GaAs Barrier Research Articles

Ultrafast photocarrier dynamics in InAs/GaAs self-assembled quantum dots investigated via optical pump-terahertz probe spectroscopy

Optical pump-terahertz probe (OPTP) spectroscopy was performed to measure the lifetime of photogenerated carriers in the barrier and the wetting layer (WL) regions of an indium arsenide on gallium arsenide (InAs/GaAs) single-layer self-assembled quantum dot (QD) sample. A modified rate equation model of carrier dynamics was proposed where possible state-filling in both QD and WL is considered. Drude model fitting was also performed to extract the time-dependent plasma frequency and phenomenological scattering time from the terahertz transmission spectra. The results of the OPTP experiment show two prominent recombination processes that occur at different timescales after photoexcitation. These two processes were attributed to carrier recombination in the GaAs barrier and the quantum well-like states of the WL based on the fitted lifetimes. Calculations using the coupled differential rate equations were also able to replicate the experimental trend at low fluence. The lack of agreement between experimental data and numerical calculations at high optical fluence was mainly attributed to the possible saturation of the GaAs density of states. Lastly, the results of the parameter fitting for the plasma frequency and scattering time indicate a transition from the barrier to the WL recombination as the dominant carrier recombination mechanism within the time scale of the OPTP scan. This further lends credence to the proposed model for carrier dynamics in SAQD systems under state-filling conditions.

Effects of parabolic barrier design for multiple GaAsBi/AlGaAs quantum well structures

The results of a comparative study on how the design of multiple quantum structures containing a parabolic barrier profile affects optical properties are presented. All quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. The investigated samples consisted of (i) double parabolic quantum wells (type A) or (ii) multiple (two or three) rectangular quantum wells surrounded by parabolic barriers (type B). The optical quality of samples was characterized performing room-temperature (RT-PL) and temperaturedependent photoluminescence (TD-PL) measurements. The investigation aimed at the optimization of a multiple quantum well (MQW) structure design for application in the gain region of near infrared (NIR) laser diodes (LDs) revealed benefits of both double parabolic quantum wells and a mixed design (rectangular MQW with parabolic barriers). The PL band position for all samples was registered in the vicinity around 1.19 eV, which corresponds to the Bi content in QW of ~4.4%. It was shown that all structures of type A exhibit an intense emission, while the intensity of photoluminescence measured for the samples of type B depends on the number of QWs. The weaker intensity of the PL signal from two QWs inserted between parabolic barriers was explained by a larger point defect density at low temperature grown inner GaAs barriers. The room-temperature PL intensity of the structure with three GaAsBi QWs embedded in one parabolic AlGaAs barrier was the highest one. The shift of PL peak position to lower energies (1.16 eV) was attributed to the slightly higher bismuth concentration, 4.9%.

Optically Induced Spin Electromotive Force in a Ferromagnetic-Semiconductor Quantum Well Structure.

Hybrid structures combining ferromagnetic (FM) and semiconductor constituents have great potential for future applications in the field of spintronics. A systematic approach to study spin-dependent transport in a GaMnAs/GaAs/InGaAs quantum well (QW) hybrid structure with a few-nanometer-thick GaAs barrier is developed. It is demonstrated that a combination of spin electromotive force measurements and photoluminescence detection provides a powerful tool for studying the properties of such hybrid structures and allows the resolution of the dynamic FM proximity effect on a nanometer scale. The method can be generalized to various systems, including rapidly developing 2D van der Waals materials.

