Separate Confinement Heterostructure Lasers Research Articles

Graded Cladding Layers in Infrared III-Arsenide Separate Confinement Heterostructure Laser Diodes

Graded aluminum composition in AlGaAs n-cladding and p-cladding layers have been shown to exhibit improved optoelectronic performance in the single quantum well separate confinement heterostructure laser diode (SQW SCH LD) in the infrared region. The output power of the laser diode is increased up to 45 mW by using graded aluminum composition in n-/p- cladding layers. In addition, the lasing threshold current density is also lowered which results in a significant optical gain. The stimulated emission rate also improves due to improved electron-hole recombination and, thus, resulting in improved lasing.

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.

Performance and reliability improvement of 905 nm high power laser diode by design, fabrication and characterization of high damage threshold mirrors

The performance improvement of 905 nm InGaAs/AlGaAs/GaAs Single Quantum Well Separate-Confinement Heterostructure Laser Diode (SQW-SCH-LD) in consequence of three-step procedure for facet passivation and protection was demonstrated. Initially, processed InGaAs/AlGaAs/GaAs SQW-SCH LD wafer was scribed and cleaved in ambient atmosphere. Secondly, the air-exposed bar facets were cleaned and impurities were removed with Ar + irradiation and bombardment. Finally, after simulation, mirrors consist of single-layer Al2O3 as Low-Reflection (LR) and multilayer |(Al2O3/Si)4| as High-Reflection (HR) coatings were deposited on front and back facets, respectively. Mirror's surface and interface properties were investigated by UV-Vis-NIR Spectroscopy, Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FESEM), Burn-in test, Life-time test and Thermography. Spectrophotometer results of coated mirrors were in appropriate adaptation with the simulation results. High magnification FESEM images reveal the precise repeatability stacked Al2O3/Si 8-layers with amorphous structure on GaAs substrate. Thermographic image of front facet after life-time reveals that front mirror retains initial quality. In the case of coated front mirror and in Continues-Wave laser mode, laser induce damage threshold (LIDT) improved and increased up to >> 45.75 kW/cm2. Improvement in mirror coating process results in enhancement laser reliability and operational parameters such as increasing output power and power efficiency, decreasing degradation rate without any catastrophic optical mirror damage.

Plasma-assisted molecular beam epitaxy of Al(Ga)N layers and quantum well structures for optically pumped mid-UV lasers on c-Al2O3

This paper reports on novel approaches developed for plasma-assisted molecular beam epitaxy of Al-rich AlGaN epilayers and quantum well heterostructures on c-sapphire, which allowed us to fabricate low-threshold optically-pumped separate confinement heterostructure lasers emitting in the mid-UV spectral range (258–290 nm) with the threshold power density below 600 kW cm−2. The optimum buffer structure has been developed which provides lowering the near-surface threading dislocation density down to 1.5 × 108 and 3 × 109 cm−2 for screw and edge types, respectively, and improving the surface morphology (rms < 0.7 nm at the area of 3 × 3 μm−2). It comprises the high-temperature (780 °C) migration enhanced epitaxy growth of a (30–70) nm thick AlN nucleation layer on c-Al2O3, followed by a 2 μm thick AlN buffer grown under the metal-rich conditions in the Al-flux modulation mode and containing several (up to 6) ultra-thin (∼3 nm) GaN interlayers grown at N-rich conditions. Proper strain engineering in AlGaN single quantum well heterostructure grown atop of the AlN buffer layer enables one to preserve dominant TE polarization of both spontaneous and stimulated emission even at shortest obtained wavelength (258 nm). The threshold power density of stimulated emission as low as 150 kW cm−2 at 289 nm for a single quantum well laser structure has been demonstrated.

