Quantum Well Intermixing Research Articles

1.3-µm identical active electro-absorption modulated laser with quantum well intermixed passive waveguide.

Quantum well intermixing (QWI) is an effective method for simple and well-defined monolithic integration of photonic devices. We introduce an identical-active electro-absorption modulated laser (IA-EML) with optimized QWI, which is applied to reduce the absorptive waveguide region. To determine the optimal intermixed IA-EML structure, we conduct a comparative analysis between the cases of an IA-EML with only an intermixed waveguide region and with both intermixed waveguide and electro-absorption modulator (EAM) regions, as well as the case without QWI. The results reveal that the intermixed region effectively inhibits the absorption in the waveguide. In particular, the IA-EML with only waveguide intermixing exhibits superior modulation characteristics with low driving voltages and a high extinction ratio. Our work provides an attractive approach for suppressing the absorptive waveguide region in the IA-EML to enhance modulation performance and to develop photonic integrated circuits with a simplified process.

Design of low-threshold photonic-crystal surface-emitting lasers with confined gain regions by using selective area intermixing

Photonic-crystal surface-emitting lasers have many promising properties over traditional semiconductor lasers and are regarded as the next-generation laser sources. However, the minimum achievable lasing threshold of PCSELs is still several times larger than that of VCSELs, and limiting its applications especially if the required power is small. Here, we propose a new design that reduces the gain region in the lateral plane by using selective quantum-well intermixing to reduce the threshold current of PCSELs. By performing theoretical calculations, we confirmed that the threshold current can be lowered by a factor of two to three while keeping the PCSEL’s advantage of small divergence angle.

Effects of Thermal-Strain-Induced Atomic Intermixing on the Interfacial and Photoluminescence Properties of InGaAs/AlGaAs Multiple Quantum Wells.

Quantum-well intermixing (QWI) technology is commonly considered as an effective methodology to tune the post-growth bandgap energy of semiconductor composites for electronic applications in diode lasers and photonic integrated devices. However, the specific influencing mechanism of the interfacial strain introduced by the dielectric-layer-modulated multiple quantum well (MQW) structures on the photoluminescence (PL) property and interfacial quality still remains unclear. Therefore, in the present study, different thicknesses of SiO2-layer samples were coated and then annealed under high temperature to introduce interfacial strain and enhance atomic interdiffusion at the barrier-well interfaces. Based on the optical and microstructural experimental test results, it was found that the SiO2 capping thickness played a positive role in driving the blueshift of the PL peak, leading to a widely tunable PL emission for post-growth MQWs. After annealing, the blueshift in the InGaAs/AlGaAs MQW structures was found to increase with increased thickness of the SiO2 layer, and the largest blueshift of 30 eV was obtained in the sample covered with a 600 nm thick SiO2 layer that was annealed at 850 °C for 180 s. Additionally, significant well-width fluctuations were observed at the MQW interface after intermixing, due to the interfacial strain introduced by the thermal mismatch between SiO2 and GaAs, which enhanced the inhomogeneous diffusion rate of interfacial atoms. Thus, it can be demonstrated that the introduction of appropriate interfacial strain in the QWI process is of great significance for the regulation of MQW band structure as well as the control of interfacial quality.

Sputtering Al2O3 enhanced bandgap engineering for integrated photonic devices

We present experimental results of the application of Sputtering enhanced Quantum Well Intermixing (QWI) process, where the bandgap of InGaAsP/InP quantum well laser microstructure is effectively modified by intermixing quantum well and potential barrier layers through sputtering Al2O3-QWI. This technique creates point defects through plasma induced disordering in the process of sputtering Al2O3. The subsequent Rapid thermal Anneal (RTA) process is performed to increase the diffusion rate and quantum well intermixing. This results in an increased transitional energy and hence it modifies the bandgap essential for the development of next generation complex photonic integrated circuits (ICs). We report a large bandgap blue shift of 144 nm is achievable with a very high increase in photoluminescence (PL) intensity. The experimental results of the bandgap shifted waveguides and laser diodes are presented. Photoluminescence (PL) measurements of InGaAsP/InP laser structure were taken before and after Al2O3-QWI process. The proposed technique can provide reliable and fast post-growth wafer level fabrication of photonic devices monolithically.