Quaternary – alloyed capping for strain and band engineering in InAs sub – monolayer quantum dots

In the last few years, a considerable amount of attention has been given to the optimization of several capping layers (CLs) in InAs/GaAs sub – monolayer (SML) quantum dots (QDs) heterostructure for a sustainable room – temperature (RT) operation in 1.3–1.55 μm telecommunication range (O, C – bands). In particular, the quaternary – alloyed capping (Q – CL: Q – Sb: InxGa1−xAsySb1−y, Q – N: InxGa1−xAsyN1−y, and Q – SbN: GaAsxSbyN1−x−y) has been explored in this work to reach the above – stated photoluminescence (PL) emission wavelength and we compared it with the conventional InAs/InxGa1−xAs QDs using Nextnano software. This tool is based on 8 – band k.p model that solves the self – consistent Schrödinger – Poisson equations analytically to obtain the envelope functions of the eigen energy levels (both electrons and holes) and finally the absorption spectra. Although Q – CL help in preserving QD morphology by strain reduction, it also allowed us create deep carrier confinement potential near conduction and valence band offsets i.e. CBO (VBO) where there is a minimum carrier escape probability towards GaAs barriers (more thermal stability) thus fostering a prolonged PL emission wavelength at 300 K. By changing atomic constituent's composition in Q – CLs, the obtained ground – state (GS) band – to – band absorption energy are 1.31 (Q – Sb22%, 1.25 (Q – N1.8%), 1.37 μm (Q – SbN2.2%))). Such deep confinement aid in the dark current reduction and can promote for high temperature operation by improving the PL quantum yield by several orders of magnitude. The inherent material properties of CL composing the surfactant effect of Sb, larger bond strength of tensile – strained dil.N, and phase separation effects of Sb–N alloys are advantageous in this regard to corroborate further PL redshift.

Role of resident electrons in the manifestation of a spin polarization memory effect in Mn delta-doped GaAs heterostructures

The spin-memory effect in the GaAs / InGaAs heterostructures with $\delta$<Mn> layer in GaAs barrier have been investigated. The effect consists in spin polarization of Mn atoms due to interaction with photogenerated spin-polarized holes. The investigation of the effect was carried out by analyzing the polarization of the probe photoluminescence pulse in the pump-probe technique. It was shown that the circular polarization degree of probe pulse generated photoluminescence is strongly affected by the interaction of hole spins with spins of Mn atoms polarized by the pump pulse. The latter leads to decrease of circular polarization degree as compared with single pulse excitation ($\delta P$ effect). The amplitude of $\delta P$-effect is most strongly affected by the concentration of resident electrons in the quantum well which is believed to be due the specific compliance with selection rules for optical transition with the participation of unpolarized resident electrons. The rest of the sample's parameters including the spatial separation between $\delta$<Mn> layer and InGaAs quantum well ($d_s$) have a minor effect on the $\delta P$ value which leads to a paradoxical situation of decreasing $\delta P$-effect with the decrease of $d_s$. The proposed experimental technique consisting in creating the significant concentration of resident electrons in the QW may serve as a reliable photoluminescence method determining the strength of this effect as well as the Mn spin relaxation time in a particular nanostructure.

Integrated fabrication of a high strain InGaAs/GaAs quantum well structure under variable temperature and improvement of properties using MOCVD technology

An InGaAs/GaAs quantum well (QW) structure was prepared by metal-organic chemical vapor deposition (MOCVD) via a new growth method, where the InGaAs well layer and the GaAs barrier layer were grown under a variable temperature. A GaAs protection layer was employed to avoid indium atom evaporation on the InGaAs surface during the temperature change. Room and low-temperature photoluminescence (RT/LT-PL), atomic force microscopy (AFM), and high-resolution X-ray diffraction (HRXRD) were carried out to investigate the effect of the variable temperature growth method. The theoretical and experimental results indicated that it could erase the 2D islands and rebuild the surface morphology to a step-flow mode surface. The quality of the InGaAs crystal layer was also improved because of the annealing-like treatment. In addition, the study found that when the thickness of the GaAs protection layer was 2 nm and the growth temperature of the InGaAs layer was 560°C, the maximal properties of the InGaAs/GaAs QW were achieved. Moreover, high-quality multiple QWs with five periods were grown with the tailored structure. The growth method will improve the properties of strained InGaAs/GaAs QW materials and provide technical support for a semiconductor laser's performance optimization.

