GaAsSb Layer Research Articles

Carrier-mediated optomechanical coupling in GaAs cantilevers

We have investigated optomechanical coupling in $n$-GaAs/$i$-GaAs bilayer cantilevers induced by optical band-gap excitation. The strain-assisted optopiezoelectric effect, which is associated with the separation of electron-hole pairs due to the built-in electric field, causes a time-delayed backaction force and influences the thermal vibration of the cantilevers. Vibration of the [110]-oriented cantilever is amplified by this optopiezoelectric backaction and the self-oscillation is induced for the strong excitation. In contrast, for the [$\overline{1}$10]-oriented cantilever, the optopiezoelectric backaction deamplifies the vibration because the direction of the piezoelectric stress is reversed. We have experimentally extracted the response time of this optopiezoelectric backaction, where the delay is on the order of the nonradiative recombination lifetime in GaAs. This optopiezoelectric backaction is maximized when the laser wavelength matches the optical absorption edge. This is because the strain-induced change in the optical absorption is maximized at the absorption edge and therefore a large force gradient results.

Growth of GaAsBi alloy under alternated bismuth flows by metalorganic vapor phase epitaxy

A successful method to epitaxy GaAsBi layer on (0 0 1) GaAs substrate is proposed. During growth, alternated trimethyl bismuth (TMBi) flows were used. These TMBi flashes were switched on for a short time. The growth was monitored in situ by laser reflectometry using a 632.8 nm beam. The reflectance signal is found to change significantly during both bismuth flashes and GaAs growth stages. High-resolution X-ray diffraction (HRXRD), secondary ion mass spectroscopy (SIMS) and photoreflectance spectroscopy (PR) have been used to characterize the obtained GaAsBi layer. HRXRD curve shows a diffraction peak that can be attributed to a GaAsBi epilayer. SIMS measurements of GaAsBi layer suggest that bismuth diffuses faster near the interface. The PR spectrum indicates the band-to-band transition in GaAsBi layer. The band gap energy was determined by adjusting the PR spectrum with a multilayer model.

Shape control of quantum dots studied by cross-sectional scanning tunneling microscopy

In this cross-sectional scanning tunneling microscopy study we investigated various techniques to control the shape of self-assembled quantum dots (QDs) and wetting layers (WLs). The result shows that application of an indium flush during the growth of strained InGaAs/GaAs QD layers results in flattened QDs and a reduced WL. The height of the QDs and WLs could be controlled by varying the thickness of the first capping layer. Concerning the technique of antimony capping we show that the surfactant properties of Sb result in the preservation of the shape of strained InAs/InP QDs during overgrowth. This could be achieved by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick, and that minor intermixing of Al with the QDs takes place.

Shape Control of QDs Studied by Cross-sectional Scanning Tunneling Microscopy

In this cross-sectional scanning tunneling microscopy study we investigated various techniques to control the shape of self-assembled quantum dots (QDs) and wetting layers (WLs). The result shows that application of an indium flush during the growth of strained InGaAs/GaAs QD layers results in flattened QDs and a reduced WL. The height of the QDs and WLs could be controlled by varying the thickness of the first capping layer. Concerning the technique of antimony capping we show that the surfactant properties of Sb result in the preservation of the shape of strained InAs/InP QDs during overgrowth. This could be achieved by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick, and that minor intermixing of Al with the QDs takes place.

An atomic scale study on the effect of Sb during capping of MBE grown III–V semiconductor QDs

The use of Sb during the capping process of quantum dots (QDs) to tune the emission wavelength has been investigated by means of cross-sectional scanning tunneling microscopy (X-STM), photoluminescence (PL) measurements and atomic force microscopy. It is found that a capping layer of GaAsSb reduces InAs/GaAs QD decomposition during capping due to the smaller lattice mismatch with the QD and due to the surfactant effect of Sb. An Sb content of 14% in the GaAsSb layer has been found to be sufficient to eliminate the QD decomposition completely and improve the PL properties. Further increase in the Sb content causes a degradation of the PL properties. Reduction of the QD decomposition has also been achieved by soaking the QDs with Sb before the capping. Furthermore, it is shown that the segregation of In and of Sb in the Ga and the As sublattices respectively are two independent processes. Similar investigations to the use of Sb to tune InAs/InP (311B) QDs are reported. Also in this case, the use of Sb, either in the form of elemental Sb or as a GaAsSb alloy, has resulted in elimination of QD decomposition.

