GaAs Cap Layer Thickness Research Articles

Tracing the two- to three-dimensional transition in InAs/GaAs sub-monolayer quantum dot growth

A two- to three-dimensional (2D to 3D) transition in InAs/GaAs sub-monolayer quantum dot (SML-QD) growth was investigated, and its evolution was traced. From the AFM results, the SML-QDs disappeared gradually and plateaus around each island formed with increasing GaAs capping layer thickness (tCL) from 1.5 ML to 4.5 ML. The SML-QDs become fully covered with a GaAs capping layer, leading to a planarization process caused by strain relaxation. In addition, when the individual InAs coverage was is insufficient to cause a 3D transition, similar plateaus formed when stacking with an increasing number of stacking layers (S) from 2 to 6. On the other hand, as the InAs coverage was increased from 0.3 to 0.5 ML, 3D dots with a comparable size to typical SK-QDs were observed beyond S = 4. This means that as the total InAs deposition thickness increases above a critical thickness (~2.0 ML), 2D-small islands transform spontaneously into 3D-pyramid QDs. Such structures then show similar optical characteristics to SK-QDs. Overall, the transition from 2D to 3D in InAs/GaAs SML-QD growth could be controlled by careful choice of the parameters tcl, θ, and S.

Effect of capping procedure on quantum dot morphology: Implications on optical properties and efficiency of InAs/GaAs quantum dot solar cells

InAs/GaAs quantum dot solar cell structures have been grown by metal organic vapor phase epitaxy, using partial capping of the quantum dots plus a subsequent thermal anneal. The optical characteristics of the InAs quantum dot layers have been studied as a function of the GaAs capping layer thickness and annealing temperature. We observe that a thinner capping layer and a higher annealing temperature result in lower non-radiative defect density and in improved quantum dot size homogeneity, leading to intense and sharp photoluminescence emission at low temperatures. We use an effective mass approximation model to correlate the photoluminescence emission characteristics to the quantum dot composition and dimensions. The resulting InAs/GaAs intermediate band solar cells show the best performance for the case of a 3 nm thick capping layer and annealing at 700 °C.

Effects of thickness and V/III ratio of low temperature capping layer growth to the optical properties of InAs quantum dots

To achieve increased device performance, the authors systematically explored the dependence of the optical characteristics of InAs quantum dot (QD) ensembles grown via molecular beam epitaxy with different V/III ratios and thicknesses of the low temperature GaAs capping layers. In addition, the paper discusses the mechanism behind the dependence. Experiments showed the QD optical properties were significantly dependent on the GaAs capping layer V/III ratio and the initial GaAs capping layer thickness. An optimized V/III ratio and GaAs capping thickness is reported.

Photoreflectance and Raman Study of Surface Electric States on AlGaAs/GaAs Heterostructures

Photoreflectance (PR) and Raman are two very useful spectroscopy techniques that usually are used to know the surface electronic states in GaAs-based semiconductor devices. However, although they are exceptional tools there are few reports where both techniques were used in these kinds of devices. In this work, the surface electronic states on AlGaAs/GaAs heterostructures were studied in order to identify the effect of factors like laser penetration depth, cap layer thickness, and surface passivation over PR and Raman spectra. PR measurements were performed alternately with two lasers (532 nm and 375 nm wavelength) as the modulation sources in order to identify internal and surface features. The surface electric field calculated by PR analysis decreased whereas the GaAs cap layer thickness increased, in good agreement with a similar behavior observed in Raman measurements (IL-/ILOratio). When the heterostructures were treated by Si-flux, these techniques showed contrary behaviors. PR analysis revealed a diminution in the surface electric field due to a passivation process whereas theIL-/ILOratio did not present the same behavior because it was dominated by the depletion layers width (cap layer thickness) and the laser penetration depth.

Photoluminescence intensity enhancement in self-assembled InAs quantum dots grown on (3 1 1)B and (1 0 0) GaAs substrates and coated with gold nanoparticles

Photoluminescence intensity enhancement has been observed for self-assembled InAs quantum dots (QDs) coated with gold nanoparticles and associated to surface plasmon (SP) coupling effects. This effect was investigated for different temperatures, excitation energy, laser power, GaAs cap layer thickness, and QD morphology. It was found that the SP–QD coupling efficiency increases with the increase of the temperature. It was also observed that this effect is enhanced for QDs grown on (311)B GaAs oriented substrates.

