Showing papers on "Quantum well published in 2001"

Recent progress in quantum cascade lasers and applications

Abstract: Quantum cascade (`QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple - often several tens of - photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance. The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.

Conversion of spin into directed electric current in quantum wells.

Abstract: A nonequilibrium population of spin-up and spin-down states in quantum well structures has been achieved applying circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicular to the direction of light propagation. Because of the spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left handed. A microscopic model is presented which describes the origin of the photon helicity driven current. The model suggests that the system behaves as a battery which generates a spin polarized current.

Optical Frequency Synthesis Based on Mode- Locked Lasers

Abstract: The synthesis of optical frequencies from the primary cesium microwave standard has traditionally been a difficult problem due to the large disparity in frequency. Recently this field has been dramatically advanced by the introduction and use of mode-locked lasers. This application of mode-locked lasers has been particularly aided by the ability to generate mode-locked spectra that span an octave. This review article describes how mode-locked lasers are used for optical frequency synthesis and gives recent results obtained using them.

Nature of charge transport in quantum-cascade lasers.

Abstract: The first global quantum simulation of semiconductor-based quantum-cascade lasers is presented. Our three-dimensional approach allows us to study in a purely microscopic way the current-voltage characteristics of state-of-the-art unipolar nanostructures, and therefore to answer the long-standing controversial question: Is charge transport in quantum-cascade lasers mainly coherent or incoherent? Our analysis shows that (i) quantum corrections to the semiclassical scenario are minor and (ii) inclusion of carrier-phonon and carrier-carrier scattering gives excellent agreement with experimental results.

Edge and screw dislocations as nonradiative centers in InGaN/GaN quantum well luminescence

Abstract: Transmission electron microscopy (TEM) and scanning electron microscope cathodoluminescence (CL) have been used to determine the influence of edge and screw dislocations on the light emitting properties of InxGa1−xN quantum wells TEM is used to locate and identify the nature of dislocations CL on the same samples is used to determine the spatial variation of the luminescence A direct correlation of CL maps with TEM has been established, showing that threading edge dislocations act as nonradiative recombination centers with an associated minority carrier diffusion length of 200 nm Threading dislocations of screw and mixed type were found to be associated with surface pits which were also nonradiative in the quantum well (QW) emission, but owing to the absence of QW growth on the pit facets The contributions of edge and screw/mixed dislocations to the reduction of the QW emission are quantified, and the wider significance of these results is discussed

Quantum-cascade lasers based on a bound-to-continuum transition

Abstract: A quantum-cascade structure combining the advantages of the three-quantum well and superlattice active regions is demonstrated. In these devices, the emission occurs between a state localized close to the injection barrier and a miniband. A low threshold current density (3.6 kA/cm2), large slope efficiency (200 mW/A for 35 periods), and peak power (700 mW) are achieved at 30 °C while a peak power of 90 mW is obtained at temperatures as high as 150 °C.

High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy

Abstract: Time-resolvedphotoluminescence (PL), at T=8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from ∼5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron–hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which we find equal to 2.45±0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples.

Excitonic transitions in ZnO/MgZnO quantum well heterostructures

Abstract: In this work we present the calculation of the excitonic transition energies in ZnO/MgZnO quantum well heterostructures, accounting for the effects of the exciton–phonon interaction. The results of our calculations clearly show that the description of the electron–hole interaction by means of the static screened Coulomb potential and the use of the polaron masses for the electron and the hole leads to a poor agreement with available experimental data. On the other hand, including the exciton–phonon interaction in the calculation of the exciton binding energies, leads to the values of the excitonic transitions which agree very well with the recently published experimental data. A critical discussion of the choice of the physical parameters used in ZnO is also presented, which leads us to suggest a value for the heavy-hole band mass of 0.78m0 and a conduction-valence band ratio in the range 60/40–70/30.

Miniband formation in a quantum dot crystal

Abstract: We analyze the carrier energy band structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function approximation. The coupling among quantum dots leads to a splitting of the quantized carrier energy levels of single dots and formation of three-dimensional minibands. By changing the size of quantum dots, interdot distances, barrier height, and regimentation, one can control the electronic band structure of this artificial quantum dot crystal. Results of simulations carried out for simple cubic and tetragonal quantum dot crystal show that the carrier density of states, effective mass tensor and other properties are different from those of bulk and quantum well superlattices. It has also been established that the properties of artificial crystal are more sensitive to the dot regimentation rather then to the dot shape. The proposed engineering of three-dimensional mini bands in quantum dot crystals allows one to fine-tune electronic and optical properties of such nanostructures.

