Gudrun Andrä

Nanowires Enabling Signal‐Enhanced Nanoscale Raman Spectroscopy

Silicon nanowires grown by the vapor-liquid-solid (VLS) mechanism catalyzed by gold show gold caps (droplets) approximately 20-500 nm in diameter with a half spherical towards almost spherical shape. These gold droplets are well suited to exploit the surface-enhanced Raman scattering (SERS) effect and could be used for tip-enhanced Raman spectroscopy (TERS). The gold droplet of a nanowire attached to an atomic force microscopy (AFM) tip could locally enhance the Raman signal and increase the spatial resolution. Used as a SERS template, an ensemble of self-organizing nanowires grown bottom up on a silicon substrate could allow highly sensitive signal-enhanced Raman spectroscopy of materials that show a characteristic Raman signature. A combination of a nanowire-based TERS probe and a nanowire-based SERS substrate promises optimized signal enhancement so that the detection of highly dilute species, even single molecules or single bacteria or DNA strands, and other soft matter is within reach. Potential applications of this novel nanowire-based SERS and TERS solution lie in the fields of biomedical and life sciences, as well as security and solid-state research such as silicon technology.

Laser Induced Crystallization of Amorphous Silicon Films on Glass for Thin Film Solar Cells

Two different methods of laser induced crystallization for preparing large grained polycrystalline silicon thin films on glass are reported. The first one is a lateral epitactic crystallization process following melting by an Ar+ laser. The second one is an explosive crystallization process. Both methods lead to crystal grains of several 10 μm in size. The films, 200 to 500 nm thick, may be used as a seed layer for an epitactic thickening process leading to solar cells.

PEDOT:PSS emitters on multicrystalline silicon thin-film absorbers for hybrid solar cells

We fabricated an efficient hybrid solar cell by spin coating poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) on planar multicrystalline Si (mc-Si) thin films. The only 5 μm thin Si absorber layers were prepared by diode laser crystallization of amorphous Si deposited by electron beam evaporation on glass. On these absorber layers, we studied the effect of SiOx and Al2O3 terminated Si surfaces. The short circuit density and power conversion efficiency (PCE) of the mc-Si/Al2O3/PEDOT:PSS solar cell increase from 20.6 to 25.4 mA/cm2 and from 7.3% to 10.3%, respectively, as compared to the mc-Si/SiOx/PEDOT:PSS cell. Al2O3 lowers the interface recombination and improves the adhesion of the polymer film on the hydrophobic mc-Si thin film. Open circuit voltages up to 604 mV were reached. This study demonstrates the highest PCE so far of a hybrid solar cell with a planar thin film Si absorber.

Preparation of thick polycrystalline silicon layers on glass by laser irradiation

Abstract For polycrystalline silicon thin film solar cells a silicon layer 50 μm thick is required consisting of grains 100 μm in diameter deposited on low cost glass substrate. We report on a preparation method combining plasma enhanced CVD of amorphous silicon and laser crystallization. We start from a-Si:H thin films 200 nm thick which are deposited on glass (Corning 7059) by a rf-CVD process. These films are irradiated by scanning with an Ar+ laser to result in crystals of several 10 μm in diameter. In order to increase the film thickness on this crystalline seed layer further amorphous silicon is deposited by the same CVD process at a rate of 20 nm/min. During the deposition the growing layer is irradiated by excimer laser pulses with about 300 mJ/cm2 at a repetition rate of less than 0.1 Hz. Each laser pulse melts the newly deposited amorphous layer down to the crystalline interface which acts as a homoepitactic substrate during resolidification. In this way the whole growing amorphous layer is converted to a polycrystal.

Silicon Nanowire Solar Cells With Radial p-n Heterojunction on Crystalline Silicon Thin Films: Light Trapping Properties

We present a concept for a core-shell silicon nanowire thin-film solar cell showing strong light trapping. Nanowires are wet chemically etched into a several micrometer-thick laser-crystallized silicon thin film on glass. The nanowires are equipped with an a-Si heteroemitter deposited as a shell around the nanowires by plasma-enhanced chemical vapor deposition to achieve a radial p-n heterojunction. The space between the nanowires is filled with ZnO:Al, acting as a transparent contact. Our core-shell nanowire solar cells reached an efficiency of 8.8%. The main emphasis of this study is on the optical properties of the nanowire solar cell system.

