Strain-compensated Multiple Quantum Wells Research Articles

Passively mode-locked Yb-doped all-fiber oscillator using self-made strain-compensated semiconductor mirrors as saturable absorbers

We report a linear-cavity mode-locked Yb-doped fiber laser using self-made semiconductor saturable absorber mirrors (SESAMs). The SESAMs are grown by means of metalorganic chemical vapor deposition, with a design of high modulation depth using strain-compensated multiple quantum wells. We choose two kinds of self-made SESAMs with different periods of top reflector in the linear cavity, and they can both be applied to stable mode-locking operation without damage. The output characteristics of the mode-locked oscillator indicate that SESAMs with high modulation depth are beneficial for mode-locked fiber lasers and the modulation depth can limit the pulse width of the output pulses.

Interfacial relaxation analysis of InGaAs/GaAsP strain-compensated multiple quantum wells and its optical property

InGaAs/GaAsP strain-compensated multiple quantum wells were prepared by metal organic chemical vapor deposition on GaAs (100) substrates with misorientation of 15° toward [111]. In order to obtain better strain-compensated abrupt heterojunction interfaces, the compressive strain and relaxation of different quantum well and the total accumulated strain were investigated by adjusting In composition and the thickness of InxGa1-xAs well and GaAs1-yPy barrier keep constant. High resolution X-ray diffraction results indicate the crystal and interfacial structures of In0.18Ga0.82As (7 nm)/GaAs1-yPy with the least relaxation and total strain mismatch are better than others. From in-situ surface reflectivity curves, we observed the slope of reflectivity curve of multiple quantum wells increases with increasing lattice relaxation. Atomic force microscopic results show surface morphologies of three samples are Volmer-Weber mode. Indium segregation at heterointerface between well and barrier were investigated by secondary ion mass spectrometry which indicate indium diffusion width increase with the increasing total strain mismatch. Finally, a shoulder peak was observed from Gaussian fitting of photoluminescence, stemming from the lattice relaxation. These results demonstrate that the relaxation process is introduced and indium segregation length widens as the relaxation increases. The experimental results will be favorable for optimizing the epitaxial growth of InGaAs/GaAsP strain-compensated quantum wells in order to obtain high quality smooth heterointerface.

Progress of Aluminum Gallium Indium Phosphide Red Laser Diodes and Beyond

High-quality aluminum gallium indium phosphide epitaxial layers for red laser diodes have been grown by the metal organic chemical vapor deposition method. The layers have some issues, such as narrowing of the band gap, low p-carrier concentration, difficulty in epitaxial growth for quantum well structures, and generating of high-density hillocks. The issues have been successfully solved by introducing (100) substrates with misorientaion toward the [011] direction. High performance transverse-mode stabilized lasers are achieved by introducing the substrates, novel strain-compensated multiple-quantum well structures, which can add large strain to the wells, and low-loss optical cavity. This article also describes their applications.

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3s at 610°C.

Suppressed lattice relaxation during InGaAs/GaAsP MQW growth with InGaAs and GaAs ultra-thin interlayers

InGaAs/GaAsP strain-compensated multiple quantum wells (MQW) with an effective bandgap of 1.2eV are promising candidate materials for solar cells applications. Since high indium content in wells is needed to achieve such a low bandgap, lattice relaxation can easily occur during crystal growth. Here, we propose graded interlayers, consisting of ultra-thin GaAs and InGaAs with lower indium content, inserted between wells and barriers to prevent relaxation during growth and to achieve better crystal quality. Lattice relaxation was detected by in-situ monitoring of surface reflectance including an anisotropic component. Our method allowed the number of MQW periods before detectable lattice relaxation to reach approximately three times that without interlayers. Over 100 periods of In0.26Ga0.74As (7.0nm)/GaAs0.60P0.40 (9.2nm) MQW, with an absorption edge at 1.2eV, were successfully piled up as a result. Lattice relaxation without interlayers was caused dominantly by dislocations due to the large lattice mismatch between a well and a barrier, and graded interlayers made the hetero-interfaces smoother so that the subsequent layers could be more easily grown. Transmission electron microscopy confirmed the existence of the interlayer at both ends of In0.26Ga0.74As layer, with graded indium content and the thickness of 1.2–1.8nm.

