Quantum Well Intermixing Process Research Articles

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.

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.

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.

Quantum well intermixing technique using proton implantation for carrier confinement of vertical-cavity surface-emitting lasers

We investigated quantum well intermixing (QWI) using proton implantation to form the carrier confinement structure in the active layer of a vertical-cavity surface-emitting laser (VCSEL). The required potential barrier height is discussed referring to the result of numerical analysis. The bandgap change due to the QWI was investigated experimentally for various quantum well structures, proton dose densities, and thermal annealing conditions. A potential barrier height of 30 meV was observed using a high-indium and thin-well structure. High crystalline quality was confirmed by photoluminescence intensity measurement, even after the QWI process, and the lasing of the fabricated QWI-VCSEL was observed without any deterioration. The proposed technique would be effective in improving the device performance in a simple fabrication process.

Photonic Integrated Circuits Based on Quantum well Intermixing Techniques

The passive sections of a monolithic device must have a wider bandgap than the active regions to reduce losses due to direct interband absorption. Such bandgap engineering is usually realized by complicated regrown butt-joint or selective-area growth techniques. We, however, have developed a simple, flexible and low-cost alternative technique – quantum well intermixing (QWI) – to increase the bandgap in selected areas of an integrated device post-growth. To verify the QWI process, we have fabricated the following demonstrators: a 40 GHz semiconductor mode-locked laser producing pulses as short as 490 fs; a 10 GHz passively mode-locked extended cavity laser integrated with surface-etched distributed Bragg reflector (DBR) which can be tuned in both wavelength and pulse repetition rate; four 10 GHz 1.55 μm AlGaInAs/InP mode-locked surfaced-etched DBR lasers integrated combiner, a semiconductor optical amplifier and modulator where the four channels can be operated separately or simultaneously; a CWDM source with 12 nm wavelength separation based on an AlGaInAs/InP integrated distributed feedback laser array; and a 1.55μm DFB laser monolithically integrated with power amplifier array. In all these applications, QWI has the advantage of eliminating crystal regrowth and the associated stringent tolerance requirements that are required in traditional integration schemes.

Fabrication of an electro-absorption modulated distributed feedback laser by quantum well intermixing with etching ion-implantation buffer layer

We report the fabrication details of a monolithically integrated electro-absorption modulated distributed feedback laser (EML) based on the ion-implantation induced quantum well intermixing (QWI) technique. To well-preserve material quality in the laser region, thermal-oxide SiO2 is deposited before implantation and the ion-implantation buffer layer is etched before annealing. Thirteen pairs quantum well and barrier are employed to compensate deterioration of the modulator’s extinction ratio (ER) caused by the QWI process. The fabricated EML exhibits an 18 dB static ER at 5 V reverse bias. The 3 dB small signal modulation bandwidth of modulator is over 13.5 GHz indicating that this EML is a suitable light source for over 16 Gb/s optical transmission links.

Chemical evolution of InP/InGaAs/InGaAsP microstructures irradiated in air and deionized water with ArF and KrF lasers

Irradiation of quantum semiconductor microstructures with ultraviolet pulsed lasers could induce surface defects and modify chemical composition of the microstructure capping material that during high-temperature annealing leads to selected area bandgap engineering through the process known as quantum well intermixing (QWI). In this work, we investigate the role of both ArF and KrF excimer lasers in the QWI process of InP/InGaAs/InGaAsP microstructures irradiated in air and deionized (DI) water. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy analysis was employed to study the chemical composition of the irradiated surface and investigate the chemical evolution of ArF and KrF laser irradiated microstructures. The results indicate that InPxOy oxides are the dominating surface products of the ArF and KrF lasers interaction with InP. Consistent with this observation is a relatively greater bandgap blue shift of ∼130nm found in the microstructures irradiated in air, in comparison to a maximum of 60nm blue shift observed in the microstructures irradiated in a DI water environment.

10-GHz Mode-Locked Extended Cavity Laser Integrated With Surface-Etched DBR Fabricated by Quantum-Well Intermixing

The 10-GHz passively mode-locked AlGaInAs/InP 1.55- μm extended cavity lasers integrated with optimized surface-etched distributed Bragg mirrors have been fabricated. A quantum-well intermixing process was used to provide low-absorption loss gratings with accurate wavelength control. The lasers produce 2.99-ps sech2-pulses with a time-bandwidth product (TBP) of 0.51.

