Nanocrystalline Ge Synthesis by Picosecond Pulsed Laser Induced Melting and Rapid Solidification
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EXPERIMENT The samples used for the present study are 50 nm-thick a-Ge films grown at room temperature on glass substrates. The films are deposited by DC-sputtering from a Ge (99.999%) target in a vacuum system with a residual pressure of 3x10, 6 Torr and with an Ar operating pressure of 4x1 0-3 Torr. Details about the characterization of the as-deposited films by means of transmission electron microscopy and ellipsometry are given elsewhere [ 11]. The as-deposited samples were irradiated by laser pulses of about 10 ps at 583 nm in order to induce melting and rapid solidification. The irradiation pulses are delivered by a synchronously pumped dye laser whose output is amplified by a pulsed dye amplifier pumped by a frequency doubled Nd-YAG laser. The amplified beam is focused on the sample surface to a size of about 1 mm while the reflectivity of the surface at 633 nm is monitored in real time with ns resolution by means of a HeNe laser beam focused at the center of the irradiated spot to a size of approximately 50 ltm. After each exposure the sample is moved to a fresh region. The fluence of the laser pulses at the sample site is varied in the 0-90 mJ/cm 2 range. Further details about the experimental setup for ps laser pulse irradiation and real time reflectivity measurements are given in Ref [9]. After irradiation the structure of the irradiated regions was analyzed by means of Raman spectroscopy [12 ,13 ] in micro-Raman configuration [14 ]. The measurements were performed with a DILOR XY Raman spectrometer attached to a metallographic microscope. The excitation beam is delivered by an Ar' laser operating at 514 nm. The laser beam is focused on the sample surface to a size of about 10 ptm, and it is sufficiently attenuated (a few mW) in order to prevent both the presence of spectral broadening or local annealing. RESULTS AND DISCUSSION Figure 1 shows two representative real time reflectivity transients obtained upon melting and rapid solidification of the as-deposited material induced by irradiation with ps laser pulses. The sharp reflectivity increase observed following the absorption of the irradiation pulse
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Time (ns) Figure 1: Real time reflectivity transients obtained upon irradiation of as-deposited a-Ge films with ps laser pulses. The arrow indicates the temporal position of the irradiation pulse and the laser pulse fluences are given in each transient. 840
indicates the formation of a molten metallic layer on the surface. For melt depths below approximately ; 25 nm the maximum transient reflectivity level is a function of the molten layer thickness. For larger melt depths, the liquid layer becomes optically thick at the probe beam wavelength leading to a constant reflectivity value. This transient reflectivity behavior has been widely reported upon laser induced melting of semiconductors like Si, Ge or GaAs [15 ]. In our 2 case the minimum pulse fluence required for inducing surface melting is about 20 mJ/cm . Upon 2 solidification and for flu
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