Multiatomic Steps Research Articles

Novel functionalities by guiding electrons in highly uniform quantum wire ensembles on multiatomic step arrays of GaAs(331) substrates

Coherently aligned multi-atomic step arrays on GaAs(331) substrates generate periodic ensembles of conductive quantum wires which are exploited for novel device functionalities. Sub-micrometer wide constrictions parallel to the wires with independent side-gate control allow switching between spatially separated electron channels. The strongly modulated periodic potential in constrictions perpendicular to the array opens new opportunities for realisation of coupled quantum dot structures.

Large positive magnetoresistance in periodically modulated two-dimensional electron gas formed on self-organized GaAs multiatomic steps

We study magneto-transport properties of two-dimensional electron gas (2DEG) subjected to a periodic potential with a period of 60–70 nm, where the potential modulation is realized by periodically corrugated InGaAs channel layer formed on self-organized GaAs multiatomic steps. We find a large positive magnetoresistance (MR) when the current is perpendicular to the potential modulation, while such positive resistance is not observed for the current parallel to the potential modulation. It is also found that the amplitude of MR oscillation for current perpendicular to the potential modulation is extremely large over 3.5 T, and is modulated in 2–3.5 T. In addition, we also find the anisotropy in electron mobility. The results can be explained by the large potential modulation in the lateral surface superlattice (LSSL) and associating modulation of Landau subbands in LSSL.

Selective incorporation of Si along step edges during delta-doping on MOVPE-grown GaAs (001) vicinal surfaces

We investigated the delta-doping (δ-doping) of Si using SiH4 on MOVPE-grown GaAs (001) vicinal surfaces to explore the possibility of selective incorporation of Si along atomic steps, and to demonstrate doping quantum wires by the combination of multiatomic steps and wire-like doping. It was found that the doping density on vicinal surfaces was enhanced as the misorientation angle was increased, which suggested the enhanced decomposition of SiH4 and the selective incorporation of Si at step edges. It was also found that this selective incorporation could be enhanced by annealing the surface prior to the δ-doping, which resulted from the reduced incorporation of Si at the terrace regions. Anisotropic electron transport properties which are expected from the wire-like incorporation along step edges are also discussed.

Reactive molecular-beam epitaxy of GaN layers directly on 6H–SiC(0001)

We investigate the quality of GaN layers directly grown on 6H–SiC(0001) substrates by reactive molecular-beam epitaxy. Despite a pure three-dimensional nucleation, step-flow growth is achieved by in situ adjusting conditions such that the (2×2) reconstruction observed during growth is maximized in intensity. The resulting surface morphology exhibits large terraces separated by mono- and multiatomic steps, and is clearly superior to that obtained by plasma-assisted growth. Furthermore, the structural and optical properties of these layers are comparable to those of layers grown by plasma-assisted molecular-beam epitaxy.

Selective Control of Electrons in Quantum Wires Formed by Highly Uniform Multiatomic Step Arrays on GaAs(331) Substrates

Coherently aligned, multiatomic step arrays on GaAs(331) substrates generate a periodic array of conductive quantum wires in a two-dimensional (2D) electron gas. A small number of wires is selected by superimposing a constriction with independent side-gate control. By tuning the gate-voltage window, wires can be selected one by one. The resulting oscillatory current transmission provides a new functionality by switching between spatially separated electron channels. The wires are coupled by a small number of 2D electrons resulting in orders of magnitude reduced conductance perpendicular to the step edges.

Novel functionality by selective guidance of electrons in quantum wires on highly uniform multiatomic step arrays on GaAs (331) A substrates

We use coherently aligned, multiatomic step arrays on GaAs (331) A substrates to generate a periodic array of conductive quantum wires in a two-dimensional electron gas. A small number of quantum wires was selected for transport measurements by superimposing a submicrometer wide constriction parallel to the wires. With independent side-gate control the current could be switched between individual, spatially separated electron channels, thus demonstrating a new device functionality.

Characterization of Potential Modulation in Novel Lateral Surface Superlattices Formed on GaAs Multiatomic Steps

We investigated the potential modulation in a novel lateral surface superlattice (LSSL) structure which was realized by utilizing multiatomic steps. The structure was grown by metalorganic vapor-phase epitaxy (MOVPE) on vicinal GaAs substrates to introduce periodic thickness modulation to the channel GaAs layer with spontaneously formed multiatomic steps. Devices having split gates were fabricated on the epitaxially grown LSSL structure. The potential modulation was investigated by a potential sweeping technique, where different bias voltages were applied to the split gates to change the minima of the potential across the multiatomic steps. We observed plateaus and oscillations of conductance as a function of the gate voltage. Systematic shifts of these positions were also observed when offset voltages between the two gates were applied and changed. These features are assumed to originate from the potential modulation induced by multiatomic steps.

