Strain-compensated Quantum Wells Research Articles

Design and fabrication of 940 nm vertical cavity surface emitting laser single-emitter device

As a key part of vertical cavity surface emitting laser (VCSEL), active region will seriously affect the threshold and efficiency of the device. To obtain the appropriate laser wavelength and material gain, the design of In<sub>0.18</sub>Ga<sub>0.82</sub>As strain compensated quantum well is optimized. The relationship between the lasing wavelength of multiple quantum wells (MQWs) and the thickness is calculated. Considering the influence between the active region temperature and the lasing wavelength, the thickness of the quantum well is chosen as 6 nm, and the quantum barrier thickness is chosen as 8 nm, corresponding to the lasing wavelength of 929 nm. The material gain characteristics of the MQWs at different temperatures are simulated by Rsoft. The material gain exceeds 3300/cm at 300 K, and the temperature drift coefficient of the peak wavelength is 0.3 nm/K. In this work, Al<sub>0.09</sub>Ga<sub>0.91</sub>As and A<sub>l0.89</sub>Ga<sub>0.11</sub>As are chosen as the high- and the low-refractive index material of distributed Bragg reflector (DBR), and 20 nm graded layer is inserted between two types of materials. The influence of the graded layer thickness of DBR on the valence band barrier and reflection spectrum are calculated and analyzed. The increase of graded layer thickness can lead the band barrier peak and the reflection spectrum bandwidth to decrease. The reflection spectrum and phase spectrum of P-DBR and N-DBR are calculated by the transmission matrix mode (TMM): the reflectance of DBR is over 99% and the phase shift is zero at 940 nm. The optical field distribution of the whole VCSEL structure is simulated, in which the standing wave peak overlaps with the active region, and the maximum gain can be obtained. Using the finite element method (FEM), the effect of oxidation confined layer on the injection current is simulated. The current in the active region is effectively limited to the position corresponding to the oxidation confined hole, and its current density is stronger and more uniform. The optical field distributions in different modes of photonic crystal-vertical cavity surface emitting laser (PC-VCSEL) are simulated, and different modes have different resonant wavelengths. The values of quality factor Q in different modes of VCSEL and PC-VCSEL are calculated, Q of the fundamental mode is higher than that of higher transverse mode. It is demonstrated that the photonic crystal air hole structure can realize the output of basic transverse mode by increasing the loss of high order transverse mode. The VCSEL and PC-VCSEL with oxidation hole size of 22 μm are successfully fabricated, in which the photonic crystal period is 5 μm, the air pore diameter is 2.5 μm, and the etching depth is 2 μm. Under continuous current test, the maximum slope efficiency of VCSEL is 0.66 mW/mA, the output power is 9.3 mW at 22 mA, and the lasing wavelength is 948.64 nm at 20 mA injection current. Multiple wavelengths and large spectrum width are observed in the spectrum of VSCEL, which is an obvious multi-transverse mode. The maximum fundamental transverse mode output of PC-VCSEL reaches 2.55 mW, the side mode suppression ratio (SMSR) is more than 25 dB, and the spectrum width is less than 0.2 nm, indicating that the photonic crystal air hole has a strong control effect on the transverse mode, and the laser wavelength is 946.4 nm at 17 mA.

Optimization of the gain chip for optically pumped semiconductor disk lasers

Strain-compensated quantum wells (QWs) could greatly improve the performance of semiconductor disk lasers, such as improving the epitaxy quality of the semiconductor gain chip and obtaining a higher gain. To optimize the output characteristics of the semiconductor gain chip, the strain-compensated thickness, well depth, band energy, emission wavelength, and peak gain versus In and P compositions are investigated in detail. The results show that increasing the P composition of the strain-compensated layer will slightly reduce the emission wavelength, deepen the well depth, and increase the peak gain. But the P composition is not the higher the better. When designing a gain chip, it should have a pre-offset between the emission wavelength of the QWs at room temperature and the target wavelength of the laser at an intense pump, and the pre-offset values of the wavelength should be designed deliberately to guarantee the resonant periodic gain structure working normally when the temperature in the active region approaches the desired value. This theoretical investigation may provide guidance for optimizing the laser performance of semiconductor disk lasers and other QW lasers.

Compact superluminescent AlGaInAs/InP strain-compensated quantum-well diodes for fibre-optic gyroscopes

Compact superluminescent diodes based on AlGaInAs/InP separate-confinement double heterostructures with strain-compensated quantum wells are studied. The characteristics of these diodes that allow their application for development of fibre-optic gyroscopes in a temperature range from – 55 °C – +70 °C are demonstrated. The devices demonstrate an acceptable reliability and a potential for further improvement.

