A low-voltage, high-reflectance-change normally off refractiveGaAs/Al0.2Ga0.8As MQW reflection modulator

We present our optimization of a normally off refractive GaAs/Al _xGa_1-xAs multiple-quantum-well (MQW) reflection modulator with respect to the on/off reflectance change, on/off contrast ratio, and operating voltage. We use optical transfer matrices, theoretically calculated refractive indices, and absorption coefficients to simulate the operation of a normal-incident Fabry-Perot MQW modulator. Our calculations suggest that a normally off refractive GaAs/Al_0.2Ga_0.8As MQW reflection modulator with a reflectance change of 42.9% and an on/off contrast ratio of 1539 for an operating voltage of only 2.44 V can be fabricated by molecular-beam epitaxy (MBE)     

20 dB contrast GaAs/AlGaAs multiple quantum-well nonresonantmodulators

The authors report a successful fabrication of nonresonant GaAs/AlGaAs multiple quantum well (MQW) modulators with a contrast ratio of more than 20 dB and a wide bandwidth of 3 nm. The optical path, as long as 8 μm, is attained by fabricating a high-purity thick MQW layer with a residual carrier concentration of less than 10¹⁴ cm^-3 and introducing a high-reflectivity AlAs/AlGaAs quarter-wavelength mirror. The driving wavelengths are located at the absorption edge, resulting in the low-absorption loss     

InGaN/GaN MQW p–n junction photodetectors

InGaN/GaN multiquantum well (MQW) p–n junction photodetectors with semi-transparent Ni/Au electrodes were fabricated and characterized. It was found that the fabricated InGaN/GaN MQW p–n junction photodetectors exhibit a 20 V breakdown voltage and a 3.5 V forward 20 mA turn on voltage. It was also found that the photocurrent to dark current contrast ratio is higher than 10⁵ when a 0.4 V reverse bias was applied to the InGaN/GaN MQW p–n junction photodetectors. Furthermore, it was found that the maximum responsivity was 1.28 and 1.76 A/W with a 0.1 and 3 V applied reverse bias, respectively.     

Design considerations for a multiple-quantum-well nonresonantsurface-normal modulator

It is shown that by using optimal quantum-well thickness, and by using a symmetric back-to-back p-i-n structure, a non-Fabry-Perot surface-normal reflection modulator based on the quantum confined Stark effect in a GaAs/AlGaAs multiple-quantum-well (MQW) can provide intensity modulation with (a) at least 10-dB contrast ratio, (b) a drive voltage less than 10 V, and (c) an active-layer thickness less than 4 μm. The drive voltage for a given contrast ratio can be minimized by using the quantum-well structure with the maximum Δα/F      

On the operational and manufacturing tolerances of GaAs-AlAs MQWmodulators

We approach the question of optimization of surface-normal p-i(multiquantum-well, MQW)-n modulators from the viewpoint of investigating their tolerance to variations in wavelength and temperature and errors in manufacture. The reflection characteristics of two high-quality samples are carefully processed to eliminate Fabry-Perot fringes, and then their spectra at any bias are characterized with six phenomenological parameters which depend on λ₀, the zero-field exciton position. The two GaAs-AlAs samples have λ₀'s of 833.8 and 842.3 nm, and so cover a range useful for modulators designed to operate near 850 nm in the normally reflecting condition, i.e., reflection decreases with field. A linear interpolation of the parameters of these two samples is used to predict the behavior of MQW diodes with λ₀'s around this range, and so a fully comprehensive examination of normally reflecting MQW modulators is performed. The performance aspect that is examined is contrast ratio as a function of nonuniformities in the devices or operating conditions given a voltage swing of 3 V. There are two operational modes discussed. If the voltage offset of the bias is allowed to vary via a feedback circuit, a contrast of 2:1 may be maintained over an operating wavelength change (Δλ) of 17 nm with local variations of wavelength of ±1 nm, which corresponds to a temperature variation of 60°C while allowing for variations of laser driver wavelength of ±1 nm. If feedback Is not permitted, we determine that, given tolerances to manufacturing errors, a contrast of 1.5:1 may be maintained over a wavelength range of ~5 nm by either using stacked diode designs or extremely shallow quantum wells     

