Surface Studies of Laser Annealed Semiconductors
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SURFACE STUDIES OF LASER ANNEALED SEMICONDUCTORS*
D. M. ZEHNER AND C. W. WHITE Solid State Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37830
ABSTRACT The surface regions of semiconductor single crystals have been examined following laser annealing in an ultrahigh vacuum environment with the output of a pulsed ruby laser. Atomically clean surfaces with impurity levels below 0.1% of a monolayer can be produced by multiple pulse irradiation. Ordered surface structures are produced on low index oriented crystals as well as crystals slightly misoriented. Metastable surface structures exhibiting (lxi) LEED patterns have been produced on (111) orientations and are believed to be a consequence of the rapid cooling rates of 109 degs/sec achieved with the laser irradiation process. The surface and subsurface regions of ion-implanted Si crystals have been examined both before and after laser irradiation and results obtained from Si samples implanted with As are discussed.
INTRODUCTION The rapid deposition of energy from Q-switched lasers into the near-surface region of semiconductors leads to melting of the crystal to a depth of several thousand angstroms, followed by liquid-phase epitaxial regrowth from the underlying substrate at growth velocities calculated to be of the order of meters per second. This processing technique provides a way of achieving 9 heating and cooling rates (approaching 10 deg/sec) which are orders of magnitude faster than those obtained with more conventional techniques. With proper annealing conditions, it has been shown that regions free of extended defects can be formed and substitutional impurities can be incorporated into the lattice far in excess of the equilibrium solubility limits. These observations of the changes that occur in the subsurface region suggest that the surface properties (impurities, structure, electronic energy levels) of laser-annealed semiconductors can be significantly altered by the high heating and cooling rates and rapid velocities of solidification that can be achieved using pulsed lasers. In this paper we discuss studies concerned with laser irradiation of semiconductor single crystals in an ultrahigh vacuum environment. We show that pulsed laser irradiation can be used to produce atomically clean surfaces. This is accompanied by the observation of ordered surface structures which indicates that the liquid-phase epitaxial regrowth process extends to the outermost monolayers of the crystal. The production of metastable (lxi) surface structures on (111) oriented crystals of Si is discused as well as the geometric and electronic properties of the Si(111) surface. We show that pulsed laser irradiation of vicinal surfaces can be used to produce a well defined stepped surface. Finally, we show that the combination of ion implantation and laser annealing can be used to produce surfaces with electronic properties that cannot be achieved by conventional methods. *Research sponsored by the Division of Materials Sciences,
U. S.
Department of
Energy under c
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