Multilayer InAs Research Articles

Analytical Model and Experimental Analysis to Estimate the Interdiffusion and Optoelectronic Properties of Coupled InAs Quantum Dots Post Rapid Thermal Processing

The stoichiometric variations in molecular beam epitaxy (MBE)-grown In(Ga)As quantum dots (QDs) after rapid thermal processing (RTP) is estimated from an analytical model assisted by transmission electron microscopy (TEM) images. A systematically grown multilayer (bi-, penta-, hepta-, and deca-layer) InAs Stranski–Krastanov (SK) QDs coupled electronically to submonolayer (SML) QDs are used in this study. The As-grown (ASG) samples are treated with RTP at temperatures of 650 °C, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$700~^{\circ }\text{C}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$750~^{\circ }\text{C}$ </tex-math></inline-formula> , and 800 °C for 30 s. Our model estimates a significant diffusion of In atoms in the growth direction compared with the in-plane direction altering the QD dimensions and composition. To check its reliability, we incorporated an eight-band k.p model on all the RTP-treated strained In(Ga)As QD heterostructures. Photoluminescence (PL) measurements reveal a strong correlation with the simulation results; furthermore, a strong PL blueshift (56 nm) and narrow linewidth from 67 nm (ASG) to 33 nm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$800~^{\circ }\text{C}$ </tex-math></inline-formula> ) are seen due to an increase in the uniformity of QDs supporting the atomic force microscopy (AFM) results. AFM images reveal the formation of highly uniform and dense QDs with increased annealing temperature. The activation energy (AE) shows a decreasing trend due to the reduction in carrier confinement with RTP, agreeing with the analytical model. This is probably the first report that presents the effect of RTP on SK-SML coupled QD devices both in terms of theoretical and experimental analyses. This analytical model can thus be extended to any annealing study and for any material with certain known parameters and is helpful for futuristic studies.

Light absorption properties of a nanowire/quantum-dot hybrid nanostructure array

The light absorption properties of a nanowire/quantum-dot hybrid nanostructure array are investigated. By growing multilayer InAs quantum dots on the sidewalls of GaAs nanowires, not only the absorption spectrum of GaAs nanowires is extended by quantum dots, but also the light absorption of quantum dots is dramatically enhanced due to the light-trapping effect of the nanowire array. By optimizing the nanowire and quantum dot parameters, a maximum photocurrent increment of 2.8 mA/cm2 is obtained by incorporating 5 layer InAs quantum dots into a 3 μm high GaAs nanowire array, 4.6 times of its thin-film counterpart. Mode analysis demonstrates that the fundamental mode in the nanowire has a dominate influence on the QD absorption in the wavelength range beyond GaAs bandgap.

Photovoltaic Performance of a Nanowire/Quantum Dot Hybrid Nanostructure Array Solar Cell

An innovative solar cell based on a nanowire/quantum dot hybrid nanostructure array is designed and analyzed. By growing multilayer InAs quantum dots on the sidewalls of GaAs nanowires, not only the absorption spectrum of GaAs nanowires is extended by quantum dots but also the light absorption of quantum dots is dramatically enhanced due to the light-trapping effect of the nanowire array. By incorporating five layers of InAs quantum dots into a 500-nm high-GaAs nanowire array, the power conversion efficiency enhancement induced by the quantum dots is six times higher than the power conversion efficiency enhancement in thin-film solar cells which contain the same amount of quantum dots, indicating that the nanowire array structure can benefit the photovoltaic performance of quantum dot solar cells.