Photoluminescence properties of GaAsBi single quantum wells with 10% of Bi

A set of single quantum well (SQW) samples of GaAs1-xBix with x ~ 0.1 and p-doped GaAs barriers grown by molecular beam epitaxy was investigated by the temperature-dependent photoluminescence (PL) spectroscopy. Those GaAsBi SQW structures showed a high crystalline quality, a smooth surface and sharp interfaces between the layers and exhibited a high PL intensity and a lower than 100 meV PL linewidth of QW structures. Temperature dependence of the optical transition energy was S-shape-free for all investigated structures and it was weaker than that of GaAs. An analysis of the carrier recombination mechanism was also carried out indicating that the radiative recombination is dominant even at room temperature. Moreover, numerical calculations revealed that a higher Be doping concentration leads to an increased overlap of the electron and heavy hole wave functions and determines a higher PL intensity.

Effects of the interfacial states on the spin-dependent tunneling of Mn3Al-based magnetic tunnel junction

The study of antiferromagnets (AFMs) has become one of the hotspots of spintronics because of their strong environmental adaptability; however, their magnetic ordering structure limits their practical application. The compensated ferrimagnet Mn3Al has the characteristics of an AFM and the magnetic moment can be controlled by a magnetic field. The successful synthesis of Mn3Al, which possesses a high Curie temperature (605 K), may lead to new applications in spintronics devices. In this work, the electronic structure of free Mn3Al (001) surfaces and the interface effects of the Mn3Al electrode and the GaAs tunnel barrier in the magnetic tunnel junction (MTJ) are investigated with the non-equilibrium Green’s function method. Although redistribution of the partial density of states between the surface and bulk is observed for the two different types of Mn3Al surfaces, the half-metallicity is retained for both the surface structures. Due to the half-metallicity of the surfaces, the tunneling magnetoresistance (TMR) of the MTJ reaches up to 107%. Moreover, the TMR dependence on the GaAs barrier thickness as well as the interface is studied. Through the visual display of the interface states, we find that the interface states can promote electron tunneling in the antiparallel configuration. The high TMR can be maintained at a small barrier width owing to the weak interface states. The results are explained with the transmission coefficient in reciprocal space. This work provides some physical insight for the design and implementation of Mn3Al-based MTJs.

Passive mode-locking of p-doped quantum dot semiconductor lasers

Quantum dot based monolithic edge-emitting semiconductor lasers at 1.25 µm are ideal sources for the generation of broad optical frequency combs for optical communication applications. In this work, InAs/InGaAs quantum dot lasers with different total laser length to absorber length ratio and with different p-doping concentrations in the GaAs barrier sections are investigated experimentally in dependence on the gain injection current and absorber reverse bias voltage. A smaller mode-locking area is found for the p-doped device in dependence on the laser biasing conditions. For the undoped active region 1.3 ps short pulse widths at a pulse repetition rate of 20 GHz with a pulse-to-pulse timing jitter of 111 fs are reported for an absorber section length of 12% to the total cavity length. For an undoped and p-doped device short pulse emission between 2.5 ps and 5.5 ps is attained and a shorter absorber section length of 8% or 5%.

Effect of In0.70Ga0.30As quantum dot insertion in the middle cell of InyGa1-yP/InxGa1-xAs/Ge triple-junction for solar cells

Abstract This paper focuses on the simulation and optimization of electrical and structural properties of high efficiency InGaP/InGaAs/Ge triple junction solar cells that incorporate In0.7Ga0.3As quantum dots with in the GaAs middle cell material. Current density-voltage (J-V), external quantum efficiency (EQE) and capacitance-voltage (C-V) characteristics have been simulated and discussed. Results show that 30 pairs of In0.70Ga0.30As (QD)/GaAs (barrier) in the middle cell provide a relative enhancement of 13% in EQE in the 900–1000 nm wavelength range. This leads to a short-circuit current of 20 mA/cm2, an open circuit voltage of 2.3 V, a fill factor of 81.73%, and a conversion efficiency of 39.03%. The C-V revealed that a relatively high number of interfacial states are present in the 3-J cell structure including the QD layers, which decreases the open circuit voltage. In this study we benefited 18% of relative efficiency.

