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We have developed an in situ mask that enables the selective formation of molecular beam epitaxially grown layers in narrow regions. This mask can be fitted to a sample holder and removed in an ultrahigh-vacuum environment; thus, device structures can be fabricated without exposing the sample surfaces to air. Moreover, this mask enables the observation of reflection high-energy electron diffraction during growth with the mask positioned on the sample holder and provides for the formation of marker layers for ensuring alignment in the processes following the selective growth. To explore the effectiveness of the proposed in situ mask, we used it to grow quantum dot (QD) structures in narrow regions and verified the perfect selectivity of the QD growth. The grown QDs exhibited high optical quality with a photoluminescence peak at approximately 1.30 mum and a linewidth of 30 meV at room temperature. The proposed technique can be applied for the integration of microstructures into optoelectronic functional devices.

The nonlinear optical phase shift in self-assembled InAs quantum dots (QDs) under resonant excitation in a ground-state transition was measured by a unique two-color pump/probe ellipsometric polarization analysis. This ellipsometric analysis makes use of the large optical birefringence of SK-QD [(SK) — Stranski-Krastanov] originating from the asymmetric structure. A phase shift of 0.5π rad was obtained at an input pump pulse energy density of 30 pJ/μm2, a detuning of 11 meV, and a time delay of 20 ps in a 1 mm long waveguide having QDs with a peak wavelength of 1290 nm, a volume density of 4×1015 cm-3, and inhomogeneous broadening of 35 meV. Analysis revealed that the phase shift is mainly attributed to the absorption saturation for TE-polarized light, though other mechanisms also could contribute at higher pumping. The calculation, based on the two-level approximation, revealed that the minimum energy density for π shift is 240 fJ/μm2, calculated under ideal conditions. [ABSTRACT FROM AUTHOR]

The optical nonlinear properties of self-assembled InAs/GaAs quantum dots (QDs) were experimentally verified by means of transient absorption measurements. A saturation pulse energy P[SUBs] of 13 fJ/μm[SUP2] and an absorption recovery time &tou;[SUBr] of 55 ps were obtained from transmission bleaching and pump/probe measurements for a waveguide sample with ten-layer-stacked QDs. An absorption saturation intensity I[SUBs] of 2.5×10[SUP4]W/cm[SUP2], calculated from P[SUBs] and τ[SUBr], was found. The saturation pulse energy is up to an order of magnitude smaller than, or at least comparable with, the reported values for excitons in quantum wells of III-V compound semiconductors. The dipole length, as calculated from the absorption cross section, is of the same order as the lattice constant of the InAs QDs. The results are expected to experimentally verify that QDs show a delta-function-like density of states. [ABSTRACT FROM AUTHOR]

Abstract: We have grown bi-layer InAs quantum dots (QDs) on GaAs substrates for extending the emission wavelength of InAs/GaAs QDs beyond 1.3μm. The QD bi-layer, which comprises two QDs layers (seed- and active-QDs) separated with a GaAs spacer layer of 10nm in thickness, exhibits light emission from active-QDs with longer wavelength. An enlargement of the active-QDs occurred by optimizing several growth parameters: growth temperature of seed-QDs, amount of InAs supplied for seed- and active-QDs layers. These optimized parameters lowered the density of the seed-QDs strain spreading upward, which resulted in an enlargement of the active-QDs. We achieved a control of the extension of emission wavelength up to approximately 1.4μm from 1.2μm. These results indicate that the QD bi-layer can be applied to a broadband light source exhibiting an emission spectrum centered at 1.3μm with a bandwidth of 200nm. [Copyright &y& Elsevier]

Abstract: Two types of nanophotonic technologies—two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)—were developed for key photonic device structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach–Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip–flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements. [Copyright &y& Elsevier]

