Angel M. Ardila

Temperature dependence of the Raman shift in GaAs conformal layers grown by hydride vapor phase epitaxy

Raman spectra between room temperature and 350 °C were measured in GaAs layers grown by hydride vapor phase epitaxy on Si substrates using the selective conformal growth method. The contributions of the thermal expansion, anharmonic phonon decay, and strain are considered in order to analyze the Raman data. The tensile strain in the conventional GaAs/Si seed and in the conformal GaAs layers was determined from the Raman spectra. It is shown that the thin SiO2 layer between the GaAs and the Si substrate is a compliant layer that plays an important role in the reduction of the dislocation density in the conformal layers. The tensile strain in conformal layers was higher than in the conventional GaAs/Si layers, in which strain is relieved by the high density of dislocations.

Self-doping near the seed/layer interface in conformal GaAs layers grown on Si

Undoped GaAs layers grown on Si substrates by the conformal method were studied by micro-Raman spectroscopy, cathodoluminescence, and diluted Sirtl solution with light (DSL) etching. The results show that nonintentional doping of conformal layers can take place near the seed/layer interface. The self-doped area presents a bright luminescence emission and shows longitudinal optic-plasmon coupled Raman modes. The nonintentional dopants were n type as deduced from Raman spectroscopy and DSL selective etching. The doped region extends only 2–3 μm from the seed and was tentatively associated with enhanced diffusion of Si in the presence of dislocations at the interface between the seed and the conformal layer.

Cell volume variation under different concentrations of saline solution (NaCL)

ABSTRACT Background: It is very important to know the volumetric relation between the cellular contraction and the concentration of saline solution (NaCl) used in clinical practice. Methods: Blood samples of healthy volunteers were exposed to different concentrations of NaCl ranging from 0.42% to 11.70% and the resultant variation in the hematocrit was observed. The variation of the cellular volume as a function of the corresponding concentration of NaCl was studied. An equation theoretically predicting the observed behavior was obtained Results: We studied 53 blood samples of 12 healthy volunteers. The relationship between cellular contraction and the concentration of NaCl shows that the maximum contraction of cellular volume is obtained when NaCl is about 5.85%. The mathematical model derived is summarized in the equation. Δ V = 1 C 4 Δ V means the cellular variation to the corresponding NaCl concentration. This model has a linear regression coefficient of 0.996 and applying chi-square goodness of fit test is observed a level of confidence superior to 99.5%. It was also noted that concentrations equal to or higher than 7.02% of NaCl provokes some degree of hemolysis. Conclusions: There is no reason to use concentrations of NaCl higher than 5.85%. Higher concentrations do not induce more contraction of cellular volume, but they do provoke cellular damage.

Cathodoluminescence study of Si complex formation in self-doped and intentionally Si-doped GaAs conformal layers

An Si complex formation is observed in unintentionally doped as well as n-type doped GaAs layers grown laterally on GaAs seeds deposited on (100) Si substrates. The free carrier concentration was accurately assessed by previous Raman studies. Particularly interesting was the observation of a doped stripe nearby the GaAs seed in the unintentionally doped layers. In this work the formation of the complexes in both kinds of doped samples was analysed by means of SEM-cathodoluminescence (CL) studies. Different bands, indicative of the presence of more than one Si-related transition, have been observed in the infrared spectral region. The distribution of some of the related complexes at the different depths along the growth axis was analysed by depth-resolved CL. The nature of the emission levels has been investigated by power dependent CL studies, allowing to correlate them to DAP transitions. The Si autodoping origin of the doped stripes on the unintentionally doped samples was also confirmed.

Study of defects in conformal GaAs/Si layers by optical techniques and photoetching

We present the characterization of crystal defects in GaAs layers grown on silicon substrates by the conformal growth method. This technique consists of GaAs lateral growth from GaAs seeds confined in between the Si substrate and an overhanging dielectric cap layer. These conformal layers grow with a reduced density of crystal defects, e.g. dislocations, which are effectively filtered by the particular geometrical configuration. The samples were analyzed by micro-Raman spectroscopy, cathodoluminescence, phase stepping microscopy and were etched by diluted stirl solution applied with light (DSL). Several structures were revealed and analyzed, e.g. a quasi-periodic array of hillocks and valleys that are spatially correlated with fluctuations of the luminescence intensity. Other revealed crystal defects were grooves, hillocks and cracks. The main properties of these defects and their possible origin are studied.