Strain in GaAsSb quantum well studied by X-ray diffraction and Rutherford backscattering/channeling

The strain state of a GaAs 1− x Sb x /GaAs single quantum well (SQW) was studied using high resolution X-ray diffraction (HR-XRD) and Rutherford backscattering/channeling (RBS/C). The results reveal that the GaAsSb quantum well has good crystalline quality and the Sb content x of the GaAsSb layer is 0.36. The 7.3 nm GaAs 0.64Sb 0.36 layer is highly strained with a perpendicular elastic strain of 2.0% and a degree of relaxation of 0.23. Photoluminescence (PL) measurement at room temperature presents a peak at 0.995 eV (1.224 μm) with a linewidth of 67 meV, revealing the optical properties of the GaAsSb SQW.

Remote Dipolar Interactions for Objective Density Calibration and Flow Control of Excitonic Fluids

In this Letter we suggest a method to observe remote interactions of spatially separated dipolar quantum fluids, and in particular, of dipolar excitons in GaAs bilayer based devices. The method utilizes the static electric dipole moment of trapped dipolar fluids to induce a local potential change on spatially separated test dipoles. We show that such an interaction can be used for model-independent, objective fluid density measurements, an outstanding problem in this field of research, as well as for interfluid exciton flow control and trapping. For a demonstration of the effects on realistic devices, we use a full two-dimensional hydrodynamical model.

Use of Sb spray for improved performance of InAs/GaAs quantum dots for novel photovoltaic structures

Photoluminescence output from InAs/GaAs quantum dots has been improved by a Sb treatment immediately prior to capping with GaAs. Spectra taken at 300 and 80 K show a significant increase in output intensity when the quantum dots are exposed for 15 s under a Sb flux of approximately 0.1 monolayers per second, but this improvement is lost when the Sb exposure is extended to 30 s. There is no significant shift in the emission energies between samples indicating strain relief due to the cap layer is not responsible for the improvement. Analysis of temperature dependent photoluminescence taken between 80 and 300 K show increased activation energies at lower temperatures when an Sb spray is used, suggesting passivation of deep defect levels. For the higher temperature activation energy, corresponding to carrier escape from the QD to the barrier, whilst a 15 s Sb spray gives a substantial increase, the longer 30 s Sb spray sees the activation energy decrease, a result deduced to be due to Sb segregation providing shallow defect levels. A band structure including a very thin GaAsSb layer adjacent to the quantum dots is used to explain these results, with the 30 s Sb spray leading to shallow Sb segregation related defects and a lower activation energy. Depth dependent X-ray photoelectron spectroscopy data support the band structure proposed to explain the photoluminescence results and also reveals the highest concentration of Sb at the sample surface suggesting a ‘floating layer’ of Sb during growth of the GaAs cap. Some of the implications of these results, for growth of quantum dot samples and for two novel solar cell proposals, the intermediate band and hot carrier solar cells, are discussed.

