Vicinal GaAs Substrates Research Articles

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520°C, and then exhibits a sudden decrease at 535°C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates.

Structural evolution in strained In0.18Ga0.82As stacking multilayers on vicinal GaAs surfaces

Strain relaxation mechanisms for the growth of m-layers of In0.18Ga0.82As/GaAs on a GaAs(100) substrate, tilted 2° towards the [0–11] direction, have been studied by molecular beam epitaxy (MBE) and atomic force microscopy (AFM). While dislocations alone provide a strain relaxation mechanism for nominal GaAs(100), additional strain relaxation mechanisms were observed for a vicinal GaAs(100) substrate. For m ⩽ 8, step bunching provided a mechanism for strain relaxation. For m ⩾10, in addition to the step bunching, bunched corners along two [051] and [0–1–5] directions provided the mechanism for strain relaxation. These surface patterns provide potential to act as a template for the growth of more uniform and organized nanostructures. For m = 16, the formation of dislocations provided an additional route for strain relaxation.

Dependence of bimodal size distribution on temperature and optical properties of InAs quantum dots grown on vicinal GaAs (100) substrates by using MOCVD

Self-assembled InAs quantum dots (QDs) are grown on vicinal GaAs (100) substrates by using metal-organic chemical vapour deposition (MOCVD). An abnormal temperature dependence of bimodal size distribution of InAs quantum dots is found. As the temperature increases, the density of the small dots grows larger while the density of the large dots turns smaller, which is contrary to the evolution of QDs on exact GaAs (100) substrates. This trend is explained by taking into account the presence of multiatomic steps on the substrates. The optical properties of InAs QDs on vicinal GaAs(100) substrates are also studied by photoluminescence (PL) . It is found that dots on a vicinal substrate have a longer emission wavelength, a narrower PL line width and a much larger PL intensity.

Investigation of magnetocrystalline anisotropy by planar Hall effect in GaMnAs epilayers grown on vicinal GaAs substrates

An asymmetric shift of the Hall resistance in the presence of an in-plane magnetic field is observed in ferromagnetic GaMnAs epilayers grown on vicinal GaAs substrates, due to the superposition of both the planar Hall effect and the anomalous Hall effect (AHE). The asymmetric shift (i.e., the difference between the values of the Hall voltage observed at magnetic field of opposite sign) is a direct manifestation of the strength of the magnetocrystalline anisotropy, which confines the magnetization to the preferred crystalline plane, thus resulting in a nonzero component of the magnetization normal to the GaMnAs layer. This is confirmed by the observation that the asymmetry in the resistance is proportional to the saturated AHE resistance measured in perpendicular high field configuration.

Growth of InAs quantum dots on vicinal GaAs (1 0 0) substrates by metalorganic chemical vapor deposition and their optical properties

The growth of InAs quantum dots on vicinal GaAs (1 0 0) substrates was systematically studied using low-pressure metalorganic chemical vapor deposition (MOCVD). The dots showed a clear bimodal size distribution on vicinal substrates. The way of evolution of this bimodal size distribution was studied as a function of growth temperature, InAs layer thickness and InAs deposition rate. The optical properties of dots grown on vicinal substrates were also studied by photoluminescence (PL). It was found that, compared with dots on exact substrates, dots on vicinal substrates had better optical properties such as a narrower PL line width, a longer emission wavelength, and a larger PL intensity.

Highly strained quantum structures grown on GaAs (0 0 1) vicinal substrate by MOCVD

In this paper, we report experimental results regarding highly strained InP and InAs/InP quantum nanostructures grown on vicinal GaAs (0 0 1) substrate with 7° miscut towards (1 1 0) orientation by MOCVD using TBA and TBP as group V sources. Formation of well-aligned wire nanostructures has been observed, which may be attributed to the effects of multi-atomic steps formed during GaAs buffer layer growth. We also observed strong photoluminescence from these nanostructures even for the type II band gap alignment of the InP/GaAs quantum wires. Our results show a possible way for fabricating low-dimensional quantum nanostructures with band gap engineering.

Real-time RHEED investigation of indium segregation in InGaAs layers grown on vicinal GaAs(001) substrates

The surface segregation of indium atoms was investigated in situ and in real time by reflection high-energy electron diffraction (RHEED) during molecular-beam epitaxy of InGaAs layers on misoriented GaAs(001) substrates. The strong damping of the RHEED oscillations was quantitatively related to the strength of the segregation process that was found to decrease with increasing miscut angle.

Self-Assembled InAs Quantum-Dot Chains on Self-Formed GaAs Mesa-Stripes by Molecular Beam Epitaxy

The self-formation of InAs quantum-dot chains (QDCs) was demonstrated on GaAs/InGaAs/GaAs(001) layers by molecular beam epitaxy (MBE). In the MBE growth of a GaAs layer on a corrugated InGaAs/GaAs buffer layer, mesa-stripe structures with the {211}A side wall were spontaneously formed along the [1 10] direction. InAs QDCs were formed on the GaAs mesa stripes along the [1 10] direction. In particular, uniform InAs QDCs were fabricated on the vicinal GaAs(001) substrate, misoriented toward the [110] direction. The self-formation of QDCs was attributed to the misfit dislocation and step structure.

