Graded Index Separate Confinement Heterostructure 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.

Decorrelation of internal quantum efficiency and lasing threshold in AlGaN-based separate confinement heterostructures for UV emission

In this paper, we study the internal quantum efficiency and lasing threshold of AlGaN/GaN separate confinement heterostructures designed for ultraviolet laser emission. We discuss the effect of carrier localization and carrier diffusion on the optical performance. The implementation of graded index separate confinement heterostructures results in an improved carrier collection at the multi-quantum well, which facilitates population inversion and reduces the lasing threshold. However, this improvement is not correlated with the internal quantum efficiency of the spontaneous emission. We show that carrier localization at alloy inhomogeneities results in an enhancement of the radiative efficiency but does not reduce the laser threshold, which is more sensitive to the carrier injection efficiency.

AlGaN/GaN asymmetric graded-index separate confinement heterostructures designed for electron-beam pumped UV lasers.

We present a study of undoped AlGaN/GaN separate confinement heterostructures designed to operate as electron beam pumped ultraviolet lasers. We discuss the effect of spontaneous and piezoelectric polarization on carrier diffusion, comparing the results of cathodoluminescence with electronic simulations of the band structure and Monte Carlo calculations of the electron trajectories. Carrier collection is significantly improved using an asymmetric graded-index separate confinement heterostructure (GRINSCH). The graded layers avoid potential barriers induced by polarization differences in the heterostructure and serve as strain transition buffers which reduce the mosaicity of the active region and the linewidth of spontaneous emission.

Design of high-performance double quantum well vertical cavity transistor lasers with GRIN base region

Different confinement structures are analyzed to achieve higher optoelectronic performances for double quantum well vertical cavity transistor laser with graded index separate confinement heterostructure. Adding the drift component to the diffusion term of the current density and solving new sets of equations, modified electro-optic performances of the device is obtained. Band-gap engineering of the original structure predicts simultaneous improvements in both current gain (more than two times) and −3 dB optical bandwidth (by 1.5 GHz). Other less critical, yet important, performance metrics including optical output power and threshold current (up to 20%) are enhanced due to applying graded layers of AlξGa1-ξAs in the base region.

Graded-Index Separate Confinement Heterostructure AlGaN Nanowires: Toward Ultraviolet Laser Diodes Implementation

High-density dislocations in materials and poor electrical conductivity of p-type AlGaN layers constrain the performance of the ultraviolet light emitting diodes and lasers at shorter wavelengths. To address those technical challenges, we design, grow, and fabricate a novel nanowire structure adopting a graded-index separate confinement heterostructure (GRINSCH) in which the active region is sandwiched between two compositionally graded AlGaN layers, namely, a GRINSCH diode. Calculated electronic band diagram and carrier concentrations show an automatic formation of a p–n junction with electron and hole concentrations of ∼1018 /cm3 in the graded AlGaN layers without intentional doping. The transmission electron microscopy experiment confirms the composition variation in the axial direction of the graded AlGaN nanowires. Significantly lower turn-on voltage of 6.5 V (reduced by 2.5 V) and smaller series resistance of 16.7 Ω (reduced by nearly four times) are achieved in the GRINSCH diode, compared with the ...

450 nm (Al,In)GaN optical amplifier with double 'j-shape' waveguide for master oscillator power amplifier systems.

In this paper we demonstrate 450 nm (Al,In)GaN graded index separate confinement heterostructure travelling wave optical amplifier with a double 'j-shape' waveguide. The length of the amplifier is 2.5 mm and the width of the ridge is 2.5 µm. The active region consists of three 3.5 nm thick quantum wells. The measured optical gain under CW operation in room temperature exceeded 29 dB for low power input signals. The saturation output power was 21 dBm for 400 mA driving current. The demonstrated amplifier, provides a good solution for the blue light, all nitrides, and master oscillator power amplifier systems.

Role of the electron blocking layer in the graded-index separate confinement heterostructure nitride laser diodes

In this work, we investigate the role of the electron blocking layer (EBL) in laser diodes based on a graded index separate confinement heterostructure. We compare two sets of devices with very different EBL aluminum composition (3% and 12%) and design (graded and superlattice). The results of electro-optical characterization of these laser diodes reveal surprisingly modest role of electron blocking layer composition in determination of the threshold current and the differential efficiency values. However, EBL structure influences the operating voltage, which is decreased for devices with lower EBL and superlattice EBL. We observe also the differences in the thermal stability of devices – characteristic temperature is lower for lasers with 3% Al in EBL.

