AlGaAs/InGaAs/GaAs quantum well doped channel heterostructure fieldeffect transistors

The results of experimental and theoretical studies of pseudomorphic AlGaAs/InGaAs/GaAs quantum-well doped-channel heterostructure field effect transistors (QW-DCHFETs) are presented. The channel doping was introduced in two ways: during growth by molecular beam epitaxy or by direct ion implantation. The latter technique may be advantageous for fabrication of complementary DCHFET circuits. Peak transconductances of 471 mS/mm and peak drain currents of 660 mA/mm in 0.6-μm-gate doped-channel devices were measured. The results show the advantages of the DCHFET over standard heterostructure FET structures and their potential for high-speed IC applications. Self-consisted calculations of the subband structure show that the potential barrier between the quasi-Fermi level in the channel and the bottom of the conduction band in the barrier layer is considerably larger for the doped channel structure than for the structure with an undoped channel. This lowers the thermionic emission gate current of the doped channel device compared to the undoped channel device     

Low-voltage phase modulation in GaAs/AlGaAs quantum well opticalwaveguides

Reports GaAs/AlGaAs quantum well waveguide phase modulators with high phase shift coefficients, as large as 520 degrees per V mm. By operating at wavelengths far below the bandedge and applying DC bias the authors achieve large electro-optic modulation with low absorption loss in device lengths on the order of 100 μm and drive voltages on the order of 1 V     

Electrooptical modulation in multiple quantum well hetero nipiwaveguides

An optical intensity modulator based on multiple quantum well hetero (MQW-H) nipi waveguides is reported. In the low optical power regime (~10^-5 W), the modulator exhibits an extinction ratio in excess of 100:1 at low drive voltage (4 V) and 5-B attenuation. Modelling and experimental results of the time response of the waveguide modulator are presented, and it is shown that the response is limited by the RC time constant of the drive circuit and the photocurrent charging up a dielectric capacitance. The modelling shows that with a loss penalty of ~1.5 dB, the speed of the present waveguide modulator is limited solely by the RC time constant that would allow modulation speeds of ~100 ps     

Anisotropic electroabsorption and optical modulation in InGaAs/InAlAs multiple quantum well structures

The first measurements to be made of large anistropic electroabsorption and modulation of long-wavelength light propagating along the plane of InGaAs/InAlAs multiple quantum well (MQW) structures grown by molecular beam epitaxy (MBE) are reported. Photocurrent response of waveguide p-i-n diodes is studied for incident light polarization parallel and perpendicular to the MQW layers. Photocurrent increase with reverse bias throughout the entire photoresponse spectrum is observed for both polarizations. The MQW p-in optical modulator shows a capacitance-limited pulse response of 250 ps and the modulation depth is 14 percent.     

GaAs/AlGaAs multiple quantum well GRIN-SCH vertical cavity surfaceemitting laser diodes

Vertical cavity surface emitting lasers (VCSELs) with GaAs/AlGaAs multiple quantum well (20 wells) graded-index separate-confinement-heterostructure (GRIN-SCH) active regions are discussed. The VCSEL structures, which also contained two Al_xGa_1-xAs/Al_yGa_1-yAs distributed Bragg reflectors, were grown by molecular beam epitaxy and had a threshold current and current density at room temperature pulsed excitation of 16 mA and 14 kA/cm², respectively, near 0.85-μm wavelength. Both single-longitudinal and fundamental transverse mode emission characteristics were observed with a light output greater than 3 mW and a slope efficiency of 0.12 mW/mA     

Optical nonlinearities in a passive GaAs/GaAlAs multiple quantum well strain-induced waveguide

Nonlinear Fabry-Perot switching effects have been observed in a 400 μm long GaAs/GaAlAs MQW strain-induced waveguide resonator. Experimental evidence showing that the mechanism responsible for the switch has a negative coefficient in the refractive index nonlinearity is described; this rules out the possibility that heating could be the cause of the observed switching. However, thermal effects were always present in the background and became more prominent at slow input-power sweep rates. Bistability due to increased absorption has also been observed in a similar nonresonant waveguide structure, at slightly longer wavelengths.     

