Multiatomic Steps Research Articles

Nucleation Transitions for InGaAs Islands on Vicinal (100) GaAs

Differences in InGaAs island nucleation on vicinal (100) GaAs surfaces as a function of miscut angle are presented. Arrhenius plots of saturation island densities show changes in activation energy and critical cluster size, which are interpreted as resulting from nucleation through different mechanisms: homogeneously (on terraces) and heterogeneously on monatomic and multiatomic steps. Furthermore, a link between step separation and island uniformity is observed. Step availability is found to be a major determinant of island uniformity at temperatures where clusters are not mobile.

Formation of multi-atomic steps and novel heterojunctions on vicinal (1 1 1)B substrates by MBE and anisotropic transport of 2D electrons

We have investigated the MBE growth of GaAs on vicinal (111)B substrates and examined in detail the multi-atomic step structures appearing at the growth front as well as at the n-AlGaAs/GaAs heterojunction. It has been found that the uniformity and periodicity of such steps depend strongly on the growth condition, especially As 4 flux, and that their morphology drastically affects transport of electrons flowing along heterojunctions. The use of high As 4 flux is effective in improving the step uniformity, and in drastically reducing the channel resistance perpendicular to the steps. Key factors for the fabrication of high-quality devices such as quantum wires on (111)B planes have been discussed.

A Novel Electron Wave Interference Device Using Multiatomic Steps on Vicinal GaAs Surfaces Grown by Metalorganic Vapor Phase Epitaxy: Investigation of Transport Properties

We have studied the transport properties of novel lateral surface superlattice electron wave interference devices in which multiatomic steps are utilized to introduce periodic potentials into two dimensional electron systems. The device structure was fabricated by metalorganic vapor phase epitaxy (MOVPE) on vicinal GaAs surfaces, and spontaneously formed multiatomic steps were periodically embedded at the heterojunction of n-AlGaAs/GaAs. Oscillations of the transconductance as a function of gate voltage were found in three different devices. Our simple analysis suggests that the oscillations found in two devices result from the periodic potentials induced by the multiatomic steps and coherent electron wave interference, whereas those in the other one arise from random interference. Our results imply the effect of height and randomness in the periodic potential on the characteristics of electron wave interference devices.

Selective area growth at multi-atomic-height steps arranged on GaAs (111)A vicinal surfaces by atomic layer epitaxy

A quantum wire array has been fabricated on GaAs (111)A vicinal substrates by continuous growth of metalorganic vapor phase epitaxy (MOVPE) and atomic layer epitaxy (ALE). The formation of multi-atomic height steps with high uniformity and continuity has been achieved by MOVPE on GaAs (111)A vicinal substrates misoriented toward [2̄11] direction. In the ALE process, the growth rate of the GaAs layer on the (111)A terrace has been controlled with the source gas feeding sequence and the growth temperature. Growth suppression on the (111)A terrace and selective adsorption of source molecules at the step region has been confirmed. A GaAs/GaAs 0.9P 0.1 quantum wire structure has been fabricated on the (111)A vicinal surface using an ALE selective growth technique.

Multiatomic step formation with excellent uniformity on vicinal (1 1 1)A GaAs surfaces by metalorganic vapor-phase epitaxy

Giant steps with high uniformity and continuity have been successfully formed by metalorganic vapor-phase epitaxy on GaAs (1 1 1)A vicinal surfaces for the application of quantum wire fabrication. The step height is more than 30 monoatomic layer at a growth temperature of 660°C. The surface morphologies obtained from scanning electron microscopy and atomic force microscopy measurements show the straight steps on (1 1 1)A substrates misoriented toward [2̄ 1 1] direction with high uniformity. Terrace uniformity and step continuity are improved as the off-angle increases. A straight section of a step exceeding 10 μm was obtained on 4° off-substrate. Fluctuation has also been suppressed within 1 monoatomic layer height on this vicinal substrate.

