Hydrogen Passivation of Grain Boundaries in Polycrystalline Silicon Deposited by Molecular Beams
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HYDROGEN PASSIVATION OF GRAIN BOUNDARIES IN POLYCRYSTALLINE SILICON DEPOSITED BY MOLECULAR BEAMS D. JOUSSE '), S.L. DELAGE, S.S. IYER and M. CROWDER * IBM T.J. Watson Research Center, P.O.Box 218, Yorktown Heights, NY 10598 *IBM San Jose, 5600 Cottle Road, San Jose, CA 95193
Absiract Grain boundary properties of polysilicon deposited by molecular beams have been investigated by electron spin resonance and conductivity measurements. The variations of the conductivity activation energy with doping can be explained by a density of states model consisting mainly of two exponential bandtails, implying that dangling bonds play a minor role. A hydrogen plasma treatment at 500 'C reduces the spin density by a factor of three but also passivates weak Si-Si bonds thus leading to steeper bandtails. The possibility of hydrogen-related intra-grain gap states is also discussed.
Introduction Polycrystalline silicon (polysilicon) has many important applications in integrated circuit technology, but has had so far very limited success as the active layer of thin film transistors (TFT's) for large scale and low temperature applications like flat panel displays. One of the main reasons has been the high OFF current associated with the small grain size and the large defect densities at grain boundaries found in the material prepared by the usual thermal decomposition of silane (LPCVD) at 625 'C. The use of an alternative technique like molecular beam deposition appears very attractive in view of the high value of the effective channel mobility reported for TFT's made by this technique [1]. For the physicist, the ultra-high vacuum conditions associated with this growth process offer a better guarantee that one is looking at fundamental properties of grain boundaries rather than impurity effects. Hydrogenation has also become very popular for the improvement of device characteristics [ 1,2] but is far from being completely understood because hydrogen may produce different effects depending on the treatment conditions: passivation of dangling bonds [3] or intragrain defects , compensation of acceptor [4] and possibly donor [5] states, and finally damaging of the top layer [6]. We report here on the study of grain boundary (GB) defects by electron spin resonance (ESR) and the variations of electrical conductivity with doping. The data obtained after hydrogenation are used for the analysis of GB defect passivation and hydrogen-dopant interaction.
Experimental The samples have been grown in a Si molecular beam epitaxy system using two different techniques, namely 0 direct formation (DF) at 750 C, and in situ solid phase crystallization (SPC) at 650 'C of an amorphous film deposited at 250 'C. As described in a separate paper [7], the latter technique produces films with a smoother morphology and higher doping efficiency. The dopant concentration was determined by Rutherford Back Scattering on highly doped samples and then extrapolated from flux to the low doping regime. The substrates were high resistivity c-Si substrates covered with a
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