H. Navarro-Contreras

Raman scattering study of photoluminescent spark-processed porous InP

Abstract Raman scattering has been used to study porous InP (001) samples prepared by the application of high voltage spark discharges in air and argon atmospheres. For photoluminescent material, Raman scattering as well as Normaski microscopy of a transversally cut sample, (011) face, show the existence of two very distinct zones, that lie at different depths: a superficial luminescent region constituted mostly by In 2 O 3 and InPO 4 oxides, and a second adjacent deeper zone formed by damaged InP. These results highlight the role that the oxidation plays in this material as source of the visible luminescence that the material emits when excited with UV radiation. The deepest region shows InP-like vibrational behavior with broad longitudinal optical (LO) and transversal optical (TO) phonon bands. A fit of the observed lineshapes of the TO and LO modes using the spatial correlation model provides an estimate of the size of the crystalline regions ( L ∼30 A).

Epitaxial Growth of Strained Ge Films on GaAs(001)

Ge films were grown epitaxially as well as pseudomorphically on GaAs(001) substrates at different temperatures using high-vacuum magnetron sputtering. The crystal quality of the resultant layers was examined by high-resolution X-ray diffraction (HRXRD). The rocking curves of Ge layer grown at temperatures greater than 470°C show clear Pendellosung oscillations, confirming the high crystalline quality of the Ge layers. The angular position of the Ge diffraction peak is observed shifted from that for a pure Ge layer grown pseudomorphically on GaAs. Calculations from the lattice parameter model are consistent with the supposition of Ge films with high concentrations of As or Ga, as reconfirmed by Hall effect measurements. Atomic Force Microscope (AFM) measurements performed to characterize the Ge surface, indicate that the RMS surface roughness is typically 30 A, but that it can be as low as 3 A for intermediate temperatures, a value that compares favorable with those obtained for molecular beam epitaxially grown material.

Dislocation densities in MBE grown ZnSe epitaxial layers on GaAs by HRXRD

Abstract In this work, we report on dislocation densities in MBE grown ZnSe/GaAs/GaAs heterostructures of different epilayer thickness. We examined the full-width at half-maximum of the reflection peaks obtained by high-resolution X-ray diffraction. We observed three regimes of dislocation generation. The first regime is present for samples of subcritical thickness, where only stacking faults are present and their formation depends on the conditions of preparation of the substrate and the initial conditions of growth. The second regime exists for samples with layer thickness greater than the critical thickness and thinner than a threshold thickness h t ≅0.3xa0μm, where a large amount of misfit and threading dislocations are generated. The third regime starts for layer thickness greater than h t . In this regime the formation of dislocations is substantially slowed down and the dislocation density follows a 1/ h dependence as predicted by the glide model. Comparing dislocation densities obtained by X-ray diffractometry with the pit density revealed by chemical etching of one sample and those observed by transmission electron microscopy of two samples, agreement exists among them within the same order of magnitude.

High purity GaSb grown by LPE in a sapphire boat

Abstract GaSb layers grown by LPE in a sapphire boat exhibit higher free hole concentrations than layers grown at the same temperature but in a graphite boat. In both cases, as the growth temperature is lowered, the free hole concentration diminishes and eventually the layers’ conductivity change to N type. The transition temperature is lower for the samples grown in the sapphire boat. This behavior is explained assuming that the residual impurities coming from the graphite boat are mainly donors, making the concentration of the native acceptors less compensated in GaSb grown in the sapphire boat. The free carrier concentrations and their mobility were determined by infrared reflectivity. Evidence of the compensation effect was obtained from the photoluminescence spectra of the samples.

Raman studies of aluminum induced microcrystallization of n+ Si:H films produced by PECVD

Abstract We performed a Raman scattering study of aluminum induced microcrystallization of thin films of phosphorous-doped hydrogenated amorphous silicon (n + a-Si:H). These thin films of heavily doped n + a-Si:H were prepared by plasma enhanced chemical vapor deposition. Afterwards, aluminum was deposited and followed by an annealing process at 523 K in a nitrogen environment during several hours. Raman results reveal the formation of microcrystalline regions distributed in the amorphous matrix, induced by the film annealing in the presence of the aluminum. We have used the spatial correlation model to estimate from the Raman signal the microcrystallite size and its relation with the annealing time. The estimated crystallite size was found to be between 6.8 and 9.5 nm and the broadening and downshift of the signals are explained in terms of the crystallite size and lattice expansion effects due to the annealing process. Conductivity values of the samples as a function of the annealing time are explained in terms of the contributions from the amorphous and from the microcrystalline phases.