Optimization of InAs quantum dots through growth interruption on InAs/GaAs quantum dot heterostructure

This study investigates the optical and structural properties of multilayer InAs QDs heterostructures designed with varying levels of growth interruption. Samples were subject to growth interruptions of 0, 25, 50, and 75s (samples A, B, C, and D, respectively). Low-temperature (9K) photoluminescence (PL) experiments revealed an initial redshift in the ground-state emission peak (from 1131 to 1145nm) as growth interruption (GI) was varied from 0 to 25s. In addition, we observed a blueshift (from 1145 to 1100nm) when the interruption lasted for more than 25s. The thermal activation energies of samples A–D, calculated from temperature-dependent (8–300K) PL experiments, were approximately 182.4, 241.5, 206.2 and 175.3meV, respectively. The photoluminescence excitation (PLE) studies reveal the existence of two strong peaks at each detection energy, one at ~ 63meV and other at ~ 140meV. These two PLE peaks arise from the InAs QDs shifting toward the higher energy (meV) levels which have lower intensities with the increase of GI. Longer GI time increases the intensities of InAs wetting layer as well as GaAs matrix. This, in turn, worsens the optical property. In our structural study, high-resolution transmission electron microscopy (HRTEM) confirmed that a short interruption (25s, Sample B) in the growth of QDs produced a better size homogeineity. We also observed that Sample B has a tensile stress along the growth direction (001), which increased the height of the InAs QDs to approximately 8nm with the interplanar distance of 0.626Å along the 001 planes. In addition, the InAs QDs maintained their geometrical shape till 50s and the shape of the dots started deterioting after 50s as a result of the decrease in QDs height of 2nm revealed from HRTEM. In contrast, high-resolution X-ray diffraction (HRXRD) found the strain energy drastically dropped to 0.4765meV/atoms (75s, Sample D) from 0.819meV/atoms (25s, Sample B), showing inhomegenous QDs size distribution. Higher strain energy of 0.819meV in the stacked heterostructure improved QDs size homogeineity. The average indium content calculated from the out-plane HRXRD in samples A–D were 0.234%, 0.288%, 0.241%, and 0.218%, respectively. From the growth model, we noticed a significant change in the surface self-diffusion rate of indium adatoms. The indium adatoms migration to larger adjacent QDs is higher when the pre-matured QDs lie within the critical distance (X; ~ 30nm). At X > 30nm, the QDs start strinking, thereby reducing the height of QDs. Thus, this paper shows how our growth method helps in optimizing the QDs homogeineity and increasing the QDs dimension.

Temperature-dependent modulated reflectance of InAs/InGaAs/GaAs quantum dots-in-a-well infrared photodetectors

We present a photoreflectance (PR) study of multi-layer InAs quantum dot (QD) photodetector structures, incorporating InGaAs overgrown layers and positioned asymmetrically within GaAs/AlAs quantum wells (QWs). The influence of the back-surface reflections on the QD PR spectra is explained and a temperature-dependent photomodulation mechanism is discussed. The optical interband transitions originating from the QD/QW ground- and excited-states are revealed and their temperature behaviour in the range of 3–300 K is established. In particular, we estimated the activation energy (∼320 meV) of exciton thermal escape from QD to QW bound-states at high temperatures. Furthermore, from the obtained Varshni parameters, a strain-driven partial decomposition of the InGaAs cap layer is determined.

Analysis of optical near-field energy transfer by stochastic model unifying architectural dependencies

We theoretically and experimentally demonstrate energy transfer mediated by optical near-field interactions in a multi-layer InAs quantum dot (QD) structure composed of a single layer of larger dots and N layers of smaller ones. We construct a stochastic model in which optical near-field interactions that follow a Yukawa potential, QD size fluctuations, and temperature-dependent energy level broadening are unified, enabling us to examine device-architecture-dependent energy transfer efficiencies. The model results are consistent with the experiments. This study provides an insight into optical energy transfer involving inherent disorders in materials and paves the way to systematic design principles of nanophotonic devices that will allow optimized performance and the realization of designated functions.

