InAs/InAlGaAs/(001) InP quantum dot lasers achieving >100°C continuous-wave operation via uniformity-enhanced MOCVD growth.

High temperature photoluminescence up to 100°C was demonstrated from the p-doped ten-layer InAs∕InGaAs quantum dot (QD) laser structure. 1.3μm InAs QD lasers were fabricated using pulsed anodic oxidation from this structure. High output power of 882mW and low transparency current density of 5.9A∕cm2∕QD layer were obtained. Ground state (GS) lasing could be maintained from a QD laser with short cavity length of 611μm, corresponding to the maximum modal gain of 23.1cm−1 from this laser system. GS continuous wave operation up to 100°C was also demonstrated from an InAs QD laser (50×2500μm2).

Summary form only given. We report direct measurements of the optical loss and gain of structures employing 1, 3, 5 and 7 layers of quantum dots using an electrically pumped multi-section technique. The structures consist of layers of InGaAs quantum dots embedded in GaAs quantum wells with an AlGaAs waveguide core whose thickness is adjusted in each case to maintain a constant average confinement factor per layer. We plot the optical mode loss for structures containing different numbers of dot layers. The results indicate that in fact the optical mode loss, /spl alpha//sub 1/, is independent of the number of quantum dot layers. On the other hand, the peak-gain does scale with the number of layers. The peak-gain per layer at low current densities is similar for structures with 1, 3, 5, and 7 layers of quantum dots. This means the net-modal gain (gain-/spl alpha//sub 1/) of the dot ground state can be increased by using multiple quantum dot layers in the active region. At high current densities the gain per layer of quantum dots does not scale with layer number in a simple fashion and we also discuss the origin of this effect.

A modified gradient approach for the growth of low-density InAs quantum dot molecules by molecular beam epitaxy

We report on the growth of quantum dot (QD) layers of InAsP alloys buried in GaAs by low-pressure metalorganic chemical vapor deposition. Ternary QDs were obtained by the addition of a PH3 flux during the InAs QD growth, exhibiting recombination energies lying between those of InAs and InP QDs. The electronic properties of these QDs, as evaluated by photoluminescence spectroscopy, could be tailored by varying both the growth rate and the PH3 flux for a constant AsH3 flux. The morphology of these QDs was investigated by transmission electron microscopy from which the formation of an InAsP ternary alloy QDs was inferred. Based on electron microscopy results, the fundamental role of As incorporation on the morphology of and on the defect nucleation associated to InAsP QDs could be then evaluated. From this optical–structural combined analysis, we were able to identify the growth conditions that produce good quality InAsP QDs embedded in GaAs.

Single and stacked layers of InP quantum dots in (AlxGa1−x)0.51In0.49P barriers were grown by metal-organic vapor-phase epitaxy for applications in semiconductor laser devices and optical gate structures. An in-plane laser structure with a single layer of InP QDs is presented, emitting at 1.942 eV and exhibiting low threshold current densities of 780 A/cm2 in electrically pulsed laser operation at 288 K for a 2000μm long device with uncoated facets. In order to realize an externally driven optical gate structure the stacking behavior of InP in (AlxGa1−x)0.51In0.49P barriers was investigated. To ensure the optical addressability of each quantum dot layer, a special double dot structure where the high energetic smaller sized quantum dot is situated above the low energetic larger sized dot, was produced. The coupling between these quantum dots can be adjusted by the thickness of the spacer layer. The structures are embedded in a ni-Schottky structure and the influence of an external electric field on the emission of the quantum dot ensemble is investigated.

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage Ur. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-A-thick or 40-A-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 A, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage Ur applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.

The authors report lasing of InAs∕InGaAsP∕InP (100) quantum dots (QDs) wavelength tuned into the 1.55μm telecom region. Wavelength control of the InAs QDs in an InGaAsP∕InP waveguide is based on the suppression of As∕P exchange through ultrathin GaAs interlayers. The narrow ridge-waveguide QD lasers operate in continuous wave mode at room temperature on the QD ground state transition. The low threshold current density of 580A∕cm2 and low transparency current density of 6A∕cm2 per QD layer, measured in pulsed mode, are accompanied by low loss and high gain with an 80-nm-wide gain spectrum.