Superluminescent diodes in the spectral range of 1.5 – 1.6 μm based on strain-compensated AlGaInAs/InP quantum wells

Superluminescent diodes based on AlGaInAs/InP separate-confinement double heterostructures with strain-compensated quantum wells are investigated. The influence of elastic strains in the active region on the output characteristics of the devices is analysed. It is shown that such a design of a superluminescent diode allows an optical power of more than 5 mW, a radiation spectrum width of more than 60 nm, a degree of output radiation polarisation up to 30 dB to be obtained at the output of a single-mode fibre, and has a great potential for further improvement.

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.

Intersubband absorption coefficients of GaN/AlN and strain-compensated InGaN/InAlN quantum well structures

Intersubband (ISB) transition absorption properties of strain-compensated InGaN/InAlN quantum well (QW) structures grown on GaN substrate were investigated using an effective mass theory. The interband dipole moment of the strain-compensated InGaN/InAlN QW structure is shown to be comparable to that of the conventional GaN/AlN QW structure. However, the strain-compensated InGaN/InAlN QW structure shows much larger intersubband absorption coefficient than the conventional GaN/AlN QW structure. This is mainly due to the fact that the former shows much larger quasi-Fermi-level separation than the latter because the electron effective mass is reduced with the inclusion of Indium. As a result, the strain-compensated QW structures could be used as ISB photodetectors for telecommunication operating at 1.55 μm with a higher absorption coefficient and a reduced strain, compared to conventional GaN/AlN QW structures.

Intersubband energies in strain-compensated InGaN/AlInN quantum well structures

Intersubband transition energies in the conduction band for strain-compensated InGaN/AlInN quantum well (QW) structures were investigated as a function of strain based on an effective mass theory with the nonparabolicity taken into account. In the case of an InGaN/AlInN QW structure lattice-matched to GaN, the wavelength is shown to be longer than 1.55 μm. On the other hand, strain-compensated QW structures show that the wavelength of 1.55 μm can be reached even for the QW structure with a relatively small strain of 0.3 %. Hence, the strain-compensated QW structures can be used for telecommunication applications at 1.55 μm with a small strain, compared to conventional GaN/AlN QW structure.

InP-based type-I quantum well lasers up to 2.9 μm at 230 K in pulsed mode on a metamorphic buffer

This work reports on up to 2.9 μm lasing at 230 K of InP-based type-I quantum well lasers. This record long wavelength lasing is achieved by applying InP-based Sb-free structures with eight periods of strain-compensated InAs quantum wells grown on metamorphic In0.8Al0.2As template layers. The continuous-wave threshold current density is 797 A/cm2 and the idealized extrapolated threshold current density for infinite cavity length is as low as 58 A/cm2 per quantum well at 120 K. This scheme is a promising pathway for extending the wavelength range of type-I quantum well lasers on InP substrates.

Light emission characteristics of blue strain-compensated InGaN/InGaN/InGaN light-emitting diodes

The light emission characteristics of blue strain-compensated InGaN/InGaN light emitting diodes (LEDs) grown on InGaN substrates were investigated using the multiband effective mass theory. The radiative recombination rate of the strain-compensated LED structure is shown to be larger than that of the conventional LED structure due to the decrease in the lattice mismatch with the substrate. Also, the light emission distribution in quantum wells (QWs) for the strain-compensated LED structure is found to be much more uniform than that for the conventional LED structure. As a result, the simulation results suggest that the efficiency droop is significantly improved when a conventional GaN barrier and substrate are replaced by an InGaN barrier and substrate. Thus, a possibility exists to employ a strain-compensated QW to improve the optical performance of InGaN-based blue LEDs.

Type-I mid-infrared InAs/InGaAs quantum well lasers on InP-based metamorphic InAlAs buffers

InAs/InGaAs quantum well laser structures have been grown on InP-based metamorphic In0.8Al0.2As buffers by gas source molecular beam epitaxy. The effects of barrier and waveguide layers on the material qualities and device performances were characterized. X-ray diffraction and photoluminescence measurements prove the benefits of the strain compensation in the active quantum well region on the material quality. The device characteristics of the lasers with different waveguide layers reveal that the separate confinement heterostructure plays a crucial role on the device performances of these metamorphic lasers. Type-I emissions in the 2–3 µm range have been achieved in these InP-based metamorphic antimony-free structures. By combining the strain-compensated quantum wells and separate confinement heterostructures, the laser performances have been improved and laser emission up to 2.7 µm has been achieved.

Study on the Luminescence Properties of the Strain Compensated Quantum Well

Compared with the conventional strained quantum well, the InGaAs/GaAsP strain compensated quantum well (SCQW) has better optical properties, as the well layer and the barrier layer lattice mismatch with each other which results in the introduction of stress. In this paper, we adopted the Kohn-Luttinger Hamiltonian, conducted some theoretical calculations using the transfer matrix method, and finally verified the following change trend of the InGaAs quantum well following the In-group through experiments: the growth of the low In-group can improve the epitaxial flatness of the active area; the growth of the high In-group can increase the wavelength as well as the strain. In this paper, we adopted the AlGaAs barrier material instead of the GaAsP, made an analysis on the level changes of the compensation quantum well, and successfully fostered the strain compensated quantum well structure using the metal-organic chemical vapor deposition (MOCVD) system which had better optical properties compared with the strained quantum wells.

