ML GaAs Research Articles

Vapor-liquid-solid nucleation of GaAs on Si(111): Growth evolution from traces to nanowires

GaAs nanowires were grown on Si(111) substrates by molecular-beam epitaxy employing Au droplets for the vapor-liquid-solid mechanism. The nucleation happens in three stages, which coincide with the abundance of one of three different GaAs manifestations: first Au-induced lateral traces, then three-dimensional islands, and finally Au-induced vertical nanowires. During deposition of the first 7 ML of GaAs no nanowires grew. By reflection high-energy electron diffraction and transmission electron microscopy, the crystal structure of the GaAs manifestations was examined. Traces and islands adopted the zinc-blende crystal structure and include twins, while nanowires grew in the hexagonal wurtzite structure. The delay in nanowire formation was observed to increase for higher growth temperatures. The gradual covering of the Si by GaAs was comparable to the case of Au-free growth and appears to be linked with the evolution from trace to nanowire growth. During trace formation, Au droplets kept in contact with the Si substrate and were pushed sideways by precipitating GaAs. The nucleation stages could thus be explained by considering that any Au droplet created a trace while in contact with the Si surface and a nanowire after having been pushed onto GaAs. This mechanism of trace growth was explained by the lower interface energy of liquid Au droplets on Si when compared with that on GaAs, as supported by numerical estimates.

Molecular beam epitaxy growth and characterization of type-II InAs/GaSb strained layer superlattices for long-wave infrared detection

The authors report on investigation of type-II InAs/GaSb and InAs/InxGa1−xSb strained layer superlattices (SLSs) for long-wave infrared detection. Growth conditions were optimized to obtain nearly lattice matched (Δa/a∼0.03%) 13 ML InAs/7 ML GaSb SLS nBn detector structure with cutoff wavelength of ∼8.5 μm (77 K). Dark current density was equal to 3.2×10−4 A/cm2 (Vb=+50 mV, 77 K) for this detector structure. Thin 10 ML InAs/6 ML In0.35Ga0.65Sb SLS was grown with zero lattice mismatch achieved by incorporation of 2.5 ML of GaAs in every SLS period.

High-power semiconductor disk laser based on InAs∕GaAs submonolayer quantum dots

An optically pumped semiconductor disk laser using submonolayer quantum dots (SML QDs) as gain medium is demonstrated. High-power operation is achieved with stacked InAs∕GaAs SML QDs grown by metal-organic vapor-phase epitaxy. Each SML-QD layer is formed from tenfold alternate depositions of nominally 0.5 ML InAs and 2.3 ML GaAs. Resonant periodic gain from a 13-fold nonuniform stack design of SML QDs allows to produce 1.4W cw at 1034nm. The disk laser demonstrates the promising potential of SML-QD structures combining properties of QD and quantum-well gain media for high-power applications.

Enhancement of the room temperature luminescence of InAs quantum dots by GaSb capping

The authors have studied the use of antimony for the optimization of the InAs∕GaAs(001) self-assembled quantum dot (QD) luminescence characteristics in the 1.3μm spectral region. The best results have been obtained by capping InAs QDs with 2 ML of GaSb grown on top of a 3 ML GaAs barrier separating the InAs and the GaSb layers. This results in an order of magnitude enhancement of the room temperature luminescence intensity at 1.3μm emission wavelength.

Terrace width distribution during unstable homoepitaxial growth of GaAs(110): An experimental study

The temporal evolution of the step bunching instability formed during GaAs homoepitaxial growth on the GaAs(110) vicinal to (111)A has been studied by atomic force microscopy (AFM) and the step–step distribution has been quantified as a function of deposition time. Analysis of the AFM data has shown that neither the terrace width distribution (TWD) nor the terrace height distribution (THD) fit to a Gaussian function in the initial stages of growth, but both evolve with time as the bunching instability develops. After deposition of 500 ML of GaAs the TWD exhibits a clear Gaussian behavior while the THD is very well fitted to a Lorentzian distribution. The GaAs surface morphology initially shows a great dispersion in terrace height and width values with a clear anisotropy along the <001> tilt direction, but evidence of self-controlled growth is observed irrespective of layer thickness.

