GaAs Quantum Wire Structures Research Articles

Strain-induced electronic energy changes in multilayered InGaAs∕GaAs quantum wire structures

Electronic states of layered InGaAs∕GaAs(001) quantum wire and quantum dot chain structures have been investigated by electroreflectance and surface photovoltage spectroscopy. Band-gap shrinkage and heavy-hole/light-hole state splitting have been observed in the GaAs barrier material. This can be understood by shear strain existing in the GaAs barrier due to strain relaxation and anisotropy within the wires or dot chains. By comparing the experimental results with theoretical calculations, we found that the strain relaxation in the direction perpendicular to the wires or the dot chains has a stronger effect on the heavy-hole–light-hole splitting than on band-gap modification in the InGaAs wires and dot chains. The piezoelectric field induced by the shear strain is also discussed.

Strain fields in InAs∕GaAs quantum wire structures: Inclusion versus inhomogeneity

This paper studies the elastic fields in InAs∕GaAs quantum wire (QWR) structures arising from the lattice mismatch between InAs and GaAs. The present treatment is different from recent analyses based on the Eshelby inclusion approach where the QWR material, for simplicity, is assumed to be the same as the matrix/substrate. Here, a more complete treatment is developed taking into account the structural inhomogeneity using the boundary integral equation method. We implement our model using discrete boundary elements at the interface between the QWR and its surrounding matrix. The coefficients of the algebraic equations are derived exactly for constant elements using our recent Green’s-function solutions in the Stroh formalism. For both (001) and (111) growth directions, our results show that while the elastic fields far from the QWR are approximated well by the homogeneous inclusion approach, for points within or close to the QWR, the differences between the fields computed with the simplified inclusion and complete inhomogeneity models can be as large as 10% for the test system. These differences in the strain fields will have strong implications for the modeling of the quantized energy states of the quantum wire nanostructures. Since the strain fields inside and close to the wire are more important than the exterior strain fields from the standpoint of the confined electronic states, we suggest that in the vicinity of the QWR, the inhomogeneity model be used with proper elastic constants, while the simple exact inclusion model be used in the bulk of surrounding medium.

Mindlin's problem for an anisotropic piezoelectric half–space with general boundary conditions

This paper considers Mindlin's problem in an anisotropic and piezoelectric halfspace with general boundary conditions, including 16 different sets of surface conditions. The Green's function due to...

Many-body renormalization of semiconductor quantum wire excitons: absorption, gain, binding, and unbinding.

We consider theoretically the formation and stability of quasi-one-dimensional many-body excitons in GaAs quantum wire structures under external photoexcitation conditions by solving the dynamically screened Bethe-Salpeter equation for realistic Coulomb interaction. In agreement with several recent experimental findings the calculated excitonic peak shows weak carrier-density dependence up to (and even above) the Mott transition density, nc approximately 3 x 10(5) cm(-1). Above nc we find considerable optical gain demonstrating compellingly the possibility of a one-dimensional quantum wire laser operation.

Direct measurements of the electron–acoustic phonon interaction in GaAs quantum wire structures

In this paper we describe new direct measurements of the emission and absorption of acoustic phonons by quasi-one-dimensional electrons in GaAs quantum wire structures. The measurements were made using heat-pulse techniques: for the absorption experiments, nonequilibrium phonons were generated using a metal-film resistive heater and the absorption detected via the change in conductivity induced in the quantum wire itself (phonoconductivity); in the emission experiments, the electrons in a quantum wire array were heated electrically and the emitted phonons detected by superconducting bolometers.

Patterned high-index substrates as templates for novel quantum wire and quantum dot arrays in hydrogen-assisted MBE

We report new approaches to fulfill the stringent criteria on the geometrical features of GaAs quantum wire and quantum dot structures to be met for useful devices. The clever combination of self-organized growth on high-index substrates with lithographic patterning and atomic-hydrogen assistance allows fabrication of GaAs quantum wires and quantum dots with superior electronic properties by molecular beam epitaxy. In addition, our method allows the precise positioning of the wires and dots on specific wafer areas.

Nanoscale mapping of confinement potentials in single semiconductor quantum wires by near-field optical spectroscopy

The quasi-one-dimensional confinement potential and its influence on carrier transport in a GaAs quantum wire structure are directly mapped by low-temperature near-field scanning optical spectroscopy with subwavelength spatial resolution. Shallow asymmetric potential barriers in the vicinity of the quantum wire are detected, and their height and width are determined quantitatively. We demonstrate the strong influence of such local barriers on carrier transport and trapping into the quantum wire, suppressing carrier trapping at low temperature.

Spin splitting in GaAs quantum wire structures

Resonant Raman scattering is used to probe electronic excitations between the two spin-split conduction bands of GaAs quantum wire structures produced by a new method using a pulsed laser interference grating. We discuss the difference of the intrasubband single particle excitations spectra of quantum wires oriented along two perpendicular directions.