Growth and optical investigation of quaternary GaInAsSb/AlGaAsSb quantum wells

Infrared photoluminescence and high sensitive absorption measurements were performed on a quaternary GaInAsSb/AlGaAsSb strained multiple quantum well (MQW), as well as single quantum well (SQW) structures grown by molecular-beam epitaxy, to investigate its band offsets and subband behavior. Strong luminescence and well-resolved excitonic absorption peaks are observed even at room temperature, which is indicative of the good quality of our quaternary sample. By fitting the experimental results to the theoretical calculations, we find that the light holes are confined in well regions for Ga_0.75In_0.25As_0.04Sb_0.96/Al_0.22Ga_0.78As_0.02Sb_0.98 QWs (type I MQW) with a conduction-band offset ratio of Q_c = 0.66 ± 0.01. The transition from type I to type II for light holes is predicted theoretically and demonstrated directly by photoluminescence spectra in the SQW structures.     

Integration of waveguide-type wavelength demultiplexingphotodetectors by the selective intermixing of an InGaAs-InGaAsPquantum-well structure

Using the selective intermixing of an InGaAs-InGaAsP multiquantum-well (MQW) structure, a wavelength demultiplexing photodetector which can demultiplex two widely separated wavelengths was fabricated. An InGaAs-InGaAsP MQW with a u-InP cladding layer and a n-InGaAs cap layer, grown by metal organic chemical vapor deposition was used. Selective area intermixing of the InGaAs-InGaAsP MQW structure was done by a rapid thermal annealing after the deposition and patterning of the SiO₂ dielectric layer on the InGaAs cap layer. The integrated structure consists of shorter and longer wavelength sections, separated by an absorber section. Shorter wavelength and absorber sections were intermixed with the SiO₂ dielectric layer. At a wavelength of 1477 nm, the output photocurrent ratio was enhanced as the length of the absorber region increased and a ratio of over 30 dB was observed, while at a wavelength of 1561 nm, an output photocurrent ratio of 18.9 dB was observed     

Polarization dependence of a 1.52 μm InGaAs/InP multiple quantumwell waveguide electroabsorption modulator

A ridge-waveguide In_0.53Ga_0.47As/InP multiple-quantum-well (MQW) electroabsorption modulator operating at a wavelength of 1.52 μm is demonstrated. The modulator exhibits large polarization-dependent electroabsorption behavior which favors the modulation of the TM mode. At a reverse bias of 10 V, the modulator has a 24.5-dB extinction ratio for the TM mode, whereas that for the TE mode is only 11.1 dB. Polarization-dependent saturation of absorption has been observed in this device for incident optical power levels of less than 1 mW     

Quantum-confined Stark effect modulators at 1.06 μm on GaAs

Electroabsorption modulation is achieved at or near a wavelength of 1.06 μm with In_xAl_yGa_1-x-yAs/In _xGa_1-xAs multiple-quantum-well (MQW) structures grown on GaAs substrates. The lattice mismatch (close to 2%) between the MQW and the substrate is accommodated by a compositional-step-graded buffer array. A dislocation density of less than 10⁷/cm² is estimated for the MQW region. For 80-to-100 Å well widths, a maximum electroabsorption coefficient of 8000 cm^-1 with an applied voltage of 15 V is obtained     

Signal stability in periodically amplified fiber transmission systems using multiple quantum well saturable absorbers for regeneration

The use of multiple quantum well (MQW) saturable absorbers (SAs) for signal regeneration in periodically amplified fiber transmission systems is explored. A systematic study of signal destabilization resulting from incomplete saturation of MQW SAs used for regeneration, and of means of overcoming such destabilization, is presented. A computer model for MQW SAs, which considers the asymmetric Fabry-Pe/spl acute/rot (AFP) cavity structure commonly employed to increase the contrast of such devices, is presented. The model is used to simulate nitrogen-implanted MQW SAs with < 5 ps recovery time in a transmission system. A comparison is made with results previously obtained for a 10 Gb/s standard single-mode fiber (SMF) recirculating loop transmission experiment using MQW SAs and temporary soliton propagation for signal regeneration. The simulations allow the benefits derived from the two parts of the regenerator to be identified, as well as their contributions to the destabilization of the propagating signal. The error-free transmission distance is improved from /spl sim/ 2000 to > 7000 km when the two components are combined.     

