Effect of GaAs/AlGaAs quantum-well structure on refractive index

We investigate the refractive index difference between the GaAs/AlGaAs quantum wells (QWs) and bulk AlGaAs. We find the refractive index difference is smaller when the electric field in the nominally intrinsic MQW region is larger, or when the well (GaAs) thickness of the QW's is larger, or when the Al fraction of the QWs is smaller. The maximum refractive index difference between the 100 Å GaAs/100 Å Al_0.2Ga_0.8As QWs at zero electric filed and bulk Al_0.1Ga_0.9As is about 0.044. Even with a small refractive index difference of 0.0132, the OFF-state reflectance of a normally-off MQW modulator at the designed photon wavelength will increase from the desired value of 0% to 90% when the reflectivity of the bottom mirror is 0.99     

A GaAs/AlGaAs asymmetric Fabry-Perot reflection modulator with veryhigh contrast ratio

Performance results for a normally on, electroabsorptive, surface-normal Fabry-Perot reflection modulator are presented. The device employs a cavity with a 100 Å GaAs/100 Å Al_0.3 Ga_0.7As multiple quantum well and top and bottom quarter-wave mirrors with 4 and 19.5 periods, respectively. Very low values of off-state reflectance were measured, giving a maximum contrast ratio >1000 (30 dB) and a maximum reflectance difference of 64.3%. The contrast ratio is, to the authors' knowledge, the largest reported to date     

InGaAs-InP P-I (MQW)-N surface-normal electroabsorption modulatorsexhibiting better than 8:1 contrast ratio for 1.55-μm applicationsgrown by gas-source MBE

We report growth of In_0.53Ga_0.47 As-InP multiple quantum well (MQW) modulators operating at 1.55 μm for fiber-to-the-home applications. By employing a 200-period InGaAs-InP MQW stack in the intrinsic region of a p-i-n structure and working in reflection, we have been able to realize surface-normal modulator devices that exhibit better than an 8:1 contrast ratio. This is the highest contrast ratio reported to date for this type of device working at this wavelength     

Electroabsorptive Fabry-Perot reflection modulators with asymmetricmirrors

A normally-on electroabsorptive surface-normal Fabry-Perot reflection modulator is reported with an on/off ratio of 22, insertion loss of 3.7 dB, and bandwidth of 3.4 nm for an operating voltage swing of 11 V. The asymmetric Fabry-Perot structure is made with asymmetric mirrors using a quarter-wavelength grating of 15 1/2-periods on the bottom and an air-semiconductor interface on the top. The active region is 1.4 μm thick and composed of 100Å GaAs/100 A Al_0.2Ga_0.8As multiple quantum wells. The structure provides both high-efficiency and bandwidth for surface-normal modulators     

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     

High-contrast/low-voltage normally on InGaAs/AlGaAs asymmetricFabry-Perot modulator

An asymmetric Fabry-Perot cavity modulator is proposed with a 50-period In_0.15Ga_0.85As/Al_0.30Ga_0.70 As strained-layer superlattice active layer. The back Bragg reflector consists of 25 periods of GaAs/AlAs layers while for the front reflector of the device the natural reflection of the air-semiconductor interface was used. Spectral measurements as a function of the applied reverse voltages showed a change in reflectivity from 33% for 0 V reverse bias to 5% for 7 V, at a wavelength of 969 nm. This gives a maximum contrast ratio of 8.3 dB and an insertion loss of 4.9 dB. For higher voltages applied across the device, the reflectivity increases again     

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      

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     

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     

Low-voltage superlattice asymmetric Fabry-Perot reflection modulator

A normally off transverse superlattice asymmetric Fabry-Perot modulator that has a contrast ratio of more than 26:1 at the Fabry-Perot resonance, and a reflection change of 33% at a wavelength of approximately 20 degrees AA away from Fabry-Perot mode for an operating voltage swing of <3 V is discussed. The structure contains an active region of 52 periods of a 30 AA GaAs-30 AA Al/sub 0.3/Ga/sub 0.7/As superlattice, embedded between top and bottom quarter-wave grating mirrors. The modulation is achieved by reducing the cavity loss at the Fabry-Perot resonance through the field-induced effective absorption edge blue-shift in the superlattice.<>     

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     

Experimental and theoretical studies of a novel hetero-nipireflection modulator

A novel GaAs/Al_0.35Ga_0.65As hetero-nipi all-optical reflection modulator has been investigated, both theoretically and experimentally. The modulation is found to be dominated by the refractive index changes that occur in the quantum wells as a result of optically induced electric field changes via the quantum-confined Stark effect. Good agreement is found between the theoretical and experimental behavior, with a maximum absolute experimental modulation of 11% and a maximum contrast ratio of 4.4:1 being observed at room temperature at optical pump densities of only 1.9 mW/cm². A numerical model of the device is developed, which predicts large reflectivity modulations of greater than 53% with a contrast ratio of 25:1 in optimized device structures     

A low-drive-voltage, high-speed monolithic multiple-quantum-wellmodulator/DFB laser light source

