Buried Quantum Dots Research Articles

Improvement in Performance of InAs Surface Quantum Dot Heterostructure-Based H2S Gas Sensor by Introducing Buried Quantum Dot Layer

In this work, we have demonstrated InAs surface quantum dot (SQD) based H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S gas sensors. The epitaxial growth of the strain-coupled and uncoupled InAs/GaAs QD heterostructures is done using the solid-source Molecular Beam Epitaxy (MBE) tool. For both types of heterostructures, the coverage of the InAs monolayer (ML) for the SQD layer varies from 0.9 ML to 2 ML. The ML coverage of the buried quantum dots (BQDs) layer for the coupled heterostructures is kept constant (2.7 ML). The atomic force microscopy results demonstrated that the coupled heterostructures have higher quantum dot density in the SQDs layer in comparison to the uncoupled one due to strain propagation from the BQDs towards the SQD layer. The sensor fabricated using the coupled heterostructure with 2 ML SQDs has demonstrated better performance than the uncoupled one for various concentrations (1 ppm -1000 ppm) of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S gas due to inter-dot carrier tunneling between BQDs and SQDs layer. The coupled InAs gas sensor showed the best sensing properties at room temperature (45.9% sensor response @ 100ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S). We have demonstrated the selectivity of the sensor towards H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S among various target gasses like CO, CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O, and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the stability over a longer period of time with only 3% deviation (within acceptable limit). These findings have the potential to promote the fabrication of high-performance gas sensors using SQDs-based coupled heterostructures.

Angle-Selective Photodetection in Ge/Si Quantum Dot Photodiodes Enhanced by Microstructured Hole Arrays

We report on the near-infrared (NIR) photoresponse of a micropatterned Ge/Si quantum dot (QD) pin photodiode at different angles of radiation incidence. The photon-trapping hole array was etched through the n+-type top contact layer to reach the buried QDs. The normal-incidence responsivity was observed to be resonantly increased at wavelengths of 1.4, 1.7, and 1.9 μm by factors of 40, 33, and 30, respectively, compared with the reference detector without holes. As the incident angle θ increases, the resonance peaks are disappeared and at θ&gt;40∘ a new resonance with a 25× enhancement arises at a wavelength of 1.8 μm. Simulation of the near-field intensity, Poynting vector distribution and wave polarization showed that at small θ, the strong electric field is primarily localized under the air holes (1.4 μm, TM mode) or between the holes (1.7 and 1.9 μm, TE modes) inside the region occupied by QDs, resulting in the strong NIR photocurrent. At large θ, the dominant resonance detected at 1.8 μm is the result of coupling between the TE and TM modes and formation of a mixed near-field state.

Novel approach for augmented carrier transfer and reduced Fermi level pinning effect in InAs surface quantum dots

The present work introduces an in-situ technique to tune the inter-dot coupling between vertically aligned InAs surface and buried quantum dots (SQD and BQD). Here, we utilize the self-assembly growth kinetics to get the most stable SQDs with lowest possible surface states. We vary the monolayer (ML) coverage of top SQD layer (from 2.2 ML to 1.6 ML), keeping monolayer coverage of BQD constant (2.7 ML). The reduced Fermi level pinning effect is observed through the monolayer coverage minimization, and the upshifting of energy level in the conduction band is obtained. The degeneracy between BQD and SQD energy levels is maximized in case of the 1.6 ML SQD and an enhanced coupling between the two QD families is observed through the photoluminescence (PL) result analysis. The type-II carrier distribution with effect of the Fermi level pinning is reduced in the 1.6 ML SQD structure, which claims the highest level of Coulomb coupling between the two QD families. Also, AFM images show lower dispersion in size and shape of the surface dot with 1.6 ML coverage. The proposed optimum QD heterostructure would promote a QD based sensor with high efficiency in effect with higher level of inter- dot carrier tunneling.

Light-Trapping-Enhanced Photodetection in Ge/Si Quantum Dot Photodiodes Containing Microhole Arrays with Different Hole Depths.

