Strain-compensated Quantum Wells Research Articles

Optical gain analysis of strain-compensated InGaN–AlGaN quantum well active regions for lasers emitting at 420–500 nm

Strain-compensated InGaN quantum wells with tensile AlGaN barriers are analyzed as improved gain media for laser diodes emitting at 420–500 nm. The band structure is calculated using the 6-band k ·p formalism, taking into account valence band mixing, strain effect, and spontaneous and piezoelectric polarizations. The optical gain analysis exhibits significant improvement in the peak optical gain and differential gain for the strain-compensated structures. The calculation also shows a significant reduction of threshold carrier density and current density for diode lasers employing the strain-compensated InGaN–AlGaN QW active regions.

Comparison of the band alignment of strained and strain-compensated GaInNAs QWs on GaAs and InP substrates

We present a comparison of the band alignment of the Ga1−xInxNyAs1−y active layers on GaAs and InP substrates in the case of conventionally strained and strain-compensated quantum wells. Our calculated results present that the band alignment of the tensile-strained Ga1−xInxNyAs1−y quantum wells on InP substrates is better than than that of the compressively strained Ga1−xInxNyAs1−y quantum wells on GaAs substrates and both substrates provide deeper conduction wells. Therefore, tensile-strained Ga1−xInxNyAs1−y quantum wells with In concentrations of x⩽0.53 on InP substrates can be used safely from the band alignment point of view when TM polarisation is required. Our calculated results also confirm that strain compensation can be used to balance the strain in the well material and it improves especially the band alignment of dilute nitride Ga1−xInxNyAs1−y active layers on GaAs substrates. Our calculations enlighten the intrinsic superiority of N-based lasers and offer the conventionally strained and strain-compensated Ga1−xInxNyAs1−y laser system on GaAs and InP substrates as ideal candidates for high temperature operation.

1.02-μm vertical-cavity surface-emitting lasers with strain-compensated InGaAs quantum wells

We demonstrate strain-compensated InGaAs/GaAsP/GaAs 1.02-μm vertical-cavity surface-emitting lasers (VCSELs) grown monolithically on a GaAs substrate using molecular beam epitaxy. VCSELs at this wavelength are optimal light sources for recently developed graded-index perfluorinated plastic optical fibers. Preliminary results show that the VCSELs oscillate in continuous-wave mode at temperatures up to 80 °C.

Room temperature CW lasing operation of monolithically grown 1.55 μm vertical external cavity surface emitting laser

We report room temperature (20 °C) continuous-wave operation of 1.55 μm vertical-external-cavity surface-emitting lasers. The optically pumped monolithic InP-based structure, grown by metal–organic chemical vapor deposition, includes a InP/InGaAsP Bragg reflector, and an active region with strain compensated quantum wells. Output power up to 4 mW is obtained at 0 °C. The thermal impedance of the structure is deduced from the experimental data.

Structural and optical properties of strain-compensated GaAsSb/GaAs quantum wells with high Sb composition

The structural and optical properties of GaAsSb/GaAs quantum wells (QWs) and strain-compensated GaAsP/GaAs/GaAsSb/GaAs/GaAsP QWs grown on a GaAs substrate by molecular beam epitaxy are investigated using high-resolution x-ray diffraction and photoluminescence (PL) measurements. We demonstrated that the insertion of tensile GaAsP layers into the active region of GaAsSb/GaAs QWs effectively improves the structural and optical quality. Even the Sb composition is as high as 0.39. The PL spectra at 11 K and room temperature indicate that the PL peak of strain-compensated QWs has a narrower linewidth and higher intensity in comparison to the sample without strain compensation. The results of PL peak blueshift with increasing excitation show the strain-compensated GaAsSb/GaAs interface characteristic of type-I band alignment.

