Strained-layer Quantum Wells Research Articles

High-temperature operation of InGaAs strained quantum-well lasers

Strained-layer quantum-well (SQW) laser structures have been investigated for avionics applications requiring high-temperature performance. The authors have successfully demonstrated InGaAs-GaAs SQW lasers capable of CW operation up to 200 degrees C with more than 5 mW single-mode optical power. These lasers have an emission wavelength of approximately=980 nm, threshold current density of 200 A/cm/sup 2/, differential quantum efficiency of 60%, high output power of approximately=1 W with 50 mu m stripe, and characteristic temperature of 130-140 K. >

200-A/cm/sup 2/ threshold current density 1.5- mu m GaInAs/AlGaInAs strained-layer GRIN-SCH quantum-well laser diodes grown by OMCVD

Summary form only given. The authors report 1.5 mu m GaInAs/AlGaInAs GRIN-SCH SL (strained layer) QW (quantum well) LDs (laser diodes) having both compressive and tensile strain in the well. They were grown by low-pressure organometallic chemical vapor deposition (OMCVD). Very low threshold current densities of 200 and 400 A/cm/sup 2/ were obtained in tensile and compressive SL QW LDs, respectively. The characteristic temperatures were 60 K for a lattice-matched QW LD, 72 K for a compressive SL QW LD, and 50 K for a tensile SL QW LD. Lasing wavelengths were in the 1.5 mu m range for all cases. >

Structures for improved 1.5 μm wavelength lasers grown by LP-OMVPE; InGaAs-InP strained-layer quantum wells a good candidate

Abstract InGaAs/InP strained-layer MQW lasers are theoretically predicted to exhibit enhanced performance over lattice matched MQW lasers as a result of the strain-induced reduction of non-radiative recombination processes in the structures. The LP-OMVPE growth of abrupt lattice matched InGaAsP/InP QWs has been optimized and high quality In 0.8 Ga 0.2 As/InP strained layers (1.8% biaxial compression) have been grown. The multiple line PL emissions at 1.5 K from single wells indicate atomically smooth and abrupt interfaces with monolayer well width variations. However, the critical thickness of strained-layer QWs was found to decrease when grown on substrates with increasing misorientation from the (001) plane. In 0.8 Ga 0.2 As/InGaAsP strained-layer MQW lasers emitting at 1.55 μm were fabricated. Excellent properties such as CW output powers as high as 200 mW, an extremely high differential external efficiency of 82%, threshold currents as low as 10 mA and a T 0 value as high as 97 K were obtained.

Two-dimensional spin confinement in strained-layer quantum wells.

We show that a two-dimensional spin system can occur for ``heavy,'' ${\mathit{m}}_{\mathit{J}}$=\ifmmode\pm\else\textpm\fi{}3/2 holes resulting from a reduction to tetragonal symmetry produced by biaxial strain or confinement in a quantum well. Evidence is produced from magnetotransport measurements on strained ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb/GaSb quantum wells, in which the spin splitting is determined only by the component of magnetic field in the growth direction, and from published work on unstrained GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunctions. Complementary behavior is predicted for ``light'' holes.

The in-plane effective mass in strained-layer quantum wells

The problem of calculating the valence-band structure of strained-layer quantum wells in the effective-mass approximation is reviewed. Using the spherical approximation and exploiting the simplicity of the infinitely deep well model we show that the in-plane effective mass is determined by two factors—a splitting contribution which is dominant at large strains, and a quantum confinement contribution. A model for finite-depth wells is presented which gives analytic expressions for the zone-center in-plane mass and associated nonparabolicity factor, and it is applied to the system InxGa1−xAs/GaAs. The model allows the computation of valence-band structure using no more than a pocket calculator. It is shown to give results in reasonable agreement with experiment.

Optical properties of III–V strained-layer quantum wells

This paper reviews recent experimental and theoretical studies of InGaAs strained layer quantum wells and superlattices grown on InP and GaAs substrates. The optical properties of a set of periodically structured samples covering a wide range of biaxially compressive and tensile strain are compared with those of lattice-matched In 0.53Ga 0.47As/InP structures. The dramatic variations observed in the optical behavior of the samples under strain are quantitatively analysed in terms of an effective mass model and bulk phenomenological deformation potential theory.

Photoluminescence in strained-layer quantum wells

Photoluminescence and thermally modulated photoluminescence have been employed in thin single quantum wells in the InP/Ga1−x Inx As/InP system (0≤x≤1). The potential use of these methods to estimate conduction and valence-band offsets in strained-layer heterostructures is discussed.