A high-speed InGaAs/InAlAs multiple-quantum-well (MQW) intensity modulator and an InGaAsP/InGaAs MQW distributed feedback laser were monolithically integrated by using a hybrid growth technique combining molecular beam epitaxy and metalorganic vapor phase epitaxy. An operating drive voltage of only 2.0 V, a 20-dB on/off ratio, and a 3-dB bandwidth greater than 15 GHz were obtained. This device operated stably in a single mode and with a side-mode suppression ratio of more than 50 dB     

Metamorphic In0.4Al0.6As/In0.4Ga0.6As HEMTs on GaAs substrate

New In_0.4Al_0.6As/In_0.4Ga_0.6 As metamorphic (MM) high electron mobility transistors (HEMTs) have been successfully fabricated on GaAs substrate with T-shaped gate lengths varying from 0.1 to 0.25 μm. The Schottky characteristics are a forward turn-on voltage of 0.7 V and a gate breakdown voltage of -10.5 V. These new MM-HEMTs exhibit typical drain currents of 600 mA/mm and extrinsic transconductance superior to 720 mS/mm. An extrinsic current cutoff frequency f_T of 195 GHz is achieved with the 0.1-μm gate length device. These results are the first reported for In_0.4 Al_0.6As/In_0.4Ga_0.6As MM-HEMTs on GaAs substrate     

Extremely low-voltage Fabry-Perot reflection modulators

A normally-on-electroabsorptive surface-normal Fabry-Perot reflection modulator with a reflection change of 47% for an operating voltage swing of only 2 V, i.e. 23%/V, is discussed. The structure has an active region of twenty-four 100-Å-GaAs/100-Å-Al_0.2 Ga_0.8As multiple quantum wells, sandwiched between two quarter-wavelength grating mirrors with the bottom one more reflective. The wavelength range over which more than half of the maximum reflection change is observed is as wide as ~7 nm for 2 V     

Observation of negative differential resistance inAl0.2Ga0.8As/Al0.4Ga0.6As/GaAs double barrier resonant tunnelling structure

Negative differential resistance has been observed in the current/voltage characteristics of a double barrier resonant tunnelling structure with Al_0.2Ga_0.8As emitters, Al_0.4 Ga_0.6As barriers and GaAs quantum well for the first time. The NDR becomes clear at low temperatures below 77 K, and the current/voltage characteristic is asymmetric. Results demonstrate that high-quality abrupt GaAs-Al_xGa_1-xAs-Al_yGa_1-yAs heterojunctions can be of use in resonant tunnelling structures     

Ga0.51In0.49P/In0.15Ga0.85As/GaAs pseudomorphic doped-channel FET with high-current densityand high-breakdown voltage

Ga_0.51In_0.49P/In_0.15Ga_0.85 As/GaAs pseudomorphic doped-channel FETs exhibiting excellent DC and microwave characteristics were successfully fabricated. A high peak transconductance of 350 mS/mm, a high gate-drain breakdown voltage of 31 V and a high maximum current density (575 mA/mm) were achieved. These results demonstrate that high transconductance and high breakdown voltage could be attained by using In_0.15Ga_0.85As and Ga_0.51In_0.49P as the channel and insulator materials, respectively. We also measured a high-current gain cut-off frequency f_t of 23.3 GHz and a high maximum oscillation frequency f_max of 50.8 GHz for a 1-μm gate length device at 300 K. RF values where higher than those of other works of InGaAs channel pseudomorphic doped-channel FETs (DCFETs), high electron mobility transistors (HEMTs), and heterostructure FETs (HFETs) with the same gate length and were mainly attributed to higher transconductance due to higher mobility, while the DC values were comparable with the other works. The above results suggested that Ga_0.51In_0.49P/In_0.15Ga_0.85 As/GaAs doped channel FET's were were very suitable for microwave high power device application     

GaAs/Al0.3Ga0.7As resonant tunneling diodes with atomically flat interfaces grown on (411)A GaAs substrates by MBE

Al_0.3Ga_0.7As (10 nm)/GaAs (5 nm)/Al_0.3Ga_0.7As (10 nm) double barrier resonant tunneling diodes (RTDs) have been fabricated on a (411)A GaAs substrate by molecular beam epitaxy (MBE), which has atomically flat GaAs/Al_0.3Ga_0.7As interfaces over a device area. The (411)A RTDs showed a larger peak to valley current ratio by about 40% at 77K comparing to RTDs grown on a conventional (100) GaAs substrate simultaneously. Reduction of the valley current (0.61 times smaller) is the main cause for this larger peak to valley current ratio, which is probably due to improved interface roughness and defects of the (411)A RTDs.     

High-speed InGaAlAs/InAlAs multiple quantum well optical modulators

High-speed modulation over 22 GHz for waveguided InGaAlAs/InAlAs multiple quantum well (MQW) optical modulators is described. A large on/off ratio of over 25 dB is demonstrated with a low-drive voltage (6 V) operating in the 1.55-μm wavelength region. The design and characteristics of MQW p-i-n modulators are discussed. The causes of large-insertion loss and the required drive voltage bandwidth figure of merit for the MQW modulator are discussed. The frequency response measurements show that the response speed is limited by the RC time constant of the device. This suggests that the speed can be further enhanced by decreasing the size and capacitance of the device