Design of a monomeric arsinogallane and chemical conversion to gallium arsenide

A monomeric arsinogallane containing a covalent gallium-arsenic bond has been prepared, and its molecular structure has been determined by x-ray crystallography. The compound reacted with tert-butanol at ambient temperature to yield the III-V semiconductor gallium arsenide as a finely divided amorphous solid. During the initial stages of the reaction small clusters of gallium arsenide were apparently present in solution. The band gaps of these particles, as observed by their absorption spectra, were larger than that of the bulk material. This work is a step toward the development of new molecular precursors for technologically important materials and the study of quantum size effects in small semiconductor particles.     

Optical studies of individual InAs quantum dots in GaAs: few-particle effects

Optical emission from individual strained indium arsenide (InAs) islands buried in gallium arsenide (GaAs) was studied. At low excitation power density, the spectra from these quantum dots consist of a single line. At higher excitation power density, additional emission lines appeared at both higher and lower energies, separated from the main line by about 1 millielectron volt. At even higher excitation power density, this set of lines was replaced by a broad emission peaking below the original line. The splittings were an order of magnitude smaller than the lowest single-electron or single-hole excited state energies, indicating that the fine structure results from few-particle interactions in the dot. Calculations of few-particle effects give splittings of the observed magnitude.     

Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.     

GaAs Clusters in the Quantum Size Regime: Growth on High Surface Area Silica by Molecular Beam Epitaxy

Molecular beam epitaxy has been used to grow microcrystalline clusters of gallium arsenide (GaAs) in the size range from 2.5 to 60 nanometers on high-purity, amorphous silica supports. High-resolution transmission electron microscopy reveals that clusters as small as 3.5 nanometers have good crystalline order with a lattice constant equal to that of bulk GaAs. Study of the microcrystallite surfaces by x-ray photoelectron spectroscopy shows that they are covered with a shell (1.0 to 1.5 nanometers thick) of native oxides of gallium and arsenic (Ga(2)O(3) and As(2)O(3)), whose presence could explain the low luminescence efficiency of the clusters. Optical absorption spectra of the supported GaAs are consistent with the blue-shifted band edge expected for semiconductor microcrystallites in the quantum size regime.     

Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot

The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons.     

Evidence of soft-mode quantum phase transitions in electron double layers

Inelastic light scattering by low-energy spin-excitations reveals three distinct configurations of spin of electron double layers in gallium arsenide quantum wells at even-integer quantum Hall states. The transformations among these spin states appear as quantum phase transitions driven by the interplay between Coulomb interactions and Zeeman splittings. One of the transformations correlates with the emergence of a spin-flip intersubband excitation at vanishingly low energy and provides direct evidence of a link between quantum phase transitions and soft collective excitations in a two-dimensional electron system.     

A photomicrodynamic system with a mechanical resonator monolithically integrated with laser diodes on gallium arsenide

A cantilever resonant microbeam, laser diodes, and a photodiode have been fabricated on the surface of a gallium arsenide substrate. The microbeam is excited photothermally by light from a laser diode. The vibration is detected with a photodiode as the variation in light output caused by the difference in optical length between the microbeam and another laser diode. A high carrier-to-noise ratio (45 decibels) is achieved with a short (3 micrometers) external cavity length. Such a small distance allows a lensless system, which increases the ease of fabrication. This work could lead to applications in which photomicrodynamic systems are monolithically integrated on a gallium arsenide substrate with surface micromachining technology.     

Two-dimensional Mott-Hubbard electrons in an artificial honeycomb lattice

Artificial crystal lattices can be used to tune repulsive Coulomb interactions between electrons. We trapped electrons, confined as a two-dimensional gas in a gallium arsenide quantum well, in a nanofabricated lattice with honeycomb geometry. We probed the excitation spectrum in a magnetic field, identifying collective modes that emerged from the Coulomb interaction in the artificial lattice, as predicted by the Mott-Hubbard model. These observations allow us to determine the Hubbard gap and suggest the existence of a Coulomb-driven ground state.     

Observation of the spin Hall effect in semiconductors

Electrically induced electron-spin polarization near the edges of a semiconductor channel was detected and imaged with the use of Kerr rotation microscopy. The polarization is out-of-plane and has opposite sign for the two edges, consistent with the predictions of the spin Hall effect. Measurements of unstrained gallium arsenide and strained indium gallium arsenide samples reveal that strain modifies spin accumulation at zero magnetic field. A weak dependence on crystal orientation for the strained samples suggests that the mechanism is the extrinsic spin Hall effect.     

Imaging coherent electron flow from a quantum point contact

Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.     

Ultrafast bond softening in bismuth: mapping a solid's interatomic potential with X-rays

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.     