Over the last decade InAs Stranski–Krastanov (S-K) quantum dots (QDs) grown on GaAs substrate have been widely explored for optoelectronic devices. In the recent past, the uncapped surface QDs are getting much attention for sensing application with adequate sensitivity to lower molar concentration of contaminants. Though non-uniform size distribution is an inherent property of the self-assembled S-K growth process, sensitivity of the surface QDs is significantly affected by this. Here, we have grown the surface QDs upon buried QD layer with a low barrier layer thickness to reduce the non-uniformity. In general, the 2D to 3D transition of the InAs QDs occurs only above 1.7 monolayer (ML). However, the 3D transition may be possible even at a lower monolayer coverage with the residual strain induced from the underlying QDs. In this study, particularly we have grown InAs surface QD layer at 1.6 ML coverage above the 2.7 ML buried QD layer with 8 nm thick GaAs spacer. The impact of vertically induced strain of the underneath InAs QD layer on the growth of surface dots has been investigated. The morphology of surface QDs is observed through Atomic force microscopy (AFM), which indicates the formation of uniform QDs with lower defects. The low temperature photoluminescence (PL) spectroscopy provides the evidence of the wave function overlap between the buried and surface QDs.

For the past few decades, silicon solar-cells have been researched to improve PCE through surface texturing, anti-reflection coating, plasma doping, selective emitter, back contact cell, local contact cell, and metallization. Eventually, silicon solar-cells has been saturated with the maximum PCE value of ~25 %.[1] Recently, the research on the energy-down-shift via implementing quantum-dots (QDs) in silicon solar-cells has been proposed to overcome the saturation of the PCE of silicon solar−cells since QDs are able to absorb UV light and emit visible light. However, they have not reported an evident PCE improvement and clear mechanism. Thus, we implemented the energy-down-shift via Cd0.5Zn0.5S/ZnS core/shell ODs on silicon solar-cells.The core/shell QDs were spin-coated on SiNX film deposited textured p-type silicon solar-cells, as shown in Fig. 1(a). They were well coated along the SiNX film textured surface where Al and Pt were deposited on the QDs layer to avoid the focus-ion-beam damage during the TEM sample preparation, as shown in Fig. 1(b). The core/shell QDs showed a spherical shape, were well crystallized, and well dispersed with each other, as shown in Fig. 1(c). The average size of the Cd0.5Zn0.5S/ZnS core/shell QDs was 6.6 nm, as shown in Fig. 1(d). The chemical composition of the core QD and shell layer coated on the core QD were Cd0.5Zn0.5S and ZnS, and the diameter of the core QD and thickness of the shell coated on the core QD were 4.2 and 1.2 nm, respectively, as shown in Fig. 1(e), analyzed by EDX line-scan profile. The Cd0.5Zn0.5S/ZnS core/shell QDs in the quartz-cuvette with 0.05 wt% absorbed 100% UV light in wavelength from 450 to 250 nm and emitted the 442 nm PL peak−signal with the quantum yield of 80%. These results evidently indicate that the Cd0.5Zn0.5S/ZnS core/shell QDs emitted the blue visual light when the UV light was absorbed, proving the energy-down-shift of the Cd0.5Zn0.5S/ZnS core/shell QDs. The dependency of the light absorption amount of the Cd0.5Zn0.5S/ZnS core/shell QDs on the concentration (wt %) of the QD solution was estimated as a function of the wavelength when the QD solutions were spin-coated on glass, as shown in Fig. 2(b), The absorption amount of the UV light in wavelength between 250 and 450 nm increased with the concentration of the QD solution. For p-type silicon solar-cells coated with the QDs layer, the dependency of PV performance on the average QDs layer thickness was shown in Fig. (3). The value of JSC increased abruptly from 34.70 to 36.94 mA/cm2 as the QDs layer thickness increased up to 8.8 nm, which was a 6.45 % increase compared to the reference without the QDs layer. Finally, PCE increased from 16.92 to 18.00 % as the QDs layer thickness increased up to 8.8 nm corresponds to relative 6.4 % PCE enhancement compared with that in the reference. These results indicate that the coating of Cd0.5Zn0.5S/ZnS core/shell QDs on the SiNX film textured surface for p-type silicon solar-cells affect JSC due to the energy-down-shift effect of the QDs but it does not affect VOCand FF.*This work was financially supported by the Brain Korea 21 plus Project in 2014, Korea.Fig. 1. Design of energy-down-shift via Cd0.5Zn0.5S/ZnS QDs coated on SiNx film textured p-type silicon solar cell.Fig. 2. Optical characteristics for Cd0.5Zn0.5S/ZnS QDs.Fig. 3. Photo-voltaic performance for p-type silicon solar-cells coated with Cd0.5Zn0.5S/ZnS core/shell QDs Reference [1] M. Tuan Trinh. et al., Nature Photonics 6, 316–321 (2012)[2] Bae. W K. et al., Chem Master 205307-5313 (2008)

