We report on the correlation between the dopant-distribution and the location of ferroelectric domains in Czochralski grown, chromium-doped LiNbO3 single crystals. Optical microscopy, scanning electron microscopy and X-ray topography have been used to show that the domain walls follow the steepest gradient of the Cr-concentration. The experimental findings are explained by model calculations assuming that the development of domains is determined by the elastic deformations due to varying dopant concentration within the crystal. The property relating spatially varying deformation to an electrical field within the crystal (which defines the domain polarity) is assumed to be the piezoelectricity of LiNbO3 below the Curie temperature.

We study the limits of pseudomorphic strain in MBE grown In0.2Ga0.8As/GaAs multiple quantum well structures and the influence of lattice relaxation on the optoelectronic properties of high-speed p-i-n photodetectors with MQWs in the intrinsic region. High-resolution X-ray diffraction yields the degree of lattice relaxation. For the detectors these results are in agreement with photocurrent spectroscopy measurements and subband calculations. The detectors yield a quantum efficiency of unity, in spite of the onset of lattice relaxation.

Dechanneling and angular scan measurements with 2 MeV 4He+ ions have been performed to investigate molecular-beam epitaxially (MBE)-grown strained InyGa1−yAs/GaAs single quantum well (SQW) and multiple quantum well (MQW) structures grown on (100) GaAs substrates. Dechanneling analysis was carried out in the 〈100〉 direction and in the (011) and (011) planes in order to determine defect densities. Since the defect densities were near the detection limit of Rutherford backscattering (RBS), it was necessary to optimize all measurement conditions. Ion channeling angular scans about the 〈110〉 axis were carried out in the (001) plane. In both SQW and MQW structures, the angular difference between the 〈110〉 directions of the substrate and the InyGa1−yAs epilayers (kink angle) was determined from the experimental data. The kink angles of the MQW targets determined by RBS are consistent with the kink angles determined by X-ray diffraction (XRD). The indium concentrations calculated from the kink angles, assuming fully commensurate growth, are lower than expected and differ from the results obtained by photoluminescence (PL).

By analyzing the photocurrent nonlinearity, observed under excitation with two successive picosecond optical pulses, we investigate the carrier transport and electric field recovery in high-speed p-i-n photodetectors containing In0.2Ga0.8As/GaAs quantum wells in the intrinsic region. The nonlinearity originates from the distortion of the electric field by the free carriers generated by the leading pulse, which changes the absorption of the subsequent pulse. The nonlinear signal is resonantly enhanced by orders of magnitude at suitable excitation wavelengths due to excitonic resonances. For the recovery time of the electric field we find values less than 15 ps.

The structural limitations of In0.35Ga0.65As/GaAs MQWs for high-speed laser structures are investigated. The gradual onset of defect formation and strain relaxation are studied in test structures and lasers with i) different numbers of QWs; ii) Be-doped MQWs, and; iii) different substrates. The gradual onset of strain relaxation is observed by PL microscopy. Prior to the onset of strain relaxation, resonant Raman scattering and Rutherford backscattering measurements show increasing defect densities with increasing numbers of QWs. Defect formation and dislocation glide are shown to be strongly influenced by the growth temperature, substrate material and dopant impurities.

We have fabricated p-i-n photodetectors with 10, 15 and 20 well In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) in the intrinsic region, whose bandwidths exceed 23 GHz. Cathodoluminescence (CL) images are characterized by rectangular grids, indicating strain relaxation through the formation of misfit dislocations. The MQW photoluminescence (PL) is strongly affected by the junction electric field, but PL from the p++ GaAs cap shows that increasing the number of QWs enhances the non-radiative recombination associated with the misfit dislocation network. Infrared absorption measurements reveal sharp, room temperature excitonic absorption at 980 nm, which shifts to 1000 nm with a 6 V reverse bias. The peak excitonic absorption intensity and linewidth improve as the number of wells increases. Room temperature dark current measurements on 250 μm x 250 μm p-i-n photodetectors yield leakage currents as low as 2.5 nA for a reverse bias of 4.4 V. These results demonstrate the usefulness the usefulness of strain-relaxed InGaAs/GaAs MQW photodetectors for high speed applications.

We have used high-resolution x-ray diffraction and photocurrent spectroscopy to investigate strain relaxation in In0.2Ga0.8As/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.

In the design of heteroepitaxial systems the knowledge of composition, strain and thickness of the various layers involved is of great importance. The determination of these structural parameters is among the applications most common in high-resolution X-ray diffraction. Numerical simulation of rocking curves has emerged as an important aid in the determination of the relevant structural parameters. Based upon the dynamical theory of X-ray diffraction a FORTRAN program has been written to simulate and refine structural parameters from rocking curve data. X-ray rocking curve analysis has been used to develop and optimize MOCVD-growth of Ga(x)In1-xP/GaAs heteroepitaxial systems. We observed superlattice peaks in samples of low misfit. With the aid of computer simulations we attribute these to unintentional periodic compositional fluctuations (Ga/In) of the order of 1%.