Strained-layer Multiple Quantum Wells Research Articles

Design optimization for 25 Gbit/s DML InGaAlAs/InGaAsP/InP SL-MQW laser diode incorporating temperature effect

In this paper, a detailed carrier dynamics model for quantum well lasers is used to study the modulation bandwidth of the directly modulated strained-layer multiple quantum well (SL-MQW) laser. The active region of the directly modulated laser (DML) is optimized in terms of the number of QWs and barrier height. To compromise the device dynamic performance at different operating temperatures, we present an overall optimized design for a 25 Gbps DML under an ambient temperature ranging from 25 to 85°C. To further enhance the modulation bandwidth, we have also proposed a mixed QWs design that increases the 3 dB bandwidth by almost 44% compared to the one without undergoing optimization. The experimental results show that the 3 dB bandwidth of the optimized DML can reach 19 GHz. A clear eye diagram with a bit rate of 25 Gbps was observed at 25°C.

Estimation of multiple-quantum-well laser parameters for simulation of dispersion supported transmission systems at 20 Gbit/s

In this paper, a set of multiple-quantum-well (MQW) laser parameters is proposed for simulation of optical transmission systems at 20 Gbit/s. The parameters have been estimated by joint fitting of a small signal intensity modulation (IM) response model to five measured IM response curves of a strained layer MQW laser, using the Levenberg–Marquardt method. Good agreement between theoretical and experimental curves was obtained. Using these laser parameters, we have assessed the performance of dispersion supported transmission systems at 20 Gbit/s incorporating an erbium doped fibre amplifier (EDFA) or a semiconductor optical amplifier (SOA) as a booster amplifier. It is shown that the use of a SOA, with an unsaturated gain of 20 dB, improves the system performance for link lengths ranging from 8 to 20 km of standard single-mode fibre (SMF) due to partial chirp compensation in the SOA, and degrades the system performance between 1.2 and 2.5 dB for link lengths ranging from 30 to 50 km. The increase of the unsaturated gain of the SOA from 20 to 25 dB is only advantageous for link lengths ranging from 8 to about 12.5 km where a small performance improvement, less than or equal to 0.8 dB, is observed. The influence on the system performance of an increase of the laser line width enhancement factor from 2 to 3 is also investigated.

Band alignment and barrier height considerations for the quantum-confined Stark effect

Strained-layer multiple quantum wells InAsP/InP and InAsP/InGaP optical modulators based on the quantum-confined Stark effect have been fabricated from layers grown by metalorganic vapor phase epitaxy on InP(001). The device layers have been characterized by complementary high resolution x-ray diffraction, transmission electron microscopy, optical absorption and photoluminescence analyses. The structural properties of the layers were deduced from the above data and an accurate determination of the band alignment of the heterostructures was made by performing multiple transition fits to the optical absorption spectra using the Marzin–Bastard envelope function model for strained-layer superlattices. The electric field-dependent redshift of the fundamental electron-heavy hole transition was measured by a photocurrent method and found to be enhanced for structures with lower valence band barrier heights. This observation leads directly to the conclusion that the overall performance of high speed, low drive voltage optical modulators may be improved by engineering the band alignment of the multiple quantum well stack towards structures with disproportionately large conduction band offsets. An optimization of the band alignment will permit more efficient optical modulation by reducing the drive field required to operate the device, which, in turn, can have direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses.

Electronic structure of [formula omitted] strained-layer quantum wells

The GaAs 1 − x P x Al y Ga 1 − y As (x=0.07–0.14, y=0.3) strained-layer multiple quantum wells grown by metal-organic vapor phase epitaxy on GaAs-(111)B substrates have been characterized by X-ray diffraction and photoreflectance (PR) spectroscopy. By fitting the experimental results with the calculation which was based on the deformation potential theory modified by the strain-induced piezoelectric field, the energy band offset ratio for the conduction band at the heterointerface of GaAs 1 − x P x Al y Ga 1 − y As was quantitatively determined to be Q c = 0.61 ± 0.03. This value was found to be nearly independent of the phosphorous composition x in the whole region that has been investigated in this study. Moreover, several anomalous phenomena observed with the PR measurement as well as the possible physical origins have also been discussed.

