Strain-compensated Multiple Quantum Research Articles

Design and fabrication of photonic crystal resonators for single-mode and vertical surface emission from strain-compensated quantum cascade lasers operating at 4.32 μm

Photonic crystal resonators with the C4v symmetry were designed and fabricated on quantum cascade lasers with a strain-compensated multiple quantum well to achieve single-mode and vertical surface emission at 4.32 μm. Their fabrication accuracy was confirmed by high-resolution reflection spectroscopy. The maximum output power was 10 mW at 77 K. A far-field pattern with a small divergence angle below 1 degree was observed. Its main peak had a donut shape, which was attributed to the spatial symmetry of the resonance mode of the photonic crystal.

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.

Morphology evolution in strain-compensated multiple quantum well structures

Morphology evolution in (In,Ga)As-Ga(As,P) strain-compensated multilayer structures is studied. The effects of nanoscale interface corrugation and phase separation are evident after the third period of the multilayer structure and become more pronounced with each new stack until the sixth period. Then, the interface stabilizes pointing to the formation of strain-balanced equilibrium interface structure. The epitaxial structure remains defect-free up to the maximum number (twenty) of periods studied. In a structure with a high lattice mismatch between the neighboring layers, In0.40Ga0.60As/GaAs0.85P0.15, clusters of dislocations are revealed already in the third period. The observed phenomena are critical for proper engineering of optoelectronic devices.

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.

InAsP/InGaAsP quantum-well 1.3 µm 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.

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.

Effect of InGaAsP intermediate layer in 1.3 μm AlGaInAs strain-compensated multiple quantum well laser diodes

The effect is investigated of introducing a linearly graded-index separate-confinement-heterostructure InGaAsP intermediate layer between the InGaAs cap layer and the InP cladding layer to improve the characteristics of 1.3 μm AlGaInAs strain-compensated multiple-quantum-well laser diodes (LDs). With the InGaAsP intermediate layer, 3 μm-ridge-strip LDs without facet coating under CW operation exhibit a threshold current of 16 mA, a resistance of 5.8 Ω, a characteristic temperature of 75 K in the range 20–70°C and 41 K at 70–100°C, a differential quantum efficiency of 50% and a low slope efficiency drop of −1.3 dB between 20 and 70°C. These characteristics are better than those of LDs without the InGaAsP intermediate layer. A wavelength swing of 0.48 nm/°C for LDs operated at 50 mA is achieved.

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.

Effects of distance between wells on band structure and characteristics of InGaAs/InGaAsP strain-compensated multiple quantum well lasers

In terms of the multi-well energy representation technique, the effects of the distance between wells on the valence band structure and characteristics are analyzed for InGaAs/InGaAsP strain-compensated multiple quantum well lasers with zero net strain. The computed result shows that a coupling effect exists between the wells, causes an energy split, and affects the properties of the laser, such as the density of states, optical gain, differential gain, threshold wavelength, threshold carrier density and threshold current density. We find that when the distance between wells equals twice the thickness of the well, the effect of the distance between wells on the characteristics of the laser becomes weak. Therefore, for the practical design of lasers, it is reasonable to take the thickness of the barrier to be twice that of the well.

Structural and optical properties of 0.98 μm InGaAs/InGaAsP strained-compensated multiple quantum well structures grown by gas-source molecular beam epitaxy

The effects of growth temperature on the structural and optical properties of gas-source molecular beam epitaxy grown 0.98 μm InGaAs/InGaAsP strain-compensated multiple quantum well structures were studied by transmission electron microscopy (TEM), double crystal X-ray diffraction and photoluminescence measurements. It was found that high quality of quantum well structures can be obtained at a lower growth temperature. A higher growth temperature caused an immiscible growth for the InGaAsP alloy from the observation of the TEM images. As a result, the optical and structural quality of the quantum well structure was drastically degraded.

High-temperature, low threshold current, and uniform operation 1×12 monolithic AlGaInAs/InP strain-compensated multiple quantum well laser array in 1.5 μm

In this paper, we describe the fabrication of a monolithically integrated 1/spl times/12 array of 1.5-/spl mu/m AlGaInAs/InP strain-compensated multiple-quantum-well (MQW) lasers, which has high reliability and highly uniform characteristics in low threshold current, slope efficiency, and lasing wavelength. Besides, each diode on the array exhibits a high characteristic temperature of 88 K and a low slope-efficiency drop of less than 1 dB between 20-80/spl deg/C and a lasing wavelength of 1510 nm at 20/spl deg/C and 20 mA. Also, the diode on the array has a maximum resonance frequency of above 8 GHz or 3-dB modulation bandwidth of 12 GHz.

Buried heterostructure InGaAsP/InP strain-compensated multiple quantum well laser with a native-oxidized InAlAs current blocking layer

An InAlAs native oxide is used to replace the p-n reverse-biased junction in a conventional buried heterostructure InP-based laser. This technique reduces the number of regrowth steps and eliminates leakage current under high-temperature operation. The InAlAs native oxide buried heterostructure (NOBH) laser with strain-compensated InGaAsP/InP multiple quantum well active layers has a threshold current of 5.6 mA, a slope efficiency of 0.23 mW/mA, and a linear power up to 22.5 mW with a HR-coated facet. It exhibits single transverse mode with lasing wavelength at 1.532 μm. A characteristic temperature (T0) of 50 K is obtained from the NOBH laser with a nonoptimized oxide layer width.

Low threshold current and high temperature operationof 1.55 µm strain-compensated multiple quantum well AlInAs/AlGaInAs laser diodes

Low threshold current and high temperature operation is achieved in a 1.55 µm graded-index separate-confinement heterostructure and strained multi-quantum well AlInAs/AlGaInAs laser diode which was fabricated from epitaxial wafers grown by low-pressure organometallic vapour phase epitaxy. Remarkable improvements in threshold current, operating temperature and characteristic temperature have been demonstrated. The threshold current is 7 mA at 20°C, the emission wavelength is 1.5542 µm and the characteristic temperature between 20 and 70°C is 84 K.

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.