Quantum-well Intermixing Technique Research Articles

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.

Multi-bandgap photonic materials and devices fabricated by UV-laser induced quantum well intermixing

Ultraviolet (UV)-laser induced quantum well intermixing (QWI) technique can generate large multiple bandgap blue shifts in III-V quantum well semiconductor heterostructure. The application of the UV-laser QWI technique to fabricate multi-bandgap photonic devices based on compressively strained InGaAsP/InP quantum well laser microstructure is reported. We show that under certain UV-laser irradiation conditions, the photoluminescence (PL) intensity can be enhanced, and the full width at half maximum (FWHM) linewidth can be reduced. The blue shift of bandgap can reach as large as 145 nm, while the PL intensity is about 51% higher than that of the as-grown material. Experimental results of post growth wafer level processing for the fabrication of bandgap-shifted waveguides and laser diodes are presented.

UV laser induced selective-area bandgap engineering for fabrication of InGaAsP/InP laser devices

Large bandgap blueshifts in III–V quantum semiconductor microstructures are achievable with UV-laser induced quantum well intermixing (QWI). We report on the application of the UV-laser QWI technique to investigate bandgap engineering of a compressively strained InGaAsP/InP quantum well laser microstructure. The attractive performance of the technique, determined by the ability of a laser to generate point defects, has been demonstrated with bandgap blueshifts reaching 142nm, with enhancement of photoluminescence intensity. We have also investigated this technique for post-growth wafer level processing designed for the fabrication of laser diodes.

Monolithically Integrated Mach-Zehnder Interferometer All-Optical Switches by Selective Area MOVPE

We achieved first dynamic all-optical signal processing with a bandgap-engineered MZI SOA all-optical switch. The wide-gap Selective Area Growth (SAG) technique was used to provide multi-bandgap materials with a single step epitaxy. The maximum photoluminescence (PL) peak shift obtained between the active region and the passive region was 192 nm. The static current switching with the fabricated switch indicated a large carrier induced refractive index change; up to 14 π phase shift was obtained with 60mA injection in the SOA. The carrier recovery time of the SOA for obtaining a phase shift of n was estimated to be 250-300 ps. A clear eye pattern was obtained in 2.5 Gbps all-optical wavelength conversion. This is the first all-optical wavelength conversion demonstration with a bandgap-engineered PIC with either selective area growth or quantum-well intermixing techniques.

Quantum-well-intermixed monolithically integrated widely tunable all-optical wavelength converter operating at 10 Gb/s

This letter reports on an InP Mach-Zehnder interferometer semiconductor optical amplifier all-optical wavelength converter monolithically integrated with a sampled grating distributed Bragg reflector laser. The device is fabricated using centered quantum wells to achieve a high modal overlap with the active region. The active and passive waveguide regions are defined using an impurity-free quantum-well intermixing technique. The device is shown to perform error-free at 10-Gb/s data rates with a 30-nm output tuning range. The device's output power is also measured to be >3 dBm across the range.

Monolithically integrated InGaAs-AlGaInAs Mach-Zehnder Interferometer optical switch using quantum-well intermixing

A highly efficient monolithically integrated Mach-Zehnder interferometer (MZI) optical switch based on a 1.5-/spl mu/m InGaAs-AlGaInAs multiple-quantum-well structure is reported. The device was fabricated by integrating phase shifter sections with bandgap-shifted low-loss waveguides obtained by a single step sputtered SiO/sub 2/-annealing quantum-well intermixing technique. Evaluation of the refractive index change induced by current injection (plasma, band-filling, band-shrinkage effects) and applied electric field (quantum confined Stark effect) based on the MZI was investigated. A device with an active length of 400 /spl mu/m in one of the MZI arms had an extinction ratio /spl ges/20 dB with a half-wavelength current (I/sub /spl pi//) and half-wavelength voltage V/sub /spl pi// as low as 3.2 mA and 3.5 V (1.9 V in push-pull configuration), respectively.