A relationship for temperature dependence of threshold current for 1.3-μm compressively strained-layer multiple-quantum-well lasers

Assuming that the differential gain is a linear function of temperature, a formula is derived to describe the threshold current as a function of temperature. A maximum operating temperature, T/sub max/ which is related to the fundamental physical properties (such as differential gain, free carrier loss and intervalence band absorption) of the lasers appears naturally in the formula, at which lasing ceases. To experimentally investigate the relationship, studies were carried out on 1.3-/spl mu/m strained-layer multiple quantum well (SL-MQW) lasers with variant 0.7% compressively strained wells. The formula shows a good correlation with threshold current versus temperature data over the temperature range 200 K to 450 K.

Experimental analysis of characteristic temperature in quantum-well semiconductor lasers

We experimentally analyzed the characteristic temperature T/sub 0/ in 1.3-/spl mu/m GaInAsP-InP strained layer multiple-quantum-well (SL-MQW) semiconductor lasers by measuring temperature dependence of gain characteristics. We developed a new approach to evaluate the characteristic temperature, where the temperature dependence of the carrier lifetime estimated from the temperature dependence of the spontaneous emission intensity was taken into account. Our result showed that the temperature dependence of the threshold current is mainly determined by the temperature dependence of the spontaneous emission efficiency which reflects nonradiative recombination, carrier overflow effect, and carrier leakage effect. As a secondary effect, the current density dependence of the internal loss also affects the degradation of the characteristic temperature. The temperature dependence of the differential gain dG/dN and the transparency carrier density N/sub tr/ in the wells are not the dominant mechanism of the low characteristic temperature in GaInAsP-InP lasers around room temperature.

1.5-μm strained-layer MQW-DFB lasers with high relaxation-oscillation frequency and low-chirp characteristics

1.5-/spl mu/m wavelength strained-layer multiple-quantum-well (SL-MQW) distributed-feedback (DFB) lasers for optical video distribution systems, including optical-fiber amplifiers, are studied with respect to the relaxation-oscillation frequency and wavelength chirp characteristics. Several types of lasers are examined as parameters of amount of strain, optical confinement, and detuning. It is confirmed that the introduction of negative detuning has an obvious effect on the increase in relaxation-oscillation frequency f/sub r/ and the reduction in chirp /spl Delta/F. The SL-MQW DFB lasers with low optical confinement and negative detuning show extremely low FM response /spl Delta/F//spl Delta/I of less than 60 MHz/mA as well as high f/sub r/ over 10 GHz for the first time. Additionally, low-chirp value /spl Delta/F of 180 MHz with modulation depth of 10% is realized at relatively low bias current (I/sub h/=I/sub th/+30 mA).

High-temperature CW operation of InGaAsP-InP semi-insulating buried heterostructure lasers using reactive ion-etching technique

We fabricated 1.5-/spl mu/m semi-insulating buried heterostructure (SI-BH) lasers with InGaAsP-InP strained-layer multiple-quantum wells using a reactive ion etching (RIE) technique for mesa definition. A very high CW operating temperature of 150/spl deg/C was obtained in a 300-/spl mu/m-long laser whose rear facet was HR-coated. >

Polarization dependence of photo-detection in strained multiple quantum-well semiconductor lasers

We investigated polarization dependence of photo-detection in strained layer multiple quantum-well (SL-MQW) lasers for time compressive multiplex (TCM) application. The polarization dependence of SL-MQW lasers is larger than that of bulk lasers because of the quantum effect. To improve the polarization dependence of photo-detection, we theoretically and experimentally analyzed the effect of strain quantity on the polarization dependence, and found that large tensile strained quantum-well structure is suitable for the small polarization independence of 0.8 dB and low threshold current characteristics. Moreover, we achieved smaller polarization dependence less than 0.5 dB by applying reverse bias voltage in the photo-detection mode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of effect of strain-compensated structure and compensation limit in strained-layer multiple quantum wells

