direct-driven fuel pellet in an ICF reactor. It is known that the HIB illumination nonuniformity is sensitive to a little pellet displacement from a reactor chamber center; a pellet displacement of only 50–100 μm was tolerable in the conventional HIB illumination schemes. In this paper by three-dimensional computer simulations a new robust HIB illumination scheme was found, in which a 200–300 μm displacement is allowed. [ABSTRACT FROM AUTHOR]

We described the photoluminescence (PL) properties of individual micelle-encapsulated single-walled carbon nanotubes (SWNTs) at room temperature. Single PL peak from isolated individual SWNT with a chiral index of (6, 5) showed a linear increase and saturation behavior of the PL intensity. Unusual PL intensity fluctuation in the temporal evolutions of the PL intensity, referred to as PL intermittency, was seen with some SWNTs, while the PL intensity with most SWNTs remained at a constant amplitude. The mechanism of the PL intermittency was discussed. [ABSTRACT FROM AUTHOR]

Optical nonlinearity in the yellow luminescence (YL) band of GaN was investigated using thick bulk samples. Transient pump-probe measurements revealed strong transmission changes due to two-photon absorption (TPA) even at the middle of the YL band. The TPA coefficient evaluated reaches ∼5 cm/GW at about 1.3 eV, which was as large as the mid-gap resonance. The TPA spectrum clearly showed that the observed large nonlinearity originated from the YL band. On the basis of efficient TPA in the YL band, relaxation processes in the multiphoton-induced photoluminescence excitation spectrum were also investigated. [ABSTRACT FROM AUTHOR]

Magnetotransport properties of modulation-doped Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures were investigated by means of magnetoresistance measurements at low temperatures and high magnetic fields. Strong Shubnikov-de Haas oscillations with the double periodicity are observed. The mobility spectrum is obtained, which demonstrates that the mobilities of the two-dimensional electron gas (2DEG) in the two subbands in the triangular quantum well at heterointerface. It is found that the mobility of the 2DEG in the second subband is much higher than that in the first one. This is explained that interface roughness scattering and alloy disorder scattering have much stronger influence on transport properties of the 2DEG in the first subband than that in the second subband in Al[sub x]Ga[sub 1-x]N/GaN heterostructures. [ABSTRACT FROM AUTHOR]

We demonstrate an enhancement of spontaneous emission coupling factor β in a nitride-based vertical-microcavity surface-emitting laser. The 2.5λ vertical microcavity, the quality factor of which reaches 740, is sandwiched between a nitride and an oxide distributed Bragg reflector. From input–output measurements and analyses of the rate equations, the β of the lasing mode at a wavelength of 396.1 nm is estimated to be 1.6×10[sup -2]. The estimated β can be well accounted for by a simple theoretical model. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Cyclotron resonance (CR) and magnetotransport experiments have been performed on modulation Si-doped Al[sub x]Ga[sub 1-x]N/GaN heterostructures with aluminum fraction x varying from 0.15 to 0.30. A clear CR absorption and Shubnikov–de Haas oscillations have been observed. The CR line shapes are analyzed by calculating the high frequency conductivity of a two-dimensional electron gas. The obtained electron effective mass m[sup *] and scattering time τ are found to depend on the aluminum fraction x. For x=0.30 the measured CR frequency shifts significantly upward, which demonstrates the formation of potential fluctuations in Al[sub x]Ga[sub 1-x]N/GaN heterostructures with large aluminum fraction x. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A way of analyzing the data in a variable stripe length method gain experiment is presented. We confirm that the stripe length dependence of the gain in In[sub 0.02]Ga[sub 0.98]N/In[sub 0.16]Ga[sub 0.84]N multiple quantum wells is caused by the change of the chemical potential along the excited stripe due to the interaction of the carrier and photon densities, and the gain threshold density is estimated. A trial function assuming a Lorentzian line shape for the stripe length dependence of the gain is compared with the edge emission intensity as a function of the stripe length. This is found to fit very well with our data, even beyond the saturation region. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the successful growing of nanometer-scale indium gallium nitride (InGaN) self-assembled quantum dots (QD) on GaN surface without any surfactant, using atmospheric-pressure metalorganic chemical vapor deposition. Average diameter of the InGaN QD; Dependence of QD properties on growth conditions; Emission seen at 2.86 eV at room temperature.

Determines the concentration of oxygen incorporated in GaAs/AlAs (gallium arsenide) interfaces and AlAs layers by secondary ion mass spectrometry (SIMS). Importance of minimizing the impurity incorporation from residual gas species in molecular-beam epitaxy; Details on the number of oxygen atoms adsorbed on the AlAs surfaces during growth interruption; Advantages of SIMS.