HIGH-FIELD MAGNETOTRANSPORT STUDIES OF FERROMAGNETIC GaAs/Mn DIGITAL ALLOYS

Magnetotransport properties of ferromagnetic GaAs / Mn digital alloys have been investigated in fields up to 33 T. A series of four GaAs / Mn digital alloys with different Mn coverages (0.15 ML - 0.5 ML) at fixed GaAs spacer thickness (9 ML), with Curie temperatures, TC, between 20 and 40 K shows hopping conduction behavior in the zero-field sheet resistance below TC. Analysis of the high field magnetotransport measurements on these samples reveals hole densities between 0.45×1013 and 1.8×1013 cm -2/ Mn layer at 5 K. In contrast, a GaAs / Mn digital alloy with slightly different parameters (0.5 ML Mn and 14 ML GaAs spacer layers) and growth conditions shows essentially metallic behavior and much higher TC (60 K). The zero-field sheet resistance, although decreasing weakly with T at low temperatures, cannot be fit by a hopping expression. From analysis of Shubnikov-de Haas oscillations observed in this sample, an effective mass of ≈0.31m0 was determined, close to the heavy hole mass of GaAs . The hole density extracted from fits to RHall at high fields is comparable to that of the insulating GaAs / Ma digital alloys at the same Mn coverage. This suggests that the increased metallicity is the most important factor in significantly enhancing TC.

The emission wavelength tuning of InAs/InP quantum dots with thin GaAs, InGaAs, InP capping layers by MOCVD

InAs quantum dots (QDs) were grown on InP substrates by metalorganic chemical vapor deposition. The width and height of the dots were 50 and 5.8 nm, respectively on the average and an areal density of 3.0×10 10 cm −2 was observed by atomic force microscopy before the capping process. The influences of GaAs, In 0.53Ga 0.47As, and InP capping layers (5–10 ML thickness) on the InAs/InP QDs were studied. Insertion of a thin GaAs capping layer on the QDs led to a blue shift of up to 146 meV of the photoluminescence (PL) peak and an InGaAs capping layer on the QDs led to a red shift of 64 meV relative to the case when a conventional InP capping layer was used. We were able to tune the emission wavelength of the InAs QDs from 1.43 to 1.89 μm by using the GaAs and InGaAs capping layers. In addition, the full-width at half-maximum of the PL peak decreased from 79 to 26 meV by inserting a 7.5 ML GaAs layer. It is believed that this technique is useful in tailoring the optical properties of the InAs QDs at mid-infrared regime.

Magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures

Magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures have been investigated by polarized neutron reflectometry and magnetometry. Tc of three samples with 20, 50, and 100 ML GaAs spacers ranges from 30 to 40 K. The saturation magnetization of three samples exhibits a pronounced tail extending over 50 K above Tc in addition to a temperature-independent background. For the 50 ML sample, PNR measurements show a similar tail but no background. These behaviors can be explained by a two-step ordering process. In the tail region, two-dimensional islands first individually become ferromagnetic. Long-range order develops as the temperature is decreased below Tc.

Self-assembled GaAs antidots growth in InAs matrix on [formula omitted] InAs substrate

We have grown GaAs antidots in InAs matrix on (100) InAs substrate successfully. The quantum-sized 3-D islands were observed clearly in both AFM and TEM measurements. From these observations, the 2-D to 3-D transition thickness is determined to be between 2.25 and 2.5ML. For 2.5ML GaAs deposition, the grown antidots have a size of about 15–35nm in base diameter and about 2–4nm in height with a density about 3–4×1010cm−2.

Growth and magnetic properties of (Ga,Mn)As as digital ferromagnetic heterostructures

(Ga,Mn)As digital ferromagnetic heterostructures (DFH) are grown by incorporating Mn delta-doping layers into GaAs using molecular beam epitaxy. Investigation of structural properties (via X-ray diffraction, TEM, RHEED) and magnetic properties (via SQUID, MOKE) as a function of growth temperature reveal the following: (1) a DFH growth window from 240 to 280 °C, (2) a strong dependence of the ferromagnetic transition temperature ( T C) on growth temperature. The dependence of T C on the spacing between Mn layers is also investigated, and non-zero T C is observed for spacing as high as 200 ML of GaAs. Furthermore, magnetic measurements on a single layer of Mn delta-doping indicate that this ultrathin film is ferromagnetic.

