V-grooved Substrates Research Articles

Characterization and modelling of the strain fields associated with InGaAs layers on V-grooved InP substrates

A series of lattice-matched or compressively strained InGaAs layers were grown by gas source molecular beam epitaxy on V grooved InP substrates, with grooves bounded by {211}A or {111}B facets. The stress field associated with the layers was measured by the degree of polarization technique, and compared to the predictions of analytical or finite element models. Good agreement was found for the {211}A V grooves, but both the nominally lattice-matched and compressively strained layers grown on {111}B V grooves displayed similar degree of polarization maps. Analytical electron microscopy demonstrated that the {211}A V-groove samples had the targeted composition, but the {111}B samples showed much higher In/Ga ratios at the bottom of the groove than the expected values. Indium enrichment at the bottom of the groove led to defect formation there, and left the V groove of both the lattice-matched and compressively strained samples under a net compressive force.

Observation of exciton states in GaAs coupled quantum wires on a V-grooved substrate

The exciton states of GaAs coupled quantum wires are investigated by the measurement of photoluminescence excitation (PLE) in comparison with those of single quantum wires. In the PLE spectra of single quantum wires (wire thickness=4.5 nm), sharp exciton peaks of the first two heavy hole-like transitions are observed with large energy difference of 47 meV, while two adjacent exciton peaks with the small energy splitting of 24 meV are observed in the coupled quantum wires (wire thickness=5 nm, barrier thickness=3 nm). From the measurements of the barrier thickness dependence, these exciton states agree well with the coupled states of the quantum wires calculated by the finite element method.

Self-limiting OMCVD growth of GaAs on V-grooved substrates with application to InGaAs/GaAs quantum wires

We demonstrate that the formation of GaAs quantum wires on self-limiting AlGaAs grown on V grooves occurs via a transient increase of the growth rates in a set of different nanofacets. Upon growth of sufficiently thick layers on AlGaAs, the GaAs surface reaches a self-limiting profile as well, through an equalization of the relative growth rates on these facets. Atomic force microscopy studies show that the step density in the facets along the groove evolves with GaAs thickness in the same way as the facets extension, thus suggesting a role of the step distribution in the establishment of the self-limiting profiles. The self-limiting GaAs groove profile is much broader than the AlGaAs one at corresponding growth temperatures; however, it can be sharpened down to a radius of curvature of 5 nm for T = 550°C. Under these conditions, GaAs was successfully used as a barrier material for growing vertical arrays of self-ordered InGaAs wires.

GaInAs/InP quantum wires grown by metalorganic vapor phase epitaxy on V-grooved InP substrates

A single nominally lattice matched GaInAs quantum well (QW)/quantum wire (QWR) structure was grown by metalorganic vapor phase epitaxy (MOVPE) in V-grooved InP substrates. Different Si0 2 etch masks with opening widths from 2 μm down to 200 nm (for application as second order DFB grating) were defined by optical and electron beam lithography. A damage-reduced wet chemical etching process enables the growth of the GaInAs QWs/QWRs without any InP buffer layer. In low temperature photoluminescence we found improved intensity for all wire structures prepared by this etching technique. A reduction of the period and opening width of the V-groove etch mask resulted in a optimized luminescence intensity ratio between QW and QWR. Decay times from time resolved luminescence measurements were compared to the decay times of wet or dry etched mesa wires before and after regrowth. The good optical properties of the GaInAs QWRs are encouraging for future application as a QWR-laser device.

Self-limiting growth of GaAs surfaces on nonplanar substrates

The growth of GaAs epitaxial layers by organometallic chemical vapor deposition on top of V-grooved substrates is found to exhibit a self-limiting behavior. As in the case of the self-limiting growth of AlGaAs on similar patterned substrates, the self-limiting GaAs profile exhibits characteristic crystallographic nanofacets. However, these facets are considerably broader than in typical, self-limiting AlGaAs profiles obtained at similar growth temperatures. Atomic force microscopy in air reveals the three-dimensional structure of the self-limiting GaAs surfaces, showing monolayer steps on the central (100) nanofacets and quasiperiodic modulation caused by step bunching on the side {311}A nanofacets. The width of the self-limiting GaAs V grooves can be reduced to less than 10 nm at sufficiently low growth temperatures, thus providing useful templates for growing, e.g., self ordered InGaAs/GaAs quantum wires.

