GaAsSb Strain Reducing Layer Research Articles

Evaluation of different capping strategies in the InAs/GaAs QD system: Composition, size and QD density features

In this work, two different strategies to preserve InAs/GaAs QDs against decomposition during the capping process have been compared structurally and optically. They are based on: (i) the control of the growth parameters of the capping layer (CL), such as growth rate, and (ii) the nature of the CL, such as the use of GaAsSb strain-reducing layers (SRL). For this, we have statistically determined the average size, composition and areal density in sample populations of dozens of QDs as well as the wetting layer features. Faster growth rates and the presence of Sb in the CL results in an increase of both average QD height as well as the In content but the thorough comparison among all the samples allowed us to draw the following important inferences. i) QD volume is a better parameter than QD height to assess QD decomposition and PL redshift; ii) higher QD volumes are not always linked to higher In contents and iii) a reduction in the QD density is observed in both strategies related to the increase of the average volume. The results are discussed in terms of the mechanisms that might operate during the capping process, such as surface coverage, intermixing or segregation.

Influence of the excitation density and temperature on the optical properties of type I InAs/GaAsSb quantum dots

In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs1−xSbx (x = 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing excitation density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs- capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the excitation power density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.

Tuning the optical properties of InAs QDs by means of digitally-alloyed GaAsSb strain reducing layers

In this work, we demonstrate the use of a digitally alloyed GaAsSb as a strain reducing layer (SRL) to tune the optical properties of InAs quantum dots grown on GaAs substrates. The GaAsSb digital alloy was made using a short period GaAs/GaSb superlattice. Increasing the Sb-shutter duty cycle (Sb-SDC) from 0 to 0.35 redshifts the emission wavelength from 1220 nm to 1492 nm at 300 K. The wavelength red shift is attributed mainly to the increased Sb content in the GaAsSb SRL as confirmed by high resolution X-ray diffraction. Power dependent photoluminescence showed a type II band alignment for the samples grown with a Sb-SDC of 0.25 and 0.35. Reversing the growth order of GaAs and GaSb layers and maintaining the same Sb-SDC (0.25) drastically alters the optical properties of the QDs. A large wavelength blue shift of 143 nm and a transformation from a type II to type I emission were observed as a result of GaSb intermixing with InAs QDs.

GaAsSb-capped InAs QD type-II solar cell structures — improvement by composition profiling of layers surrounding QD

Type-II band alignment offers several advantages for proposed intermediate band solar cell structures. We focused on the quantum dot (QD) solar cell structures based on type-II InAs/GaAs QD layers capped with GaAsSb strain reducing layers. The GaAsSb strain reducing layers were prepared with or without graded Sb concentration. Strong enhancement of photocurrent was achieved by adding an InGaAs buffer layer under the type-II QD structure, thanks to improved electron extraction from QDs. For comparison, a structure with GaAs-capped InAs QDs was prepared, too. Properties of all structures are compared and the mechanism of carrier extraction or relaxation is discussed. Gradient of antimony concentration in a strain reducing layer (SRL) significantly improved resulting properties of solar cell structures. It is shown that in a multiple-QD structure with a GaAsSb SRL, electrons and holes have non-intersecting trajectories which prevents carrier recombination and improves the efficiency of solar cell structures. NextNano band structure calculations of different types of structures support our experimental results.

Comparison of MOVPE grown GaAs, InGaAs and GaAsSb covering layers for different InAs/GaAs quantum dot applications

InAs/GaAs quantum dot (QD) heterostructures with different covering layers (CLs) prepared by MOVPE are compared in this work. The recombination energy of a structure covered only by GaAs depends nonlinearly on CL thickness. Experimental data of photoluminescence (PL) were supported by theoretical simulations. These simulations prove that the strain plays a major role in the structures. InGaAs strain reducing layer (SRL) was studied as well. Due to the strain reduction, the recombination energy is decreased, so the structure has longer PL wavelength. By theoretical simulations it was shown that for high content of In in InGaAs covering layer (approximately 45% and more), the heterostructure is type II, which would normally be unreachable for flat layers. For the structure with GaAsSb SRL, the band alignment is highly dependent on the SRL composition. The type I/type II transition occurs for approximately 15% of Sb; this value also slightly depends on the QD size. All structures were also studied by HRTEM to show different behavior of the CLs on the interface with InAs which highly influences the structure quality.

