Showing papers on "Quantum well published in 2000"

Large Rashba splitting in InAs quantum wells due to electron wave function penetration into the barrier layers.

Abstract: We report on zero-field spin splitting of two-dimensional electron systems. Though absent in the unbiased InAs square asymmetric quantum well (SAQW), the Rashba splitting becomes pronounced by applying a positive back-gate voltage. In our SAQW, the Rashba parameter α increases with electron density and is tuned by a factor of about 2 using an additional front gate without charging the well. We argue that the band-edge profile provides the important contribution for spin-orbit interaction due to barrier penetration of the envelope wave function. This mechanism can provide the potential for high speed implementation in spintronics.

Theoretical Study of Orientation Dependence of Piezoelectric Effects in Wurtzite Strained GaInN/GaN Heterostructures and Quantum Wells

Abstract: We calculated the crystal orientation dependence of piezoelectric fields in wurtzite strained Ga0.9In0.1N/GaN heterostructures. The highest longitudinal piezoelectric field of 0.7 MV/cm can be generated in (0001)-oriented biaxial-strained Ga0.9In0.1N layer coherently grown on GaN. On the contrary, no longitudinal piezoelectric field is induced in strained layers grown along orientations at an off angle of 39° or 90° from (0001). The high symmetry planes with these angles are, for instance, (1124) and (1012) for 39°, and (1120) and (1010) for 90°. We also calculated the crystal orientation dependence of the transition probability in a 3-nm strained Ga0.9In0.1N/GaN quantum well, which indicated that the transition probability with these non-(0001) orientations becomes 2.3 times larger than that with the (0001) orientation. We conclude that high-performance strained nitride-based optical devices can be obtained by control of the crystal orientation.

Robust electrical spin injection into a semiconductor heterostructure

Abstract: We report efficient electrical injection of spin-polarized carriers from a non-lattice-matched magnetic contact into a semiconductor heterostructure. The semimagnetic semiconductor ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}$ is used as a spin-injecting contact on a GaAs-based light-emitting diode. Spin-polarized electrons are electrically injected across the II-VI/III-V interface, where they radiatively recombine in a GaAs quantum well and emit circularly polarized light. An analysis of the optical polarization which includes quantum confinement effects yields a lower bound of 50% for the spin injection efficiency.

Room temperature continuous wave InGaAsN quantum well vertical cavity lasers emitting at 1.3 um

Abstract: Selectively oxidized vertical cavity lasers emitting at 1294 nm using InGaAsN quantum wells are reported for the first time which operate continuous wave at and above room temperature. The lasers employ two n-type Al{sub 0.94}Ga{sub 0.06}As/GaAs distributed Bragg reflectors each with a selectively oxidized current aperture adjacent to the optical cavity, and the top output mirror contains a tunnel junction to inject holes into the active region. Continuous wave single mode lasing is observed up to 55 C. These lasers exhibit the longest wavelength reported to date for vertical cavity surface emitting lasers grown on GaAs substrates.

Intersubband electroluminescence from silicon-based quantum cascade structures.

Abstract: The quantum cascade laser, which uses electronic transitions within a single band of a semiconductor, constitutes a possible way to integrate active optical components into silicon-based technology. This concept necessitates a transition with a narrow linewidth and an upper state with a sufficiently long lifetime. We report the observation of intersubband electroluminescence from a p-type silicon/silicon-germanium quantum cascade structure, centered at 130 millielectron volts with a width of 22 millielectron volts, with the expected polarization, and discernible up to 180 kelvin. The nonradiative lifetime is found to depend strongly on the design of the quantum well structure, and is shown to reach values comparable to that of an equivalent GaInAs/AlInAs laser structure.

Material parameters of InGaAsP and InAlGaAs systems for use in quantum well structures at low and room temperatures

Abstract: The set of material parameters for quantum well structures is of immense importance because of its usage in the development of theories, extraction of experimental data, and the proper design of devices. In particular, the (Al,In)GaAs/GaAs, InGaAs/InP and (In,Ga)AlAs/InGaAs quantum well systems have drawn a lot of attention. They form the center core of materials used for fundamental basic research and device applications. Despite the presence of some review articles and reference books, there is a lack of clear reference on the accurate determination of the material parameters for quantum wells. This review aims to provide a comprehensive and systematic set of material parameters for the above quantum well systems grown on (1 0 0) substrates at two different temperatures, below 10 K and at around 300 K. The parameters are compared against experimental data from various fabrication sources, measurement techniques, and quantum well structures. The values presented here serve as an accurate and up to date source of reference.

