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We have extracted the ratio between the carrier capture and escape times, eta, for In(0.25)Ga(0.75)As-GaAs lasers containing one, two, or three quantum wells, from high-frequency subthreshold impedance measurements at different temperatures. Our results show that the carrier capture process dominates over the diffusion along the confinement region in the overall transport/capture process. The obtained value for eta is comparable to unity, and this fact has to be taken into account to obtain real material parameters, such as the carrier lifetime and the radiative recombination coefficient.

Short-haul fiber-optic communication systems require high-speed semiconductor lasers that can operate uncooled over a wide temperature range. In this letter, we describe high-speed short-cavity InGaAs-GaAs multiple-quantum-well lasers operating at 1.1-mu m wavelength. The Fabry-Perot lasers were fabricated in a triple-mesa geometry suitable for on-wafer probing. With 3 x 200 mu m(exp 2) ridge-waveguide lasers, which showed the best compromise between high-temperature and high-speed performance, a 3-dB modulation bandwidth of 14.5 GHz at 130 deg C was achieved. Uncooled 20-Gb/s operation of these lasers is presented over a wide-temperature range from 25 deg C to 130 deg C without automatic power control.

The carrier profile for MBE grown ln(ind 0,35)Ga(ind 0.65)As/GaAs multiquantum well laser structures with nominally undoped and beryllium-doped active regions was determined by using the capacitance- voltage (C-V) technique at room temperature. A simple theoretical model was used to extract the impurity concentration and the quantum-well carrier density from the experimental profiles. We obtained a high carrier concentration in nominally undoped devices caused by a strong growth temperature dependent Be diffusion from the p-cladding layer, and no difference between doped samples with different nominal dopant location.

We demonstrate record direct modulation bandwidth from MBE-grown In(0.35)Ga(0.65)As-GaAs multiple-quantum-well lasers with undoped active regions and with the upper and lower cladding layers grown at different growth temperatures. Short-cavity ridge waveguide lasers achieve CW direct modulation bandwidths up to 40 GHz for 6 x 130 micron(exp 2) devices at a bias current of 155 mA, which is the damping limit for this structure. We further demonstrate large-signal digital modulation up to 20 Gb/s (limited by the measurement setup) and linewidth enhancement factors of 1.4 at the lasing wavelength at threshold of ~1.1 micron for these devices.

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/Alx Ga1-y As and pseudomorphic Iny Ga1-y As/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer hickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO2 films of varying thickness, chemical-vapor-deposited SiOxNy films of varying composition, and spin-on SiO2 films were used as cap layers during rapid thermal annealing (850-950 degree). Photoluminescence at 10K has been employed to determine the interdiffusion-induced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO2) up to a limit determined by saturation of the outdiffused Ga concentration in the SiO2 caps. A maximum concentration of [Ga] = 4-7 x 10 (19) ccm in the Si02 caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiOx Ny cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In0.2 Ga0.8 As/GaAs interdiffusion coefficients, activation energies EA and prefactors D0 were estimated to ranging from 3.04 to 4.74 eV and 5 x 10(3) to 2 x 10(5) square centimeter/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.

We present results from In sub0.35 Ga sub 0.65 As-GaAs 4 QW lasers whose Al sub0.8 Ga sub0.2 As binary short-period superlattice (SPSL) cladding layers were grown at either 700 degree Celsius or 620 degree Celsius. The threshold current densities (J subth) are about 3 times smaller for the lasers whose AlGaAs cladding layers were grown at 620 degree Celsius. At the same time, the internal differential quantum efficiency (n subi) increases from 6O % to 7O %. We attribute the lower J subth and increased n subi to a significant decrease in the concentration of point defects in the MQW active region, consistent with the observed improvement of the diode ideality factor. Temperaturedependent photoluminescence (PL) results suggest that these nonradiative recombination centers are related to point defects recently observed with Raman scattering. Ridge lasers grown with low-temperature SPSL Al subx Ga sub1-x As have achieved 3-dB modulation bandwidths of 24 GHz at record low bias currents of o nly 25 mA.