Amplitude and phase modulation in a 4 μm-thick GaAs/AlGaAsmultiple quantum well modulator

The experimental results of a 4 μm-thick GaAs/AlGaAs MQW modulator show an ~10:1 on/off ratio with an applied voltage of 20 V (Δα~6×10³/cm at E~50 kV/cm) and ~0.4 πrad of phase shift with an applied voltage of 10 V (Δn~0.04 at E~25 kV/cm). Such high electro-optical modulations have previously been reported only in MQW optical waveguide modulators     

1.3-μm InAsP modulation-doped MQW lasers

The effect of both n-type and p-type modulation doping on multiple-quantum-well (MQW) laser performances was studied using gas-source molecular beam epitaxy (MBE) with the object of the further improvement of long-wavelength strained MQW lasers. The obtained threshold current density was as low as 250 A/cm² for 1200-μm-long devices in n-type modulation-doped MQW (MD-MQW) lasers. A very low CW threshold current of 0.9 mA was obtained in 1.3-μm InAsP n-type MD-MQW lasers at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest value for lasers grown by all kinds of MBE in the long-wavelength region. Both a reduction of the threshold current and the carrier lifetime in n-type MD MQW lasers caused the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay time are very attractive for laser array applications in high-density parallel optical interconnection systems. On the other hand, the differential gain was confirmed to increase by a factor of 1.34 for p-type MD MQW lasers (N_A=5×10¹⁸ cm ^-3) as compared with undoped MQW lasers, and the turn-on delay time was reduced by about 20% as compared with undoped MQW lasers. These results indicate that p-type modulation doping is suitable for high-speed lasers     

Integration of InAlGaAs/InGaAs MODFETs on MQW modulators on GaAssubstrates

An In_xAl_yGa_1-x-y device layer structure that enables the monolithic integration of In_0.25Al _0.75As/In_0.15Ga_0.85As MODFETs and In _0.25Al_0.35Ga_0.40As/In_0.25Ga _0.75As MQW modulators is reported. Current gain cutoff frequencies of 10 GHz are measured for 1 μm gate length MODFETs. MQW modulators operating at 1.05 μm demonstrate 20% transmission modulation for an applied 8 V     

A comparative study of temperature sensitivity of InGaAsP andAlGaAs MQW lasers using numerical simulations

We used numerical simulation to compare the temperature sensitivity of an InGaAsP MQW laser emitting at 1.55 μm and an AlGaAs MQW laser at 0.82 μm. By artificially changing the InGaAsP laser gradually into a structure similar to the AlGaAs laser, we gained quantitative insight into how each material or structural parameter causes the relatively low T₀ of the InGaAsP MQW laser. Using a typical MQW structure we demonstrated the relative importance of parameters involving Auger recombination, current leakage over the quantum barrier, optical confinement and band offset. We found that if these parameters were made the same as the AlGaAs laser, the T₀ of the InGaAsP laser was even better than that of the AlGaAs laser. Our numerical simulation confirmed that the Auger recombination is the main cause of low T₀ in MQW InGaAsP lasers. We also discovered that thermal current leakage over the barrier and Auger recombinations are correlated with each other and both factors must be improved to increase the T₀ of InGaAsP lasers to that of AlGaAs lasers     

Novel GaAs/AlGaAs multiquantum-well Schottky-junction device andits photovoltaic LWIR detection