The growth of InAs quantum dots (QDs) by organometallic vapor phase epitaxy (OMVPE) for use in GaAs based photovoltaics devices was investigated. Growth of InAs quantum dots was optimized according to their morphology and photoluminescence using growth temperature and V/III ratio. The optimized InAs QDs had sizes near 7×40 nm with a dot density of 5(±0.5)×1010 cm-2. These optimized QDs were incorporated into GaAs based p-i-n solar cell structures. Cells with single and multiple (5x) layers of QDs were embedded in the i-region of the GaAs p-i-n cell structure. An array of 1 cm2 solar cells was fabricated on these wafers, IV curves collected under 1 sun AM0 conditions, and the spectral response measured from 300-1100 nm. The quantum efficiency for each QD cell clearly shows sub-bandgap conversion, indicating a contribution due to the QDs. Unfortunately, the overarching result of the addition of quantum dots to the baseline p-i-n GaAs cells was a decrease in efficiency. However, the addition of thin GaP strain compensating layers between the QD layers, was found to reduce this efficiency degradation and significantly enhance the subgap conversion in comparison to the un-compensated quantum dot cells.

Wavelength tunable InAs/InP(1 0 0) quantum dots in 1.55-μm telecom devices

We report on the InAs quantum dots (QDs) laser in the 1.55μm wavelength region grown by gas source molecular-beam epitaxy. The active region of the laser structure consists of fivefold-stacked InAs QD layers embedded in the InGaAsP layer. Ridge waveguide lasers were processed and continuous-wave mode operation was achieved between 20 and 70°C, with characteristic temperature of 69K. High internal quantum efficiency (56%) and low infinite length threshold current density (128A∕cm2 per QD layer) was obtained for the as-cleaved devices at room temperature. The lasing wavelength range between 1.556 and 1.605μm can be covered by varying the laser cavity length.

Influence of quantum dot growth on the electrical properties of Au/GaAs Schottky diode structures containing self-assembled InAs quantum dots fabricated via atomic layer molecular beam epitaxy is investigated. Current-voltage characteristics and low frequency noise measurements were performed and analyzed. Employing four different structures; containing single quantum dot layer, without quantum dot layer for a reference, thicker capping layer with single quantum dot layer, three quantum dot layers, we find the diode containing single quantum dot layer show largest leakage current and all the dots show 1/f behavior in low frequency noise characteristics. Current dependence of the noise current power spectral density shows that all the dots have linear current dependence at low bias which is explained by the mobility and diffusivity fluctuation. The Hooge parameter was determined to be in the range of 10 -7 to 10 -8 . At high bias, the diodes containing quantum dot layer(s) show I F β dependence with the value of β larger than 2 (3.9, and 2.7), and the diode without quantum dot layer and thicker capping layer show the value of β smaller than 2 (1.6). The deviation of the values of β from two is explained by the random walk of electrons involving interface states at the metal-semiconductor Schottky barrier interface via barrier height modulation. It seems that the growth of quantum dots induces generation of the interface states with its density increasing towards the conduction band edge. The value of β smaller than 2 means that the interface states density is increasing towards the midgap. Typical value of the interface states density was found to be on the order of 10 11 to 10 12 cm 2 /Vs.

Wavelength tuning of single and vertically stacked InAs quantum dot (QD) layers embedded in InGaAsP∕InP (100) grown by metal organic vapor-phase epitaxy is achieved by controlling the As∕P surface exchange reaction during InAs deposition. The As∕P exchange reaction is suppressed for decreased QD growth temperature and group V-III flow ratio, reducing the QD size and photoluminescence (PL) emission wavelength. The As∕P exchange reaction and QD PL wavelength are then reproducibly controlled by the thickness of an ultrathin (0–2 ML) GaAs interlayer underneath the QDs. Submonolayer GaAs coverages result in a shape transition from QDs to quantum dashes at low group V-III flow ratio. Temperature dependent PL measurements reveal excellent optical properties of the QDs up to room temperature with PL peak wavelengths in the technologically important 1.55μm region for telecom applications. Widely stacked QD layers are reproduced with identical PL emission to increase the active volume, while closely stacked QD layers reveal a systematic PL redshift and linewidth reduction due to vertical electronic coupling which is proven by the linear polarization of the cleaved-side PL changing from in plane to isotropic.

The electroluminescence and stimulated emission of lasers with one layer of InAs quantum dots (QD’s) grown in a single molecular-beam epitaxial process on vicinal GaAs(001) surfaces misoriented in the direction [010] by 2, 4 and 6° are investigated. It is discovered that an increase in the misorientation angle leads to a blue shift and a decrease in the full width at half maximum (FWHM) of the electroluminescence spectrum. This effect is attributed to a decrease in the size of the quantum dots and improvement in their size uniformity. A strong dependence of the threshold current density on the width of the spontaneous luminescence spectrum is discovered. The room-temperature threshold current density of the lasers with one layer of quantum dots and the spontaneous luminescence spectrum having the smallest FWHM (54 meV) equals 210 A/cm2.