High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers

Abstract: Quantum cascade (QC) lasers, based on intersubband transitions in semiconductor quantum wells, are characterized by ultrafast (picosecond) carrier lifetimes. An important consequence of this unique property is the expected absence of relaxation oscillations in the transient response of these devices. Here, we discuss and experimentally verify this prediction by measuring the modulation response of several 8-μm-QC lasers, properly processed and packaged for high-speed operation, up to 10 GHz.

On the origin of carrier localization in Ga1−xInxNyAs1−y/GaAs quantum wells

Abstract: We have investigated by temperature-dependent photoluminescence (PL) spectroscopy as-grown GaInNAs, InGaAs, and GaAsN quantum wells (QWs) embedded in a GaAs matrix. The evolution of the PL peak position and of the PL linewidth shows evidence of a strong carrier localization for the GaInNAs QWs only. The high delocalization temperature, in the 150 K range, indicates the presence of a high density of possibly deep-localizing potential wells. In addition, a higher density of nonradiative recombination centers appears to result in stronger carrier localization. Transmission electron microscopy reveals well defined, flat interfaces, in these comparatively high N-content (yN∼0.04–0.05) QWs. Our results thus demonstrate that the origin of localization in GaInNAs QWs is the concomitant presence of both In and N, which may result in strain and/or composition fluctuations.

InGaAsNSb/GaAs quantum wells for 1.55 μm lasers grown by molecular-beam epitaxy

Abstract: InGaAsNSb/GaAs quantum wells (QWs) were grown by solid-source molecular-beam epitaxy using a N2 radio frequency plasma source. The effect of adding Sb during growth of InGaAsN/GaAs QWs was studied. X-ray diffraction, reflection high-energy electron diffraction and transmission electron microscopy studies indicate that Sb suppresses the three-dimensional growth and improves the interface of the QWs. X-ray diffraction and secondary ion mass spectroscopy analysis show that Sb gets incorporated into the quantum well, which becomes a quinternary compound that was previously unexplored. The introduction of Sb during growth of InGaAsN/GaAs QWs significantly enhances the optical properties of the QWs. 1.53 μm room-temperature photoluminescence was obtained from InGaAsNSb/GaAs QWs, which demonstrates the potential of fabricating 1.55 μm InGaAsNSb/GaAs QW lasers for long-haul applications.

A dual-wavelength indium gallium nitride quantum well light emitting diode

Abstract: We have designed and implemented a monolithic, dual-wavelength blue/green light emitting diode (LED) consisting of two active indium gallium nitride/gallium nitride (InGaN/GaN) multiple-quantum-well segments. The segments are part of a single vertical epitaxial structure in which a p++/n++ InGaN/GaN tunnel junction is inserted between the LEDs, emitting in this proof-of-concept device at 470 nm and 535 nm, respectively. The device has been operated as a three-terminal device with independent electrical control of each LEDs to a nanosecond time scale.

Grating-tuned external-cavity quantum-cascade semiconductor lasers

Abstract: Grating-coupled external-cavity quantum-cascade lasers were studied for temperatures from 80 to 230 K. At 80 K, a tuning range of ∼65–88 nm are obtained for 4.5 and 5.1 μm laser amplifiers, respectively. The tuning ranges for both narrowed substantially with increasing temperature, to ∼23 nm at 203 K. The threshold varied slowly versus wavelength, while the efficiency appeared to be close to optimum toward wavelengths shorter than the free running wavelength.

Quantum dot lasers

Abstract: A quantum dot active region is disclosed in which quantum dot layers are formed using a self-assembled growth technique. In one embodiment, growth parameters are selected to control the dot density and dot size distribution to achieve desired optical gain spectrum characteristics. In one embodiment, the distribution in dot size and the sequence of optical transition energy values associated with the quantum confined states of the dots are selected to facilitate forming a continuous optical gain spectrum over an extended wavelength range. In another embodiment, the optical gain is selected to increase the saturated ground state gain for wavelengths of 1260 nanometers and greater. In other embodiments, the quantum dots are used as the active region in laser devices, including tunable lasers and monolithic multi-wavelength laser arrays.