The role of quantum-well states and carrier scattering times in discontinuities of opto-electrical characteristics of SCH lasers

Drift-diffusion computer simulations model available in Synopsys’ Sentaurus TCAD is used to study electrical, I-V, and optical, I-L, characteristics of separate-confinement heterostructure laser based on AlGaAs. We investigate the role of the width and depth of Quantum Well (QW) active region, below and above the lasing threshold. The device properties depend on both, the number of bound QW states and on closeness of the highest bound states to conduction or valence band offset. The lasing action may not exist at certain widths or hights of QW, and the threshold current is a discontinuous function of these parameters. The effects are more pronounced at low temperatures. Discontinuities in characteristics are found, at certain conditions, in temperature dependencies as well. The carriers scattering time on QW is shown to have the crucial role on amplitude of discontinuities.

First principles investigation of optical properties of zinc-blende Al xGa 1− xAs 1− yN y materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al x Ga 1− x As 1− y N y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al x Ga 1− x As and Al x Ga 1− x As 1− y N y with x≤0.375 and y up to 4% are presented. Al x Ga 1− x As on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al x Ga 1− x As has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C MN=2.32 and a N level of E N=(1.625+0.069 x) eV. The results show that Al x Ga 1− x As can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al x Ga 1− x As 1− y N y could be an alternative to Al x Ga 1− x As when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.

Temperature sensitivity (T) of tensile-strained GaAsP/(AlGa)As double-barrier separate confinement heterostructure laser diodes for 800 nm band

The temperature sensitivity of the double-barrier separate confinement heterostructure (DBSCH) laser diodes (LDs), intended for high-power, low vertical beam divergence emission, was shown to be somewhat lower than that of the large optical cavity (LOC) devices of similar beam divergences. For the lowest beam divergences, it still remains considerably high, resulting in low characteristic temperatures T0 not exceeding 80 K in the LDs of the vertical beam divergences below 15°. In this work, the decrease in the T0 values of such tensile-strained GaAsP/(AlGa)As DBSCH LDs has been found primarily due to an increased, thermally activated occupation of the heavy-hole subband and, to a minor degree, occupation of the second quantum level, both giving rise to transitions that do not contribute to the optical gain. This is caused by the higher and more temperature-dependent at-threshold band filling for devices of reduced confinement factor (Γ) aimed at achieving low beam divergence. This phenomenon seems to be common for all the tensile-strained LDs and acts in addition to other mechanisms that decrease the T0, such as the carrier escape from a quantum well to surrounding layers. However, in DBSCH LDs no carrier escape has been detected, presumably because of their thin waveguide layers. This can explain the somewhat higher T0 values of these devices when compared to their LOC counterparts.

Photo-selective chemical etching of InAs and GaSb to manufacture microscopic mirrors

1 IntroductionPhotochemical processing of materials, particularly insemiconductors, has been a relatively well-known techniqueused for some time with relatively few applications [1].However, devices demanding new processing techniques,suchasMEMS[2],Leds[3]andlaserswithintegratedlensesor gratings, etc. have renewed interest in this technique.The technique can be described as follows [4]: a semi-conductor is immersed in an electrolyte, forming a spacecharge region at the solution–semiconductor interface; at thesame time the semiconductor is illuminated with light (nor-mally a laser) of energy greater than the material’s band gap,creating electron-hole pairs. The electric field due to bandbending at the semiconductor surface attracts one type ofcarrier to the surface and repels the other type of carriertowardsthebulk.Normally,inn-typematerial,theholesgotothesurfaceandtheelectronstothebulk.Inthiscasetheexcessholes increase the oxidation state of the surface atoms,increasingtheetchingrate.Ifthereactionrateislimitedbythesupplyofphoto-generatedholesatthesurface,theetchingratecan be adjusted simply by changing the light intensity.Since the technique uses a beam of monochromaticphotons (laser) to locally control reactive interactions nearthe solid surface, it is particularly useful to performlocalized material deposition (writing), solid doping,alloying and/or, as in our case, material removing (etching)without masking assistance [5]. Due to these unique char-acteristics Laser-Assisted Chemical Etching (LACE) offersadvantageous distinctiveness that makes it preferred toother materials processing techniques [6].Of particular interest in this work is the photochemicaletching of semiconductors, also called LACE. This techniquehas been already used successfully to produce channels withapproximately parabolic cross section along GaAs substrates,creating a refraction index negative gradient inside a Graded-Index Separate Confinement Heterostructure (GRIN-SCH)laser structure. This effect was used to deviate the higher orderlateral modes out of the laser str ucture keeping only the centralone[7].Inthatworkthefeasibilityofanewtypeofhigh-powercoherent semiconductor laser with lateral emission was dem-onstrated. As far as we know this was the first time LACE wasused successfully to build structures in laser diodes thatsimultaneously improved their power output and coherence.In this work we show the results of applying LACE ton-type GaSb substrates to create cavities with nearly par-abolic cross sectioned profiles that could be used as micromirrors or micro lenses to concentrate or deviate lightbeams emitted from a surface emitting laser diode. Aparabolic profile of the mirror is best suited to obtain singlemode operation in an unstable resonator [8, 9].GaSb is an interesting material for the production oflasers and photodetectors in the near and mid infraredregion. However, the use of GaSb requires the developmentof some processing techniques such as etching andpolishing. Chemical processing of GaSb is particularly