Formation and characterization of InGaAs strained quantum wires on GaAs multiatomic steps grown by metalorganic vapor phase epitaxy

Abstract We investigate the formation of self-organized InGaAs quantum wires (QWRs) on coherent GaAs multiatomic steps grown on vicinal GaAs substrates. In our previous study, GaAs QWRs were formed at the edge of AlAs multiatomic steps grown by metalorganic vapor phase epitaxy (MOVPE). We also found that coherent multiatomic steps formed on GaAs tend to be disturbed during AlAs growth. In the present investigation, we clarify that a thin InGaAs layer with less than 0.15 In content grows in step flow growth mode from the edge of GaAs multiatomic steps, while that with more than 0.2 In content grows in three-dimensional nucleation and growth mode at the edge of multiatomic steps due to the strain effect. Next, we fabricate InGaAs GaAs strained QWRs at the edge of coherent GaAs multiatomic steps. Locally thick InGaAs QWR structures are observed by a transmission electron microscope. Photoluminescence (PL) measurements reveal that the PL spectra of InGaAs QWR samples show lower energy shift (red-shift) by about 30 meV compared to those of quantum well ones due to the formation of InGaAs QWRs at the edge of coherent GaAs multiatomic steps.

Large conductance anisotropy in a novel two-dimensional electron system grown on vicinal (111)B GaAs with multiatomic steps

A novel selectively doped AlGaAs/GaAs heterojunction was successfully formed on a vicinal (111)B substrate, in which both quasiperiodic and aperiodic multiatomic steps with the average spacings of ∼20 nm are introduced. It is found that the electrical conductance G⊥ of two-dimensional electrons across the steps is far lower than that of G∥ along the steps and the ratio G∥/G⊥ exceeds 100 at 18 K. While G∥ is almost independent of temperature T below ∼70 K, G⊥ increases exponentially with 1/T with the activation energy of ∼5 meV, indicating the presence of potential barriers for the electron motion across the aperiodic steps.

A near-field scanning optical microscopy study of the photoluminescence from GaN films

We have achieved spatially resolved photoluminescence from GaN films using a near-field scanning optical microscope (NSOM). GaN films grown by hydride vapor phase epitaxy (HVPE) and metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates have been studied. We have performed spatial scans of topography, band edge, and yellow luminescence signals. Atomic force microscopy measurements were also made and compared with the NSOM topography. We have found spatial variations in photoluminescence characteristics at the submicron scale for both HVPE and MOVPE GaN. The observed enhancement of yellow luminescence at multiatomic step edges on the HVPE GaN surface suggests that the yellow luminescence is associated with chemical impurities incorporated during the growth of GaN films.

Theoretical and experimental investigation of an electron interference device using multiatomic steps on vicinal GaAs surfaces

We propose a new, lateral surface superlattice (LSSL) type of electron interference devices, where the period of LSSL is typically 60 nm, by utilizing multiatomic steps on a vicinal GaAs(0 0 1) surface. Conductivity of the device is theoretically studied by taking the effect of randomness in the LSSL into account. We also investigate its drain and transconductance characteristics experimentally at low temperatures, and found clear oscillations in g m − V G characteristics, which were ascribed to the electron interference effect.

Stepped growth and etching of (001) diamond

We have observed, for the first time, step-flow etching of (001) diamond substrates using hydrogen plasma treatment and have succeeded in step-flow epitaxial growth of diamond films on these hydrogen plasma-treated substrates. By subjecting diamond (001) substrates to hydrogen plasma, the corrugated structure observed before the treatment almost disappears and atomic steps (several atoms high) running over the surface parallel to the [110] direction appear. This shows that lateral propagation of multiatomic steps takes place during etching. In the step-flow epitaxial growth, multi-atomic steps with a terrace width of approximately 8 nm run over the surface parallel to the [110] direction, suggesting that the migration length of precursors under plasma conditions is longer than previously reported for hot filament growth.

Surface Smoothness and Step Bunching on GaAs (111)B Facets Formed by Molecular Beam Epitaxy

GaAs trapezoidal structures grown by molecular beam epitaxy on mesa-patterned (001) substrates have been studied by atomic force microscopy to clarify the morphology of (111)B facet planes. It is found that multi-atomic steps of 1–4 nm in height Δ appear on the facets with typical periods Λ of 30–60 nm. When the mis-orientation angle Δθ of mesa-stripes with respect to [110] increases from 2° to 7°, the corrugation height Δ is found to increase monotonically. Thus the mis-orientation angle Δθ must be minimized to form smooth facet structures.