Carrier dynamics and recombination in silicon doped InAs/GaAs quantum dot solar cells with AlAs cap layers

The effects of doping InAs quantum dots (QDs) with Si on charge carrier dynamics and recombination in the InAs/GaAs QD solar cells (QDSCs) with AlAs cap layers was investigated. Non-radiative and radiative recombination paths in the doped cells were identified by changes in emission intensity, long-wavelength photovoltage (PV) as well as time-resolved PV and photoluminescence (PL) measurements. We find that the reduction of long-wavelength PV and PL with n-doping is due to the electron population of the QD ground states (GSs) and shrinkage of the depletion layer. The time constants, derived from the time-resolved PV, grow non-monotonically with increasing of the doping density in the QDs due to redistribution of electrostatic potential in the intrinsic region of p-i-n diode and electron population of EL2 defect states of GaAs barriers. We also find that the GS emission from the InAs QDs decreases with n-doping. The results show that PL traces depend on carrier dynamic in the top QD layers populated partially with electrons from ionized impurities, whereas PV transients were found to be strongly dependent on recombination via QD and defect states located outside the depletion layer. We conclude that the non-radiative recombination of photogenerated electrons and holes via defects is suppressed due to the spatial separation by the local electric fields in and around doped AlAs/InAs QDs, as the potential profile of the intrinsic region is modulated spatially by built-in charges. The interpretation of experimental data suggests limiting mechanisms in the InAs/GaAs QDSCs operation and sheds light on possible approaches for their further improvement.

Effect of p-doping on the intensity noise of epitaxial quantum dot lasers on silicon.

This work experimentally investigates the impact of p-doping on the relative intensity noise (RIN) properties and subsequently on the modulation properties of semiconductor quantum dot (QD) lasers epitaxially grown on silicon. Owing to the low threading dislocation density and the p-modulation doped GaAs barrier layer in the active region, the RIN level is found very stable with temperature with a minimum value of -150dB/Hz. The dynamical features extracted from the RIN spectra show that p-doping between zero and 20 holes/dot strongly modifies the modulation properties and gain nonlinearities through increased internal losses in the active region and thereby hinders the maximum achievable bandwidth. Overall, this Letter is important for designing future high-speed and low-noise QD devices integrated in future photonic integrated circuits.

Photoelectric and deep level study of metamorphic InAs/InGaAs quantum dots with GaAs confining barriers for photoluminescence enhancement

Metamorphic InAs/InGaAs quantum dots (QDs) have been proposed as active elements for optoelectronic light-emitting devices operating in the infrared range. However, advanced structure design to allow efficient and stable enhancement of quantum yield at room temperature are still needed. Here, we compare a metamorphic InAs/In0.15Ga0.85As QD heterostructure with and without GaAs confining barriers, to investigate the effect of introducing GaAs barriers on the photo- and thermo-electrical properties. GaAs confining barriers allow to enhance the QD photoluminescence intensity at 1.3 µm, i.e. second telecommunication window, by more than two orders of magnitude at room temperature and at 80 K. We also discuss the effect of GaAs barriers on the carrier transport and on defect-related levels detected by means of photocurrent and deep level thermally stimulated current spectroscopies. GaAs confining barriers decrease the thermal escape rate of electrons confined in QD and wetting layer, thereby highly increasing the radiative recombination and also quenching the photocurrent. At low temperatures, the barriers also reduce the capture of electrons generated in InGaAs by the QD layer and, on the other hand, prevent the trapping of electrons outside the QD layer, decreasing carrier lifetimes. The deep levels identified as point and extended defects have been detected in the InGaAs layers. There are no new types of defects introduced in the structure by the addition of the barriers, but this causes a weakly increased density of traps near QDs. Our results show that InAs/InGaAs QDs with GaAs confining barriers can be efficient light emitters with only a slight increase of defects in the structure. Hence, such advanced design for metamorphic QDs can be of relevant interest for applications in energy-efficient QD lasers, optical amplifiers and single-photon emitters operating at 1.3–1.55 µm.

