Ion-Beam-Induced Epitaxy and Solute Segregation at the Si Crystal-Amorphous Interface
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ION-BEAM-INDUCED EPITAXY AND SOLUTE SEGREGATION AT THE Si CRYSTAL-AMORPHOUS INTERFACE J. M. POATE*, D. C. JACOBSON*, F. PRIOLO*(a), and MICHAEL THOMPSON** *AT&T Bell Laboratories, Murray Hill, NJ 07974 "**Department of Materials Science, Cornell University, Ithaca, NY 14853
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ABSTRACT Segregation and diffusion of impurities in amorphous Si during furnace and ion-beam-induced epitaxy will be discussed. The use of ion beams to enhance the crystal growth process has resulted in novel behavior for fast diffusers such as Au. Diffusion is enhanced in the temperature range 300-700 K with activation energies -0.3 eV. Segregation and trapping are analogous to behavior at liquid-solid interfaces. INTRODUCTION The epitaxial crystal growth of Si is a subject of widespread fundamental and technological importance, encompassing crystal growth and dopant segregation phenomena from the liquid, solid and vapor phase. These processes are important in Si technology for both bulk crystal growth and for the formation of thin electrically doped epitaxial layers. In this review we will concentrate on a somewhat novel aspect of Si epitaxy - the use of energetic ion beams to induce solid phase epitaxy[1t . Three condensed phases of Si are commonly recognized; crystal (c), amorphous (a) and liquid. Amorphous Si is thermodynamically unstable in contact with crystalline Si and furnace heating will cause the amorphous layer to recrystallize 21 . If the interface between the crystal and amorphous phase is clean, epitaxial growth ensues. Using ion implantation to produce essentially ideal a-Si layers, solid phase epitaxy has been measuredf31 over the temperature range 500-1200'C. This crystal growth process is quite remarkable. The interface motion remains planar and its velocity is characterized by a single activation energy of 2.7 eV over a range of 10-10 cm/sec to 10-1cm/sec. Despite this knowledge, the basic mechanisms at the heart of solid phase epitaxy are almost completely unknown at the level of the interfacial defects and bond breaking processes. Much of this problem stems from a lack of knowledge about basic defect creation and diffusion processes in a-Si. Solid phase epitaxy can be extended to even lower temperatures by means of heavy ion irradiation. Figure 1 shows the a-c interface velocity as a function of temperature for thermal (i.e. furnace) and ion-beam conditions. The a-Si layers on (100)Si are typically several thousand A thick. For this example, 2.5 MeV Ar ions were used to induce crystal growth at a dose rate of 7x10 13 ions/cm 2 sec. The interface motion is characterized by an activation energy of 0.3 eV with enormous velocity enhancement over the simple, thermal epitaxy for temperatures
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