Vicinal GaAs Substrates Research Articles

Growth and characterization of high quality B xGa 1− xAs/GaAs(0 0 1) epilayers

In a first step towards the growth of BInGaAs, we have grown and characterized the B x Ga 1− x As/GaAs ternary compound with boron composition up to x=0.06 on GaAs(0 0 1) vicinal substrates. The incorporation behavior of boron has been studied as a function of growth temperature, diborane flux, gallium precursor and carrier gas (hydrogen and nitrogen). A maximum for boron incorporation ( x≈0.04–0.06) is found at 550–600°C depending on the precursor and the carrier gas. The epilayers have good crystalline quality as measured by X-ray rocking curve of the (0 0 4) diffraction peak (full-width at half-maximum of 38 arcsec for x=0.035). However, the surface morphology is very sensitive to the diborane supersaturation in the gas phase. At high diborane flow rate, the surface appears as though it is covered in dust. A low surface roughness of 0.4 nm was measured by atomic force microcopy (AFM) in the best growth conditions. AFM images also show a cross-hatch pattern for the highest boron composition.

Fabrication of single- or double-row aligned self-assembled quantum dots by utilizing SiO2-patterned vicinal (001) GaAs substrates

We investigated the formation of In0.8Ga0.2As self-assembled quantum dots (SAQDs) grown on SiO2-patterned 1°, 2°, and 5°-off (001) GaAs substrates by selective area metalorganic vapor phase epitaxy technique. The SiO2 patterns were filled with various stripe opening windows along the misorientation direction of the substrates. During the growth of the GaAs buffer layer on the opening regions, the steps on the (001) top facet was affected by the widths of the (001) top facet and the misorientation angles of the substrates. Single- or double-row aligned In0.8Ga0.2As SAQDs having definite interval were successfully fabricated on the (001) top facet with optimized top width and periodicity of step bunching. These results indicate that the selective growth technique of SAQDs by utilizing SiO2-patterned vicinal substrates is promising for nanoelectronic device applications such as single-electron memory devices.

Photoabsorption Characterization on Surface InAs Nanostructures Using Light-Illuminated Scanning Tunneling Microscopy

We characterized surface InAs nanostructures grown on a GaAs vicinal substrate by using scanning tunneling microscopy (STM) under light illumination. In local photoinduced current imaging with a DC bias voltage, the photoabsorption signals on InAs nanowires could not be detected when the illuminated photon energy was adjusted between the band gaps of GaAs and InAs, due to electrical isolation of the surface InAs from the conductive substrate. On the other hand, imaging an optical response of differential conductance with AC bias modulation revealed a clear contrast between InAs nanowires and the surrounding GaAs region without the generation of photocarriers in the underlying GaAs.

Optical properties of remotely doped AlAs/GaAs coupled quantum wire arrays. II. Fermi-edge singularities

We perform low-temperature optical experiments on remotely doped lateral superlattices epitaxially grown on vicinal GaAs substrates. This quantum wire system provides a degenerate electron gas of density ${n}_{s}\ensuremath{\simeq}8\ifmmode\times\else\texttimes\fi{}{10}^{11} {\mathrm{cm}}^{\ensuremath{-}2}$ subjected to a tunable one-dimensional periodic potential of amplitude $\ensuremath{\simeq}20 \mathrm{meV}$ and period $\ensuremath{\simeq}30 \mathrm{nm},$ i.e., at the scale of its Fermi energy ${(E}_{F}\ensuremath{\simeq}25 \mathrm{meV})$ and Fermi wavelength $({\ensuremath{\lambda}}_{F}\ensuremath{\simeq}30 \mathrm{nm}).$ The enhancement of Fermi-edge singularities in tilted lateral superlattices [T. M\'elin and F. Laruelle, Phys. Rev. Lett. 76, 4219 (1996)] is examined in the framework of the Fano resonance scheme that we recently proposed for two-dimensional systems in presence of alloy disorder [T. M\'elin and F. Laruelle, Phys. Rev. Lett. 85, 852 (2000)]. Including periodic intersubband couplings as extrinsic scatterings in the Fano model, we obtain a nice agreement with experimental data. This thus rules out possible many-body scenarios for the enhancement of Fermi-edge singularities in lateral superlattices, where multiple Coulomb diffusions of Fermi-sea electrons by charged photocreated valence holes would be boosted by the reduced dimensionality.

