Impurity-free Vacancy Diffusion Research Articles

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.

Controllable red and blue shifting of InGaAsP quantum well bandgap energy for photonic device integration

We demonstrate bandgap tuning of InGaAsP multiple quantum well structures by utilizing an impurity-free vacancy diffusion technique. Substantial modification of the bandgap energy toward the red and blue parts of the spectrum has been observed using SiO2/SiOyNx/SiNx capping layers and by controlling the associated oxygen and nitrogen content. The resulting degree of tuning, up to 120 nm red shift and 140 nm blue shift of the band-to-band wavelength emission, has been studied using room-temperature photoluminescence, in agreement with the emission spectra obtained from semiconductor optical amplifier waveguide strips.

Impurity free vacancy diffusion induced quantum well intermixing based on hafnium dioxide films

Impurity free vacancy diffusion (IFVD) induced quantum well intermixing (QWI) based on electron beam evaporation hafnium dioxide (HfO2) in the red light diode laser wafer is firstly demonstrated in this work. The red light diode laser wafer had an active region of two 6nm-thick GaInP quantum wells and three 8nm-thick AlGaInP quantum barriers. 135nm thick HfO2 film was evaporated on the surface of the diode laser wafer at 200°C. The QWI processes were induced by rapid thermal annealing (RTA) for 20s at different temperatures. The intensity and the full width at half maximum (FWHM) of the active region emitting wavelength were found increasing and decreasing with the increasing annealing temperature, respectively. When the sample was annealed at 1000°C, a blue shift of 18nm was found by the HfO2 IFVD induced QWI. Moreover, the diffusion lengths and inter-diffusion coefficients were calculated based on concentration distribution in the active region, and the inter-diffusion coefficient values were higher than the results in the Zn impurity diffusion induced QWI.

Microstructural and compositional modification of In0·53Ga0·47As/In0·52Al0·48As multiquantum wells using rapid thermal annealing process

Quantum well intermixing of In0·53Ga0·47As/In0·52Al0·48As multiquantum wells (MQWs) in an impurity free vacancy diffusion (IFVD) mechanism was investigated to observe the intermixing aspect and the effect of defects in MQWs with regard to the microstructural aspect using transmission electron microscopy. The MQWs were grown on a GaAs (100) substrate using compositionally graded buffer layers via molecular beam epitaxy, and the MQWs were annealed at 750 and 900°C for 30 s via rapid thermal annealing for quantum well intermixing using IFVD method. In the fabricated MQWs, defects, such as stacking faults, twins and dislocations, were not generated at 750°C. The diffusion of Ga in the well layer for the quantum well intermixing started from the top well layer, because the SiO2 layer that supplied vacancies for the quantum well intermixing was at the top of the sample. Additionally, in the same well layer, the intermixing did not show equality, because these vacancies were not supplied homogeneously. Especially, in the 900°C annealed case, many dislocations were generated from the cladding layer. These dislocations contributed to new vacancy generation sites, thus the quantum well intermixing was accelerated.

Effect of intermixing on bulk and interface Raman modes in GaAs:AlAs superlattice waveguide structures

Spatially resolved Raman spectroscopy at room temperature is used to study quantum well intermixing in GaAs:AlAs superlattice structures. Phonon modes are probed from the side facet along the [110] direction. The intermixing leads to the appearance of interfacial alloy modes and degraded the intensity of the superlattice interface (IF) modes, which can be used as a sensitive indicator of superlattice quality. These changes in the Raman spectra, along with spatially resolved photoluminescence, are used to distinguish the degree of intermixing in samples intermixed by impurity free vacancy diffusion at 850–950 °C, and to investigate the bandgap modulation in a periodically intermixed bandgap grating fabricated using ion implantation induced disordering. The shift of the GaAs-transverse optic mode to the GaAs-like transverse optic alloy mode and the degradation of the GaAs-IF mode are shown to provide the best indication of the extent of intermixing. Due to the large contrast in the GaAs-IF mode intensity between as-grown and intermixed superlattice, using IF modes is found to be a promising route for characterizing the resolution of the superlattice bandgap grating.

