Graded-index Separate-confinement Heterostructure Lasers Research Articles

Ge1-xSnx/Si1-ySny SLs lattice-matched to Ge for 1.55 μm lasers

GeSn is a promising material for fabricating light sources on Si integrated circuits. Because the lattice constant of GeSn is larger than that of Ge, GeSn lasers are grown on relaxed GeSn buffers or Ge virtual substrates, with edge dislocations in the interfaces. A structure of Ge1-xSnx/Si1-ySny SLs lattice-matched to Ge is proposed to overcome this drawback. Band structure was calculated with different Sn contents and thicknesses of GeSn and SiSn. Light emission at 1.55 μm was achieved with 0.9 nm GeSn0.11/ 3.9 nm SiSn0.195 SLs. A graded-index separate-confinement heterostructure laser was designed with the proposed SLs of 8 periods and its performance was investigated. The TE modal gain was dominant over TM modal gain, due to the top valence band is heavy hole-like. The transparency and threshold current density was 3.2 kA/cm2 and 4.1 kA/cm2, respectively at room temperature. This study provided a feasible way to fabricate CMOS compatible light sources monolithically integrated on Si platform.

Systematic study on the confinement structure design of 1.5-μm InGaAlAs/InP multiple-quantum-well lasers

An in-dept analysis on the separate confinement heterostructure (SCH) design parameters of 1.5-μm InGaAlAs/InP multiple quantum-well (MQW) lasers is reported. Theoretical calculations show a drastic enhancement on threshold current and slope efficiency from step-index SCH (STEP-SCH) to graded-index SCH (GRIN-SCH) design, but the effect ceases beyond a critical number of grading steps. This finding implies ease of the growth process and reduction in cost. The overall GRIN-SCH’s thickness is found to exert great influence over the achievable laser’s threshold current and slope efficiency. An average of 27 mA threshold current reduction and more than 32% of slope efficiency increment were achieved by optimizing the GRIN-SCH thickness. Increasing the grading energy range of the GRIN-SCH decreases the slope efficiency, but is found to effectively reduce carrier leakage at elevated temperature leading to a less temperature-sensitive threshold current MQW ridge lasers were fabricated and characterized out of two laser materials, one with a reference STEP-SCH and another with a proposed optimized GRIN-SCH profile. The laser with optimised SCH design has shown a 36% reduction in room temperature threshold current as compared to that with the STEP-SCH design, which is in good agreement to the theoretical simulation. In addition, a record high characteristic temperature (T0) of 105 K was obtained on the GRIN-SCH laser structure, which is more than three fold increment as compared to the STEP-SCH design.

High-performance InGaAs-GaAs-AlGaAs broad-area diode lasers with impurity-free intermixed active region

Broad-area InGaAs-GaAs-AlGaAs double-quantum-well graded-index separate-confinement heterostructure lasers with as-grown and intermixed active regions were fabricated and characterized. An impurity-free vacancy diffusion method was used to intermix the quantum wells. Light-current characteristics of both types of lasers were used to extract information about the effects of intermixing process on threshold current density, internal optical loss, internal quantum efficiency, material gain, etc. Comparison between these parameters indicates comparable device performance, even though intermixing involved annealing at 1000°C which resulted in a 42-nm wavelength blueshift.

Room temperature oscillation of self-organized In0.2Ga0.8As/GaAs quantum wire lasers grown on (221)A GaAs substrates by molecular beam epitaxy

Self-organized In0.2Ga0.8As/GaAs quantum wires (QWRs) grown on a (221)A GaAs substrate by molecular beam epitaxy (MBE), which has the narrowest linewidth of photoluminescence peak among self-organized QWRs reported so far, have been applied to an active region of a graded-index separate confinement heterostructure laser structure. Since Si is not a good n-type dopant for (221)A GaAs in the MBE growth, a Sn-doped GaAs/AlAs superlattice layer was used for an n-type cladding layer. The laser operated at room temperature at an emission wavelength of 894 nm under pulsed current conditions with a threshold current of 182 mA (the threshold current density of 2.8 kA/cm2). This result indicates that (221)A InGaAs/GaAs QWRs have sufficient quality for laser devices.

Capture and confinement of light and carriers in graded-index quantum well laser structures

We investigated graded-index separate-confinement heterostructure lasers and compared their performance with a conventional quantum well laser by using Raman and time-resolved photoluminescence spectroscopy. We found that grading the index of the Ga 1− x Al x As/GaAs waveguide and increasing the Al content improves considerably both, the light and carrier trapping and reduces carrier escape from the active region.

Effect of heterobarrier leakage on the performance of high-power 1.5 μm InGaAsP multiple-quantum-well lasers

Broad stripe 1.5 μm InGaAsP/InP multiple-quantum-well graded-index separate-confinement heterostructure lasers with different waveguide widths and doping profiles were designed, fabricated, and characterized. A record value of more than 16 W of pulsed optical power was obtained from lasers with a broadened waveguide design. Studies of the characteristics of lasers with different p-doping profiles as well as modeling data show that the heterobarrier electron leakage is responsible for the effect of the device optical power saturation with current.

