Electrical characterisation of UHV-bonded silicon interfaces
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Electrical characterisation of UHV-bonded silicon interfaces A. Reznicek, S. Senz, O. Breitenstein, R. Scholz and U. Gösele Max-Planck-Institute of Microstructure Physics Weinberg 2, D-06120 Halle, Germany
Abstract Direct wafer bonding can be used to mechanically and electrically connect semiconductors. In our experiments two 100 mm diameter (100) Si wafers (n-doping: 1014 cm −3 ) are first cleaned by standard chemical cleaning (RCA 1, 2). The surface is terminated by hydrogen after a HF dipping. The wafers are prebonded in air to protect the surface. After introduction into the ultra high vacuum (UHV) system the wafers are separated again. The hydrogen termination is released in a heating chamber. RHEED confirmed a surface reconstruction. The wafers are then cooled down to room temperature and bonded in UHV. The bonding energy is very close to the bulk bonding energy. Measurements of whole n-n wafers showed a linear relationship of voltage and current at a low current density of 0.05 A/cm2. The current flow is inhomogeneous, which is visible in IRthermography images. Above 0.1 V the current density first saturates, but increases superlinearly for higher voltages. The electrical properties of a grain boundary can be modeled by a double Schottky barrier. The barrier height decreases with increasing applied voltage. C-V measurements show a strong dependence of capacitance on frequency, temperature and applied voltage. The capacitance increases with higher temperature and lower frequency. The interface state density can be estimated from the low temperature and high frequency capacitance limit as D it = 1⋅ 1011 cm-2eV-1 assuming a constant density of states. We can conclude that in order to avoid the undesirable effect of the potential barrier and trap states at the bonding interface a high doping near the interface is required for the application of wafer bonding to devices with a high current density across the bonded interface.
Introduction Grain boundaries in silicon and other semiconductors have been investigated since many years. Most of the interest concentrates on materials for polycrystalline solar cells or voltage dependent resistors (varistors). The electrical properties of these devices are determined by the grain boundaries [1, 2]. The usual processing includes a high temperature step, e.g. solidification of a silicon melt to produce solar cells. Due to this high temperature process most of the impurities are concentrated at the grain boundaries. This results in a high density of electrically active states and a double sided depletion layer. In our experiments we produce an artificial grain boundary by bonding of two silicon single crystals. The most important distinction to other experiments is that we create this grain boundary at room temperature. The bonding is performed in UHV and allows direct formation of covalent bonds. I4.4.1
Experimental For the experiments 100 mm CZ-grown silicon wafers were used. The p-doping is 1⋅ 1015 cm -3 and the n-doping is 3 ⋅ 1014 cm 3 . On the backside the wafers ar
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