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.

Growth and properties of the MOVPE GaAs/InAs/GaAsSb quantum dot structures

This review paper summarizes some of results achieved during last years of our quantum dot (QD) research.We show that the QD shape (aspect ratio and elongation) significantly influences the QD photoluminescence (PL) spectrum. Magnetophotoluminescence can be used for determination of the anisotropy of QDs. While the calculated shifts in magnetic field of the energies of higher radiative transitions are found to be sensitive to the lateral elongation, the shift of the lowest transition is determined mainly by the exciton effective mass. This behavior can be used for determining both the effective mass and the elongation fairly reliably from the magnetophotoluminescence spectra displaying at least two resolved bands.Lateral shape of InAs/GaAs QDs in vertically correlated structures was also studied. We found the ways to control the QD elongation and consequently the energy difference between PL transitions by adjusting properly the spacer layer thickness.The main goal was to redshift QD PL emission towards telecommunication wavelengths of Metal Organic Vapor Phase Epitaxy prepared InAs/GaAs QDs using InGaAs or GaAsSb covering strain reducing layer (SRL). Our results proved that GaAsSb SRL improves the QD PL properties and the type I or type II band alignment can be controlled by both, GaAsSb composition and QD size. Maintaining the type I heterostructure is important for high luminescence efficiency and emission wavelength stability of QD structure. The simulation of electron structure in InAs QDs covered with GaAsSb SRL and experimental results reveal the importance of increasing QD size for obtaining a longer wavelength PL from the type I heterostructure.The type II structure of InAs/GaAs QDs covered by GaAsSb SRL with Sb content near 30% enabled us to achieve extremely long emission wavelength at 1.8µm. The high amount of antimony in the SRL causes the preservation of QD size, and increased QD size prolongs the PL wavelength. The type II heterostructures with ground state electrons confined in InAs QDs and ground state holes in GaAsSb SRL have a strong potential in detector and solar cell applications as is demonstrated by photocurrent measurement.The dependence of built in electric field on QD PL properties is presented since this knowledge is necessary for application of QDs in different kind of electronic structures.We have experimentally demonstrated that the type I/type II transition of band structure of InAs QD with GaAsSb SRL does not depend only on the composition of the SRL, but also on other structure parameters, like QD size or intensity of electric field.

Growth of InAs/GaAs quantum dots covered by GaAsSb in multiple structures studied by reflectance anisotropy spectroscopy

In this work we present results obtained on quantum dot (QD) and multiple QD structures with InAs/GaAs QDs covered by GaAsSb strain reducing layer (SRL) prepared by MOVPE. The growth of structures was studied in situ by reflectance anisotropy spectroscopy (RAS), which offers direct observation of processes during the structure growth such as QD formation, and dissolution or surfacting of In and Sb atoms. Enhanced In surfacting was observed for structures with GaAsSb SRL. Possible ways, how to suppress surfacting of both types of atoms and how to prevent their transport on epitaxial surface to the subsequent QD layer in MQD structures, are suggested. Different interruptions, growth rates and temperatures of the separation layer growth are discussed with respect to the suppression of undesired surfacting of In and Sb atoms. The conclusions derived from RAS measurements are supported by HRTEM, AFM or PL results.

