The disease caused by the influenza virus is a global public health problem due to its high rates of morbidity and mortality. Thus, analysis of the information generated by epidemiological surveillance systems has vital importance for health decision making. A retrospective analysis was performed using data generated by the four molecular diagnostic laboratories of the Mexican Social Security Institute between 2010 and 2016. Demographics, influenza positivity, seasonality, treatment choices and vaccination status analyses were performed for the vaccine according to its composition for each season. In all cases, both the different influenza subtypes and different age groups were considered separately. The circulation of A/H1N1pdm09 (48.7%), influenza A/H3N2 (21.1%), influenza B (12.6%), influenza A not subtyped (11%) and influenza A/H1N1 (6.6%) exhibited well-defined annual seasonality between November and March, and there were significant increases in the number of cases every 2 years. An inadequate use of oseltamivir was determined in 38% of cases, and the vaccination status in general varied between 12.1 and 18.5% depending on the season. Our results provide current information about influenza in Mexico and demonstrate the need to update both operational case definitions and medical practice guidelines to reduce the inappropriate use of antibiotics and antivirals.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

The effects of AlN overgrowth on the structural properties of GaN nanostructures grown at 750°C by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy and transmission electron microscopy. The capping process induces a remarkable change in the dimension of the nanostructures. We demonstrate that the thickness/size reduction occurs at the first stage of AlN overgrowth and affects only the top GaN/AlN interface. This phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures and depends on the strain state of the nanostructures.

In this paper we present experimental and theoretical study of intersubband transitions at telecommunication wavelengths in GaN/AlN hexagonal-phase quantum wells grown by molecular beam epitaxy on sapphire substrates. Crossed structural and photoluminescence experiments show that strong in-plane carrier localization occurs due to thickness fluctuations at GaN/AlN interfaces. Fourier transform infrared spectroscopy and photo-induced absorption spectroscopy performed on doped and undoped samples reveal a systematic blue-shift of the e1-e2 transitions with doping due to many body interactions. A good agreement is achieved between experiments and self-consistent Schrödinger-Poisson calculations.

Growth conditions, structural and optical properties of GaN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value and on growth temperature, the growth mode of GaN deposited on AlN can be either of the Stranski-Krastanow or of the Frank-Van der Merwe type. It will be shown that vertical correlation results in a red shift and in a narrowing of the photoluminescence spectra.

Growth of Eu-doped GaN quantum dots embedded in AlN will be described. Intense photoluminescence associated with Eu has been measured, with no GaN band-edge emission, as an evidence that carrier recombination mostly occurs through rare earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of temperature has been put in evidence, as a further confirmation of the recombination of confined carriers through Eu ion excitation.

Epitaxial growth of quaternary AlGalnN compounds by plasma-assisted molecular beam epitaxy has been demonstrated. Two-dimensional growth is achieved under In excess, with a monolayer of In segregating at the growth front. The maximum In incorporation is significantly affected by the substrate temperature and the Al mole fraction of the alloy. This behavior has been attributed to the enhancement of In segregation due to the high binding energy of A1N compared to InN and GaN.

We propose a new procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum from a GaN 2D layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the GaN coverage. We demonstrate that the QD density can be controlled in the 1010 cm−2 to 2 × 1011 cm−2 range. It is shown that beyond a given GaN thickness there is a coexistence between elastic and plastic relaxation.

In this paper, we study the surfactant capability of In for the growth of AlGaN/GaN heterostructures by plasma-assisted molecular beam epitaxy. Growth conditions were determined to have a self-regulated 1×1 In adlayer on AlxGa1-xN (0001). The presence of this In film favors two dimensional growth of AlGaN under stoichiometric conditions, and inhibits the formation of metal droplets on the surface. The quality of these layers was assessed by high resolution X-ray diffraction, atomic force microscopy and photoluminescence.