High-Performance $\hbox{In}_{0.5}\hbox{Ga}_{0.5} \hbox{As/GaAs}$ Quantum-Dot Lasers on Silicon With Multiple-Layer Quantum-Dot Dislocation Filters

Compound-semiconductor-based lasers grown directly on silicon substrates would constitute an important technology for the realization of on-chip optical interconnects. The characteristics of GaAs-or InP-based devices on silicon can be degraded by the large density of propagating dislocations resulting from the large lattice mismatch (> 4%). The use of multiple layers of self-organized In(Ga, Al)As/GaAs quantum dots (QDs) as a 3D dislocation filter to impede the propagation of dislocations and to reduce dislocation density in GaAs/Si lattice-mismatched heterostructures has been investigated. The effectiveness of this technique, depending on QD composition, size, areal density, and number of dot layers, is analyzed by a quasi-3D model of strain-dislocation interaction. It is found that ten layers of InAs QDs of size ~20-30 nm constitute the most effective dislocation filter. This is experimentally verified by cross-sectional transmission electron microscopy, photoluminescence, and performance characteristics of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> As/GaAs QD separate confinement heterostructure lasers on Si. The lasers exhibit J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ~900 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 273 K, the large characteristic temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> =278 K) is in the temperature range of 5degC-85degC, and the output slope efficiency (~0.4 W/A) is independent of temperature in the range of 5degC-50degC.

Improvement of quantum efficiency of MBE grown AlGaAs/InGaAs/GaAs edge emitting lasers by optimisation of construction and technology

In this paper, we compare theoretically, as well as experimentally, four constructions of quantum well (QW) AlGaAs/InGaAs/GaAs semiconductor lasers: one standard, simple separate confinement heterostructure (SCH) laser and three different GRIN SCH (graded index separate confinement heterostructure) lasers. By optimising the construction and technology of epitaxy, an increase of laser quantum efficiency was obtained for the wavelength 980 nm. The highest measured quantum efficiencies approach those predicted theoretically. The recommended growth conditions ensuring obtaining the best laser parameters are given.

Analysis of the Linewidth-Enhancement Factor of Long-Wavelength Tunnel-Injection Quantum-Dot Lasers

We have studied the linewidth-enhancement factor of 1.3-mum tunnel-injection quantum-dot (QD) lasers utilizing a rate-equation model that takes into account the injection of electrons directly into the QDs from a coupled quantum well, the presence of wetting layer states, and nonequilibrium carrier relaxation in the QDs. In a conventional separate confinement heterostructure QD laser, plasma effects, which result from a large portion of the injected carriers preferably occupying the barrier and wetting layer states, largely determine the values of the linewidth-enhancement factor and lead to a strong dependence of the linewidth-enhancement factor on injection current. In a tunnel-injection QD laser, however, due to the injection of "cold" electrons directly into the lasing states of the QDs, both the values of linewidth-enhancement factor and the dependence on injection current are substantially reduced. The calculated linewidth-enhancement factors of conventional separate confinement heterostructure and tunnel-injection QD lasers are in excellent agreement with reported experimental values. Our analysis elucidates the role of tunnel injection in achieving near-zero alpha-parameter, which would be important in the design of chirp-free high-speed QD lasers