Ripening suppression and large photoluminescence blueshift in aligned InGaAs quantum dots on a vicinal (100) GaAs substrate

We observe ripening suppression in aligned InGaAs quantum dots (QDs) along multiatomic steps on a vicinal (100) GaAs substrate. By varying the growth interruption times, a study of QD morphologies by atomic force microscope reveals a clear Ostwald ripening suppression effect on QD formation. On the other hand, we observe a regular ripening for InGaAs QDs on an exact (100) substrate. In addition, n-QD chains aligned along multiatomic steps are observed. An inhomogeneously broadened photoluminescence (PL) spectrum with larger spectral width is obtained for a QD ensemble on a vicinal (100) substrate as compared to those on an exact (100) substrate although statistical analyses of QD size distributions show similar size dispersions. The spectral broadening is attributed to the lateral quantum coupling of aligned n-QD chains. PL spectra for QDs on vicinal (100) substrates show a larger rate of blueshift with increasing interruption times, which is interpreted as a lack of the PL compensation effect by size enlargement of the QDs during growth interruption. Thus the result supports the ripening suppression effect which is observed in the morphology study.

Natural formation of multiatomic steps on patterned vicinal substrates by MOVPE and application to GaAs QWR structures

In this paper, we investigate multiatomic steps naturally formed on patterned vicinal GaAs substrates during metal organic vapor phase epitaxial (MOVPE) growth. The substrates are (0 0 1)GaAs misoriented by 2.0° and 5.0° toward [−1 1 0] direction. Periodic trenches are formed on the substrate with periods of 0.4–4.0 μm along the [1 1 0] direction using electron beam lithography and wet etching techniques. Periodic multiatomic step structures are clearly observed on AlGaAs/GaAs grown surfaces which are almost equal to the periods of the underlying substrate patterns. From cross-sectional scanning electron microscope (SEM) images, the growth modes are observed. As the growth proceeds, the growth modes change and can be divided into three regions; the multiatomic step formation, transition and multiatomic step dissociation regions. The multiatomic step features also depend on the pattern periods. GaAs grown layer thicknesses at the edges of the multiatomic steps are thicker than those on the terraces, and this effect is prominent for shorter periodicity patterns. GaAs/AlGaAs quantum wells are grown on patterned vicinal substrates with a 0.4 μm period. The cathodoluminescence (CL) spectrum at 7.0 K has two emission peaks. From the surface and cross-sectional CL images, the lower energy peak corresponds to the quantum well formed at the edges of the multiatomic steps and the higher energy peak corresponds to that on the terrace, consistent with the results of cross-sectional SEM observations. Using optimized patterning and growth condition, we can fabricate highly uniform quantum wire (QWR) arrays at the step edges.

Formation and characterization of modulated two-dimensional electron gas on GaAs multiatomic steps grown by metalorganic vapor phase epitaxy

We used multiatomic steps spontaneously formed on vicinal surfaces during metalorganic vapor phase epitaxial (MOVPE) growth to introduce lateral potential modulation into two-dimensional electron gas (2DEG) and to realize a novel lateral surface superlattice (LSSL) with period of around 70 nm. Their electron transport properties were investigated at low temperatures and large anisotropies in the mobility and transconductance characteristics were found for the current parallel and perpendicular to the multiatomic steps. The potential modulation was further applied to form multiple quantum dots and tunnel junctions in combination with the squeezing of the width of the channel defined across the multiatomic steps. The Coulomb blockade type conductance oscillations were observed at low temperature transport measurement.

Anisotropic magneto-resistance of laterally modulated GaAs/AlGaAs systems with a 15–20 nm periodicity formed on vicinal (1 1 1)B substrates

Laterally modulated two-dimensional electron systems with a 15–20 nm periodicity have been successfully formed at corrugated n-AlGaAs/GaAs heterojunctions. The lateral modulation is provided by multi-atomic steps formed on the surfaces of GaAs grown on vicinal (111)B substrates. We have observed a highly anisotropic magneto-resistance Rxx of the system. When the current flow is parallel (//) or perpendicular (⊥) to the steps, the oscillation amplitude of Rxx// was much smaller than that of Rxx⊥. This result can be explained by an anisotropic scattering rate between edge-states through anisotropic bulk-states modulated by the step structure.