Multiple-wavelength integration in InGaAs-InGaAsP structures using pulsed laser irradiation-induced quantum-well intermixing

In this paper, we present the characteristics of a quantum-well intermixing technique using pulsed-photoabsorption-induced disordering. Photoluminescence, micro-Raman spectroscopy, and transmission electron microscopy were used to characterize the process. Using this technique, a differential wavelength shift between the intermixed and nonintermixed regions of over 160 nm has been observed from InGaAs-InGaAsP heterostructures. It was found from the micro-Raman measurements that a spatial resolution of better than 2.5 /spl mu/m can be achieved. A theoretical model has been developed to estimate the spatial resolution limit. Theoretical analysis has also been performed to investigate the effect of laser irradiation on the degree of intermixing in InGaAs-InGaAsP structures. To verify the capability of this process in monolithic photonic integration, high-quality bandgap tuned lasers, two-section extended cavity lasers, and multiple-wavelength laser chips have been fabricated.

Enhanced wavelength shift in partly gain-coupled distributed feedback lasers using quantum well intermixing

In this paper, we present a theoretical study on the properties of wavelength shift due to quantum well (QW) intermixing in the partly gain-coupled distributed feedback (DFB) lasers. A maximum possible wavelength red-shift of 9.2 nm is predicted if using a proper DFB grating, whose period is smaller than that in the conventional DFB lasers. In contrast to the index-coupled λ/4-shifted DFB lasers, no increase in carrier density is required to compensate for the gain reduction due to QW intermixing, as the threshold gain in the partly gain-coupled DFB laser decreases with the QW intermixing because of the increase in the coupling coefficient. The injected threshold current density for lasing is considerably flat in the wavelength red-shift range. The result is important as it indicates that the QW intermixing technique may be used in the fabrication of the equal threshold DFB lasers with different wavelengths in the monolithically integrated laser arrays.

Monolithic fabrication of 2 /spl times/ 2 crosspoint switches in InGaAs-InAlGaAs multiple quantum wells using quantum-well intermixing

In this letter, we report the fabrication of 2 /spl times/ 2 crosspoint switches, which monolithically integrate passive waveguides, electro-absorption modulators and optical amplifiers onto one chip using sputtered SiO/sub 2/ quantum-well intermixing technique. The switches have low insertion loss to be about 4-5 dB and extinction ratios up to 26 dB.

Demonstration of a DFB laser with an integrated electro-absorption modulator produced using a novel quantum-well intermixing technique

Bandgap engineering is used extensively in the telecommunications field. Quantum well intermixing (QWI) is a post-growth method of bandgap engineering that has been studied for the integration of photonic devices (Marsh et al., J. Vac. Sci. Technol. A16 (1998) 810). A novel method of QWI has been discovered that uses layers of indium phosphide grown by helium-plasma-assisted gas source molecular beam epitaxy, subsequently referred to as He*-InP, in combination with thermally induced QWI. The He*-InP has been shown to contain large numbers of defects that can act as fast non-radiative recombination centres. These can be used with the thermally induced QWI to produce regions with a larger bandgap and containing the fast, non-radiative defect centres. This novel intermixing process has been used to fabricate integrated distributed feedback lasers and electro-absorption modulators.

Dual-wavelength laser source monolithically integrated with Y-junction coupler and isolator using quantum-well intermixing

A dual-wavelength laser source monolithically integrated with an isolator and a Y-junction coupler is fabricated by using a new quantum-well intermixing technique. The technique employs a buried Ge layer between the sample surface and the spin-on silica film to control the bandgap tuning in selective areas across a wafer. The integrated isolator can avoid the crosstalk between the two channels of the device. Two distinct lasing wavelengths of 950 and 969 nm are coupled into one single output port through the transparent Y-junction coupler. The two channels have similar threshold current and slope efficiency.