Nanoscale effects on the thermal and mechanical properties of AlGaAs/GaAs quantum well laser diodes: influence on the catastrophic optical damage

In this work we study the catastrophic optical damage (COD) of graded-index separate confinement heterostructure quantum well (QW) laser diodes based on AlGaAs/GaAs. The emphasis is placed on the impact that the nanoscale physical properties have on the operation and degradation of the active layers of these devices. When these laser diodes run in continuous-wave mode with high internal optical power densities, the QW and guide layers can experiment very intense local heating phenomena that lead to device failure. A thermo-mechanical model has been set up to study the mechanism of degradation. This model has been solved by applying finite element methods. A variety of physical factors related to the materials properties, which play a paramount role in the laser degradation process, have been considered. Among these, the reduced thicknesses of the QW and the guides lead to thermal conductivities smaller than the bulk figures, which are further reduced as extended defects develop in these layers. This results in a progressively deteriorating thermal management in the device. To the best of our knowledge, this model for laser diodes is the first one to have taken into account low scale mechanical effects that result in enhanced strengths in the structural layers. Moreover, the consequences of these conflicting size-dependent properties on the thermo-mechanical behaviour on the route to COD are examined. Subsequently, this approach opens the possibility of taking advantage of these properties in order to design robust diode lasers (or other types of power devices) in a controlled manner.

Graded index separate confinement heterostructure transistor laser: analysis of various confinement structures

A new configuration of the confinement structure is utilized to improve optoelectronic performance, including threshold current, ac current gain, optical bandwidth, and optical output power of a single quantum well transistor laser. Considering the drift component in addition to the diffusion term in electron current density, a new continuity equation is developed to analyze the proposed structures. Physical parameters, including electron mobility, recombination lifetime, optical confinement factor, electron capture time, and photon lifetime, are calculated for new structures. Based on solving the continuity equation in separate confinement heterostructures, the threshold current reduces 67%, the optical output power increases 37%, and the −3 dB optical bandwidth increases to 21 GHz (compared to 19.5 GHz in the original structure) when the graded index layers of AlξGa1−ξAs (ξ:0.05→0 in the left side of quantum well, ξ:0→0.02 in the right side of quantum well) are used instead of uniform GaAs in the base region.

Effect of indium in Al0.65Ga0.35N/Al0.8Ga0.2N MQWs for the development of deep‐UV laser structures in the form of graded‐index separate confinement heterostructure (GRINSCH)

AlGaN‐based graded‐index‐separate‐confinement‐heterostructures (GRINSCHs) with Al0.65Ga0.35N/Al0.8Ga0.2N MQWs in their active region were grown by plasma‐assisted MBE on (0001) 6H‐SiC substrates. The MQWs were grown under excess Ga. Indium flux was used in addition in some of the samples in order to study the role of indium on the growth mode of such MQWs. Transmission electron microscopy (TEM) provides evidence that the bottom compositionally graded AlGaN layer in the GRINSCH structure may also be serving as a strain transition buffer layer, by blocking threading defects in the vicinity of the AlN/AlGaN heterointerface. X‐ray diffraction (XRD) indicates that the interfaces in the MQWs grown in the presence of indium are sharper. However, photoluminescence (PL) studies indicate evidence of stimulated emission at relatively low threshold only in the sample grown without indium, which is attributed to deep band structure potential fluctuations in this sample. The presence of indium during growth led to more homogeneous AlGaN MQWs, which require higher excitation flux to produce population inversion.

Deep-UV optical gain in AlGaN-based graded-index separate confinement heterostructure

We propose AlGaN-based GRaded-INdex Separate Confinement Heterostructure as a candidate for electrically pumped deep-UV semiconductor laser. Strong compositional fluctuations were introduced in the active region (75 nm-thick Al0.72Ga0.28N film grown by RF Plasma- assisted Molecular Beam Epitaxy) to obtain net modal optical gain peaked at 257 nm in excess of 80 cm −1 . We measured an optical gain threshold of 14 µJ/cm 2 . Because of polarization-doping of the compositionally graded AlGaN regions, which automatically leads to the formation of p-i-n junction, these results pave the way for lasing under electrical injection.

MBE growth of P-doped 1.3 μm InAs quantum dot lasers on silicon

The authors report on the growth of InAs/GaAs quantum dots lasers on silicon emitting in the 1.3 μm wavelength regime with p-doped active regions. A growth optimization procedure guided by a combination of high and low excitation photoluminescence is presented for the InAs quantum dot growth. Growth conditions derived from this procedure are used to produce high optical quality quantum dots embedded in a GaAs/AlxGa1−xAs graded index separate confinement heterostructure waveguide. Ridge waveguide lasers fabricated from the as-grown material achieve room temperature continuous wave lasing at low thresholds, with high output power and elevated T0.