Structural study of metastable (GaAs)1-X(Ge2)X thin films grown by RF magnetron sputtering

Abstract Epitaxial metastable (GaAs) 1− X (Ge 2 ) X thin films have been grown on GaAs(1xa00xa00) in a RF planar magnetron sputtering system (MS), without As overpressure, throughout most of the full compositional X range. The structural and compositional properties were investigated by high resolution X-ray diffraction (HRXRD) and secondary ion mass spectrometry (SIMS). The SIMS depth profiles demonstrate good homogeneity of Ge concentration for all the samples. HRXRD measurements show that the lattice constant dependence on the concentration X does not obey Vegards law and that a transition between zinc-blende and diamond crystal structures occurs at X ≈0.35

Stress in GaAs at the hetero-interface of ZnSe/GaAs/GaAs: a possible effect of pit filling and difference in thermal expansion coefficients

We report on the observation of stress effects on GaAs at the ZnSe–GaAs hetero-interface. Samples with the structure ZnSe/GaAs/GaAs(100) were grown by molecular beam epitaxy (MBE). ZnSe epilayers thickness ranged from 0.08 to 0.6 μm. Hetero-interfacial stress effects were investigated by photoreflectance (PR) and reflectance-difference spectroscopy (RDS). From a comparison between PR spectra and the second energy-derivative of the RDS spectra (SDRD) we conclude that both PR and RDS spectra have two components: (1) a bulk-like signal as for bare GaAs and (2) a signal coming from a strained region near the ZnSe–GaAs hetero-interface. From the theory of PR we estimate that the observed compressive strain giving rise to the second component has a value e≅−0.0010±0.0004, independent of the thickness of the ZnSe epilayer. Atomic force microscopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnSe growth revealing an almost uniform density of pits for all samples observed. These have irregular cross-section profiles, a situation that tends to preclude coherent growth between the ZnSe and the GaAs. We calculate that there has to be present a strain in the upper atomic layers of the GaAs due to the incoherent growth of ZnSe inside the GaAs pits and to the difference in thermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of same magnitude as that observed by PR.

Excitonic transitions in (GaAs)1−x(Ge2)x/GaAs multilayers grown by magnetron sputtering

We report on excitonic transitions observed at room temperature in multilayers of (GaAs)1−x(Ge2)x/GaAs grown by magnetron sputtering. We attribute the observation of the exciton absorption at room temperature to confinement effects on the bound electron–hole pairs inside the Ge-rich layers of the (GaAs)1−x(Ge2)x alloys. From secondary ions mass spectroscopy these layers are measured to be 50, 33, and 30 nm in full width at half maximum (FWHM), for three different samples. These layers alternate with (GaAs)1−x(Ge2)x alloy layers of very low Ge concentration that can be considered of plain GaAs, ∼360u2009nm in thickness for two samples and 240 nm for a third one. The observed transition energies of the exciton are very close to ≈1u2009eV both at 300 and 4 K. A calculation of exciton energy-level spacing and transition probabilities provides acceptable values for the exciton transition energies observed, considering a triangular quantum well as an approximation to the Ge profile in the confinement layers. The observ...

Strain in GaAs at the heterointerface of ZnSe/GaAs/GaAs

GaAs at interfaces of molecular beam epitaxy (MBE) grown ZnSe/GaAs/GaAs films with ZnSe layers of different thicknesses is studied by photoreflectance (PR) spectroscopy. We can separate two different near-band-edge optical features originating from two different regions of the heterostructure by using in-phase and out-phase PR measurements as well as the results of two different wavelength pumping lasers. One of the transitions is a bulk-like signal as for bare GaAs and another signal is attributed to a strained region adjacent to the ZnSe/GaAs interface. The bulk-like signal originates in a region that encompasses the buffer layer/substrate GaAs interface, as is also revealed by the observation of Franz--Keldysh oscillations in the transitions from this region, which manifest the existence of an electric field within it. An electric field which is larger in magnitude is also visible in the PR signal from the heterointerface. Results for the second derivative of reflectance difference spectra (SDRD) further supports the existence of two spatially separated regions in the GaAs that produces two independent overlapping optical modulated signals in these heterostructures. From the theory of PR we estimate that the observed compressive strain giving rise to the second component has a value , which is independent of the thickness of the ZnSe epilayer. Atomic force microscopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnSe growth revealing an almost uniform density of pits for all samples observed. These have irregular cross section profiles, a situation that tends to preclude coherent growth between the ZnSe and the GaAs. We calculate that a strain in the upper atomic layers of the GaAs has to be present due to the incoherent growth of ZnSe inside the GaAs pits and to the difference in thermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of the same magnitude as that observed by PR.

Lattice vibrations study of Ga1-xInxAsySb1-y quaternary alloys with low (In, As) content grown by liquid phase epitaxy

Raman scattering spectroscopy was used to measure and analyze the lattice vibrations in some quaternary Ga1−xInxAsySb1−y alloys with low (In, As) contents, (0.03 <x< 0.12 and 0.03 <y< 0.10). The layers were grown by liquid phase epitaxy on (001) GaSb substrates at 540°C. High Resolution X-Ray Diffraction results showed profiles associated with a quaternary layer lattice matched to the GaSb substrate as obtained from the (004) reflection. The experimental diffractograms were simulated to estimate alloy composition, thickness and lattice mismatch of the layer. Raman scattering results show phonon frequencies associated to the TO and LO GaAs-like modes as well as GaSb + InAs-like mode, which are characteristic of this quaternary alloy. The As content dependence of the phonon frequency measured in this alloy for low (In, As) contents agree well with the modified Random-Element Isodisplacement (REI) model and also with other available experimental reports. This method can also be used to estimate alloy compositions for this kind of quaternary alloys.