LATERAL MAPPING OF ATOMIC SCALE INTERFACE MORPHOLOGY AND DISLOCATIONS IN QUANTUM WELLS BY CATHODOLUMINESCENCE IMAGING

Our present knowledge of the atomic scale structural, chemical and electronic properties of semiconductor interfaces is inversely proportional to their importance for a whole generation of novel electronic and photonic quantum well devices. It is the purpose of this paper to demonstrate how wavelength- and time-resolved cathodoluminescence imaging (CLI) provides a one-to-one image of the crystallographic island structure of the heterointerfaces which are the boundaries of the quantum well. A detailed description of the fully computer controlled cathodoluminescence (CL) system is given. The accelerating voltage, which is choosen to be 30 kV in our standard CL work, is lowered to 3 kV in order to reduce the diameter of the carrier generation volume resulting in an increase of the lateral CL resolution. The exciting electron beam is digitally scanned over a sample area divided in up to 512 x 400 pixels. The CL signal is detected in the visible and near infrared regime (GaAs quantum wells) by a cooled photomultiplier using the technique of time resolved single photon counting. The total time resolution is better than 250 ps. Our data acquisition technique has the unique advantage that simultaneously up to 14 time Windows can be activated, enabling us to record up to 14 spectra or up to 14 images corresponding to different times with respect to the start of the exciting pulse in a single run. Results on AlGaAs/GaAs/AlGaAs quantum wells are presented as a typical example. Direct images of growth islands differing by one monolayer height (2.8 A) at GaAs/AlGaAs heterointerfaces and of the columnar structure of GaAs QWs are observed. The dependence of the lateral extension of these islands, which for certain growth conditions exceeds 6-7 µm, on the parameters of crystal growth is investigated. A transition from 2 dimensional to 3 dimensional crystal growth due to an increase of the MBE (molecular beam epitaxy) growth temperature from 600° C to 660° C is clearly observed. Spectrally- and time-resolved CLI experiments directly visualize the lateral diffusion of the quasi two dimensional carriers along the quantum well interfaces and provide a measure of the in-plane diffusion velocity in quantum well structures. The CLI detection setup is further extended to the infrared regime, by adapting a Ge-PIN-diode and avalanche photodiodes to the photon counting System. This enables CLI and time resolved CLI investigation of InGaAs quantum wells and related material Systems. Strain induced dislocations in pseudomorphic strained layer quantum wells (e.g. GaAs/InGaAs/GaAs QWs) are directly visualized by infrared CLI. Time resolved experiments yield lifetime images around these dislocations.

Optical studies of InxGa1−xAs/GaAs strained-layer quantum wells

Intense single-peak photoluminescences of free-exciton origin were observed in InxGa1−x As/GaAs strained-layer quantum wells with x ranging from 0.09 to 0.20. Band lineups in these strained-layer heterostructures were determined by the dependence of luminescence energy on the well thickness. The heterojunction discontinuity in the heavy hole valence bands was found to be about 30% of the gap difference and independent of x. Exciton-phonon coupling was determined by temperature-dependent absorption measurements and the coupling strength was found to be similar to that of unstrained GaAs/AlxGa1−x As quantum wells.

Electronic structures of In1−xGaxAs-InP strained-layer quantum wells

The band structures of In1−xGaxAs-InP strained-layer quantum wells are investigated theoretically in the bond-orbital model. For small x, the well material is subject to a compressive biaxial strain which lifts the HH1 subband further apart from the LH1 subband, resulting in smaller in-plane effective mass for holes. For large x, the strain becomes tensile and the LH1 subband is lifted upward with respect to the HHl subband. For x near the critical value, where the HHl and LHl energy levels cross each other, the valence-band structure undergoes a direct-to-indirect transition.

Strained InGaAs/GaAs quantum wells with a 1.3 µm bandedge at room temperature

AbstractThe fabrication of long wavelength opto-electronic devices on GaAs substrates is an attractive method for the monolithic integration of optical and electronic devices on a single chip for applications in telecommunications. InGaAs strained layer quantum wells provide one way of engineering the bandgap needed for 1.3 µm devices at room temperature. We have grown and characterized quantum well structures with bandgap of .96 eV using a novel technique, called spatial strain separation, to avoid the stringent limitation of critical thickness which limits the achievable bandgap in this material system. Moreover we have studied the stability of such structures with respect to thermal processing, including oven and rapid thermal annealing. We have observed no degradation of the quantum wells after thermal annealing as judged from the PL spectra shown here.

Guided Wave Devices Based on III-V Heterojunctionand Quantum Well Structures

ABSTRACTGuided wave technology based on III–V compound semiconductor materials is important for optical signal processing, fiber communication and sensor applications. The provision of heterojunction and quantum well materials by LPE, MBE, and MOCVD have led to new concepts and techniques in tailoring the properties of discrete devices and optoelectronic integrated circuits. This paper discusses the electro-optical properties of heterojunction, quantum well, superlattice based on two novel effects, the Franz-Keldysh effect in bulk materials and the quantum confined Stark effect in semiconductor quantum wells. The electroabsorption and electrorefraction properties resulted from these effects are utilized in the realization of some waveguide modulators. Materials based on InGaAsP/InP heterojunctions, InGaAs/GaAs strained layer quantum wells and InGaAs/InP quantum wells will be emphasized.