Study of the X-Ray Attenuation as a Function of the Density and Thickness of the Absorbent: Cortical Bone and BaSO4

In the present paper a non-metallic material to attenuate X-rays is proposed, mainly for medical imaging applications in order to reduce the radiation dose received by patients due to dispersion. For this purpose, the filing of cortical bone and barium sulfate (BaSO4) were characterized using X-ray diffraction, energy-dispersive X-ray analysis and Raman spectroscopy techniques. Attenuation capacity of the X-rays was determined using an X-rays equipment (10–30 kV) and a Geiger-Muller detector, bearing in mind that the intensity of the transmitted radiation depends on the thickness and density of the material, having a 1 mm lead sheet as reference. In addition, a radiation attenuation comparison dose emmited by a dental X-ray generator using TLD-100 thermoluminescent crystals and periapical radiographic plates is presented, identifying that BaSO4 is the material that attenuates the better this type of radiation compared to cortical bone.

Microcharacterization of Conformal GaAs on Si Layers by Spatially Resolved Optical Techniques

The growth of III-V layers on Si substrates has the interest to combine the high performance of these semiconductors with the mature technology of Silicon. However, the large lattice mismatch and the difference in the thermal expansion coefficients prevent the obtention of layers free of crystal defects for minority carrier devices. Recently, a growth technique, the so-called conformal growth, allows the obtention of nearly defect free GaAs and AlGaAs layers on Si substrates. This method is basically a confined lateral growth. Layers of submicrometric thickness with very low defect concentration are obtained. We present herein a study of some GaAs conformal layers on Si. The layers are characterized by spatially resolved techniques as PhotoLuminescence Imaging (PLI), Cathodoluminescence (CL) and microRaman. Different properties of these layers are studied. In particular, the stress distribution, the existence of growth failures or the free carrier distribution in doped layers. Some others aspects as the influence of the seed orientation, the existence of nucleations and the crystalline order are also discussed.

Properties of AlGaAs layers grown on Si by the conformal method

High quality AlGaAs/Si layers were obtained by metal organic vapour phase epitaxy using the conformal growth technique. These layers were characterized by means of cathodoluminescence, microRaman, photoluminescence imaging and optical interferometry in the phase stepping mode. The main issues regarding these layers are studied, e.g. Al distribution, quality of the layers, growth front, homogeneity, etc. Special attention was paid to the incorporation of Al. The results herein presented show that an improvement of the quality of AlGaAs/Si heterostructures can be achieved by the conformal growth method.

Study of doping in GaAs layers by local probe techniques: micro-Raman, micro-photoluminescence and cathodoluminescence

The free carrier concentration of GaAs layers grown by MOCVD either on GaAs or Si substrates, by the conformal method in the last case, was obtained from the micro-Raman spectra using the hydrodynamic approach to fit the LO phonon-plasmon coupled Raman modes. The results on homoepitaxial layers were used as a calibration of the fitting method. The measurements in the selectively doped conformal layers were then compared with data obtained by micro-photoluminescence and cathodoluminescence spectroscopy and imaging. The doping data are compared with those deduced from the room temperature micro-photoluminescence and cathodolumiescence spectra.

Doping profiles of n-type GaAs layers grown on Si by the conformal method

Abstract : We study doping profiles in selectively Si-doped GaAs layers grown by the conformal method, This growth technique allows to obtain GaAs/Si with optoelectronic quality. The samples are laterally grown, and selective doping with Si is carried out in such a way that doped stripes are intercalated with undoped ones. The study of the doping profiles was carried out by cathodoluminescence (CL) and micro-Raman (Micro-R) spectroscopy. Abrupt doping profiles between doped and undoped stripes were demonstrated by monochromatic CL images. Deep level related CL bands can be observed between 1000 and 1400 nm, evidencing the complex mechanism for Si incorporation at the growth temperature (730 deg C). Net doping concentrations and mobilities across the layers were determined from the analysis of the phonon-plasmon coupled modes in the Micro-R spectra obtained with a lateral resolution better than 1 micrometer.