Long wavelength strain-engineered InAs multi-layer stacks quantum dots laser diode on GaAs substrate

The growth of GaAs based 1.5 μm multi-layer stacked InAs quantum dots (QDs) has been investigated by solid-source molecular beam epitaxy (SSMBE), which was very important devices for transmission window. Owing to a strong electronic coupling between the QDs layers and the quantum wells (QWs), and antimony (Sb) introduced by for long-wavelength semiconductor lasers were obtained. The device structure for QDs laser diodes (LDs) with a cavity length of 1000 μm and stripe width of 100 μm as well as the device fabrication results will also be presented. The output performance was achieved with continuous wave (CW) operation, the measurement were from 20 to 60°C with a temperature step of 10°C. The threshold current density was 168 A/cm2, and the CW operating up to 20 mW at room temperature (RT) was achieved.

Long wavelength strain-engineered InAs five stacks quantum dots laser diode growth by molecular beam epitaxy

In this paper the molecular beam epitaxy (MBE) growth of GaAs based 1.5μm transmission window InAs quantum dots (QDs) was systematically studied. GaAs based InAs QDs with 1.5μm emission were fabricated successfully by multi-layer stacked InAs QD structures. Owing to a strong electronic coupling between the QD layers, the QDs and the quantum wells (QWs), strain-engineered QDs introducing antimony (Sb) with emission wavelength up to 1.5μm at room temperature were achieved. The QD laser diodes (LDs) were fabricated, with cavity lengths of 1000μm and stripe width of 100μm. The output performance was achieved with the threshold current densities of the device was as low as 164A/cm2, the continuous wave operating up to 22mW at room temperature (RT) was achieved.

Effect of layer separation, InAs thickness, and rapid thermal annealing on the optical emission from a multi-layer quantum wire structure

Multi-layer InAs quantum wire stacks with different layer separations (8, 15, and 25 nm) and InAs thicknesses (3, 4, 5, and 7 monolayers [ML]) were grown on and embedded in In0.53Ga0.27Al0.20As barrier/spacer layers lattice-matched to an InP substrate. For the samples with 4 ML of InAs and different layer separations, double peak photoluminescence was observed from quantum wire stacks separated by 8 nm, and with a 15 nm spacer layer a long wavelength component was observed extending from the main peak. Only a single peak was found as the spacer layer thickness was increased to 25 nm. For the quantum wire stacks with different InAs layer thicknesses and a separation of 8 nm, double peak photoluminescence spectra were observed in the sample with 4 ML of InAs, and a main peak with a long wavelength component was obtained from the sample with 3 ML of InAs. Only a single peak was detectable for the InAs layer thicknesses of 5 and 7 ML. The optical emission features were studied via temperature and excitation laser power dependent photoluminescence. Based on the photoluminescence and transmission electron microscopy observations, photoluminescence spectral features can be attributed to a bi-modal height distribution in certain samples. In order to extend the optical emission to room temperature, the sample with 5 ML of InAs and an 8 nm spacer layer was subjected to post-growth rapid thermal annealing at different temperatures. The emission wavelength was tunable from 1.63 to 1.72 μm at room temperature.

Multilayer InAs Quantum Dot with GaNAs Strain Compensation Layers Partly Inserted in a Thin Spacer Layer

Multilayer InAs quantum dot with a thin spacer prepared using a GaNAs strain compensation layer was investigated by metalorganic chemical vapor deposition. The GaNAs tensile strained layer was inserted partly in the spacer without contacting with the compressively strained InAs dot and GaInAs cover layer. The stacking number of up to 5 with a thin spacer of 18 nm was realized at a wavelength of 1.4 µm without severe degradation of optical quality although a slight dot size increase in the upper layer was confirmed. The result indicates the advantageousness of GaNAs for a thin spacer structure of the multilayer quantum dot.

Multilayer InAs Quantum Dot with GaNAs Strain Compensation Layers Partly Inserted in a Thin Spacer Layer

Multilayer InAs quantum dot with a thin spacer prepared using a GaNAs strain compensation layer was investigated by metalorganic chemical vapor deposition. The GaNAs tensile strained layer was inserted partly in the spacer without contacting with the compressively strained InAs dot and GaInAs cover layer. The stacking number of up to 5 with a thin spacer of 18 nm was realized at a wavelength of 1.4 µm without severe degradation of optical quality although a slight dot size increase in the upper layer was confirmed. The result indicates the advantageousness of GaNAs for a thin spacer structure of the multilayer quantum dot.