Simulation of an AlGaInAs/InP Electro-Absorption Modulator Monolithically Integrated with Sidewall Grating Distributed Feedback Laser by Quantum Well Intermixing

A novel AlGaInAs/InP electro-absorption modulated laser (EML) with a simple fabrication process is proposed, in which the electro-absorption modulator (EAM) has a 10 nm blueshift induced by quantum well intermixing (QWI) and is monolithically integrated with a sidewall grating distributed-feedback (DFB) laser working at 1.55 μm wavelength. The extent of the QWI process is characterized by a diffusion length. The quantum confined Stark effect (QCSE) is simulated in terms of extinction ratio (ER) and chirp for bias electric fields from 0 kV/cm to 200 kV/cm and for different amounts of intermixing. The results indicate that for a 150 µm-long EAM with a 10 nm blueshift induced by QWI, an ER of 40 dB is obtained at 2.5 V reverse bias with no penalty in chirp compared to an as-grown quantum well (QW) and the insertion loss at 0 V bias is 0.11 dB for 1.55 µm operation wavelength. The simulated –3 dB bandwidth of the electrical to optical power response is 22 GHz.

Monolithically Integrated 8 × 8 Transmitter-Router Based on Tunable V-Cavity Laser Array and Cyclic Arrayed Waveguide Grating Router

A monolithic 8 × 8 transmitter-router chip fabricated on InGaAlAs-InP multi-quantum well wafer for distributed wavelength routing network in the O-band with a channel spacing of 400 GHz is experimentally demonstrated. A tunable V-cavity laser (VCL) array, a cyclic-arrayed waveguide grating router (AWGR) and a semiconductor amplifier (SOA) array are integrated using the quantum-well intermixing (QWI) technique for its fabrication simplicity and cost effectiveness. A 200 GHz-spaced compact-size VCL is demonstrated with a 27 nm tuning range and a side-mode suppression ratio (SMSR) around 40 dB using a single-electrode controlled tuning. The chip output power is 24.6 mW. Measurement results of the transmitter-router chip show that all 64 input-output combinations have cyclic response spectra. After passing through the SOA with 80 mA bias current, an output power of about 9.6 mW and optical signal-to-noise ratio (OSNR) up to 39 dB have been measured. The demonstrated transmitter-router chip is fabricated on the same quantum well structure without requiring multiple epitaxial growth and complex grating fabrication. The characteristics of multi-wavelength and multi-port transmissions enable the distributed optical routing networks with flexible bandwidth allocation, which can find wide application in datacenters and high-performance computers.

GaInAs/GaAs quantum well intermixing based on SiO2–Cu composite film

Quantum well intermixing (QWI) is widely used in bandgap tuning of optoelectronic materials. In this paper, GaInAs/GaAs QWI is studied based on SiO2–Cu composite layers. The effects of annealing parameter and Cu layer thickness on the photoluminescence (PL) characteristics are analyzed. The results show that the PL blue shift gradually increases with the rise of annealing temperature, and the PL intensity exhibits a peak rise at a temperature that is related to the Cu layer thickness. It is also found that the PL blue shift increases when the thickness of the Cu layer becomes large in the composite film. In addition, multi-cycle annealing can be used to improve the PL properties of the samples, while the PL can have a large blue shift. Compared with SiO2 cap layer, SiO2–Cu composite film exhibits a more significant effect on QWI for GaInAs/GaAs quantum well (QW) materials.

Monolithic integration of electro-absorption modulators and photodetectors on III-V CMOS photonics platform by quantum well intermixing.