Carrier Recombination Dynamics Investigations of Strain-Compensated InGaAsN Quantum Wells

The time evolution of the photoluminescence (PL) of 1300-nm emitting InGaAsN/ GaAs/GaAsP strain-compensated single quantum well (QW) in the temperature range of T 1/4 10 K - 300 K is investigated. The PL spectra observed at the early stages of carrier recombination is dominated by two transitions. These two transitions are identified as the first quantized electron state to heavy-hole state (e1-hh1) and electron to light-hole state (e1-lh1) from the analysis of polarized photocurrent measurements in combination with k · p simulation of the band structure. At longer time delays, the dilute-nitride QW exhibits carrier localization at low temperatures and faster recombination time at higher temperatures. The PL dynamics characteristics observed in the InGaAsN QW are different from those measured from the InGaAs QW.

Quantum-confined Stark effect at 13 μm in Ge/Si_035Ge_065 quantum-well structure

Room-temperature quantum-confined Stark effect in a Ge/SiGe quantum-well structure is reported at the wavelength of 1.3 μm. The operating wavelength is tuned by the use of strain engineering. Low-energy plasma-enhanced chemical vapor deposition is used to grow 20 periods of strain-compensated quantum wells (8 nm Ge well and 12 nm Si(0.35)Ge(0.65) barrier) on Si(0.21)Ge(0.79) virtual substrate. The fraction of light absorbed per well allows for a strong modulation around 1.3 μm. The half-width at half-maximum of the excitonic peak of only 12 meV allows for a discussion on physical mechanisms limiting the performances of such devices.

Narrow linewidth 1118/559 nm VECSEL based on strain compensated GaInAs/GaAs quantum-wells for laser cooling of Mg-ions

We report on the development of an optically-pumped vertical external-cavity surface-emitting laser emitting near 1120 nm using strain compensated quantum wells. The development is motivated by the need to achieve narrow linewidth emission at ~280 nm via fourth harmonic generation, which is required to cool Mg+ ions. The gain mirror had a top-emitting geometry, was grown by molecular beam epitaxy and comprised GaInAs/GaAs quantum wells strain compensated by GaAsP layers; the strain compensation was instrumental for achieving a dislocation free epitaxial structure without dark lines. We demonstrate VECSEL operation at a fundamental wavelength close to 1118 nm with a linewidth of less than 300 kHz. Using a lithium triborate crystal we achieved frequency doubling to ~559 nm with an output power of 1.1W.

Spontaneous emission rate of green strain-compensated InGaN/InGaN LEDs using InGaN substrate

Optical properties of strain-compensated InGaN/InGaN quantum well (QW) structures using a InGaN substrate are investigated using the multiband effective mass theory. These results are compared with those of conventional InGaN/GaN QW structures using a GaN substrate. The strain-compensated QW structure shows that a smaller well thickness is needed to obtain the transition wavelength of 530 nm than the InGaN/GaN QW structure. The spontaneous emission peak of a strain-compensated QW structure is shown to be much larger than that of a conventional QW structure. This is mainly attributed to the fact that the internal field is reduced due to the decrease in the lattice mismatch with a substrate.

Enhancement of light power for strain-compensated hybrid InGaN/InGaN/MgZnO light-emitting diodes

Electronic and optical properties of strain-compensated InGaN/InGaN/MgZnO quantum well (QW) structures using a MgZnO substrate are investigated using the multiband effective mass theory. A strain-compensated InGaN/InGaN/MgZnO QW structure with a larger strain shows larger matrix element than that with a smaller strain. The spontaneous emission peak rapidly increases with increasing compressive strain because the matrix element is enhanced for the strain-compensated QW structure with a larger strain. In addition, we find that the strain-compensated QW structure with the larger Mg composition in the substrate has greater spontaneous emission peak than the strain-compensated QW structure with the smaller Mg composition in the substrate.

Self-Consistent Analysis of Strain-Compensated InGaN–AlGaN Quantum Wells for Lasers and Light-Emitting Diodes

Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.

Electronic and optical properties of 530 nm strain-compensated hybrid InGaN/InGaN/ZnO quantum well light-emitting diodes

Electronic and optical properties of 530 nm strain-compensated hybrid InGaN/InGaN/ZnO quantum well (QW) light-emitting diodes (LEDs) using a ZnO substrate are investigated using the multiband effective mass theory. These results are compared to those of conventional InGaN/GaN QW LEDs using a GaN substrate. A strain-compensated QW structure is found to have much larger spontaneous emission than an InGaN/GaN QW structure. This can be explained by the fact that a strain-compensated QW structure has much larger matrix element than an InGaN/GaN QW structure due to the reduction in the internal field.

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.