Fabrication of self-aligned ridge waveguide lasers with spot-size converter for quantum well intermixed superluminescent diodes

We fabricated InGaAs/GaAs multiple quantum well ridge waveguide lasers with a spot-size converter (SSC) in a single epitaxial growth step by using conventional lithography and reactive ion etching. The SSC was based on the laterally tapered rib adiabatic waveguide to expand the mode size. The narrow tapered ridge top of ∼0.7 µm was opened by a self-aligned method associated with benzocyclobutene planarization and overall etch-back processes. For an uncoated 1.5 mm long cavity, the far-field beam divergence angle from the SSC facet was about 15° (parallel) and 27° (perpendicular) at 20 °C under continuous-wave (CW) operation, indicating a significant reduction compared to that from the laser facet (i.e., 32° × 41°). The device exhibited a maximum output power of 62 mW/facet with a threshold current of 73 mA and a slope efficiency of 0.34 mW mA−1. A selective intermixing technique using impurity-free vacancy diffusion was applied to the SSC laser structure to achieve a superluminescent light source via an optimized anti-reflectivity coating. The CW output power up to 1.4 mW at 20 °C was observed and the spectral bandwidth was approximately 24 nm.

Impurity-Free Vacancy Diffusion of InGaAsP/InGaAsP Multiple Quantum Well Structures Using SiH4-Dependent Dielectric Cappings

We report the impurity-free vacancy diffusion (IFVD) behaviors of In0.86Ga0.14As0.64P0.36/In0.88Ga0.12As0.25P0.75 multiple quantum well (MQW) structures using dielectric films as a capping layer. The effects of SiOx and SiNx films deposited using different SiH4 flow rates on the IFVD are investigated by photoluminescence measurements and ellipsometry analysis. The properties of dielectric capping layer play a key role in the intermixing of this MQW structure, indicating that the increased porosity of SiOx and SiNx layers enhances the bandgap energy shift of MQWs. A significant blue shift of 173 nm (i.e., 106 meV) for the sample capped with a relatively more porous SiNx, which was deposited using 20 sccm SiH4, was obtained after a thermal annealing of 850 °C for 30 s. The mechanism was analyzed by using transmission electron microscopy and energy dispersive X-ray spectroscopy. It is believed that the enhanced blue shift is ascribed to the dominant interdiffusion of group V atoms because the compositions of group III atoms are kept almost identical in the wells/barriers.

Effect of the property of dielectric capping layers on impurity-free vacancy diffusion in InGaAs/InGaAsP MQW structures

A dependence of the interdiffusion in InGaAs/InGaAsP multiple quantum well (MQW) structures on the stoichiometry of SiOx and SiNx capping layers, which were deposited with different SiH4 flow rates by plasma enhanced chemical vapor deposition (PECVD), has been studied. Decreasing the SiH4 flow rate during PECVD deposition produces relatively more voids inside SiOx or SiNx film. For both SiOx and SiNx capping layers, the blue shift of intermixed samples after rapid thermal annealing (RTA) becomes larger due to the enhancement of the interdiffusion of well/barrier caused by the increased porosity of dielectric capping layers as SiH4 flow rate decreases from 300 sccm to 20 sccm. Using SiOx (SiNx) deposited with different SiH4 flow rates, the magnitude of blue shift is varied by about 98 nm (75 nm) for the samples annealed at 850 °C for 30 s. The blue shifts of 138 nm and 98 nm of photoluminescence peak wavelength for the SiOx (deposited with 20 sccm SiH4) capped MQWs without and with an InGaAs cap layer, respectively, are observed after RTA of 850 °C for 30 s. A possible mechanism is discussed for these phenomena.

Influence of impurity-free vacancy diffusion on the optical and optoelectronic properties of the In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells electroabsorption modulator structure

The effects of impurity-free vacancy diffusion using a SiO2 capping layer on the optical and optoelectronic properties of the In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells (MQW) electroabsorption (EA) modulator structure are investigated. A significant improvement (about 20 times compared to an as-grown sample) of photoluminescence (PL) intensity was observed after thermal treatments at temperatures of above 700 °C. In this structure, however, a red shift was observed after rapid thermal annealing (RTA) using the SiO2 capping layer, in contrast to the blue shift in conventional interdiffused MQW. A red shift of about 27 meV was obtained at an annealing temperature of 800 °C for 45 s without noticeable PL linewidth broadening. We believe that the red shift is attributed to the exchange between Ga of an InGaAs well and In of an InAlAs barrier. InGaAs/InAlAs MQW EA modulators were fabricated on as-grown and annealed substrates. The basic characteristics of the devices fabricated before and after RTA were evaluated by current–voltage and photocurrent measurements.