InAs quantum wire induced composition modulation in an In0.53Ga0.37Al0.10As barrier layer grown on an InP substrate

Composition modulations are observed by transmission electron microscopy in In0.53Ga0.37Al0.10As barrier layers that overgrow both single- and multilayer InAs quantum wire structures grown on an InP substrate. Indium-rich (gallium-deficient) regions were observed in the region of the barrier layer lying directly above individual quantum wires, while indium-deficient (gallium-rich) regions were detected in the barrier above the gaps between adjacent underlying quantum wires. The magnitude of such modulation was typically 7% (atomic percent) for both indium and gallium as estimated from the energy dispersive x-ray analysis. The origin of such composition modulations was determined by modeling the chemical potential distribution for indium and gallium on the growth front of the barrier layer at the initial capping stage of the quantum wires with finite element simulations. It is found that the number and positions of the indium-rich regions are determined by the combined effects of strain and surface energy distributions on the barrier material capping the quantum wires. Moreover the estimated magnitudes of the composition modulation for both indium and gallium from the finite element models are in good agreement with the experimental observations. This method provides a simple way to understand the origin of, and to estimate the magnitude of the quantum wire-induced composition modulation in the barrier layer.

Morphology and optical properties of single- and multi-layer InAs quantum dots

An understanding of the structural and optical properties of quantum dots (QDs) is critical for their use in optical communication devices. In this study, single- and multi-layer self-organized InAs QDs grown on (001) GaAs substrates by molecular beam epitaxy (MBE) were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the lateral size of multi-layer InAs QDs are larger and flatter than single-layer InAs QDs, which are oval-shaped. The change in shape and size may be attributed to the presence of InGaAs spacer layers in multi-layer InAs QDs. Reciprocal spacer mapping and fast Fourier transformation images clearly show that InGaAs spacer layers present in the multi-layer InAs QDs structures help to release the strain originally existing in the QDs. In addition, the photoluminescence peak of the multi-layer InAs QDs is broader than QD in the single-layer one, which implies that the multi-layer InAs QDs size variation is more random than the single-layer one and this corresponds with the HAADF-STEM images. These results prove that spacer layers release strain influencing the morphology and optical properties of the QDs.

Stacking pattern of multi-layer InAs quantum wires embedded in In 0.53Ga 0.47− xAl xAs matrix layers grown lattice-matched on InP substrate

Multi-layer InAs quantum wires were grown on, and embedded in In 0.53Ga 0.47− x Al x As (with x=0, 0.1, 0.3 and 0.48) barrier/spacer layers lattice matched to an InP substrate. Correlated stacking of the quantum wire arrays were observed with aluminum content of 0 and 0.1. The quantum wire stacks became anti-correlated as the aluminum content was increased to 0.3 and 0.48. The origin of such stacking pattern variation was investigated by finite element calculations of the chemical potential distribution for indium on the growth front surface of the capping spacer layer. It is shown that the stacking pattern transition is determined by the combined effect of strain and surface morphology on the growth front of the spacer layers.

Stacking of multilayer InAs quantum dots with combination capping of InAlGaAs and high temperature grown GaAs

We are reporting the growth of multilayer stacks of quantum dots (10 periods) with a combination capping of In 0.21Al 0.21Ga 0.58As (30 Å) and GaAs (70–180 Å) grown by solid source molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) has been used for the insitu monitoring of quantum dot (QD) formation in heterostructure samples. The samples were also characterized by other exsitu techniques like cross sectional transmission electron microscopy (XTEM) and photoluminescence measurements (PL). For a heterostructure sample with thin barrier thickness (<100 Å), an XTEM image showed the stacking of QDs only up to the 5th layer and in the upper layers there was hardly any formation of dots. We presume the stoppage of dot formation is due to the uneven surface of the InAlGaAs alloy overgrown on the InAs QDs, as a result of the local compositional deviations of the Group-III atoms. Samples grown with thicker barriers (>100 Å of GaAs) showed good stacking of islands until the tenth layer. The thick GaAs layer overgrown on the InAlGaAs at 590 ∘C is believed to remove the surface modifications of the quaternary layer thereby creating a smoother surface front for the growth of subsequent QD layers.

