ZnO nanorods for efficient third harmonic UV generation.

ZnO nanorods grown by both high temperature vapour phase transport and low temperature chemical bath deposition are very promising sources for UV third harmonic generation. Material grown by both methods show comparable efficiencies, in both cases an order of magnitude higher than surface third harmonic generation at the quartz-air interface of a bare quartz substrate. This result is in stark contrast to the linear optical properties of ZnO nanorods grown by these two methods, which show vastly different PL efficiencies. The third harmonic generated signal is analysed using intensity dependent measurements and interferometric frequency resolved optical gating, allowing extraction of the laser pulse parameters. The comparable levels of efficiency of ZnO grown by these very different methods as sources for third harmonic UV generation provides a broad suite of possible growth methods to suit various substrates, coverage and scalability requirements. Potential application areas range from interferometric frequency resolved optical gating characterization of few cycle fs pulses to single cell UV irradiation for biophysical studies.

Shutterless deposition of phosphorous doped microcrystalline silicon by Cat-CVD

In this paper we present results on phosphorous-doped μc-Si:H by catalytic chemical vapour deposition in a reactor with an internal arrangement that does not include a shutter. An incubation phase of around 20 nm seems to be the result of the uncontrolled conditions that take place during the first stages of deposition. The optimal deposition conditions found lead to a material with a dark conductivity of 12.8 S/cm, an activation energy of 0.026 eV and a crystalline fraction of 0.86. These values make the layers suitable to be implemented in solar cells.

Study of post-deposition contamination in low-temperature deposited polysilicon films

The presence of hydrogen in polysilicon films obtained at low temperatures by hot-wire CVD and the post-deposition oxidation by air-exposure of the films are studied in this paper. The experimental results from several characterization techniques (infrared spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry and wavelength dispersive spectroscopy) showed that hydrogen and oxygen are homogeneously distributed at grain boundaries throughout the depth of the films. Hydrogen is introduced during the growth process and its concentration is higher in samples deposited at lower temperatures. Oxygen diffuses along the grain boundaries and binds to silicon atoms, mainly in Si 2O groups.

Hot wire configuration for depositing device grade nano-crystalline silicon at high deposition rate

The University of Barcelona is developing a pilot-scale hot wire chemical vapor deposition (HW-CVD) set up for the deposition of nano-crystalline silicon (nc-Si:H) on 10 cm × 10 cm glass substrate at high deposition rate. The system manages 12 thin wires of 0.15-0.2 mm diameter in a very dense configuration. This permits depositing very uniform films, with inhomogeneities lower than 2.5%, at high deposition rate (1.5-3 nm/s), and maintaining the substrate temperature relatively low (250 °C). The wire configuration design, based on radicals' diffusion simulation, is exposed and the predicted homogeneity is validated with optical transmission scanning measurements of the deposited samples. Different deposition series were carried out by varying the substrate temperature, the silane to hydrogen dilution and the deposition pressure. By means of Fourier transform infrared spectroscopy (FTIR), the evolution in time of the nc-Si:H vibrational modes was monitored. Particular importance has been given to the study of the material stability against post-deposition oxidation.

Desenvolupament d'hidrogels superficials que contenen motius d'adhesió cel·lular

Projecte de recerca elaborat a partir d’una estada al Departament d’Enginyeria Química del Massachusetts Institute of Technology entre abril i octubre del 2006. S’ha dissenyat i sintetitzat uns nous films polimèrics, amb aplicacions en l’àmbit de l’enginyeria de teixits, utilitzant la tècnica anomenada iCVD (initiated Chemical Vapor Deposition), prèviament desenvolupada pel grup receptor. Es tracta d’uns hidrogels superficials de gruix controlable, que incorporen un monòmer fluorat, el qual s’havia estudiat extensament en el grup d’origen. Aquest monòmer es caracteritza per reaccionar molt fàcilment amb pèptids, de manera que aquests queden units covalentment a la superfície. Diferents estratègies pel desenvolupament d’aquests copolímers han estat avaluades, tant des del punt de vista purament sintètic com de la pròpia aplicació. Les condicions de polimerització han estat optimitzades i els hidrogels s’han caracteritzat químicament per tècniques espectroscòpiques (FTIR, XPS), i físicament per angle de contacte i el·lipsometria. D’aquesta manera, s’ha estudiat la capacitat dels hidrogels d’absorbir aigua i alhora augmentar el seu gruix, depenent de la quantitat d’agent reticulant introduït i de la incorporació del nou monòmer. A continuació, s’han optimitzat les condicions de reacció d’aquestes superfícies amb pèptids que incorporen una molècula fluorescent, la qual permet detectar fàcilment per microscòpia de fluorescència si la reacció ha tingut lloc. Una vegada la plataforma ha estat posada a punt, s’han iniciat assajos cel·lulars tant amb fibroblasts embriònics de ratolí com amb cèl·lules humanes umbilicals. Els resultats preliminars suggereixen una morfologia diferent de les cèl·lules segons si es cultiven sobre films modificats amb pèptids que promouen l’adhesió cel·lular o sobre les seves seqüències permutades no actives. Però, el més interessant és que també s’han observat certes diferències depenent si els films contenen el component hidrogel o no, fet que suggeriria un paper actiu d’aquests noves superfícies en el comportament cel·lular.

