Engineering laser gain spectrum using electronic vertically coupled InAs-GaAs quantum dots

Continuous large-broad laser gain spectra near 1.3 /spl mu/m are obtained using an active region of electronic vertically coupled (EVC) InAs-GaAs quantum dots (QDs). A wide continuous electroluminescence spectrum, unlike that from conventional uncoupled InAs QD lasers, was obtained around 230 nm (below threshold) with a narrow lasing spectrum. An internal differential quantum efficiency as high as 90%, a maximum measured external differential efficiency of 73% for a stripe-length of L=1 mm, and a threshold current density for zero total optical loss as low as 7 A/cm/sup 2/ per QD layer were achieved.     

Optical absorption and scattering at one-particle states of charge carriers in semiconductor quantum dots

A theory is developed for the interaction of an electromagnetic field with one-particle quantum-confined states of charge carriers in semiconductor quantum dots. It is shown that the oscillator strengths and dipole moments for the transitions involving one-particle states in quantum dots are rather large, exceeding the corresponding typical parameters of bulk semiconductor materials. In the context of dipole approximation it is established that the large optical absorption cross sections and attenuation coefficients in the quasi-zero-dimensional systems make it possible to use the systems as new efficient absorbing materials.     

Optical anisotropy of InGaAs quantum dots

Polarization studies of InGaAs/GaAs quantum dots (QDs) synthesized in the submonolayer deposition mode (SMLQDs) on a singular GaAs (100) surface are carried out using photoluminescence spectroscopy. The influence of the effective In content in InGaAs SMLQDs and the effect of a wide-gap AlGaAs matrix on the optical anisotropy of the QDs are investigated. The highest degree (>15%) of optical anisotropy between the [011] and [0 $ \bar 1 $ 1] directions in the emission corresponding to the ground state of InGaAs/GaAs SMLQDs is observed for an effective In content of ～40%. The use of a wide-gap AlGaAs matrix resulted in an increase in the optical anisotropy of InGaAs SMLQDs by a factor of 1.5. It is found that vertical stacking of In(Ga)As/AlGaAs SMLQDs in the vertical-coupling mode (with spacer-layer thicknesses of 5–10 nm) leads to a further increase in the degree of optical anisotropy, which becomes as high as 25% on average. According to the data of transmission electron microscopy, the optical anisotropy of the ground-state photo-luminescence is predominantly caused by the anisotropy of the lateral dimensions of QDs in the [011] and [0 $ \bar 1 $ 1] directions.     

Optical study of InP quantum dots

Results of photoluminescence (PL) studies of self-organized nanoscale InP islands (quantum dots, QDs) in the In_0.49Ga_0.51P matrix, grown on a GaAs substrate by metalorganic vapor phase epitaxy (MOVPE), are presented. Dependences of the PL efficiency on temperature in the range 77–300 K and on excitation level at pumping power densities of 0.01–5 kW/cm² have been obtained. The PL spectra are a superposition of emission peaks from QDs and the wetting layer. Their intensity ratio depends on the pumping power and temperature, and the emission wavelength varies in the range 0.65–0.73 μm. At 77 K and low excitation level, InP QDs exhibit high temperature stability of the emission wavelength and high quantum efficiency. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 2, 2001, pp. 242–244. Original Russian Text Copyright ? 2001 by Vinokurov, Kapitonov, Nikolaev, Sokolova, Tarasov.     

Enhanced spontaneous emission using quantum dots and an aperturedmicrocavity

Spontaneous emission characteristics from apertured microcavities are studied using quantum-dot light emitters. Spatial averaging over the emitter positions within the apertures significantly impacts the measured lifetime changes, but lifetime changes due to the aperture modes are still readily observed. A maximum increase of a factor of ~2.5 over the cavity-free emission rate is measured. We argue that, for a narrow bandwidth emitter, the microcavity enhancement can lead to nearly-single-mode and high-speed operation even in the spontaneous regime     

Size dependence saturation and absorption of PbS quantum dots

The saturation intensity for lead sulfide quantum dots in a titanium dioxide-glycerol matrix (PbS/TiO₂-glycerol) on a glass substrate has been studied as a function of quantum cluster concentration and cluster size. The saturation intensity in these materials is strongly dependent on the size of the semiconductor nanocrystals, their concentration, or the sample thickness. The samples are reflective at a certain range of the incoming intensity of the optical field and become transparent over a threshold intensity beyond which the output and input intensities are linearly related. The system studied involves very dilute distribution of PbS QD of dimension∼10 nm embedded in a matrix of TiO₂-glycerol. Since the distribution is relatively constant in each deposited layer, the total number of QDs in a given area is proportional to the thickness. We found that the threshold of power separating absorption-bleaching, Pth is linearly related to the thickness, with values of Pth ∼few mW/cm², more than 2-3 orders of magnitude below that of QDs with dimension∼1 μm, representing a typical solid.     