Temperature and hydrostatic pressure effects on the electronic structure, optical properties of spherical segment quantum dot/wetting layer and group velocity of light

To investigate hydrostatic pressure and temperature effects on electron energy levels of an InAs spherical segment quantum dot with wetting layer embedded in GaAs barrier, we use the finite element method to solve the Schrödinger equation in the effective mass approximation. Results show that the ground, the first and the second excited state energies decrease (increase) as the hydrostatic pressure (temperature) increases for the constant temperature (pressure). Moreover, given the optical properties of the system interacting with two laser fields, red (blue) shifts are observed in the linear and nonlinear absorptions and dispersions for the probe pulse as the hydrostatic pressure (temperature) increases. Furthermore, as the hydrostatic pressure increases, the maximum of the group velocity of light inside the slow light frequency range increases and the slow light frequency range shifts to the lower probe frequencies.

Effect of thermal annealing on the optical and structural properties of (311)B and (001) GaAsBi/GaAs single quantum wells grown by MBE

The effect of Furnace Annealing (FA) and Rapid Thermal annealing (RTA) on the structural and optical properties of GaAs1 − xBix/GaAs single quantum wells grown on (001) and (311)B substrates by molecular beam epitaxy was investigated. The structural properties were investigated by high-resolution x-ray diffraction (HR-XRD) and Transmission Electron Microscopy. The Bi concentration profiles were determined by simulating the HR-XRD 2θ−ω scans using dynamical scattering theory to estimate the Bi content, lattice coherence, and quality of the interfaces. The Bi composition was found to be similar for both samples grown on (001) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi quantum well (QW) layer but also extends out of the GaAsBi QW toward the GaAs barrier. Photoluminescence (PL) measurements were performed as a function of temperature and laser power for samples with a nominal Bi composition of 3%. PL spectra showed that (001) and (311)B samples have different peak energies at 1.23 eV and 1.26 eV, respectively, at 10 K. After RTA at 300 °C for 2 min, the PL intensity of (311)B and (001) samples was enhanced by factors of ∼2.5 and 1.75, respectively. However, for the (001) and (311)B FA samples, an enhancement of the PL intensity by a factor of only 1.5 times could be achieved. The enhancement of PL intensity in annealed samples was interpreted in terms of PL activation energies, with a reduction in the alloy disorder and an increase in the Bi cluster.

An Analytical Approach to Study Annealing Induced Interdiffusion of In and Ga for Truncated Pyramidal InAs/GaAs Quantum Dots

A strain-assisted model for studying the annealing induced interdiffusion of constituent atoms in truncated pyramidal InAs/GaAs self-assembled quantum dot (QD) is developed. In our model, we have considered the equiprobable diffusion of Ga and In atoms in and out of the QD system, thereby resulting in the formation of In rich In1-xGaxAs as the QD material and a combination of Ga rich InxGa1-xAs and GaAs barrier material. This assumption is supported by the high-resolution cross-sectional transmission electron microscopy (HRTEM) images and high-resolution X-ray diffraction (HRXRD) plots of the as-grown and annealed samples. First principle calculations are utilized to evaluate the strain and annealing temperature dependent activation energy, which further assists to solve the Fickian diffusion model. Our simplistic model provides a detailed insight into the effect of annealing phenomenon on the bimodal Photoluminescence (PL) spectra of molecular beam epitaxy (MBE) grown InAs/GaAs QDs, which have evolved as a potential system for innumerous applications including single photon sources, quantum computers, spin LEDs, LASERs and Infrared Photodetectors.

Vertically Coupled Hybrid InAs Sub-Monolayer on InAs Stranski–Krastanov Quantum Dot Heterostructure: Toward Next Generation Broadband IR Detection