Shape and surface morphology changes during the initial stages of encapsulation of InAs/GaAs quantum dots

The change in shape and surface morphology of InAs/GaAs(0 0 1) quantum dots (QDs) during their initial encapsulation by GaAs has been studied using reflection high energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM). The shape of the QDs changes significantly during the earliest stages of overgrowth. In situ RHEED measurements show a significant chevron angle change after deposition of only 2 ML of GaAs, before losing all crystallographic structure as more GaAs is deposited. STM results indicate significant surface mass transport, the height of the QDs decreases faster than the rate at which the GaAs capping layer is deposited and the QDs effectively “collapse” during the earliest stages of encapsulation. The area of the partially capped QDs increases significantly with increasing GaAs deposition and there is an anistropic increase in shape with significant elongation along the [ 1 ̄ 1 0] azimuth.

Photoluminescence and AFM Studies on Blue Shifted InAs/AlyGa1?yAs Quantum Dots

Photoluminescence and structural properties of self-assembled InAs quantum dots, grown on AlyGa1—yAs, are studied for y = 0, 0.3, and 0.5. A maximum blue shift of 150 meV for the ground state emission energy is determined with increasing y for the samples as grown. Increased surface density and size inhomogeneity is observed for growth on AlGaAs compared to growth on GaAs. Rapid thermal annealing at temperatures ranging from 550 to 850 °C is used to further increase the ground state emission energy with respect to the as grown samples. We find a shift up to a maximum emission energy of 1.9 eV for InAs quantum dots (QDs) embedded in Al0.3Ga0.7As compared to 1.4 eV for GaAs matrix material. To combine the higher spectral shift by annealing for InAs QDs embedded in AlGaAs with the lower dot density for the growth on GaAs, favored for single dot spectroscopy, we investigated InAs QDs grown on AlGaAs separated by 2 ML GaAs. For these samples we observe a maximum shift by annealing up 1.4 eV and intense room temperature PL.

Free-standing and overgrown InGaAs/GaAs nanotubes, nanohelices and their arrays

The possibility is shown to fabricate a wide class of free-standing nano-objects based on few monolayers thick scrolled heterostructures. Using an ultra-thin film (1 ML GaAs:1 ML InAs), nanotubes with an inside diameter of ≈2 nm have been obtained, which constitutes the limiting size for this system. Molecular-beam-expitaxy overgrown structures with nanotubes embedded into GaAs have been obtained.

Excitation transfer in novel self-organized quantum dot structures

We report on studies of excitation transfer processes in vertically self-organized pairs of unequal-sized quantum dots (QDs), created in InAs/GaAs bilayers having differing InAs deposition amounts in the first (seed) and subsequent layer. The former and latter enable independent control, respectively, of the density and the size distribution of the second layer QDs. This approach allows us to enhance the average volume and improve the uniformity of InAs QDs, resulting in low-temperature photoluminescence at 1.028 eV with a linewidth of 25 meV for 1.74 ML (seed)/3.00 ML InAs stacking. The optical properties of the formed pairs of unequal-sized QDs with clearly discernible ground-state transition energy depend on the spacer thickness and composition. Photoluminescence results provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer in such asymmetric QD pairs. Transfer times down to 20 ps (36 ML GaAs spacer) are estimated, depending exponentially on the GaAs spacer thickness.

Strain effects on the surface optical transitions of GaAs

The surface optical properties of GaAs on GaP-strained layers have been analyzed in situ by chemical modulation spectroscopy. A clear surface-related optical transition has been identified, and the effect of strain on this transition has been studied, showing a blueshift of 30 meV even for epitaxies as thin as 1 ML of GaAs deposited on top of GaP. This small shift is consistent with a transition between dimer states and dangling-bond states of the Ga-Ga dimers.

Excitation transfer in self-organized asymmetric quantum dot pairs

Excitation transfer processes within self-organized quantum dot (QD) pairs in bilayer InAs/GaAs QD samples are investigated. QDs in samples with a 1.74-ML InAs seed layer and a 2.00 ML InAs second layer are found to self-organize in pairs of unequal sized QDs with clearly discernible ground-state transition energy. Photoluminescence (PL) and PL excitation results for such asymmetric QD pairs provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer. Variations in the optical behavior as a function of the spacer thickness and composition are attributed to the barrier-dependent tunnel probability. Tunneling times down to 20 ps (36 ML GaAs spacer) are estimated, depending exponentially on the GaAs spacer thickness.

