GaAs/GaN heterostructures were grown by molecular-beam epitaxy using GaN/supphire (0001) templates. In spite of a ~20% lattice mismatch, epitaxial growth was realized, so that the GaAs films showed good adhesion and their surface had a large mirror-like area. The GaAs films were as thick as 1μm. The surface profile was characterized by atomic-force microscopy, which gave an average roughness of 10 nm for a 5×5 μm scan. Micro-Raman characterization and transmission electron microscopy (TEM) showed that the epitaxial GaAs films had zincblende lattice with (111) orientation, whereas the GaN substrates had wurtzite symmetry. The GaAs/GaN interface was found to be flat and abrupt. A large number of defects have been observed which originated from relaxation of the large lattice mismatch. The defects included misfit dislocations and nanocavities at the interface, as well as dislocations and stacking faults in the bulk of the GaAs film. Sharp interference fringes and characteristic behavior were observed for the ψ and Δ parameters of spectroscopic ellipsometry in the range of 0.75-5.3 eV. Simulation of the optical properties of the GaAs/GaN/sapphire heterostructure indicated a reasonably good optical quality of the layers and interfaces. Photoluminescence (PL) spectra recorded at the temperatures from 17 to 300 K revealed wide and weak radiative bands. Non-radiative processes dominated in recombination of non-equilibrium carriers. The observed PL broadening originated from the band tails that were a result of the high density of charged defects.

"Negative Resistance Device', IBM Technical Disclo sure Bulletin, 31(7):20 (1988). Baratte, H., "Stain Layer Gated Selective Area En hancement and Depletion Mode Semiconductor Field Effect Transistors' IBM Technical Disclosure Bulletin, 31(7):17 (1988). Molecular-Beam Epitaxial Growth and Characteriza tion of Strained GanAs/AllinAs and InAS/GaAs "Quantum Well Two-Dimensional Electron Gas Field Effect Transistors', Seventh Molecular Bearn Epitaxy Workshop, Cambridge, Mass., 20-22 (Oct. 1986). Gavrilovic, P., et al., “Resonant Tunnelling in a GaAs 1-PGaAs Strained Layer Quantum Well Heterostruc ture', Solid State Communications, 52(3):237 (1984).

We have investigated the low-frequency 1/f noise of both suspended and on-substrate graphene field-effect transistors and its dependence on gate voltage, in the temperature range between 300 and 30 K. We have found that the noise amplitude away from the Dirac point can be described by a generalized Hooge's relation in which the Hooge parameter α(H) is not constant but decreases monotonically with the device's mobility, with a universal dependence that is sample and temperature independent. The value of α(H) is also affected by the dynamics of disorder, which is not reflected in the DC transport characteristics and varies with sample and temperature. We attribute the diverse behavior of gate voltage dependence of the noise amplitude to the relative contributions from various scattering mechanisms, and to potential fluctuations near the Dirac point caused by charge carrier inhomogeneity. The higher carrier mobility of suspended graphene devices accounts for values of 1/f noise significantly lower than those observed in on-substrate graphene devices and most traditional electronic materials.

We demonstrate a light emitting p-i-n diode made of a highly aligned film of separated (99%) semiconducting carbon nanotubes, self-assembled from solution. By using a split gate technique, we create p- and n-doped regions in the nanotube film that are separated by a micron-wide gap. We inject p- and n-type charge carriers into the device channel from opposite contacts and investigate the radiative recombination using optical micro-spectroscopy. We find that the threshold-less light generation efficiency in the intrinsic carbon nanotube film segment can be enhanced by increasing the potential drop across the junction, demonstrating the LED-principle in a carbon nanotube film for the first time. The device emits infrared light that is polarized along the long axes of the carbon nanotubes that form the aligned film.

We have grown single-crystal (Ga1−xZnx)(N1−xOx) solid-solution nanowires using nanostructured ZnGa2O4 precursor prepared by a sol-gel method. From electrical transport measurements in individual nanowire field-effect transistors, we have identified the conduction as n-type and obtained a background carrier density (∼1019 cm−3) and an electron mobility (∼1 cm2/V s) that are consistent with chemical disorder and a large number of charge traps, as confirmed by the devices’ photocurrent response. From the dependence of the device photoresponse on incident light wavelength, we have determined the energy band gap of (Ga0.88Zn0.12)(N0.88O0.12) to be as much as ∼0.6 eV lower than that of GaN or ZnO.