Carrier quantum confinement in self-orderedAlxGa1−xAsV-groove quantum wells

Observation of quantum confined states and optical anisotropy of the interband absorption in self-ordered ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ vertical quantum wells (VQW's) grown on V-grooved substrates is reported. The variation in Al composition across the VQW's was determined by parallel electron energy-loss spectroscopy. It was then employed to calculate the eigenstates of the confined carriers in these structures, with valence-band mixing included using a $4\ifmmode\times\else\texttimes\fi{}4$ $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Luttinger model. Polarized photoluminescence excitation spectra of the structure show features attributed to transitions between electron and heavy-hole or light-hole confined eigenstates, with measured energies in good agreement with the calculated ones. Polarization anisotropy associated with confined electrons and holes is evidenced by the measured spectra and explained by valence-band mixing in the (011) self-ordered quantum well.

Fabrication and photoluminescence of AlGaAs/GaAs quantum wire superlattices on V-grooved substrate

An AlGaAs(90 Å)/GaAs(45 Å) quantum wire superlattice (QWR-SL) with excellent size uniformity is grown on a 4.8 μm pitch V-grooved substrate by flow rate modulation epitaxy at a specific growth temperature (∼630 °C). In the low temperature (12 K) photoluminescence spectrum of the AlGaAs(90 Å)/GaAs(45 Å) QWR-SL, two excited subband emission peaks are observed at an excitation power density at least about four orders of magnitude lower than that needed for the observation of similar peaks from the single quantum wire (SQWR) reference sample. Another two side-peaks are also observed at the long wavelength side of the ground subband peak whose energy separations from the excited subband peaks are very close to the energies of GaAs optical phonons, implying the possibility of participation of optical phonons in the radiative transition processes of the QWR-SL. The ground subband emission peak of the QWR-SL sample shows a radiative lifetime (∼2.1 ns) almost four times longer than that of the SQWR sample. The above observations suggest that the carrier relaxation processes and the radiative transition probability of the QWR-SL seem to be considerably modified against the SQWR structure.

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.

InGaAs layer effect on the growth of AlGaAs/GaAs quantum wires on V-grooved GaAs substrates

We have studied an effect of InGaAs layer on the structural and optical properties of AlGaAs/GaAs quantum wires (QWRs) grown on V-grooved GaAs(100) substrates by metalorganic chemical vapor deposition. The In 0.15Ga 0.85As layer with thickness of 2–4 μm were prepared on GaAs(100) substrates, and the V-grooves were defined with 20 μm intervals by photolithographic method. The quantum structures with five periods of 5 nm GaAs and 25 nm Al 0.5Ga 0.5As layers were grown on V-grooved InGaAs/GaAs substrates. From the scanning electron microscope and transmission electron microscope measurements, it appeared that the thick InGaAs layer caused to form several facets in the side walls of V-grooves. Especially, the side walls near the bottom were convexly shaped, resulting in narrowing the width near the bottom of the V-groove. In the photoluminescence (PL) spectra, QWRs grown on this substrate showed a blue shift, while any PL signal from the top-quantum wells on InGaAs layer has not appeared. From these results, it was suggested that the InGaAs layer plays an important role for the lateral tightening of QWRs.