Multiatomic step formation on GaAs(001) vicinal surfaces during thermal treatment

We observed the surface morphology of vicinal GaAs(001) after thermal treatment in AsH 3 H 2 atmosphere by atomic force microscopy (AFM). Clear multiatomic steps were formed under the high temperature thermal treatment. Next, we investigated the mechanism of step bunching during thermal treatment by two experiments from the view point of Ga atom evaporation. One is the selective thermal treatment using a partially masked GaAs wafer, and the evaporation amount of Ga atoms was estimated by AFM. The other is the investigation of photoluminescence (PL) peak energy shifts for AlGaAs GaAs single quantum wells with a thermal treatment process at the top of the GaAs quantum well layer, compared to those without thermal treatment. These results indicate that the evaporation hardly occurs during the thermal treatment process. Therefore, step bunching phenomena on GaAs(001) vicinal surfaces during thermal treatment are probably caused by migration of the atoms detached from upside steps and their re-incorporation to downside steps.

Quantum Well Wire Fabrication Method Using Self-Organized Multiatomic Steps on Vicinal (001) GaAs Surfaces by Metalorganic Vapor Phase Epitaxy

Coherent multiatomic steps with extremely straight edges are naturally formed on vicinal (001) GaAs surfaces during metalorganic vapor phase epitaxial growth. GaAs quantum well wires (QWWs) are formed on these self-organized multiatomic steps. In our previous study, a thin AlGaAs layer was grown on GaAs with multiatomic steps as a lower barrier of QWWs. However, the height and spacing of the steps slightly fluctuate on AlGaAs layer surfaces. Therefore, in this experiment, AlAs layer instead of AlGaAs layer was used as a lower barrier layer to improve the uniformity of the height and spacing of the steps. Atomic force microscopy observations and photoluminescence (PL) measurements at 20 K revealed that the underlying coherent GaAs multiatomic steps were well traced by the AlAs barrier layer rather than the AlGaAs barrier layer. Furthermore, we measured the polarization anisotropy of the PL spectra from the QWWs with AlAs. These results suggest that uniform QWWs are successfully formed using multiatomic steps on vicinal (001) GaAs surfaces.

Formation of InGaAs Quantum Dots on GaAs Multi-Atomic Steps by Metalorganic Chemical Vapor Deposition Growth

Aligned InGaAs quantum dots as small as 20 nm were naturally formed at multi-atomic step edges by metalorganic chemical vapor deposition growth. In addition, it was found that anisotropic structure of InGaAs along the step edges toward the [110]A direction appears with the increase of growth time of InGaAs. This phenomenon may be useful in the formation of quantum wires.

In situ fabrication of self-aligned InGaAs quantum dots on GaAs multiatomic steps by metalorganic chemical vapor deposition

Self-alignment of InGaAs quantum dots was achieved by growing the quantum dots on the multiatomic steps in metalorganic chemical vapor deposition. In this technique, first GaAs epilayer with multiatomic step structures along straight lines was grown on a vicinal GaAs substrate under appropriate growth conditions. Then, the InGaAs quantum dots were grown selectively on the multiatomic step edges using strain effects. This growth technique results in spontaneously aligned InGaAs quantum dots without any preprocessing technique prior to the growth.

Structural analysis of AlGaAs quantum wires on vicinal (110)GaAs by transmission electron microscopy and energy dispersive X-ray spectroscopy

Giant step structures consisting of coherently aligned multi-atomic steps were naturally formed during the molecular beam epitaxy growth of Al 0.5Ga 0.5As/GaAs superlattices (SLs) on vicinal (110)GaAs surfaces misoriented 6° toward (111)A. The growth of AlAs/Al x Ga 1− x As/AlAs quantum wells (QWs) on the giant step structures realized Al x 0 Ga 1− x 0 As ( x 0 < x) quantum wires (QWRs). We studied the giant step structures and the QWRs by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). TEM observations revealed that the QWRs were formed at the step edges. The cross sections of the QWRs were as small as 10 nm × 20 nm and the lateral distances between them were about 0.15 μm. We clarified the roles of the SLs to form the coherent giant step structures. From EDX analysis, it was estimated that the AlAs composition in the Al 0.5Ga 0.5As layers varied from 0.5 (terrace) to 0.41 (step edge). In the AlAs/Al x Ga 1− x As/AlAs QWs, the AlAs compositional modulation and the confinement by the AlAs barriers led to the embedded Al x 0 Ga 1− x 0 As regions. These results supported the existence of the Al x 0 Ga 1− x 0 As QWRs on the giant step structures.