Photodetection based on assemblies of quantum dots (QDs) is able to tie the advantages of both the conventional photodetector and unique electronic properties of zero-dimensional structures in an unprecedented way. However, the biggest drawback of QDs is the small absorbance of infrared radiation due to the low density of the states coupled to the dots. In this paper, we report on the Ge/Si QD pin photodiodes integrated with photon-trapping hole array structures of various thicknesses. The aim of this study was to search for the hole array thickness that provided the maximum optical response of the light-trapping Ge/Si QD detectors. With this purpose, the embedded hole arrays were etched to different depths ranging from 100 to 550 nm. By micropatterning Ge/Si QD photodiodes, we were able to redirect normal incident light laterally along the plane of the dots, therefore facilitating the optical conversion of the near-infrared photodetectors due to elongation of the effective absorption length. Compared with the conventional flat photodetector, the responsivity of all microstructured devices had a polarization-independent improvement in the 1.0–1.8-μm wavelength range. The maximum photocurrent enhancement factor (≈50× at 1.7 μm) was achieved when the thickness of the photon-trapping structure reached the depth of the buried QD layers.

Anomalous photoluminescence of InAs surface quantum dots: intensity enhancement and strain control by underlying quantum dots

Strain control and photoluminescence (PL) enhancement of InAs surface quantum dots (SQDs), exposed to ambient conditions, have been achieved by introducing underlying buried quantum dots (BQDs). The PL wavelength has been tuned from 1270 to as long as 1780 nm, redshifted as the size of the SQDs is reduced. This is in strong contrast to standard QDs, in which blueshift is observed from smaller QDs following basic quantum mechanics. Here, smaller SQDs, both in height and base area, as observed by atomic force microscopy, were obtained with wider GaAs spacer thickness between the SQDs and BQDs. The result strongly suggests that strain and related effects dominate the electronic properties of the SQDs rather than their size, and that a change in the complex strain field occurs through the spacer. The underlying BQDs also serve as effective carrier reservoirs. A PL intensity enhancement of 17 fold was observed as the GaAs spacer thickness was reduced from 150 to 10 nm. A large portion of the photoexcited carriers is initially captured and stored in the BQDs. When sufficient carriers are transferred to fill non-radiative surface states, the excess may be transferred to the SQDs enhancing the luminescence.

Carrier Injection to In0.4Ga0.6As/GaAs Surface Quantum Dots in Coupled Hybrid Nanostructures

Stacking growth of the InGaAs quantum dots (QDs) on top of a carrier injection layer is a very useful strategy to develop QD devices. This research aims to study the carrier injection effect in hybrid structures with a layer of In0.4Ga0.6As surface quantum dots (SQDs), coupled to an injection layer of either one layer of In0.4Ga0.6As buried QDs (BQDs) or an In0.15Ga0.85As quantum well (QW), both through a 10 nm GaAs thin spacer. Spectroscopic measurements show that carrier capture and emission efficiency for SQDs in the BQD injection structure is better than that of the QW injection, due to strong physical and electrical coupling between the two QD layers. In the case of QW injection, although most carriers can be collected into the QW, they then tunnel into the wetting layer of the SQDs and are subsequently lost to surface states via non-radiative recombination. Therefore, the QW as an injection source for SQDs may not work as well as the BQDs for stacking coupled SQDs structures.

Droplet epitaxy of InAs/InP quantum dots via MOVPE by using an InGaAs interlayer

InAs quantum dots (QDs) are grown on an In0.53Ga0.47As interlayer and embedded in an InP(100) matrix. They are fabricated via droplet epitaxy (DE) in a metal organic vapor phase epitaxy (MOVPE) reactor. Formation of metallic indium droplets on the In0.53Ga0.47As lattice-matched layer and their crystallization into QDs is demonstrated for the first time in MOVPE. The presence of the In0.53Ga0.47As layer prevents the formation of an unintentional non-stoichiometric 2D layer underneath and around the QDs, via suppression of the As-P exchange. The In0.53Ga0.47As layer affects the surface diffusion leading to a modified droplet crystallization process, where unexpectedly the size of the resulting QDs is found to be inversely proportional to the indium supply. Bright single dot emission is detected via micro-photoluminescence at low temperature, ranging from 1440 to 1600 nm, covering the technologically relevant telecom C-band. Transmission electron microscopy investigations reveal buried quantum dots with truncated pyramid shape without defects or dislocations.

