Longwave generation in laser structures based on InGaAs(N) quantum wells on GaAs substrates

The MBE growth regime has been optimized for the obtaining of laser structures based on InGaAs(N)/GaAs quantum wells (QWs) with high indium content. Structures containing InGaAs and InGaAsN isolated QWs exhibit low-threshold longwave emission at room temperature. Lasers based on QWs of the In_0.35GaAs and In_0.35GaAsN_0.023 types are characterized by the radiation wavelengths λ=1.085 and 1.295 μm at a threshold current density of 60 and 350 A/cm², respectively.     

Analysis of optical gain and threshold current density of 980 nm InGaAs/GaAs compressively strained quantum well lasers

The valence hole subbands, optical gain spectra and threshold current density of InGaAs/GaAs compressive-strained quantum wells (QWs) were studied using a numerical approach. We found that a higher In composition in the quantum well and a thicker well give longer emitting wavelength; a narrower well and higher In composition lead to higher TE mode peak gain. The result also shows a suitable combination of In composition, QW thickness and number of QWs should be selected to achieve low threshold current density.     

InGaAs quantum dots grown by molecular beam epitaxy for light emission on Si substrates

The aim of this study is to achieve homogeneous, high density and dislocation free InGaAs quantum dots grown by molecular beam epitaxy for light emission on silicon substrates. This work is part of a project which aims at overcoming the severe limitation suffered by silicon regarding its optoelectronic applications, especially efficient light emission device. For this study, one of the key points is to overcome the expected type II InGaAs/Si interface by inserting the InGaAs quantum dots inside a thin silicon quantum well in SiO2 fabricated on a SOI substrate. Confinement effects of the Si/SiO2 quantum well are expected to heighten the indirect silicon bandgap and then give rise to a type I interface with the InGaAs quantum dots. Band structure and optical properties are modeled within the tight binding approximation: direct energy bandgap is demonstrated in SiO2/Si/InAs/Si/SiO2 heterostructures for very thin Si layers and absorption coefficient is calculated. Thinned SOI substrates are successfully prepared using successive etching process resulting in a 2 nm-thick Si layer on top of silica. Another key point to get light emission from InGaAs quantum dots is to avoid any dislocations or defects in the quantum dots. We investigate the quantum dot size distribution, density and structural quality at different V/III beam equivalent pressure ratios, different growth temperatures and as a function of the amount of deposited material. This study was performed for InGaAs quantum dots grown on Si(001) substrates. The capping of InGaAs quantum dots by a silicon epilayer is performed in order to get efficient photoluminescence emission from quantum dots. Scanning transmission electronic microscopy images are used to study the structural quality of the quantum dots. Dislocation free In50Ga50As QDs are successfully obtained on a (001) silicon substrate. The analysis of QDs capped with silicon by Rutherford Backscattering Spectrometry in a channeling geometry is also presented.     

Experimental determination of the optimum number of quantum wells in multiwell heterolasers with radiation leakage into a substrate

The dependence of the mode structure and threshold characteristics of laser diodes with significant (～94%) radiation leakage into a substrate on the number of quantum wells (QWs) in a laser-diode heterostructure has been studied. It is established that laser diodes with a small number of QWs generate on the fundamental waveguide mode. As the number of QWs increases, the first waveguide mode begins to dominate and the threshold current increases, which is probably related to a weak filling of central QWs with nonequilibrium carriers and increased losses of the fundamental mode.     

Controlling the wavelength of InGaAs/GaAs/InGaP lasers by ion implantation

The possibility of controlling the wavelength of emission from an InGaAs/GaAs/InGaP laser heterostructure with strained quantum wells (QWs) using medium-energy proton implantation followed by thermal annealing has been studied. It is established that the optimum proton energy is related to the arrangement of QWs in the structure (e.g., 150 keV for QWs at a depth of ≈1.3 μm). Proton irradiation to a total dose of 6 × 10¹⁴ cm⁻² followed by annealing at 700°C allows the wavelength of emission from the modified region to be decreased by 8–10 nm at minimum losses in the output intensity. The observed effect can be used to obtain two-band emission from the same chip and has good prospects for use in the development of new optoelectronic schemes.     