Describes the preparation of T-shaped gallium arsenide quantum wire (T-QWR) structure with aluminum arsenide barriers using the cleaved edge growth method. Observation of three photoluminescence (PL) peaks; Determination of the T-QWR binding energy; Comparison between the effective binding energy and thermal energy.

Presents a correction to the article "Photoluminescence intermittency in an individual single-walled carbon nanotube at room temperature" published in a 2005 issue of "Applied Physics Letters".

High-quality GaN with smooth surface morphology has been grown on vicinal a-plane sapphire substrates by metalorganic chemical vapor deposition. The misorientation angles of vicinal a-plane sapphire substrates were changed systematically and the results were compared with the growth on exact a- and c-plane sapphire substrates. Surface morphology and crystalline qualities are found to be very sensitive to misorientation angles of a-plane sapphire substrates and the misorientation angle was optimized to be 0.25°. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectra were measured for sub-nanometer-thick GaN quantum wells (QWs) with Al[sub 0.8]Ga[sub 0.2]N barriers, which were grown by atmospheric-pressure metal-organic chemical-vapor deposition. The thickness of the GaN QW layers was systematically varied from 1 to 4 ML. We clearly observed a PL peak at room temperature at a wavelength as short as 247 nm (5.03 eV) from 1-ML-thick QWs. The effective confinement energy, or difference between this recombination energy and the band gap of bulk GaN, is as large as 1.63 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report photoluminescence (PL) and time-resolved PL measurements of GaN/Al[sub x]Ga[sub 1-x]N multiple quantum wells with barriers of high aluminum content, x=0.5. In wells of width 1-2 nm, low temperature recombination appears to be dominated by radiative processes with lifetimes ∼0.5 ns. Dependence of lifetime on emission energy is very small compared to InGaN quantum wells, indicating that carrier localization is very slight and interface quality is high. In 4 nm wells, PL emission at an energy below the bulk GaN band gap and long recombination lifetimes result from the polarization field across the wells. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Influences of the thickness of the Si-doped n-type Al[sub 0.22]Ga[sub 0.78]N barrier and the thickness of the Al[sub 0.22]Ga[sub 0.78]N spacer on mobility and density of the two dimensional electron gas (2DEG) in modulation-doped Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures were investigated. 2DEG mobilities of 1274 cm2/V s at 300 K and 4495 cm2/V s at 77 K were reached. Both 2DEG mobility and density decrease dramatically when the Al[sub 0.22]Ga[sub 0.78]N barrier becomes partially relaxed, indicating that transport properties of the 2DEG are influenced significantly by the piezoelectric polarization of the Al[sub 0.22]Ga[sub 0.78]N layer. From our results, the critical thickness of an Al[sub 0.22]Ga[sub 0.78]N layer on GaN is estimated to be between 65 and 75 nm, which is much higher than that predicted by theoretical calculation. This may be attributed to the interaction of misfit dislocations and the presence of a high density of extended defects in the Al[sub 0.22]Ga[sub 0.78]N layer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) of modulation-doped Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures was investigated. The PL peak related to recombination between the two-dimensional electron gases (2DEG) and photoexcited holes is located at 3.448 eV at 40 K, which is 45 meV below the free excitons (FE) emission in GaN. The peak can be observed at temperatures as high as 80 K. The intensity of the 2DEG PL peak is enhanced significantly by incorporating a thin Al[sub 0.12]Ga[sub 0.88]N layer into the GaN layer near the heterointerface to suppress the diffusion of photoexcited holes. The energy separation of the 2DEG peak and the GaN FE emission decreases with increasing temperature. Meanwhile, the 2DEG peak energy increases with increasing excitation intensity. These results are attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface, which becomes stronger when temperature or excitation intensity is increased. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quarter-wave reflectors consisting of sets of GaN and Al[sub 0.34]Ga[sub 0.66]N layers have been grown on sapphire substrates by atmospheric-pressure metal organic chemical vapor deposition. A periodic structure with flat interfaces was observed by high-resolution scanning electron microscopy. X-ray diffraction measurements were performed to characterize the structures, from which the Al content x in the Al[sub x]Ga[sub 1-x]N layers was determined to be 0.34. No cracks could be seen on the surface of the reflectors by optical microscopy. The measured peak reflectivity at 390 nm increases with the number of pairs and reaches as high as 96±2% in the 35-pair reflector. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]