We report on electron and hole capture times in n-type GaAs/AlAs/Al0.3Ga0.7As double-barrier quantum well (DBQW) structures in an electric field applied perpendicular to the layers. We have measured the time-dependent photoluminescence (PL) originating from the GaAs wells and the Al0.3Ga0.7As barrier layers. The experimental capture times are obtained from least-squares fits of appropriate model functions to the observed PL transients. We have theoretically determined electron capture times for the investigated DBQWs, by calculating the electron wavefunctions and taking into account various scattering processes, including impurity scattering, optical phonon-assisted tunnelling and intervalley scattering. We find evidence that electron capture occurs by Gamma -X intervalley transfer via X-point subbands localized in the AlAs layers.

The cap layer dependence and selectivity of impurity free interdiffusion in molecular beam epitaxially grown pseudomorphic InyGa1-yAs/GaAs multiple quantum well structures with y=0.2 and 0.35 have been studied. The influence of the cap layer composition and deposition technique on the degree of interdiffusion has been investigated using photoluminescence. Whereas variations in compositon are found to result only in small variations in the interdiffusion induced band gap shift, a degree of selectivity suitable for monolithic laser/modulator integration can be achieved by combining electron beam evaporated SiO2 and chemical vapour deposited SiON films as cap layers. In addition, the compatibility of the impurity free interdiffusion process with the fabrication of devices containing highly strained Ino-35Gao-65As/GaAs multiple quantum wells has been demonstrated. Using photoluminescence microscopy and Raman scattering measurements, photoluminescence energy shifts of up to 200 meV have bee n observed without strain relaxation or generation of misfit dislocations, accompanied by a substantial reduction of the photoluminescence line width.

Highly localised carbon doping is demonstrated within the active region of strained InGaAs/GaAs MQW lasers. As predicted by numerical beam propagation simulations, the smaller linewidth enhancement factor arising from the combination of strain and p-doping leads to reduced filamentation in tapered laser structures, as compared to devices fabricated from otherwise identical epilayer structure containing undoped active regions.

We have investigated electron and hole capture times in n-type double-barrier quantum well (DBQW) infrared (3-5 mym) detector structures. Photoluminescence upconversion allows us to observe the relaxation of photoexcited carriers from AlAs/(AlGa)As/AlAs barriers into GaAs quantum wells. For 2 nm AlAs, 25 nm Alsub0.3Gasub0.7As and 5 nm GaAs, the electron capture process is non-resonant, with a capture time of 47 ps at the temperature of 5 K and 28 ps at 77 K. Theoretical calculations show that electron tunneling across the 2 nm AlAs layers mainly occurs via the Gamma-minimum, and that the Gamma-X contributin is negligible.

We show that the spatial distribution of the dopants strongly influences the transport asymmetry and the photovoltage observed in double barrier quantum well intersubband photodetectors. This influence can be quantitatively explained by the local space-charge fields arising from an asymmetry of the doping profile with respect to the well centers. The resulting transport model correctly predicts both the observed transport asymmetry of the photocurrent and the opposite asymmetry of the dark current.

Using GaAs/AlAs/GaAs heterostructures we report measurements of complete hemispheric Augerelectron diffraction patterns originating from single well- defined subsurface sites within a crystal lattice. With this data set a detailed experimental analysis of multiple elastic-scattering effects is performed. We investigate the dependence of the forward enhancement factor along atomic chains on the length and on the internal structure of the chain. No randomization of the elastically scattered electrons is observed even for long traveling path lengths. The angle-dependent features related to elastic scattering as well as those originating from inelastic scattering are evaluated independently to obtain depth profiles of the electron-emitting atoms. Both types of depth profiles indicate a microscopic redistribution of Al and Ga in a surface region of one bilayer thickness.

Resonant Raman scattering by longitudinal optical (LO) phonons in the GaAs barriers of InxGa1-xAs/GaAs multiple quantum well structures has been used to study the onset of strain relaxation as te number of quantum wells is increased. The intensity of scattering from one and two GaAs LO phonons for excitation in resonance with the GaAs E0+delta0 band gap is found to be highly sensitive to the formation and/or redistribution of defects and the buildup of strain in the barriers, and thus to strain relaxation in the InxGa1-xAs quantum wells. This behavior allows us to detect the onset of strain redistribution, even in samples where the frequencies of both the GaAs LO phonon in the barrier layers and the GaAs-like LO phonon in the InxGa1-xAs quantum wells show no measurable shift. Here, the GaAs LO phonon frequency observed in structures with fully strained InxGa1-xAs wells and unstrained GaAs barriers is used as a reference.