Spatial Bandgap Tuning in Long Wavelength InAs Quantum Dots-in-Well Laser Structure

ABSTRACTWe employed the postgrowth impurity-free vacancy diffusion technique to selectively tune the bandgap of the InAs/InGaAlAs dots-in-well laser structure grown on (100) InP substrate. A blueshift up to 170 nm with a significant decrease in the photoluminescence linewidth has been observed. Spatial control of the bandgap shifts has been achieved using SiO2 and SixNy layers as annealing caps. A differential wavelength shift of 76 nm has been observed after a rapid thermal annealing step at 750 °C for 30 s. In contrast to most reported results in other material systems using similar process, we observed a larger bandgap shift from the SixNy capped samples than from the SiO2 capped samples. Our theoretical calculation indicates that the unusual intermixing behavior in this material system is governed by different interdiffusion rates of group-III atoms.

High-performance InGaAs-GaAs-AlGaAs broad-area diode lasers with impurity-free intermixed active region

Broad-area InGaAs-GaAs-AlGaAs double-quantum-well graded-index separate-confinement heterostructure lasers with as-grown and intermixed active regions were fabricated and characterized. An impurity-free vacancy diffusion method was used to intermix the quantum wells. Light-current characteristics of both types of lasers were used to extract information about the effects of intermixing process on threshold current density, internal optical loss, internal quantum efficiency, material gain, etc. Comparison between these parameters indicates comparable device performance, even though intermixing involved annealing at 1000°C which resulted in a 42-nm wavelength blueshift.

High-power AlGaAs/GaAs broad-area lasers grown by MBE

A new broad area (BA) structure laser has been designed with a weak lateral index waveguide, an optical coupling layer and nonabsorbing windows to improve the light output properties of BA lasers. The wafer has been grown successfully by MBE and the BA stripe has been obtained mainly by an impurity-free vacancy diffusion (IFVD) technique. The prepared devices have been measured with a maximum output power of 3.2 W. A satisfactory far field ( θ ∥) output property is also obtained.

Impurity-Free Vacancy Diffusion Technique forInGaAsP/InP Multiple Quantum Well Laser Structure

Bandgap tuning of the InGaAsP/InP multiple quantum well (MQW) laser structureby the impurity-free vacancy diffusion (IFVD) is investigated usingphotoluminescence. It has been demonstrated that the effects of the plasmabombardment to the sample surface involved in the IFVD technique can enhance theintermixing of the InGaAsP/InP MQW laser structure. The reliability of the IFVDtechnique, particularly the effects of the surface decomposition and the intrinsicdefects formed in the growth or preparation of the wafer, has been discussed. PACS: 73. 61. Ey, 81. 05. Ea

Quantum well intermixing ofIn1-xGaxAs/InP and In1-xGaxAs/In1-xGaxAs1-yPy multiple-quantum-well structures byusing the impurity-free vacancy diffusion technique

We investigated the influence of strain and barriercomposition on the quantum well intermixing (QWI) in In1-xGaxAs/In1-xGaxAs1-yPy multiple-quantum-well (MQW) structures by usingthe impurity-free vacancy diffusion technique. A compressivelystrained MQW structure showed a higher degree of intermixingthan a lattice-matched one due to lower thermal stabilityand larger bandgap difference between the quantum well (QW) andthe barrier. Also, the photoluminescence blueshift increases withincreasing difference of bandgap energy between the QW andthe barrier. In addition, a highly selective QWI with a large bandgapshift difference of 123 meV (195 nm) using an identical silica caphas been achieved from samples capped with InGaAs/SiO2 andInP/SiO2 capping layers. This behaviour may beattributed to the difference in thermal expansion coefficientbetween InGaAs and InP at the annealing temperature.

GaAs/AlGaAs quantum well intermixing using buried Al-oxide layer

Using buried wet-oxidised Al/sub x/O/sub j/ layers to enhance impurity free vacancy diffusion, the intermixing of GaAs/AlGaAs quantum wells has been achieved. A 70 /spl Aring/ thick GaAs quantum well shows a blueshift of 59 meV when the sample is annealed at 950/spl deg/C for 120s. By cathodoluminescence measurements, it is confirmed that the bandgap transition region is localised laterally within 1 /spl mu/m of the oxide/nonoxide interface.