The present paper gives an overview over different sensor applications of GaN thin films and AlGaN/GaN heterostructures. The response of Pt-GaN Schottky diodes towards hydrogen and hydrogen containing gases is analysed and their gas sensitivity is characterized from room temperature up to 600°C. In addition, the sheet carrier density of a two dimensional electron gas confined at the heterointerface in AlGaN/GaN high electron mobility transistors (HEMTs) is shown to be significantly influenced by changes in the electronic properties of the device surface. This effect is successfully exploited for the realization of ion detectors and sensors for fluid monitoring based on AlGaN/GaN HEMTs with non-metalized gate areas. Promising possibilities of fabricating monolithically integrated sensor devices for wireless signal transmission are demonstrated by the realization of SAW devices on epitaxial AlN-films.

We propose to model the spectral response of p-n junction photodetectors based on gallium nitride and related AlGaN alloys. The model is based on the resolution of the differential equations that govern the excess carrier variation in each layer of the photodiode taking into account all the physical parameters, in particular the presence of deep trap levels in the forbidden gap. We notice that the theoretical results are in good agreement with the experiments. We have also analysed the effect of the recombination velocity at the illuminated surface, as well as the impact of the thickness and the doping density of the p-type layer (illuminated zone) on the spectral response magnitude.

Au/GaN and Cu/GaN Schottky contacts have been studied using X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). Clean and stoechiometric GaN samples were obtained using in situ hydrogen plasma treatment and Ga deposition. The growth of Cu and Au follows Stranski-Krastanov and Frank van der Merwe modes respectively. The interfaces are sharp and non-reactive. Schottky barriers of 1.15eV for Au/GaN and 0.85eV for Cu/GaN were measured using XPS.

Au/GaN and Cu/GaN Schottky contacts have been studied using X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). Clean and stoechiometric GaN samples were obtained using in situ hydrogen plasma treatment and Ga deposition. The growth of Cu and Au follows Stranski-Krastanov and Frank van der Merwe modes respectively. The interfaces are sharp and non-reactive. Schottky barriers of 1.15eV for Au/GaN and 0.85eV for Cu/GaN were measured using XPS.

Schottky barrier photovoltaic detectors based on Si-doped AlxGa1-xN (0 ≤ × ≤ 0.22) have been fabricated and characterized. Samples were grown on basal plane sapphire by LP-MOVPE. Schottky contacts were made with Au. Responsivities are independent of the diode size and of the incident power in the range measured (10mW/m2 to 2KW/m2). The spectral response shows an abrupt cutoff that shifts linearly to higher energy with increasing Al content. A visible rejection of 3 to 4 orders of magnitude is observed in AlxGa1-xN Schottky photodiodes. Device time response is RC-limited, and a minimum decay time as short as 15ns have been estimated in unbiased Al0.22Ga0.28N diodes. This time response can be further reduced by reverse biasing.

The electrical and electroluminescent properties of MOVPE GaN p-n homojunctions have been analyzed as a function of temperature and bias. Electroluminescence is observed for V>3 V under dc and ac conditions. The main emission at low T is a donor-acceptor transition involving shallow acceptors, though it disappears at higher T due to the ionization of the acceptors and compensation by ionized donors. Room temperature dc and ac electroluminescence spectra evolve under increasing bias from a blue-shifting visible band involving deep states at the p-type side of the p-n junction, to a band-to-band UV recombination at high bias. In agreement, the superlinear dependence of light-current characteristics at low current injection becomes linear when the defects are saturated. Time analysis of the spectra vs pulse duration and duty cycle allows the determination of the visible radiative recombination and relaxation times associated to the Mg-related deep states, which are found to behave as acceptors lying 0.55 eV above the valence band. A simple 3-level model is able to explain the visible emission, which involves the conduction band (or shallow donor) and those deep acceptors in the p-layer. Optimum UV/visible ratio emission requires intense and relatively long pulses, with a high duty cycle to impede visible recombination.