Observation of energy transfer between InAs quantum dots by pump-and-probe micro-photoluminescence measurement

Using pump-and-probe micro-photoluminescence measurement, we observed the nonlinear photoluminescence (PL) of multilayer InAs quantum dots (QDs), which originated from energy transfer between dots. In double- and five-layered InAs QDs, the differential PL intensity, Δ I/I probe, became negative at certain pump intensity. With changes in the pump intensity, Δ I/I probe decreases or increases. This is explained by the generation of a non-radiative path and the saturation of non-radiative sites by the pump pulses. Our experimental results suggest that InAs QDs can act as AND- and NOT-gates by using suitable thresholds and control lights.

Emission characteristics improvement in structures with InAs/GaAsN/InGaAsN superlattices

The influence of some parameters of nitrogen-containing heterostructures InAs/GaAsN/InGaAsN with strain-compensated superlattices (SCSL) on their emission characteristics has been studied. It is established that the net strain in the structure affects the photoluminescence (PL) linewidth, internal quantum efficiency, intensity, and wavelength. The maximum PL intensity and minimum full width at half maximum (FWHM) of the PL line were achieved with small strains (0–0.2%), whereas the maximum wavelengths (∼1.76 μm) observed for large strain (about +1%). By adding multilayer InAs inserts in the active InGaAsN quantum well in combination with using strain-compensated GaAsN/InGaAsN superlattices, it is possible to control the room-temperature emission wavelength in the range of 1.45–1.76 μm without significantly deteriorating the emissiion characteristics.

Effects of the thickness of GaAs spacer layers on the structure of multilayer stacked InAs quantum dots

The effects of the thickness of GaAs spacer layers on the structure of multilayer stacked InAs quantum dots (QDs) grown by molecular-beam epitaxy were studied using transmission electron microscopy. To investigate QD structure depending on spacer layer growth, first uncapped free-standing QDs were grown and their structure compared with that of multilayer stacked QDs. In addition, vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The uncapped QDs were found to form a lens-shaped structure with side facets. Upon capping with a GaAs spacer, the apex of nonaligned QDs flattened by In diffusion. However, the aligned QDs maintained their lens-shaped structure with round apex after capping. It is believed that their apex did not flatten because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers. The results demonstrate the possibility of controlling QD structure by adjusting the thickness of spacer layers.

On the spectral response of quantum dot infrared photodetectors: Postgrowth annealing and polarization behaviors

Effects of the postgrowth rapid thermal annealing on the device properties of a multilayer InAs∕GaAs quantum dot infrared photodetector are investigated. Clear shift in the spectral photoresponse toward smaller energy region with increasing annealing temperatures is shown. Polarization behaviors of the photoresponse peaks are presented.

Doping characterization of InAs∕GaAs quantum dot heterostructure by cross-sectional scanning capacitance microscopy

In order to better understand dopant incorporation in quantum dot infrared photodetectors, the application of cross-sectional scanning capacitance microscopy (SCM) has been used to investigate carrier occupation/distribution in a multilayer InAs∕GaAs quantum dot (QD) heterostructure for different doping techniques. The doping schemes in the QD structure include direct doping (in InAs QD layers) and remote doping (in GaAs barrier layers), each with different doping concentrations. The SCM image suggests that large band bending occurs due to highly doped, remote-doping layers, thereby causing electron redistribution in direct-doping layers. The experimental result is supported by a band structure calculation using the Schrödinger–Poisson method by NEXTNANO3.