Laser-Induced Synthesis of Thin GeSe 2 Films
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LASER-INDUCED
SYNTHESIS
OF THIN GeSe
2
FILMS
C. ANTONIADIS, M.C. JOLIET, R. ANDREW, L.D. LAUDE AND M. WAUTELET I.R.I.S. Mons, Facult6 des Sciences, Universitd de l'Etat, B-7000 Mons, Belgium.
ABSTRACT Thin films of GeSe2 are synthesized by laser irradiation of Ge-Se sandwiches, with 2Se:IGe atomic proportion. The synthesized free-standing films are demonstrated to have the monoclinic crystal structure of GeSe2. Moreover, they exhibit a preferential orientation, with the c-axis perpendicular to the plane of the film. Films synthesized on glass substrate are characterized by an indirect optical energy gap of 2.3 eV, in agreement with published data on GeSe2. The melting point of Ge is not attained in the case of films on glass substrate.
INTRODUCTION Laser irradiation of thin films has been demonstrated to induce the crystallization of elemental films (Si, Ge) [ 1, 2] and the synthesis of semiconducting III-V and Il-VI compounds, starting from the elemental constituents [3-5]. In previous work, the problem of the synthesis was "relatively" simple, in the binary phase diagram. In since only one compound exists this work, we choose to go one step further, i.e. we examine the laser-induced synthesis of a binary system (Ge-Se) for which two stable compounds exist (GeSe, GeSe 2 ) [6]. As compared to previous work, the problem is further complicated by the fact that the melting points, Tm, of the elemental constituents are very different (Ge: 1210 K; Se: 490 K). It is also interesting to know whether GeSe forms before or after GeSe2. In order to test the feasability of the laser-induced synthesis of GeSe and/or GeSe 2 , two regimes are considered : i) free-standing films irradiated under pulsed conditions (t = Is); ii) continuous irradiation of films deposited on glass. The results are analyzed by transmission electron microscopy (TEM) and electron diffraction (case i), and by optical spectrometry (case ii).
EXPERIMENT 1000 A thick films made of Ge/Se sandwiches, with 2:1 Se:Ge atomic proportion, are vacuum evaporated onto either glass substrates or freshly cleaved NaCl crystals, in a vacuum of the order of 10-6 torr. The Ge-Se films evaporated onto NaCl are encapsulated between two 200 A thick SiO layers in order to protect them from air. These films, called now free-standing films, are floated off and deposited onto TEM copper grids with 80 x 80 um mesh size. All films are irradiated in air using an Ar+ laser operating on all green lines. The spatial profile of Mat. es. Soc. Symp. P-c. Vol. 23 (1984) @Elsevier science Publishing Co., Inc.
754
the beam is gaussian, with a diameter (1/e) on the sample of about 1.6 mm. The pulsed irradiations are performed by chopping the laser beam. RESULTS Free-standing
films
Free-standing films are irradiated for I s with a laser beam of power equal to 4.5 W. In the irradiated zone, we find crystal platelets up to several microns in diameter, giving the selected area diffraction pattern shown in figure 1. In conjunction with an Al calibration standard, this pattern is readil
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