Nanostructuració d'òxids mitjançant mètodes de solució química avançats i nanolitografia tova

Un dels reptes tecnològics més importants del segle XXI és el desenvolupament i organització de materials funcionals a escala nanomètrica ja que permet modificar-ne les propietats fonamentals i generar-ne de noves. En el cas dels òxids complexos aquesta tecnologia ha generat grans perspectives en diferents àrees d’estudi perquè presenten propietats molt interessants com la magnetoresistència colossal, superconductivitat o multiferroicitat. En particular, en l’òxid complex superconductor YBa2Cu3O7 (YBCO) s’ha demostrat que la formació de capes superconductores sobre plantilles nanoestructurades o bé la formació de nanocompostos en una matriu superconductora permet millorar de manera espectacular les seves propietats (corrent crític). Aquests resultats introduiran canvis paradigmàtics en la tecnologia de l’energia elèctrica (cables, motors, generadors) i en totes aquelles aplicacions que requereixin camps magnètics intensos. Ara bé, cal aconseguir-ho mitjançant tècniques de baix cost i que permetin un fàcil escalat. Durant els 4 mesos que s’ha disfrutat de l’ajut BP s’han preparat amb èxit noves capes primes superconductores nanoestructurades mitjançant la introducció d’una fase secundària nanomètrica complexa, Ba2YTaO6 (BYTO), dins la matriu d’YBCO pel mètode de baix cost de deposició de solucions químiques. Aquesta nova composició ha donat lloc a un augment significatiu de les propietats superconductores comparat amb la fase tradicional d’YBCO i per tant tenen gran potencial per fabricar futures cintes superconductores.

Bipolar pulsed excitation of erbium-doped nanosilicon light emitting diodes

High quantum efficiency erbium doped silicon nanocluster (Si-NC:Er) light emitting diodes (LEDs) were grown by low-pressure chemical vapor deposition (LPCVD) in a complementary metal-oxide-semiconductor (CMOS) line. Erbium (Er) excitation mechanisms under direct current (DC) and bipolar pulsed electrical injection were studied in a broad range of excitation voltages and frequencies. Under DC excitation, Fowler-Nordheim tunneling of electrons is mediated by Er-related trap states and electroluminescence originates from impact excitation of Er ions. When the bipolar pulsed electrical injection is used, the electron transport and Er excitation mechanism change. Sequential injection of electrons and holes into silicon nanoclusters takes place and nonradiative energy transfer to Er ions is observed. This mechanism occurs in a range of lower driving voltages than those observed in DC and injection frequencies higher than the Er emission rate.

Development of laser-fired contacts for amorphous silicon layers obtained by Hot-Wire CVD

In this work we study aluminium laser-fired contacts for intrinsic amorphous silicon layers deposited by Hot-Wire CVD. This structure could be used as an alternative low temperature back contact for rear passivated heterojunction solar cells. An infrared Nd:YAG laser (1064 nm) has been used to locally fire the aluminium through the thin amorphous silicon layers. Under optimized laser firing parameters, very low specific contact resistances (ρc ∼ 10 mΩ cm2) have been obtained on 2.8 Ω cm p-type c-Si wafers. This investigation focuses on maintaining the passivation quality of the interface without an excessive increase in the series resistance of the device.

Uniformity Study of Amorphous and Microcrystalline Silicon Thin Films Deposited on 10cmx10cm Glass Substrate using Hot Wire CVD Technique

The scaling up of the Hot Wire Chemical Vapor Deposition (HW-CVD) technique to large deposition area can be done using a catalytic net of equal spaced parallel filaments. The large area deposition limit is defined as the limit whenever a further increment of the catalytic net area does not affect the properties of the deposited film. This is the case when a dense catalytic net is spread on a surface considerably larger than that of the film substrate. To study this limit, a system able to hold a net of twelve wires covering a surface of about 20 cm x 20 cm was used to deposit amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon over a substrate of 10 cm x 10 cm placed at a filament-substrate distance ranging from 1 to 2 cm. The uniformity of the film thickness d and optical constants, n(x, λ) and α(x,¯hω), was studied via transmission measurements. The thin film uniformity as a function of the filament-substrate distance was studied. The experimental thickness profile was compared with the theoretical result obtained solving the diffusion equations. The optimization of the filament-substrate distance allowed obtaining films with inhomogeneities lower than ±2.5% and deposition rates higher than 1 nm/s and 4.5 nm/s for (μc-Si:H) and (a-Si:H), respectively.

