InGaAs Strain-reducing Layer Research Articles

Impact of dislocations in InAs quantum dot with InGaAs strain-reducing layer structures on their optical properties

InAs quantum dots with InGaAs strain-reducing layer on GaAs(001) grown at three different temperatures were investigated from the aspect of both structural and optical properties. Dislocations originated from the InAs quantum dot (QD) layer were observed at growth temperatures of 490 °C, 500 °C, and 510 °C. Their densities are relatively larger in the cases of 490 °C and 510 °C, where they are caused by strain accumulation at larger-size InAs quantum dots during cover layer growth. Photoluminescence lifetimes at 6 K are almost the same in the three samples. On the other hand, that of the 500 °C-grown sample is an order of magnitude larger than the other two samples at 300 K. This indicates that dislocations act as a non-radiative center to deteriorate optical characteristics. Growth around 500 °C suppresses the growth of larger-size InAs QDs and reduces the InAs strain accumulation, which leads to the dislocation formation at the cover layer.

Short-wave infrared (SWIR) photodetection by InAs/GaAs quantum dot heterostructures grown on Ge (100) substrate without Migration Enhanced Epitaxy layer

Integration of III-V based optoelectronic devices with Silicon (Si) technology is the prime choice of researchers for the futuristic Si-photonics. Profusion research is going on for direct growth of III-V heterostructures on the Germanium (Ge) substrate. Also, monolithic integration of III-V based device structures on Si using Ge layer and Migration Enhanced Epitaxy (MEE) technique have been endeavored. In this study, InAs based quantum dot infrared photodetectors (QDIPs) are grown on Ge substrate without MEE layers. The GaAs/AlAs superlattice buffer (SLB) layers are used to suppress the defects and dislocations. Two different QDIPs with GaAs capped and In 0.15 Ga 0.85 As/GaAs capped InAs QD are grown and characterized using photoluminescence (PL), high-resolution X-Ray Diffraction (HRXRD), and Raman spectroscopy (RS). Red-shifted and narrow PL spectrum of the InGaAs/GaAs capped QDIP with respect to the GaAs capped QDIP attributes to formation of larger dots and homogeneous size distribution. Low strain and high activation energy in the InGaAs capped QDs signify better strain relaxation via the InGaAs strain reducing layer (SRL) and better carrier confinement. Interband-assisted near-infrared (NIR) photoresponse is obtained from both devices till room temperature. However, the InGaAs/GaAs capped QDIP shows shortwave infrared (SWIR) response, which promulgates the formation of QDs with better crystallinity due to the presence of InGaAs SRL. The room temperature (300 K) dark current density of the GaAs and InGaAs capped QDIP is 1.2 × 10 −1 A/cm 2 and 6.6 × 10 −3 A/cm 2 respectively with −2 V bias voltage. This work demonstrates the pathway to uncomplicated growth of high quality QDIP structures on Ge substrate. • A novel technique is used to grow defect free InAs QD structures on Ge substrate. • GaAs/AlAs SLB layers are used to suppress the defects and dislocations. • InAs QDs with GaAs and InGaAs/GaAs capping are investigated and compared. • Both structures show low dark current at room temperature. • Infrared photoresponse till room temperature is obtained for both QDIPs.

Tailoring the optical properties of InAs/GaAs quantum dots by means of GaAsSb, InGaAs and InGaAsSb strain reducing layers

Self-organized InAs QDs have been grown by Molecular Beam Epitaxy with different strain reducing layers (SRL) having nearly similar lattice mismatches to GaAs. The used SRLs are GaAsSb, InGaAs and InGaAsSb. Atomic Force Microscopy (AFM), transmission electron microscopy (TEM), high resolution X-ray diffraction (HRXRD), power dependent photoluminescence (PL) from low to room temperature have been employed for the characterization of the grown samples. Our results revealed that the emission wavelength reaches 1.32 μm when the InAs QD are either covered by InGaAs or GaAsSb with a PL spectrum broadening when using GaAsSb. However, the incorporation of only 1% of Sb in the InGaAs SRL extends further the wavelength to 1.37 μm without altering the PL spectrum. For more quantitative analysis of the observed results, the QDs size dependence on the SRL type has been estimated by tuning the theoretical transition energies, obtained by solving the single band effective mass Schrodinger equation for an ellipsoidal QD through changing the dot size to fit the experimental transition energies. Indeed, in addition to the QDs strain reduction, the type of alloy capping layer is found to seriously alter the QD size and aspect ratio and consequently the energy separation between the first and second excited state. However, from the temperature dependence of the PL, the thermionic emission activation energies were found to be close to separation between the ground and first excited state independently of the SRL type.

