Optical-phonon modes and Frohlich potential in quantum dots

By extending the microscopic dipole model on optical-phonon modes as applied in quantum wells and quantum wires, to rectangular quantum dots (QD), optical phonon modes and their accompanying Frohlich potentials in QD are calculated and classified. When the bulk phonon dispersion is ignored, the optical phonon modes in QD can be clearly divided into the confined LO- and TO-bulk-like modes and the extended interface-like modes. Among the interface-like modes, a special attention is given to the corner modes, whose anisotropic behavior is depicted in the long wavelength limit. Based on the numerical results, a set of analytical formula are proposed to approximately describe the bulk-like modes, for which both the optical displacements and Frohlich potentials vanish at the interfaces. (C) 2000 Elsevier Science Ltd. All rights reserved.

Temperature and excitation power dependence of the optical properties of InAs self-assembled quantum dots grown between two Al0.5Ga0.5As confining layers

We have investigated the temperature and excitation power dependence of photoluminescence properties of InAs self-assembled quantum dots grown between two Al0.5Ga0.5As quantum wells. The temperature evolutions of the lower-and higher-energy transition in the photoluminescence spectra have been observed. The striking result is that a higher-energy peak appears at 105 K and its relative intensity increases with temperature in the 105-291 K range. We demonstrate that the higher-energy peak corresponds to the excited-state transition involving the bound-electron state of quantum dots and the two-dimensional hole continuum of wetting layer. At higher temperature, the carrier transition associated with the wetting layer dominates the photoluminescence spectra. A thermalization model is given to explain the process of hole thermal transfer between wetting layer and quantum dots. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Quantum-dot superluminescent diode: A proposal for an ultra-wide output spectrum

We propose a novel superluminescent diode (SLD) with a quantum dot (QD) active layer, which should give a wider output spectrum than a conventional quantum well SLD. The device makes use of inhomogeneous broadness of gain spectrum resulting from size inhomogeneity of self-assembled quantum dots grown by Stranski-Krastanow mode. Taking a design made out in the InxGa1-xAs/GaAs system for example, the spectrum characteristics of the device are simulated realistically, 100-200 nm full width of half maximum of output spectrum can be obtained. The dependence of the output spectrum on In composition, size distribution and injection current of the dots active region is also elaborated.

Ultrafast low-temperature grown AlGaAs/GaAs photorefractive quantum wells using point defects as capture centers

At a medium substrate temperature of 400 degrees C and a lower As flux, we have grown an ultrafast AlGaAs/GaAs photorefractive multiple quantum well (MQW) structure by molecular beam epitaxy. The as-grown sample exhibits strong photorefractive effect under the transverse Frantz-Keldysh geometry. A peak electroabsorption of 2100 cm(-1) is measured in the as-grown sample in an 11 kV/cm dc electric field, and the peak photorefractive diffraction efficiency can be 1.2%. After postgrowth annealing, the photorefractive effect becomes weak and disappears in samples annealed above 700 degrees C. Using optical transient current spectroscopy, deep levels are measured in these samples. It is found that deep levels are stable against annealing until 700 degrees C. Using a pump-probe technique, carrier lifetimes are measured at room temperature. We find that the as-grown sample has a lifetime of 20 ps, while the 700 degrees C annealed sample has a lifetime of more than 200 ps. The ultrafast lifetime in the as-grown sample is caused by point defects, not by As clusters. Our result show that AlGaAs/GaAs MQW structure grown around 400 degrees C has better performance of the photorefractive effect. (C) 1999 American Institute of Physics. [S0003-6951(99)04036-X].

Effect of defects on optical phonon Raman spectra in SiC nanorods

Fabricated one-dimensional (1D) materials often have abundant structural defects. Experimental observation and numerical calculation indicate that the broken translation symmetry due to structural defects may play a more important role than the quantum confinement effect in the Raman features of optical phonons in polar semiconductor quantum wires such as SiC nanorods, (C) 1999 Elsevier Science Ltd. All rights reserved.

Influence of (001) vicinal GaAs substrates on the optical properties of defects in low-temperature grown GaAs/AlGaAs multiple quantum wells

Photoluminescence (PL) spectroscopy and carrier lifetime measurement has been used to characterize optical properties of defects in the low-temperature (LT) grown GaAs/AlGaAs multiple quantum well structures. Two sets of samples were grown at 400 degrees C by molecular beam epitaxy on nominal (001) and miscut [4 degrees off (001) towards (111) A] GaAs substrates, respectively. After growth, samples were subjected to 30 s rapid thermal annealing at 600-800 degrees C. It is found that after annealing, two defect-related PL features appear in the samples grown on nominal (001) GaAs substrates, but not in those grown on miscut (001) GaAs substrates. The carrier lifetimes are about 31 and 5 ps in as-grown samples grown on nominal and miscut (001) GaAs substrates, respectively. The different PL spectra and carrier lifetimes in two sets of samples are attributed to different structures of the As-Ga-like defects formed during LT growth. (C) 1999 American Institute of Physics. [S0003-6951(99)00230-2].