Strain-compensated 1.3-μm AlGaInAs quantum-well lasers with multiquantum barriers at the cladding layers

Strain-compensated 1.3-/spl mu/m AlGaInAs graded-index separate confinement heterostructure (GRINSCH) lasers with multiquantum barrier (MQB) at both the nand p-cladding layers are comprehensively studied and compared with the conventional GRINSCH lasers. It is found that the lasers with MQBs exhibit lower threshold current, higher maximum output power and better temperature characteristics because of the enhanced barrier height for carrier leakage. The characteristic temperature is improved as much as 10 K and the vertical far-field angle is also reduced from 38/spl deg/ to 32/spl deg/ as compared to the conventional counterpart.

Characteristics of multistack multiquantum barrier and its application to graded-index separate confinement heterostructure lasers

The enhancement of electron barrier height by multistack multiquantum barrier structure is simulated using the transfer matrix method. The validity and feasibility of this concept is verified by the experimental results on GaAs-AlAs multistack multiquantum barriers. Based on the simulated results, both 0.78 and 1.3 /spl mu/m graded-index separate confinement heterostructure (GRIN-SCH) lasers with predicted enhanced carrier and optical confinements using graded multistack multiquantum barriers are designed. Lower threshold current, higher modulation bandwidth as well as higher characteristic temperature are expected for these lasers.

AlGaAs/GaAs buried quantum well laser diodes by one time selective metalorganic chemical vapor deposition growth on dielectric window stripes

AlGaAs graded-index separate-confinement heterostructure lasers with lateral mode confinement are realized by one time metalorganic chemical vapor deposition with silicon dioxide stripes as selective masks and current blocking layers. The effect of the active region width on lasing characteristics is investigated. The threshold current and external quantum efficiency are around 12 mA and 70%, respectively, for narrow lasers. Elevated waveguide structure formed by the growth-rate difference between (100) and sidewall planes was effective for stable lateral mode characteristics.

Uniform MBE growth of 2 inch diameter wafer for GaAs/AlGaAs and InGaAs/AlGaAs quantum well lasers

Uniform GaAs/AlGaAs and InGaAs/AlGaAs quantum well lasers have been grown by molecular beam epitaxy at high temperatures, where the desorption of Ga or In cannot be neglected. This is achieved by the uniform temperature distribution in our newly developed In-free holder. The graded-index separate-confinement heterostructure (GRIN-SCH) laser with a GaAs quantum well grown at 720°C has shown the threshold current I th of 72±4 mA and the emission wavelength λ of 854.7±0.8 nm. The GRIN-SCH laser with an InGaAs strained single quantum well has been grown at 570°C for an InGaAs well and at 720°C for AlGaAs cladding and GRIN layers without growth interruption. I th is 53±3 mA and λ is 975.5±5.3 nm. The larger variation of λ results from the nonuniform temperature transient just before the growth in InGaAs.

Techniques and applications of graded-composition InGaAlAs alloys

As molecular-beam epitaxy crystal growth techniques steadily progress from the research stage into full-scale production, the need for precise control of the crystal growth process becomes increasingly important. This is particularly true for epitaxial layer structures requiring precisely controlled compositional gradients. We have developed techiques for the growth of linearly graded InGaAlAs, while precisely maintaining lattice matching to the InP substrate, such that x-ray full widths at half-maximum close to the theoretical limit have been achieved. Through the use of these techniques, heterojunction bipolar transistors and graded-index separate confinement heterostructure laser diodes have been grown, fabricated, and tested.

High-power single-mode strained single quantum well InGaAs/AlGaAs lasers grown by molecular beam epitaxy on nonplanar substrates

Strained single quantum well InGaAs/AlGaAs graded-index separate confinement heterostructure lasers have been grown by molecular beam epitaxy over nonplanar substrates. In addition to the low threshold currents provided in situ by lateral current blocking pn junctions obtained by plane-dependent doping of the amphoteric Si dopant, we observe variations in lasing wavelength, efficiency, and internal absorption as a function of the central (100) facet length. These variations are associated with increased indium composition in the strained quantum well which arises from incorporation of adatoms migrating from the low-growth (311)A side facets to the preferential growth (100) active area facets. Uncoated devices (750 μm×4 μm) have been found to have threshold currents as low as 6 mA (Jth=320 A/cm2) and exhibit single-mode behavior to greater than 100 mW at a wavelength of ∼1.0 μm when reflectivity modified (90%/10%).

Mapping of local temperatures on mirrors of GaAs/AlGaAs laser diodes

Spatially resolved Raman scattering measurements (<1 μm) have been performed to determine the surface temperature distribution on coated and uncoated facets of ridge-waveguided GaAs/AlGaAs single quantum well graded-index separate-confinement heterostructure lasers. A strong nonlinear temperature versus output-power dependence is observed for cleaved, uncoated mirrors (ΔT>100 K for P>1 MW/cm2). Raman line scans show hot spot regions at the facets. Degradation strength correlates with facet heating. Disorder-activated Raman phonon modes indicate strong crystal damage. Laser mirrors with λ/2-Al2O3 coatings withstand up to 4–5 times the power density without significant heating and degradation. Local electroluminescence measurements along the cavity confirm increasing temperatures when approaching the facets and show that the resonator bulk material remains cold (ΔT<5 K).

