MQW Active Region Research Articles

Impact of a prestrained graded InGaN/GaN interlayer towards enhanced optical characteristics of a multi-quantum well LED based on silicon substrate.

This paper presents alternate pairs of InGaN/GaN prestrained layers with varying indium compositions, which are inserted between the GaN/InGaN MQW active region and the n-GaN layer in a light-emitting diode (LED) nanostructure in order to obtain enhanced optical characteristics. The device is mounted on a silicon substrate followed by a GaN buffer layer that promotes charge injection by minimizing the energy barrier between the electrode and active layers. The designed device attains more than 2.897% enhancement in efficiency when compared with the conventional LED, which is attributed to the reduction of a polarization field within the MQW region. The proposed device with 15% indium composition in the prestrained layer attains a maximum efficiency of 85.21% and a minimized efficiency droop of 3.848% at an injection current of 40mA, with high luminous power in the output spectral range. The device also shows a minimum blueshift in the spectral range, indicating a decrease in the piezoelectric polarization.

Improving Output Efficiency of InGaN-Based MQW Green Laser Diodes by Modulating Indium Content of Quantum Barriers and Using Composite Lower Waveguide Layers.

Potential barriers between the waveguide layer and MQW active region may influence injection efficiency significantly, which is important in improving output characteristics of GaN-based green laser diodes (LDs). In this study, potential barriers and injection efficiency of LDs are investigated by simulation methods. It is found that different indium content in quantum barrier layers results in different potential barrier heights, leading to different recombination rates in upper and lower waveguide layers, and the injection efficiency can be modulated effectively. An eclectic choice of indium content can suppress recombination in two waveguide layers, improving the output characteristics of green LDs. Additionally, a composite lower waveguide layer structure is proposed to reduce the negative effect of potential barriers. High output power and low threshold current are achieved owing to the reduction in electron injection blockage and hole leakage effects.

Optimized waveguide coupling of an integrated III-V nanowire laser on silicon

The recent integration of III-V semiconductor nanowire (NW) lasers on silicon waveguides marked a key step toward their usage as coherent light sources for future silicon photonics applications. However, the low index contrast between III-V semiconductors and silicon results in a weak modal reflectivity, calling for improved design structures that enable both low-threshold lasing and good in-coupling efficiency into waveguides. Here, we perform numerical simulations to explore how the alternating refractive index of a silicon waveguide with a thin SiO2 interlayer can be used to significantly improve the reflectivity at the III-V–silicon interface to values of up to 83%. We further investigate the frequency dependencies of the end-facet reflectivity and in-coupling efficiency as a function of the nanowire and waveguide dimensions. Our results are kept general by the normalization to the nanowire radius R and show for a waveguide width of 2.75⋅R a maximum coupling efficiency of 50%. Variations in waveguide height or SiO2 interlayer thickness by ±0.1⋅R increase the coupling efficiency by a factor of 2, with little effect on the end-facet reflectivity. Ultimately, a prototypical NW-laser structure consisting of a 1.3-μm emitting InGaAs MQW active region in a core-multishell structure was simulated, showing an optimized low-threshold gain of <500 cm−1 for a TE01 mode with a coupling efficiency of ∼13%. By simplified approximations, we illustrate that these analyses can be adapted to a variety of material systems and serve as guidelines in the construction of optimized nanowire lasers on silicon-on-insulator waveguides for future on-chip optical interconnects.

Magnetic-induced PL modulation of InGaN/GaN MQWs by a CoFeB ferromagnetic cap layer

III-nitride semiconductors show wide applications in general illumination, displays, visible-light communication, power electronics and so on. Until recently, it has been rather hard to experimentally realize magnetic modulation or coupling due to the lack of magnetism in GaN-based semiconductors. Here, we have fabricated the nonpolar a-plane InGaN/GaN MQWs capped by ferromagnetic CoFeB thin film. After magnetizing a CoFeB cap layer under a low magnetic field of 100 mT, the peak PL wavelength of nonpolar a-plane InGaN/GaN MQWs has a red shift from 460 to 475 nm, while its PL polarization degree shows an effective enhancement from 29% to 41% by increasing the CoFeB thickness. The magnetic-induced carrier redistribution has played a great role on the PL modulation of nonpolar InGaN/GaN MQW active region. Therefore, our proposed method exhibits a great potential to develop the highly polarized light emitters, optical polarization modulation, polarization-sensitive detectors, magnetic-optical modulation and coupling sensing in future multi-field applications.

Effect of profile and size of isolation trench on the optical and electrical performance of GaN-based high-voltage LEDs

Four types of HV-LEDs with different isolation trench width were presented. The isolation trench with an oblique angle of 45.6° was obtained using a combination of Cl2/BCl3 plasma chemistry and a thermally reflowed photoresist mask layer, enabling conformal metal lines coverage across the isolation trench. The effect of isolation trench width on the optical and electrical characteristics of HV LEDs was also investigated. A quantitative model was developed to analyze light coupling propagation phenomenon occurring within HV LEDs. The suppression of light coupling propagation among adjacent LED cells was achieved by extending isolation trench width from 3.81μm to 12.30μm, which improved light extraction efficiency and thus increased light output power of HV LEDs. However, the significantly increasing loss of MQW active region area, which was caused by further extending isolation trench width from 12.30μm to 40.49μm, decreased light output power of HV LEDs.

Integrated Duo Wavelength VCSEL Using an Electrically Pumped GaInAs/AlGaAs 980 nm Cavity at the Bottom and an Optically Pumped GaInAs/AlGaInAs 1550 nm Cavity on the Top.

In this work, an integrated single chip dual cavity VCSEL has been designed which comprises an electrically pumped 980 nm bottom VCSEL section fabricated using GaInAs/AlGaAs MQW active region and a 1550 nm top VCSEL section constructed using GaInAs/AlGaInAs MQW active region but optically pumped using half of the produced 980 nm light entering into it from the electrically pumped bottom cavity. In this design, the active region of the intracavity structure 980 nm VCSEL consists of 3 quantum wells (QWs) using Ga0.847In0.153As, 2 barriers using Al0.03Ga0.97As, and 2 separate confinement heterostructures (SCH) using the same material as the barrier. The active region of the top emitting 1550 nm VCSEL consists of 3 QWs using Ga0.47In0.52As, 2 barriers using Al0.3Ga0.17In0.53As, and 2 SCHs using the same material as the barrier. The top DBR and the bottom DBR mirror systems of the 1550 nm VCSEL section plus the top and bottom DBR mirror systems of the 980 nm VCSEL section have been formed using GaAs/Al0.8Ga0.2As. Computations show that the VCSEL is capable of producing 8.5 mW of power at 980 nm from the bottom side and 2 mW of power at the 1550 nm from top side.

Enhanced performance of GaN-based light-emitting diodes with InGaN/GaN superlattice barriers

GaN-based multiple quantum well light-emitting diodes (LEDs) with conventional and superlattice barriers have been investigated numerically. Simulation results demonstrate using InGaN/GaN superlattices as barriers can effectively enhance performances of the GaN-Based LEDs, mainly owing to the improvement of hole injection and transport among the MQW active region. Meanwhile, the improved electron capture decreases the electron leakage and alleviates the efficiency droop. The weak polarization field induced by the superlattice structure strengthens the intensity of the emission spectrum and leads to a blue-shift relative to the conventional one.

Inversion by metalorganic chemical vapor deposition from N- to Ga-polar gallium nitride and its application to multiple quantum well light-emitting diodes

We demonstrate a metalorganic chemical vapor deposition growth approach for inverting N-polar to Ga-polar GaN by using a thin inversion layer grown with high Mg flux. The introduction of this inversion layer allowed us to grow p-GaN films on N-polar GaN thin film. We have studied the dependence of hole concentration, surface morphology, and degree of polarity inversion for the inverted Ga-polar surface on the thickness of the inversion layer. We then use this approach to grow a light emitting diode structure which has the MQW active region grown on the advantageous N-polar surface and the p-layer grown on the inverted Ga-polar surface.

Emission enhancement in GaN-based light emitting diodes with shallow triangular quantum wells

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.

1550 nm InGaAsP LD 광송신회로의 PSPICE 모델 및 광변조 특성 해석

The PSPICE equivalent circuit elements of a 1550 nm InGaAsP laser diode were derived by using multi-level rate equations. The device parameters were extracted by using a self-consistent numerical method for the optical gain properties of the MQW active regions. The resulting equivalent circuit model is also applied to an actual optical transmitter, and its PSPICE simulation results show good agreement with the measured results once the parasitic capacitance due to the packaging is taken into account.

XPM Response of Multiple Quantum Well chirped DFB-SOA All Optical Flip-Flop Switching

In this paper, based on the coupled-mode and carrier rate equations, derivation of a dynamic model and numerically analysis of a MQW chirped DFB-SOA all-optical flip-flop is done precisely. We have analyzed the effects of strains of QW and MQW and cross phase modulation (XPM) on the dynamic response, and rise and fall times of the DFB-SOA all optical flip flop. We have shown that strained MQW active region in under an optimized condition into a DFB-SOA with chirped grating can improve the switching ON speed limitation in such a of the device, significantly while the fall time is increased. The values of the rise times for such an all optical flip-flop, are obtained in an optimized condition, areas tr=255ps.

Room temperature polariton lasing in a GaN∕AlGaN multiple quantum well microcavity

The authors report room temperature polariton lasing at λ∼345nm in a hybrid AlInN∕AlGaN multiple quantum well microcavity (MQW-MC) containing a GaN∕AlGaN MQW active region, i.e., the achievement under nonresonant optical excitation of coherent light emission of a macroscopic population of polaritons occupying the lowest energy state of the lower polariton branch. This was made possible by taking advantage of the efficient relaxation of polaritons in a MQW-MC exhibiting a large vacuum Rabi splitting ΩVRS=56meV.

