Microstructure evolution in amorphous Ge/Si multilayers grown by magnetron sputter deposition
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Microstructure evolution in amorphous GeySi multilayers grown by magnetron sputter deposition K. J¨arrendahl,a) I. Ivanov,b) and J-E. Sundgren Department of Physics, Thin Film Physics Division,c) Link¨oping University, S-581 83 Link¨oping, Sweden
G. Radn´oczi and Zs. Czigany Research Institute for Technical Physics of the Hungarian Academy of Sciences, P.O. Box 76, H-1325 Budapest, Hungary
J. E. Greene Department of Materials Science, the Coordinated Science Laboratory, and the Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801 (Received 26 August 1996; accepted 11 March 1997)
Microstructure evolution in amorphous GeySi multilayers grown by dual-target dc magnetron sputtering was investigated by cross-sectional transmission electron microscopy, x-ray diffraction, and growth simulations. In films grown under low intensity ion-irradiation conditions, the structure is columnar with low-density regions along column boundaries where layer intermixing was observed. By increasing the low-irradiation intensity (controlled by an applied negative substrate-bias), structures with smooth and well-defined layers could be grown. This was achieved at bias voltages between 80 and 140 V, depending on the sputtering gas pressure. As the ion-irradiation intensity is further increased, ion-induced intermixing degrades the layer interfaces and finally an amorphous Si1–x Gex alloy forms. The combination of x-ray diffraction measurements and reflectivity calculations reveals an asymmetry between the GeySi and SiyGe interface widths due, primarily, to a corresponding asymmetry in incident particle energies during the growth of alternate layers.
I. INTRODUCTION
While multilayers (or one-dimensional superlattices1 ) were first fabricated more than 60 years ago,2,3 it was not until the beginning of the 1970s that this field of materials science started to be of importance.4 After a decade of semiconductor multilayer growth investigations concentrated on lattice-matched epitaxial materials, it was also shown that amorphous semiconductor multilayers5,6 could be realized in high-quality structures with sharp interfaces. The interest in amorphous multilayers now includes electronic,7 optical,5,7 and mechanical8 properties. As in the case of their crystalline counterparts, amorphous multilayers exhibit unique new features due to their reduced dimensionality.9 They are also important in an increasing number of applications, e.g., solar cells,10 thin-film transistors,11 and uv and x-ray mirrors.12 a)Current
address: Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 276957907; Electronic mail: kenneth [email protected] b) Current address: 225 Walter Scott Engineering Center, Department of Mechanical Engineering, University of Nebraska – Lincoln, Lincoln, Nebraska 68588-0656. c) WWW home page: http://www.ifm.liu.se/Thinfilm 1806
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 7, Jul 1997
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