Photoluminescence of InAs quantum dots embedded in graded InGaAs barriers

The effects of the top barrier and the dot density on photoluminescence (PL) of the InAs quantum dots (QDs) sandwiched by the graded In_xGa_1−xAs barriers grown by metal-organic vapor phase epitaxy (MOVPE) have been studied. Two emission peaks corresponding to the ground state and the 1st excited state transitions of the QD structures have been observed, which matches well to the theoretical calculation. The PL emission linewidth and intensity of the InAs QDs structure are improved by reducing the Indium/Gallium composition variation of the graded In_xGa_1−xAs top barrier layer of the structure. The QDs’ ground states filling excitation power depends on the crystal quality of the InGaAs barrier layer and the QD density. The extracted thermal activation energy for the QDs’ PL emission is sensitive to the QD size.     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

Morphology response to strain field interferences in stacks of highly ordered quantum dot arrays

Twofold stacked InGaAs/GaAs quantum dot (QD) layers are grown on GaAs(001) substrates patterned with square arrays of shallow holes. We study the surface morphology of the second InGaAs QD layer as a function of pattern periodicity. Comparing our experimental results with a realistic simulation of the strain energy density E(str) distribution, we find that the second InGaAs QD layer sensitively responds to the lateral strain-field interferences generated by the buried periodic QD array. This response includes the well-known formation of vertically aligned QDs but also the occurrence of QDs on satellite strain energy density minima. Our calculations show that base size and shape as well as lateral orientation of both QD types are predefined by the E(str) distribution on the underlying surface.     

Regular array of InGaAs quantum dots with 100-nm-periodicity formed on patterned GaAs substrates

Regular arrays of InGaAs quantum dots (QDs) with a 100-nm-periodicity have been successfully fabricated by controlling the nucleation sites on artificially prepared nano-hole arrays. The nucleation probability of a single QD at each nano-hole reached 100% by depositing InGaAs at low temperature and subsequent annealing. Four InGaAs QD layers were vertically stacked while conserving the regularity, and the stacked QD array has shown a clear photoluminescence peak at room temperature. We discuss the effects of several growth conditions on the nucleation probability of QDs.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

Spontaneous one-dimensional lateral alignment of multistacked InGaAs quantum dots on GaAs (n 1 1)B substrates

In_0.45Ga_0.55As/GaAs multistacking quantum dot (QD) structures were fabricated on a GaAs (n 1 1)B (n=2–4) substrate by metalorganic vapor-phase epitaxy. QDs spontaneously aligned in the [0 1 1] direction were observed on stacked QDs, whereas QDs were randomly distributed in the initial In_0.45Ga_0.55As layer growth. The formation mechanism of this self-alignment was studied by changing the number of In_0.45Ga_0.55As/GaAs multilayers and crystallographic arrangement. Photoluminescence spectra showing clear polarization dependence indicate carrier coupling in the QD arrays. This growth technique results in spontaneously aligned InGaAs QDs without any preprocessing technique prior to growth.     

Selective growth of stacked InAs quantum dots by using the templates formed by the Nano-Jet Probe

We have demonstrated the selective area growth of stacked self-assembled InAs quantum dot (QD) arrays in the desired regions on a substrate and confirmed the photoluminescence (PL) emission exhibited by them at room temperature. These InAs QDs are fabricated by the use of a specially designed atomic force microscope cantilever referred to as the Nano-Jet Probe (NJP). By using the NJP, two-dimensional arrays with ordered In nano-dots are fabricated in the desired square regions on a GaAs substrate and directly converted into InAs QD arrays through the subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs are stacked. These stacked QDs exhibit the PL emission peak at a wavelength of 1.02 μm.     

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.     

Temperature-dependent photoluminescence of vertically stacked self-assembled CdSe quantum dots in ZnSe

We studied the optical properties of multiple layers of self-assembled CdSe quantum dots (QDs) embedded in ZnSe, grown by molecular beam epitaxy. The ZnSe barrier thicknesses separating the QD layers ranged from 30 to 60 monolayers (ML). For stacks with thinnest ZnSe barriers photoluminescence (PL) measurements reveal blue shifts as large as 180 meV relative to PL observed for single QD layers. The amount of blue shift decreases with increasing barrier thickness, and for the 60 ML spacer the PL energy returns to that emitted by a single layer of QDs. Temperature dependence of the integrated intensity of the emission spectra reveals that the activation energy for PL quenching is largest for barrier thicknesses of 30 and 45 ML. We tentatively attribute these effects to a decrease in the size of the vertically stacked QDs when the thickness of the barrier layers is small.     

