GaAs Spacer Thickness Research Articles

Vertical spin transport in MnAs/GaMnAs heterostructures

We have studied the effect of barrier strength on the tunneling magnetoresistance of MnAs/GaMnAs heterostructures with double AlAs barriers. The epitaxial structures were grown by low-temperature molecular beam epitaxy. The vertical magnetotransport properties were studied for various GaAs spacer thicknesses. We find that the junction resistivities of double barrier samples increase exponentially as the barrier strength increases, implying that direct tunneling process governs the transport properties. In contrast, the tunneling magnetoresistance depends primarily on the number of interfaces rather than on the barrier strength.

Tailoring of high-temperature photoluminescence in InAs∕GaAs bilayer quantum dot structures

Temperature-dependent photoluminescence is investigated in bilayer InAs∕GaAs quantum dot structures with constant InAs deposition θ1 in the seed layer, but variable deposition θ2 in both the second layer and the GaAs spacer layer. It is shown that interlayer coupling, leading to the formation of asymmetric quantum dot pairs, strengthens the high-temperature photoluminescence and strongly influences carrier relaxation channels. We report that radiative recombination and carrier capture efficiency by the quantum dots in the second layer can be tailored using the deposition θ2 and the GaAs spacer thickness.

Chemical profile and magnetoresistance ofGa1−xMnxAs/GaAs/AlAs/GaAs/Ga1−xMnxAstunnel junctions

We have investigated the manganese diffusion depth and the tunneling magnetoresistance (TMR) properties in ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$/GaAs/AlAs/GaAs/${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ tunnel junctions. Auger electron spectroscopy and transmission electron microscopy analysis show that the Mn diffusion depth is less than 15 \AA{}. TMR measurements have been performed on tunnel junctions where different GaAs spacer thicknesses are inserted between the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ electrode and AlAs tunnel barrier. Our results suggest that the GaAs thickness plays a crucial role on the temperature dependence of the TMR.

Spectroscopic investigations of GaAsSb/GaAs based structures for 1.3 μm VCSEL applications

A spectroscopic investigation of GaAsSb/GaAs quantum well (QW) structures grown on GaAs substrates by molecular beam epitaxy is presented. Besides studying their temperature-dependent photoluminescence (PL), the low temperature PL of a series of GaAsSb/GaAs/AlGaAs structures, with varying GaAs spacer thickness is investigated. This latter study was undertaken to investigate the GaAsSb/GaAs band alignment. Blue shifts in the PL peak as a function of excitation laser intensity at a temperature of 10 K for the range of spacer thickness variation are studied. It is observed that significant blue shifting occurs only for spacers thicker than /spl sim/2 nm. It is tentatively suggested that this is indicative of a transition from the electrons and holes being predominantly confined in the same layer (the QW) to being more strongly confined in adjacent layers, as the spacer thickness increases. The angle-dependent photomodulated reflectivity (PR) of similar samples is investigated. Here strong low energy interference oscillations (LEIO) are encountered, which tend to obscure any PR signals arising from the QW. The latter are exploited to estimate the refractive index of the layer responsible for the LEIO, and thus identify it. However, a way to avoid the LEIO is shown, by shortening the laser excitation wavelength, which results in measurable PR signals from the QW region, yielding several QW transition wavelengths.

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.

Mn diffusion in Ga1−xMnxAs∕GaAs superlattices

Ga 1 − x Mn x As ∕ GaAs superlattices with Mn concentrations of 1% and 5% in the Ga1−xMnxAs layers and a GaAs spacer thickness of 4 and 60 GaAs monolayers have been studied by cross-sectional scanning tunneling microscopy. By achieving atomic resolution of the superlattices, we observe individual Mn atoms in the Ga1−xMnxAs layers and in the GaAs spacer. We find that about 20% of the total amount of Mn diffuses from the GaMnAs layers into the GaAs spacer layers. Our results can be related to previous measurements of the magnetic properties of short period Ga1−xMnxAs∕GaAs superlattices.

Analytical investigation of dead space effect under near-breakdown conditions in GaInP/GaAs composite double heterojunction bipolar transistors

The dead space effect under near-breakdown conditions in GaInP/GaAs composite collector double heterojunction bipolar transistor (DHBT) is investigated analytically. Using the dead space corrected model, the breakdown voltage is found to decrease with GaAs spacer thickness as reported from experiments. The common-mode emitter IV characteristics for the DHBT are simulated until the onset of multiplication with good agreement with reported experimental results [IEEE Elec. Dev. Lett. 15 (1994) 10]. A proposed optimised structure is simulated with comparably good turn-on I– V characteristics and improved breakdown performance.

Hall effect in InAs/GaAs superlattices with quantum dots: identifying the presence of deep level defects

We have carried out van der Pauw resistivity and Hall effect measurements on a series of Molecular Beam Epitaxy InAs/GaAs superlattice samples containing InAs quantum dots. Three growth parameters were varied, the InAs coverage, the number of repetitions of the InAs/GaAs layers, and the GaAs spacer thickness. The results can be grouped in two sets, those samples presenting low and high resistivity. The group presenting low resistivity is composed by the samples with GaAs spacer of 30 monolayers (ML) and InAs coverage of 1.9 monolayers. The group presenting high resistivity is composed of samples with GaAs spacer of 40 ML. We claim that the high resistivity characteristic is due to the presence of deep level. Increasing the spacer from 30 to 40 ML decouples the InAs planes favouring the deep level formation.