Effect of Cap Thickness on InAs/InP Quantum Dots Grown by Droplet Epitaxy in Metal–Organic Vapor Phase Epitaxy

InAs quantum dots (QDs) are grown on bare InP(001) via droplet epitaxy (DE) in metal–organic vapor phase epitaxy (MOVPE). Capping layer engineering, used to control QD size and shape, is explored for DE QDs in MOVPE. The method allows for the tuning of the QD emission over a broad range of wavelengths, ranging from the O‐ to the L‐band. The effect of varying the InP capping layer is investigated optically by macro‐ and micro‐photoluminescence (PL, µPL) and morphologically by transmission electron microscopy (TEM). A strong 500 nm blueshift of the QD emission wavelength is observed when the capping layer is reduced from 20 to 8 nm, which is reflected by a clear size reduction of the buried QDs.

Protected Long-Distance Guiding of Hypersound Underneath a Nanocorrugated Surface.

In nanoscale communications, high-frequency surface acoustic waves are becoming effective data carriers and encoders. On-chip communications require acoustic wave propagation along nanocorrugated surfaces which strongly scatter traditional Rayleigh waves. Here, we propose the delivery of information using subsurface acoustic waves with hypersound frequencies of ∼20 GHz, which is a nanoscale analogue of subsurface sound waves in the ocean. A bunch of subsurface hypersound modes are generated by pulsed optical excitation in a multilayer semiconductor structure with a metallic nanograting on top. The guided hypersound modes propagate coherently beneath the nanograting, retaining the surface imprinted information, at a distance of more than 50 μm which essentially exceeds the propagation length of Rayleigh waves. The concept is suitable for interfacing single photon emitters, such as buried quantum dots, carrying coherent spin excitations in magnonic devices and encoding the signals for optical communications at the nanoscale.

Strong sulfur passivation effects on the gas sensitivity of an In0.3Ga0.7As surface quantum dots coupling structure

The strong sulfur passivation effects on the gas sensitivity of a coupling structure is investigated. The coupling structure is consisted of an In0.3Ga0.7As surface quantum dots (SQDs) layer being stacked on 5-layers buried quantum dots (BQDs) and is grown by solid source molecular beam epitaxy. With the increase of sulfur passivation time, both the height of SQDs and the gas response of the coupling structure are significantly reduced. Simultaneously, the PL emission from SQDs is suppressed deeply and the decay time of carriers in BQDs layers are risen evidently. After 20mintues passivation, the gas response is reduced to one fourth of that of un-passivated sample, the decay time of carriers in BQDs layers rose from 134 ps to 181 ps and the PL emission from SQDs almost disappeared. This work is an important meaning for the development of high stability and repeatability gas sensors.

Carrier localization in self-organized quantum dots: An interplay between quantum and solid mechanics

Localization of carriers in a self-organized quantum dot is a problem of quantum mechanics to be solved with the localizing potential for electrons and holes determined by the geometry, chemical composition, and built-in mechanical stress–strain field. We show that changes in the aspect ratio of a buried pyramidal quantum dot result in a substantial redistribution of the mechanical field components, which in turn provides certain non-trivial conditions for the strongest carrier localization at any given volume of the quantum dot.

Photoluminescence characterization of wetting layer and carrier dynamics for coupled InGaAs/GaAs surface quantum dot pair structures.

The optical properties are investigated by spectroscopic characterizations for bilayer InGaAs/GaAs quantum dot (QD) structures consisting of a layer of surface quantum dots (SQDs) separated from a layer of buried quantum dots (BQDs) by different GaAs spacers with thicknesses of 7 nm, 10.5 nm and 70 nm. The coupling from the BQDs to SQDs leads to carrier transfer for the two samples with thin spacers, 7 nm and 10.5 nm, in which QD pairs are obtained while not for the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from BQDs to SQD through the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at the wavelength of ∼ 960 nm, while the excitation intensity dependent PL and PLE spectra show that this is from the wetting layer (WL) of the SQDs. This WL is very important for carrier dynamics in bilayer structures of BQDs and SQDs, including for carrier generation, capture, relaxation, tunneling, and recombination. These results provide useful information for understanding the optical properties of InGaAs SQDs and for using such hybrid structures as building blocks for surface sensing devices.