Strain distribution and electronic states in stacked InAs/GaAs quantum dots with dot spacing 0–

We have calculated the strain distribution and electronic structures in stacked InAs/GaAs quantum dots (QDs) with the dot spacing 6–. We used the elastic continuum theory for the strain distribution, and the 8-band k·p theory for the electronic structures. For the triply stacked QDs, the light-hole (LH) component of the hole ground state increases with decreasing the dot spacing. The LH component in the columnar QD (dot spacing ) reaches 21.1% which is 4.8 times larger than that in the single QD due to the reduction of the biaxial strain. Further increase of the LH component (up to 28.6%) is obtained in the fivefold-stacked columnar QD. This result suggests a possibility of increase in the TM-mode transition in the columnar QDs.     

Numerical simulation of a coupling effect on electronic states in quantum dots

Lasers operating at 1.3 μm have attracted considerable attention owing to their potential to provide efficient light sources for next-generation high-speed communication systems. InAs/GaAs quantum dots (QDs) were pointed out as a reliable low-cost way to attain this goal. However, due to the lattice mismatch, the accumulation of strain by stacking the QDs can cause dislocations that significantly degrade the performance of the lasers. In order to reduce this strain, a promising method is the use of InAs QDs embedded in InGaAs layers. The capping of the QD layer with InGaAs is able to tune the emission toward longer and controllable wave-lengths between 1.1 and 1.5 μm. In this work, using the effective-mass envelope-function theory, we investigated theoretically the optical properties of coupled InAs/GaAs strained QDs based structures emitting around 1.33 μm. The calculation was performed by the resolution of the 3D Schrödinger equation. The energy levels of confined carriers and the optical transition energy have been investigated. The oscillator strengths of this transition have been studied with and without taking into account the strain effect in the calculations. The information derived from the present study shows that the InGaAs capping layer may have profound consequences as regards the performance of an InAs/GaAs QD based laser. Based on the present results, we hope that the present work make a contribution to experimental studies of InAs/GaAs QD based structures, namely the optoelectronic applications concerning infrared and mid-infrared spectral regions as well as the solar cells.     

Inhibition of emission wavelength blueshift in annealed InAs/GaAs quantum dot stacks: an important observation for their potential application in photovoltaic devices

We have investigated the effect of post growth rapid thermal annealing on self-assembled InAs/GaAs multilayer QDs (MQD) overgrown with a combination barrier of InAlGaAs and GaAs for their possible use in photovoltaic device application. The samples were characterized by transmission electron microscopy and photoluminescence measurements. We noticed a thermally induced material interdiffusion between the QDs and the wetting layer in the MQD sample up to a certain annealing temperature. The QD heterostructure exhibited a thermal stability in the emission peak wavelength on annealing up to 700 ^°C temperature. A phenomenological model has been proposed for this stability of the emission peak. The model considers the effect of the strain field, propagating from the underlying QD layer to the upper layers of the multilayer QD and the effect of indium atom gradient in the combination barrier layer due to the presence of a quaternary InAlGaAs layer.     

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.     

InAs/AlAs quantum dots with InGaAs insertion layer: dependence of the indium composition and the thickness

We have systematically studied the effect of an In_xGa_1−xAs insertion layer (IL) on the optical and structural properties of InAs quantum dot (QD) structures. A high density of 9.6×10¹⁰ cm⁻² of InAs QDs with an In_0.3Ga_0.7As IL has been achieved on a GaAs (1 0 0) substrate by metal organic chemical vapor deposition. A photoluminescence line width of 25 meV from these QDs has been obtained. We attribute the high density and high uniformity of these QDs to the use of the IL. Our results show that the InGaAs IL is useful for obtaining high-quality InAs QD structures for devices with a 1.3 μm operation.     

Confinement-enhanced dots-in-a-well QDIPs with operating temperature over 200 K

LWIR InAs/Al_0.3Ga_0.7As/In_0.15Ga_0.85As confinement-enhanced DWELL (CE-DWELL) QDIPs with operation temperatures higher than 200 K are reported. A thin Al_0.3Ga_0.7As barrier layer was inserted above the InAs QDs to improve the confinement of QD states in the In_0.15Ga_0.85As DWELL structure and the device performance. The better confinement of the electronic states increases the oscillator strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases for more than 20 times and the detectivity is an order of magnitude higher at 77 K. With better device parameters of CE-DWELL, it is possible to achieve high quantum efficiency, high operating temperature and long wavelength detection at the same time.     

The effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs quantum dots grown on GaAs(3 1 1)B substrate

We fabricated multiple stacked self-organized InGaAs quantum dots (QDs) on GaAs (3 1 1)B substrate by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) to realize an ordered three-dimensional QD array. High quality stacked QDs with good size uniformity were achieved by using strain-compensation growth technique, in which each In_0.35Ga_0.65As QD layer was embedded by GaNAs strain-compensation layer (SCL). In order to investigate the effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs QDs, the thickness of GaNAs SCL was varied from 40 to 20 nm. We observed that QDs were vertically aligned in [3 1 1] direction when viewed along [0 1 −1], while the alignment was inclined when viewed along [−2 3 3] for all samples with different SCL thickness. This is due to their asymmetric shape along [−2 3 3] with two different dominant facets thereby the local strain field around QD extends further outward from the lower-angle facet. Furthermore, the inclination angle of vertical alignment QDs became monotonously smaller from 22° to 2° with decreasing SCL thickness from 40 to 20 nm. These results suggest that the strain field extends asymmetrically resulting in vertically tilted alignment of QDs for samples with thick SCLs, while the propagated local strain field is strong enough to generate the nucleation site of QD formation just above lower QD in the sample with thinner SCLs.     

Demonstration of 640 × 512 pixels long-wavelength infrared (LWIR) quantum dot infrared photodetector (QDIP) imaging focal plane array

We have exploited the artificial atom-like properties of epitaxially grown self-assembled quantum dots (QDs) for the development of high operating temperature long wavelength infrared (LWIR) focal plane arrays (FPAs). QD infrared photodetectors (QDIPs) are expected to outperform quantum well infrared detectors (QWIPs) and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow multi-layer LWIR dot-in-a-well (DWELL) structures based on the InAs/InGaAs/GaAs material system. This hybrid quantum dot/quantum well device offers additional control in wavelength tuning via control of dot-size and/or quantum well sizes. DWELL QDIPs were also experimentally shown to absorb both 45° and normally incident light. Thus we have employed a reflection grating structure to further enhance the quantum efficiency. The most recent devices exhibit peak responsivity out to 8.1 μm. Peak detectivity of the 8.1 μm devices has reached 1 × 10¹⁰ Jones at 77 K. Furthermore, we have fabricated the first long-wavelength 640 × 512 pixels QDIP imaging FPA. This QDIP FPA has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60 K operating temperature.     

Fabrication and characterization of a vertical pillar structure including a self-assembled quantum dot and a quantum well

We fabricate and characterize a novel vertical pillar structure including a self-assembled InAs quantum dot (QD) and an InGaAs quantum well (QW). The vertical current through both the InAs QD and an electrostatically defined QD made in the InGaAs QW can be measured by adjusting the position of the InGaAs QD in the QW plane relative to the InAs QD with two side-gate voltages applied independently. We study optical response of the current through the vertical double QD by irradiating light, which is assumed to be mainly absorbed in the InAs QDs. We successfully probe a time-dependent energy level shift due to the Coulomb interaction from holes trapped in the vicinity of the pillar.     

Selective growth of GaAs quantum dots and vertical quantum wires in two-dimensional V-grooves

We propose and demonstrate a novel technique for the fabrication of quantum dot (QD) structures using metal organic chemical vapor deposition (MOCVD). The GaAs quantum dots are grown at the bottom of the two-dimensional V-groove (2DVG) structures which are composed of (1 1 1)A and (1 1 1)B-facets on GaAs(1 0 0). The 2DVG is formed by MOCVD selective growth on a SiO₂ patterned substrate. It should be noted that the 2DVGs cannot be formed by a chemical wet etching technique because the facet's anisotropy of etching ratios are different. By changing the growth condition, we can obtain GaAs QD structures which have a size of less than 10 nm, and vertical GaAs quantum wires (V-QWRs) in 2DVGs. We have observed photoluminescence from each structure. We have also demonstrated stacking of GaAs QDs in the 2DVG on GaAs (1 0 0).     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.