Abstract: The selective-area-growth (SAG) of InAs quantum dots (QDs) with different emission wavelengths on different areas was carried out using the metal-mask (MM) method during conventional molecular beam epitaxy (MBE) growth. Two SAG areas (SAG-1 and SAG-2) of the QDs were obtained using the rotational MM having square windows; first, the MM was mounted on a substrate for SAG-1, and then the same MM was used for SAG-2 after being rotated it by 180°. The absorption wavelengths of the selectively grown QDs were controlled by inserting strain-reducing layers of a different thickness on each grown QD layer. Photoluminescence measurements revealed the successful SAG of QDs in the neighboring a few hundred-micron-square areas with different emission wavelengths, e.g., 1250 and 1300nm. These techniques are promising for applications of QD-based optical and electronic devices, including our proposed photonic crystal (PC) and QD-based all-optical digital flip–flop (FF) device, i.e., PC-FF. [Copyright &y& Elsevier]

Abstract: A metal-mask (MM) molecular beam epitaxy technique for selective-area-growth (SAG) of self-assembled InAs-quantum dots (QDs) on a GaAs substrate was developed for realizing ultra-small and ultra-fast all-optical switches (PC-SMZ). Growth conditions of the QD were optimized by virtue of the MM with a large window enabling real-time observations of reflection-high-energy-electron-diffraction patterns and growth temperatures. Successful SAG was confirmed by atomic force microscope observations of the high-density (4×1010 cm−2) QD grown only on the unmasked area, while high uniformity of the QD was examined by room-temperature photoluminescence (PL) measurement. It resulted in a 1250nm peak in the center wavelength, 38meV in FWHM, comparable to that of the QD grown without the MM. In addition, insertion of a strain-reducing layer of GaInAs on the QD reported previously was confirmed to be effective for controlling the PL peak wavelength of the QD between 1250 and 1320nm without degrading the optical quality of QDs. The MM method shown here provides a possibility of an advanced PC-SMZ, i.e., an ultra-fast all-optical flip-flop (PC-FF), which requires a QD ensemble with a different absorption-peak wavelength in a different area. [Copyright &y& Elsevier]

Abstract: We have successfully grown ordered InGaAs quantum dots (QDs) with perfect selectivity; these QDs were formed in a nano-hole region and not in any other flat regions. To achieve perfect selectivity, InGaAs was grown at a high temperature of 530°C and a low growth rate of 0.035ML/s. A scanning tunneling microscope (STM) image showed the formation of the ordered InGaAs QDs with a density of 1×1010 cm−2, whereas only monolayer steps were observed and no QDs were formed in the flat region. Further, we stacked InAs QD arrays by using the InGaAs QDs on the first layer as a template. Another STM image revealed the formation of small InAs QDs with a high density. QDs with an octagonal lattice arrangement were also formed in a region that was limited in a small area. This arrangement of InAs QDs is probably due to strain field of the underlying InGaAs QDs. [Copyright &y& Elsevier]

Abstract: We have demonstrated the selective area growth of high density InAs quantum dots (QDs) by using site-controlled InAs dots that were formed in the desired regions as templates. The fabrication of the InAs dots for the templates was enabled by the use of a specially designed atomic-force-microscope (AFM) cantilever, referred to as the nano-jet probe (NJP). Using the NJP, two-dimensional (2D) arrays of ordered In nano-dots were reproducibly fabricated in the desired regions on a GaAs substrate. These In nano-dots were directly converted to InAs QD arrays by subsequent annealing with irradiation of arsenic flux. By using the converted QD arrays as strain templates, self-organized InAs QDs were formed in the selected regions. [Copyright &y& Elsevier]

We have successfully fabricated high-density two-dimensional indium (In) nano-dot arrays on a GaAs substrate with periods of 50 nm. These nano-dot structures were formed using an atomic force microscope (AFM) probe with a specially designed cantilever (the Nano-Jet Probe) having a hollow pyramidal tip with a sub-micron size aperture on the apex and an In-reservoir tank within the stylus. These ordered In nano-dot arrays can be directly converted to InAs quantum dot (QD) arrays by subsequent irradiation of arsenic flux in the molecular beam epitaxy chamber, which is connected to the AFM chamber through an ultra-high-vacuum (UHV) tunnel. [Copyright &y& Elsevier]

