Epitaxial pulsed laser crystallization of amorphous germanium on GaAs

Abstract

We have investigated the crystallization of amorphous germanium films on GaAs crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial films are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a ͑GaAs͒ 1Ϫx Ge 2x epitaxial alloy with a graded composition profile at the interface with the substrate. Since Ge and GaAs are thermodynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling process. Lower laser fluencies lead to polycrystalline layers with a patterned surface structure. The latter is attributed to the freeze-in of instabilities in the melt during the fast solidification process.

SixGe1−x films and heterojunctions produced by epitaxial crystallization of a-SixGe1−x alloys on GaAs

“…Prior to the deposition, the substrates were heated to 350°C and bombarded by a 30 eV hydrogen ion beam in order to remove the natural surface oxide [1]. The efficiency of this process was verified using X-ray photoelectron spectroscopy.…”

“…One alternative approach consists in using crystalline Si x Ge 1Àx layers obtained by laser crystallization (LC) of amorphous (a) Si x Ge 1Àx films on GaAs. The almost perfect lattice matching between Ge and GaAs ensures the formation of high quality, dislocation-free epitaxial Ge layers by LC [1]. The band gap can be varied in a controlled manner by introducing Si in the Ge layers.…”

Laser interference structuring of a-Ge films on GaAs

“…The crystallization experiments were performed using single pulses ͑pulse width of ϳ7 ns͒ from a frequency-doubled Nd:YAG laser ( L ϭ532 nm). 9 The laser beam profile was homogenized using a vacuum spatial filter, leading to a Gaussian-like intensity profile with a diameter a͒ 0 ϳ4 mm on the sample surface. The integrated intensity of the laser pulses was adjusted using an attenuator consisting of a half wave plate and a polarizer and measured by deviating a portion of the laser beam to a detector.…”

“…6 The lateral temperature gradient induced by the selective illumination has also been applied to induce a lateral solidification of amorphous silicon films, which permits the control of grain size and orientation of the crystallized material. 3,4,7,8 In a previous work, 9 we have demonstrated that amorphous ͑a-͒ Ge films deposited on crystalline GaAs substrates can be epitaxially crystallized when irradiated by short laser pulses. The Ge/GaAs system is ideal for laser crystallization studies since the two materials are almost perfectly matched with respect to lattice constants and thermal expansion coefficients.…”

“…We found, however, that the fast cooling rates after laser irradiation may lead to the formation of a nonequilib-rium (GaAs) 1Ϫx Ge 2x alloy at the interface between the Ge and the GaAs film. 9 In this work we investigate the LIC of a-Ge films grown on ͑100͒-oriented crystalline GaAs substrates. We demonstrate that submicrometer lines of epitaxial Ge on GaAs can be produced by the LIC technique by using single pulses from a frequency-doubled Nd-YAG laser.…”