Optically pumped GaN∕AlGaN separate-confinement heterostructure laser grown along the (112¯0) nonpolar direction

This letter concerns experiments on optically pumped GaN∕AlGaN separate-confinement heterostructure laser structures grown by plasma assisted molecular beam epitaxy. The structures were grown along the (112¯0) nonpolar crystallographic direction on a bulk GaN substrate. Different widths of GaN quantum wells were applied in the studied structures. Laser action is clearly demonstrated by the spontaneous emission saturation, abrupt line narrowing, and strong TE polarization of output light. A lasing threshold was reached at an excitation power density of 260kW∕cm2 for a 700-μm-long cavity at room temperature.

Properties of InGaN blue laser diodes grown on bulk GaN substrates

High-pressure growth from solution is at present the only method able to provide true bulk GaN monocrystals. In this paper, we would like to demonstrate that in spite of their small, centimeter range size, they can become a technological platform for the realization of high-quality violet and near-UV laser diodes. We used MOVPE technique to deposit InGaN/AlGaN/GaN layers forming a separate confinement heterostructure laser diode. These devices are characterized by a low density of dislocations (∼10 5 cm −2) and a high optical output power of 1.9 W, measured in short pulses (30 ns) to prevent the structure from overheating. We will briefly discuss the challenges we face during the process of optimization of these devices.

Growth and in situ annealing conditions for long-wavelength (Ga, In)(N, As)/GaAs lasers

The conjugated effect of growth temperature and in situ thermal annealing on the photoluminescence properties of In0.4Ga0.6As0.985N0.015/GaAs quantum wells (QWs) grown by molecular-beam epitaxy has been investigated. The interplay between growth temperature and annealing effects is such as the optimum growth temperature is not the same for as-grown and annealed samples. By using the combination of a low growth temperature and a high in situ annealing temperature, separate confinement heterostructure laser diodes with a single In0.4Ga0.6As1−xNx (x=0.015–0.021)/GaAs QW have been grown. The broad area devices emit from 1.34 to 1.44 μm at room temperature with a threshold current density of 1500–1755A∕cm2.

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.

Operation and Catastrophic Optical Degradation of II-VI Laser Diodes at Output Powers larger than 1 W

An output power of 1.55 W, the highest ever reported, at 20 °C has been achieved for a ZnCdSSe/ZnSSe/ZnMgSSe separate-confinement heterostructure laser diode under short-pulsed operation. A differential quantum efficiency of 28% and a maximum conversion efficiency of 1.5% were obtained. The output power was limited by catastrophic optical damage (COD) of the facets, as proved by atomic force microscopy (AFM).

ABOUT KINKS IN AlGaAs LASERS LIGHT-CURRENT CHARACTERISTIC

The origin of the kinks in semiconductor lasers light-current characteristic (L(l)) is theoretically alld experimentally analyzed. The devices are straight separate confinement heterostructure lasers. We have developed a model to calculate L(l) which considers two thermal loss mechanisms: the leakage current and Auger recombination. It is shown that the kinks in the light-current characteristic appear at temperatures at which the considered mechanisms crossover.

Characterization of etched facets for GaN-based lasers

Dry-etching of laser facets is commonly used for (InAl)GaN/sapphire-based structures since the epitaxial planes of the nitride layers are rotated with respect to the substrate planes making cleaving impractical. To achieve steep and smooth facets by chemically assisted ion beam etching, a 3-layer resist system is developed for patterning. Characterization by scanning electron microscopy and atomic force microscopy shows facets with root-mean-square roughnesses of 7 nm and inclination angles of 2–4°. Optically pumped lasers yield low threshold excitation densities for fully doped separate confinement heterostructure lasers.