Carrier dynamics of strain-engineered InAs quantum dots with (In)GaAs surrounding material

The present study reports on the optical properties of epitaxially grown InAs quantum dots (QDs) inserted within an InGaAs strain-reducing layer (SRL). The critical energy states in such QD structures have been identified by combining photoluminescence (PL) and photoluminescence of excitation (PLE) measurements. Carrier lifetime is investigated by time-resolved photoluminescence (TRPL), allowing us to study the impact of the composition of the surrounding materials on the QD decay time. Results showed that covering the InAs QDs with, or embedding them within, an InGaAs SRL increases the carrier dynamics, while a shorter carrier lifetime has been observed when they are grown on top of an InGaAs SRL. Investigation of the dependence of carrier lifetime on temperature showed good stability of the decay time, deduced from the consequences of improved QD confinement. The findings suggest that embedding or capping the QDs with SRL exerts optimization of their room temperature optical properties.

GaAsSb/InAs/(In)GaAs type II quantum dots for solar cell applications

We focused on design of suitable underlying and covering layers of InAs/GaAs quantum dots (QDs) with the aim to increase the carrier extraction rate in the QD solar cell structures. Covering QDs by a GaAsSb strain reducing layer (SRL) with type II band alignment significantly improves photogenerated carrier extraction from InAs QDs. An additional thin InGaAs SRL below InAs QDs further enhances the extraction of photogenerated carriers. Properties of QD structures without any SRL, with GaAsSb covering SRL, and with combination of thin below-QDs InGaAs and GaAsSb covering SRLs are compared and the mechanism of carrier extraction is discussed. We showed that thin below-QDs InGaAs SRL together with increasing profile of antimony concentration in covering GaAsSb SRL can significantly improve the resulting properties of solar cell structures with InAs QDs.

Broadband Light Emission from Chirped Multiple InAs Quantum Dot Structure

Broadband light emission is obtained from a chirped multiple InAs/InGaAs/GaAs quantum dot (QD) structure. The thickness of the InGaAs strain-reducing layer (SRL) is used as the tuning parameter to adjust the light emission property of each QD layer in the chirped structure. It is shown from the photoluminescence (PL) measurement that the SRL thickness has a strong influence on the PL peak position, linewidth, and intensity. By constructing the chirped QD structure comprising five groups of QD layers with different SRL thicknesses, a broadband electroluminescence emission with the full width at half maximum of 202 nm is realized, indicating the feasibility of chirped multiple InAs QD layers on broadening the emission spectrum.

Ground-state energy trends in single and multilayered coupled InAs/GaAs quantum dots capped with InGaAs layers: Effects of InGaAs layer thickness and annealing temperature

Vertically coupled, multilayered InAs/GaAs quantum dots (QDs) covered with thin InGaAs strain-reducing layers (SRLs) are in demand for various technological applications. We investigated low temperature photoluminescence of single and multilayered structures in which the SRL thickness was varied. The SRL layer was responsible for high activation energies. Deviation of experimental data from the Varshni (1967) model, E(T)=E−∞T2/T+β, suggests that the InAs-layered QDs have properties different from those in bulk material. Anomalous ground-state peak linewidths (FWHM), especially for annealed multilayer structures, were observed. A ground-state peak blue-shift with a broadened linewidth was also observed. Loss of intensity was detected in samples annealed at 800°C. Presence of SRLs prevents formation of non-radiative centers under high temperature annealing. The results indicate the potential importance of such structures in optoelectronic applications.

The effect of InGaAs strain-reducing layer on the optical properties of InAs quantum dot chains grown on patterned GaAs(100)

We report the influence of InGaAs strain-reducing layers on the optical properties of quantum dot chains grown on groove patterns oriented along the [011], [01-1], and [010] directions. The site-controlled InAs quantum dot chains were grown by molecular beam epitaxy on GaAs(100) substrates patterned by nanoimprint lithography. The InGaAs capping causes a redshift of photoluminescence, which depends on the groove orientations. Based on the analysis of the surface morphology before and after capping, we attribute this to variation of composition and effective thickness of the InGaAs layer in grooves with different orientations. Furthermore, we analyze the effect of the InGaAs cap thickness on the in-plane polarization anisotropy of the photoluminescence emission and show that the [01-1]-oriented quantum dot chains experience a significant increase of polarization anisotropy with increasing InGaAs cap thickness. The increase of polarization anisotropy is attributed to enhanced interdot coupling due to a reduction of the barrier height and piezoelectronic potential.

Redshift and discrete energy level separation of self-assembled quantum dots induced by strain-reducing layer

We theoretically studied the role of a InGaAs and InAlAs strain-reducing layer (SRL) with different thicknesses and indium compositions covered on InAs/GaAs self-assembled quantum dots (QDs). The ground-state transition wavelength increases as the thickness and indium composition of the SRL increase. The energy separation between ground state and excited state can achieve the maximum by a proper design. The redshift is due to (1) the strain reducing in QD, (2) the potential barrier, and (3) effective mass reducing in SRL, but the latter two tend to cancel each other and the energy level separation is mainly determined by (2) and (3).