Dielectric properties of GaAs AlGaAs multiple quantum well waveguides

GaAs/AlGaAs multiple quantum well (MQW) waveguides are investigated using the grating coupling technique. Large polarization dependent effects of the two-dimensional excitons are seen in the TE/TM dispersion relation of single mode slab waveguides. By treating the MQW as a layered composite dielectric, we deduce the oscillator strengths of the heavy and light hole excitons, and the background dielectric constant of the MQW without the excitonic contribution, for fields polarized parallel and perpendicular to the MQW layers.     

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     

Single quantum well strained InGaAs/GaAs lasers with large modulation bandwidth and lower damping

Strained In/sub 0.2/Ga/sub 0.8/As/GaAs single quantum well polyimide buried ridge waveguide lasers with 3 dB modulation bandwidths of 15 GHz and gain compression factors in as small as 9.98*10/sup -18/cm/sup -3/ have been fabricated. The authors have also observed that the gain compression factor is not larger than in lasers with bulk active areas, and is lower for structures with a smaller number of wells and shorter cavity lengths.<>     

Intersubband absorption characteristics in OMVPE grown delta-doped GaAs/AlGAs multiple quantum well structures

A study was carried out using a series of multiple quantum well structures grown by organometallic vapor phase epitaxy (OMVPE) to evaluate the intersubband absorption characteristics, thus enabling a direct correlation of the delta doping parameters to device performance. Significant improvements in the absorption characteristics were obtained by adopting a localized delta-doping profile in the absorption quantum wells as opposed to homogeneously doped structures. The transition linewidths were observed to decrease from 40 meV in the uniformly doped sample to 20 meV in devices in corporating delta-doped quantum wells. In addition, the measured absorption strengths of the intersubband resonance grew in a linear fashion as the carrier sheet density in the well was increased to approximately 2 × 10¹² cm⁻².     

Room temperature excitonic nonlinear absorption and refraction in GaAs/AlGaAs multiple quantum well structures

We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.     

Barrier width dependence of leakage currents in InGaAs/ GaAs multiple quantum well P-I-N diodes

A range of InGaAs/GaAs strained layer multiple quantum well pin diode structures with different barrier dimensions has been grown and characterized. High quality low leakage structures (< 2 × 10⁻⁴ A/cm² @ 0.9 V _BD ) with a high degree of strain (∼2%) have been produced. An important factor affecting the leakage for a fixed well composition and dimension is found to be the barrier width.     

Cathodoluminescence study of pyramidal facets in piezoelectric InGaAs/GaAs multiple quantum well pin photodiodes

A study of the optical and electrical activity of defects at the surface of InGaAs/GaAs multiple quantum well pin photodiodes was done on two different misoriented (111)B GaAs substrates: substrate A, misoriented 1° towards [2̄11] and substrate B, misoriented 2° towards [21̄1̄]. In this article, we report the existence of different faceted defects at the surface with the misorientation and their influence on the optical performance. The surface of photodiodes grown on substrate A shows inclined pyramids with two enhanced facets. The electron beam induced current measurements showed that these pyramids act as high efficient collector of the e-beam excited electron–hole pairs (e–h). This behaviour agrees with the reduction of the cathodoluminescence emission efficiency at the facets. In contrast, a different pyramid type with one enhanced facet is observed at the surface of diodes grown on substrate B. However, these facets have not shown either optic or electric activity. In these diodes, the dislocations localised at the active region degrade the device performance acting as non-radiative recombination centres.     

Modulation spectroscopy study of a strained layer GaAs/GaAsP multiple quantum well structure