SEXAFS Study of the GaAs/InP Interface

Extended X-Ray Absorption Fine Structure measurements have been performed at the Ga and As K-edge of a nominal 3 ML GaAs grown by Molecular Beam Epitaxy on a InP(100) substrate deoxidized in As atmosphere. We exploited the in-plane linear polarization of the synchrotron radiation measuring SEXAFS spectra taken with the polarization vector parallel and perpendicular to the growth direction. By comparing the ratio of the ooordination numbers of the pairs As-Ga, As-In, Ga-As, taken at the two different angles of the polarization vector, a quantitative model of the interface is obtained. Two well defined layers form on the InP substrate giving rise to the following sample structure; 3 ML GaAs/3 ML InAS/InP. This rules out the formation of InGaAs or InGaAsP layers at the interface.

Formation of GaAs/AlAs(001) interfaces studied by scanning tunneling microscopy.

We have investigated the interface formation of molecular-beam epitaxy (MBE) grown GaAs/AlAs(001) heterostructures by in situ scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). Starting from a smooth As-rich (2\ifmmode\times\else\texttimes\fi{}4) reconstructed GaAs surface the formation of the normal interface was studied at AlAs layer thicknesses ranging from 1 to 50 ML. With increasing AlAs thickness the reconstruction changes from (2\ifmmode\times\else\texttimes\fi{}4) to (2\ifmmode\times\else\texttimes\fi{}3) via a (1\ifmmode\times\else\texttimes\fi{}1) and a (1\ifmmode\times\else\texttimes\fi{}3) symmetry under the conditions used. For the inverted interface, the RHEED pattern changes immediately after the deposition of only 1 ML of GaAs on AlAs from a (2\ifmmode\times\else\texttimes\fi{}3) to a clear (2\ifmmode\times\else\texttimes\fi{}4) symmetry. The STM images of both interfaces show that the interface formation is accompanied by a disordering of the (2\ifmmode\times\else\texttimes\fi{}4) reconstruction by forming kinks in the missing dimer rows that can be related to intrinsic point defects. We attribute the different incorporation of these intrinsic point defects to the different electronic properties of the two interfaces. Large-scale STM images also reveal a different surface roughness for the normal and inverted interface. \textcopyright{} 1996 The American Physical Society.

In situ observation of indium segregation by reflectance difference spectroscopy in single monolayer heterostructures grown by atomic layer epitaxy

Single monolayers of InAs in GaAs and GaAs in InAs have been grown by atomic layer epitaxy (ALE) at 50 Torr. In situ reflectance difference spectroscopy monitoring of the surface during each stage of the growth showed a strong asymmetry in the surface behavior between the two systems. Following insertion of an InAs monolayer in GaAs, approximately 20 ML of GaAs are required to recover an In-free, As-stabilized GaAs surface at 390 °C. On the other hand, following the insertion of 1 ML of GaAs in InAs, the spectrum returns to a Ga-free, As-stabilized InAs surface after only 1 ML of InAs deposition. This behavior shows clear evidence of the presence of segregation caused by thermodynamic factors even at the very low growth temperatures used for ALE.

Role of Steps in GaAs Heteroepitaxial Growth on InAs(001) Surfaces

The role of steps during the initial stages of GaAs growth on InAs surfaces was investigated by scanning tunneling microscopy (STM). The surface structures of a nominally (001) InAs substrate and a misoriented (001) InAs substrate tilted by 1° towards the [111]B direction were observed by STM after a certain amount of GaAs deposition. In the case of a nominally (001) surface, a growth mode transition from two-dimensional (2D) to 3D island growth occurred when more than 0.75 ML GaAs was deposited. On the other hand, in the case of a vicinal surface, a growth mode transition did not occur when the same amount of GaAs was deposited onto the surface. In this case, GaAs-selective growth attached to the step edges and crack formation extending in the [11̄0] direction were observed in the STM images. These results indicate that the initial growth stages of GaAs heteroepitaxy on an InAs vicinal surface are different from those on a nominally (001) InAs surface due to the existence of steps.