Long-wavelength strain-compensated GaAsSb quantum-well heterostructures laser grown by metalorganic chemical vapor deposition

We report data on strain-compensated GaAsSb double-quantum-well lasers having a type-I band alignment and grown by metalorganic chemical vapor deposition on GaAs substrates. In order to compensate for strain effects and to establish a type-I band alignment, tensile-strained higher-band-gap GaAsP quantum-well barriers have been employed. A lasing wavelength of λ∼1.200 μm from a 500 μm long device at 1.15Ith has been achieved at room temperature in pulsed-mode operation. The maximum output power was 20 mW for the same device. A low threshold current density of 608.0 A/cm2 was obtained for 1370 μm long and 60 μm stripe lasers. The calculated internal quantum efficiency and internal loss were 33.8% and 8.4 cm−1, respectively. We also calculated an infinite cavity length threshold current density of 265 A/cm2. From these data, a gain constant G0∼1728 cm−1 and transparency current density of 135 A/cm2 were calculated. The result showed that the strain-compensated GaAsSb quantum wells are promising active materials for 1.3 μm vertical-cavity surface-emitting lasers.

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.

InGaAs/InGaAs strain-compensated quantum well cells for thermophotovoltaic applications

Strain-compensated layers in photovoltaic devices can yield unique advantages as the absorption threshold can be extended towards longer wavelengths beyond that of the lattice-matched material, which is particularly important for thermophotovoltaic (TPV) applications. In such a nanostructure, where InGaAs barriers and InGaAs quantum wells of appropriate compositions are strain compensated on an InP substrate, the absorption of a quantum well cell (QWC) can be extended to ∼2 μm . Due to the higher band-gap barriers, the dark current remains at a low level more appropriate to lattice-matched InGaAs. Great care has to be taken in design and growth to achieve a situation that is close to strain balance with zero stress. Results are presented on a strain-compensated QWC that absorbs out to 1.77 μm . Predictions show that strain-compensated InGaAs/InGaAs QWCs have superior performance when compared with bulk InGaAs on InP monolithic interconnected modules and GaSb TPV cells.

Room-temperature operation of photopumped monolithic InP vertical-cavity laser with two air-gap Bragg reflectors

We demonstrate a long wavelength (λ=1.56 μm) vertical-cavity laser built on a low-loss resonator formed by two InP/air-gap Bragg reflectors. The monolithic, InP-based structure uses a periodic gain active region with six strain-compensated quantum wells. The photopumped vertical-cavity laser requires record low power density of only 370 W/cm2 to reach threshold at 25 °C. The equivalent threshold current density is estimated to be as low as 400 A/cm2. Continuous-wave operation is demonstrated up to 32 °C despite the low heat conductivity of the reflectors. The emission is single mode and a power of up to 110 μW has been coupled into a single-mode fiber.

Valence Subband Structures and Optical Properties of Strain-Compensated Quantum Wells

The strain compensation effects on the valence subbands and on the optical properties of GaInAs/AlGaInAs quantum well structures are theoretically studied for the first time. In the case of compressive-strained quantum wells, where the top valence subbands are always formed with heavy hole (HH) subbands, the compensatingly tensile-strained barriers shift the first light hole (LH) subbands upward increasing the valence band mixing between them, and significantly reducing the transverse electric (TE) gain. In contrast, in the tensile-strained quantum wells whose top valence subband is formed with LH subband, the compensatingly compressive-strained barriers shift the top LH subband downward and on some occasions the top LH subband is replaced with the first HH one. The increase of the TE gain is relatively small due to the strong valence band mixing. The strain of the barrier layers is found to play an important role in the valence subband structures and optical properties.

High-performance strain-compensated InGaAs-GaAsP-GaAs (/spl lambda/=1.17 μm) quantum well diode lasers

This letter reports studies on highly strained and strain-compensated InGaAs quantum-well (QW) active diode lasers on GaAs substrates, fabricated by low-temperature (550/spl deg/C) metal-organic chemical vapor deposition (MOCVD) growth. Strain compensation of the (compressively strained) InGaAs QW is investigated by using either InGaP (tensile-strained) cladding layer or GaAsP (tensile-strained) barrier layers. High-performance /spl lambda/=1.165 μm laser emission is achieved from InGaAs-GaAsP strain-compensated QW laser structures, with threshold current densities of 65 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 1500-μm-cavity devices and transparency current densities of 50 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The use of GaAsP-barrier layers are also shown to significantly improve the internal quantum efficiency of the highly strained InGaAs-active laser structure. As a result, external differential quantum efficiencies of 56% are achieved for 500-μm-cavity length diode lasers.