Area selectivity of InGaAsP-InP multiquantum-well intermixing by impurity-free vacancy diffusion

Area selectivity of bandgap tuning in the InGaAsP-InP multiquantum-well structure has been investigated using low temperature photoluminescence (PL). The bandgap blue-shift in the intermixed region was as much as 170 meV for a rapid thermal anneal of 30 s at 850/spl deg/C, and was controllable using annealing temperature and time. From samples with SiO/sub 2/ stripe patterns, clearly separated PL peaks were observed centered at 0.95 and 1.08 eV, each representing signals originating from the dielectric capped and exposed areas, respectively. In samples with stripes intervals less than 6 /spl mu/m, PL signals did not separate, but formed one broad spectrum due to lateral diffusion. The lateral diffusion was found less than 3.0 /spl mu/m.

Band-gap blue shift by impurity-free vacancy diffusion in 1.5-μm-strained InGaAsP/InP multiple quantum-well laser structure

The effects of defect-enhanced, impurity-free, quantum-well (QW)-barrier compositional intermixing caused by the SiO2 cap annealing at 750 °C of a 1.5-μm InGaAsP/InP multiple quantum-well (MQW) laser structure have been studied by photoluminescence (PL). A substantial band-gap blue shift, as much as 112 nm (∼66 meV), was found in the structure and the value of the shift can be controlled by the anneal time. The amount of the shift does not depend on the thickness of the SiO2 cap layer. Ridge-waveguide lasers were fabricated on the different areas of the wafer, with and without a SiO2 cap during a 60 s anneal. The lasing wavelength of the laser produced with the SiO2 cap has a 78 nm blue shift over that of the laser without the SiO2 cap.

High-quality extended cavity ridge lasers fabricated by impurity-free vacancy diffusion with a novel masking technique

By using phosphorous doped (5% wt P) silica as masking material and standard silica capping to promote quantum well interdiffusion, GaAs-AlGaAs ridge waveguide QW lasers with integrated transparent waveguides were fabricated. With a selective differential blue-shift of 30 nm in the absorption edge, devices with 400-μm/2.73-mm-long active/passive sections exhibited threshold currents of 8 mA in CW operation, only 1 mA higher than that for normal lasers of the same active length and from the same chip. This 14% increase in threshold current was accompanied by a slope efficiency decrease of 40%. Losses of 3.2 cm/sup -1/ were measured in the passive waveguides at the lasing wavelength using the Fabry-Perot resonance method. This value is among the lowest reported so far using an impurity-free disordering technique.

Selective quantum-well intermixing in GaAs-AlGaAs structures using impurity-free vacancy diffusion

Impurity-free vacancy disordering (IFVD) using SiO/sub 2/ and SrF/sub 2/ dielectric caps to induce quantum-well (QW) intermixing in the GaAs-AlGaAs system is studied. The intermixing rate of IFVD was found to be higher in n-i-p and intrinsic than in p-i-n structures, which suggests that the diffusion of the Group III vacancy is not supported in p-type material. Single-mode waveguides have been fabricated from both as-grown and bandgap-tuned double-quantum-well (DQW) laser samples. Propagation losses as low as 8.5 dB cm/sup -1/ were measured from the bandgap-tuned waveguides at the lasing wavelength of the undisordered material, i.e., 860 nm. Simulation was also carried out to study the contribution of free-carrier absorption from the cladding layers, and the leakage loss induced by the heavily p-doped GaAs contact layer. It was found that the leakage loss contributed by the GaAs cap layer is significant and increases with wavelength. Based on IFVD, we also demonstrate the fabrication of multiple-wavelength lasers and multichannel wavelength division multiplexers using the one-step selective intermixing in selected area technique. This technique enables one to control the degree of intermixing across a wafer. Lasers with bandgaps tuned to five different positions have been fabricated on a single chip. These lasers showed only small changes in transparency current, internal quantum efficiency, or internal propagation loss, which indicates that the quality of the material remains high after being intermixed. Four-channel wavelength demultiplexers based on a waveguide photodetector design have also been fabricated. Photocurrent and spontaneous emission spectra from individual diodes showed that the absorption edge was shifted by different degrees due to the degree of QW intermixing. The results obtained also imply that the technique can be used in the fabrication of broad-wavelength emission superluminescent diodes.

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. >