We have successfully and reproducibly fabricated uniform indium (In) nano-dots at a selected point. Nano-dot formation was realized using an atomic force microscope (AFM) probe with a specially designed cantilever, which was equipped with a hollow pyramidal tip with a sub-micron size aperture on the apex and an In-reservoir tank within the stylus. The In nano-dots formed in this study can be directly converted to InAs quantum dots by subsequent irradiation of arsenic flux in the molecular beam epitaxy chamber, which is connected to the AFM chamber through an ultra-high-vacuum tunnel. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

A technique for controlling the height of InAs quantum dots (QDs) on GaAs surfaces has been investigated by combining layer-by-layer in situ AsBr3 etching and molecular beam epitaxy (MBE). One layer of InAs QDs in the Stranski–Krastanow (SK) growth mode is first formed on a GaAs surface by MBE. After a thin GaAs film is deposited on the InAs SK QDs, in situ etching creates an array of nanoholes on the etched GaAs surface. The nanoholes offer highly selective nucleation sites for the subsequent growth of InAs QDs. Finally, second-layer QDs are grown by providing InAs of only about 1 monolayer (ML). Atomic force microscopy surface observations indicate that a planar growth surface is obtained with the supplied amount of ∼1 ML of InAs. Cross-sectional structural observations conducted using a scanning transmission electron microscope and energy dispersive X-ray analysis reveal that vertically overlapping InAs QDs are achieved. These results demonstrate the formation of height-controlled InAs QDs. [Copyright &y& Elsevier]

A spatial site-control method for InAs quantum dots (QDs) is proposed and demonstrated using ultrahigh vacuum in situ processing. Regular InAs QD arrays are initially prepared as strain templates on GaAs surfaces by the scanning tunneling microscope (STM) probe-assisted site-control technique. Multiple layers of InAs Stranski–Krastanov QDs are then grown with GaAs spacers for strain-induced vertical alignment above the regular QD arrays, resulting in three-dimensional QD lattices. Step-by-step STM observations reveal the growth mode of QDs in the vertical alignment process. [Copyright &y& Elsevier]

Abstract: Multi-color quantum dot (QD) ensembles were grown by selective-area growth method to realize a shape-controlled broadband light source. By using a metal-mask, QD ensembles and strain reducing layer (SRL) were formed in selective areas on a wafer. The SRL thickness was varied to achieve appropriate shifts in the peak wavelength of the QD emission spectrum up to 90nm. A summation of PL spectra obtained from the multi-color QD ensembles shows a broadband emission spectrum with a width of approximately 120nm, even though this spectrum is attributed to the ground state emissions of these QD ensembles. A current-induced broadband light source such as a superluminescent diode (SLD) based on the multi-color QD ensembles is expected to have an emission spectrum with a width of more than 120nm owing to the combination of excited state emissions. Furthermore, a desired shape of the SLD spectrum can be obtained by controlling the injection current applied to each QD ensemble. This approach is promising for a shape-controlled broadband SLD, and it is particularly applicable to optical coherence tomography (OCT). [Copyright &y& Elsevier]

Abstract: We have developed a nanoprobe-assisted bottom-up technique that allows to fabricate site-controlled quantum dots (SC-QDs), using a specially designed atomic force microscope probe, referred to as the Nano-Jet Probe (NJP). Beginning with the fabrication of regular QD arrays by using the NJP, we vertically aligned self-assembled InAs QDs using the strain-induced stacking method and produced assembled QD stacks. In order to improve the optical property of the SC-QDs, an AlGaAs barrier layer was introduced in the spacer layer of the stacked structures. The photoluminescence measurements of the fabricated structures revealed that they had good crystallographic qualities. [Copyright &y& Elsevier]

Abstract: 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 1.02μm. [Copyright &y& Elsevier]

Abstract: We have demonstrated the selective area growth of self-assembled InAs quantum dots (QDs) in the desired regions by using a template composed of InAs QD arrays. These InAs QDs were fabricated by the use of a specially designed atomic force microscope cantilever, referred to as the Nano-Jet Probe (NJP). Using the NJP, two-dimensional arrays of ordered In nano-dots were fabricated in the selected square regions on a GaAs substrate and directly converted to InAs QD arrays by subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs were formed in the selected square regions. These InAs QD arrays were used for the further stacking of QDs, and photoluminescence emission was confirmed from the fabricated stacked QD structure. [Copyright &y& Elsevier]