Optical properties of remotely doped AlAs/GaAs coupled quantum wire arrays. I. Periodic quantum confinement and localization of minority carriers

We perform low-temperature optical experiments on remotely doped lateral superlattices epitaxially grown on vicinal GaAs substrates. This quantum wire system provides a degenerate electron gas of density n s ≃ 8 X 10 1 1 cm - 2 subjected to a tunable one-dimensional periodic potential of amplitude ≃20 meV and period ≃30 nm, i.e., at the scale of its Fermi energy (E F ≃25 meV) and Fermi wavelength (λ F ≃30 nm). We show that the one-dimensional quantum confinement strongly affects both photoluminescence and photoluminescence excitation properties of the degenerate electron system. Through a careful analysis of the optical emission and absorption linear polarization spectra together with their temperature dependence, we distinguish between features due to (i) the periodic one-dimensional confinement and (ii) the localization anisotropy of photocreated valence-band carriers.

SPIN-RESOLVED PHOTOELECTRON SPECTROSCOPY OF ULTRATHIN Fe FILMS ON GaAs(001)

Thin epitaxial Fe films were grown on singular and vicinal GaAs(001) substrates, and their magnetic and electronic structures were investigated by synchrotron-based spin-resolved and spin-integrated photoelectron spectroscopy with different Fe thickness. There were two types of substrates: one was a Si-doped n-type GaAs(001) surface with doping concentration of 2 × 1018 cm -3 (singular substrate), and the other was orientated by 3° toward the (111)A direction (vicinal substrate). Spin polarization of the secondary electron peak at different growth stages of Fe coverage for the singular substrate sample and the vicinal one were measured. In the case of singular substrates, there was a dependence of their initial surface reconstruction, which is associated with complex domain structure, while no such the dependence was observed in the case of vicinal substrates. The result from the vicinal sample suggests the geometrical influence of the initial surface stoichiometry of the substrate.

Increased epitaxial thickness limit in low-temperature GaAs grown on a vicinal substrate

We show that the use of vicinal GaAs(0 0 1) substrates with high misorientation angle inhibits the nucleation of pyramidal defects and hence delays the breakdown of the perfect epitaxial growth of LT-GaAs layers grown under identical growth conditions. The experimental results indicate that surface roughness plays the key role in the breakdown of perfect epitaxy, since our investigations are performed on samples with the same amount of strain. Hence, from the obtained data and taking into account the previously reported results of kinetic growth simulations we conclude that the breakdown of perfect epitaxial growth in LT-GaAs is driven by kinetics.

HRTEM observation of CdSe/ZnSe SQWs grown on vicinal GaAs substrate

Microscopic structural properties of CdSe/ZnSe single quantum wells grown on vicinal GaAs substrate are investigated by cross-sectional high-resolution transmission electron microscope. The interface of ZnSe buffer and GaAs substrate, the surface of top ZnSe, and CdSe well are observed. At the ZnSe/GaAs interface, the lattice structure is strongly smeared, indicating that structures other than ZnSe and GaAs are formed. Stacking faults are generated at this region. A distinct step and terrace structure is observed at the surface of top ZnSe. The averaged terrace width coincides with the nominal value of 5° vicinal surface. Well layer is tilted with regard to the (2 0 0) lattice plane by 5°, and no misfit dislocation is found. Strain mapping of the obtained lattice image is performed.

Surface morphology of low temperature grown GaAs on singular and vicinal substrates

The evolution of the surface morphology of epitaxial GaAs layers grown at low substrate temperatures (LT-GaAs) on singular and vicinal (001) GaAs substrates is studied by means of kinetic Monte-Carlo simulations. The simulation model includes the effects of Ehrlich–Schwoebel barriers at step-edges as well as anisotropic surface diffusion. We find that the surface morphology is dominated by a pattern of elongated growth mounds, which are organized into columns parallel to [1̄10]. The formation of this pattern is gradually suppressed on vicinal substrates as the misorientation angle increases. Simulated surface morphologies are compared to atomic force microscopy measurements on LT-GaAs epilayers grown on singular GaAs(001) substrates at different temperatures and good quantitative agreement is found. We propose to use vicinal substrates for LT-GaAs growth in order to overcome the known problem of epitaxial breakdown above a certain epitaxial thickness.