Effect of Annealing on Optical Properties of InAs Quantum DotsGrown by MOCVD on GaAs (100) Vicinal Substrates

Thermal annealing effect on InAs quantum dots grown on vicinal (100)GaAs substrates is studied in comparison with dots on exact (100) GaAssubstrates. We find that annealing acts stronger effect on dots withvicinal substrates by greatly accelerating the degradation of materialquality, as well as slightly increasing the blueshift of the emissionwavelength and the narrowing of PL linewidth. It is attributed to thehigher strain in the dots formed on the vicinal substrates.

Electronic, optical, and structural properties of quantum wire superlattices on vicinal (111) GaAs substrates

Electronic, optical, and structural properties of quantum wire superlattices on vicinal (111) GaAs substrates

Comparative study of InAs quantum dots grown on different GaAs substrates by MOCVD

We present a comparative study of InAs quantum dots grown on Si-doped GaAs (1 0 0) substrates, Si-doped GaAs (1 0 0) vicinal substrates, and semi-insulating GaAs (1 0 0) substrates. The density and size distribution of quantum dots varied greatly with the different substrates used. While dots on exact substrates showed only one dominant size, a clear bimodal size distribution of the InAs quantum dots was observed on GaAs vicinal substrates, which is attributed to the reduced surface diffusion due to the presence of multiatomic steps. The emission wavelength is blueshifted during the growth of GaAs cap layer with a significant narrowing of FWHM. We found that the blueshift is smaller for QDs grown on GaAs (1 0 0) vicinal substrates than that for dots on exact GaAs (1 0 0) substrates. This is attributed to the energy barrier formed at the multiatomic step kinks which prohibits the migration of In adatoms during the early stage of cap layer growth.

Complex quantum dot arrays formed by combination of self-organized anisotropic strain engineering and step engineering on shallow patterned substrates

One-dimensional (In,Ga)As quantum dot (QD) arrays are created on planar singular, vicinal, and shallow mesa-patterned GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As∕GaAs quantum wire (QWR) superlattice template in molecular beam epitaxy. On planar singular substrates, highly uniform single QD arrays along [0−11] are formed. On shallow [0−11] and [011] stripe-patterned substrates, the generated type-A and -B steps distinctly affect the surface migration processes which are crucial for QWR template development, i.e., strain-gradient-driven In adatom migration along [011] and surface-reconstruction-induced Ga∕In adatom migration along [0−11]. In the presence of both type-A and -B steps on vicinal substrates misoriented towards [101], the direction of adatom migration is altered to rotate the QD arrays. This establishes the relationship between self-organized anisotropic strain and step engineering, which is exploited on shallow zigzag-patterned substrates for the realization of complex QD arrays and networks with well-positioned bends and branches, exhibiting high structural and optical quality.

Properties of GaAs/AlGaAs quantum wells grown by MOVPE using vicinal GaAs substrates

Low-temperature photoluminescence (PL) was used to optimize the structural and optical quality of 2–15 nm thick GaAs/AlGaAs quantum wells (QWs). GaAs wafers, misoriented off (1 0 0) towards 〈1 1 1〉A and 〈1 1 1〉B were used, covering the 0–0.6° misorientation range in steps of 0.1°. A dramatic reduction in the 10 K PL linewidth is observed for all 0.2–0.3° misorientations, as compared to nominally oriented wafers: on 2, 5 and 15 nm thick QWs, linewidths fall from 23, 8 and 3 meV to 6, 3 and 2.5 meV, respectively, among the narrowest reported to date for similar structures. Above 0.3° misorientation, the emission broadens to become, at 0.6°, comparable or worse than for the exact (1 0 0) samples. The 4 K electron mobility of two-dimensional electron gases grown on similar, misoriented substrates increases steadily and then decreases again at larger misorientations.

Carrier diffusion in InGaAs/GaAs quantum wells grown on vicinal [formula omitted] substrates

We have investigated the influence of vicinal GaAs substrates on the optical and electronic properties of InGaAs/GaAs quantum wells (QWs). A single In 0.10Ga 0.90As QW was grown by molecular-beam epitaxy on a vicinal GaAs(0 0 1) substrate with a miscut angle of 0° (nominal), 2°, 4° and 6° towards [1 1 0] . The carrier diffusion was obtained by a micro-photoluminescence scan technique that permits to observe the effective diffusion length characterized by the lateral spread of carriers in the QW followed by radiative recombination. The carrier diffusion length was obtained parallel ( L ||) and perpendicular ( L ⊥) to the atomic steps. The diffusion length decreases as the temperature increases up to 100 K . Above this temperature we found different behaviours that depend on the sample miscut angle.

