Subsurface Processing of Electronic Materials Assisted by Atomic Displacements
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MRS BULLETIN/JUNE 1992
amorphous layers at very low temperatures, to form ultrapure amorphous silicon for studying thermodynamic properties of this phase,9 or to mix films with semiconductors and form stable compounds such as silicides.10 Indeed, ion damage has been used to electrically isolate devices,3 to form optical waveguides and cavities,11 and to improve the junction properties of deeply doped layers.12 These issues are briefly reviewed in this article.
Damage Production and Amorphization of Semiconductors At low temperatures, where irradiationproduced defects are immobile, lattice disorder in semiconductors is controlled by collisional processes which depend solely on the integrated energy deposited in nuclear collisions. At higher temperatures when defects are mobile, defect annihilation and agglomeration can occur. At this stage, defect production competes with dynamic defect annealing in determining the resultant disorder structure. Hence, the ability to amorphize a semiconductor can depend on the defect density and rates of defect production and annealing, involving many parameters, such as ion mass, energy, fluence and flux, and substrate temperature.13 Examples of how the resultant defect structure can vary with energy deposition density and substrate temperature are given schematically in Figure 1. Figure la shows a typical energy deposition density as a function of depth (e.g., for 100 keV As implanted Si to a dose of ~1014/cm2). At low temperatures (Figure lb), the energy
deposition is sufficient to exceed the threshold for amorphization14 at certain depths (see lower broken line in Figure la). This generates a buried amorphous layer with incomplete amorphization (amorphous zones, defect clusters, and isolated displacements) on either side of this layer, where the energy deposition is subthreshold. If the temperature is raised, the effective threshold energy density for amorphization increases (middle dashed line in Figure la) as a result of dynamic annealing. Figure lc illustrates the resultant disorder structure: a thinner buried amorphous layer with sharp amorphous-crystalline (a-c) interfaces and surrounding disordered layers consisting of defect clusters within crystalline silicon. If the temperature is raised further (Figure Id), amorphization does not occur. In this case, defect annealing dominates defect production: Figure Id illustrates a situation in which a buried disordered layer consisting of small loops and clusters is formed. The transition from the structure illustrated in Figure lc to that in Figure Id is critically dependent on temperature as illustrated in the TEM cross-section micrographs in Figure 2.15 Figure 2a shows a typical buried a-Si layer formed by 1.5 MeV Xe irradiation to a fluence of 5 X 1015/cm2 at 210°C. Raising the temperature during irradiation by only 30°C (but keeping all other parameters the same) resulted in the structure shown in Figure 2b, which consists of a buried crystalline layer containing a dense band of extended defects. Note that the surface amorph
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