Study of InGaP/InGaAs double delta-doped channel heterostructure field-effect transistors

The device properties of InGaP/InGaAs double delta-doped channel heterostructure field-effect transistors (DDDCHFETs) are comprehensively analyzed and demonstrated. Based on the variations of delta-doped densities of InGaAs double channels and GaAs spacer thickness, the dc and rf characteristics are compared and studied. Due to the employed InGaAs DDDC structure and Schottky behaviors of InGaP “insulator,” good pinch-off and saturation characteristics, higher and linear transconductance, and good rf performances are obtained. Experimentally, for comparison, a practical DDDCHFET with good device properties is fabricated successfully. It is known that the experimental results are very consistent with theoretical simulation data.

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E 1–HH 1) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.

Dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/GaAs quantum dots

Temperature-dependent photoluminescence (PL) measurements have been carried out to investigate the dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/GaAs quantum dots (QDs). The PL spectra showed that the peak corresponding to the interband transitions form the ground electronic subband to the ground heavy-hole band (E 0-HH 1) of the InAs QDs shifted to a higher energy side with increasing the GaAs spacer thickness and that the full width at maximum (FWHM) of the (E 0-HH 1) peak rapidly decreased with increasing a spacer thickness. The position energy and the FWHM of the (E 0-HH 1) peak for the InAs/GaAs QDs at several temperatures were observed. The present observations can help improve understanding of the dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/GaAs QDs.

Formation and stacked layer of quantum dots

Three-dimensional formation occurring at the initial stage of the heteroepitaxial growth of a lattice-mismatched system is reviewed. It is experimentally proved that the formation of InAs islands on the GaAs substrate is due to the Stranski–Krastanov (S–K) growth mode. This formation has been proposed to realize a mesoscopic island structure which can be utilized as a quantum dot (QD). They are naturally formed with the S–K growth mode. They have been called naturally- or self-formed QDs. The strain energy of the QD is calculated with the valence-force field model, and then the possible formation of dislocation-free QDs is described. The experimental results of InAs/GaAs and InP/GaP QDs are presented. In the stacked layer of InAs/GaAs QDs, the GaAs spacer thickness for the vertical ordering of QDs is investigated. The ordering, dependent on the spacer thickness, is found by the transmission electron microscope images of cross-sectional stacked layers. The QD can be neither stacked nor vertically ordered when the thickness is greater than 2 h where h denotes the QD height. The QDs are crushed when the spacer is thinner than h. The vertically ordered QDs are stacked when the GaAs spacer thickness is between h and 2 h.

Optical properties of wetting layers in stacked InAs/GaAs quantum dot structures

The optical transitions of the wetting layers in two-fold self-assembled InAs/GaAs quantum dot samples are studied as a function of GaAs spacer thickness by various methods. The absorption related studies by photoreflectance and selective photoluminescence excitation spectroscopy reveal already for thick barriers, for which coupling effects can be excluded, two energetically separated heavy-hole transitions. This splitting indicates the formation of two wetting layers during growth with a 10% difference in width and reflects strain field interaction between the island layers. Thin spacer layer samples show in addition the expected wetting layer coupling as confirmed by subband calculations.

Photoluminescence characterization of InAs/GaAs quantum dot bilayers

We have investigated the emission from InAs/GaAs quantum dots (QD) bilayer samples with different GaAs spacer thickness. For large spacers, one peak is observed, and the layers are independent. For small spacers, one peak is observed and the layers are then electronically coupled. For intermediate spacers (≈120 Å), two emission peaks are observed and these can be made coincident by tuning the amount of InAs deposited in the second layer. We present a technique using two different excitation wavelengths, which enables us to attribute the emission to each layer, and to show that the shifts are not due to electronic coupling. Moreover, resonant excitation shows that the wetting layers are also different in each layer. These results indicate that the presence of the first QD layer strongly influences the growth of the second one, leading to very different properties.

Inhomogeneous broadening in quantum dots with ternary aluminum alloys

We study how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when using AlGaAs barriers to increase the carrier confinement. The inhomogeneous broadening of the QD ensemble is found to increase when ternary aluminum alloys are used next to or within the QDs. By growing thin GaAs spacers to separate the QDs from the AlGaAs barriers, we obtain QD ensembles which exhibit little photoluminescence quenching and well-defined excited states up to room temperature. Postgrowth rapid thermal annealing is then used to intermix these InAs/GaAs/AlGaAs QDs and diffuse the Al towards the QDs. In contrast with QDs having thick binary GaAs barriers, the inhomogeneous broadening of QDs with nearby AlGaAs barriers is not decreased with intermixing, leading to unresolved excited state peaks when the interdiffusion length becomes comparable to the GaAs spacer thickness.

Effects of Spacer Thickness on the Performance of InGaAs/GaAs Quantum Dot Lasers

ABSTRACTIt is found that the performance of self-assembled In0.5Ga0.5As/GaAs multi-stack quantum dot lasers is sensitive to the GaAs spacer thickness between the dots. Reducing the spacer thickness from 30 nm to 10 nm leads to narrow photoluminescence linewidth, low threshold current, high characteristic temperature and high internal quantum efficiency. This behavior is attributed to inhomogeneous broadening caused by dot size fluctuation related to spacer thickness.

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.

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.

Optical properties of vertically aligned self-assembled InGaAs quantum dot layers on (311)A/B and (100) GaAs substrates

In this work, we present substrate orientation effects on optical properties in vertically stacked In0.5Ga0.5As layers grown by molecular beam epitaxy on (311)A/B and reference (100) GaAs substrates. Samples were grown for different GaAs spacer thicknesses. The spacer thickness variation shows the influence on PL spectra for all planes. The differences in peak shape, peak position, amplitude and integrated luminescence have been observed for all surfaces. These differences suggest that indium migration in spacer layers is caused by the strain fields induced by islands buried below, and is different at the three surfaces. Vertical electronic coupling between quantum dots is confirmed by photoluminescence temperature dependence.