R. Dassow

Nucleation and Growth of Crystalline Silicon Films on Glass for Solar Cells

Understanding nucleation and growth of crystalline Si films on glass is of prime importance in order to tailor and optimize semiconductor properties for electronic devices such as solar cells. Commercial glass limits the maximum processing temperature of Si films to 600 C. Solid phase and laser crystallization, or a combination of both techniques, are thus primarily used to crystallize Si films on glass. While random nucleation and growth processes always result in the formation of a log± normal size distribution, control of nucleation sites allows one to determine the location of grain growth or even the crystallographic orientation of grains. Via sequential lateral solidification using a copper vapor laser, we obtain Si crystallites on glass of several tens of mm in length.

From polycrystalline to single crystalline silicon on glass

Thin film transistors and solar cells from polycrystalline silicon suffer from the influence of grain boundary charges. This contribution reviews three possibilities to suppress these charges: (i) by the selection of specially oriented grain boundaries in small-grained silicon. Analysis of solar cells demonstrates that (110)-textured films must contain electrically low-active boundaries. (ii) Special laser crystallization yields large-grained Si of excellent quality. (iii) Transfer techniques allow one to fabricate single crystal films on foreign substrates. In addition to these three methods, we discuss joined-wafer silicon for growth of single crystalline Si films and layers of essentially unlimited size.

Nd:YVO 4 Laser Crystallization for Thin Film Transistors with a High Mobility

We crystallize amorphous silicon films with a frequency doubled Nd:YVO 4 laser operating at a repetition frequency of up to 50 kHz. A sequential lateral solidification process yields polycrystalline silicon with grains longer than 100 μm and a width between 0.27 and 1.7 μm depending on film thickness and laser repetition frequency. The average grain size is constant over the whole crystallized area of 25 cm 2 . Thin film transistors with n- type and p-type channels fabricated from the polycrystalline films have average field effect mobilities of μ n = 467 cm 2 /Vs and μ p = 217 cm 2 /Vs respectively. As a result of the homogeneous grain size distribution, the standard deviation of the mobility is only 5%.

High mobility thin film transistors by Nd:YVO4-laser crystallization

Abstract Laser crystallization of amorphous silicon is one of the most interesting ways to obtain high-quality polycrystalline silicon films on glass. We crystallized the channel region of n- and p-type thin film transistors (TFTs) with a frequency-doubled Nd:YVO 4 laser utilizing a sequential lateral solidification process. The high repetition rate of the laser of up to 100 kHz allows for high scanning speeds of up to 5 cm s −1 . The laser irradiation was performed in air at room temperature. The resulting polycrystalline films showed longitudinally elongated grains with a length of up to 100 μm in the scanning direction of the laser beam and a width of up to 2 μm perpendicular to the scanning direction. Due to the anisotropic grain dimensions, the TFT performance depends on the orientation of the channel with respect to the scanning direction. Furthermore, a scale down of the TFT dimensions results in a better device performance because the number of grain boundaries within the channel of a TFT is reduced. For example, a decrease in the width W and length L of the channel from W =63 and L =22 μm to W =30 and L =15 μm increases the field-effect electron mobility μ N of the TFTs from μ N =410 to 510 cm 2 V −1 s −1 . The high mobility μ and low sub-threshold slope S =0.45 V decade −1 obtained with a gate oxide thickness of 100 nm show the high quality of laser-crystallized polycrystalline silicon.

Microstructure of laser-crystallized silicon thin films on glass substrate

We investigate the microstructure of polycrystalline silicon films (grain size, orientation distribution, and grain boundary population). These films are produced by laser crystallization of amorphous silicon on glass substrates by a frequency doubled Nd:YVO4 laser operating at a wavelength of 532 nm. Transmission electron microscopy reveals that the grains have an average width between 0.25 and 5 μm depending on the crystallization parameters and a length of several 10 μm. Electron backscattering diffraction experiments show that the grain orientation of the poly-Si films is textured. Type and extent of texture depend in a complex way on the thickness of the crystallized amorphous silicon layer, on the repetition rate of the laser pulses, and on whether or not an additional buffer layer is present on the glass substrate. In any case, the grain boundary population is dominated by first and second order twin boundaries.

Large-grained polycrystalline silicon on glass by copper vapor laser annealing

We crystallized 400 nm thick amorphous silicon films on low cost glass substrates using a pulsed copper vapor laser. The moderate pulse energy (60 μJ) and high repetition rate (20 kHz) of the laser allow crystallization scanning rates up to 10 mm/s. The resulting polycrystalline silicon films show large elongated grains 3 μm wide and several tens of microns in length.

Grain populations in laser-crystallised silicon thin films on glass substrates

We investigate the polycrystalline microstructure, i.e. grain size and orientation distribution, that forms during laser crystallisation of amorphous silicon on glass substrates by a frequency doubled Nd:YVO -laser operating at a wavelength of 532 4 nm. Transmission electron microscopy reveals that the grains have an average width from 0.25 to 3 m and a length of several 10 m. Electron back-scattering diffraction indicates that the grain orientation of the poly-Si films is textured. Type and extent of texturing depend in a complex way on the thickness of the crystallised amorphous silicon layer and on whether or not a buffer layer is present. 2001 Elsevier Science B.V. All rights reserved.

Low-Temperature Processing of Crystalline Si Films on Glass for Electronic Applications

The present paper gives an overview of the material properties and the technology of the low-temperature preparation and modification of crystalline Si films on glass. Electronic properties of Si films strongly depend on the film structure, which thus determines possible areas of devices applications. In detail, we discuss i) high-throughput pulsed laser crystallization using a solid state laser that enables the formation of Si films with elongated grains having a length of several ten μm, ii) low-temperature epitaxy at temperatures around 600°C with a rate up to 0.5 μm/min using ion assisted deposition, and iii) the formation of quasi-monocrystalline Si films via crystallization of porous Si. This innovative thin film transfer technology permits reuse of Si wafers and produces films with a thickness-dependent hole mobility of up to 78 cm2/Vs and effective internal light trapping.

Reproducible high field effect mobility polysilicon thin film transistors involving pulsed Nd:YVO/sub 4/ laser crystallization

We fabricated low temperature TFTs on low-cost glass substrates for large area electronic applications. A newly developed crystallization method uses a pulsed diode pumped Nd:YVO/sub 4/ laser. The high repetition capability of up to 100 kHz of this laser results in high throughput rates associated with a scanning speed of up to 5 cm/s. One of the remarkable features-of the TFTs is a field effect mobility mean value of 350 cm/sup 2//Vs with a standard deviation of 5%.

Numerical Modeling of High Repetition Rate Pulsed Laser Crystallization of Silicon Films on Glass

This contribution investigates the crystallization behavior of amorphous silicon films on glass by using pulsed lasers with very high repetition rates up to 100 kHz. We determine the influence of the laser repetition rate f and of the film thickness d on the grain width g of the resulting polycrystalline silicon films. Our experimental results indicate a strong dependence of the grain width g on film thickness d as well as on the repetition rate f of the laser. The grain width rises from g = 0.27 µm to g = 3.59 µm if the film thickness increases from d = 50 nm to d = 300 nm and the repetition rate f from f = 20 kHz to 100 kHz. We use a purpose developed two- dimensional finite difference numerical model to calculate the evolution of the temperature in the silicon film and in the glass substrate. An increase of both, the film thickness d , and the repetition rate f decrease the solidification velocity v of the film. A comparison of the solidification velocity v s and the measured grain width g shows a linear correlation.