Optical and electrical study of cap layer effect in QHE devices with double-2DEG

ABSTRACTIn this work we report on the characteristics of GaAs/AlGaAs heterostructures with a symmetric double two-dimensional electron gas (D-2DEG). Optical characterization was made by room temperature photoreflectance (PR) spectroscopy as well as electrical properties were determinated using the quantum Hall effect measurements at 2K. In order to study the surface effects on the conduction band profile, three samples with different GaAs cap layer thickness (25, 60 and 80 nm) were grown by the molecular beam epitaxy. Photoreflectance spectra at room temperature show the wide-period Franz-Keldysh oscillations between 1.42 and 1.70 eV originated by the surface electric field. The analysis of these oscillations shows that the surface electric field varies from 503 to 120 kV/cm whereas the thickness of the cap layer increases that was produced by the reduction of the depletion zone near the surface. Using QHE measurements we found that electron density increases if the surface electric field decreases.

InAs/GaAs quantum dot capping in kinetically limited MOVPE growth regime

InAs/GaAs quantum dot (QD) properties can be significantly influenced by the growth conditions of the QD capping layer. We have studied the effect of a group III partial pressure in the reactor on the QD capping process and on the QD photoluminescence when the capping layer is grown under the kinetically limited regime. Two types of capping layers were prepared: GaAs and InGaAs. The GaAs capping layer growth rate decrease did not influence QD dissolution, but increased the dissolution of big hillocks. Influence of the GaAs capping layer thickness on QD photoluminescence is also demonstrated. The composition of the ternary strain reducing InGaAs capping layer can be considerably changed depending on the V/III ratio under kinetically limited growth.

Influence of a thin GaAs cap layer on structural and optical properties of InAs quantum dots

In this letter we investigate the changes in the surface morphology and emission wavelength of InAs quantum dots (QDs) during initial GaAs encapsulation by atomic force microscopy and photoluminescence. The density (2.9×1010 cm−2) and height (7.9±0.4 nm) of the uncapped QDs decrease and saturate at 0.6×1010 cm−2 and 4 nm, respectively, after the deposition of 4 monolayers (MLs) of GaAs. A model for the evolution of surface morphology is proposed. Photoluminescence spectra of the surface dots show a wavelength shift from 1.58 to 1.22 μm when the GaAs capping layer thickness increases from 0 to 8 MLs.

Optimizing the growth procedure for InAs quantum dot stacks by optical in situ techniques

The influence of the GaAs cap layer thickness on the surface morphology and stoichiometry of InAs quantum dot (QD) layers grown on GaAs(0 0 1) was investigated by real-time reflectance anisotropy spectroscopy (RAS) and ellipsometry. A redistribution of InAs from partially covered islands to the surface of the GaAs cap layer was found. Introducing a well-defined growth interruption (GRI) during cap layer growth, a narrower size distribution of the InAs islands could be achieved and, moreover, large clusters and dislocations in the sample could be completely avoided. Transmission electron microscopical images of stacked QD layers substantiate the growth model of island redistribution by showing a fractional second wetting layer. Using this procedure of island redistribution dislocation free 5-fold InAs QD stacks were realized.

Confinement effects in strain-induced InGaAs/GaAs quantum dots

The effects of both lateral and vertical confinements in strain-induced InGaAs/GaAs quantum dots were investigated by varying the InP stressor size, the width and composition of the quantum well as well as the GaAs caplayer thickness. The energy-level spacing decreases with increasing the stressor diameter and very large stressor leads to a barrel-shaped lateral confining potential rather than a parabolic one. The hydrostatic strain along the central axis of the stressor decays exponentially with a characteristic depth of about 25 nm. The quantum-dot state energies can be tuned easily by the quantum-well width and composition. If the QW is thin or the indium composition is small, the quantum dot states may be leaky due to weak vertical confinement.

In 0.5 Ga 0.5 As quantum dot intermixing and evaporation in GaAs capping layer growth

Our study of GaAs growth over self-assembled In0.5Ga0.5As quantum dots grown by metalorganic vapor-phase epitaxy showed that GaAs capping layer surface morphology at the onset strongly depended on temperature. Incompletely capped In0.5Ga0.5As islands were elongated toward [110], indicating anisotropy in intermixing. During higher-temperature growth interruption, islands show craters in quantum dot centers. Craters become hexagonal holes whose depth matches GaAs capping layer thickness. Postannealing photoluminescence spectra show no peak corresponding to overly large quantum dot radiation, indicating that growth interruption after capping layer formation at a certain thickness eliminates overly large quantum dots.

ABNORMAL LASING BEHAVIORS IN THIN p-CLAD InGaAs QUANTUM WELL LASERS

Thin p-clad (250 nm) InGaAs quantum well (QW) lasers with 200 nm p +-GaAs cap layer thickness and various device configurations are studied. For wide stripe ( w=50 μm) lasers, abnormal lasing characteristics of very high thresholds, current on-time dependent lasing and micro-second long delays are observed. With 300 nm ridge-height, micro-second long lasing delays are found in w=6 μm lasers, and Q-switching in w=2.5 μm and 3.5 μm lasers, respectively. The unusual lasing behaviors are attributed to the decrease of transverse optical confinement caused by the increase of p +-GaAs cap layer thickness. To improve device performance and eliminate lasing delay, p +-GaAs cap layer thickness of wide stripe lasers and stripe width of narrow stripe devices have to be not greater than 170 nm and 1.5 μm, respectively, in thin p-clad (250 nm) laser structure.