Using contactless electroreflectance (CER) and piezoreflectance at 300 K we have characterized a GaAs/GaAs_1?xP_x multiple quantum well (MQW) structure, “GaAs” (nominal) and GaAsP epilayers grown by chloride transport chemical vapor deposition on GaAs (001) substrates. From a detailed lineshape fit to the CER data from the epilayers we have determined the energies of the fundamental band gap and hence the phosphorous composition. The nominal “GaAs” epilayers were found to have phosphorous compositions of about 2.5–3.2%, a result of the phosphorous diffusion between growth chambers in the reactor. The GaAs_1?xP_x epilayer had x=0.29. For the GaAs_0.97P_0.03/GaAs_0.71P_0.29MQW comparison between the experimentally observed energies of a number of quantum transitions with a theoretical envelope function calculation, including the effects of strain in the barriers, made it possible to evaluate the unstrained conduction band offset parameter Q_c=0.50±0.05. Our value for this parameter is discussed in relation to other works. Atomic force microscopy was employed to investigate the surface morphology of the 230 Å GaAsP top layer of the MQW in addition to a 2000 Å GaAsP epilayer. From the absence of any cross-hatch pattern associated with misfit dislocations on the former we concluded that the GaAsP in the MQW is pseudomorphic. On the other hand the 2000 Å epilayer exhibited signs of strain relaxation.     

Electron mobility in a AlGaAs/GaAs/AlGaAs quantum well

An oscillatory dependence of the electron mobility on the quantum well (QW) thickness in a AlGaAs/GaAs/AlGaAs heterostructure with double-sided modulation doping has been observed experimentally. A steep decrease in mobility with increasing electron concentration in the QW is established. The conditions for an increase in mobility on introducing a thin barrier into the QW are determined. The first experimental observation of an increase in mobility by a factor of 1.3 in a QW of thickness L=26 nm upon introducing a thin (1–1.5 nm) AlAs barrier is reported.     

Experimental evidence for hole transport limited intensity modulation response in quantum well lasers

Direct experimental evidence is presented to show that it is the hole transport across wide separate confinement heterostructures and not the electron quantum capture that leads to the low-frequency rolloff in the intensity modulation response of quantum well lasers.<>     

Tradeoff between mobility and subthreshold characteristics in dual-channel heterostructure n- and p-MOSFETs

The mobility and subthreshold characteristics of TiN-gate, dual-channel heterostructure MOSFETs consisting of strained-Si-Si/sub 0.4/Ge/sub 0.6/ on relaxed Si/sub 0.7/Ge/sub 0.3/ are studied for strained-Si cap layer thicknesses ranging from 3 to 10 nm. The thinnest Si cap sample (3 nm) yields the lowest subthreshold swing (80 mV/dec) and the highest hole mobility enhancement (2.3X at a vertical effective field of 1 MV/cm). N-MOSFETs show the expected electron mobility enhancement (1.8X) for 10- and 5-nm-thick Si cap samples, which reduces to 1.6X for an Si cap thickness of 3 nm. For Si cap and gate oxide thicknesses both equal to 1 nm, simulations predict a moderate degradation in p-MOSFET subthreshold swing, from 73 to 85 mV/dec, compared to that for the Si control.     

Critical thickness in strained-layer GaInAs/GaAs quantum well lasers

The critical thickness in strained-layer GaInAs/GaAs quantum well lasers was studied by measuring the dependence of the threshold current on the number of quantum wells. The critical thickness for 20% In composition was found to be around 30 nm, which is twice as large as predicted by the Matthews-Blakeslee model.<>     

GaAlAs/GaAs single quantum well gain-coupled distributed feedbacklaser

A novel gain-coupled distributed feedback laser with a single quantum well (SQW) active layer is proposed and fabricated. The gain perturbation for the gain coupling is due to the periodically perturbed SQW active layer. The characteristics under continuous-wave (CW) operation are presented, included an excellent single-mode property and a very high yield in the single-mode oscillation. A CW threshold current of 31 mA and a high side-mode suppression ratio of 47 dB were achieved. The high single-mode-oscillation yield near 100% indicated the dominance of the gain-coupled optical feedback in the lasers     

Relative intensity noise reduction in InGaAs/InP multiple quantum well lasers with low nonlinear damping

The authors demonstrate theoretically and experimentally that the relative intensity noise (RIN) of laser diodes can be dramatically reduced by decreasing nonlinear damping in the laser. Four types of InGaAs/InP multiple quantum well (MQW) lasers with different well widths, barrier widths, and numbers of wells were fabricated. By comparing these four types of devices, it is shown that MQW lasers with wider wells, narrower barriers, and larger numbers of wells have smaller nonlinear damping and lower RIN.<>