Wafer-bonded 1.3-μm vertical-cavity surface-emitting lasers

Numerical analysis of the current spreading and carrier diffusion problem for four popular long wavelength vertical-cavity surfaceemitting lasers (VCSELs) structures is presented. The results show that current confinement for p-mirror VCSELs is twice as effective as the corresponding n-mirror VCSELs. We also demonstrate the design, fabrication and device characterization for long wavelength 1.3 ?m VCSELs, where oxygen-implanted current-confinement regions were formed in a GaAs/AlGaAs Bragg reflector bottom mirror, which was wafer-bonded to an AlGaInAs/InP cavity consisting of nine straincompensated quantum wells. Room temperature continuous-wave (cw) operation of 1.3 ?m VCSELs with a record low threshold current density of 1.57 kA/cm2 and a record low threshold current of 1 mA is realized. Also, a record low pulsed threshold current density of 454 A/cm2 and a threshold current of 0.83 mA at 20°C are achieved. The maximum cw and pulsed operating temperatures are 40°C and above 100°C, respectively.

Modeling of gain, differential gain, index change, and linewidth enhancement factor for strain-compensated QWs

The gain, differential gain, index change, and linewidth enhancement factor for strain-compensated InGaAs-GaAsP-InGaP quantum wells (QWs) are modeled. The model we have developed builds upon the model-solid theory for determining the band offsets, the k/spl middot/p method for calculating the matrix elements of dipole moment, and the density matrix approach for computing the complex susceptibility of strain compensated QWs. We also incorporate bandgap renormalization. The calculated results based on the model are consistent with available experimental results in the literature. It is shown that InGaAs-GaAsP-InGaP strain-compensated QWs could offer much higher gain, higher differential gain, and lower linewidth enhancement factor than AlGaAs-GaAs conventionally compressively strained QWs, but more because of its larger hand offset than anything else.

High-performance 1.06-μm selectively oxidized vertical-cavity surface-emitting lasers with InGaAs-GaAsP strain-compensated quantum wells

We present the first room-temperature continuous-wave operation of high-performance 1.06-μm selectively oxidized vertical-cavity surface-emitting lasers (VCSEL's). The lasers contain strain-compensated InGaAs-GaAsP quantum wells (QW's) in the active region grown by metalorganic vapor phase epitaxy. The threshold current is 190 μA for a 2.5×2.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device, and the threshold voltage is as low as 1.255 V for a 6×6 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device. Lasing at a wavelength as long as 1.1 μm was also achieved. We discuss the wavelength limit for lasers using the strain-compensated QW's on GaAs substrates.

Long wavelength (1.3 μm) vertical-cavity surface-emitting lasers with a wafer-bonded mirror and an oxygen-implanted confinement region

We proposed and demonstrated a novel design for long wavelength (1.3 μm) vertical-cavity surface-emitting lasers (VCSELs). In this design, oxygen-implanted current-confinement regions were formed in a GaAs/AlGaAs Bragg reflector which is the bottom mirror wafer bonded to an AlGaInAs/InP cavity consisting of nine strain-compensated quantum wells. Room- temperature continuous-wave (cw) operation of 1.3 μm-VCSELs with a record low cw threshold current density of 1.57 kA/cm2 and a record low cw threshold current of 1 mA have been realized.

Low-threshold proton-implanted 1.3-μm vertical-cavity top-surface-emitting lasers with dielectric and wafer-bonded GaAs-AlAs Bragg mirrors

We demonstrate a new structure for long-wavelength (1.3-μm) vertical-cavity top-surface-emitting lasers using proton implantation for current confinement. Wafer bonded GaAs-AlAs Bragg mirrors and dielectric mirrors are used for bottom and top mirrors, respectively. The gain medium of the lasers consists of nine strain-compensated AlGaInAs quantum wells. A record low room temperature pulsed threshold current density of 1.13 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> has been achieved for 15-μm diameter devices with a threshold current of 2 mA. The side-mode-suppression-ratio is greater than 35 dB.