Broadly Tunable Grating-Coupled External Cavity Laser With Quantum-Dot Active Region

A broadly tunable and high-power grating-coupled external cavity laser with a tuning range of more than 200 nm and a ~ 200-mW maximum output power was realized, by utilizing a gain device with the chirped multiple quantum-dot (QD) active layers and bent waveguide structure. The chirped QD active medium, which consists of QD layers with InGaAs strain-reducing layers different in thickness, is beneficial to the broadening of the material gain spectrum. The bent waveguide structure and facet antireflection coating are both effective for the suppression of inner-cavity lasing under large injection current.

Influence of strain reducing layers on electroluminescence and photoluminescence of InAs/GaAs QD structures

We present a comparison of photo- (PL) and electro-luminescence (EL) spectra of quantum dot (QD) structures with different strain reducing layers (SRL). Simple QD structures without SRL have negligible difference between the PL and EL maxima, which are near 1250 nm. InGaAs and GaAsSb SRLs were used to shift the luminescence maximum towards telecommunication wavelengths at 1.3 or 1.55 μm. We have found that MOVPE prepared QD structures with SRL exhibit an EL maximum at a considerably shorter wavelength than the PL maximum measured on similar samples without doping in the absence of built-in electric field. A mechanism to explain this phenomenon is proposed for both types of SRLs. The GaAsSb SRL is more suitable for long wavelength EL due to the higher confinement potential of electrons compared to InGaAs SRL. EL maximum at 1300 nm and PL maximum at 1520 nm were achieved on InAs QD structures with GaAs 0.87Sb 0.13 SRL (type I heterojunction).

InGaAs and GaAsSb strain reducing layers covering InAs/GaAs quantum dots

We compare properties of InAs/GaAs quantum dots (QDs) covered by InGaAs or GaAsSb strain reducing layers (SRLs) prepared by metal organic vapor phase epitaxy. Stronger red shift of QD emission was achieved with InGaAs SRL as compared to GaAsSb one with similar strain in the structure. This can be caused by the increase of QD size during InGaAs SRL growth. The heterojunction between InAs QDs and GaAsSb SRL changes from type I to type II between 13% and 15% of Sb in the SRL. Important advantage of GaAsSb SRL can be the possibility to change the overlap of electron and hole wave functions and QD dipole moment orientation by the composition of GaAsSb. We have achieved the highest PL intensity suggesting best wave function overlap near 13% of Sb in the SRL. Band alignment, transition probability and transition energy were calculated to help the interpretation of achieved results.

Thermal Activation of Carriers in InGaAs/InAs/GaAs Quantum Dots

InAs self-assembled quantum-dot (QD) structures with overgrowth InGaAs strain-reducing layers (SRLs) grown by using two di erent growth modes with molecular-beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) were fabricated on GaAs substrates in a solid-source MBE system and their optical properties were investigated by characterizing the temperature-dependent photoluminescence (PL). The activation energies for thermal quenching were derived from tting the temperature-dependent PL data with the assumption of three thermal quenching processes, including escape of carriers from the SRL to the GaAs barrier. The highest activation energy for strong thermal quenching was shown to be comparable to the energy di erence between the PL peak of the ground state of the QDs and that of the SRL, indicating that the escape of electron-hole pairs from QDs to the SRL is probably the dominant process for quenching in the high-temperature region.

Computational Study on the In-Plane Symmetry of Electron Wavefunctions in Self-Assembled InAs/GaAs Quantum Dots

We made a computational study of electron wavefunction symmetry in two types of InAs/GaAs self-assembled quantum dot (QD): one is a normal domical and pyramidal Stranski-Krastanow(SK)-type QD but embedded in InGaAs strain-reducing layer, and the other is a columnar-shaped QD fabricated via direct perpendicular stacking of SK-type QDs. Our calculations based on the three-dimensional finite element method suggested that the in-plane symmetry of the electron wavefunction is superior to that of the crystallographic QD structure. The presence of an InGaAs strain-reducing layer helps to improve the symmetry of SK-type QD. The higher the indium composition of the strain-reducing layer, the greater the improvement. The improvement is greater with domical QD than with pyramidal QD. We also found that the multiple wetting layers around the columnar-shaped QD structure act as a strain-reducing layer, so that the improvement of symmetry is dependent on the average indium composition of the multiple wetting/interval layers. The symmetry improvement was explained by the wavefunction overflow outside the QDs. We found that, to achieve highly symmetric wavefunction, the strain-reducing-layer-embedded QDs are adequate more than the columnar-shaped QDs. These results will aid in the development of an entangled-photon generator that requires symmetric in-plane QD structure.