We investigated the effect of the strain-compensated structure of InGaAs(+)/InGaAs(-) multiple quantum wells (MQWs). Photoluminescence (PL) spectroscopy and thickness fringes by a transmission electron microscopy were used to evaluated the optical and structural crystal quality. For the +0.5/ −0.5% strain-compensated cases, lattice distortion was not observed, and PL intensity increased when the number of periods increased from 5 to 15. Crystal quality was improved by using the +0.5/ −0.5% strain-compensated MQW instead of the +0.5/0% compressively strained MQW. It was possible to increase the stack thickness of the strained MQWs. However, crystal quality was degraded with increase in strain. For the +1.25/ −1.25% cases, PL intensity decreased when the number of periods increased from 5 to 15, and defects were observed in a 15-period sample. Moreover, crystal quality was degraded by using the +1.25/ −1.25% strain-compensated MQW instead of the +1.25/0% compressively strained MQW.

Carrier screening effects in piezoelectric strained InGaAs/GaAs quantum wells grown on the [111]B axis

Excitation-power-dependent blue shifts in photoluminescence and electroluminescence are studied in piezoelectric strained layer multiple quantum wells (MQWs) incorporated in p-i-n diodes. By investigating MQWs with different geometric structures and controlling external bias it is demonstrated that, in contrast to previous studies, these blue shifts cannot always be attributed to long-range screening across a MQW and that screening must take place due to charge redistribution within individual wells. The results provide design rules to ensure this latter screening mechanism, which is subject to fast recovery.

Ultraviolet lasing and excitonic gain in Cd xZn 1−xS-ZnS strained-layer multiple quantum wells

Ultraviolet stimulated emission has been observed from Cd x Zn 1− x S-ZnS strained-layer multiple quantum wells under both optical pumping at room temperature and pulsed current injection at 30 K. An exciton-related mechanism of optical gain formation was also studied on the basis of a partial phase-space-filling effect of excitons localized at the lower energy states of inhomogeneously broadened exciton resonance. It is proposed that the localization of excitons contributes to the formation of optical gain in wide-bandgap II–VI semiconductor quantum wells.

Electroreflectance spectra of Ge x Si 1- x /Si strained layer multiple-quantum wells

We have investigated the optical transitions above the fundamental gap of a set of GexSi1-x/Si strained layer multiple-quantum wells by electroreflectance (ER). The samples were grown by molecular beam espitaxy (MBE). The thickness of the strained layer of GexSi1-x was 5nm with Ge concentration x in the range from 0.4 to 0.5, and the Si barrier layer greater than 16nm. Considering the energy shift caused by strain and quantum well confinement, we were able to clearly recognize the transitions from different quantum well structures associated with the critical points E0, E'0, and E1. The transitions of the critical points E0 and E'0, which are very weak in bulk materials, are apparently enhanced in the quantum well structures.

Comparison of Relaxation Process of Compressive and Tensile Strains in InGaAs Lattice-Mismatched Layers on InP Substrates

This paper investigates the difference in crystal quality between strained-layer multiple quantum wells with compressive (+0.5%) and tensile strains (-0.5 %). For the compressive strain, the photoluminescence intensity decreased and the length of fringe bending increased from 250 Å to 500 Å when the number of periods increased from 5 to 15. The amount of fringe bending increased when the InP thickness decreased, especially when the strain was compressive. We also investigated the relaxation process in an InGaAs layer as a function of the layer thickness (from 25 nm to 2 µ m). For a compressive strain (+1.1%), misfit dislocations were observed near the interface between InGaAs and InP substrate. On the other hand, for a tensile strain (-1.1 %), we observed cracks instead of misfit dislocations. Moreover, the cracks were considered to increase the X-ray full width at half maximum of both the InGaAs lattice-mismatched layer and the InP substrate.