High characteristic temperature 1.5 µm wavelength laser diode via Sb-based quantum dots in quantum wells

The advent of laser diodes in the 1.55?µm wavelength region is becoming a hot topic in the field of telecommunications. The growth of strain-engineered Sb-based multi-stacks quantum dots (QDs) on GaAs by molecular beam epitaxy (MBE) is advantageous to restrain its drawback of self-absorption and thus beneficial for preparing efficient laser diodes (LDs). Moreover, owing to a strong electronic coupling between the QDs layers and the quantum wells (QWs), strain-engineered QDs introducing antimony (Sb) with emission wavelength up to 1.5?µm were achieved at room temperature. The p-type doping can substantially increase the QD laser’s ground state gain at room temperature. Based on this simple process, high efficient LD was obtained. The LD was fabricated with a cavity length of 1000?μm and a stripe width of 100?μm. The output performance was achieved with threshold current densities of the device as low as 135?A/cm², and with high Characteristic Temperatures of 118?K or higher in the temperature range between 20°C and 80°C. The continuous wave operating up to 32?mW were achieved at room temperature (RT).     

Effects of non-uniform size distribution on the spectral optical gain properties of InGaAs/InGaAsP quantum dots

We present a theoretical study of the optical gain, emission wavelength, and threshold current density for edge emitting laser structures with an active region based on InGaAs/InGaAsP quantum dots. The analysis of the effects of the size distribution of the dots is also presented. Spectral gain curves are generated for InGaAs/InGaAsP dots where high optical gain and high independence of spectral characteristics are obtained for a uniform distribution of dots. With typical non-uniform distribution, we show a reduction in gain by a factor of 6. Also, we predict the onset of new transition peaks and a red shift in the most probable operating lasing wavelength. Finally, we demonstrate that there is a large range of full width at half maximum (FWHM) of the dots size distribution where variations in the maximum gain and associated wavelength, as well as threshold current density, are minimum.     

Long-wavelength quantum-dot lasers

Quantum dots (QD) of (InGaAs/GaAs) on GaAs substrate with long-wavelength emission (1300 nm) have been fabricated using metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) for use in surface-emitting laser diodes. QDs are obtained by employing two different approaches, seeding and overgrowth with a quantum well, yielding similar recombination spectra. Despite the shift to long wavelengths, a large separation (80 meV) between excited states is maintained. The introduction of such QDs into a vertical cavity leads to a strong narrowing of the emission spectrum. Lasing from 1300-nm QD VCSEL is reported.     

The effects of both surface segregation of In atoms and strain on the confinement profile of InGaAs/GaAs multi quantum wells

The InGaAs/GaAs quantum wells (QWs) have been investigated by optical measurements at different growth conditions. Growth temperature of samples with different layer thickness and nominal indium composition results in a graduation of the indium concentration (In) at the interfaces between the quantum wells and the barriers. The modification of the indium composition then distorts the potential profiles for higher temperature growth and can be attributed to In segregation effect. This leads to a blue-shift of the transition energies compared to a perfectly square quantum well. However, we also observe a clear dependence of the transition energies, inconsistent with a simple adjustment of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the QWs, we obtain good agreement with experiment.     

Fabrication of surface metal nanoparticles and their induced surface plasmon coupling with subsurface InGaN/GaN quantum wells

Based on the fabrication of Ag nanoparticles (NPs) with controlled geometry and surface density on an InGaN/GaN quantum well (QW) epitaxial structure, which contains indium-rich nano-clusters for producing localized states and free-carrier (delocalized) states in the QWs, and the characterization of their localized surface plasmon (LSP) coupling behavior with the carriers in the QWs, the interplay behavior of LSP coupling with carrier delocalization in the QWs is demonstrated. By using the polystyrene nanosphere lithography technique with an appropriate nanosphere size and adjusting the post-fabrication thermal annealing condition, the induced LSP resonance wavelength of the fabricated Ag NPs on the QW sample can match the QW emission wavelength for generating the coherent coupling between the carriers in the QWs and the induced LSP. The coupling leads to the enhancement of radiative recombination rate in the QWs and results in increased photoluminescence (PL) intensity, red-shifted PL spectrum, reduced PL decay time, and enhanced internal quantum efficiency. It is found that the observed effects are mainly due to the LSP coupling with the delocalized carriers in the QWs.     