Low temperature amorphous and nanocrystalline silicon thin film transistors deposited by Hot-Wire CVD on glass substrate

Amorphous and nanocrystalline silicon films obtained by Hot-Wire Chemical Vapor Deposition have been incorporated as active layers in n-type coplanar top gate thin film transistors deposited on glass substrates covered with SiO 2. Amorphous silicon devices exhibited mobility values of 1.3 cm 2 V - 1 s - 1, which are very high taking into account the amorphous nature of the material. Nanocrystalline transistors presented mobility values as high as 11.5 cm 2 V - 1 s - 1 and resulted in low threshold voltage shift (∼ 0.5 V).

Substrate influence on the properties of doped thin silicon layers grown by Cat-CVD

We present structural and electrical properties for p- and n-type layers grown close to the transition between a-Si:H and nc-Si:H onto different substrates: Corning 1737 glass, ZnO:Al-coated glass and stainless steel. Structural properties were observed to depend on the substrate properties for samples grown under the same deposition conditions. Different behaviour was observed for n- and p-type material. Stainless steel seemed to enhance crystallinity when dealing with n-type layers, whereas an increased crystalline fraction was obtained on glass for p-type samples. Electrical conduction in the direction perpendicular to the substrate seemed to be mainly determined by the interfaces or by the existence of an amorphous incubation layer that might determine the electrical behaviour. In the direction perpendicular to the substrate, n-type layers exhibited a lower resistance value than p-type ones, showing better contact properties between the layer and the substrate.

Optimisation of doped microcrystalline silicon films deposited at very low temperatures by Hot-Wire CVD

In this paper we present new results on doped μc-Si:H thin films deposited by hot-wire chemical vapour deposition (HWCVD) in the very low temperature range (125-275°C). The doped layers were obtained by the addition of diborane or phosphine in the gas phase during deposition. The incorporation of boron and phosphorus in the films and their influence on the crystalline fraction are studied by secondary ion mass spectrometry and Raman spectroscopy, respectively. Good electrical transport properties were obtained in this deposition regime, with best dark conductivities of 2.6 and 9.8 S cm -1 for the p- and n-doped films, respectively. The effect of the hydrogen dilution and the layer thickness on the electrical properties are also studied. Some technological conclusions referred to cross contamination could be deduced from the nominally undoped samples obtained in the same chamber after p- and n-type heavily doped layers.

Effect of the nanoparticles on the structure and crystallization of amorphous silicon thin films produced by rf glow discharge

Thin films of nanostructured silicon (ns-Si:H) were deposited by plasma-enhanced chemical vapor deposition in the presence of silicon nanoparticles at 100 C substrate temperature using silane and hydrogen gas mixture under continuous wave (cw) plasma conditions. The nanostructure of the films has been demonstrated by diverse ways: transmission electron microscopy, Raman spectroscopy and x-ray diffraction, which have shown the presence of ordered silicon clusters (1!=2 nm) embedded in an amorphous silicon matrix. Due to the presence of these ordered domains, the films crystallize faster than standard hydrogenated amorphous silicon samples, as evidenced by electrical measurements during the thermal annealing.

Atomic structure of the nanocrystalline Si particles appearing in nanostructured Si thin films produced in low-temperature radiofrequency plasmas

Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced chemical vapor deposition. The term "polymorphous" is used to define silicon material that consists of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever the plasma conditions, the electron diffraction images always revealed the existence of a well-defined crystalline structure different from the diamondlike structure of Si. The formation of nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room temperature.

Calorimetry of hydrogen desorption from a-Si nanoparticles

The process of hydrogen desorption from amorphous silicon (a-Si) nanoparticles grown by plasma-enhanced chemical vapor deposition (PECVD) has been analyzed by differential scanning calorimetry (DSC), mass spectrometry, and infrared spectroscopy, with the aim of quantifying the energy exchanged. Two exothermic peaks centered at 330 and 410 C have been detected with energies per H atom of about 50 meV. This value has been compared with the results of theoretical calculations and is found to agree with the dissociation energy of Si-H groups of about 3.25 eV per H atom, provided that the formation energy per dangling bond in a-Si is about 1.15 eV. It is shown that this result is valid for a-Si:H films, too.