In the present article, we are introducing a novel heterogeneously coupled InAs Submonolayer (SML) on Stranski-Krastanov (SK) quantum dot (QD) heterostructure with reduced strain accumulation. Theoretical comparison manifests the superiority of this hybrid SML-SK QDs over bilayer SK QDs. Photoluminescence and photoluminescence excitation spectroscopy are employed to characterize the electronic interaction and carrier tunneling in between this hybrid quantum dot assembly. Growth rate is optimized at 0.1 ML/sec. To tune the coupling, the barrier layer thickness is varied from 5 nm to 10 nm. Even up to the highest barrier thickness, no signature of SML peak is obtained in the PL response. But, PLE depicts the presence of SML peak, which is partially overlapped with the SK 2nd excited peak. The sample with 7.5 nm GaAs barrier shows perfect resonance between SML ground state and SK 2nd excited state. Based on experimental and analytical results, this SML on SK (SML-SK) configuration is compared with SK on SML (SK-SML) configuration; where SML-SK exhibits better inter dot electronic interaction than its reciprocal configuration. For ex situ tailoring of this heterogeneous electronic coupling, rapid thermal annealing has been done. It shows tunability of inter dot interaction with improved crystalline quality. Finally, we have shown a SML-SK quantum dot infrared photodetector with a broad spectral response (from SWIR to near LWIR).

Investigation of optical properties of QD-InAs/GaAs heterostructures obtained by ion-beam deposition

In this paper, we investigate heterostructures with an array of InAs quantum dots encapsulated by GaAs barrier layers obtained by ion-beam epitaxy. The thickness of the layers was less than 30 nm. It is shown that this technique allows to obtain quantum dots with lateral dimensions up to 50 nm with a height of 10 nm. The density of the obtained array of quantum dots was 109 cm−2. The studies performed using photoluminescence methods revealed the peaks of the main transitions for quantum dots at 1.1 eV (1150 nm) for samples with GaAs barrier, which corresponds to the near-infrared. The width of the main peak of the samples was about 0.2-0.25 eV, which is associated with the dispersion of quantum dots sizes. Dark current-voltage characteristics of the structures proved that the value of dark current density at 90 K is about 10−6 A/cm2. The asymmetry of the dark current curves at positive and negative shifts is determined. The samples also showed that an increase of temperature leads to degradation of characteristics. When the operating temperature rises to 300 K, the density of the dark current changes in the range from 0.1 to 0.01 A/cm2.

Photoluminescence of InGaAs/GaAs Quantum Nanodisk in Pillar Fabricated by Biotemplate, Dry Etching, and MOVPE Regrowth

The III–V compound semiconductor quantum dot (QD) photonic devices have attracted considerable attention due to their stable operation at high temperature, low threshold current, and high-speed modulation. The photoluminescence (PL) energy of QDs depends on the dot sizes and the energy height of barriers. It is difficult to encourage the growth of low indium content QDs because of small lattice distortions between the InGaAs well regions and GaAs barrier regions. In conventional self-organized QD devices produced by Stranski–Krastanow growth methods, the injected carriers are first captured by the wetting layers and then dropped to the QDs with relaxation to the ground state for QD devices. Our proposed unique quantum disk structures have no wetting layers because the quantum well (QW) region is fully etched by using a low-damage dry etching process. We produced 30 nm diameter and 9 nm thick InGaAs nanodisks using a biotemplate via neutral beam etching and metal–organic vapor phase epitaxy (MOVPE). We observed the InGaAs nanodisks via scanning electron microscopy and measured their photoluminescence (PL). To confirm the quantum confinement effects of InGaAs nanodisks, their energies and transient PL behaviors were measured as a function of temperature. The combined low-damage dry etching and MOVPE regrowth processes form an important technique, as they promise a low-damage interface and regrowth ability. This unique technology is attractive for carrier transportation dynamics and spintronics of hybrid QDs and QW nanostructures, and it is easy to adapt for industrial production systems.

Magnetically controlled exciton transfer in hybrid quantum-dot–quantum-well nanostructures

A magnetophotoluminescence study of the carrier transfer with hybrid InAs/GaAs quantum dot(QD)-InGaAs quantum well (QW) structures is carried out where we observe an unsual dependence of the photoluminescence (PL) on the GaAs barrier thickness at strong magnetic field and excitation density. For the case of a thin barrier the QW PL intensity is observed to increase at the expense of a decrease in the QD PL intensity. This is attributed to changes in the interplane carrier dynamics in the QW and the wetting layer (WL) resulting from increasing the magnetic field along with changes in the coupling between QD excited states and exciton states in the QW and the WL.