J.G. Mendoza-Alvarez

Analysis of depletion edge translation lightwave modulators

Presents a complete analysis of waveguide phase modulators based on the depletion-edge-translation concept. The phenomena taking place inside the depletion region which contribute to changing the refractive index there are studied. It is shown that the behavior of these modulators can be understood in terms of two electric field-related and two carrier-related effects: linear electrooptic, electrorefractive, plasma, and band filling. The sum of the refractive index variations produced by each one of these effects, taking into account the waveguide geometry, accounts quantitatively for the experimental phase shifts measured in the devices. No fitting parameters are used and a very good agreement between theory and experiment is obtained. Based on this theory, an analysis of the device is made in terms of the optimum values for the doping in the waveguide, and also in terms of the wavelength dependence of the device phase modulation properties. >

Refractive index dependence on free carriers for GaAs

This work reports on the influence of the injected free carriers on GaAs refractive index N at 297 °K. The variation of N caused by injected free carriers was theoretically calculated in a more complete way than has been performed earlier. New results were obtained rather than those of a reducing effect on N in a linearlike dependence on the injected free carrier concentration n. They are: (1) linearlike dependence of N on n occurs only beyond a certain value n1 and (2) an increasing effect on N is caused by the injected free carriers for concentrations in the range between n=0 and a certain value of n=nc (nc<n1) . In the range nc<n<n1 a nonlinear decreasing effect was obtained. Effect (2) has not been noticed up to now, so far as the authors know.

Temperature dependence of the band gap energy of crystalline CdTe

In this paper we report the band gap energy value in CdTe single crystals by measuring the free exciton energy in the photoluminescence spectra taken at different temperatures and for two laser light intensities. The results have been analyzed using the Manoogian-Wooley (M-W) equation; by comparing them with the expression proposed by Vina; and using Varshnis expression for temperature variation of the band gap energy. The best fit was obtained with the M-W equation, and in relation with the UT s term in this equation, our analysis showed an s-value close to the unity, as has been reported for other materials. The value of U was very close to the theoretical prediction involving the product of the thermal expansion coefficient and the deformation potential of the material. From the term related to the electron-phonon interaction, we obtained a Debye temperature close to the tabulated value. Fitting the experimental data to the theoretical expression for EgOTU; the band gap energy for a temperature of 4.2 K was calculated to be 1.6065 eV in agreement with the well-established value for this material; a band gap energy of 1.5125 eV is proposed for a temperature of 300 K. q 2000 Elsevier Science Ltd. All rights reserved.

ac impedance analysis on crystalline layered and polycrystalline bismuth titanate

A comparison is made on the dielectric properties of the ferroelectric perovskite bismuth titanate for both crystalline (along different directions) and polycrystalline (with and without flux) samples, using the complex ac impedance technique. Results for the impedance, conductivity activation energies, bulk dielectric constant, and temperature behavior of the low‐frequency dielectric constant are presented. We discuss these results in terms of the conduction mechanisms depending on the nature of the sample, and compare results for crystalline and polycrystalline samples. Also, we show the important influence of the flux on the dielectric properties of the different samples, which must be taken into account in the use of these materials as substrates for the growth of superconducting thin‐film ceramic oxides.

Photoluminescence measurements in the phase transition region for CdS thin films

CdS polycrystalline thin films were grown by the chemical bath deposition technique at 80 °C onto glass substrates. The films grow in the cubic crystalline structure as determined by x‐ray diffraction analysis. After thermal annealing in S2 and Ar atmospheres, the CdS changes from the metastable zinc blende phase to a stable wurtzite one. The cubic‐to‐hexagonal transition temperature has been determined to be 370 °C, as seen by the photoluminescence spectra and the x‐ray diffraction patterns of the different samples. These spectra show the well‐known green emission band of the CdS centered at 2.4 eV for the as‐grown sample, which shifts to 2.25 eV for the sample annealed at 365 °C just before the phase transition takes place. For the sample annealed at 374 °C, an abrupt blueshift of the green band occurs going back to an energy value of 2.4 eV, when the crystalline phase transition occurs.