Molecular beam epitaxial growth of quantum wires on V-grooved InP substrates with (1 1 1) sidewalls

Abstract Bulk InGaAsP and InGaAs InGaAsP quantum well structures have been grown by gas source molecular beam epitaxy on V-grooved InP substrates having (1 1 1)A and (1 1 1)B sidewalls. The growth of the InGaAsP layer in a (1 1 1)A V-groove results in a flat and wide bottom, which rules out the possibility of the formation of InGaAs InGaAsP quantum wires (QWRs). However, the growth of the InGaAsP layer in (1 1 1)B V-grooves results in a sharper bottom so that crescent-shaped InGaAs InGaAsP quantum wells are formed such that QWR behaviour is possible. Transmission electron microscopy shows that the crescent-shaped InGaAs layer has a very large thickness variation at the V-groove bottom and that all layers at the V-groove bottom are free of extended defects. The photoluminescence from different spatial regions of the sample has been identified using a selective etching technique. Two peaks are associated with the QWRs at V-groove bottom, with the higher energy one apparently being from the second lateral subband transition. The energy separation between these two peaks is consistent with values calculated from the measured thickness variation. Consistent with QWR behaviour, the photoluminescence is polarized, with a stronger signal along the wire direction than perpendicular to the wire direction.

Photoconductivity study of crescent-shaped GaAs/GaAlAs quantum wires grown by flow rate modulation epitaxy

We present photoconductivity data on GaAs quantum wires grown on a V-grooved substrate by flow rate modulation epitaxy. They show that a moderate excitation power density, ∼1 W/cm2, allows the observation of the absorption structure of a single GaAs quantum wire embedded in a p-i-n diode. Furthermore, by increasing the number of active wires inside a diode, the photoconductivity signal is enhanced and additional details of the absorption structure are evidenced. And, finally, a rough quantitative agreement is obtained between the experimental absorption transitions and a simple calculation of the one-dimensional excitonic states using the envelope function approximation.

The morphology of InP/InGaAs grown by molecular beam epitaxy onto V-grooved InP substrates

Gas source molecular beam epitaxial growths have been performed onto V-groove patterned InP substrates with (1 1 1)A and (1 1 1)B faceted sidewalls in order to investigate the dependence of epilayer morphology of InP/InGaAs quantum well structures on the growth conditions. It is found that the V/III flux ratio significantly affects the shape and roughness of a V-groove bottom and the roughness of the sidewall. The growth temperature and growth rate also affect the morphology. With the growth conditions optimized, InP/InGaAs quantum well structures can be grown in the grooves with sharp bottoms and smooth surfaces. The sharpness of the bottom of the groove is related to growth conditions that decrease the growth rate at the bottom by limiting the supply of the group V component.

Electronic States in Crescent-Shaped GaAs Coupled Quantum Wires

The electronic states in crescent-shaped GaAs coupled quantum wires on V-grooved substrate have been investigated by theoretical analysis using the finite element method (FEM). The doublet state, namely the symmetric and antisymmetric states, and the transverse electron states in crescent-shaped coupled quantum wires are clarified. The conditions of the coupling and the effect of the thickness disparity between two quantum wires are also clarified. The preliminary experiment on the photoluminescence (PL) characteristics of the crescent-shaped coupled quantum wires, fabricated on V-grooved substrate, are compared to the results obtained by the theoretical analysis. The observed PL peak splittings have been explained by the energy splitting between the symmetric and the antisymmetric states in the theoretical analysis. The experimental results are in good agreement with the results of theoretical analysis for weakly coupled quantum wires.

Proposal of a New Self-Limited Growth and Its Application to the Fabrication of Atomically Uniform Quantum Nanostructures

A new self-limited growth technique based on control of surface migration in flow rate modulation epitaxial growth on nonplanar substrates is proposed. The proposed self-limited growth is experimentally confirmed by growing AlGaAs/GaAs quantum wires on V-grooved substrates. Using this new self-limited growth technique, atomically uniform semiconductor quantum nanostructures are expected to be fabricated easily.