Improved carrier transfer in vertically coupled surface and buried InAs Stranski-Krastanov quantum dot system via ex-situ surface state passivation

In this work, the authors have investigated the optical and morphological characteristics of uncapped InAs quantum dots (QDs) and their sensitivity to the surface passivation. The temperature dependent carrier transfer mechanism between self-assembled vertically coupled InAs surface quantum dot (SQD) and underneath buried quantum dot (BQD), separated by a thin spacer layer is investigated. Oxide layer etching followed by surface passivation has been carried out to deplete the surface states and improve the structural/optical behaviour of uncapped InAs SQDs. The passivation time has been varied from 0.5 to 2.5 h and its effect on the optical and morphological properties is investigated. The larger SQDs are resized to smaller ones and show lower size dispersion after the etching process. Additionally, an increased energy level overlapping between SQD and BQD is obtained after the etching process. This partial overlap of energy levels further corroborates the enhanced electronic communication achieved between the surface and underneath buried dots. The sample, with 1 h of surface passivation, unveils as the best among all in terms of stronger luminescence from the SQD. Through this study, the SQDs are projected as the primary receptor, whereas the BQDs behave like carrier reservoirs. Hence, the BQDs would provide the carriers to enhance the sensitivity of the SQDs, which would impact the performance of QD based sensors.

Gas Sensitivity of In0.3Ga0.7As Surface QDs Coupled to Multilayer Buried QDs

A detailed analysis of the electrical response of In0.3Ga0.7As surface quantum dots (SQDs) coupled to 5-layer buried quantum dots (BQDs) is carried out as a function of ethanol and acetone concentration while temperature-dependent photoluminescence (PL) spectra are also analyzed. The coupling structure is grown by solid source molecular beam epitaxy. Carrier transport from BQDs to SQDs is confirmed by the temperature-dependent PL spectra. The importance of the surface states for the sensing application is once more highlighted. The results show that not only the exposure to the target gas but also the illumination affect the electrical response of the coupling sample strongly. In the ethanol atmosphere and under the illumination, the sheet resistance of the coupling structure decays by 50% while it remains nearly constant for the reference structure with only the 5-layer BQDs but not the SQDs. The strong dependence of the electrical response on the gas concentration makes SQDs very suitable for the development of integrated micrometer-sized gas sensor devices.

Lateral carrier transfer for high density InGaAs/GaAs surface quantum dots

In(Ga)As surface quantum dots (SQDs) are promising candidates for surface-sensitive detection applications. As such, studies of the carrier dynamics are indispensable when attempting to understand both the physics and the device performances based on such SQD structures. Here, lateral carrier transfer among neighboring quantum dots (QDs) has been investigated through spectroscopic measurements for high density InGaAs/GaAs SQDs in a hybrid nanostructure with similar buried QDs (BQDs). Photoluminescence (PL) and temporal-resolved PL (TRPL) spectra demonstrate that the SQDs exhibit intra-layer exciton transfer and also strong carrier transfer to the surface states at 10 K. This imparts different optical characteristics to the SQDs with regards to the band filling and emission-energy-dependent carrier lifetime. In addition, PL excitation (PLE) spectra reveal a wetting layer absorption feature for the SQDs, which provides a channel for thermally activated carriers to transfer among the SQDs and to ultimately escape from the SQD layer. These investigations enrich our understanding of carrier dynamics for InGaAs SQDs, which may help to develop novel surface sensitive sensors.

Size and Shape Evolution of GaAsSb-Capped InAs/GaAs Quantum Dots: Dependence on the Sb Content

Capping InAs/GaAs quantum dots (QDs) with a thin GaAsSb layer alters the QDs structural properties, leading to considerable changes in their optical properties. The increase of the Sb content induces a redshift of the emission energies, indicating a change in the buried QDs shape and size. The presence of well-defined ground- and excited-state emission bands in all the photoluminescence spectra allow an accurate estimation of the buried QDs size and shape by numerical evaluation and tuning of the theoretical emission energies. For an Sb content below 14%, the QDs are found to have a type I band alignment with a truncated height pyramidal form. However, for higher Sb content (22%), the QDs are present in a full pyramidal shape. The observed behavior is interpreted in terms of increasing prevention of InAs QDs decomposition with increasing the Sb content in the cap layer.

