Local Bonding Structure of Hydrogen in Crystalline Silicon: NMR and TEM Studies

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LOCAL BONDING STRUCTURE OF HYDROGEN INCRYSTALLINE SILICON: NMR AND TEM STUDIES J. B. BOYCE, N. M. JOHNSON, S. E. READY, J. WALKER, and G.B.ANDERSON Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT Nuclear magnetic resonance (NMR) and transmission electron microscopy have been used to obtain information on the local structure of deuterium in singlecrystal silicon, both for deuterated, phosphorous-doped Si that contains plateletstructured defects and for deuterated boron-doped silicon. In both cases, the NMR spectrum consists of two components, a narrow doublet and a central, unsplit line. The doublet arises from D bonded to Si with the Si-D bond along the < 111 > directions. The central line, which contains more D than does the doublet, is ascribed primarily to molecular D2 that resides in regions of the crystal where translation and tumbling are inhibited and possibly to some D in weak Si-D bonds. INTRODUCTION Hydrogen, introduced into single-crystal silicon (c-Si), can generate extended structural defects as well as passivate defects and dopants [1]. Dependent upon the hydrogenation and doping conditions, H can form dopant-H complexes with either donors or acceptors or it can generate platelet-structured defects. For the latter case, these defects are planar in shape, are aligned predominantly along {111} crystallographic planes, and involve the coordinated formation of Si-H bonds [1-4]. Despite the fact that these phenomena have been well-studied, much remains unknown about the local hydrogen bonding structure. Nuclear magnetic resonance (NMR) has become one of the major techniques for structural studies of hydrogen in materials. Yet it has not been applied extensively to hydrogen in crystalline semiconductors due to sensitivity limitations, that is, it requires a large total number of spins while standard hydrogenation techniques introduce H in substantial amounts only near the exposed surface. EXPERIMENT Two strategies were implemented to achieve a sufficient signal-to-noise ratio for the NMR measurements. First, deuterium was substituted for hydrogen since its lower natural abundance (0.0156%) reduces the background signal to a negligible level. D also provides an additional probe of the local environment via its quadrupole moment which is zero for H. Secondly, since D is introduced only through an exposed surface, it was necessary to stack together a set of thin Si (100) wafers, 3 mils thick, deuterated on both sides. A set of 76 such wafers, rectangular in shape (0.64 cm by 1.9 cm) formed the NMR sample (total volume = 0.68 cm3). Two such samples were studied: (1) n-type Si (2x 1018 phosphorous cm- 3) deuterated with a two-step treatment, namely, 1750C for 20 minutes and 225°C for 60 min [5]. This procedure has been shown to generate a high density of platelets (see Fig. 1).

Mat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society

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(2) p-type Si (1.5X 1019 boron cm- 3) deuterated at 350°C for at least 210 minutes. A negligible fraction of the D is expec

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