Formation of uniform GaAs multi-atomic steps with 20–30 nm periodicity and related structures on vicinal (111)B planes by MBE

We studied morphology of GaAs surfaces and the transport properties of two-dimensional electron gas (2DEG) on vicinal (111)B planes. Multi-atomic steps (MASs) are found on the vicinal (111)B facet grown by molecular beam epitaxy, which will affect electron transport on the facet. We also studied how the morphology of GaAs epilayers on vicinal (111)B substrates depends on growth conditions, especially on the As4 flux. The uniformity of MASs on the substrates have been improved and smooth surfaces were obtained when the GaAs was grown with high As4 flux, providing step periodicity of 20 nm. The channel resistance of the 2DEG perpendicular to the MASs is reduced drastically with this smooth morphology. These findings are valuable not only for fabricating quantum devices on the (111)B facets but also those on the vicinal (111)B substrates.

Novel magneto-resistance oscillations in laterally modulated two-dimensional electrons with 20 nm periodicity formed on vicinal GaAs (1 1 1)B substrates

Laterally modulated two-dimensional electron gas has been formed in quasi-periodically corrugated AlGaAs/GaAs heterojunctions, where multi-atomic steps with about 20 nm periodicity are generated by growing GaAs epi-layer on vicinal (1 1 1)B substrates. We have measured two- and multi-terminal magneto-resistance R x perpendicular to the steps and R y parallel to them, and found novel oscillations different from the ordinary Shubnikov–de Haas oscillations. In the oscillation, R y has exhibited peaks at magnetic fields where R x shows minima, revealing the phase shift of 180° between them. We speculate that this anomaly is caused by van Hove singularity in the density of states of Landau subband in the periodically modulated system.

Different paths to tunability in III–V quantum dots

Tunability in the concentration and average dimensions of self-forming semiconductor quantum dots (QDs) has been attained. Three of the approaches examined here are: variations with temperature, group V partial pressure and with substrate miscut angle. Thermally activated group III adatom mobilities result in larger diameters and lower concentrations with increasing deposition temperatures. These variations are presented for InGaAs/GaAs and AlInAs/AlGaAs, where striking differences were seen. Tunability in the InGaAs/GaAs QD concentration was also obtained in metalorganic chemical vapor deposition by varying the arsine flow. The latter gave widely varying concentrations and similar sizes. Substrate orientation was found to also be a key factor in island nucleation: Changes in vicinal orientation near (100) can be used to exploit the preferential step edge nucleation at mono and multi-atomic steps, so varying miscut angle (θm) can be used to change island densities and sizes. Anisotropies in island nucleation producing n-dot strings aligned with multiatomic step edges are observed for θm⩾0.75° and up to 2°. Quantum mechanical coupling from such island strings result in non-Gaussian shapes in the inhomogeneously broadened photoluminescence peaks. The effects of some of the other morphological differences presented here on the luminescence emission from QD ground states is discussed for InGaAs/GaAs QDs.

Optical characterization and laser operation of InGaAs quantum wires on GaAs multiatomic steps

We have been demonstrating natural formation and characterization of InGaAs strained quantum wire (QWR) structures on coherent GaAs multiatomic steps grown by metalorganic vapor phase epitaxy (MOVPE). In this paper, we report on the optical properties of InGaAs QWRs. First, photoluminescence (PL) and photoluminescence excitation (PLE) spectra were taken at 77 K. PLE spectra showed the polarization anisotropy for excitation light with electric field parallel or perpendicular to the QWRs. Next, we measured the PL of InGaAs QWRs on GaAs multiatomic steps at 4.2 K, in magnetic fields, to examine the lateral confinement. It was found that the diamagnetic coefficient is 16 μeV/T 2, and is much smaller than that of quantum well (QWL) on singular (001) GaAs substrates (45 μeV/T 2), which suggests the presence of lateral confinement. We also succeeded for the first time in the pulsed laser operation of InGaAs QWRs on GaAs multiatomic steps at 77 K by current injection.