Highly Reliable High-Speed 1.1-$\mu$m-Range VCSELs With InGaAs/GaAsP-MQWs

In this paper, we describe high-speed 1.1-μm-range oxide-confined vertical-cavity surface-emitting lasers (VCSELs) for large-scale optical interconnection applications. For achieving high data rates up to 25 Gbit/s under high temperature, we applied InGaAs/GaAsP strain-compensated multiple quantum wells (SC-MQWs) as the active layer. The developed device showed 25 Gbit/s error-free operation at 100°C. We also examined reliability of the VCSELs through accelerated life tests. The result showed an extremely long lifetime of about 10 thousand hours in MTTF under an ambient temperature of 150°C and a current density of about 19 kA/cm2. The level of reliability either equaled or surpassed that of conventional 0.85-μm VCSELs. Moreover, we revealed a typical failure mode of the device, which was caused by ≪110≫ dark line defects (DLDs) generated in the n-DBR layers under the current aperture area.

Growth and characterization of strain-compensated InGaAs/GaAsSb type II multiple quantum wells on InP substrate

Strain-compensated InGaAs/GaAsSb type II multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) and their optical and electrical properties were studied. High-quality strain-compensated type II MQWs were successfully grown, which have longer emission wavelength than that of lattice-matched type II MQWs. PL peak energy at 300 K of the strain-compensated type II MQWs, where the InGaAs layer has 0.6% tensile strain and GaAsSb layer has 0.6% compressive strain, shows a red-shift of 43 meV, which is 12 meV larger than the calculated energy shift of 31 meV. In addition, the PL intensity and the electron mobility of the strain-compensated MQWs are comparable to those of the lattice-matched MQWs, suggesting that the crystal quality of the strain-compensated MQWs is good and are very promising for low dark current photodiodes in the 2 μm wavelength region.

InAsP/InGaAsP quantum-well 1.3 [micro sign]m vertical-cavity surface-emitting lasers

1.3 mum vertical-cavity surface-emitting lasers based on a novel gain media consisting of InAsP/InGaAsP strain-compensated multiple quantum wells are reported. SiO 2 /TiO 2 dielectric thin-film pairs and wafer-bonded GaAs/Al(Ga)As distributed Bragg reflectors are used as the top and bottom cavity mirrors, respectively. The device with a 5 mum-diameter selectively etched tunnel-junction aperture exhibits submilliampere threshold current as low as 0.54 mA and single-transverse mode emission. Maximum output optical power of 1.9 mW was observed in multimode lasing devices.

25 Gbit/s 100°C operation of highly reliable InGaAs/GaAsP-VCSELs

1.1 µm-range high-speed oxide-VCSELs with InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) have been developed. 25 Gbit/s 100°C operation and high reliability over 3 000 h under 150°C were demonstrated.

Optimization of Barrier Structure for Strain-Compensated Multiple-Quantum-Well AlGaInP Laser Diodes

A theoretical analysis of a strain-compensated multiple-quantum-well (MQW) AlGaInP laser diode (LD) has been conducted to minimize the operation current and enhance the operation temperature. The effect of the barrier height on the tensile-strain quantum barriers was studied under the same optical confinement and emission wavelength. The simulation results suggest that a more uniform and higher carrier distribution inside the MQW region can be obtained for a lower barrier height owing to the improvement in carrier injection. The performance of the AlGaInP LD is thus improved. However, when the barrier height is too small, higher spontaneous rates in the quantum barrier region deteriorate the LD performance instead. Theoretical analysis shows that an Al composition of 0.1 for the 0.5%-tensile-strain AlxGayIn1-x-yP barrier is the optimal value for strain-compensated MQW AlGaInP LDs.

Effects of n-type modulation-doping barriers and a linear graded-composition GaInAsP intermediate layer on the 1.3μm AlGaInAs∕AlGaInAs strain-compensated multiple-quantum-well laser diodes

We report the fabrication, characterization, and comparison of four 1.3μm AlGaInAs∕AlGaInAs strain-compensated multiple-quantum-well (SC-MQW) laser structures: (1) sample A—with only an undoped SC-MQW active region, (2) sample B—with an undoped SC-MQW active region and a linear graded-composition (LGC) GaInAsP intermediate layer, (3) sample C—with an n-type modulation-doping (MD) SC-MQW active region, and (4) sample D—with an n-type MD-SC-MQW active region combined with a LGC GaInAsP intermediate layer. The inclusion of either n-type modulation-doped SC-MQW active region or LGC GaInAsP intermediate layer can improve the performance of a laser diode (LD). The LD sample D, which includes both an n-type MD-SC-MQW active region and a LGC GaInAsP intermediate layer, exhibits the best overall performance including a threshold current of 12.5mA, a characteristic temperature of 85K in 20–80°C temperature range, a lasing wavelength shift of 0.38nm∕K, and a relaxation frequency response of 9.9GHz.

