D. Toet

Dynamics of lateral grain growth during the laser interference crystallization of a-Si

Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed laser crystallization with holography, enables the fabrication of periodic arrays of polycrystalline silicon (poly-Si) lines with lateral dimensions between 0.5 and 20 μm. The lines consist of grains with well-defined grain boundary locations and lateral dimensions that are appreciably larger than the thickness of the initial a-Si:H film (up to 2 μm for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulations of the heat transport and phase transitions during laser crystallization. The theoretical results were compared to: (i) measurements of the crystallization kinetics, determined by recording the transient changes of the reflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission electron microscopy. The simulations indicate that the...

Laser crystallization and structuring of amorphous germanium

The short-pulse laser crystallization and interference structuring of amorphous germanium films were investigated by time resolved reflection measurements and Raman spectroscopy. We demonstrate that submicrometer crystalline structures with very sharp lateral interfaces can be produced by laser interference crystallization of nonhydrogenated samples. In hydrogenated films, on the other hand, the film surface disrupts upon laser exposure leading to the formation of a free-standing crystalline membrane. The Raman spectra of laser crystallized germanium display effects of finite crystallite size and stress.

Growth of polycrystalline silicon on glass by selective laser‐induced nucleation

Polycrystalline silicon on glass substrates was grown by a method based on the creation of nucleation sites using laser crystallization of amorphous silicon followed by thermal annealing at temperatures below 600 °C. Annealing induces the crystallization of the material around the seeds, eventually leading to coalescence of adjacent domains before spontaneous nucleation sets in. Micro‐Raman spectroscopy shows that the seeds experience a tensile stress, which causes a radial birefringence in the surrounding amorphous silicon, detected by optical anisotropy measurements. We conjecture that this stress facilitates the crystallization of the material around the seed upon thermal annealing.

Lateral Grain Growth during the Laser Interference Crystallization of a‐Si

We used laser interference (LIC), a combination of pulsed laser crystallization and holography, to fabricate polycrystalline silicon lines in an amorphous silicon (a-Si) film. Under appropriate conditions, the grains in the lines reach in-plane dimensions appreciably larger than the thickness of the initial a-Si film (up to 1.7 μm for a 300 nm a-Si film). Atomic force and transmission electron microscopy indicate that these large grains result from the lateral solidification of the silicon lines melted by the laser pulse. The lateral growth is well reproduced by simulations of the LIC dynamics by means of a two-dimensional melting and solidification model.

Growth mechanisms in laser crystallization and laser interference crystallization

The processes involved in the pulsed laser crystallization of amorphous silicon thin films were studied using transient reflection measurements. A model of the melting and solidification induced by the laser exposure, based on a one-dimensional calculation of the heat flow, was used to simulate the time-dependent reflectivity, yielding agreement with the experiments. Two laser beams interfering on the sample surface lead to the growth of long grains (up to 1.5 μm), with a well-defined orientation. We conclude that this lateral growth results from explosive crystallization combined with liquid phase growth.

Short-pulse laser-induced crystallization of intrinsic and hydrogenated amorphous germanium thin films

We report on the laser crystallization of intrinsic (a-Ge) and hydrogenated (a-Ge:H) amorphous germanium thin films using short, i.e., ns range, laser pulses. The influence of hydrogen on the phase transitions was investigated by monitoring the reflectance of the sample during laser irradiation. We determined the thresholds for melting (36 mJ/cm2) and for surface damage (66 mJ/cm2) of the a-Ge film. In a-Ge:H, hydrogen effuses on a short time scale (10 ns) upon laser irradiation. The effusion leads to the formation of a lifted-off (100 nm thick) crystalline Ge membrane, leaving behind a rough and incompletely crystallized surface. In a-Ge, on the other hand, no surface disruption is observed. The Raman spectra of hydrogenated samples are dominated by stress effects, while those corresponding to non-hydrogenated samples are dominated by crystallite size distribution effects. We also conclude that laser-induced annealing, carried out by applying several pulses with increasing intensity, can be used as a too...

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/

Short-pulse laser crystallization and structuring of a-Ge

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Laser induced nucleation and growth of polycrystalline silicon

Laser pulses in the nanosecond range were used to crystallize and structure (lateral dimensions≤1 μm) amorphous germanium thin films. The crystallized material consists of grains with sizes increasing from about 5 to more than 20 nm as a function of laser pulse energy. Arrays of polycrystalline Ge dots (diameter ∼1 μm, period ∼5 μm) were produced by bringing three laser beams to interference on the sample surface. These arrays can be used as seeds for solid-phase growth of polycrystalline areas by thermal annealing below 450°C.

Optical diffraction gratings produced by laser interference structuring of amorphous germanium–nitrogen alloys

We present a new method for growing large area polycrystalline silicon films from amorphous silicon at temperatures below 550°C, based on the selective creation of point-like polycrystalline nucleation seeds using laser crystallization and subsequent lateral growth by thermal crystallization. The growth rate depends on the laser intensity used to create the seed, as well as on the doping concentration and the thickness. Atomic force microscopy reveals grains of average diameter 60 nm in the thermally crystallized areas.