Quantum wire structures incorporating (GaAs) m(GaP) n short-period superlattice fabricated by atomic layer epitaxy

Lateral confined systems of (GaAs) m (GaP) n short-period superlattice (SPSL) fabricated by using atomic layer epitaxy (ALE), for composition control in quantum nano-structures, are demonstrated. (GaAs) m (GaP) n SPSL were grown on V-grooved GaAs substrates by the localized-ALE on nanometer scale. The lateral confined systems composed of well-defined (GaAs) m (GaP) n SPSL have been confirmed via transmission electron microscopy (TEM) and photoluminescence (PL) measurements. Also GaAs/(GaAs) m (GaP) n -SPSL quantum wire structures were fabricated by ALE growth, and composition control in the structures was successfully realized for the first time.

Fabrication of highly uniform [formula omitted] quantum wire superlattices by flow rate modulation epitaxy on V-grooved substrates

A growth condition (growth temperature) which could maintain an identical growth profile of AlGaAs grown on a V-grooved substrate by metalorganic vapor phase epitaxy independent of the growth thickness is established. This condition is applied to the fabrication of highly uniform AlGaAs GaAs quantum wire superlattices (QWR-SLs) by vertical stacking of multiple QWRs on a V-grooved substrate. An AlGaAs barrier layer thickness of about twice that of the GaAs QWR layer is found to be necessary for the complete recovery of the distorted V-groove bottom curvature. A series of 20 period AlGaAs GaAs QWR-SLs are successfully grown by flow rate modulation epitaxy under the developed condition for a stable AlGaAs growth profile. The excellent size uniformity of the vertically stacked QWRs is confirmed by transmission electron microscopy observation and the narrow full width at half maximum of an AlGaAs(135 A ̊ ) GaAs(45) A ̊ QWR-SL photoluminescence peak.

Low-pressure OMCVD growth of AlGaAs vertical quantum wells on non-planar substrates

AlGaAs vertical quantum well structure were formed by low-pressure (20 mbar) organometallic chemical vapor deposition on V-grooved substrates. Ga segregation in the wells of these structures was evidenced by means of atomic force microscopy and photoluminescence experiments. Transmission electron microscopy cross sections resolve different branches in the well. This substructure, whose definition is enhanced in low-pressure growth, is associated with the formation of specific facets at the bottom of the grooves. The dependence of the growth rate on the crystallographic facet and the Al mole fraction is the key mechanism for the establishment of the self-limiting profile in the V-grooves.

Perpendicular giant magnetoresistance of Co/Cu multilayers on V-grooved substrates: Dependence on deposition method

The perpendicular giant magnetoresistance of Co/Cu multilayers grown at an angle onto V-grooved InP substrates is studied using two different deposition techniques. Samples are grown both at ultra-high vacuum in a molecular beam epitaxy system and in a classical high vacuum electron beam evaporation system. Using the two spin-channel model for electron transport, the spin-asymmetry parameters were determined. The absolute value and temperature behavior of the spin-asymmetry parameters of the Co bulk scattering and the interface scattering are comparable for both series. We find that the interface scattering of the e-beam evaporated multilayers is relatively less spin-dependent.

Low pressure metal-organic chemical vapor deposition of InP/InAlAs/InGaAs quantum wires

We report the fabrication of InGaAs quantum wires in the InP/InAlAs material system using low pressure metal-organic chemical vapor deposition. The use of a low temperature InP buffer layer on V-grooved InP substrates is the key to grow either directly an InGaAs layer for quantum wire formation or to deposit InAlAs layers that show a resharpening effect in the V-groove. This self-limiting growth is the prerequisite for stacking quantum wires, which we report here for the first time in this material system. We further report on structural and optical properties of the InGaAs quantum wires obtained from transmission and scanning electron microscopy, photo- and cathodoluminescence measurements.

GMR in corrugated Co/Cu multilayers

A new corrugated material comprising antiferromagnetically coupled Co/Cu multilayers was prepared using V-grooved Si substrates. Scanning electron microscopy and X-ray diffraction observations indicate that the corrugated multilayers were successfully grown with the formation of smooth and clear interfaces. Magnetoresistance change with the sense current normal to the grooves was larger than that with the current parallel to the grooves. The MR enhancement due to the difference in measurement geometry increased as the width of the grooves decreased.