Extending the epitaxial thickness limit in low-substrate-temperature- grown GaAs

A method for extending the epitaxial thickness limit in low-temperature-grown GaAs (LT-GaAs) is presented. It is shown that the use of vicinal GaAs(001) substrates with a high misorientation angle reduces the surface roughness of LT-GaAs and inhibits the nucleation of defects which cause the breakdown of perfect epitaxial growth. Kinetic Monte Carlo simulations are used to describe the influence of the vicinal substrate on the growth mode and to estimate the appropriate misorientation angle.

Investigation of optical and structural properties of In xGa 1− xAs/GaAs quantum wells grown on vicinal GaAs(0 0 1) substrates

Optical and structural properties of In 0.15Ga 0.85As/GaAs quantum wells (100, 200 and 300 Å wide), grown by molecular beam epitaxy on nominal and vicinal GaAs(0 0 1) substrates, have been investigated by low-temperature photoluminescence, high-resolution X-ray diffraction and transmission electron microscopy. All the experimental data pointed out towards the presence of step bunching on vicinal substrate. The blueshift of the optical emissions observed in the semiconductor structure grown on the vicinal substrate was explained in terms of a simple model that includes both indium segregation and strain modulation.

Influence of the temperature and excitation power on the optical properties of InGaAs/GaAs quantum wells grown on vicinal GaAs(001) surfaces

Photoluminescence experiments were performed as a function of temperature and excitation intensity in order to investigate the optical properties of In0.1Ga0.9As/GaAs quantum wells grown on vicinal GaAs(001) substrates with different miscut angles. The misorientation of the surface played an important role and influenced the intensity, efficiency, energy, and full width at half maximum of the optical emission, as well as the segregation of indium atoms. It is shown that at high temperature the optical properties of InGaAs quantum wells grown on vicinal substrates are slightly inferior to ones of the same structure grown a nominal surface because of the faster escape of the carriers.

The initial stage of InGaAs growth by MOVPE on multiatomic-stepped GaAs structures

We investigate the formation of self-organized InGaAs quantum wires (QWRs) on coherent multiatomic-stepped GaAs that had been grown on GaAs vicinal substrates by metal-organic vapor phase epitaxy. The key issues in the formation of InGaAs QWRs are the uniformity of the multiatomic steps and the large modulation in the lateral thickness of InGaAs on multiatomic-stepped GaAs structures. We use images obtained by atomic force microscopy and cross-sectional images of InGaAs layers obtained by transmission electron microscopy to investigate the surface morphology of InGaAs layers on multiatomic-stepped GaAs structures. As a result, we found that in the initial stages of InGaAs growth, Ga and In atoms preferentially attach themselves to the bottom edges of the multiatomic steps, and multiatomic-stepped structures with their own characteristic period and then reappear. These results suggest that the Schwoebel barrier, which causes the step bunching, may be decreased by the change of surface reconstruction after the introduction of small amount of In atoms to GaAs surfaces in the initial stage of growth. Furthermore, multiatomic-stepped structures formed during the growth of InGaAs layers have different periods from those on the underlying GaAs layer. The difference between the multiatomic-stepped structures on GaAs and InGaAs is thought to be caused by the difference of Schwoebel barriers between these two stepped surfaces.

Correlation of the structural and optical properties of GaN grown on vicinal (0 0 1) GaAs substrates with the plasma-assisted MBE growth conditions

Molecular beam epitaxy, using a radio frequency nitrogen plasma source, was investigated for the direct growth of GaN thin films on vicinal (0 0 1) GaAs substrates. It has been found possible to grow GaN thin films with various types of crystal structure. Cubic (0 0 1) GaN with epitaxial relationship to the GaAs substrate could be grown without GaAs nitridation and under stoichiometric N/Ga flux ratio conditions. Hexagonal mixing in the cubic GaN films occurred mainly as domains with the [0 0 0 1] oriented parallel to the GaAs surface normal. Their origin was related to irregularities at the GaAs surface, produced by interaction with the N-beam. Layers which exhibited intense 17 K photoluminescence with the main peak at approximately 3.42 eV, presented a polycrystalline hexagonal structure with a domain width of several 100 nm. These layers did not exhibit a “yellow-band” luminescence.