Ion channeling effect on lattice relaxation in strained In1−xGaxAs/InP multiple-quantum-well structure

The ion channeling phenomena for the strained and strain partially relaxed In1−xGaxAs/InP multiple-quantum-well (MQW) structures, which are grown by metalorganic vapor phase epitaxy, are presented. In addition to a well-known channeling phenomenon of strained-layer MQW structures, which cause an enhanced dechanneling along 〈110〉, it was found that dechanneling in the substrate covered with strained MQW structure is enhanced in comparison with the strain partially relaxed MQW structure. This phenomenon is observed clearly for the 〈100〉 oriented substrate rather than that along the inclined 〈110〉 axis. It is suggested that a distortion perpendicular to the 〈100〉 growth direction is induced near the interface between the buffer layer and the MQW structure by the strain stored in MQW structure. This effect is less observed in the strain partially relaxed MQW structure than in the strained MQW structure.

Scanning tunneling microscopy of doping and compositional III–V homo- and heterostructures

Scanning tunneling microscopy (STM) was used to study the (110) cross-sectional surfaces of molecular-beam epitaxially grown III–V homo- and heterostructures, which include GaAs multiple p–n junctions, (InGa)As/GaAs strained-layer multiple quantum wells, and (AlGa)As/GaAs heterojunctions. Both doping and compositional effects can be resolved by the topographic contrasts of constant-current STM images. The samples were prepared by either cleaving in ultrahigh vacuum or cleaving ex situ followed by sulfide [(NH4)2S] passivation. Sulfide passivated samples have been found to be advantageous for the measurements of scanning tunneling spectroscopy.

Low-chirp highly linear 1.55 mu m strained-layer MQW DFB lasers for optical analog TV distribution systems

The design and realization of highly linear 1.5- mu m distributed-feedback (DFB) lasers for optical analog TV distribution systems based on dispersive single-mode fibers are reported. A simple and comprehensive model relating grating characteristics to laser chirp was developed and used for optimization of the DFB grating. The model predicts that lasers emitting at wavelengths located in the positive slope sections of the DFB-reflectivity-wavelength characteristic have higher resonance frequencies and reduced chirp. The lasers were realized by using strained-layer multiple quantum wells for high differential gain and low alpha factor, antireflection (AR) coating of one laser facet for reduction of internal photon density, and a special grating design for low spatial hole burning and chirp reduction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Glancing Angle EXAFS Investigation of InGaAs/GaAs Strained Layer Multiple Quantum Wells: Strain and Bond Distances

The strain distribution in quantum wells of InxGa1-xAs/GaAs (x<0.25) is investigated by glancing-angle EXAFS. The measures have been performed at glancing angles below the critical angle for total reflection in order to avoid substrate contributions to the signal. The results show that a number of Ga-As bond lengths are stretched. Indeed, two Ga-As bonds distances coexist: 2.43±0.01 Å and 2.62±0.04 Å, which correspond to the Ga-As and In-As bond distances in the binary compounds GaAs and InAs, respectively. This result is independent of the In molar fraction in the strained alloy layers.

Optical properties and electronic structure of thin (Ga,In)As-AlAs multiple quantum wells and superlattices under internal and external strain fields.

We report on studies of the optical properties of a series of (Ga,In)As-AlAs thin strained-layer multiple quantum wells and superlattices submitted to high uniaxial stress perpendicular to the growth axis of the structures. Samples having both type-I and type-II configurations for their conduction-band to valence-band potential profiles have been studied. Explanation of the experimental data has been made in the context of an envelope-function approach, which explicitly takes into account the mixing of the upper valence-band states with the split-off valence-band states. The inclusion of spin-dependent deformation potentials is necessary to interpret the values of the zero-stress energies as well as to explain their behavior as a function of the external stress.

Linewidth enhancement factor of 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well DFB lasers

The linewidth enhancement factor alpha in a 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well (SL-MQW) distributed feedback (DFB) laser has been evaluated from the relation between the frequency and intensity modulation indexes, and the spontaneous emission spectra below threshold current. It is demonstrated that the measured alpha -parameter of a 1.3 mu m SL-MQW DFB laser is about two, and is much smaller than that in a conventional bulk DFB laser. From the resonance frequency dependence on the output power, it is concluded that this reduction of the alpha -parameter originates in the increased differential gain. The reduction of wavelength chirping, as a result the low alpha -parameter, was experimentally confirmed for the SL-MQW DFB laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>