UV emitters based on an AlGaN p–n junction in the form of graded-index separate confinement heterostructure

AlxGa1−xN-based UV emitters in the form of a graded-index separate confinement heterostructure were grown on 6H-SiC. The devices consist of 100 nm AlxGa1−xN compositionally graded from x = 0.3 to 0 and x = 0 to 0.3 in two sides. Owing to polarization doping from naturally occurring or intentionally incorporated dopants, the two sides are of the doped p- and n-types, giving rise to a p–n junction. The doping level was calculated to be 1018 cm−3 for the p- and n-sides. It was found experimentally that this doping level requires the intentional incorporation of Mg and Si, and those devices show better rectification and stronger electroluminescence emission.

High power single mode 980 nm AlGaInAs/AlGaAs quantum well lasers with a very low threshold current

To achieve low threshold current as well as high single mode output power, a graded index separate confinement heterostructure (GRIN-SCH) AlGaInAs/AlGaAs quantum well laser with an optimized ridge waveguide was fabricated. The threshold current was reduced to 8 mA. An output power of 76 mW was achieved at 100 mA current at room temperature, with a slope efficiency of 0.83 W/A and a horizon divergent angle of 6.3°. The maximum single mode output power of the device reached as high as 450 mW.

A neutron radiation-hardened superluminescent diode

We present a novel superluminescent diode (SLD) with high optical performances for hardened neutron irradiation. The degradation of the light output from the SLDs is caused by a reduction of the minority carrier lifetime resulting from displacement damage after high-energy neutron irradiation. The SLDs with a higher pre-irradiation light output will be less sensitive to radiation. We have selected an InGaAsP/InP multi-quantum well (MQW) as the active region structure for its performance, its high optical gain and minute active region. Graded-index separate-confinement-heterostructure (GRIN-SCH) has been applied for the waveguide structure. A specific absorbing region and anti-reflective coatings have been designed and optimized. Moreover, the radiation test results indicate that the SLD with an InGaAsP/InP MQW structure has better neutron hardening ability than the SLD with DH structures after a 6 × 1013 − 1 × 1014 n/cm2 1 MeV neutron irradiation.

An extensive study on simple and GRIN SCH-based In0.71Ga0.21Al0.08As/InP lasing heterostructures

In this paper, we have proposed a model for simple and graded index (GRIN) separate confinement heterostructure (SCH)-based In0.71Ga0.21Al0.08As/InP single-quantum-well lasing heterostructures and studied the energy band structure along with conduction and valence band envelope functions and band offsets. Under theoretical simulation, we have determined the behavior of quasi-Fermi levels in both the conduction and valence bands. Moreover, we have also calculated various important lasing characteristics and the optical gain of the lasing structure as a function of lasing wavelength in transverse electric (TE) and transverse magnetic (TM) modes. For both types of lasing structures, optical gain in the TE mode has been reported to be larger than that in the TM mode. In the TE mode the maximum optical gain for simple and GRIN structures has been achieved at lasing wavelengths of 1.55 and 1.38 μm, respectively, while in the TM mode the values of maximum optical gain for both types of the structures are found to be the same, i.e. at 1.34 μm. Since the optical attenuation within the waveguide is found to be minimum at 1.55 μm, the simple SCH-based In0.71Ga0.21Al0.08As/InP lasing structures are suggested to be preferred to the GRIN SCH-based lasing structures of InGaAlAs/InP material systems.

Design and Simulation of an 808 nm InAlAs/AlGaAs GRIN-SCH Quantum Dot Laser Diode

Quantum dots were designed within a GRIN-SCH(Graded index - Separate confinement Heterostructure) heterostructure to create a high power InAlAs/AlGaAs laser diode. 808 nm light emission was with a quantum dot composition of In0.665Al0.335As and wetting layer composition of Al0.2Ga0.8As by LASTIP simulation software. Typical characteristics of GRIN structures such as high confinement ratios and Gaussian beam profiles were shown to still apply when quantum dots are used as the active media. With a dot density of 1.0x1011 dots/cm2, two quantum dot layers were found to be good enough for low threshold, high-power laser applications.

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.

GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

Theoretical calculations of the mode characteristics of an equilateral-triangle resonator (ETR) with a 10μm cavity side length show that the fundamental mode, with longitudinal mode index of 25, has a wavelength of 2.185μm and a longitudinal mode separation of 100nm. This mode has a quality factor (∼2×105) that is much larger than the first (∼5×104) and second (∼3×104) order modes, indicating that single fundamental mode lasing should be accessible over a broad wavelength tuning range. An electrically injected ETR based on this design is fabricated from an InGaAsSb∕AlGaAsSb∕GaSb, graded-index separate-confinement heterostructure, laser diode wafer with a 2.1μm emission wavelength. This device achieved single mode, continuous wave operation at 77K with a threshold current of 0.5mA and a single mode wavelength tuning range of 3.25nm, which is accomplished by varying the injection current from 0.5to6.0mA.

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