Efficient Red Perovskite Light‐Emitting Diodes Based on Solution‐Processed Multiple Quantum Wells

This paper reports a facile and scalable process to achieve high performance red perovskite light‐emitting diodes (LEDs) by introducing inorganic Cs into multiple quantum well (MQW) perovskites. The MQW structure facilitates the formation of cubic CsPbI₃ perovskites at low temperature, enabling the Cs‐based QWs to provide pure and stable red electroluminescence. The versatile synthesis of MQW perovskites provides freedom to control the crystallinity and morphology of the emission layer. It is demonstrated that the inclusion of chloride can further improve the crystallization and consequently the optical properties of the Cs‐based MQW perovskites, inducing a low turn‐on voltage of 2.0 V, a maximum external quantum efficiency of 3.7%, a luminance of ≈440 cd m^?2 at 4.0 V. These results suggest that the Cs‐based MQW LED is among the best performing red perovskite LEDs. Moreover, the LED device demonstrates a record lifetime of over 5 h under a constant current density of 10 mA cm^?2. This work suggests that the MQW perovskites is a promising platform for achieving high performance visible‐range electroluminescence emission through high‐throughput processing methods, which is attractive for low‐cost lighting and display applications.     

The effect of ferromagnetic Mn-delta-doped layer on the emission properties of GaAsSb/GaAs and InGaAs/GaAsSb/GaAs heterostructures

We have studied the emission characteristics and circularly polarized electroluminescence of light-emitting diodes based on heterostructures with a single (GaAs/GaAsSb/GaAs) or two-layer (GaAs/InGaAs/GaAsSb/GaAs) quantum well (QW) and a Mn-delta-doped layer in the GaAs barrier. The ferromagnetic effect of the delta-layer of Mn on the spin polarization of carriers in QWs based on type-II heterostructures has been observed and studied for the first time. The observed phenomena are described using a model of the exchange interaction of Mn ions in the barrier and holes in the QW.     

ZnO 1-D nanostructures: Low temperature synthesis and characterizations

ZnO is one of the most important semiconductors having a wide variety of applications in photonic, field emission and sensing devices. In addition, it exhibits a wide variety of morphologies in the nano regime that can be grown by tuning the growth habit of the ZnO crystal. Among various nanostructures, oriented 1-D nanoforms are particularly important for applications such as UV laser, sensors, UV LED, field emission displays, piezoelectric nanogenerator etc. We have developed a soft chemical approach to fabricate well-aligned arrays of various 1-D nanoforms like nanonails, nanowires and nanorods. The microstructural and photoluminescence properties of all the structures were investigated and tuned by varying the synthesis parameters. Field emission study from the aligned nanorod arrays exhibited high current density and a low turn-on field. These arrays also exhibited very strong UV emission and week defect emission. These structures can be utilized to fabricate efficient UV LEDs.     

Molecular beam epitaxy growth methods of wavelength control for InAs/(In)GaAsN/GaAs heterostructures

We discuss the molecular beam epitaxy (MBE) growth methods of emission wavelength control and property investigations for different types of InAs/(In)GaAsN/GaAs heterostructures containing InGaAsN quantum-size layers: (1) InGaAsN quantum wells deposited by the conventional mode in a GaAs matrix, (2) InAs quantum dots deposited in a GaAsN matrix or covered by an InGaAs(N) layer, and (3) InAs/InGaAsN/GaAsN strain-compensated superlattices with quantum wells and quantum dots. The structures under investigation have demonstrated photoluminescence emission in a wavelength range of ～1.3-1.8?μm at room temperature without essential deterioration of the radiative properties.     

