J. Krinke

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.

Large grained polycrystalline silicon films by solid phase crystallization of phosphorus-doped amorphous silicon

For the first time, polycrystalline Si films with a grain diameter exceeding 10 μm are prepared by solid phase crystallization of phosphorus-doped, amorphous Si films on glass. Low pressure chemical vapour deposition from Si2H6PH3 serves to form amorphous Si films on glass substrates which are then subsequently solid phase crystallized at a temperature of 600°C. Transmission electron microscopy shows a log-normal grain size distribution in the crystallized films. The average grain size increases with the phosphorus concentration in the films and reaches 3.1 μm at an electron concentration of 1.5 × 1020cm−3. The area weighted average grain size is 5.9 μm. Hall effect measurements yield an electron mobility of about 50 cm2/(V s) for electron concentrations above 1019 cm–3.

Growth of polycrystalline silicon films on glass by high-temperature chemical vapour deposition

Covering glass substrates with polycrystalline Si films for electronic devices such as solar cells still presents a great challenge. In a two-step process, we first coat a novel high-temperature resistant glass substrate with a thin film of amorphous Si, which is then solid-phase crystallized at . In the second step, atmospheric pressure chemical vapour deposition at serves to deposit a several micron thick light-absorbing film. The minority carrier diffusion length in our films correlates with the area weighted grain size determined by transmission electron microscopy. We obtain a hole mobility of after hydrogen passivation and an electron diffusion length of .

Large area polycrystalline silicon thin films grown by laser-induced nucleation and solid phase crystallization

Abstract We present investigations on a two-step technique for the growth of polycrystaIline silicon thin fiIms on gIass substrates. In the first step, a lattice of artificial seeds is created by laser crystallization of amorphous silicon. In the second step, crystallites are grown around the seed by thermal annealing below 600 °C. For seed separations of 7 ~m, adjacent crystallites can coalesce before spontaneous nucleation becomes apparent. Transmission electron microscopy reveals that the crystallites are star shaped, consisting of domains separated by twin boundaries. Microscopic reflection difference spectroscopy measurements reveal the existence of a radial stress pattern around the seeds. We conjecture that this stress drives the lateral growth around the seeds during thermal annealing. Keywords: Crystallization; Silicon; EIectron microscopy; Optical properties 1. Introduction The ability to grow large area thin films of polycrystalline silicon (poly-Si) on inexpensive substrates such as glass is of great interest for the fabrication of solar cells and thin film transistors. The growth procedures investigated in the past primarily involve the recrystallization of amorphous silicon (a-Si). They can be divided in two categories: the first cate- gory concerns methods based on spontaneous nucleation, such as thermal annealing [ 1] and laser crystallization [2], while the second concerns recrystallization techniques involving artificially created nucleation centers. The latter methods characteristically provide a better control over the resulting film quality than the former ones. Artificial nuclei can be fabricated, for example, by solid state agglomeration [3] or self-ion implantation [4]. The work presented here concerns investigations of a growth procedure based on the creation of seeds in an a-Si film using laser crystallization, followed by a solid-phase lateral growth using thermal annealing. This artificial nucle- ation method requires processing temperatures below 600 °C, enabling the use of common glass substrates. It also has the advantage of being relatively simple, since it does not involve Present address: School of Electrical Engineering, University of New South Wales, Kensington 2052, Sydney, Australia. 0040-6090/97/

Transport analysis for polycrystalline silicon solar cells on glass substrates

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Deposition and characterization of polycrystalline silicon films on glass for thin film solar cells

The authors fabricate polycrystalline silicon solar cells on glass by Si deposition on solid phase crystallized seed layers and derive an effective diffusion length L/sub eff,QE/=3 /spl mu/m from quantum efficiency measurements. Three-dimensional transport modeling reveals that L/sub eff,QE/ differs from the diffusion length L/sub eff,IV/ in the diode saturation current j/sub o/=(q n/sub i//sup 2/D)/(N/sub A/ L/sub eff,IV/). Here q, n/sub i/, D, and N/sub A/ denote the elementary charge, intrinsic carrier concentration, diffusion constant and doping concentration, respectively. However, the difference is small for their polycrystalline Si solar cells. Dominant recombination in the space charge region limits the open circuit voltage to 340 mV.

Crystalline silicon films by chemical vapor deposition on glass for thin film solar cells

The authors deposit phosphorus-doped, amorphous Si by low pressure chemical vapor deposition and subsequently crystallize the films by furnace annealing at a temperature of 600 C. Optical in-situ monitoring allows one to control the crystallization process. Phosphorus doping leads to faster crystallization and a grain size enhancement with a maximum grain size of 15 {micro}m. Using transmission electron microscopy they find a log-normal grain size distribution in their films. They demonstrate that this distribution not only arises from solid phase crystallization of amorphous Si but also from other crystallization processes based on random nucleation and growth. The log-normal grain size distribution seems to be a general feature of polycrystalline semiconductors.

Solution growth of silicon on multicrystalline Si substrate: growth velocity, defect structure and electrical activity

Covering glass substrates with polycrystalline Si films having an electronic quality suitable for thin-film solar cells still presents a great challenge. In a two-step process, the authors first coat a novel high-temperature resistant glass substrate with a thin film of amorphous Si, which is then solid phase crystallized. In the second step, atmospheric pressure chemical vapor deposition serves to deposit a several micron thick light absorbing film. They obtain an area weighted average grain size of 2.5 /spl mu/m in their films. Hall mobilities in their p-type Si films are 68 cm/sup //Vs, the minority carrier diffusion length is 2 /spl mu/m. They introduce a new pyramidal film texture and numerically calculate an efficiency potential of 12 to 15% depending on surface recombination velocity for thin film solar cells using a-Si films on glass.

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

Thin silicon films, solution-grown on cast silicon wafers, are examined as an example of liquid phase epitaxy of silicon on a polycrystalline seed layer. We investigate the structural and electrical properties of grain boundaries in the epilayers by transmission electron microscopy and electron-beam-induced current measurements. We find that growth close to thermodynamic equilibrium leads to low electrical activity of defects and to a low-energy geometry of grain boundaries. Layers grown with different growth rates show no difference in electrical activity. Trenches at grain boundary sites are typical surface features of the epilayers. An increased growth rate leads to a reduction in trench depth, which is an advantage for the contact metallization of solar cells.