In-situ diagnostics for preparation of laser crystallized silicon films on glass for solar cells

Polycrystalline silicon thin film solar cells require coarse grained silicon layers on a glass substrate. The preparation starts with a layer of amorphous silicon some hundred nanometers thick. By laser crystallization it is converted into a seed layer consisting of grains several ten μm in size. We report on in situ diagnostics by time resolved reflection and transmission (TRRT) measurements during the preparation process. Joint diagnostics by different lasers and the comparison with optical and electron micrographs of the resulting films give unique information about the crystallization processes. Even if different processes occur in hydrogenated or hydrogen free amorphous silicon films during the heating induced by different irradiation parameters the results of crystallization are quite similar.

Temperature dependent optical properties of amorphous silicon for diode laser crystallization.

The temperature dependent optical parameters n and k of amorphous silicon deposited by electron beam evaporation were determined at the wavelength of 808 nm. This was achieved by fitting an optical model of the layer system to reflection values of a fs-laser beam. From n(T) and k(T) the absorption of a-Si layers as depending on thickness and temperature were calculated for this diode laser wavelength. By heating the layers to 600 °C the absorption can be increased by a factor of 4 as compared to room temperature, which allows for diode laser crystallization of layers down to 80 nm in thickness.

Varying the layer structure in multicrystalline LLC-silicon thin-film solar cells

Multicrystalline silicon thin-film solar cells with grains exceeding 100 μm were prepared by layered laser crystallization. The layer system is generated in two steps. In the first step a multicrystalline seed layer is fabricated on a low cost glass substrate. This is achieved by depositing a-Si followed by scanning a diode laser beam for crystallization. In a second step this seed layer is epitaxially thickened by electron beam evaporation of a-Si combined with repeatedly applying pulses of an excimer laser. p⁺pn⁺ and p⁺nn⁺ superstrate cells with 2 μm thick absorber were prepared with different doping levels and different thickness of the seed layer. Without reflector these cells, after hydrogen passivation, delivered V<inf>oc</inf> up to 514 mV and I<inf>sc</inf> of 17.5 mA/cm² if deposited directly onto the glass substrate. With an additional SiN<inf>x</inf> antireflection layer I<inf>sc</inf> reached 20 mA/cm².

Experimental setup for investigating silicon solid phase crystallization at high temperatures

An experimental setup is presented to measure and interpret the solid phase crystallization of amorphous silicon thin films on glass at very high temperatures of about 800 °C. Molybdenum-SiO(2)-silicon film stacks were irradiated by a diode laser with a well-shaped top hat profile. From the relevant thermal and optical parameters of the system the temperature evolution can be calculated accurately. A time evolution of the laser power was applied which leads to a temperature constant in time in the center of the sample. Such a process will allow the observation and interpretation of solid phase crystallization in terms of nucleation and growth in further work.

A new technology for crystalline silicon thin film solar cells on glass based on laser crystallization

A technology is proposed to prepare crystalline silicon thin film solar cells on glass as a superstrate. In a first step an a-Si:H layer is deposited by PECVD onto borosilicate glass. By scanning an Ar/sup +/-laser beam, this layer is crystallized with grains several 10 /spl mu/m in size and is at the same time p/sup +/-doped by boron from the glass so that a transparent electrode layer is formed. In the next step further a-Si is deposited and repeatedly irradiated by an excimer laser during deposition. In this way, a p-absorber layer is grown epitaxially from the underlying electrode which is acting as a seed layer. Finally, by excimer laser doping, a n/sup +/-emitter is fabricated to result in a p/sup +/-p-n/sup +/-layer sequence. Onto the silicon, a metal is deposited as the second electrode acting as a back reflector. Results on the characterization of the different layers are presented with the emphasis on crystallographic and chemical properties. Challenges in preparing the proposed layer sequence are discussed.