Near-field spectroscopy of quantum wires grown by selective growth method

Using near-field optical microscopy we present experimental results on spatially resolved PL spectra of GaAs quantum wire (QWR) structures with a lateral width of 15 nm fabricated by selective growth method. The geometries of the samples make it possible to observe the luminescence from a single QWR structure. The typical fluctuations of the PL peak energy from different QWRs is estimated to be about 5 meV. To reduce the diffusion of carriers from barrier regions, PL spectra are observed by selective excitation of the QWR. A PL line width of 5 meV, which is about ten times narrower than that of the conventional PL measurement, is observed.

Effects of As 2 flux for fabrication of GaAs/AlGaAs quantum wires on V-grooved substrates in molecular beam epitaxy

GaAs/AlGaAs quantum-wire structures on V-grooved substrates have been fabricated under As 2 flux by molecular beam epitaxy. Under As 2 flux, a smaller number of Ga atoms migrate than those under As 4 flux to the V-groove bottom from the side-wall surface. The Al 0.6Ga 0.4As layer on the V-grooved GaAs substrate grown under As 2 flux preserves the V-shape, whereas the V-shape cannot be preserved and the quantum-wire structures cannot be fabricated under As 4 flux. The GaAs quantum-wire structures which have Al 0.6Ga 0.4As barrier layers grown under As 2 flux have good optical quality.

Mapping of the Local Confinement Potential in Semiconductor Nanostructures by Near-Field Optical Spectroscopy

The optical properties of a new quantum-well-embedded GaAs quantum wire structure grown on patterned (311)A GaAs surfaces are directly mapped by low temperature near-field scanning optical microscopy (NSOM). Photoluminescence excitation spectroscopy with subwavelength spatial resolution allows a quantitative analysis of the local confinement potential of such structures which exhibit shallow barriers close to the quantum wire. This local potential influences the carrier transport dynamics within the embedding quantum well and ‐‐ in particular ‐‐ carrier trapping from twodimensional quantum well into one-dimensional wire states as is directly revealed in steady-state and time-resolved photoluminescence experiments at variable temperatures.

Difference in Diffusion Length of Ga Atoms under As2 and As4 Flux in Molecular Beam Epitaxy

The surface diffusion length of Ga adatoms under As2 or As4 flux has been measured using V-grooved GaAs (001) substrates in molecular beam epitaxy. The diffusion length on the (001) surface toward the [110] direction, of Ga adatoms under As2 flux is about half of that under As4 flux. A smaller number of Ga atoms under As2 flux migrate to the (001) ridge surface from the sidewall surface than those under As4 flux. Furthermore, the Al0.6Ga0.4As layer on the V-grooved GaAs substrate grown under As2 flux preserve the V-shape, whereas the V-shape cannot be preserved during the growth under As4 flux. The GaAs quantum wire structures which have Al0.6Ga0.4As barrier layeres are fabricated under As2 flux.

Effects of anodic oxide induced intermixing on the structural and optical properties of quantum wire structure grown on nonplanar GaAs substrate

Effects of anodic oxide induced intermixing on the structural and optical properties of stacked GaAs quantum wire (QWR) structures grown on a sawtooth-type nonplanar GaAs substrate are investigated. Cross-sectional transmission electron microscope (XTEM) observation, temperature dependent photoluminescence (PL) and cathodoluminescence (CL) imaging were used. Intermixing was achieved by pulsed anodic oxidation of the GaAs cap layer and subsequent rapid thermal annealing, was verified by XTEM analysis. A significant enhancement of QWR PL is observed accompanied by a notable blueshift of the sidewall quantum well (SQWL) PL due to the intermixing. Furthermore, an extended necking region is observed after the intermixing by spatially resolved CL. The temperature dependence of the PL intensities of both SQWL and QWR show maxima at approximately T∼110 K indicating the role of the extended necking region in feeding carriers to SQWL and QWR.

Energy relaxation of an excited electron gas in quantum wires: Many-body electron-LO-phonon coupling.

We theoretically study energy relaxation via LO-phonon emission in an excited one-dimensional electron gas confined in a GaAs quantum wire structure. We find that the inclusion of phonon renormalization effects in the theory extends the LO-phonon dominated loss regime down to substantially lower temperatures. We show that a simple plasmon-pole approximation works well for this problem, and discuss implications of our results for low temperature electron heating experiments in quantum wires.

Fabrication of [formula omitted] quantum wires on a non-(111) V-grooved GaAs substrate by chemical beam epitaxy

We have successfully grown InGaAs GaAs quantum wire (QWR) structures on a non-(111) V-grooved GaAs(100) substrate by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and unprecracked monoethylarsine. To improve the lateral confinement, we have used non-(111) V-grooves which have planes close to (322), where (111) planes are formed near the bottom region during growth, confining QWRs effectively at the bottom of V-grooves. An optimum growth temperature of InGaAs GaAs QWRs was found by examining the temperature-dependent growth rates corresponding to various crystal orientations. Three significant PL peaks were observed at 77 K and related to side QWLs, top QWLs and QWRs with peak energies of 1.439, 1.384 and 1.240 eV, respectively. The luminescence from the QWRs could be still observed even at 200 K. These results suggest that our method using non-(111) V-grooves and unprecracked MEAs is useful method for fabrication of InGaAs GaAs QWRs.