Improved modulation contrast of asymmetric Fabry–Perot field-effect transistor self-electro-optic effect devices by in situ thickness corrections

We present an in situ optical method for thickness correction during the gas-source molecular beam epitaxial growth of asymmetric Fabry–Perot cavities. Separate growth corrections are made to center the spectrum of the mirror reflectivity, and to place the cavity resonance at the desired wavelength, all by measuring the reflectivity spectrum without breaking the vacuum. We choose to use an AlInP/InGaP quarter-wave mirror to demonstrate that the correction can be made after the growth of only three periods. By adding additional GaAs cap layer thickness and thereby adjust the positions of the Fabry–Perot minima of a AlGaAs/GaAs field-effect transistor self-electro-optic effect devices, improvement of modulation contrast from 3:1 to 9:1 is demonstrated.

Breaking up of misfit dislocations in GaAs/In0.3Ga0.7As/GaAs heterostructure

The effect of GaAs cap layer with different thicknesses in the GaAs/In0.3Ga0.7As/GaAs heterostructure on misfit dislocation is investigated with transmission electron microscopy, and it is found that lines of misfit dislocation break up and move out of the structure when the GaAs cap layer thickness exceeds a certain amount. The breaking up and moving out of misfit dislocations, initially confined in the (001) substrate/InGaAs epilayer interface, occur mainly along the [110] direction on the interface in the structure.

Absorption spectroscopy studies of strained InGaAs/GaAs single-quantum wells

Strained In0.20Ga0.80As/GaAs single-quantum well (SQW) structures with the GaAs capping layer thickness ranging from 5 to 500 nm have been studied directly by absorption spectroscopy. The absorption peaks are shifted to lower energy while the GaAs capping layer thickness decreases due to the strain relaxation in InGaAs/GaAs SQW structures induced by the GaAs capping layer. The best fit gives the conduction-band offset ratio Qc=0.70±0.05. The pronounced exciton peaks are observed in the absorption spectra at room temperature. The strength of the exciton–phonon coupling is determined from linewidth analysis and is found to be much larger than that of strained InGaAs/GaAs MQW structures.

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In0.20Ga0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.

Influence of cap layer thickness on optical quality in In0.2Ga0.8As/GaAs single quantum wells

Influences of GaAs cap layer thickness on residual strain in partially relaxed, 25-nm-thick In0.2Ga0.8As/GaAs single quantum wells have been investigated by photoluminescence and photoreflectance at 77 K. It was found that the residual strain increased and the optical quality improved with increasing cap layer thickness. Therefore, both quantum well and cap layer thicknesses determine the optical quality in lattice-mismatched semiconductor heterostructures.

Optical studies of strained InGaAs/GaAs single quantum wells

Photoluminescence and adsorption experiments on strained In 0.20Ga 0.80As/GaAs single quantum well (SQW) structures are reported as a function of the GaAs capping layer thickness which ranges from 5 to 500 nm. The luminescence peak is shifted to lower energy as the GaAs capping layer thickness decreases, which is the first demonstration of the effect of the GaAs capping layer thickness on the strain of InGaAs/GaAs single quantum wells. Excitonic transitions up to the recombination between the fourth electron subband and the fourth hole subband (4e-4hh), both allowed and forbidden, are observed in the absorption spectra. The strain of each sample has been deduced. The calculated results, taking into account both the strain and the quantum well effect, are in good agreement with the measured values. The minimum GaAs capping layer thickness for the growth of strained In 0.20Ga 0.80As(25 nm)/GaAs SQW structures is estimated to be 5–10 nm.

Strain recovery in partially relaxed In0.2Ga0.8As/GaAs single quantum wells with increasing GaAs cap layer thickness

The effect of cap layer thickness on residual strain in partially relaxed, 250 AA thick In0.2Ga0.8As/GaAs single quantum well structures was examined by photoluminescence, transmission electron microscopy and high-resolution x-ray diffraction. When the cap layer thickness increased from 50 to 5000 AA, the residual strain increased from 0.88% to the misfit value of 1.4%. As a consequence the cap layer thickness should be taken as a parameter, along with misfit and growth temperature, to determine critical layer thickness in lattice-mismatched single quantum well structures.

Reduction of emitter thickness in AlGaAs/GaAs heterojunction bipolar transistor

The effects of AlGaAs layer thickness and GaAs cap layer thickness in the emitter of abrupt npn AlGaAs/GaAs heterojunction bipolar transistors on the performance of devices were investigated experimentally. It was found that only 500 Å of A10.25Ga0.75As layer in the emitter is enough to produce high injection efficiency and a small signal current gain of more than 1100