Suppression of Ostwald ripening inIn0.5Ga0.5Asquantum dots on a vicinal (100) substrate

A comparative study of the morphology of self-assembled ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dots grown by atmospheric pressure metal-organic chemical vapor deposition on the exact (100) and 2\ifmmode^\circ\else\textdegree\fi{}-off (100) GaAs substrates as a function of growth interruption time (0--1200 sec) is presented. The dots are randomly distributed on the exact (100) substrate, whereas the dots on the 2\ifmmode^\circ\else\textdegree\fi{}-off (100) substrate are aligned along multiatomic steps. As the interruption time $t$ is increased, the density of dots on the exact (100) substrate decreases and their average volume progressively increases with $\ensuremath{\sim}{t}^{3/4}$ dependence, indicating a regular Ostwald-ripening process. By contrast, the average volume of dots on the 2\ifmmode^\circ\else\textdegree\fi{}-off (100) substrate saturates for interruption times over 200 sec and shows obvious suppression of ripening. In particular, the size of dots on the 2\ifmmode^\circ\else\textdegree\fi{}-off (100) substrate is limited within the atomic terrace width $(\ensuremath{\sim}55\mathrm{nm}).$ These results demonstrate that the density and size of dots could be controlled by interruption time and substrate miscut angle.

InGaAs/GaAs quantum nanostructure fabrication on GaAs (111)A vicinal substrates by atomic layer epitaxy

InGaAs/GaAs quantum wire (QWR) and dots were fabricated on GaAs (111)A vicinal substrates by the atomic layer epitaxy (ALE) technique. In0.25Ga0.75As QWR structures were formed on metal–organic vapor phase epitaxy grown homogeneous multiatomic height steps on GaAs (111)A vicinal substrates misoriented toward the [112̄] direction. In photoluminescence measurement only one spectrum, which shows strong polarization dependence, was observed. The result and its narrow full width at half maximum of 10 meV imply that the elimination of the wetting layer and excellent size uniformity are realized by the ALE technique. Spontaneous alignment of InGaAs quantum dots was also achieved by the ALE method. Boxlike shaped dot arrays of which height is restricted by the step height indicate that the ALE growth on the (111)A vicinal surface has an effect on not only the arrangement but also on the size and shape control of quantum nanostructures.

Self-organised InGaAs quantum wire lasers on GaAsmulti-atomic steps

The authors report the first successful demonstration of self-organised InGaAs quantum wire (QWR) laser diodes (LDs) utilising GaAs multi-atomic steps at 77 K by pulsed current injections. The lasing wavelength of InGaAs QWR-LDs is consistent with the peak position of photoluminescence spectra at 77 K. When a cavity direction is perpendicular to the QWR's array direction, the threshold current density of InGaAs QWR LDs is smaller than that of quantum well LDs.

Delta-Doping and the Possibility of Wire-like Incorporation of Si on GaAs Vicinal Surfaces in Metalorganic Vapor Phase Epitaxial Growth

We have investigated the delta-doping of Si on GaAs vicinal surfaces on which self-organized multiatomic steps are formed during metalorganic vapor phase epitaxy (MOVPE) and the possibility of its wire-like selective incorporation into step edges to form doping quantum wires (DQWRs). We evaluated the electrical characteristics of delta-doped layers by Hall and C-V measurements and investigated their dependence on misorientation angles of GaAs vicinal substrates and doping time. The incorporation of Si is enhanced by steps. This effect is particularly important at the initial stage of delta-doping when the surface coverage of Si is not high. Our results also suggest that the doping density at multiatomic step regions is higher than at terrace regions. Therefore, it is expected that the selective wire-like incorporation of Si takes place at the step edges, and DQWRs can be realized under suitable growth conditions.

Uniform multiatomic step arrays formed by atomic hydrogen assisted molecular beam epitaxy on GaAs (331) substrates

Coherently aligned multi-atomic step arrays are naturally formed during molecular beam epitaxy (MBE) of GaAs/(AlGa)As heterostructures on GaAs (331) substrates. The step height systematically increases with the substrate temperature while the lateral periodicity remains almost unchanged. With atomic hydrogen irradiation the step height is larger by more than a factor of 2 compared to that in conventional MBE which is attributed to a larger surface migration length of adatoms. The higher uniformity in atomic hydrogen assisted MBE allows the formation of step arrays, 12–13 nm high with a lateral periodicity around 250 nm, and straight step edges over 10 μm length. The step arrays reveal a strong influence on the electron transport of Si-modulation-doped GaAs/(AlGa)As heterostructures with the conductivity parallel to the step edges at cryogenic temperatures more than one order of magnitude larger than that perpendicular to the steps.