Optical gain and luminescence of a ZnO-MgZnO quantum well

The optical gain and the luminescence of a ZnO quantum well with MgZnO barriers is studied theoretically. We calculated the non-Markovian optical gain and the luminescence for the strained-layer wurtzite quantum well taking into account the excitonic effects. It is predicted that both optical gain and luminescence are enhanced for the ZnO quantum well when compared with those of the InGaN-AlGaN quantum well structure due to the significant reduction of the piezoelectric effects in the ZnO-MgZnO systems.     

Optical properties and structure of InAs quantum dots in near-infrared band

The InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are studied as a function of growth temperature at a specific InAs coverage of 2.7 ML. The QDs density is significantly reduced from 8.0 × 1010 to 5.0 × 109 cm-2 as the growth temperature increases from 480℃ to 520℃, while the average QDs diameter and height becomes larger. The effects of the growth temperature on the evolution of bimodal QDs are investigated by combining atomic force microscopy (AFM) and photoluminescence (PL). Results show that the formation of the bimodal QDs depends on the growth temperature: at a growth temperature of 480℃,large QDs result from the small QDs coalition; at a growth temperature of 535℃, the indium desorption and InAs segregation result in the formation of small QDs.     

Effect of size dispersion on the optical absorption of an ensemble of semiconductor quantum dots

The linear optical absorption of an ensemble of semiconductor quantum dots randomly positioned in an insulating matrix is studied theoretically and experimentally for the CdTe/glass system. The calculation of the effective dielectric function of the system, whose imaginary part determines the absorption, is based on a modified Maxwell-Garnett formalism using a diagram technique to calculate the average renormalized polarizability of spherical quantum dots. This approach takes into account both the fluctuations of the polarization interaction due to the random positions of the spheres and their size dispersion. A comparison of the theoretical and experimental spectra permits determination of the mean quantum dot size and concentration. The dependence of these parameters on the postgrowth annealing time of the samples is not consistent with the existing theories on the spinodal decomposition of solid solutions. Fiz. Tekh. Poluprovodn. 32, 1378–1383 (November 1998)     

半导体单量子点增益与吸收特性的研究

采用密度矩阵的方法进行数值模拟计算紧束缚态的半导体单量子点的增益与吸收行为.其动力学数值结果显示,在某些不同时刻的增益谷和吸收峰的左右分布保持不变对称性,且有上下镜像反转的现象发生;在在长时间演化中发现该量子点系统的增益与吸收发生激烈的振荡而呈现出了量子光学中典型的崩塌与复苏的有趣物理现象.     

Photoinduced and equilibrium optical absorption in Ge/Si quantum dots

The results of studies of the optical absorption spectra in Ge/Si quantum dot structures in the mid-infrared region are reported. Two types of structures different in terms of the method used for quantum dot formation and in terms of barrier layer thickness are explored. The photoinduced absorption associated with the nonequilibrium population of hole states and optical absorption in structures doped to different levels are investigated. Specific features that are associated with occupation of the ground and excited states of quantum dots and exhibit a polarization dependence are observed. From the experimental data, the energy spectrum of holes is determined for structures of both types.     

Electronic coupling and structural ordering of quantum dots using InAs quantum dot columns

In this article, recent investigations of vertically aligned quantum dot columns conducted at Stanford University are reviewed. The quantum dots are InAs in a matrix of GaAs. Both the quantum dots and quantum dot columns are formed through strain-induced islanding, without lithography. Two aspects of these columns are discussed. First, the electronic coupling of quantum dots within columns of up to ten quantum dots is demonstrated. The coupling is adjusted and improvements to a simple light-emitting diode are shown. Second, increased uniformity of a surface quantum dot layer is shown when a subsurface layer of these columns are used. The most impressive results occur when the columns contain a large number of islands. Reduced variations in average ensemble height and diameter, called size uniformity, and average nearest neighbor distances, called structural uniformity, are shown. A surface unit cell of islands is demonstrated and the lack of a surface lattice is discussed.     

Structural and infrared absorption properties of self-organized InGaAs/GaAs quantum dots multilayers

Self-organized InGaAs/GaAs quantum dots (QDs) stacked multilayers have been prepared by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy shows that the InGaAs QDs are nearly perfectly vertically aligned in the growth direction [100]. The filtering effect on the QDs distribution is found to be the dominant mechanism leading to vertical alignment and a highly uniform size distribution. Moreover, we observe a distinct infrared absorption from the sample in the range of 8.6–10.7 μm. This indicates the potential of QDs multilayer structure for use as infrared photodetector.     