Passively mode-locked diode-pumped surface-emitting semiconductor laser

Abstract: A surface-emitting semiconductor laser has been passively mode locked in an external cavity incorporating a semiconductor saturable absorber mirror. The gain medium consists of a stack of 12 InGaAs-GaAs strained quantum wells, grown above a Bragg mirror structure, and pumped optically by a high-brightness diode laser. The mode-locked laser emits pulses of 22-ps full-width at half maximum duration at 1030 nm, with a repetition rate variable around 4.4 GHz.

Dependence of composition fluctuation on indium content in InGaN/GaN multiple quantum wells

Abstract: The information on the variations of indium composition, aggregation size, and quantum-well width is crucially important for understanding the optical properties and, hence, fabricating efficient light-emitting devices. Our results showed that spinodal decomposition could occur in InGaN/GaN multiple quantum wells with indium content in the range of 15%–25% (grown with metal–organic chemical-vapor deposition). A lower nominal indium content led to a better confinement of indium-rich clusters within InGaN quantum wells. The InGaN/GaN interfaces became more diffusive, and indium-rich aggregates extended into GaN barriers with increasing indium content. It was also observed that indium-rich precipitates with diameter ranging from 5 to 12 nm preferred aggregating near V-shaped defects.

Ultrafast intersubband relaxation (⩽150 fs) in AlGaN/GaN multiple quantum wells

Abstract: The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.

Electronic states and band alignment in GalnNAs/GaAs quantum-well structures with low nitrogen content

Abstract: We investigate the electronic states in strained Ga0.62In0.38N0.015As0.985/GaAs multiple- quantum-well structures using photoluminescence and (polarized) photoluminescence excitation measurements at low temperature. From a theoretical fit to the experimental data, a type-I band alignment for the heavy holes with a strained conduction-band offset ratio of about 80% is obtained, while the light holes show an approximately flat band alignment. Additionally, our results suggest an increased effective electron mass in GaInNAs, possibly due to the interaction of the conduction band with nitrogen-related resonant states, an observation prospectively of benefit for GaInNAs-based diode lasers.

Evidence of strong carrier localization below 100 K in a GaInNAs/GaAs single quantum well

Abstract: We report an anomalous temperature dependence of the photoluminescence (PL) spectrum of a 7 nm Ga0.72In0.28N0.028As0.972/GaAs single quantum well. The PL peak energy exhibits an inverted S-shape dependence with temperature. Below 100 K, the PL integrated intensity shows a temperature dependence similar to that of amorphous semiconductors. The observed anomalous behavior is explained by a strong localization of carriers at low temperatures that could be induced by the presence of nitrogen. Thermal annealing does not significantly change the anomalous temperature dependence.

Low chirp observed in directly modulated quantum dot lasers

Abstract: We have examined the dynamic properties of high-aspect-ratio InAs-quantum-dot (QD) lasers at room temperature. A novel characteristic of low chirp in the lasing wavelength under 1-GHz current modulation was found in the quantum dot lasers. This is more than one order of magnitude less than the typical chirp (0.2-nm) found in a conventional quantum well laser that we used as a reference. Low chirp was obtained not only in the ground state lasing but in the second level lasing of quantum dots as well.

Bound states in optical absorption of semiconductor quantum wells containing a two-dimensional electron Gas

Abstract: The dependence of the optical absorption spectrum of a semiconductor quantum well on two-dimensional electron concentration n(e) is studied using CdTe samples. The trion peak (X-) seen at low n(e) evolves smoothly into the Fermi edge singularity at high n(e). The exciton peak (X) moves off to high energy, weakens, and disappears. The X,X- splitting is linear in n(e) and closely equal to the Fermi energy plus the trion binding energy. For Cd0.998Mn0.002Te quantum wells in a magnetic field, the X,X- splitting reflects unequal Fermi energies for M = +/-1/2 electrons. The data are explained by Hawrylak's theory of the many-body optical response including spin effects.

Quantum-dot heterostructure lasers

Abstract: Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications.

