Growth of Large Diameter Silicon-Germanium Monocrystals
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Growth of Large Diameter Silicon-Germanium Monocrystals Richard H. Deitch, Stephen H. Jones and Thomas G. Digges, Jr. Virginia Semiconductor, Inc., 1501 Powhatan Street, Fredericksburg, VA 22401, U.S.A.
ABSTRACT
Si-Ge monocrystals up to 50 mm diameter and up to 17 at% germanium were grown using a modified Czochralski technique. Pre-grown large diameter silicon seeds with various crystallographic orientations were used as templates for the alloy solidification to reduce cap crystallization time and insure monocrystallinity at desired diameters. Discussed are the influences that seed preparation, crystal growing parameters, and post-growth processing have on the material that was produced using this new technique.
INTRODUCTION
A Si-Ge alloy is of interest because it has higher carrier mobility than silicon and can be tailored to have a bandgap energy at any value between silicon and germanium. Monocrystals can be used as substrates for epitaxy, for X-ray analyzers, solar cells, thermoelectrics, photodetectors, and other electronic devices. Since the 1995 review of Si-Ge crystal growth by Schilz and Romanenko [1], there have been numerous advances made in growing crystals using the Czochralski [2-6] and other methods [7-11]. To be a practical substrate, a material must have appropriate physical and chemical properties, and be capable of being processed into sufficiently large wafers having flat surfaces. The intention of this investigation was to determine if it was possible to grow a 25 mm diameter or larger Si-Ge monocrystal with a relatively high germanium content without making modifications to a standard crystal pulling furnace or to its control systems. As seen in Figure 1, the equilibrium phase diagram for this binary alloy shows that there is complete solubility of germanium and silicon in both the liquid and solid states. When a liquid of any composition is at its liquidus temperature, a solid having higher silicon content than that of the liquid can form. As the Si-Ge solidifies, germanium is rejected into the liquid and the amount of germanium in both the liquid and the solid increases. A monocrystal can be grown when the correct combination of thermal conditions and growth rates are imposed. Since the equilibrium phase diagram has a relatively large liquidus-solidus gap, pulling rates on the order of a few mm per hour are required to prevent dendritic growth that may lead to polycrystallinity. It is not known to what extent ordering can occur in Si-Ge alloys, and whether microsegregation can be completely avoided.
O5.7.1
Atomic % Silicon Distribution Coefficients
kSi > 1 kGe < 1
Si mp ~ 1414 C
LIQUID
SOLID
Ge mp ~ 938 C
Weight % Silicon Figure 1. The Si-Ge equilibrium phase diagram
When a standard Czochralski method was used to initiate growth, it was found that randomly oriented crystals would nucleate at the circumference of a melted-in seed. This made it impossible to grow a monocrystal cap to a size that was significantly larger than the seed diameter. It was observed, however, that these
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