The antiguiding parameter in mid-infrared optically pumped semiconductor lasers

We describe measurements of the antiguiding parameter, α, for several optically pumped semiconductor lasers. Three laser structures were investigated; two of the lasers utilize W-quantum wells (QWs) in which 14 InAs/In0.4 GaSb/InAs QWs are imbedded in lattice-matched In0.25 GaAsSb layers. The emission wavelengths of the W lasers were ∼3.5 and 4.5 μm, respectively. The other laser, a double heterostructure (DH) design, contained a ∼1.5 μm InAsSb active region embedded in ∼2.5 μm thick AlAsSb clad regions. The emission wavelength of the DH was λ∼3.8 μm. We employed the Hakki–Paoli method [B. W. Hakki and T. L. Paoli, J. Appl. Phys., 44, 4113, (1973)] in conjunction with a Fourier transform infrared spectrometer to measure subthreshold gain and index variations as a function of pump intensity. To reduce errors associated with incoherent background emission a full spectral curve fit was used to determine the differential gain and index. The results reveal the antiguiding factor in the W lasers to be low with α∼1.0. The antiguiding factor for the DH was markedly larger with α=9.4±1.3. We attribute the low α for the W lasers to the higher QW gain as well as to inhomogeneous broadening induced by the 14 QWs. The differing well widths and the independent optical pumping of the wells, leads to a net gain spectrum that is symmetrical about the gain peak. This symmetry, in turn, leads to small differential index shifts at the gain peak; the result of the small differential index and large differential gain is low antiguiding

High optical property vertically aligned InAs quantum dot structures with GaAsSb overgrown layers

This study investigates the feasibility of growing high quality columnar InAs/GaAsSb quantum dots (QDs) on a GaAs (100) substrate using a molecular beam epitaxial system. Structural and photoluminescence (PL) studies are conducted on vertically aligned, ten-period InAs quantum dot (QD) stacks with two different overgrown layer designs. Experimental results indicate an increased dot density of 5×1010cm−2 with completely suppressed coalescences in vertically aligned InAs quantum dots capped by a GaAsSb layer. This finding demonstrates a columnar dot structure with an enhanced luminescent intensity, activation energy, and narrow spectral line width of 22meV.

Achievement of InSb Quantum Dots on InP(100) Substrates

The formation of InSb quantum dots within a GaAs0.51Sb0.49 matrix lattice matched to InP is investigated. We show that the deposit of InSb on GaAs0.51Sb0.49 alloy surface, allows the achievement of a high density of InSb islands without dislocations. A strong dissolution of InSb quantum dots occurs during the capping with a GaAsSb layer. Reflection high energy electron diffraction analysis shows that InSb island dissolution occurs during the growth interruption under As and Sb. We propose a procedure based on the deposit of a thin GaSb capping layer on top of InSb islands to prevent the As/Sb exchange. Optical properties are investigated using photoluminescence. Electronic properties are discussed within an improved tight-binding model.

Photoreflectance and photoluminescence study of annealing effects on GaAsBi layers grown by metalorganic vapor phase epitaxy

The optical properties of GaAsBi layers grown by atmospheric pressure metalorganic vapor phase epitaxy on p-type GaAs substrates and annealed at different temperatures are investigated by photoreflectance and photoluminescence spectroscopies. Photoreflectance spectra show no significant shift in the band gap energy of GaAs0.965Bi0.035 with annealing temperature except 600 °C for which the band gap energy reaches a minimum value corresponding to a red shift of 60 meV. The low temperature photoluminescence spectra of GaAsBi layers show a broad band centered at ∼1.36 eV. The temperature dependence of the 1.36 eV emission band in addition to the increasing intensity of this band with bismuth flow suggests that it is originated from Bi clusters or from complex defects probably located at the surface/interface of the GaAsBi epilayer.

Effect of annealing on the structural and optical properties of (3 1 1)B GaAsBi layers

The influence of post-growth annealing on the microstructure and photoluminescence (PL) of GaAsBi alloys grown on (3 1 1)B GaAs is analyzed. Conventional transmission electron microscopy (TEM) performed on as-grown samples evidence the presence of structural defects and a mosaic structure in the GaAsBi layer. A sequence of stacking faults at regions close to the GaAs/GaAsBi interface are observed in high resolution TEM images. After annealing at 473 K during 3 h the mosaic structure disappears, the presence of defects is reduced and the PL peak intensely enhances.