Quantum-well Intermixing Process for Large Blueshifting in an Ion-implanted InGaAs/InGaAsP Multiple-quantum-well Structure Using Two-step Annealing

Photonics/Sensor System Center, Korea Institute of Science and Technology (KIST), Seoul 136-791(Received 13 September 2010, in final form 6 October 2010)We describe studies on implantation-enhanced quantum-well intermixing in a lattice-matchedInGaAs/InGaAsP multiple-quantum-well p-i-n heterostructure. Samples are implanted with a doseof 5 × 10

Cap Layer Influence on Impurity-Free Vacancy Disordering of InGaAs/InP Quantum Well Structure

Quantum well intermixing (QWI) by the impurity-free vacancy disordering (IFVD) technique is an important and effective approach for the monolithic integration of optoelectronic devices based on InGaAs/InP quantum well structures. We experimentally investigate the influence of the capping layer SiO2 and Si3N4 on the QWI by IFVD. The results show that for all the samples with three-types differently doped (P, N and I) top InP layers, Si3N4 can always induce a larger photoluminescence blueshift than SiO2 in the IFVD QWI process, which attributes more to the group III and V vacancies point defects created in the interface of Si3N4-InP than that of SiO2-InP, proved by the SIMS measurements. The inherent mechanisms for explaining these properties are further discussed.

Quantum well intermixing of a quantum well structure grown on an InAsP metamorphic pseudosubstrate on InP

InGaAsP quantum well (QW) structures, with ∼1% compressive strain, have been grown on an InAs0.21P0.79 metamorphic substrate layer (MSL) deposited on an InP substrate. For comparison, a similar QW structure is grown directly on InP. A consequence of growth on the MSL is to move the QW and barrier compositions outside the spinodal isotherm resulting in a significant reduction in phase separation. This is shown to increase the photoluminescence wavelength and improve its quality in terms of linewidth and intensity. Thermally induced QW intermixing (QWI) has also been carried out on these structures both with and without a low-temperature InP capping layer. The defects present in the underlying metamorphic buffer layers are shown to have no effect on the QWI process. However, the samples with the MSL are shown to exhibit more intermixing either due to a higher diffusion or greater P–As exchange probability in the different composition QW and barrier layers compared to the QW structure grown without the MSL, and possibly influenced by the large reduction in phase separation.

Design of a Grating-Assisted Lateral Directional Coupler by Impurity-Induced Quantum-Well Intermixing of InGaAs/GaAs

An asymmetric waveguide grating-assisted coupler, which is suitable for a coarse wavelength division multiplexing system, based on a quantum-well intermixing process in InGaAs/GaAs, has been designed. The proposed device processing is similar to that used in nominal microelectronics processing. The device has been modeled from the first principle of refractive- index change due to F-implantation and anneal. The coupler has been analyzed by quasi-static effective index as well as finite difference methods in conjunction with an orthonormal-mode beam propagation method. A power-coupling ratio of ~90% at a wavelength of 1.51 mum with a coupled wavelength bandwidth of ~22 nm has been obtained. The coupler lengths are within a few millimeters, and the device is suitable for multiwavelength integration.

UV laser controlled quantum well intermixing in InAlGaAs/GaAs heterostructures

The influence of surface irradiation of GaAs with a KrF excimer laser on the magnitude of the quantum well intermixing (QWI) effect has been investigated on GaAs/AlGaAs and GaAs/AlGaAs/InAlGaAs QWs heterostructures. The selective area irradiation through a SiOx mask was carried out in an atmospheric environment. Following the 1000 pulses irradiation at 100 mJ/cm2, the samples were annealed in a rapid thermal annealing furnace at 900 °C. Photoluminescence mapping and cathodoluminescence measurements show that significant laser-induced suppression of the QWI process can be achieved with lateral resolution of the order of 1μm.

Analysis of inductively coupled argon plasma-enhanced quantum-well intermixing process for multiple bandgap implementation

The inductively coupled argon (Ar) plasma-enhanced quantum-well intermixing has been investigated for the implementation of multiple bandgaps in an InGaAs/InGaAsP quantum-well laser structure. Multiple bandgap control is achieved by controlling the local defect concentrations without manipulating the critical rapid thermal annealing process using a multi-step plasma exposure or a spatial defect modulation. The interdiffusion process is found to obey Fick's law and enhanced by the near-surface point defects after a short plasma exposure time. For a prolonged exposure, the enhancement is reduced attributed to the possible formation of defect clusters. With single-step plasma exposure, the defect concentrations, thus intermixing degree, can be controlled using a variable window opening. This work demonstrates the practical approaches of multiple bandgap creation suitable for the photonic-integrated circuit application.

