Structural changes in GaAs induced by ultrafast (fs) laser pulses
- PDF / 141,868 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 38 Downloads / 225 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Structural changes in GaAs induced by ultrafast (fs) laser pulses L. N´anai, R. Vajtai, and Cs. Beleznai Department of Experimental Physics, J´ozsef Attila University, H-6720 Szeged, D´om t´er 9, Hungary
J. Remes and S. Lepp¨avuori Microelectronics and Materials Physics Laboratories, University of Oulu, SF-90570 Oulu, Finland
Thomas F. George Office of the Chancellor/Departments of Chemistry and Physics & Astronomy, University of Wisconsin – Stevens Point, Stevens Point, Wisconsin 54481-3897 (Received 23 January 1997; accepted 19 December 1997)
Ultrafast changes in the crystal structure of GaAs induced by intense femtosecond laser pulses are detected and investigated. Atomic force microscopy and Raman microprobe analysis of the laser-treated area show centrosymmetric (disordered) features which are different from the original zinc-blend structure of the GaAs lattice. The frozen-in structure shows evidence for a special heat transfer from the laser-induced crater to the boundary, namely the heat has been transferred ballistically by a high-density electron-hole plasma.
I. INTRODUCTION AND BACKGROUND
Several years ago, laser annealing of ion-implanted semiconductors sparked a surge of increased interest in the study of laser-induced ultrafast structural phase transitions. Theoretical and experimental work led to the general agreement that pulsed laser annealing might be considered as a rapid thermal process unless the laser pulse length is shorter than a few tens of picoseconds.1–3 Intense femtosecond laser pulses, however, can transfer energy in the carrier system over a time interval shorter than the time of phonon emission, creating a dense free-carrier plasma. As a result of these very short pulses, a lattice can be disordered by electronic excitation while the lattice modes remain vibrationally cold. This phenomenon is of special interest for the case of diamond-type (Si) and zinc-blend-type (GaAs) crystals because of their practical importance. The structural changes might be investigated experimentally using a femtosecond pump-and-probe technique to measure changes in optical reflectivity (R) and, for the case of noncentrosymmetric crystals, second-harmonic generation (SHG). Several groups have undertaken time-resolved R and SHG investigations of Si4–6 and GaAs.7–9 In addition, Rota and co-workers carried out luminescence measurements to obtain information about the distribution of carriers.10 Experimental investigations revealed that, for the case of Si under the influence of intense fs pulses, the crystalline order becomes disordered with melting of the surface layer on a time scale less than several ps.6 Tom and co-workers found that the order-dependent 1808
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 7, Jul 1998
Downloaded: 13 Mar 2015
SHG signals disappear after 150 fs, which is too short of a time for the excited electronic states to heat the lattice to the equilibrium melting temperature.4 Measurements made on GaAs are very important b
Data Loading...