New frontiers in quantum cascade lasers and applications

Abstract: Recent advances and new directions in quantum cascade (QC) lasers are discussed. Invented in 1994 following many years of research on band-structure engineered semiconductors and devices grown by molecular beam epitaxy, this fundamentally new laser has rapidly advanced to a leading position among midinfrared semiconductor lasers in terms of wavelength agility as well as power and temperature performance. Because of the cascaded structure, QC lasers have a slope efficiency proportional to the number of stages. Devices with 100 stages having a record peak power of 0.6 W at room temperature are reported. QC lasers in the AlInAs-GaInAs lattice matched to InP material system can now be designed to emit in the whole midinfrared range from 4 to 20 /spl mu/m by appropriately choosing the thickness of the quantum wells in the active region. Using strained AlInAs-GaInAs, wavelengths as short as 3.4 /spl mu/m have been produced. New results on QC lasers emitting at 19 /spl mu/m, the longest ever realized in a III-V semiconductor laser, are reported. These devices use innovative plasmon waveguides to greatly enhance the mode confinement factor, thereby reducing the thickness of the epitaxial material. By use of a distributed feedback (DFB) geometry, QC lasers show single-mode emission with a 30-dB side-mode suppression ratio. Broad continuous single-mode tuning by either temperature or current has been demonstrated in these DFB QC lasers at wavelengths in two atmospheric windows (3-5 and 8-13 /spl mu/m), with continuous-wave linewidths <1 MHz when free running and /spl sim/10 KHz with suitable locking to the side of a molecular transition. These devices have been used in a number of chemical sensing and spectroscopic applications, demonstrating the capability of detecting parts per billion in volume of several trace gases. Sophisticated band-structure engineering has allowed the design and demonstration of bidirectional lasers. These devices emit different wavelengths for opposite bias polarities. The last section of the paper deals with the high-speed operation of QC lasers. Gain switching with pulse widths /spl sim/50 ps and active modelocking with a few picosecond-long pulses have been demonstrated. Finally, a new type of passive modelocking has been demonstrated in QC lasers, which relies on the giant and ultrafast optical Kerr effect of intersubband transitions.

GaAs-based long-wavelength lasers

Abstract: The present paper reviews recent achievements in the fabrication of diode lasers for the near-infrared range on GaAs substrates. 1.3??m light emitters are currently widely used in fibre-optic communication systems. GaAs-based devices are potentially advantageous compared to their InGaAsP counterparts in several aspects, such as improvement of thermal stability, possibility to grow vertical-cavity surface-emitting lasers in a single growth run and the use of large-area high-quality inexpensive GaAs substrates. Three main approaches have been suggested so far to achieve the 1.3??m emission from structures grown on GaAs substrates. They are InGaAs and GaAsSb quantum wells, GaInAsN quantum wells and InAs/GaAs quantum dots. In the present paper we discuss all these approaches including material growth, optical properties and laser characteristics. The results obtained by these methods are compared and their potential advantages discussed.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

Abstract: The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's /spl eta//sub i/ for the dot densities studied.

Direct determination of electron effective mass in GaNAs/GaAs quantum wells

Abstract: Electron effective mass (m*e) in GaNxAs1-x/GaAs quantum wells (QWs) is investigated by the optically detected cyclotron resonance technique. The m*e values of 0.12m0 and 0.19m0 are directly determi ...

Spontaneous polarization and piezoelectric field in G a N / A l 0.15 Ga 0.85 N quantum wells: Impact on the optical spectra

Abstract: We have investigated the effects of the built-in electric field in ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}$ quantum wells by photoluminescence spectroscopy. The fundamental electron heavy-hole transition redshifts well below the GaN bulk gap for well widths larger than 3 nm for the specific quantum wells investigated and exhibits a concomitant reduction of the intensity with increasing well thickness. The experimental data are quantitatively explained by means of a self-consistent tight-binding model that includes screening (either dielectric or by free-carriers), piezoelectric field and spontaneous polarization field. The impact of the built-in field on the exciton stability is discussed in detail. We demonstrate that the exciton binding energy is substantially reduced by the built-in field, well below the values expected from the quantum size effect in the flat band condition.

Low-threshold current density 1.3-μm InAs quantum-dot lasers with the dots-in-a-well (DWELL) structure

Abstract: The wavelength of InAs quantum dots in an In/sub 0.15/Ga/sub 0.85/As quantum-well (DWELL) lasers grown on a GaAs substrate has been extended to 1.3-/spl mu/m. The quantum dot lasing wavelength is sensitive to growth conditions and sample thermal history resulting in blue shifts as much as 73 nm. The room temperature threshold current density is 42.6 A cm/sup -2/ for 7.8-mm cavity length cleaved facet lasers under pulsed operation.