Structural and optical characterization of InAs nanostructures grown on high-index InP substrates

The structural and optical properties of InAs layers grown on high-index InP surfaces by molecular beam epitaxy are investigated in order to understand the self-organization of quantum dots and quantum wires on novel index surfaces. Four different InP substrate orientations have been examined, namely, (1 1 1)B, (3 1 1)A, and (3 1 1)B and (1 0 0). A rich variety of InAs nanostructures is formed on the surfaces. Quantum wire-like morphology is observed on the (1 0 0) surface, and evident island formation is found on (1 1 1)A and (3 1 1)B by atomic force microscopy. The photoluminescence spectra of InP (1 1 1)A and (3 1 1)B samples show typical QD features with PL peaks in the wavelength range 1.3-1.55 mu m with comparable efficiency. These results suggest that the high-index substrates are promising candidates for production of high-quality self-organized QD materials for device applications. (C) 1999 Elsevier Science B.V. All rights reserved.

Suppression of sequential tunneling current by a perpendicular magnetic field in a three-barrier, two-well heterostructure

When an intersubband relaxation is involved in vertical transport in a tunneling heterostructure, the magnetic suppression of the intersubband LO or LA phonon scattering may also give rise to a noticeable depression of the resonant tunneling current, unrelated to the Coulomb correlation effect. The slowdown of the intersubband scattering rate makes fewer electrons able to tunnel resonantly between two adjacent quantum wells (QWs) in a three-barrier, two-well heterostructure. The influence of the magnetic field on the intersubband relaxation can be studied in an explicit way by a physical model based on the dynamics of carrier populations in the ground and excited subbands of the incident QW. (C) 1998 American Institute of Physics. [S0003-6951(98)00925-5].

Modification of InAs quantum dot structure by the growth of the capping layer

InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications. (C) 1998 American Institute of Physics.

Growth and characterization of InGaAs/InAlAs/InP high-electron-mobility transistor structures towards high channel conductivity

High-quality InGaAs/InAlAs/InP high-electron-mobility transistor (HEMT) structures with lattice-matched or pseudomorphic channels have been grown by molecular-beam epitaxy (MBE). The purpose of this work is to enhance the channel conductivity by changing the epitaxial structure and growth process. With the use of pseudomorphic step quantum-well channel, the highest channel conductivity is achieved at x = 0.7, the corresponding electron mobilities are as high as 12300 (300 K) and 61000 cm(2)/V.s (77 K) with two-dimensional electron gas (2DEG) density of 3.3 x 10(12) cm(-2). These structures are comprehensively characterized by Hall measurements, photoluminescence, double crystal X-ray diffraction and transmission electron microscopy. Strong room-temperature luminescence is observed, demonstrating the high optical quality of the samples. We also show that decreasing the In composition in the InyAl1-yAs spacer is very effective to increase the 2DEG density of PHEMT structures. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

Numerical evaluation of energy loss rate for hot carriers in quantum wells

Using the Frohlich potential associated with realistic optical phonon modes in quantum well systems, the energy loss rates of hot electrons, holes, and electron-hole pairs are calculated, with special emphasis on the effects of carrier density, hot phonon population, quantum well width, and phonon dispersion on the hot-carrier relaxation process in quasi-two-dimensional systems. (C) 1998 Academic Press Limited.

Electroabsorption-modulated DFB laser integrated with dual-waveguide spot-size converter - art. no. 60200N

A 1.55-mu m ridge DFB laser and electroabsorption modulator monolithically integrated with a buried-ridge-stripe dual-waveguide spot-size converter at the output port for low-loss coupling to a cleaved single-mode optical fiber was fabricated by means of selective area growth, quantum well intermixing and dual-core technologies. These devices exhibit threshold current of 28 mA, side mode suppression ratio of 38.0 dB, 3-dB modulation bandwidth of 12.0 GHz, modulator extinction ratios of 25.0 dB dc. The output beam divergence angles of the spot-size converter in the horizontal and vertical directions are as small as 8.0 degrees x 12.6 degrees, respectively, resulting in 3.2 dB coupling loss with a cleaved single-mode optical fiber.

Defect influence on luminescence efficiency of GaN-based LEDs

Wafers with normal light-emitting diode structure were grown by metal organic chemical vapor deposition system. The pressure and temperature were varied during growth of buffer layer in order to grow different types of epilayers. The cathodoluminescence results show that the interface distortion of quantum well plays an important role in radiant efficiency. The electroluminescence detections indicate that the dislocations also influence the external quantum efficiency by lowering the electron injection efficiency. (c) 2006 Elsevier Ltd. All rights reserved.

Calculation of valence subband structures for strained GaInP/AlGaInP quantum wells without axial approximation

Usually in the calculation of valence subband structure for III-V direct bandgap material, axial approximation had been used in the Luttinger-Kohn model to simplify the computational efforts. In this letter, the valence subband structure for the GaInP/AlGaInP strained and lattice-matched quantum wells was calculated without axial approximation, on the basis of 6x6 Luttinger-Kohn Hamiltonian including strain and spin-orbit splitting effects. The numerical simulation results were presented with help of the finite-difference methods. The calculation results with/without axial approximation were compared and the effect of axial approximation on the valence subband structure was discussed in detail. The results indicated that there was a strong warping in the GaInP valence band, and axial approximation can lead to an error when k was not equal to zero, especially for compressively strained and lattice-matched GaInP/AlGaInP quantum wells.