Structure of Perturbed Twin Boundaries in Silicon
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STRUCTURE OF PERTURBED TWIN BOUNDARIES IN SILICON ANITA GARG, W.A.T. CLARK and J.P. HIRTIt Department of Metallurgical Engineering, The Ohio State University, 116 W. 19th Avenue, Columbus, Ohio 43210 ABSTRACT Silicon bicrystals containing Y =3(60 /[111]12) twin boundaries have been fabricated by sintering together single crystal (IMi) :vafers (which deviated from the exact (111) by upto ±4 '). Transmission electron microscopic investigation revealed the boundaries to be near-twin boundaries having a superposed tilt component. The grain boundary dislocation structure of these boundaries has been studied in a transmission electron microscope (TEM) and interpreted as arising from the interaction of a large tilt component with a smaller twist component, to give the observed low energy configuration. INTRODUCTION In recent years the study of grain boundary structure in Si and Ge has contributed significantly to understanding the influence of these boundaries on the performance of various semi-conducting devices. Several high and low angle twist and tilt boundaries have been fabricated, under controlled orientations, and their structure examined in the TEM 1-3j. The grain boundary dislocations observed in the vicinity of coincidence misorientations have been found to be consistent with those predicted theoretically ý41. In practice, a grain boundary may not be a pure twist or tilt boundary but may be a combination of both. It is of interest to know how different dislocations would interact to modify the structure of such a boundary. Here, we describe the structure of two near-twin boundaries, with a large superposed tilt component of 3.9'. EXPERIMENTAL DETAILS Silicon bicrystals for this study were fabricated from two Si (Ill) wafers, each having a 50 mm diameter and an approximate thickness of 250 gm. In order to produce the desired twin boundaries, the wafers were rotated about 111 by 60' relative to each other and square pieces, 8 mm on a side, were cut from both wafers. These pieces were then stacked alternately and pressure sintered using an adapted method of Foil and Ast 2", which is based on the original technique of Schober and Balluffi '1 . The sintering was carried out at a temperature of 1250' C and a pressure of 2 M|Pa for 3 hours in a reduced atmosphere of hydrogen. Specimens for electron microscopy were prepared by cutting slices from the stack, mechanically grinding them to 100 pm and chemically thinning them to electron transparency. TEM characterization was carried out in a JEOL 200CX microscope operating at 200 KeV. using the weak beam imaging technique wherever possible. RESULTS Bicrystal 1 The diffraction patterns of the two crystals conitaining the twin boundary for I(al arid (b) respectively. A large beam direction IIl are shown in Figs Mat. Res. Soc. Syrup. Proc. Vol. 106. ' 1988 Materials Research Society
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(a)
Wb)
FIG. 1. Diffraction pattern for beam direction [11111 in (a) crystal 1, and (b) crystal 2
FIG. 2. Bright field micrograph of the boundary under multibeam conditions, with the el
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