Intersubband absorption in degenerately doped GaN/AlxGa1−xN coupled double quantum wells

Abstract: Intersubband absorption in coupled GaN/AlGaN double quantum wells (DQWs) has been measured. The samples were grown by molecular-beam epitaxy on a sapphire substrate and with large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 and 1.52 μm were measured for a symmetric DQW of 12 A wide wells coupled by a 10 A wide barrier, which also showed evidence of excited-state anticrossing. As expected, asymmetric DQWs displayed no such anticrossing, and the ground-state anticrossing energies were found to be much smaller, as a result of the comparatively large effective electron mass, than the energy broadening of individual transitions. Degenerate doping of the DQWs was used to establish a common reference energy at the Fermi level, which allows overcoming uncertainties related to intrinsic internal electric fields. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm.

Recombination mechanisms in GaInNAs/GaAs multiple quantum wells

Abstract: Recombination processes in Ga1−xInxNyAs1−y/GaAs multiple quantum wells (MQWs) were investigated as function of the nitrogen molar fraction. We found a pronounced S-shaped behavior for the temperature-dependent shift of the photoluminescence emission similar to the ternary nitrides InGaN and AlGaN. This is explained by exciton localization at potential fluctuations. Time-resolved measurements at 4 K reveal an increase of the decay time with decreasing emission energy. A model based on lateral transfer processes to lower-energy states is proposed to explain this energy dependence. The formation of tail states in the Ga1−xInxNyAs1−y/GaAs MQWs is attributed to nitrogen fluctuations.

Design and simulation of terahertz quantum cascade lasers

Abstract: Strategies and concepts for the design of THz emitters based on the quantum cascade scheme are analyzed and modeled in terms of a fully three-dimensional Monte Carlo approach; this allows for a proper inclusion of both carrier–carrier and carrier–phonon scattering mechanisms. Starting from the simulation of previously published far-infrared emitters, where no population inversion is achieved, two designs are proposed. The first one follows the well-established chirped-superlattice scheme whereas the second one employs a double-quantum well superlattice to allow energy relaxation through optical phonon emission. For both cases a significant population inversion is predicted at temperatures up to 80 K.

Bright blue electroluminescence from an InGaN/GaN multiquantum-well diode on Si(111): Impact of an AlGaN/GaN multilayer

Abstract: We present an electroluminescence test structure which consists of an InGaN/GaN multiquantum well as active region on the top of an AlGaN/GaN multilayer grown by metalorganic vapor phase epitaxy on Si(111) substrate. The integral room-temperature electroluminescence spectrum reveals a peak emission wavelength of 467 nm and a significantly higher brightness than an identical reference structure on sapphire substrate. In microelectroluminescence imaging, two emission peaks at 465 and 476 nm can be separated originating from locally different areas of the diode. Cathodoluminescence measurements in cross section and high-resolution x-ray diffraction measurements show that the structure is less strained than a sample without the AlGaN/GaN multilayer. The AlGaN/GaN multiple layer sequence which has a total thickness of 1.5 μm causes lattice relaxation during growth after a thickness of around 0.9 μm as directly visualized by cathodoluminescence line scans across the diode.

Lasing Characteristics of Low-Threshold GaInNAs Lasers Grown by Metalorganic Chemical Vapor Deposition

Abstract: We report on the lasing characteristics of low-threshold long-wavelength GaInNAs double quantum well (DQW) lasers grown by metalorganic chemical vapor deposition (MOCVD). We have achieved a threshold current density of 450 A/cm2 for a 1.28-µm-emitting laser. This is the lowest value for 1.3-µm-range GaInNAs lasers grown by MOCVD. We also observed high characteristic temperatures (T0) of 210 K and 130 K for 1.25 µm and 1.28 µm lasers, respectively. In addition, we investigated the gradual change in lasing characteristics under pulsed operation. The blue shift of an emission wavelength and a threshold current reduction were observed, which is similar to that observed in the thermal annealing of GaInNAs.

Band-structure modulation of nano-structures in an electric field

Abstract: A method to electronically modulate the energy gap and band-structure of semiconducting carbon nanotubes is proposed. Results show that the energy gap of a semiconducting nanotube can be narrowed when the nanotube is placed in an electric field perpendicular to the tube axis. Such effect in turn causes changes in electrical conductivity and radiation absorption characteristics that can be used in applications such as switches, transistors, photodetectors and polaron generation. By applying electric fields across the nanotube at a number of locations, a corresponding number of quantum wells are formed adjacent to one another. Such configuration is useful for Bragg reflectors, lasers and quantum computing.