Quantum well intermixing (QWI) on a III-V-on-insulator (III-V-OI) substrate is presented for active-passive integration. Shallow implantation at a high temperature, which is essential for QWI on a III-V-OI substrate, is accomplished by phosphorus molecule ion implantation. As a result, the bandgap wavelength of multi-quantum wells (MQWs) on a III-V-OI substrate is successfully tuned by approximately 80 nm, enabling the monolithic integration of electro-absorption modulators and waveguide photodetectors using a lateral p-i-n junction formed along the InP/MQW/InP rib waveguide. Owing to the III-V-OI structure and the rib waveguide structure, the parasitic capacitance per unit length can be reduced to 0.11 fF/µm, which is suitable for high-speed and low-power modulators and photodetectors. The presented QWI can extend the possibility of a III-V complementary metal-oxide-semiconductor (CMOS) photonics platform for large-scale photonic integrated circuits.

Monolithically Integrated AlGaInAs MQW Polarization Mode Converter Using a Stepped Height Ridge Waveguide

An AlGaInAs multiple-quantum-well (MQW) polarization mode controller (PMC) using a stepped height ridge waveguide is presented, which is monolithically integrated with a sidewall grating distributed feedback laser using quantum well intermixing (QWI). QWI is used to create a 100 nm blueshift in the PMC and to partially eliminate the anisotropy and birefringence of the MQW structure. The PMC structure is modelled and optimized using a 3D full-vectorial Finite-Element Method package. The maximum polarization conversion efficiency (PCE) is around 96% for a 537-μm-long PMC operating at a wavelength of 1550 nm. To maintain a PCE of ≥90%, the fabrication tolerances of the dry-etch corner and ridge waveguide widths are ± 0.05 μm and ± 0.03 μm respectively. The main advantages of the proposed design are that only a single step of MOPVE and two steps of dry etching are required for the whole integrated device, significantly reducing complexity and cost.

Influence of SixNy Deposition Parameters on Intermixing of Quantum Wells

Influence of SixNy Deposition Parameters on Intermixing of Quantum Wells

Design and Optimization of 1.55 μm AlGaInAs MQW Polarization Mode Controllers

A 1.55 μm AlGaInAs multi-quantum-well (MQW) ridge waveguide polarization mode controller (PMC) is proposed. The design is based on an asymmetric half-ridge waveguide structure in which the ridge is shallow etched on one side and has a deeply etched mesa structure on the other side. The Finite-Element Method (FEM) was used to simulate the PMC and optimize its structural parameters comprehensively. Furthermore, the fabrication tolerances were also investigated in detail. The optimized PMC has a polarization conversion efficiency (PCE) of around 92.5% with a half-beat length of 1250 μm. When the PMC length was fixed at 1250 μm, to achieve a PCE derivation less than 8%, the tolerances for the ridge waveguide width and shallow etch height were 1.60 μm to 1.65 μm and 2.13 μm to 2.18 μm, respectively. In order to reduce interband gap absorption loss, the quantum well intermixing (QWI) technique was used in the model to realize a blueshift (200 nm) in the PMC. QWI is a simple, flexible, and low-cost technique for fabricating a PMC integrated with a laser diode and reduces parasitic reflections, which would otherwise degrade the overall performance. QWI also eliminates MQW material anisotropy and alleviates the birefringence effect without the need for regrowth, achieving nearly uniform properties as a bulk material.

Study on the proximity of QWI in InP-based AlGaInAs MQWs using the IFVD method and its application in single frequency teardrop laser diodes.

This paper presents our research on quantum well intermixing (QWI) of InP-based AlGaInAs/AlGaInAs multi-quantum wells using impurity-free vacancy-disordering (IFVD) and the QWI mask proximity effect and its application in the design and fabrication of a teardrop laser. Using a Si3N4 film deposited by plasma-enhanced chemical vapor deposition (PECVD) as a QWI promoter mask and annealing under 700°C for 2 minutes, a 70 nm wavelength blue shift of a FP laser is achieved using InP-based AlGaInAs quantum well laser material. It is found that a 5 µm separation is needed between the QWI mask edges and the non-QWI area during the QWI process. Based on the QWI technique and proximity effect, the designed and fabricated teardrop laser demonstrated continuous wave (CW) lasing above 40 mA and single frequency operation with a side mode suppression ratio of 32.6 dB at 77.3 mA.