Low-temperature synthesis of zintl compounds with a single-source molecular precursor

Thermolysis of the heterobimetallic phosphinidene complex [Sb(PCy)3]2- Li6.6HNMe2 (Cy = C6H11) at 303 to 313 kelvin gives Zintl compounds containing (Sb7)3- anions. The complex thus constitutes a stable molecular single-source precursor to Zintl compounds and provides a potential low-temperature route to photoactive alkali metal antimonates. The new chemical reaction involved, which is driven thermodynamically by the formation of P-P bonds, has implications in the low-temperature synthesis of other technologically important materials (such as gallium arsenide).     

Coupled quantum dots fabricated by cleaved edge overgrowth: from artificial atoms to molecules

Atomically precise quantum dots of mesoscopic size have been fabricated in the gallium arsenide-aluminum gallium arsenide material system by cleaved edge overgrowth, with a high degree of control over shape, composition, and position. The formation of bonding and antibonding states between two such "artificial atoms" was studied as a function of quantum dot separation by microscopic photoluminescence (PL) spectroscopy. The coupling strength within these "artificial molecules" is characterized by a systematic dependence of the separation of the bonding and antibonding levels, and of the PL linewidth, on the "interatomic" distance. This model system opens new insights into the physics of coupled quantum objects.     

Crystal Structures at High Pressures of Metallic Modifications of Compounds of Indium, Gallium, and Aluminum

X-ray diffraction shows that the high-pressure modifications (at 22 to 130 kilobars) of the antimonides of indium, gallium, and aluminum are analogous to white tin. The arsenide and phosphide of indium transform to NaCl type. The transformation of these semiconductors to their metallic states is empirically related to their energy gap under normal conditions.     

The effect of spin splitting on the metallic behavior of a two-dimensional system

Experiments on a constant-density two-dimensional hole system in a gallium arsenide quantum well revealed that the metallic behavior observed in the zero-magnetic-field temperature dependence of the resistivity depends on the symmetry of the confinement potential and the resulting spin splitting of the valence band.     

Conductance quantization at a half-integer plateau in a symmetric GaAs quantum wire

We present data from an induced gallium arsenide (GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e2/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.     

In-Plane Structure of the Liquid-Vapor Interface of an Alloy: A Grazing Incidence X-ray Diffraction Study of Bismuth:Gallium

The liquid-vapor interface of a bismuth-gallium mixture (0.2 percent bismuth and 99.8 percent gallium) at 36 degrees C has been studied by grazing incidence x-ray diffraction. The data show, in agreement with thermodynamic arguments, that bismuth is heavily concentrated in the liquid-vapor interface. The x-ray diffraction data are interpreted with the assistance of a simple model that represents the interface as a partial monolayer of bismuth. This analysis leads to the conclusion that the bismuth concentration in the interface is about 80 percent, that there is no significant mixing of gallium and bismuth in the interface, and that the structure function of the interfacial bismuth is like that of supercooled bulk liquid bismuth.     

Heterogeneous three-dimensional electronics by use of printed semiconductor nanomaterials

We developed a simple approach to combine broad classes of dissimilar materials into heterogeneously integrated electronic systems with two- or three-dimensional layouts. The process begins with the synthesis of different semiconductor nanomaterials, such as single-walled carbon nanotubes and single-crystal micro- and nanoscale wires and ribbons of gallium nitride, silicon, and gallium arsenide on separate substrates. Repeated application of an additive, transfer printing process that uses soft stamps with these substrates as donors, followed by device and interconnect formation, yields high-performance heterogeneously integrated electronics that incorporate any combination of semiconductor nanomaterials on rigid or flexible device substrates. This versatile methodology can produce a wide range of unusual electronic systems that would be impossible to achieve with other techniques.     

Soft X-ray-driven femtosecond molecular dynamics

The direct observation of molecular dynamics initiated by x-rays has been hindered to date by the lack of bright femtosecond sources of short-wavelength light. We used soft x-ray beams generated by high-harmonic upconversion of a femtosecond laser to photoionize a nitrogen molecule, creating highly excited molecular cations. A strong infrared pulse was then used to probe the ultrafast electronic and nuclear dynamics as the molecule exploded. We found that substantial fragmentation occurs through an electron-shakeup process, in which a second electron is simultaneously excited during the soft x-ray photoionization process. During fragmentation, the molecular potential seen by the electron changes rapidly from nearly spherically symmetric to a two-center molecular potential. Our approach can capture in real time and with angstrom resolution the influence of ionizing radiation on a range of molecular systems, probing dynamics that are inaccessible with the use of other techniques.     

Suppressing spin qubit dephasing by nuclear state preparation

Coherent spin states in semiconductor quantum dots offer promise as electrically controllable quantum bits (qubits) with scalable fabrication. For few-electron quantum dots made from gallium arsenide (GaAs), fluctuating nuclear spins in the host lattice are the dominant source of spin decoherence. We report a method of preparing the nuclear spin environment that suppresses the relevant component of nuclear spin fluctuations below its equilibrium value by a factor of approximately 70, extending the inhomogeneous dephasing time for the two-electron spin state beyond 1 microsecond. The nuclear state can be readily prepared by electrical gate manipulation and persists for more than 10 seconds.