MOVPE prepared InAs/GaAs quantum dots covered by GaAsSb layer with long wavelength emission at 1.8 µm

Preparation and properties of InAs/GaAs quantum dots (QDs) prepared by the MOVPE technology covered by GaAsSb strain reducing layer (SRL) with extremely long emission wavelength at 1.8µm will be presented. Increase of the emission wavelength was achieved by the introduction of GaAsSb SRL with Sb content of about 30% in the solid phase. The high Sb concentration in the SRL causes the preservation of QD size, which is about 15nm wide at the base and 5nm high. Increased QD size increases the photoluminescence (PL) wavelength. Furthermore, high content of antimony leads to a creation of type II heterostructure for which a redshift of the PL wavelength and decrease of the PL intensity are typical. Low PL intensity may complicate light emitting applications; however fast separation of carriers in the type II structure is an advantage for detector or solar cell application, especially with the long working wavelength. With respect to the perspective application of this structure, the photocurrent (PC) measurement was chosen as the complementary characterization method. A depression of PC for quantum well wavelength region (approximately 900–1200nm) was observed for positive bias, while the PC from QDs (over 1200nm) is not sensitive to the electric field orientation at all. An explanation of this unexpected phenomenon is suggested.

Enhancing optical characteristics of InAs/InGaAsSb quantum dot structures with long-excited state emission at 1.31 μm.

In this study, the optical properties of InAs quantum dots (QDs) with various strain-reducing layers (SRLs) of GaAsSb and InGaAsSb are characterized using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The room-temperature PL results for the InAs/InGaAsSb QDs revealed stronger emission intensities than InAs QDs capped with an GaAs(1-x)Sb(x) (x = 20%)SRL, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. The InAs/InGaAsSb QDs showed a significant elongation of emission wavelengths to 1450 and 1310 nm for the ground and first-excited state at room temperature. The energy band alignment of the InAs QD heterostructures was found tailoring from type II to type I as the GaAsSb SRL was replaced by InGaAsSb layer, which improved the radiative efficiency and was verified by power-dependent PL and TRPL measurements. Post-growth rapid thermal annealing was applied on the InAs/InGaAsSb QDs to further enhance the QD quality and PL emission efficiency. The greatly improved PL intensity, reduced linewidth, shortened radiative lifetime, with increasing annealing temperature were demonstrated, and InAs/InGaAsSb QDs exhibited enhanced optical characteristics for long-wavelength emission applications.

Capping InAs quantum dots with an InGaAsSb strain-reducing layer to improve optical properties and dot-size uniformity

The superior operation characteristics of quantum dot (QD) lasers with excited-state (ES) emission, namely high saturation gain and modulation bandwidth, are advantageous for applying lasers in optical-fiber communication. The InAs QDs in this study were covered with an InGaAsSb capping layer to reduce In–Ga intermixing, thereby increasing the QD density and dot-size uniformity. However, the extension of ES emission to 1310nm in InAs/GaAs QD heterostructures is difficult because of the internal compressive strain and In–Ga intermixing. This study used an InGaAsSb strain-reducing layer (SRL) to cap InAs QDs, elongating the ground-state and excited-state (ES) emission wavelengths to 1426 and 1310nm, respectively. The optical properties of the InAs/(In)GaAsSb QD structures were measured using power-dependent photoluminescence and time-resolved photoluminescence measurements. The energy band alignment of the InAs QD heterostructure changed from a type-II to Quasi-type I confinement (transition state from type-II to type-I) when the GaAsSb SRL was replaced with an InGaAsSb layer under a constant Sb flux during the epitaxy process. Therefore, an enhanced radiative recombination efficiency was observed in the InAs/InGaAsSb QDs at the long first ES emission at 1310nm.

Light emitting diodes with InAs/GaAsSb self-assembled quantum dot layer embedded in GaAs

Luminescence properties of metalorganic vapor phase epitaxy grown light emitting diodes (LEDs) with active InAs/GaAs quantum dot (QD) layer covered by GaAsSb strain reducing layer (SRL) were investigated at temperatures from 10 to 400K. Results show that the use of GaAsSb SRL with up to 14% Sb strongly increases luminescence, redshifts the emission maximum up to 1.4μm while keeping the type I transition, narrows the luminescence linewidth and keeps the separation energy between the ground and excited state as in InAs/GaAs QD LEDs without SRL. The strong ground state emission which persists up to 400K is dominated by recombination of electrons and holes confined in QDs while the broad emission from excited states, which dominates below 250K, involves recombination from the states located in QDs and SRL. The ground state electroluminescence shows the typical Stranski–Krastanov dot temperature properties: the thermally induced narrowing of the electroluminescence linewidth and the increased QD gap energy shrinkage with temperature. The electroluminescence intensity of the ground and excited state transitions increases with temperature (up to 80K) due to the thermal escape of electrons from the wetting layer which reduces radiative recombination via wetting layer states. The dominant mechanism responsible for the thermal quenching of electroluminescence at elevated temperatures is the escape of electrons from QDs to the GaAs barrier.

