Initial Observation of the Crystal-Amorphous Transition and the Formation of Ripple Patterns on Silicon Induced by 7 ps
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INITIAL OBSEVATION OF TME CRYSTAL-AMORPHOUS TRANSITION AND THE FORMATION OF RIPPLE PATTERNS ON SILICON INDUCED BY 7 ps PULSES AT 1.05 Jm
IAN W. BOYD, STEVEN C. MOSS, THOMAS F. BOGGESS, Center for Applied Quantum Electronics Department of Physics North Texas State University Denton, Texas 76203
AND ARTHUR L. SMIRL
ABSTRACT We describe our preliminary observations of the interaction of short pulse 1 pm radiation with crystalline In particular we find that for 7 ps pulses, but silicon. not for 46 ps pulses, the crystal-amorphous transition can be induced on the Si surface. Additionally, we have measured the single shot melting threshold for c-Si to be 2 0.6 ± 0.2 J cmfor 7 ps pulses. The formation of ripple patterns and other surface structures with pulses as short as 4 ps is also reported.
INTRODUCTION The novel thermodynamic processes and extreme degrees of electronic excitation associated with the application of laser radiation to semiconductors have resulted in widespread activity in the field of laser processing over the past decade [13. For example, it is known from various experimental observations [21 that extremely fast resolidification rates can be achieved under typical conditions afforded by nanosecond and picosecond pulse irradiation of silicon. Rapid cooling rates have also given rise to a novel transition from the melt to an amorphous state. It has been proposed [3] that the criterion for producing this phase lies in the capability of creating a thermal gradient such that following melting the randomly coordinated structure freezes before recrystallization can take place. This disordered state has been formed in a number of ways. Tsu et al. [4] applied 10 ns, uv pulses to Si, taking advantage of the extremely high absorption coefficient, in order to obtain the necessary thermal gradients. Cullis et al. [5] have used similar pulses on samples held at 77 K, thereby further increasing the quench rates and producing much thicker amorphous regions. Picosecond pulses can also be used to this end. For example, Liu et al. [31 have used 30 ps pulses at X= 533 and 266 nm to produce amorphous layers on a single crystal Si surface. Rozgonyi et al. [61 and Gamo et al. [7] have reported the formation of concentric ring patterns on the amorphous surface of ion-implanted c-Si following irradiation by 25-30 ps pulses at X= 354 and 532 nm [6] and 1.06 Vm [7]. One of these rings has been identified as a-Si. However, to date the formation of amorphous layers on c-Si using 1 Jim radiation has not been reported. Such quanta are only weakly absorbed at room temperature, and although sufficient energy densities to anneal (i.e., to melt and recrystallize the c-Si) can be reached, the thermal gradients induced by 30 ps pulses are not conducive to freezing surface disorder. In this paper, we report the first observation of such a transition from single crystal to an amorphous state using X = 1.05 im pulses of 7 ps duration. We also compare the threshold fluence required by these pulses to induce this transformation with the w
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