Argon Ion Bombardment During Molecular Beam Epitaxy of Ge (001)
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ARGON ION BOMBARDMENT DURING MOLECULAR BEAM EPITAXY OF Ge (001) ERIC CHASON, K.M. HORN, J.Y. TSAO, AND S.T. PICRAUX Sandia National Laboratory, Albuquerque, NM 87185 ABSTRACT Using in situ, real-time reflection high energy electron diffraction (RHEED), we have measured the evolution of Ge (001) surface morphology during simultaneous molecular beam epitaxy and Ar ion beam bombardment. Surprisingly, low-energy Ar ions during growth tend to smoothen the surface. Bombardment by the ion beam without growth roughens the surface, but the surface can be reversibly smoothened by restoring the growth beam. We have measured the effect of such "ion beam growth smoothening" above and below the critical temperature for intrinsic growth roughening. At all measured growth temperatures the surface initially smoothens, but below the critical roughening temperature the final surface morphology is rough whereas above this temperature the final morphology is smooth. INTRODUCTION In MBE growth of single component materials, the crystal grower is generally limited in the number of parameters under his control to substrate preparation (including orientation, miscut and cleanliness), substrate temperature and deposition rate. It has been suggested that low energy ion beams might be used to extend the range for good epitaxy by modifying the growing surface and providing another adjustable parameter to control the growth kineticsrl,21. Low energy ion beams have already been used as a method of improving non-epitaxial thin film growth by enhancing nucleation and densification of the film, relieving stress, and enhancing thin film adhesion[lI]. Atomic cleaning of surfaces by ion beams has been shown to reduce epitaxial growth temperatures[3,4]. Direct ion beam deposition of epitaxial films has been demonstrated[5,6], and the lowest reported temperature for which Si has been grown epitaxially has been by this technique[7]. Recently, we found that low-energy Ar ions during growth of Ge on Ge (001) have a remarkable smoothening effect[8]. This effect is most apparent when surfaces are "extremely" rough, i.e., characterized by spotty, transmission-like RHEED patterns. Such rough surfaces smoothen, during simultaneous ion bombardment and growth, nearly an order of magnitude f~ter than during growth alone, and are now a routine step in our initial sample preparation sequence. Although there have been these demonstrations of ion beam-enhanced growth effects, there is little understanding of the effect of the beam on the surface during growth. To understand the mechanism by which ion beams assist surface smoothening during growth, we have systematically measured the effects of ion beams on growth morphology at 300, 400 and 500 C. Whereas previous measurements of ion beam effects have involved post mortem (ex-situ) techniques, the measurements described here were conducted in situ in real-time. EXPERIMENT The measurements were performed in an ultra-high vacuum growth chamber of our own design. Base pressure is typically 3x10-1 ° torr rising to 4x10-9 t
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