Elimination of mode grouping in InGaAsP∕InP ridge waveguide laser using quantum-well intermixing

In this letter, we report the observation of the longitudinal mode grouping effect in the InGaAsP∕InP multiquantum-well ridge waveguide laser. Two mode groups were observed and showed different behaviors with the changing of the injection current. The higher order lateral modes and the current spreading in the ridge waveguide structure is proposed to be responsible for this mode grouping phenomena. A one-step quantum-well intermixing process using the SiO2 electrical isolation layer itself was introduced and proved to be effective in improving the lateral electrical and optical confinement in a ridge waveguide laser structure as well as eliminating the mode grouping.

Influence of composition diffusion on the band structures of InGaNAs∕GaAs quantum wells investigated by the band-anticrossing model

We investigate the influence of quantum-well intermixing (QWI) on the electronic band structure of GaInNAs∕GaAs multiquantum wells. The band structures and optical transitions have been calculated based on the band-anticrossing (BAC) model and Fick’s interdiffusion law for both intermixed and nonintermixed samples, respectively. The calculated results are consistent with the true optical transitions observed by photoluminescence excitation spectroscopy and secondary ion mass spectroscopy. Our investigation indicates that BAC model is valid for interdiffused quantum wells and verifies that the QWI process in GaInNAs∕GaAs multiquantum wells is induced mainly by the interdiffusion of In–Ga between the quantum wells and barriers.

Current injection tunable monolithically integrated InGaAs-InAlGaAs asymmetric Mach-Zehnder interferometer using quantum-well intermixing

A compact highly efficient wavelength (de)multiplexer with channel spacing of 100 GHz has been realized with a monolithically integrated asymmetric 2 /spl times/ 2 Mach-Zehnder interferometer using a single step sputtered SiO/sub 2/-annealing quantum-well intermixing process. Wavelength tunability and asymmetric loss rebalancing of the device is demonstrated using current injection. Wavelength tuning over 0.8 nm is achieved with only 6 mA of injected current. An extinction ratio of more than 27 dB is obtained.

InGaAs/InGaAsP Quantum-Well Engineering for Multiple Regrowth MOVPE Process

ABSTRACTFor the heterogeneous integration of several layer structures for absorber, gain and passive waveguide sections in a monolithically-integrated mode locked laser diode, the bandgap of the absorber section has to be matched to the emission wavelength of the gain section. Because of the use of a multiple regrowth process for optical butt-coupling, the first grown multiple quantum-well gain material undergoes a quantum-well intermixing process, resulting in a blue shift of the emitting optical wavelength. Experimental results show, that the blue shift is dependent on the process details and cannot be investigated by simple thermal cycling of unprocessed quantum well-structures. With the introduction of an effective quantum-well width computed from the emission wavelength we found a linear relationship between the effective quantum well width shrinkage and the cumulated regrowth heating time of 8.3Å/h at a growth temperature of 630°C. Therefore knowing the cumulated regrowth time for a laser fabrication, we could successfully design the initial quantum well thickness that yields the targeted emitting wavelength and excellent matching to the absorber bandedge.

Improvement in spatial resolution of plasma-enhanced quantum-well intermixing by stress-inducing dielectric mask

We report the use of a stress-inducing dielectric mask to improve the spatial resolution of the proximity quantum-well intermixing process. Photoluminescence and Raman spectroscopy were used to study the band gap modification and the spatial resolution using Ar plasma in the InGaAs∕InGaAsP laser structure. A spatial resolution of 2.4μm has been achieved with the presence of an SixNy annealing cap as a stress-inducing mask. The simple technique provides a promising approach of lateral band gap tuning with a high spatial resolution for high-density photonic integrated circuits.

Enhancement of the Spectral Width of High-Power1.5 μm Integrated Superluminescent Light Source by Quantum WellIntermixing Process

A novel type of integrated InGaAsP superluminescent light source wasfabricated based on the tilted ridge-waveguide structure withselective-area quantum well (QW) intermixing. The bandgap structurealong the length of the device was modified by impurity free vacancydiffusion QW intermixing. The spectral width was broadened from the 16 nm ofthe normal devices to 37 nm of the QW intermixing enhanced devices at the sameoutput power level. High superluminescent power (210 mW) was obtainedunder pulsed conditions with a spectral width of 37 nm.