Multiatomic step formation mechanism of metalorganic vapor phase epitaxial grown GaAs vicinal surfaces and its application to quantum well wires

The multiatomic steps formed on GaAs vicinal surfaces by metalorganic vapor phase epitaxy (MOVPE) are studied by atomic force microscopy (AFM). An AFM image of an epitaxially grown GaAs surface showed coherent multiatomic steps with extremely straight edges over a wide area. The average height and spacing of the multiatomic steps are 1.2–8 and 30–110 nm, respectively. These terrace widths change with the growth conditions. Narrower terrace widths are obtained at higher growth rates, and under higher AsH3 partial pressures and higher impurity doping conditions. The results suggest that the migration distance of a Ga atom on the terrace and the sticking coefficient at the step sites depend on these growth conditions. Using multiatomic steps, GaAsAlGaAs quantum well wires (QWWs) were grown on a GaAs vicinal surface. Cross-sectional transmission electron microscopy and photoluminescence show the successful fabrication of QWWs.

Formation and photoluminescence characterization of quantum well wires using multiatomic steps grown by metalorganic vapor phase epitaxy

GaAs/AlGaAs quantum well wires (QWWs) were successfully fabricated using multiatomic steps on GaAs vicinal substrates by metalorganic vapor phase epitaxy (MOVPE). Coherent multiatomic steps with extremely straight step edges were observed on GaAs and AlGaAs epitaxially grown layers on vicinal substrates over a wide observation area by atomic force microscopy (AFM). Formation of QWW structures is due to the fact that the GaAs growth rate on AlGaAs with multiatomic steps is much larger at the corners of steps than on the terraces. GaAs QWWs at the corners of steps accompanied by quantum wells (QWs) on the terraces were observed in cross-sectional transmission electron microscope (TEM) images. Photoluminescence (PL) of QWWs was measured at 20 K. The PL peak energy of the QWW structures grown on 5.0°-misoriented substrates was 23 me V smaller than that of a reference QW structure on an exactly oriented substrate. Since the total amount of the grown material is basically the same for both structures, this peak energy shift indicates the formation of quantum-wire-like structures at the corners of multiatomic steps. Furthermore, this observed red shift is in good agreement with a simple theoretical estimate of the QWW structures observed by TEM. These results suggest that the present novel fabrication method of QWWs is very promising for the formation of uniform nano-meter size quantum wires without any processing damage.

Growth Behavior and Mechanism of Alkyl-Desorption-Limited Epitaxial Growth of GaAs on Exactly Oriented and Vicinal Substrates

New atomically controlled epitaxial growth, called alkyl-desorption-limited epitaxial (ADLE) growth, is studied on (001) exactly oriented and vicinal GaAs substrates. In ADLE growth, the growth rate is limited by the desorption rate of alkyl from organometals rather than by saturation of alkyl adsorption. First, the proposed ADLE growth mechanism is quantitatively confirmed by comparing a new theory of growth based on the alkyl-desorption rate equations with the experimental growth data taken on (001) exactly oriented substrates. Next, the behavior of multi-atomic steps on vicinal substrates is studied using an atomic force microscope (AFM). It is found that the multi-atomic-step heights are reduced during ADLE growth. This new phenomenon is explained by the ADLE growth mechanism.

Mechanism of Multiatomic Step Formation during Metalorganic Chemical Vapor deposition Growth of GaAs on (001) Vicinal Surface Studied by Atomic Force Microscopy

The detailed behavior and mechanism of multiatomic step formation processes during metalorganic chemical vapor deposition (MOCVD) of GaAs on vicinal surfaces are systematically investigated using an atomic force microscope (AFM). Under the low growth rate condition, the step flow growth mode with regular stripe is obtained, and the final terrace width is almost independent of the substrate misorientation angle. The result suggests that the barrier height difference for surface migrating Ga atoms between the terrace site and the step site is small, and the final terrace width is mainly determined by a migration length. Three-dimensional nucleation and growth mode with irregular steps are also obtained under the high growth rate condition. However, as the AsH3 partial pressure increases, irregular steps are no longer observed. For these results, we discuss the multiatomic step formation mechanism.