Optical thresholds, that correspond to a level located at 1 eV above the valence band, are observed by photocapacitance techniques in n-type Mg-doped GaN. In undoped GaN, this level has been previously related to the yellow emission detected by photoluminescence. In Mg-doped GaN, this yellow luminescence is only observed for excitation energies below the Mg-related band (2.9 - 3 eV). This result evidences that Mg-doping may reduce but not avoid the formation of the yellow band related defects in n-type and semiinsulating Mg-doped samples. The fact that the yellow luminescence is not observed for excitation energies above the bandgap may be justified by a higher efficiency of the Mg-related recombination path.

In this work high gain GaN photoconductive UV detectors have been fabricated and characterized, and a novel gain mechanism, dominant in these detectors, is described. DC responsivities higher than 103A/W have been measured for an incident power of lW/m2 at room temperature. The photoconductive gain depends directly on the bias voltage and scales with incident power as P−k (k ≈ 0.9) for more than five decades. A decrease of both gain and k parameter with temperature has also been observed. As a consequence of the slow non-exponential transient response, AC gain measurements result in lower values for gain and k parameter, which are frequency dependent. The high responsivity, non-linear behavior and slow non-exponential transient response, are all modeled taking into account a modulation mechanism of the layer conductive volume. Such spatial modulation is due to the photovoltaic response of the potential barriers related to the surface and charged dislocations arrays.

High quality AlN layers with full widths at half maximum values of 10 arcmin and average surface roughness (rms) of 48Å were grown by molecular beam epitaxy on Si(111) substrates. A systematic study and optimization of the growth conditions was performed in order to use these AlN layers as buffers in the growth of GaN films. Atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques were employed to determine the surface and structural quality of the layers. Best AlN films were obtained at high substrate temperatures (Tsubs>900°C) and III/V ratios close to stoichiometry. Growth conditions with III/V ratios beyond stoichiometry (Al-rich) did not further improve the crystal quality. In these cases a higher substrate temperature is needed to prevent condensation of Al on the surface. GaN films with full width at half maximum of 10 arcmin and improved optical properties were grown on top of optimized AlN buffer layers.

We have developed a lipid-based microcylinder for the controlled release of biological response modifiers. Lipid microcylinders are composed of 1,2-ditricosa-10,12-diynoyl-sn-glycerol-3-phosphocholine which form hollow cylinders 50–250 μm in length with a diameter approximately 0.5–1.0 μm. Amphiphilic molecules such as ganglioside (5–6 mol%) can be incorporated into the bilayers of the microcylinder, modulating surface characteristics of the structure.

In this study, we have examined the biocompatibility of lipid microcylinders and lipid microcylinders containing 6 mol% ganglioside. The interaction of microcylinders with peripheral blood monocytes and three cell lines: U937, a histocytic monocyte; K562, an erythroblast, and; a Jurkat derivative, a lymphoblast was assessed. Toxicity, as measured by proliferative status of the cell lines, was not evident at lipid concentrations up to 100 μg/ml lipid. Peripheral blood monocytes were observed to closely associate, but not engulf lipid microcylinders. However, when ganglioside was present in the lipid microcylinders this interaction was markedly decreased.

We have begun to measure the release rates of growth factors such as transforming growth factor-beta (TGF-β) from lipid microcylinders. Release of TGF-β from the microcylinders follows first-order kinetics. The rate of release can be modulated by increasing the temperature which results in a thermotrophic phase transition of the lipid at 43 °C.

Transmission electron microscopy observations were conducted on some zwitterionic polymers. Our observations were concentrated on the morphology of the solid state of poly (4-vinyl pyridine-sulfopropyl bctaines), with different degrees of quaternization. Among the morphological features, a microstructure arising from aggregated regions in the range of 50 to 200 Å is observed. Quite striking is the fact that this structure is absent in the micrographs for the base polymer, indicating that the aggregates are due to the interactions of the ion pairs of the polyzwitterions, as in the ionomer case. It is also found that the density of the aggregates increases when we increase the zwitterionic content of the copolymers.