Electric Field Effects on the Optical Properties of InxGa1-xAs/GaAs Strained Quantum Wells and Superlattices

AbstractWe have used photoluminescence excitation and photocurrent spectroscopy to investigate the electronic properties of InxGa1-xAs/GaAs strained layer quantum wells and superlattices. In quantum wells, sharp excitonic transitions between discrete energy levels are observed both in excitation and near flatband photocurrent spectra whereas superlattices show heavy-hole to conduction miniband transitions at the Brillouin mini-zone centre and edge, directly giving the electron miniband width. Applying a longitudinal electric field to the quantum wells produces a red shift of the excitons due to the quantum confined Stark effect, while in superlattices, photocurrent spectra at finite applied electric fields show for the first time in this system, the effects of Wannier-Stark quantization. The analysis of the spectra provides a precise determination of the band offset.

Excitonic Properties of ZnSe-ZnS Strained-Layer Superlattices and A Fibonacci Sequence

ABSTRACTExcitonic properties of ZnSe-ZnS strained-layer quantum wells (SLQWs) with type I band lineups are reviewed on the basis of our recent results of temperature- and strain-dependent photoluminescence and absorption spectra. In order to estimate the conduction and valence band offsets as a function of ZnSe well thickness, we have modified the “model-solid” theory in which the valence bands (heavy-hole band in ZnSe and light-hole band in ZnS) are relatively moved with strains. Temperature and high excitation dependent studies of the n=1 heavy-hole excitons suggest a localization of excitons and reveals the important evidence on scatterings of excitons with acoustic and optical phonons. The thermal quenching of the exciton emission is caused by thermal dissociation of quasi-two-dimensional excitons through electrons and holes, from which the activation energy for this dissociation is 4 times larger than Ea.3D (a binding energy of bulk exciton) of ZnSe. A new superlattice structure with a quasiperiodic crystal which is derived from a finite Fibonacci sequence, has been fabricated by a low-pressure MOCVD method and its photoluminescence properties are for the first time introduced.

The strain effect on the electronic structures of In1−xGaxAs/GaAs quantum wells

The electronic structures of In 1−x Ga x As/GaAs strained-layer quantum wells are investigated systematically based on the bond-orbital model. The theoretical results are in reasonable agreement with experiments. It is found that the compressive stress which develops between InGaAs and GaAs will result in the heavy hole subbands being shifted up relative to the light hole subbands. Therefore, more structures in the spectroscopic spectra and novel device properties, such as high speed p-channel field effect transistors, are expected. Furthermore, the variations of hole effective mass with strain are discussed. A criterion useful for device design is obtained.

InAs strained-layer quantum wells with band gaps in the 1.2–1.6 μm wavelength range

Strained In0.52Al0.48As/InAs single quantum wells have been grown by molecular beam epitaxy on InP substrates and their structural quality and commensurability demonstrated by transmission electron microscopy. Intense photoluminescence in the wavelength range of 1.2–1.6 μm for well widths between 10 and 30 Å was obtained, indicating a very efficient carrier collection into the highly (3.4%) biaxially compressed InAs wells.

SHALLOW IMPURITIES IN SEMICONDUCTOR QUANTUM WELLS

Energy spectra of shallow impurities (donors and acceptors) in quantum wells are studied within the effective mass approximation. New theoretical calculations on donor states in quantum wells are presented. A model potential which allows a simple and fairly accurate estimate of the entire spectrum of donors in quantum wells with intermediate width (20–300 A) is introduced. New developments for acceptors in strained-layer quantum wells (GaAs–Ga 1– x In x As) are reported. The effect of biaxial strain on the electronic structure of the strained-layer quantum well is properly included.

Shallow impurities in semiconductor quantum wells

Energy spectra of shallow impurities (donors and acceptors) in quantum wells are studied within the effective mass approximation. New theoretical calculations on donor states in quantum wells are presented. A model potential which allows a simple and fairly accurate estimate of the entire spectrum of donors in quantum wells with intermediate width (20–300 Å) is introduced. New developments for acceptors in strained-layer quantum wells (GaAsGa 1- x In x As) are reported. The effect of biaxial strain on the electronic structure of the strained-layer quantum well is properly included.

Energy Loss Rates for Light Holes in Ingaas/GaAs Strained-Layer Single Quantum Wells

ABSTRACTEnergy relaxation rates for light holes in InGaAs/GaAs strained layer quantum wells are measured. Two techniques were used to measure light hole temperatures as a function of power dissipated in the hole gas. For Th < 20K, Shubnikov-de Haas oscillations were used and for Th> 20K, a photoluminescence technique was employed.