Laser photosublimation of compound semiconductors
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I. INTRODUCTION Interaction between semiconductors and incident bandgap radiation takes place via electron-hole pair generation. The photogenerated charge carriers may thermalize on a rapid time scale, thereby converting the photon energy into heat and ultimately leading to various thermal processes, including sublimation. Alternatively, nonthermal effects may become dominant if the interaction of charge pairs with the lattice phonons is slower than other charge related processes. In this article we report on studies of laser-induced sublimation of CdS, CdTe, Hg O8 Cd O2 Te, and GaAs, which were aimed at identifying the dominant interaction mechanisms. In addition, a phenomenon particular to laser etching, whereby the laser builds its own singlemode waveguide by a dynamic light guiding effect, is described. For studies of the etching mechanism, the experimental configuration consisted of a cw argon-ion laser, line tunable in the range 458-515 nm, which was weakly focused (1 mm spot) onto the semiconductor surface. The crystals were held under a moderate vacuum ( ~ 10~ 6 Torr) and could be resistively heated to a bias temperature Tb = 500 °C. The desorbed species were detected with a quadrupole mass spectrometer (QMS). The same laser was more strongly focused to a 2.4 to 3 //m spot for studies of laser-etched microstructures.
II. DYNAMIC LIGHT GUIDING IN CdTe We have previously1 described a preliminary study of the laser etching of CdTe. In particular, a "dynamic light guiding" effect resulting in deep etched grooves of 0.4/xm width, i.e., less than the optical wavelength, was observed. The model for this process involves an active feedback between the laser-generated structure and the coupling to the light beam. We summarize the findings from a more complete study below.
a)
On leave from Dipartimento di Ingegneria Elettrica, Universita di Palermo and IAIF-CNR, Viale delle Seienze, 90128 Palermo, Italy.
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J. Mater. Res. 2 (2), Mar/Apr 1987
http://journals.cambridge.org
Figures 1 (a) and 1 (b) are scanning electron micrographs (SEM's) of deep grooves generated with a 488 nm beam focused to 2.4/zm on the top surface of a/?-type crystal. The etched features consist of two distinct sections: a tapered part at the top, which is ~ 13 fim wide at the surface and =;0.4//m wide at a depth of ;s20//m, and a vertical narrow groove ^0.4 ^ m wide and s 12 ^ m deep. Such grooves were obtained under a variety of experimental conditions. In most instances they were straight, as the one in Fig. 1 (b). Occasionally, the narrow groove was found to bend sideways. Note that the tapered part is relatively rough in contrast to the much smoother narrow groove; this taper was found to become shallower and wider with increasing laser power. The width of the grooves varies in range 0.3-1.0 /tm depending on focusing conditions. Figures 2 and 3 show SEM top views of a series of closely spaced or intersecting grooves. A study was also made of the orientation dependence of the mechanism by etching a star pattern in a CdTe (111) surface a
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