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We report the experimental demonstration of second-harmonic generation in periodically poled GaN by first-order quasiphase matching. The periodically poled structure was grown by plasma-assisted molecular-beam epitaxy. We observed about 9 μW of second-harmonic power from a fundamental input laser wavelength of 1658.6 nm with a normalized conversion efficiency of 12.76% W−1 cm−2. The ability to perform nonlinear wavelength conversion by periodic poling and the fact that GaN has a very wide window of transparency, pave the way for GaN to be used for nonlinear optical devices in telecommunications as well as a nonlinear light source for biochemical detection in the far infrared and deep ultraviolet.

We report on the growth and transport characteristics of high-density (∼10¹³cm⁻²) two-dimensional electron gases confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on sapphire substrates. For structures consisting of a 25 nm Al ₀.₃₀Ga₀.₇₀N barrier deposited on a 2 μm insulating GaN buffer, room-temperature mobilities averaging 1400cm²/Vs at a sheet charge density of 1.0×10¹³cm⁻² are consistently achieved. Central to our approach is a sequence of two Ga/N ratios during the growth of the insulating GaN buffer layer. The two-step buffer layer allows us to simultaneously optimize the reduction of threading dislocations and surface morphology. Our measured sheet resistivities as low as 350/ compare favorably with those achieved on sapphire or SiC by any growth method. Representative current-voltage characteristics of high-electron-mobility transistors fabricated from this material are presented. © 2002 American Institute of Physics.

Using electroabsorption measurements, we have studied the effects of atomic superlattice ordering on the electronic band structure of InGaAs for different growth parameters. We have observed ordering-induced polarization anisotropy, valence-band splitting and band gap reduction strongest for 550 °C growth and 2°[111]B tilted substrates. Back-folded conduction-band states show an ordering dependent energy shift. The position of the split-off valence-band, however, is almost unaffected. An extension to extremely low growth temperatures exhibits ordering also for 450 °C growth. Atomic force microscopy measurements reveal a temperature-dependent change of InGaAs surface from step-like to island formation.

Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111).

For physical studies of correlated electron systems and for realizing novel device concepts, electrostatic modulation of metal-insulator transitions (MITs) is desired. The inherently high charge densities needed to modulate MITs make this difficult to achieve. The high capacitance of ionic liquids are attractive but, voltages are needed that can be in excess of the electrochemical stability of the system. Here, we show temperature/resistivity data that suggest electrostatic modulation of the MIT temperature of NdNiO3 in a wide regime. However, additional voltammetric and x-ray photoelectron spectroscopy measurements demonstrate the electrochemical impact of the electrostatic doping approach with ionic liquids.

This letter presents a simple, quick, nondestructive method for the determination of ordering in Ga0.51In0.49P layers using x‐ray diffractometry. The skew, symmetric arrangement of x‐ray reflection was used for this purpose enabling the measurements of all allowed (hkl) reflections. Ordered Ga0.51In0.49P layers grown by metalorganic vapor‐phase epitaxy at different substrate temperatures were studied. The {1/2,1/2,1/2}, {1/2,1/2,3/2}, and {1/2,1/2,5/2} reflections of the ordering‐induced monolayer superlattices have been observed. The experimental results are discussed in detail, demonstrating that x‐ray diffractometry is a powerful means to evaluate ordering in semiconductors.

The existence of bistability, gap solitons, and self‐localized solitons in a magnetostatic periodic structure are predicted. The very low threshold power necessary for the observation of such phenomenas at microwave frequencies makes these structures very attractive for microwave signal processing.

We propose and investigate a surface plasmon photodetector concept, based on the enhancement of electrical near-field in low-defect, low-doped In0.53Ga0.47As detection volumes located in the gaps of an array of metal nanodipole antennas. We report enhancement in responsivity in the presence of nanodipoles and predict a maximum responsivity of ∼100 mA/W at wavelengths near 1550 nm. The 3 dB electrical bandwidth of the device is estimated based on its RC rise time and the hole transit time through the detection volume for the cases of conventional and ballistic transport in InGaAs and is found to range from ∼0.7 to 4 THz. Also, trends are observed relating the responsivity to the gap dimensions, revealing a trade-off between the field-enhancement in the gap and its volume, and leading to an optimum gap length producing the maximum responsivity.

Resistance profiles along as-grown GaAs nanowires were measured with a multi-tip scanning tunneling microscope used as a nanoprober. The nanowires were grown in the vapor-liquid-solid growth mode in a two-temperature-step mode and doped with Zn. Using a transport model, the resistance profile was converted to a dopant profile. The dopant distribution along the nanowires was found to correlate with the temperature during different phases of nanowire growth. The nanowire base grown at higher temperature exhibits a decreased dopant concentration. Mechanical stress by intentional bending of a nanowire was shown not to influence nanowire conductance.

It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample.

We present a study of the morphology and the thermoelectric properties of short-pulse laser-sintered (LS) nanoparticle (NP) thin films, consisting of SiGe alloy NPs or composites of Si and Ge NPs. Laser-sintering of spin-coated NP films in vacuum results in a macroporous percolating network with a typical thickness of 300 nm. The Seebeck coefficient is independent of the sintering process and typical for degenerate doping. The electrical conductivity of LS films rises with increasing temperature, best described by a power-law and influenced by two-dimensional percolation effects., Comment: 4 pages, 4 figures

We report on an experimental evidence of a significantly different dynamic nuclear polarization (DNP) for localized and itinerant electrons in n-GaAs. Optically injected spin-polarized electrons are used to generate dynamic nuclear polarization via electron-nucleus hyperfine interaction. Using time resolved Kerr rotation measurements for probing the transient Overhauser field, the DNP time constants for itinerant and localized electrons are extracted to be 10 min and less than 1 min, respectively. This is attributed to a rapid DNP occurring in the vicinity of the donors followed by a delayed nuclear spin polarization in between the donor sites.