Lateral short range ordering of step bunches in InGaAs/GaAs superlattices

In the present paper we report on structural investigations of fivefold In0.2Ga0.8As/GaAs superlattices which have been grown by means of metal organic chemical vapor deposition on vicinal GaAs(001) substrates. Cross-sectional transmission electron micrographs exhibit an initially flat and nonfaceted grooved surface, while step bunching occurs during subsequent growth stages with an inclined vertical inheritance approximately 45° off the (001) direction. A reconstructed sample cross section on the base of high resolution x-ray diffraction data qualitatively confirms the local morphology proved by transmission electron microscopy. Moreover, a line shape analysis of diffusely scattered intensity using Gauss profiles indicates a lateral short range ordering of step bunches.

Formation of GaAs wire structures and position-controlledIn0.8Ga0.2As quantumdots on SiO2-patterned vicinal (001) GaAs substrates

We investigated In0.8Ga0.2As self-assembled quantum dots (SAQDs) and their underlying GaAswire structures grown by metalorganic vapour phase epitaxy onSiO2-patterned exact and vicinal (001) GaAs substrates. The directions of themisorientation of the vicinal (001) GaAs substrate and the patterned wireSiO2 mask were . During the growth of the GaAs layer onSiO2-patterned vicinal(001) GaAs substrates, {110}, {111}A,and {113}A facets were evolved by the opening layer width of the masks and themisorientation angles of the substrates. The ratio of the growth rate on the{111}A sidewalls to that on theneighbouring facets, (G{111}A), was increased by using the vicinal substrate having the highermisorientation angle, hence the surface migration of Ga adatoms from the{111}A sidewalls to the top region decreased. In0.8Ga0.2As SAQDs having regular periodicity on a narrow (001) top terrace were formed on aSiO2-patterned 5°-off (001) GaAs substrate.

Light-illuminated STM studies on photo-absorption in InAs nanowires

We fabricated surface InAs nanowires along GaAs giant step structures on GaAs (1 1 0) vicinal substrates, and their photo-absorption properties were studied by scanning tunneling microscopy (STM) under light illumination. In photo-induced current detection, it was difficult to identify the photo-absorption signals in the InAs nanowires because of the electrical isolation of the surface nanostructures from the conductive substrate. On the other hand, optical response measurements of differential conductance under small AC bias modulation in STM enabled us to obtain the photo-absorption signals from the surface nanostructures with avoiding such an electrical isolation problem. We successfully observed optical response images of differential conductance with a clear contrast between InAs nanowires and surrounding GaAs region by the appropriate tuning of the illuminating photon energy to excite the InAs nanowires selectively.

Growth of InAs on GaAs(1 0 0) by low-pressure metalorganic chemical vapor deposition employing in situ generated arsine radicals

InAs was grown by low-pressure metalorganic chemical vapor deposition on vicinal GaAs(1 0 0) substrates misoriented by 2° toward [0 0 1]. We observed InAs crystal growth, at substrate temperatures down to 300°C, employing in situ plasma-generated arsine radicals as the arsenic source. The in situ generated arsine was produced by placing solid arsenic downstream of a microwave driven hydrogen plasma. Trimethylindium (TMIn) feedstock carried by hydrogen gas was used as the indium source. The Arrhenius plot of InAs growth rate vs. reciprocal substrate temperature displayed an activation energy of 46.1 kcal/mol in the temperature range of 300–350°C. This measured activation energy value is very close to the energy necessary to remove the first methyl radical from the TMIn molecule, which has never been reported in prior InAs growth to the best of authors’ knowledge. The film growth mechanism is discussed. The crystallinity, infrared spectrum, electrical properties and impurity levels of grown InAs are also presented.

High-Temperature PbTe Thin Films for Use in Cascade Thermoelectric Power Generation

ABSTRACTPbTe-based thin films were deposited by thermal evaporation at temperatures ranging from ambient temperature to 430°C on different vicinal GaAs (100) substrates and BaF2 (111). This materials system is being evaluated as a potential candidate thermoelectric material for a mid-temperature stage in a cascade power generation module. Pure PbTe, PbSe, and multilayer PbTe/PbSe films were investigated. All films deposited on different vicinal GaAs (100) substrates were found to be polycrystalline when deposited at 250°C or lower. A subtle effect of substrate orientation and multilayer periodicity appears to contribute to the more randomly oriented polycrystallinity, which also lowers the thermal conductivity. These results are compared with PbTe epitaxial results on BaF2 (111).

Growth of spatially ordered InAs quantum dots on step-bunched vicinal GaAs (1 0 0) substrates

Spatially ordered growth of InAs quantum dots (QD) was demonstrated via metalorganic vapour phase epitaxy on step-bunched vicinal GaAs substrates. Regular terraces of step-bunched surfaces were achieved by growing on GaAs (1 0 0) substrates off-oriented 2°, 4° and 6° towards the 〈1 1 0〉 and 〈1 1 1〉 directions, thus serving as in situ templates for the growth of the QD layer. Multilayer stacks of strain-aligned QDs were successfully grown to improve the size homogeneity and therefore optoelectronic properties. Furthermore, the possibility of fabricating laterally ordered quantum wires (QWR) in the same manner was shown.