Selective growth and other applications of hydrogen-assisted molecular beam epitaxy

The usefulness of atomic hydrogen in molecular beam epitaxy has been demonstrated, centering around selective growth. Atomic hydrogen is effective for low-temperature cleaning of substrates, surfactant effects such as restrain of island growth and suppression of the surface migration of the adatoms and selective growth on masked or V-grooved substrates. These effects are dependent on substrate temperatures. The selective growth of GaAs has been successfully demonstrated at the conventional growth temperature and growth rate with the aid of atomic hydrogen. The main mechanism of the selective growth is the re-evaporation of Ga and As from mask materials such as SiN x or SiO 2 . Selective growth has also been observed on low-index crystal facets. On (111)A and (110) facets, no GaAs was deposited in the presence of atomic hydrogen, the flux of which is approximately the same as that of Ga. GaAs quantum wire structures have been fabricated on the substrates with V-shaped grooves. The efficient capture and confinement of carriers into wire regions have been observed by photolumenescence.

Selective growth and other applications of hydrogen-assisted molecular beam epitaxy

The usefulness of atomic hydrogen in molecular beam epitaxy has been demonstrated, centering around selective growth. Atomic hydrogen is effective for low-temperature cleaning of substrates, surfactant effects such as restrain of island growth and suppression of the surface migration of the adatoms and selective growth on masked or V-grooved substrates. These effects are dependent on substrate temperatures. The selective growth of GaAs has been successfully demonstrated at the conventional growth temperature and growth rate with the aid of atomic hydrogen. The main mechanism of the selective growth is the re-evaporation of Ga and As from mask materials such as SiN x or SiO 2. Selective growth has also been observed on low-index crystal facets. On (111)A and (110) facets, no GaAs was deposited in the presence of atomic hydrogen, the flux of which is approximately the same as that of Ga. GaAs quantum wire structures have been fabricated on the substrates with V-shaped grooves. The efficient capture and confinement of carriers into wire regions have been observed by photolumenescence.

MBE fabrication of GaAs quantum wire structures on mesa stripes along the [001] direction

We propose a novel mesa geometry for the fabrication of nanostructures by molecular beam epitaxy. We have found that (110) and ( 1 110 ) facets can be formed by GaAs growth on [001] oriented mesa structures. Smooth surface morphology and extremely low growth rate on the {110} facets are the advantages of using this mesa direction over the conventional [ 0 11 ]− and [ 0 11 ]-cleaved directions. The width of the mesa top (100) plane, which is limited by the {110} facets, can be narrowed to the nanometer scale by GaAs growth. Quantum wire like structures, as narrow as ∼30 nm with a thickness of ∼30 nm at its center, were successfully fabricated.

Room Temperature Performance of (311) GaAs Quantum-Wire Structures

Quantum wire structures are directly formed during the growth of GaAs/AlAs multilayer structures by molecular beam epitaxy due to the natural evolution of nanometer-scale corrugations on (311)-oriented GaAs and AlAs surfaces. The pronounced optical anisotropy clearly observable at 300 K reaches a value as high as 30% for the heavy-hole exciton resonance in 43 Å GaAs quantum-wire structures. The luminescence intensity of (311) GaAs quantum-wire structures with average GaAs layer thicknesses below 66 Å is generally higher than that of conventional (100) GaAs quantum-well structures. In 56 Å GaAs quantum-wire structures no reduction of the integrated luminescence intensity is observed up to 400 K. In the regime of unsaturated optical gain, the amplified luminescence intensity of (311) GaAs quantum-wire structures exceeds that of (100) GaAs quantum-well structures by fourfold. These findings highlight the potential of these novel low-dimensional structures for advanced semiconductor device concepts.

Improved GaAs/AlAs multilayer structures grown by MBE on patterned GaAs (100) substrates with ridges along the (001) direction

GaAs/AlAs multilayer structures were grown by MBE on patterned GaAs(100) substrates with submicrometre-width ridges in conventional (011) and new (001) directions, and their cross-sectional structures were investigated by TEM. On the (001) ridges, (110) side facets with very good morphology were formed on both sides of the (100) terrace, in contrast with the rather poor morphology of (311)A side facets for the conventional (011) ridge case. GaAs quantum wire (QWR) structures (with a triangular cross section, 26 nm wide by 18 nm high) surrounded by very flat AlAs(110) side facets and a (100) plane were formed on the top of the (100) terrace on the (001) ridges. This indicates that the (001) ridge pattern is highly desirable for fabricating high-quality QWR structures. The migration length of Al atoms on the (100) plane during MBE growth at 570 degrees C was determined for the first time to be 8 nm by TEM observation.