Synthesis and characterization of ceria quantum dots using effective surfactants

The stable and crystalline phase of different surfactants (CTAB, PEG and SDS) capped CeO₂ nanoparticles were directly synthesized by chemical precipitation method at room temperature. The effects of surfactants on the structural and optical properties of nanoparticles are characterized. The optical properties of the nanoparticles were investigated by UV–visible and PL spectroscopy. The effects of surfactants with observed band shifts are due to quantum confinement effect. The optical band gap values are determined by simple energy wave equation and Tauc plot method. The observed particle sizes are very closer to the Bohr exitonic radius. The emission bands such as violet, blue, green and orange are observed in PL spectra. The PL integrated intensity ratio of the UV emission to the deep-level green emission (I_UV/I_DLE) for CTAB, PEG and SDS capped CeO₂ nanoparticles are observed. The XRD measurement shows that CeO₂ has cubic fluorite structure having the particle size 6–10 nm. The lattice strains were detected by Williamson–Hall plot method. The surface morphology of the nanoparticles is studied by SEM and FESEM analysis. TEM images show that the particles are nearly spherical in shape with diameter of 5–10 nm. Using FTIR spectra, the functional groups of the ceria are identified.     

Tuning of electronic states in self-assembled InAs quantum dots using an ion implantation technique

We report the tunability of up to 150 meV of the ground state transition of self-assembled InAs quantum dots (QDs) using Mn ion implantation and subsequent annealing. Because of the exciton localization in the quantum dots, the photoluminescence efficiency (T=12K) of the quantum dot transition remains at 80% of its original value after implantation with a Mn dose of 1×10¹³ cm⁻²ions. Strong luminescence still remains at room temperature. At a high implantation dose (1×10¹⁵ cm⁻²) and rapid thermal annealing (700°C for 60s) about 25% of the QD luminescence intensity is recovered at T=12K.     

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.     

OMVPE of InAs quantum dots on an InGaP surface

The organometallic vapor phase epitaxy of InAs quantum dots has been investigated by comparing the effect the underlying surface has on the quantum dot physical characteristics. Atomic force microscopy measurements were used to identify the InAs QDs coalesce to significantly larger size when deposited on an InGaP surface compared to a GaAs surface. Quantitative assessment of the total QD volume on different surfaces such as GaAs, InGaP, and GaAsP implicates the role of indium in the underlying surface for the increase in QD size on InGaP surfaces.     

Optical phenomena in InAs/GaAs heterostructures with doped quantum dots and artificial molecules

Spectra of intraband absorption of polarized mid-IR light were investigated in undoped, p-, and n-doped InAs/GaAs quantum dots (QDs) covered with an InGaAs layer. Optical matrix elements for intraband electron and hole transitions in QDs have been calculated for different polarizations of light, and a good agreement with the experimental data is obtained. It is shown that the intraband absorption of light by electrons strongly exceeds the absorption by holes. Photoluminescence spectra and TEM images of structures with artificial molecules formed by pairs of QDs were studied.     

Side-light-injection MQW bistable laser using saturable absorption and gain quenching

The characteristics of set-on and set-off operations by input light of the same wavelength have been observed in a side-light-injection MQW bistable laser. In this structure, the main bistable laser was located perpendicular to two waveguides for amplifying input light. Two intersections, the gain quenching region and the saturable absorption region, were spatially separated. Input light of 40 mu W results in saturable absorption in one intersection biased at +0.65 V, and 570 mu W causes gain quenching in the other intersection biased at +0.93 V.<>     

Optical properties of CdSe quantum dots grown on ZnSe and ZnBeSe by molecular beam epitaxy

We report detailed studies of the optical properties of CdSe quantum dots (QDs) grown on ZnSe and ZnBeSe by molecular-beam epitaxy (MBE). We performed steady-state and time-resolved photoluminescence (PL) measurements and observe that nonradiative processes dominate at room temperature (RT) in the CdSe/ZnBeSe QDs structures, though these nonradiative processes do not dominate in the CdSe/ZnSe QDs structures up to RT. We performed secondary ion-mass spectrometry (SIMS) measurement and propose that the oxygen incorporation in the ZnBeSe layers (possibly caused by the reactivity of Be) may contribute to the dominant nonradiative processes at high temperatures in the QDs grown on ZnBeSe.