Surface-plasmon-enhanced light emitters based on InGaN quantum wells

Since 1993, InGaN light-emitting diodes (LEDs) have been improved and commercialized, but these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their light-emission efficiencies have been limited. Here we describe a method to enhance this efficiency through the energy transfer between quantum wells (QWs) and surface plasmons (SPs). SPs can increase the density of states and the spontaneous emission rate in the semiconductor, and lead to the enhancement of light emission by SP-QW coupling. Large enhancements of the internal quantum efficiencies (eta(int)) were measured when silver or aluminium layers were deposited 10 nm above an InGaN light-emitting layer, whereas no such enhancements were obtained from gold-coated samples. Our results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.     

Stimulated Emission at 1.3-μm Wavelength in Metamorphic InGaAs/InGaAsP Structure with Quantum Wells Grown on Ge/Si(001) Substrate

A laser structure comprising metamorphic InGaAsP layer and InGaAs quantum wells on a non-inclined Si(001) substrate with relaxed Ge buffer layer has been grown for the first time by metal-organic vapor phase epitaxy (MOVPE). The optically pumped lasers exhibit stimulated emission at a wavelength of 1.3 μm. At liquid-nitrogen temperature, the threshold power density of pumping at 0.8 μm amounted to 250 kW/cm².

     

Shape control of InGaAs nanostructures on nominal GaAs(001): dashes and dots

A method for fabricating self-assembled InGaAs quantum dashes on a nominal GaAs(001) substrate is presented. InGaAs layers were grown on nominal GaAs(001) substrates at a low temperature to suppress the Stranski-Krastanov transition as well as indium segregation and indium desorption, then annealed at high temperatures to induce self-assembly. While typical direct growth at the annealing temperature has yielded only quantum dot shapes, our approach has enabled us to control the shape of self-assembled nanostructures from quantum dashes to quantum dots and eventually quantum dot-chains. The major factor controlling the shape of InGaAs nanostructures was found to be the thickness of the pseudomorphic In(0.4)Ga(0.6)As layer.     

Separately bounded InGaAsP high-power laser heterostructures obtained by VPE of organometallic compounds

InGaAsP/InP laser heterostructures with two stressed quantum wells operating in the wavelength range 1.3–1.55 μm were obtained by VPE of organometallic compounds. An optical emission power of 2.4 W was reached for the laser diodes with 100-μm wide strips operated in the continuous lasing mode at 20°C. A minimum threshold current density was 260 A/cm². A differential quantum efficiency η_d= 40% was obtained with a 1.9-mm-long Fabry-Perot resonator. The internal optical losses of the heterostructures are reduced to 2.6–4.2 cm⁻¹.     

Large aperture vertical cavity surface emitting laser with distributed-ring contact

To overcome the serious current crowding effect in top-emitting vertical cavity surface emitting lasers (VCSELs) with large aperture, a distributed-ring-contact (DRC) VCSEL is proposed and demonstrated. A maximal cw light output power of more than 0.3 W and a wall-plug efficiency of 17.4% are achieved for a 300 μm-diameter VCSEL. The DRC VCSEL exhibits a more homogeneous emission profile, and the laser emits at 803.3 nm with a narrow spectrum (less than 0.2 nm FWHM).     

Polarization dependence of the stark shift in the absorption edge of InGaAs/GaAs quantum dot heterostructures

The Stark effect has been studied in multilayer InGaAs/GaAs laser structures with self-assembled quantum dots (QDs). A shift in the absorption edge depending on the reverse bias voltage has been measured in a two-section laser diode. The QD absorption edge shifts toward longer wavelengths with increasing electric field strength. It is established that the QD absorption depends on the polarization of light. The intensity at which TE-polarized luminescence in laser structures is studied is more than ten times higher than that of the TE-polarized emission component, which is explained by higher amplification of the TE mode.