Influence of Cd vacancies on the photoluminescence of CdTe

Cd vacancies are identified as responsible for both the 1.55‐eV narrow band and the 1.591‐eV bound exciton peak of the CdTe photoluminescence spectrum. The characteristic 1.44‐eV broadband does not seem to be related to luminescent centers involving those vacancies.

Photoacoustic determination of non-radiative carrier lifetimes

From photoacoustic (PA) experiments we determine the nonradiative carrier lifetime in direct band-gap semiconductors. We use the Rosencwaig and Gerscho model to calculate the PA signal in semiconductors taking into account the distinction between non-radiative and radiative carrier lifetimes. We have assumed that for our high quality crystalline samples, the main contribution to the non-radiative processes comes from CHCC and CHSH Auger recombination for n and p-type materials, respectively. For GaAs, InSb and GaSb samples, the experimental data obtained by means of an open photoacoustic cell were fitted to the theoretical model and we show that the values we determined for the non-radiative recombination lifetime agree well with those reported in the literature.

Contribution of the band‐filling effect to the effective refractive‐index change in double‐heterostructure GaAs/AlGaAs phase modulators

In this work we have theoretically estimated the effective refractive‐index variation Δneff due to the removal of carriers in the depletion region of a double‐heterostructure (DH) n‐AlGaAs/n‐GaAs/p‐AlGaAs phase modulator when a reverse bias is applied to this structure. Δneff is calculated by a Kramers–Kronig analysis of the absorption spectrum, which changes around the absorption edge because of the band‐filling effect associated with the presence of carriers. We also calculated the phase shift Δφ due to Δneff, and we showed that this contribution due to the band‐filling effect can account for up to 28% of the total phase shift for a specific DH structure, showing in this way that the band‐filling effect can no longer be ignored in the analysis of phase modulators.

On the thermal characterization of two-layer systems by means of the photoacoustic effect

In this work, the problem of the thermal characterization of two-layer systems by means of the photoacoustic technique is discussed. For a two-layer system under rear-side illumination conditions, we have applied the Rosencwaig and Gersho model for calculating the pressure fluctuation in the photoacoustic gas chamber. The limiting cases in which both layers are thermally thin, thermally thick and one layer is thermally thin and the other is thermally thick are discussed. When both layers are thermally thin, a consistent equation for the heat capacity is obtained and an effective thermal diffusivity equation is derived when both layers are thermally thick. In order to test our theoretical results, we apply them to two-layer systems consisting of AlGaAs layers of different Al concentrations, grown by liquid phase epitaxy on GaAs substrates. The results of our measurements are in good agreement with the theoretical predictions. Our results show the general character of the expression for the effective thermal diffusivity of two-layer systems reported by Mansanares et al (1990 Phys. Rev. B 42 4477).

On the bowing parameter in Cd1−xZnxTe

Cd1−xZnxTe thin films were prepared on 7059 Corning glass substrates using an rf magnetron sputtering system and CdTe(1−y)+ZnTe(y) targets. The concentration (x) of Zn in the films did not coincide with the relative weight (y) of the ZnTe powder in the compressed targets. Values of x were in the range 0–0.30 as determined from x-ray diffraction patterns. The band gap energy (Eg) of the Cd1−xZnxTe samples was calculated from the photoreflectance spectra measured on the films. The position of the experimental points in the Eg versus x plot show a deviation from the phenomenological quadratic relation Eg=Eg0+ax+bx2 calculated within the virtual crystal approximation (VCA). The depart of the Eg values for higher Zn concentrations from the expected VCA model is probably due to a larger clustering of Zn atoms and/or a percolation phenomena. We obtain fitted values for the parameters a and b within the VCA approach. Comparison with data reported by other authors is made.