Utilizing small-signal direct modulation and relative intensity noise measurements, we investigate changes in the modulation response, the differential gain Deltag/Deltan, the nonlinear gain coefficient e, and the damping factor K, which result from the following three structural modifications to GaAs-based multiple quantum well lasers: 1) the addition of strain in the quantum wells; 2) an increase in the number of quantum wells, and 3) the addition of p-doping in the quantum wells. These modifications are assessed in terms of their potential for reducing the drive current required to achieve a given modulation bandwidth, for increasing the maximum intrinsic modulation bandwidth of the laser, and for improving the prospects for monolithic laser/transistor integration. The differential gain is increased both by replacing unstrained GaAs-Al0.25Ga0.75As QW's with strained In0.35Ga0.65As-GaAs QW's and by increasing the number of strained QW's, ultimately leading to substantial improvements in modulation bandwidth at a given drive current. However, in both cases, the increased differential gain is offset by corresonding increases in the nonlinear gain coefficient, leading to relatively constant values of K and hence little variation in the maximum intrinsic modulation bandwith. By further adding p-doping to the In0.35Ga0.65As-GaAs MQW active region, we have been able to simultaneously increase Deltag/Deltan and decrease K, yielding very efficient high-speed modulation (20 GHz at a dc bias current of 50 mA) and the first semiconductor lasers to achieve a direct modulation bandwidth of 30 GHz under dc bias (heat-sink temperature is equal 25 degree C). Since our laser structures show no significant carrier transport limitations, the measured K factor for the p-doped devices implies a maximum intrinsic 3 dB modulation bandwidth of 63 GHz.

We present detailed experimental results on the properties of multiple-quantum-well waveguide modulators based on strained InGaAs/GaAs layers. Transmission and photocurrent measurements are performed using a tunable Ti-sapphire-laser. The spectra reveal an absorption edge shift as large as 60 nm at 5 V reverse bias. Optimum performance is achieved around a wavelength of 1 Mym, where an extinction ratio of up to 20 dB is obtained with an absorption loss of less than 2 dB/cm. The overall insertion loss of the modulator approaches a constant value of 6.5 dB at higher wavelengths (Lambda equal or bigger than 980 nm) which is shown to be mainly affected by coupling losses.

We have used high-resolution x-ray diffraction and photocurrent spectroscopy to investigate strain relaxation in Insub0.2Gasub0.8As/GaAs multiple quantum wells and its influence on the optoelectronic and electronic properties of high-speed p-i-n photodetectors. In combination with numerical simulations and subband calculations, both methods allowed us to determine the degree of lattice relaxation. The results consistently show that lattice relaxation does not occur abruptly, but that the degree of relaxation increases gradually with increasing number of wells. In spite of the onset of lattice relaxation, these photodetectors exhibit a quantum efficiency of unity and recombination lifetimes in excess of 500 ps.

We report on time-resolved photocurrent measurements in InGaAs/GaAs-quantum well p-i-n photodetector structures in which the photocurrent is optically excited and electrically detected at different locations on the sample. As the electrical pulse propagates out from the point of excitation, a temporal broadening of the signal is induced by the in-plane resistance of the doped contact layers. This temporal broadening agrees quantitatively with analytical solutions obtained from the diffusion equation used to describe the lateral voltage propagation. We discuss the significance of this temporal behavior for the optimization of high-speed photodetectors. Finally, we point out the relationship between our optical-pump/electrical- probe experiments and the all-optical pump/probe experiments reported by Livescu et al.

The authors demonstrate p-type modulation-doped strained-layer Insub0.35Gasub0.65As/GaAs multiquantum well lasers which achieve a 3 dB direct modulation bandwidth of 20 GHz at a low CW drive current of 50 mA in a simple 3 x 200 My square meter mesa structure. For the same device dimensions, a modulation bandwidth of 30 GHz was measured at a CW drive current of 114mA. This is the highest direct modulation bandwidth reported for any semiconductor laser.