Low-Loss Buried-Heterostructure Optical Waveguide Based on Impurity-Free-Vacancy-Diffusion Quantum Well Intermixing

A new method for fabricating a high-quality buried-heterostructure optical waveguide using quantum well intermixing (QWI) has been demonstrated. By patterning a SiO2 thin film on top of a multiple quantum well (MQW) heterostructure, rapid thermal annealing (RTA) could induce laterally local QWI, resulting in a bandgap blueshift and a simultaneous decrease in the refractive index. Both lateral bandgap and index engineering could be attained along the MQW plane, which could be used for a buried-heterostructure optical waveguide. Two SiO2 strips with 3, 5 and 7 μm windows were fabricated for waveguide on a 1540 nm InGaAsP MQW sample. A 120 nm blueshift under the SiO2 area was observed, leading to the index contrast of 0.07. Far-field optical diffraction measurements were also performed to yield angles of 13.9°, 12.8° and 10.6°. A narrower window resulted in a narrower optical waveguide width and exhibited a larger diffraction angle, suggesting that QWI defined the buried optical waveguide. In addition, an electroabsorption modulator was also made by buried waveguide. A −10 dB low optical insertion loss and a 15 dB high extinction ratio in a 500 μm long waveguide were obtained, indicating that a buried heterostructure could be used for photonic devices and integration applications.

High-Power Indium Phosphide Photonic Integrated Circuits

Indium phosphide (InP) is the most developed platform for photonic integrated circuits (PICs). Of interest is the advancement of this platform for applications that demand high performance, especially high output power, including free space communications and microwave photonics. In this paper, we summarize development of InP-based PIC transmitters. Two transmitter types were fabricated: one based on an offset quantum wells (OQW) platform and the other on a quantum well intermixing (QWI) platform. The OQW-based transmitter consists of a widely tunable laser, a high-speed semiconductor optical amplifier (SOA), a Mach–Zehnder modulator, and an output SOA. This transmitter demonstrates a 44-nm tuning range, >45 dB side mode suppression ratio, 14.5 dBm off -chip power, and a data rate of 7 Gbps. The second transmitter, based on QWI, utilizes an alternate epitaxial structure to achieve a lower confinement factor for higher SOA output saturation power. This QWI transmitter consists of a distributed Bragg reflector laser, a high-speed SOA, an electroabsorption modulator, and an output SOA. The measured off -chip power is 19.5 dBm, and a data rate of 20 Gbps is demonstrated. Based on the improved performance with the new epitaxial structure, a novel platform for high-power PIC transmitters integrated with low confinement and high-power SOAs is described.

Tunable V-Cavity Lasers Integrated With a Cyclic Echelle Grating for Distributed Routing Networks

We demonstrate a monolithic InP-based photonic integrated circuit (PIC) consisting of four tunable V-cavity lasers (VCLs), a cyclic echelle diffraction grating router (EDGR) and four semiconductor optical amplifiers (SOAs). Quantum-well intermixing (QWI) technique is used to produce three different bandgap energies on the chip. Single VCL with 30 nm tuning range at 1.3 μm wavelength band and up to 40 dB side-modesuppression ratio (SMSR) is experimentally demonstrated. The chip has a 4.5 nm channel spacing and 18 nm free-spectral range (FSR). Measured spectrum of all the 16 input-output combinations show cyclic response.

Monolithically integrated InGaAsP multiple quantum well tunable laser diode for integrated optic surface plasmon resonance sensing

We demonstrate an InGaAsP multiple quantum well tunable laser diode consisting of two gain sections with different bandgap energy levels. This tunable laser is used as a probing source for an optical sensor that makes use of surface plasmon resonance (SPR). Lasers fabricated on a single monolithic substrate can have widely differing output wavelengths. Therefore, by controlling the extent of the intermixing, the bandgap energy of the material can be accurately tuned over a broad spectral range. The regions of different bandgap energies are achieved using selective area intermixing of the multiple quantum wells, through impurity-free vacancy-induced disordering. By varying the current combination that was injected to each section, the laser wavelength can be effortlessly tuned from 1538 to 1578 nm with relatively constant output power. The free spectral range of the tunable laser is found to be 0.25 nm. This tunable laser was launched into an optical SPR sensor head that was fabricated on an inverted rib dielectric waveguide to provide an input light source for the SPR sensor. The average sensitivity of the SPR sensor devices was determined to be S = 334 nm RIU − 1.