We have measured the shot noise of GaAs-(GaAl)As double-well resonant-tunneling diodes in which the barrier between the wells is so narrow (1.5 mu m) that there is a strong quantum-mechanical coupling between the wells' energy states. When compared with results in uncoupled double-well structures, our experiments show that there is no significant difference between coupled- and uncoupled-well diodes, thus casting doubt about calculations that predict that the amount of shot noise should depend on whether tunneling is sequential or coherent.

We have found experimentally that when the motion of electrons is correlated in multibarrier tunneling semiconductor heterostructures, the current spectral density of the shot noise is non-Poissonian. That the value is smaller or larger than 2eI depends on whether the correlation is negative or positive. Although existing models can qualitatively explain our results, they cannot account quantitatively for some of the findings, suggesting an incomplete understanding of noise in multi-barrier structures..

We have found strong deviations from Poissonian behavior in the low-temperature noise characteristics of triple-barrier and superlattice heterostructures. Although our results can be explained qualitatively by existing models, a quantitative comparison between experiment and theory suggests an incomplete understanding of shot noise in multi-barrier systems.

1-μm-thick zinc-blende GaAs (111) films were grown by molecular-beam epitaxy on wurtzite GaN∕sapphire (0001) templates. In spite of a ∼20% lattice mismatch, epitaxial growth was realized, so that the GaAs films showed good adhesion and their surface had a larger mirror-like area with an average surface roughness of 10 nm. Transmission electron microscopy revealed a flat and abrupt epitaxial GaAs∕GaN interface with some nanocavities and a large number of dislocations. Reasonably good crystalline quality of the GaAs films was confirmed by Raman characterization. Spectroscopic ellipsometry showed sharp interference fringes and characteristic parameters in the range of 0.75–5.3 eV. Photoluminescence study revealed extended band tails and dominance of non-radiative carrier recombination.

We have found experimentally that the noise of ballistic electron transport in a superconductor/semiconductor/superconductor junction is enhanced relative to the value given by the general relation, S_V=2eIR^2coth(eV/2kT), for two voltage regions in which this expression reduces to its thermal and shot noise limits. The noise enhancement is explained by the presence of large charge quanta, with effective charge q*=(1+2Delta/eV)e, that generate a noise spectrum S_V=2q*IR^2, as predicted in Phys. Rev. Lett. 76, 3814 (1996). These charge quanta result from multiple Andreev reflections at each junction interface, which are also responsible for the subharmonic gap structure observed in the voltage dependence of the junction's conductance.

We have found experimentally that the shot noise in InAlAs-InGaAs-InAlAs Triple-Barrier Resonant-Tunneling Diodes (TBRTD) is reduced over the 2eI Poissonian value whenever their differential conductance is positive, and is enhanced over 2eI when the differential conductance is negative. This behavior, although qualitatively similar to that found in double-barrier diodes, differs from it in important details. In TBRTDs the noise reduction is considerably larger than predicted by a semi-classical model, and the enhancement does not correlate with the strength of the negative differential conductance. These results suggest an incomplete understanding of the noise properties of multiple-barrier heterostructures.

We report tunneling phenomena in double In$_{0.53}$Ga$_{0.47}$As quantum-well structures that are at odds with the conventional parallel-momentum-conserving picture of tunneling between two-dimensional systems. We found that the tunneling current was mostly determined by the correlation between the emitter and the state in one well, and not by that between those in both wells. Clear magnetic-field-dependent features were first observed before the main resonance, corresponding to tunneling channels into the Landau levels of the well near the emitter. These facts provide evidence of the violation of in-plane momentum conservation in two-dimensional systems.

We have observed that the tunneling magnetoconductance between two-dimensional (2D) electron gases formed at nominally identical InAs-AlSb interfaces most often exhibits two sets of Shubnikov-de Haas oscillations with almost the same frequency. This result is explained quantitatively with a model of the conductance in which the 2D gases have different densities and can tunnel between Landau levels with different quantum indices. When the epitaxial growth conditions of the interfaces are optimized, the zero-bias magnetoconductance shows a single set of oscillations, thus proving that the asymmetry between the two electron gases can be eliminated.