Strain-compensated InGaAsSb multiple quantum-wells with digital AlGaAsSb barriers for midinfrared lasers

Midinfrared InGaAsSb-AlGaAsSb strain-compensated multiple quantum-wells (SCMQW) have been grown by solid-source molecular beam epitaxy. Short-period (AlGaAsSb)/sub y/--(AlGaSb)/sub 1-y/ digital barriers were employed to avoid growth interruptions at the barrier-well interfaces, thereby significantly improving the structural and optical properties of the InGaAsSb SCMQW as evidenced by X-ray diffraction and photoluminescence measurements. Based on these high-quality SCMQW, a room-temperature threshold current density as low as 163 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved for 1000-μm-long broad-area lasers emitting at 2.38 μm in pulsed mode. The 880-μm-long lasers retained a high characteristic temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) of 165 K up to 80/spl deg/C and could operate at temperatures above 100/spl deg/C. A typical wavelength blueshift of 38 meV was observed in the SCMQW laser samples compared to the SCMQW-only samples.

GSMBE growth and characterizations of AlInP/InGaAsP strain-compensated multiple-layer heterostructures

We have grown long-period AlInP/InGaAsP strain-compensated multiple-layer heterostructures (SCMLHs) and SCMLHs combined with InAsP/InGaAsP strain-compensated multiple quantum wells (SC-MQWs) by gas source molecular beam epitaxy. Etch pit density (EPD) for both structures are in magnitude of ∼10 5 cm −2. The increment of EPD with increase of period number is small, indicating low sensitivity of the dislocation density to the increase of period number of SCMLHs. High resolution X-ray diffraction and photoluminescence (PL) characterizations of the two structures demonstrate that crystal quality remains high due to strain compensation. Diffusion and segregation of indium were clearly observed in both InGaAsP and AlInP layers by secondary ion mass spectroscopy. As the thickness of epilayers increases In content increases, while Al decreases. PL for the structure of 20-pair AlInP/InGaAsP SCMLHs+InAsP/InGaAsP SC-MQWs shows strong luminescence and narrow line width. The strain-compensated technique can effectively suppress the formation of misfit dislocations in AlInP/InGaAsP SCMLHs although the thicknesses of epilayers are above the critical thickness of consisting materials, which may render its potential application in optoelectronic devices.

Luminescent properties of annealed and directly wafer-bonded InAsP/InGaAsP multiple quantum wells

InAsP/InGaAsP strain-compensated multiple quantum wells (SC-MQWs) were grown using gas source molecular beam epitaxy. The luminescent properties of the SC-MQWs after thermal annealing and direct wafer-bonding onto a GaAs substrate were studied. It is shown that photoluminescence (PL) intensities of the samples are improved by a factor of 4.1 upon annealing at 620 °C. The luminescence intensities of the samples bonded at 580 and 650 °C under ∼5 MPa pressure and 35 min annealing process are comparable with that of the as-grown ones. A proposed mechanism of defects annihilation relating to the evolution of nanoscale As-rich and P-rich clusters is then used to account for the dependence of PL intensities and the full width at half-maximum of InAsP/InGaAsP SC-MQWs on annealing temperatures.

Fabrication and Characteristics of High-Speed Oxide-Confined VCSELs Using InGaAsP–InGaP Strain-Compensated MQWs

This paper presents the fabrication and characteristics of high-performance 850-nm InGaAsP-InGaP strain-compensated multiple-quantum-well (MQW) vertical-cavity surface-emitting lasers (VCSELs). The InGaAsP-InGaP MQW's composition was optimized through theoretical calculations, and the growth condition was optimized using photoluminescence. These VCSELs exhibit superior performance with characteristics threshold currents /spl sim/0.4 mA and slope efficiencies /spl sim/0.6 mW/mA. The threshold current change with temperature is less than 0.2 mA, and the slope efficiency drops less than /spl sim/30% when the substrate temperature is raised from room temperature to 85/spl deg/C. A high modulation bandwidth of 14.5 GHz and a modulation current efficiency factor of 11.6 GHz/(mA)/sup 1/2/ are demonstrated. The authors have accumulated life test data up to 1000 h at 70/spl deg/C/8 mA.