The authors have demonstrated photovoltaic detection for a multiple-quantum-well (MQW) long-wavelength infrared (LWIR) detector. With a blocking layer, the MQW detector exhibits Schottky I-V characteristics with extremely low dark current and excellent ideality factor. The dark current is 5×10^-14 A for a 100×100 μm² detector (designed for 10-μm response) at 40 K, nearly nine orders of magnitude lower than that of a similar MQW LWIR detector without the blocking layer. The ideality factor is ~1.01-1.05 at T=40-80 K. The measured Schottky-barrier height is consistent with the energy difference between first excited states and ground states, or the peak of spectral response. The authors also report a measured effective Richardson constant (A**) for a GaAs/AlGaAs heterojunction using this blocking layer structure. The A** is ~2.3 A/cm²/K ²     

Diffusion-induced disordering ofGa0.47In0.53As/InP multiple quantum wells withzinc

Diffusing zinc into Ga_0.47In_0.53As/InP MQW layers is found to cause strong intermixing of the group III elements, which changes the composition in the quantum wells and barriers. As a result of this disordering the MQW bandgap is reduced in energy and the photoluminescence emission peak moves to longer wavelength     

3D defect distribution induced by focused ion beam irradiation at variable temperatures in a GaAs/GaAlAs multi quantum well structure

In this study, we propose a new characterisation method of the damage distribution using a GaAs/GaAlAs multi quantum well structure (MQW). Three quantum wells are used as high sensibility sensors, in relation to the damage created during the FIB irradiation through photoluminescence (P.L.) intensity measurements.

We have compared the effect of a Ga⁺ focused ion beam irradiation on the MQW structure, which was kept either at room temperature, or near the liquid nitrogen temperature (80 K) during the bombardment. The surface ion dose was kept constant at 10¹⁵ ions/cm². No subsequent annealing was performed. Irradiated samples were characterised using low temperature (1.7 K) and spatially resolved photoluminescence experiments.     

High-speed 1.5-μm compressively strained multi-quantum wellself-aligned constricted mesa DFB lasers

A great improvement in the high-speed characteristics for compressively strained multi-quantum-well (MQW) distributed-feedback (DFB) lasers with self-aligned constricted mesa structures is described. Negative wavelength detuning is an important factor in making possible the extraction of potential advantages for the compressively strained MQW DFB lasers. A 17-GHz bandwidth, which is the highest among the 1.5-μm MQW DFB lasers, is demonstrated. A wavelength chirp width of 0.42 nm at 10 Gb/s is obtained due to a reduced linewidth enhancement factor that has a magnitude of less than 2. Nonlinear damping K factor in a DFB laser with 45-nm negative detuning has drastically decreased to 0.13 ns, about half of that for unstrained MQW lasers. This is mainly due to an enhanced differential gain as large as 6.9×10 ^-12 m³/s. The estimated intrinsic maximum bandwidth is 68 GHz     

Effect of parameter variations on the performance of GaAs/AlGaAsmultiple-quantum-well electroabsorption modulators

A theoretical investigation is presented of the dependence of electroabsorption in GaAs/Al_xGa_1-xAs multiple-quantum-well (MQW) structures on the MQW parameters (Al mole fraction x, well thickness L_z barrier thickness L_b and interface quality) and on the applied electric field studied. The on/off ratio of a modulator using MQWs with x=0.45, L_z=75 Å, and L _b=78 Å is predicted to increase by 20% compared to that of a modulator using MQWs with x=0.3, L_z =100 Å, and L_b=100 Å, when the MQW total active region thickness is 1 μm     

ZnO–ZnMgO Multiple Quantum-Well Ridge Waveguide Lasers

ZnO-ZnMgO multiple quantum-well (MQW) thin-film waveguides with ridge structures have been fabricated on quartz substrates. Low-temperature deposition of high-quality ZnO-ZnMgO MQW thin films was achieved by filtered cathodic vacuum arc technique. A ridge is defined on the thin film by plasma etching. Room-temperature lasing with a peak wavelength at 378 nm of 1.5-nm well width was observed under 355-nm optical excitation. Exciton-exciton scattering was attributed to the amplified spontaneous emission observed from the MQW waveguide. The net optical gain can be larger than 80 cm^-1 at a pump intensity of 2 MW/cm² .