Growth and characterization of strain-compensated InGaAs/GaAsSb type II multiple quantum wells on InP substrate

Strain-compensated InGaAs/GaAsSb type II multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) and their optical and electrical properties were studied. High-quality strain-compensated type II MQWs were successfully grown, which have longer emission wavelength than that of lattice-matched type II MQWs. PL peak energy at 300 K of the strain-compensated type II MQWs, where the InGaAs layer has 0.6% tensile strain and GaAsSb layer has 0.6% compressive strain, shows a red-shift of 43 meV, which is 12 meV larger than the calculated energy shift of 31 meV. In addition, the PL intensity and the electron mobility of the strain-compensated MQWs are comparable to those of the lattice-matched MQWs, suggesting that the crystal quality of the strain-compensated MQWs is good and are very promising for low dark current photodiodes in the 2 μm wavelength region.

Time‐resolved photoluminescence of type‐II InAs/GaAs quantum dots covered by a thin GaAs1–xSbx layer

AbstractWe investigate carrier lifetimes of InAs/GaAs quantum dots (QDs) covered by a thin GaAs1–xSbx layer by time‐resolved photoluminescence (PL). Both the power dependent PL peak shift and the longer decay time confirm the type‐II band alignments. Different recombination paths have been identified by temperature dependent measurements. At low temperatures, the long‐range recombination with holes trapped in the GaAsSb layer is significant, resulting in non‐single‐exponential decays. The short‐range recombination with holes confined in the band‐bending region surrounding the InAs QDs is important at higher temperatures. The variation in decay time across the ground‐state and the temporal PL peak redshift further confirm the localization of holes in the GaAsSb layer. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots

We report the effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb layer. Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent photoluminescence measurements. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type-II QDs.

Giant frictional drag in strongly interacting bilayers near filling factor one

We study the frictional drag in high mobility, strongly interacting GaAs bilayer hole systems in the vicinity of the filling factor $\nu=1$ quantum Hall state (QHS), at the same fillings where the bilayer resistivity displays a reentrant insulating phase. Our measurements reveal a very large longitudinal drag resistivity ($\rho^{D}_{xx}$) in this regime, exceeding 15 k$\Omega/\Box$ at filling factor $\nu=1.15$. $\rho^{D}_{xx}$ shows a weak temperature dependence and appears to saturate at a finite, large value at the lowest temperatures. Our observations are consistent with theoretical models positing a phase separation, e.g. puddles of $\nu=1$ QHS embedded in a different state, when the system makes a transition from the coherent $\nu=1$ QHS to the weakly coupled $\nu=2$ QHS.

Surface Fermi level in GaAsSb structures grown by molecular beam epitaxy on InP substrates

Photoreflectance (PR) spectroscopy is performed to investigate the Fermi level pinning at the surface of GaAsSb, in a series of epitaxial structures with different Sb concentration. PR spectra exhibit Franz–Keldysh oscillations, originating from the built-in electric field in the GaAsSb layer. Experimental results indicate that the surface Fermi level is pinned in the lower half bandgap. The surface Fermi level is determined versus the Sb concentration between 38% and 52%.

Carrier dynamics of type-II InAs∕GaAs quantum dots covered by a thin GaAs1−xSbx layer

Carrier dynamics of InAs∕GaAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer were investigated by time-resolved photoluminescence (PL). Both the power dependence of PL peak shift and the long decay time constants confirm the type-II band alignment at the GaAsSb–InAs interface. Different recombination paths have been clarified by temperature dependent measurements. At lower temperatures, the long-range recombination between the QD electrons and the holes trapped by localized states in the GaAsSb layer is important, resulting in a non-single-exponential decay. At higher temperatures, optical transitions are dominated by the short-range recombination with the holes confined to the band-bending region surrounding the QDs.

Carrier lifetimes in type-II InAs quantum dots capped with a GaAsSb strain reducing layer

Carrier lifetimes have been measured for long-wavelength emitting InAs quantum dots (QDs) capped with a thin GaAsSb layer. Above a critical Sb composition, a type-II system is formed, resulting in an increase in the carrier lifetime. The carrier lifetime in a strongly type-II structure is increased by a factor ∼54 in comparison to the lifetime in a type-I structure. In addition, the type-II carrier lifetime varies across the inhomogeneously broadened ground-state emission, while the type-I QD lifetime is invariant.