Difference of luminescent properties between strained InAsP/InP and strain-compensated InAsP/InGaAsP MQWs

Strained InAs 0.43P 0.57/InP and strain-compensated InAs 0.43P 0.57/In 0.7Ga 0.3As 0.43P 0.57 multiple quantum well (MQW) structures were grown by gas source molecular beam epitaxy. The observations of up to 5 satellite-diffraction peaks from the high-resolution X-ray diffraction measurements demonstrate good crystalline quality for both structures. The temperature dependence of the 1e–1hh transition energies, the line width of photoluminescence (PL) spectra and emission efficiency η of the two quantum well structures are compared by low-temperature PL measurements. The temperature dependence of the 1e–1hh transitions of the two quantum well structures is similar to that of InAs 0.43P 0.57 bulk material. The thermal activation energies obtained for strain-compensated MQW are larger than those obtained for the strained one. Consequently, the PL emission efficiency decays much slower for the strain-compensated MQW than that for the strained one when temperature increases, indicating the superior temperature stability of luminescent efficiency for the strain-compensated MQW. The obtained results can be used as references to the design and fabrication of optoelectronic devices.

Gas discovery in Brunei Darussalam

InGaAs/GaAsP strain-compensated multiple quantum wells were prepared by metal organic chemical vapor deposition on GaAs (100) substrates with misorientation of 15° toward [111]. In order to obtain better strain-compensated abrupt heterojunction interfaces, the compressive strain and relaxation of different quantum well and the total accumulated strain were investigated by adjusting In composition and the thickness of InxGa1-xAs well and GaAs1-yPy barrier keep constant. High resolution X-ray diffraction results indicate the crystal and interfacial structures of In0.18Ga0.82As (7 nm)/GaAs1-yPy with the least relaxation and total strain mismatch are better than others. From in-situ surface reflectivity curves, we observed the slope of reflectivity curve of multiple quantum wells increases with increasing lattice relaxation. Atomic force microscopic results show surface morphologies of three samples are Volmer-Weber mode. Indium segregation at heterointerface between well and barrier were investigated by secondary ion mass spectrometry which indicate indium diffusion width increase with the increasing total strain mismatch. Finally, a shoulder peak was observed from Gaussian fitting of photoluminescence, stemming from the lattice relaxation. These results demonstrate that the relaxation process is introduced and indium segregation length widens as the relaxation increases. The experimental results will be favorable for optimizing the epitaxial growth of InGaAs/GaAsP strain-compensated quantum wells in order to obtain high quality smooth heterointerface.

Strain effect and characteristics of GaInP/AlGaInP strain-compensated multiple quantum wells

Ga 0.6In 0.4P/(Al 0.8Ga 0.2) 0.4In 0.6P strain-compensated multiple quantum wells (SCMQW) and Ga 0.5In 0.5P/(Al 0.4Ga 0.6) 0.5In 0.5P unstrained MQW were grown by MOVPE to investigate the shift of photoluminescence (PL) peak wavelength and the carrier lifetime caused by strain effect in SCMQW. The PL peaks of SCMQW and unstrained MQW were 608 and 628 nm, respectively. From the rocking curve of SCMQW, it indicated that the active layers of SCMQW were not zero net strain. The residual strain caused the emission intensity of SCMQW to be about four times weaker than that of unstrained MQW. The carrier lifetimes of SCMQW and unstrained MQW were measured to be 3 and 6.02 ns, respectively. It was conjectured that the residual strain in SCMQW induced a decrease in the carrier lifetime by a factor of two with respect to that of unstrained MQW.

Hole distribution in InGaAsP 1.3-μm multiple-quantum-well laser structures with different hole confinement energies

We have investigated the hole distribution in strained InGaAsP multiple-quantum-well (MQW) structures by direct hole transport measurements with time-resolved photoluminescence spectroscopy. The results show that the hole transport time over the MQW primarily depends on the hole confinement energy in the wells and increases sharply with the well depth. A simple thermionic emission model indicates that the heavy holes escape predominantly over the light-hole barrier edge for strain-compensated MQW structures. The results are corroborated with observed laser performance data.

High-Temperature and Low-Threshold-Current Operation of 1.5 µm AlGaInAs/InP Strain-Compensated Multiple Quantum Well Laser Diodes

High-temperature and low-threshold-current of 1.5 µm strain-compensated multiple-quantum-well (SC-MQW) graded-index separate-confinement-heterostructure (GRIN-SCH) AlGaInAs/InP laser diodes have been successfully fabricated. An epitaxial wafer was grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) and the 2-µm-wide ridge-waveguide (RWG) structure with self-aligned process was adopted. Significant improvements in threshold current high-temperature operation and characteristic temperature have been demonstrated. Under CW operation, these laser diodes exhibited a threshold current of 5.4 mA and 17 mA at 20°C and 100°C, respectively. Output power exceeding 5 mW was obtained at a temperature as high as 120°C. Also they have a characteristic temperature T0 of 90 K between 20°C and 70°C which is as good as the best values of the conventional InGaAsP/InP lasers.