Two-dimensional coherent laser arrays using grating surface emission

The concepts, fabrication, and operating characteristics of monolithic two-dimensional, coherent AlGaAs laser arrays are presented. The arrays consist of 100 (10*10) active elements fabricated from a single-quantum-well graded-index separate-confinement heterostructure laser geometry. A surface relief grating is used for feedback and outcoupling. The elements of the array are index-guided ridge lasers with evanescent or Y coupling in the lateral direction and injection coupling longitudinally. The far field emanating from a 60- mu m*5-mm aperture, measures 0.01 degrees *1 degrees . These arrays emit more than 1 W peak power into a 2-AA wavelength interval. By adjusting the drive current to the electrically independent gain sections of these arrays, the angular position of the far-field beam can be steered. >

High efficiency single quantum well graded-index separate-confinement heterostructure lasers fabricated with MeV oxygen ion implantation

Single quantum well AlGaAs/GaAs graded-index separate-confinement heterostructure lasers have been fabricated using MeV oxygen ion implantation plus optimized subsequent thermal annealing. A high differential quantum efficiency of 85% has been obtained in a 360-μm-long and 10-μm-wide stripe geometry device. The results have also demonstrated that excellent electrical isolation (breakdown voltage of over 30 V) and low threshold currents (22 mA) can be obtained with MeV oxygen ion isolation. It is suggested that oxygen ion implantation induced selective carrier compensation and compositional disordering in the quantum well region as well as radiation-induced lattice disordering in AlxGa1−xAs/GaAs may be mostly responsible for the buried layer modification in this fabrication process.

Experimental determination of the relation between modal gain and current density for AlGaAs single quantum well lasers grown by metalorganic vapor phase epitaxy

The relation between modal gain and current density has been determined for a 20 Å single quantum well in a graded-index separate-confinement heterostructure laser. The measurements were performed at 153 K in order to avoid leakage of carriers out of the wells which might cause an apparent saturation of the gain. A sublinear dependence of gain on current density has been observed, agreeing rather well with theoretical calculations taking band mixing into account.

High-power (1.4 W) AlGaInP graded-index separate confinement heterostructure visible (λ∼658 nm) laser

Pulsed operation of an AlGaInP graded-index separate confinement heterostructure laser grown by organometallic vapor phase epitaxy is reported. The laser active region consists of a single 100 Å Ga0.5In0.5P quantum well and 1600 Å graded index regions on both sides of the well. The graded index regions were produced by lattice-matched graded composition (AlyGa1−y)0.5In0.5P quaternary alloys. This structure reduces the broad-area threshold current compared to a double heterostructure laser, with pulsed thresholds as low as 1050 A/cm2. Total pulsed power of 1.4 W at 658 nm is available from an 80 μm×300 μm mesa-stripe laser. A differential quantum efficiency of ∼56% is measured. By examining the cavity length dependence of the threshold current density and quantum efficiency, it is apparent that the quantum well gain has not saturated in these structures. This suggests that devices containing a thinner single quantum well active region may result in a further reduction in threshold current density for visible lasers.

GaAs-GaAlAs graded-index separate confinement heterostructure laser diodes selectively grown by molecular beam epitaxy on SiO2-masked substrates

The GaAs-GaAlAs graded-index separate confinement heterostructure was grown selectively by molecular beam epitaxy on a SiO2-masked GaAs (100) substrate. The stripe windows on the SiO2 mask were 10 μm in width and were oriented along [011̄] direction. The laser diodes thus fabricated lased in a single longitudinal mode with a side mode suppression ratio of 95:1. Both the longitudinal mode and the single-lobe far-field pattern were stable up to 4Ith.

Very low threshold current densities (under 100 A/cm2) in AlGaAs/GaAs single-quantum-well GRINSCH lasers grown by molecular beam epitaxy

Threshold current densities as low as 80 A/cm2 for 3.3 mm-long cavity lasers, and 93 A/cm2 for 520 μm-long cavity lasers have been obtained in AlGaAs/GaAs graded-index separate-confinement heterostructure (GRINSCH) single-quantum-well lasers with quantum-well widths between 65–165 A grown by molecular beam epitaxy. The structures were prepared on (100) GaAs substrates and do not display the earlier reported dependence of lasing threshold characteristics on the quantum-well thickness in the range studied (65–165 A).

Calculation of carrier capture time of a quantum well in graded-index separate-confinement heterostructures

The electron capture time into the quantum well of a graded-index separate-confinement heterostructure laser has been calculated at 77 and 300 K. The results (at 77 K) show oscillations between several picoseconds and several tens of picoseconds with increasing quantum-well thickness, qualitatively similar to the behavior in simple separate-confinement structures. Quantitatively, the difference in capture times for optimal structures of each kind does not explain the lower threshold of the graded-index structure.