Supercritical fluid-enhanced IFVD quantum well intermixing for the regrowth-free photonic integration of EAM and SOA

In this study, we propose and demonstrate the enhancement of impurity-free vacancy diffusion (IFVD) quantum well intermixing (QWI) using supercritical fluid (SCF) treatment for photonic integration. Using an InGaAsP-based multiple quantum well (MQW) template for local bandgap engineering, SCF CO2 with H2O2 on a patterned SiOx capping layer could lower the QWI temperature to get a constant wavelength blueshift. Higher oxygen concentration was found in SiOx through high diffusivities and pressures of SCF and dehydration, enabling highly activated Ga2O3 reactions for more vacancies. Additionally, different schemes of semiconductor optical amplifier (SOA)-integrated electroabsorption modulators (EAM) were fabricated. SCF treatment could realize a 9- and 6-dB improvement in extinction ratio and gain, respectively, and then high-speed 45Gb/s modulation, greatly reducing the common issue of p-type dopant (zinc) diffusion during QWI.

Tunable laser diode using selectively intermixed InGaAsP multiple quantum wells

We demonstrate a tunable laser consisting of two gain sections with different bandgap energies using selective intermixing of InGaAsP multiple quantum wells. The selectivity in the intermixing process is achieved using different capping films during a high-temperature rapid annealing process. The region that was capped by a SiOyNx film had its bandgap energy blueshifted to 1530 nm while the region that was capped by a SiNx film had its bandgap energy unchanged at 1560 nm. Wavelength tuning is achieved by injecting current separately into the two gain sections. When the currents injected into the two sections are varied, the combination of the gain spectra leads to a laser wavelength in the overall effective gain whose peak position depends on the magnitudes of the two injected currents. The fabricated device was capable of producing laser emission that was tunable from 1538 to 1578 nm with almost constant output power from 1544 to 1566 nm.

Conservation of quantum efficiency in quantum well intermixing by stress engineering with dielectric bilayers

In semiconductor lasers, quantum well intermixing (QWI) with high selectivity using dielectrics often results in lower quantum efficiency. In this paper, we report on an investigation regarding the effect of thermally induced dielectric stress on the quantum efficiency of quantum well structures in impurity-free vacancy disordering (IFVD) process using photoluminescence and device characterization in conjunction with microscopy. SiO2 and SixO2/SrF2 (versus SrF2) films were employed for the enhancement and suppression of QWI, respectively. Large intermixing selectivity of 75 nm (125 meV), consistent with the theoretical modeling results, with negligible effect on the suppression region characteristics, was obtained. SixO2 layer compensates for the large thermal expansion coefficient mismatch of SrF2 with the semiconductor and mitigates the detrimental effects of SrF2 without sacrificing its QWI benefits. The bilayer dielectric approach dramatically improved the dielectric–semiconductor interface quality. Fabricated high power semiconductor lasers demonstrated high quantum efficiency in the lasing region using the bilayer dielectric film during the intermixing process. Our results reveal that stress engineering in IFVD is essential and the thermal stress can be controlled by engineering the dielectric strain opening new perspectives for QWI of photonic devices.

An efficient method based on FIR filtering and fourier transform for solving the eigen-problems in optoelectronic devices

This paper presents a highly efficient method based on finite impulse response filtering and Fourier transform techniques to solve the eigen-problems, especially with smoothly varying inhomogeneous-core, such as solving optical eigen modes in graded-index optical waveguides and electronic eigen states in intermixed quantum wells. This type of structure is normally less efficient to be dealt with in space domain directly, but bears with a narrow spectrum in spatial frequency domain. Simulation examples show that the computation cost of the proposed method is approximately at least one order of magnitude smaller than that of the conventional finite difference method solved by the most efficient multiple relatively robust representations method.