Chapter 1 Quantum wells and quantum wires for potential thermoelectric applications

Abstract: In this chapter, the predictions of an enhancement in the thermoelectric figure of merit of low-dimensional material systems relative to their corresponding bulk counterparts, as well as the present state of experimental confirmation of these predictions, are reviewed. Progress with specific quantum well, quantum wire, and quantum dot materials systems, such as the lead salts, Si-Ge, and bismuth is discussed. To date most of the effort has gone into proof-of-principle studies, though actual demonstration of the highest thermoelectric figure of merit ( Z 3D T ) of any material to date has been seen in the low-dimensional system PbSe 0.98 Te 0.02 -PbTe, where the enhancement is attributed to quantum dot formation associated with the interface between the PbTe and PbSe 0.98 Te 0.02 . In this chapter, particular attention is given to a discussion of the structure and properties of bismuth quantum wires, which are still at an early state of research. Bismuth nanowires, however, offer significant promise for practical applications, because they can be self-assembled and are predicted to have desirable thermoelectric properties when they have wire diameters in the 5- to 10-nm range. Though temperature-dependent resistance measurements have been carried out for Bi nanowires in this diameter range, reliable thermoelectric measurements have not yet been reported.

Ultraviolet Light-Emitting Diodes at 340 nm using Quaternary AlInGaN Multiple Quantum Wells

Abstract: An ultraviolet light-emitting diode with peak emission wavelength at 340 nm is reported. The active layers of the device were comprised of quaternary AlInGaN/AlInGaN multiple quantum wells, which were deposited over sapphire substrates using a pulsed atomic-layer epitaxy process that allows precise control of the composition and thickness. A comparative study of devices over sapphire and SiC substrates was done to determine the influence of the epilayer design on the performance parameters and the role of substrate absorption.

Photoluminescence up-conversion in single self-assembled InAs/GaAs quantum dots.

Abstract: Microphotoluminescence measurements under cw excitation reveal the existence of a strong photoluminescence up-conversion from single InAs/GaAs self-assembled quantum dots and also from the InAs wetting layer. Excitation spectroscopy of the up-converted photoluminescence signal shows identical features from the wetting layer and the single quantum dots, i.e., a band tail coming from the deep states localized at the rough interfaces of the wetting layer quantum well. This observation of photoluminescence up-conversion demonstrates the influence on the quantum dot properties of the environment, and highlights the limitations of the artificial atom model for a semiconductor quantum dot.

Effects of interface roughness and phonon scattering on intersubband absorption linewidth in a GaAs quantum well

Abstract: We experimentally and theoretically study the effects of interface roughness and phonon scattering on intersubband absorption linewidth in a modulation-doped GaAs/AlAs quantum well. Quantitative comparisons between experimental results and theoretical calculations make it clear that interface roughness scattering is the dominant scattering mechanism for absorption linewidth in the temperature range below 300 K. Even at room temperature, phonon scattering processes contribute little to linewidth, while polar-optical phonon scattering limits electron mobility.

High-performance InAs quantum-dot lasers near 1.3 μm

Abstract: High-performance quantum dot (QD) lasers near 1.3 μm were fabricated using four stacks of InAs QDs embedded within strained InGaAs quantum wells as an active region and a reactive-ion-etched 5-μm-ridge waveguide design. For a 1.5-mm-long cavity QD laser, ground-state continuous-wave (cw) lasing has been achieved with a single facet output power of 15 mW at temperatures as high as 100 °C, while at room temperature having a differential quantum efficiency of 55% and a single facet output power of 50 mW. The characteristic temperature T0 for ground-state cw lasing is 78 K up to our temperature measurement limit of 100 °C.

Electron-spin decoherence in bulk and quantum-well zinc-blende semiconductors

Abstract: A theory for longitudinal ${(T}_{1})$ and transverse ${(T}_{2})$ electron spin coherence times in zinc-blende semiconductor quantum wells is developed based on a nonperturbative nanostructure model solved in a fourteen-band restricted basis set. Distinctly different dependences of coherence times on mobility, quantization energy, and temperature are found from previous calculations. Quantitative agreement between our calculations and measurements is found for GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.