Formation and properties of InGaN QDs: Influence of substrates

We examine the formation and properties of InGaN quantum dots (QDs) on free-standing GaN and GaN/sapphire templates, with and without buried InGaN/GaN QD superlattices (SLs). We use scanning tunneling microscopy (STM) and scanning tunneling spectroscopy to image the QDs and measure their electronic states. As the number of layers preceding the QDs increases (i.e., increasing substrate complexity), the total QD density increases. For free-standing GaN, STM reveals a mono-modal QD-size-distribution, consistent with a limited density of substrate threading dislocations serving as heterogeneous nucleation sites. For GaN/sapphire templates, STM reveals a bimodal QD-size-distribution, presumably due to the nucleation of additional ultra-small InN-rich QDs near threading dislocations. For multi-period QD SLs on GaN/sapphire templates, an ultra-high density of QDs, with a mono-modal size distribution is apparent, suggesting that QD nucleation is enhanced by preferential nucleation at strain energy minima directly above buried QDs. We discuss the relative influences of strain fields associated with threading dislocations and buried QD SLs on the formation of InGaN QDs and their effective bandgaps.

Two dimensional photonic crystal nanocavities with InAs/GaAs quantum dot active regions embedded by MBE regrowth

We demonstrate two dimensional photonic crystal nanocavities embedding spatially-isolated active regions containing InAs/GaAs self-assembled quantum dots. We fabricated the GaAs-based structures by a combination of electron-beam lithography, dry/wet etching and crystal regrowth using molecular beam epitaxy. The fabricated nanocavity based on a photonic crystal double heterostructure supports a cavity mode with a high experimental quality factor over 10,000, despite the existence of a mound structure on the nanocavity formed during the regrowth. Using electromagnetic simulations, we discuss the origin of the cavity mode and a direction for further increasing the quality factor. The two dimensional photonic crystal nanocavities with buried quantum dot active regions will provide a useful platform for developing high performance nanolasers.

Strong Influence of Temperature and Vacuum on the Photoluminescence of In0.3Ga0.7As Buried and Surface Quantum Dots

The strong influences of temperature and vacuum on the optical properties of In0.3Ga0.7As surface quantum dots (SQDs) are systematically investigated by photoluminescence (PL) measurements. For comparison, optical properties of buried quantum dots (BQDs) are also measured. The line-width, peak wavelength, and lifetime of SQDs are significantly different from the BQDs with the temperature and vacuum varied. The differences in PL response when temperature varies are attributed to carrier transfer from the SQDs to the surface trap states. The obvious distinctions in PL response when vacuum varies are attributed to the SQDs intrinsic surface trap states inhibited by the water molecules. This research provides necessary information for device application of SQDs as surface-sensitivity sensors.

3‐D Strain Fields in Low‐Dimensional III–V Semiconductors: A Combined Finite Elements and HRTEM Approach

A versatile route toward the study of strain fields of low‐dimensional III–V semiconductor nanostructures is presented, by combining quantitative high‐resolution transmission electron microscopy (HRTEM) observations with the finite elements method (FEM). FEM facilitates a fast and straightforward three‐dimensional (3‐D) analysis of elastic properties for various growth orientations and compositional profiles down to the nanoscale. FEM calculations are employed to simulate elastic stress–strain fields of III–V cubic heterostructures comprising InAs surface and buried quantum dots (QDs) grown on GaAs(211)B substrates, and (111)‐oriented GaAs/AlxGa(1−x)As core–shell nanowires (NWs) on Si. The results are compared with experimental strain maps obtained from HRTEM images by geometric phase analysis (GPA), as well as with molecular dynamics (MD) atomistic simulations. In the former, the compositional grading along the growth axis was considered, and, in the latter, elastic fields were calculated as a function of the shell's chemical composition and shell‐to‐NW diameter ratios. The agreement between FEM calculations with experimental and theoretical results implies that the plane‐stress state can adequately describe the encountered elastic fields. Most importantly, through the determined stress–strain state, strain fields can be translated into 3‐D maps of chemical composition in the nanostructures, extracted from 2‐D experimental projections.