Broad optical bandwidth InGaAs-InAlGaAs light-emitting diodes fabricated using a laser annealing process

The use of a laser-induced quantum-well intermixing technique in the InGaAs-InAlGaAs material system is presented. We report blue shifts of up to 240 nm in the 1.55-μm emission wavelength, generated by exposure to a Nd:YAG laser. Variations in the optical intensity across the irradiating beam were used to laterally grade the bandgap along a sample. We used this technique to fabricate broad optical bandwidth, light-emitting diodes. The devices showed an increase in the full width half maximum of the emission spectrum from 125 nm in undisordered devices to over 260 nm in intermixed material. The output spectrum was also observed to be flat-topped (within 5%) across a wavelength range of 140 nm.

Modulation of the second-order nonlinear tensor components in multiple-quantum-well structures

In this paper, we present experimental results which demonstrate that quantum-well intermixing techniques can be used to modulate the magnitude of the second-order nonlinear coefficient /spl chi//sup (2)/. Impurity-free vacancy disordering with SiO/sub 2/ and Ga/sub 2/O/sub 3/ caps was used to modulate the position of the band edge and hence, the magnitude of /spl chi//sub eff//sup (2)/. Using a coupled quantum-well structure we were able to demonstrate modulation of the d/sub 33/ tensor components associated with the asymmetric structure and of the d/sub 14/ component associated with the bulk crystal structure.

Monolithic integration via a universal damage enhanced quantum-well intermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO/sub 2/, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3/spl deg/) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 /spl mu/m are demonstrated with low loss (/spl alpha/=4.1 cm/sup -1/) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration.

Monolithic integration in InGaAs-InGaAsP multiple-quantum-well structures using laser intermixing

The bandgap of InGaAs-InGaAsP multiple-quantum-well (MQW) material can be accurately tuned by photoabsorption-induced disordering (PAID), using a Nd:YAG laser, to allow lasers, modulators, and passive waveguides to be fabricated from a standard MQW structure. The process relies on optical absorption in the active region of the MQW to produce sufficient heat to cause interdiffusion between the wells and barriers. Bandgap shifts larger than 100 meV are obtainable using laser power densities of around 5 W/spl middot/mm/sup -2/ and periods of illumination of a few minutes to tens of minutes. This process provides an effective way of altering the emission wavelengths of lasers fabricated from a single epitaxial wafer. Blue shifts of up to 160 nm in the lasing spectra of both broad-area and ridge waveguide lasers are reported. The bandgap-tuned lasers are assessed in terms of threshold current density, internal quantum efficiency, and internal losses. The ON/OFF ratios of bandgap-tuned electroabsorption modulators were tested over a range of wavelengths, with modulation depths of 20 dB obtained from material which has been bandgap-shifted by 120 nm, while samples shifted by 80 nm gave modulation depths as high as 27 dB. Single-mode waveguide losses are as low as 5 dB/spl middot/cm/sup -1/ at 1550 mm. Selective-area disordering has been used in the fabrication of extended cavity lasers. The retention of good electrical and optical properties in intermixed material demonstrates that PAID is a promising technique for the integration of devices to produce photonic integrated circuits. A quantum-well intermixing technique using a pulsed laser is also demonstrated.

Fabrication of multiple wavelength lasers in GaAs-AlGaAs structures using a one-step spatially controlled quantum-well intermixing technique

We have applied a new technique, based on impurity-free vacancy diffusion, to control the degree of intermixing across a wafer. Bandgap tuned lasers were fabricated using this technique. Five distinguishable lasing wavelengths were observed from five selected intermixed regions on a single chip. These lasers showed no significant change in transparency current, internal quantum efficiency or internal propagation loss, which indicates that the material quality was not degraded after intermixing. >

Fabrication of photonic integrated circuits using quantum well intermixing

Intermixing the wells and barriers of quantum well structures generally results in an increase in the band gap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing, have been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer. Such process offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICs). Recent progress in QWI techniques is reviewed, concertrating on processes which are compatible with PIC applications, and illustrated with device demonstrators.

Quantum well intermixing

Intermixing the wells and barriers of quantum well structures generally results in an increase in the band gap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing has been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer; such processes offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICS). Recent progress in QWI techniques is reviewed, concentrating on processes which are compatible with PIC applications, in particular the achievement of low optical propagation losses.