Single photon emission from an epitaxially grown quantum dot at room temperature is presented. CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K. Under continuous wave optical excitation, the autocorrelation function g(2)(t) exhibits a sharp dip at (t = 0) with g(2)(0) = 0.16 ± 0.15 at T = 300 K, revealing excellent suppression of multiphoton emission even at room temperature.

Optically detected nuclear magnetic resonance (NMR) with micrometer resolution is demonstrated in n-GaAs using an on-chip microcoil. To trace the Overhauser field, the electron Larmor frequency is monitored via time-resolved magneto-optical Kerr rotation. Sweeping the frequency of the rf magnetic field induced by an on-chip microscale current loop, nuclear spin depolarization is achieved for each isotope species. The experimental data indicate an impact of a local quadrupole field, most likely caused by ionized donors, on the amplitude and linewidth of the NMR spectrum. By applying rf pulse sequences, the Rabi oscillation of A75s nuclear spins is obtained with an effective dephasing time of ∼200 μs.

A thermopile structure is proposed for the detection of microwave/millimeter wave radiation. The thermopairs in the suggested linear arrangement function as antennas. 5.58 V/W responsivity was achieved at 100 GHz with 40 serial connected thermopairs. The experimentally observed polarity and frequency dependence convincingly verify the proper detector operation.

We report on the fabrication of high-speed GaNAsSb photodetector for multigigabit operation at 1.3μm wavelength. The 2μm thick bulk GaNAsSb photoabsorption layer contains 3.3% of N and 8% of Sb, resulting in optical band gap of ∼0.9eV suitable for near infrared operation up to wavelengths of about 1380nm. By using the bulk photoabsorption layer, we have fabricated GaNAsSb photodetector with high dc responsivity of 0.12A∕W at 1.3μm wavelength. The photodetector exhibits 3dB cutoff frequency at 1.3μm wavelength of 4.5GHz. Furthermore, 5Gb∕s fiber-optic transmission at 1.3μm wavelength is demonstrated using the GaNAsSb photodetector.

The emission of a two-dimensional organic photonic crystal structure is investigated. A photon emitted by a molecule in this structure can take two different coupled pathways. It can either be emitted directly into the surroundings or first be transferred to a discrete state of the photonic crystal structure from where it is then emitted. This unique coupling of the pathways results in a particular asymmetric spectral line shape referred to as Fano resonance. By studying the Fano line shape as a function of the quality factor, the authors can gain insights into the coupling between the pathways within the photonic crystal structure.

The authors report on picosecond pulse response GaNAsSb∕GaAs p-i-n photodetectors grown by molecular beam epitaxy in conjunction with a rf plasma-assisted nitrogen source. The 2μm thick GaNAsSb photoabsorption layer contains 3.3% of N and 8% of Sb resulting in a dc photoresponse up to 1380nm wavelength. Dark current densities at 0 and −5V are 1.6×10−5 and 13A∕cm2, respectively. The GaNAsSb photodiodes exhibit a record pulse response width of only 40.5ps (full width at half maximum) corresponding to a 4.5GHz bandwidth.

The authors report lattice distortion and “ω-like” structural short-range order (SRO) of the β-Ti phase in a Ti–Fe–(Sn) bulk nanoeutectic composite prepared by slow cooling from the melt. The nanoeuetctic phases are chemically homogeneous, but the addition of Sn releases the local lattice strain, modifies the structural SRO, and prevents the formation of stacking faults in the body centered cubic (bcc) β-Ti phase resulting in improved plastic deformability. The elastic properties and the structural SRO of the β-Ti phase are proposed to be important parameters for developing advanced high strength, ductile Ti-base nanocomposite alloys.

An organic complementary metal–oxide–semiconductor (O-CMOS) inverter is presented, which is based on a single pentacene layer acting both as n- and p-type organic semiconductors. The circuit consists of two spatially separated transistors realized by pairs of calcium and gold source and drain electrodes, respectively. The p transistor is obtained by utilizing the conventional pentacene∕SiO2 channel interface whereas the n transistor is realized by doping the SiO2 interface with traces of calcium prior to pentacene deposition. Both, n and p transistors work exclusively in unipolar mode within the range of the supply voltage of 60 V. The O-CMOS inverter works reliably with a gain in between 17 and 24, and the respective electron and hole mobilities were found around 0.1cm2V−1s−1. The circuit shows hysteresis, which can be explained by a gate voltage-dependent electron trapping in the n channel. Electron accumulation can also be realized by the application of a polymethylmethacrylate interlayer between SiO2 ...

Nonlinear optical cavities are known to exhibit bistability and multistability. In this letter the properties of microwave Fabry–Perot and ring resonators with second‐order dispersive nonlinearity are analyzed. Experimental results such as multiple‐valued output versus input characteristics, resonance curves, and waveforms are described. On the basis of harmonic and subharmonic generation a new mechanism is found leading to multistability, which is quantitatively attributed to well‐defined soliton modes propagating in the resonator.

Deep level analysis in the ohmic regime of ungated n-InGaAs/p-InP junction field-effect transistor structures was made by low-frequency noise measurements. The noise spectra exhibit two deep trap levels in the n-InGaAs channel with activation energies of 0.49 and 0.37 eV. The related capture cross sections are 7×10⁻¹⁵ and 4×10⁻ ¹⁶ cm², respectively.