Optical studies on InAs/InGaAs/GaNAs strain-compensated quantum dots grown on GaAs (0 0 1) by molecular beam epitaxy

We have investigated the growth of 10 stacked layers of self-assembled InAs quantum dots (QDs) on GaAs (0 0 1) substrates, which were embedded in a combined set of a GaNAs strain-compensating layer (SCL) and an InGaAs strain-reducing layer (SRL). The internal compressive strain induced by each InAs QD layer was compensated by a tensile strain due to a GaNAs SCL. Consequently, the photoluminescence (PL) measured at 30 K showed a narrower linewidth of 60.3 meV for the sample with GaN 0.008As 0.992 SCL compared to 94.2 meV for InAs/GaAs uncontrolled sample due to an improved QD size uniformity. In order to reduce the nominal lattice mismatch at the heterointerface, we propose to insert an InGaAs SRL between the InAs QD layer and GaNAs SCL. We observed a PL peak redshift up to 1230 nm at room temperature, and increased PL intensity for InAs QD/InGaAs SRL/GaNAs SCL samples.

Suppression of the Polarization Dependence of the Vertical Photoluminescence from InAs/GaAs Quantum Dots by InGaAs Strain-Reducing Layer

We report the linear polarization features of the vertical photoluminescence from InAs/GaAs self-assembled quantum dots (QDs) embedded in an InGaAs strain-reducing layer. The QD samples were grown by molecular beam epitaxy. We observed the polarization dependence of the vertical photoluminescence intensity that is expected to originate in the in-plane asymmetry of the QD structure. The polarization dependence of the photoluminescence intensity of the QDs was suppressed by increasing the indium composition and the thickness of the strain-reducing layer. We computed the electron wavefunction based on the three-dimensional finite element method to explain the results of the experiments. We found that the symmetry of the wavefunction in the embedded QD is superior to that of the crystallographic QD structure, and that the improvement is attributed to the asymmetric permeation of the wavefunction into the strain-reducing layer. These results will aid in the development of vertical-light-emitting QD devices such as surface-emitting lasers and entangled-photon generators.

Self-assembled InAs island formation on GaAs (1 1 0) by metalorganic vapor phase epitaxy

Formation of self-assembled InAs 3D islands on GaAs (1 1 0) substrate by metal organic vapor phase epitaxy has been investigated. Relatively uniform InAs islands with an average areal density of 10 9 cm −2are formed at 400 ° C using a thin InGaAs strain reducing (SR) layer. No island formation is observed without the SR layer. Island growth on GaAs (1 1 0) is found to require a significantly lower growth temperature compared to the more conventional growth on GaAs (1 0 0) substrates. In addition, the island height is observed to depend only weakly on the growth temperature and to be almost independent of the V/III ratio and growth rate. Low-temperature photoluminescence at 1.22 eV is obtained from the overgrown islands.

Reduced temperature sensitivity of lasing wavelength in near-1.3 µm InAs/GaAs quantum-dot laser with stepped composition strain-reducing layer

An InGaAs strain-reducing capping layer with a stepped composition is shown to significantly reduce the temperature sensitivity of the lasing wavelength in a 1.3 µm InAs/GaAs quantum-dot laser. With this technique, the sensitivity is reduced from 0.48 nm/K for a laser with standard capping layer to 0.11 nm/K for the new design over the temperature range 20–130°C.

MBE growth of InAs self-assembled quantum dots embedded in GaNAs strain-compensating layers

We fabricated self-assembled InAs quantum dots (QDs) embedded with a combined set of a GaNAs strain-compensating layer (SCL) and an InGaAs strain-reducing layer (SRL) by using atomic hydrogen-assisted molecular beam epitaxy (H-MBE) with a RF plasma nitrogen source. By inserting an InGaAs SRL between the InAs QD layer and GaNAs SCL, we were able to achieve a significant improvement of optical properties. A 1.3 μ m -range photoluminescence (PL) emission was clearly obtained at 300 K from QDs embedded in a 5 nm-thick In 0.15Ga 0.85As SRL followed by a 20 nm-thick GaN 0.008As 0.992 SCL. Further, PL emission from the first excited state of QDs was also observed under a high excitation intensity of ∼ 20.0 W / cm 2 .

Enhanced thermal stability and emission intensity of InAs quantum dots covered by an InGaAsSb strain-reducing layer

An InGaAsSb overgrown layer, i.e., strain-reducing layer (SRL), is adopted to increase the emission intensity of InAs quantum dots (QDs) and extend the emission wavelength to as long as 1.42μm. InAs QDs capped with InGaAsSb SRL also exhibit a thermal activation energy of 534meV, which is much higher than that of InAs QDs with an InGaAs SRL. The increase in luminescence efficiency and thermal stability is attributed to the improved carrier confinement of the GaAs∕InAs∕InGaAsSb heterostructure.