Type I–type II band alignment of a GaAsSb/InAs/GaAs quantum dot heterostructure influenced by dot size and strain-reducing layer composition

The aim of this work is to red shift quantum dot (QD) photoluminescence (PL) towards telecommunication wavelengths by engineering the metalorganic vapour phase epitaxy (MOVPE) prepared structure of InAs/GaAs QDs covered by a GaAsSb strain-reducing layer. Our results proved that type I or type II band alignment can be controlled by both GaAsSb composition and QD size. Maintaining type I heterostructure is important for high luminescence efficiency and emission wavelength stability of the QD structure. The simulation of electron structure in InAs QDs covered with a GaAsSb strain-reducing layer as well as experimental results suggest the importance of increasing QD size for obtaining a longer wavelength PL from the type I heterostructure. The PL maximum wavelength 1371 nm was achieved for the MOVPE prepared type I QD structure with 14% of Sb in GaAsSb. This type of structure exhibits seven times higher PL intensity, twice narrower PL peak and 85 meV redshift in comparison with similarly prepared QDs covered by GaAs.

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.

Optical transitions in type-II InAs∕GaAs quantum dots covered by a GaAsSb strain-reducing layer

The excitation power dependence of the ground and excited state transitions in type-II InAs-GaAs0.78Sb0.22 quantum dot structure has been studied. Both transitions exhibit a strong blueshift with increasing excitation power but their separation remains constant. This behavior indicates a carrier-induced electric field oriented predominantly along the growth axis, which requires the holes to be localized in the GaAsSb above quantum dots. An accelerated blueshift of the ground state emission is observed once the excited state in the dots starts to populate. This behavior can be explained by a smaller spontaneous recombination coefficient for the excited state transition.

Long-wavelength light emission and lasing from InAs∕GaAs quantum dots covered by a GaAsSb strain-reducing layer

The effects of a thin GaAsSb strain-reducing layer on the optical properties of InAs∕GaAs quantum dots (QDs) are investigated. With increasing Sb composition, the room-temperature emission wavelength of the InAs QDs increases to ∼1.43μm. For Sb compositions above 14%, the system becomes Type II, with a decrease of the photoluminescence (PL) efficiency. At a composition of 14%, the room-temperature PL efficiency is maximized, and is also significantly enhanced when compared to that of conventional InGaAs-capped InAs QDs grown under the same conditions. Room-temperature ground-state lasing at 1.292μm is demonstrated for an InAs∕GaAsSb∕GaAs structure.

Strong photoluminescence and laser operation of InAs quantum dots covered by a GaAsSb strain-reducing layer

GaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs is reduced due to the tensile strain induced by the GaAsSb SRL, resulting in a redshift of photoluminescence (PL) peaks of the InAs QDs. A strong PL signal around a wavelength of 1.3 μm was observed even at room temperature. A laser diode containing InAs QDs with GaAsSb SRLs in the active region was fabricated, which exhibits laser oscillation in pulsed operation at room temperature. These results indicate that GaAsSb SRLs have a high potential for fabricating high efficient InAs QDs laser diodes operating at long-wavelength regimes.

Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates

We fabricated InAs quantum dots (QDs) with a GaAsSb strain-reducing layer (SRL) on a GaAs(0 0 1) substrate. The wavelength of emission from InAs QD is shown to be controllable by changing the composition and thickness of the SRL. An increase in photoluminescence intensity with increasing compositions of Sb and thickness of the GaAsSb SRL is also seen. The efficiency of radiative recombination was improved under both conditions because the InAs/GaAsSb/GaAs hetero-interface band structure more effectively suppressed carrier escape from the InAs QDs.