Self-assembled InGaN quantum dots grown by molecular-beam epitaxy

Abstract: Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.

Photoluminescence and radiative lifetime of trions in GaAs quantum wells

Abstract: Electron-trion photoluminescence spectra were measured in undoped high-quality GaAs quantum wells. The obtained line shape depends on temperature and is asymmetric with a tail towards lower photon energies. For a detailed understanding, we have solved numerically the trion Schrodinger equation in the quantum well. Both electron and hole trions are considered. Good agreement is found for the trion binding energy and for the luminescence line shape at different temperatures. The radiative lifetime of thermalized trions is found to increase linearly with temperature. Analytical results are given for both types of trions taking into account the finite photon wave vector (light-cone effect).

Optically pumped InAs quantum dot microdisk lasers

Abstract: We have achieved lasing in InAs quantum dot embedded GaAs microdisks under optical pumping. Above the lasing threshold, a drastic increase of emission intensity is accompanied by a decrease of the spectral linewidth of the whispering gallery modes. The laser light is linearly polarized. The polarization direction is parallel to the disk plane. The wide gain spectrum of quantum dots allows simultaneous lasing in several whispering gallery modes of a microdisk.

Spontaneous polarization effects in wurtzite GaN/AlGaN quantum wells and comparison with experiment

Abstract: Electronic and optical properties of wurtzite GaN/AlGaN quantum well (QW) structures with the spontaneous (SP) and piezoelectric (PZ) polarizations are investigated Although the PZ field in the well is zero where there is no strain if the QW structures are grown on a thick GaN layer, there may still exist a strong field in the well due to the difference between the SP polarizations in the well and barrier regions It is shown that the transition energies have significant dependence on both the well and the barrier widths and the many-body optical gain is reduced largely due to the SP polarization In particular, in the case of a QW structure with a large well width, the reduction of the optical gain is dominant due to larger spatial separation between the electron and hole wave functions These results suggest that a QW structure with a thin well width below 30 A is desirable for QW lasers We show that the theoretical transition energies agree very well with the experimental results for several Al compo

Stimulated emission from donor transitions in silicon

Abstract: Semiconductor sources of stimulated emission which are based on intraband optical transitions of highly nonequilibrium charge carriers are the object of an extensively developing field in physics. The new approaches in the development of unipolar active media are promising to bridge the wavelength range from l 1 m mu p to l 1000 mm of the electromagnetic spectrum. Examples of sources based on bulk material are germanium hot hole lasers [1] and Er-doped silicon lasers [2]. Emitters based on the properties of a two-dimensional electron gas in different kinds of heterostructures are quantum well “cascade” [3] and “fountain” lasers [4] as well as miniband Bloch oscillators in superlattices [5]. We propose a new far-infrared (FIR) (55 mm ,l,

Low-threshold quantum dot lasers with 201 nm tuning range

Abstract: A grating-coupled external-cavity quantum dot laser is tuned across a 201 nm range at a maximum bias of 2.87 kA/cm/sup 2/. One order of magnitude less than the bias required for comparable tuning of quantum well lasers. The tuning range increases for higher cavity losses of the quantum dot laser.

Characterization of InAs quantum dots in strained InxGa1-xAs quantum wells

Abstract: The properties of InAs quantum dots placed in a strained InGaAs quantum well are investigated. The structures are grown by solid-source molecular beam epitaxy on GaAs substrates and are characterized using photoluminescence and atomic force microscopy. Emission wavelength and the optical quality of the quantum dots vary with growth temperature and also depend on the position of the dots in the well. A strong dependence of the dot properties on the capping conditions is established. A postgrowth anneal similar to a typical laser cladding growth results in a large photoluminescence (PL) blueshift and reduces the PL intensity by more than two orders of magnitude. It is shown that these dots-in-a-well structures have superior optical properties as compared to conventional InAs dots in a GaAs matrix, and their emission wavelength can be tuned past the technologically important wavelength of 1.3 μm.

Advanced Ti:Er:LiNbO/sub 3/ waveguide lasers

Abstract: This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO/sub 3/ waveguide lasers emitting at wavelengths around 1.5 /spl mu/m. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (/spl ap/3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained.