Separate Confinement Heterostructure Research Articles

III-Arsenide separate confinement heterostructure infrared laser diodes with engineered the cladding layers

In this work, III-Arsenide laser diodes (LDs) show better optoelectronic properties when the aluminum (Al) concentration in the cladding layer is increased up to 95 % in a separate confinement heterostructure (SCH). These laser diodes show their peak gain the infrared lasing wavelength of ∼840 nm. Enhancing the Al in the n- and p-cladding layers, both separately and simultaneously in LDs, shows that the optoelectronic properties are reasonably improved. The output light power has been increased to 50 mW at a current density of 13 × 104A/cm2, with 95 % aluminum in both the n- and p-claddings. The modal field intensity has increased by 5 %, and the optical confinement factor (OCF) has improved from 0.0102 to 0.0105 in the proposed device. Consequently, the engineered cladding design, incorporating varied aluminum concentrations, is recognized as pivotal for optimizing the performance of arsenide laser diodes.

Optimal waveguide structure for low-threshold InGaN/GaN-based photonic-crystal surface-emitting lasers

We analyzed the optimal waveguide structure of two types of InGaN-based photonic crystal surface-emitting lasers (PCSELs) to suppress the coupling with leaky modes via mode simulations. To minimize the threshold material gain (gth), we calculated the confinement factor and quality factor of PCSELs with varying waveguide layer thicknesses in the separate confinement heterostructure (SCH) layer. The optical mode intensity profile revealed the coupling between the fundamental mode of SCH and parasitic leaky modes in the cladding layer or substrate as the primary root cause of the low-quality factor and high threshold gain of PCSELs. The asymmetric nature of the SCH structure yielded the optimal waveguide structure to be dependent on the position of the air holes. With a proper waveguide thickness and air hole depth, the optimized threshold modal gain of PCSELs with the n-side air holes can be less than 30 cm−1.

Investigations on the effect of separate confinement heterostructure width on the distortion performance of tunnel injection based transistor laser

Investigations on the effect of separate confinement heterostructure width on the distortion performance of tunnel injection based transistor laser

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.

III–V Microwires with Reversed Ridge Waveguides Selectively Grown on Pre-Patterned Si Substrates

To construct functional photonic integrated circuits, an efficient and compact laser is expected to be incorporated into the complementary metal–oxide–semiconductor platform. Monolithic integration of III–V lasers on pre-patterned Si substrates by the aspect ratio trapping method is a promising solution. Here, microwires with reversed ridge waveguides (RRWs) on pre-patterned Si substrates were reported. By metal–organic chemical vapor deposition, high-quality InP microwires with RRWs were obtained, and InGaAs/InGaAsP multi-quantum-well structures with InGaAsP separate confinement hetero structure (SCH–MQW) were successfully achieved. The SCH–MQW structure was buried in the InP microwire, which was beneficial for transferring the heat generated in the active region. The micron size also contributes to the efficiency of thermal diffusion. Further, simulation results showed that the metal absorption loss could be less than 4 dB/cm by properly controlling the contact area between metal electrodes and microwires. This proposed structure opens up an alternative pathway for electrically driven III–V lasers seamlessly interfaced with Si-photonics.

Optical net gain measurement on Al0.07Ga0.93N/GaN multi-quantum wells.

We report net gain measurements at room temperature in Al0.07Ga0.93N/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm-1, obtained at the maximum pumping power density of the experimental setup (743 kW/cm2). The net gain threshold was attained at 218 kW/cm2 using 193 nm optical pumping. From these experiments, we predict an electron-beam-pumped lasing threshold of 370 kW/cm2 at room temperature, which is compatible with the use of compact cathodes (e.g. carbon nanotubes). In some areas of the sample, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 µm) and high pumping power (superior to 550 kW/cm2), leading to an overestimation of the net gain value. We attribute such a phenomenon to the optical feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers.

Reduction of the lasing threshold in optically pumped AlGaN/GaN lasers with two-step etched facets

We report a two-step process to obtain smooth and vertical {10−10} m-plane facets in AlGaN/GaN separate confinement heterostructures designed to fabricate ultraviolet lasers emitting at 355 nm. The process combines inductively coupled plasma reactive ion etching with crystallographic-selective wet etching using a KOH-based solution. The anisotropy in the wet etching allows the fabrication of flat, parallel facets without degradation of the multilayer ensemble. The optical performance of the lasers is strongly improved (reduction of the lasing threshold by a factor of two) when using the two-step process for the definition of the cavity, in comparison to cavities fabricated by mechanical cleaving.

Direct Imaging of the Carrier Capture into Individual InP Quantum Dots of a Semiconductor Disk Laser Membrane.

We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport, i.e., the efficient lateral capture of carriers into an individual QD. We observe a characteristic change of the spectral fingerprint during this line scan, while the electron beam is approaching and subsequently receding from the quantum dot position. This directly correlates to the increase and decrease of the numbers of excess carriers reaching the dot, i.e., altering the quantum dot population. The characteristic shift of emission energies visualize the renormalization of the ground-state energy of the single dot, and the intensity ratio of the excitonic recombinations verifies this change of the occupation and the state-filling.

Improvement of the current – voltage performance of broadened asymmetric waveguide InGaAs/AlGaAs/GaAs semiconductor lasers (λ = 940 – 980 nm)

The purpose of this work is to improve the current – voltage (I – V) performance of semiconductor lasers based on broadened asymmetric waveguide InGaAs/AlGaAs/GaAs separate-confinement double heterostructures. We analyse the effect of AlGaAs waveguide layer composition on the output characteristics of the lasers and demonstrate that the decrease in the series resistance of the lasers and the threshold voltage of their I – V characteristic as a result of a decrease in the percentage of AlAs in the waveguide layers shifts the drop in the differential quantum efficiency of the lasers to higher pump currents, despite the decrease in the energy depth of the quantum wells in the active region.

Gallium Arsenide Photonic Integrated Circuit Platform for Tunable Laser Applications

An active-passive integration technique for operation near a wavelength of 1030 nm has been developed on a gallium arsenide (GaAs) photonic integrated circuit platform. The technique leverages quantum wells (QWs) that are slightly offset vertically from the center of the waveguide, and selectively removed prior to upper cladding regrowth to form active and passive regions. The active region consists of indium gallium arsenide (InGaAs) QWs, gallium arsenide phosphide (GaAsP) barriers, GaAs separate confinement heterostructure layers, and aluminum gallium arsenide (AlGaAs) cladding. Fabry Perot lasers with various widths were fabricated and characterized, exhibiting high injection efficiency of 98.8%, internal active loss of 3.44 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , and internal passive loss of 4.05 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for 3 μm wide waveguides. The 3 μm, 4 μm, and 5 μm wide lasers demonstrated greater than 50 mW output power at 100 mA continuous wave (CW) current and threshold current as low as 9 mA. 20 μm wide broad area lasers demonstrated 240 mW output power, 35.2 mA threshold current under CW operation, and low threshold current density of 94 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 2 mm long lasers. Additionally, these devices exhibit transparency current density of 85 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and good thermal characteristics with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 205 K, and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</sub> = 577K.

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.

Large-Signal Equivalent Circuit for Datacom VCSELs

Increasing the baud rate in optical interconnects (OIs) will require the use of more sophisticated driver and receiver electronics. This will help overcome the stagnated bandwidth of the Vertical-Cavity Surface-Emitting Laser (VCSEL) and the pin-photodetector. Next generation OIs operating at single lane rates of 50+ Gbaud will therefore require careful co-optimization of the electronics and the optoelectronics. To facilitate this work there is a need of an accurate equivalent circuit for the optoelectronic components, functioning over a broad drive current and ambient temperature range. The VCSEL is the most important and complex to model due to its non-linear behavior and strongly varying characteristics with drive current and ambient temperature. For this purpose, a large-signal equivalent circuit dedicated for high-speed datacom VCSELs has been developed and is presented here. The distributed electrical parasitics in the device layout are carefully considered, the intrinsic speed limitation from carrier transport effects in the Separate-Confinement-Heterostructure (SCH) and the carrier-photon interaction in the Quantum Wells (QWs) are included, and self-heating effects in the device are monitored. The circuit is purposely based on physical instead of empirical models so that it can provide usable feedback to VCSEL designers. For circuit demonstration, it is implemented in Keysight's Advanced Design System (ADS) software and thereafter applied to replicate the performance of a state-of-the-art 28-GHz-bandwidth VCSEL at different temperatures and drive currents. Comparison is made between simulated and measured steady-state characteristics, small-signal behavior, and large-signal response under 28 Gbaud On-Off-Keying (OOK) and Pulse-Amplitude-Modulation 4 (PAM4) modulation, showing good agreement.

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.

A Compact High-Efficient Equivalent Circuit Model of Multi-Quantum-Well Vertical-Cavity Surface-Emitting Lasers for High-Speed Interconnects

Due to their low power consumption, high modulation speed, and low cost, vertical-cavity surface-emitting lasers (VCSEL) dominate short-reach data communications as the light source. In this paper, we propose a compact equivalent circuit model with noise effects for high-speed multi-quantum-well (MQW) VCSELs. The model comprehensively accounts for the carrier and photons dynamisms of a MQW structure, which includes separate confinement heterostructure (SCH) layers, barrier (B) layers, and quantum well (QW) layers. The proposed model is generalized to various VCSEL designs and accommodates a flexible number of quantum wells. Experimental validation of the model is performed at 25 Gb/s with a self-wire-bonded 850 nm VCSEL.

1.5 – 1.6 μm semiconductor lasers with an asymmetric periodic optically coupled waveguide

High-power 1.5 – 1.6-μm semiconductor lasers with an asymmetric periodic optically coupled waveguide are developed and their current – voltage, light – current, and spectral characteristics are experimentally studied. The characteristics of these lasers are compared with the characteristics of traditional lasers based on double separate-confinement heterostructures. It is shown that the developed lasers have lower divergence and almost the same threshold and power characteristics as conventional lasers with Fabry – Perot resonators. The developed lasers with a cavity length of 1.6 mm and a mesa-stripe contact width of 3 μm mounted in a housing 11 mm in diameter have a power no lower than 200 mW at a pump current not exceeding 700 mA with a divergence of 25 – 35° versus 45° typical for conventional lasers.

The influence of waveguide doping on the output characteristics of AlGaAs/GaAs lasers

The influence of doping of waveguide layers on the output characteristics of lasers based on AlGaAs/GaAs double separate-confinement heterostructures is analysed. The heterostructures with narrow and broadened waveguides are studied. Samples of laser diode bars with undoped and doped waveguide layers are experimentally fabricated and compared. It is shown that the latter type of structures with a broadened waveguide allows one to increase the output power of the laser diode bars by 10 % – 15 %, all other conditions being equal.

О токовой зависимости эффективности инжекции и относительном вкладе скорости эмиссии и внутренних оптических потерь в насыщение ватт-амперной характеристики мощных импульсных лазеров (&amp;lambda;=1.06 мкм)

The results of numerical simulation of the current dependence of the injection efficiency in the active area of the laser based on separate confinement heterostructures are presented. The feature of the transfer of charge carriers through isotype N-n heterotransitions on the interface boundary of waveguide and active areas is shown. Using the classic dependencies of the Drude-Lorentz theory, the cross-section of electrons and holes for the GaAs waveguide was evaluated. The resulting values of σe= 1.05∙10-18 cm2 and σp= 1.55∙10-19 cm2 and current dependencies of the injection efficiency allowed to determine the root-cause reason for the pulse power saturation of semiconductor lasers. It has been established that saturation of power-current characteristics is dominated by holes escape from the active region to the waveguide and internal optical losses are lower confinement factors.

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.