Scanning tunneling microscopy study of InAs islanding on GaAs(0 0 1)

Scanning tunneling microscopy (STM) connected to molecular beam epitaxy is used to investigate InAs islanding on GaAs(0 0 1) substrates. From the time evolution of STM images taken for the surfaces covered with wetting layers and islands, it is found that InAs islanding initially occurs at monatomic steps on wetting layers, and that even at room temperature, monatomic steps move around. As a result, InAs islands are usually observed both on terraces and on steps by STM long after InAs growth. Moreover, islands are aligned with monatomic steps on vicinal GaAs(0 0 1) substrates inclined toward [1 1 0]. Our findings provide new insight into manipulating spatial arrangement of quantum dots by utilizing islanding of InAs.

Second harmonic generation by reflection from vicinal surfaces ofepitaxial layers of cadmium mercury telluride

Reflection second harmonic generation is employed to examine thesurfaces of epitaxial layers of CdxHg1-xTe (CMT) grown on vicinal GaAs(100) substrates. Different vicinal tilts, with respect to the GaAssubstrates, are observed in the epitaxial CMT layers. We also report here onthe measurement of the second-order nonlinear coefficient (d36) of CMT.Because CMT is strongly absorbing at the 1.06 µm wavelength, thismeasurement was performed by comparing the second harmonic intensity reflectedfrom the CMT surface to that measured for a quartz sample in transmission.Directly comparable expressions for the reflected and transmitted secondharmonic intensities are derived from which a value ofd36 = 365±15 pm V-1 is obtained. This value is much larger thanthose reported for similar zinc-blende-type materials and is attributed to anelectronic resonance enhancement.

MBE growth of different hexagonal GaN crystal structures on vicinal (100) GaAs substrates

GaN thin films of different hexagonal crystal structures have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy (RFMBE) on vicinal (100) GaAs substrates. Polycrystalline hexagonal material occurred for high temperature (630°C) nitridation of the GaAs surface or low temperatures of the initial GaN buffer layer deposition. On the contrary, initial GaN growth at 540°C gave hexagonal single crystals with [0001] axis either inclined at approximately 43° from the growth axis or aligned parallel to it. The GaN orientation depended on the annealing or not, respectively, of the initial low temperature buffer layer.

Radiative Recombination Features of Metastable Quantum Dot Array

Metastable quantum dot (QD) arrays were grown by submonolayer migration enhanced epitaxy (SMEE) mode on GaAs(100) singular and vicinal substrates. It has been established that two QD groups are formed on the vicinal surface. These groups – conjunct QDs (CQD) and isolated QDs (IQD) – demonstrate a basically different behavior in their photoluminescence spectra. The CQDs form chain-like structures. They lay on the wetting layer which connects them to each other. The IQDs are formed due to wetting layer rupture directly on the GaAs surface at the terrace edges. They are isolated and not connected to the rest QD array.

Kelvin probe force microscopy on InAs thin films grown on GaAs giant step structures formed on (110) GaAs vicinal substrates

Surface potential measurements on InAs thin films grown on GaAs giant steps were performed by Kelvin probe force microscopy. We found that the removal of the water-related layer from both surfaces on a sample and a tip was very effective to improve the reliability of the surface potential measurements. The measured potential distribution corresponds to the surface corrugation of the InAs thin films. In addition, the InAs layer thickness dependence of the surface InAs Fermi levels is investigated, indicating that the surface Fermi level shifts toward the vacuum level as the increase of the InAs layer thickness.

Ultrathin epitaxial Fe films on vicinal GaAs(001): A study by spin-resolved photoelectron spectroscopy

Thin epitaxial Fe films have been grown on vicinal GaAs(001) substrates and their remanent magnetic properties and the degree of substrate atom diffusion investigated using synchrotron-based photoelectron spectroscopy. The vicinal Fe films, though exhibiting greater As diffusion than their singular homologues, displayed better film quality both from the structural and the magnetic points of view. The spin-resolved valence spectra of the vicinal films resemble those for crystalline bulk Fe at lower film thicknesses than for singular films.