Self-Consistent Analysis of the Relative Intensity Noise Characteristics in the Strained AlGaInN Laser Diodes with the High Frequency Current Modulation Effects

The relative intensity noise (RIN) characteristics in 405 nm blue laser diodes grown on wurtzite AlGaInN multiple quantum well structures were investigated using the rate equations with the quantum Langevin noise model. The device parameters were extracted from the optical gain properties of the MQW active region using the self-consistent numerical method developed for calculating the multiband Hamiltonian in the strained wurtzite crystal. These methods have been applied to laser diodes for various conditions including the external feedback and the high frequency current injection.

Nitride-based LEDs with MQW active regions grown by different temperature profiles

Nitride-based light-emitting diodes (LEDs) with multiple quantum-well active regions were separately prepared by metal-organic vapor phase epitaxy in different temperature profiles. Compared with conventional samples, the reduced reverse leakage current and improved electrostatic discharge characteristics of the LEDs can both be achieved using temperature ramping and temperature cycling methods. However, using the temperature ramping may degrade the optical properties of devices due to desorption of In atoms and/or impurity incorporation. With an emission wavelength of 465 nm, the 20-mA output powers measured were 5.5, 6.0, and 7.9 mW for temperature ramping LED, conventional LED, and temperature cycling LED, respectively.

1.5 μm VCSEL structure optimization for high-power and high-temperature operation

MOVPE epitaxial growth and design optimisation of 1.5 μm vertical cavity surface emitting lasers (VCSELs) for high power, single mode and high-temperature operation are presented. The VCSEL structure comprises a strain-compensated InAlGaAs MQW active region, a p ++/n ++ In(Al)GaAs tunnel junction and top and bottom AlGaAs/GaAs fused distributed Bragg reflectors. All epitaxial layers were grown by low-pressure MOVPE in nitrogen atmosphere. Excellent optical and electrical confinement is obtained by wafer-bonding of the top mirror on tunnel junction pre-etched mesas. Devices with 7 μm aperture show single transverse mode CW operation up to 4 mW at room temperature, with 30–35 dB side-mode suppression ratio and a far field divergence angle of 9°. At 85 °C, a record single mode high emission power up to 1.7 mW is demonstrated.

40 Gbit/s wavelength conversion over 24.6 nm usingFWM in a semiconductor optical amplifier with an optimised MQW active region

An MQW SOA which has been designed for four-wave-mixing (FWM) applications is reported. In 40 Gbit/s system experiments, a 2 mm device was able to perform wavelength conversion over 24.6 nm. This is the largest reported wavelength translation at 40 Gbit/s using FWM in an SOA.

64°C continuous-wave operation of 1.5-μm vertical-cavity laser

We report on 64/spl deg/C continuous-wave (CW) operation of a 1.5-/spl mu/m vertical-cavity laser. This laser consists of two fused AlGaAs-GaAs mirrors with a strain-compensated InGaAsP-InP MQW active region. Selective lateral oxidation is used for current confinement. Minimum room-temperature threshold current is as low as 0.8 mA, and maximum CW output power is as high as 1 mW at 15/spl deg/C. Pulsed operation is achieved up to 100/spl deg/C. Current spreading losses and device heating are analyzed in detail. Dynamic parameters such as maximum 3-dB parameters such as maximum, 3-dB bandwidth (4.7 GHz), alpha factor (4.0), and linewidth (39 MHz) are also investigated.

MOVPE-grown (AlGa)As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence

We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified “double-barrier multiquantum well” (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.92-2.1 kA cm−2 for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.

MOVPE growth of ternary and quaternary tensile strained MQW structures for polarization insensitive devices

Devices for future all-optical network systems will have a strong requirement for polarization independent operation, which can be achieved by introducing biaxially tensile strain into MQW active regions. We report on InP based low pressure MOVPE growth of MQW structures for 1.55 μm wavelength with tensile strained InGaAs and InGaAsP wells and lattice matched InGaAsP barrier layers. Taking into account measurement results from X-ray and PL together with theoretical evaluations we were able to grow polarization insensitive device structures. For all-optical wavelength converters we used MQW stacks with 5 tensile strained ternary wells and achieved λ conversion with a polarization dependence of less than 1 dB. Using quaternary wells a polarization insensitive electroabsorption modulator was realized. The residual polarization dependence is < 0.4 dB for 1550 nm and < 1 dB in the wavelength range from 1540 to 1560 nm (extinction ratio 10 dB).

Very-low-threshold, highly efficient, and low-chirp 1.55-μm complex-coupled DFB lasers with a current-blocking grating

Low-threshold current DFB lasers have been fabricated based on a complex-coupled structure with a current blocking grating. A threshold current as low as 5 mA was obtained, which enabled high temperature operation up to 105/spl deg/C. Asymmetric facet coatings were applied to obtain a high efficiency of 0.3 W/A. The strained InGaAsP MQW active region and the antiphase coupling has resulted in a low modulation chirping. By direct modulation of the device at 2.488 Gb/s and using optical amplifiers, we have demonstrated digital transmission over a 235 km-long standard single-mode fiber with a power penalty of 1.55 dB.