Quantum Size Effect in Polysilicon Gates
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Quantum Size Effect in Polysilicon Gates. N. L.fshitz
S. La-yi
T. T. Sheng AT&T Bell Laboratories Murray Hill, New Jersey 07974
ABSTRACT It has been observed by several authors that metal-oxide-semiconductor devices with polycrystalline Si (polySi) gates behave differently depending on the doping species in polySi: the work-function difference between the silicon substrate and the gate %,) is higher when the gates are doped with arsenic than when they are
doped with phosphorus. We believe that the different behavior of t. can be explained by different grain textures at the polySi/SiO 2 interface. Our transmission electron microscoey of the films indicates that while P-doped material consists of large (=3000A) grains, As-doped polySi preserves its as-deposited columnar structure - even after a high temperature anneal. Moreover, at the interface with the gate oxide an as-deposited microstructure with very small (=100A) "embrionic" grains is preserved. On the basis of these observations, we suggest a model for the different behavior of t. The model is based on a quantum-size effect which becomes important for such small grain dimensions at the interface in As-doped polySi. This effect drastically reduces the number of states available in the conduction band at low energies. The resulting shift of the Fermi level provides a qualitative explanation for the observed puzzling difference between the work-functions of Asand P- doped polySi.
Polycrystalline silicon (polySi) has been used as the gate material of the MOS transistors for almost two decades. It is known that due to grain boundaries the transport properties of polySi are different from those of the single crystal material. However, it has been usually assumed that the grain boundaries do not change the band structure of the material, so that the Fermi level in polySi at any given carrier concentration is equal to that in crystalline Si with the same concentration. The work-function difference between the silicon substrate and a polySi gate (%t.,) is usually estimated on this basis. The tp, is an important parameter of the MOS system because it contributes into the threshold voltage of the field effect transistor. The MOS system with polySi gates is shown schematically in Fig. la, and its energy diagram at zero voltage on the gate is shown in Fig. lb. In equilibrium the Fermi level is constant throughout the system, and the work-function difference is defined as the distance between the conduction band edges. The ýp, is usually determined experimentally at the so-called flat-band condition show in Fig. 1c, when a voltage applied to the gate is sufficient to compensate the work-function difference and "unbend" the bands. This voltage is called the flat-band voltage and the 4p is determined as the distance between the Fermi levels in the substrate and the gatel[]. In a recent work [21 we correlated the tp, with both the doping level and carrier concentration in polySi (the substrate doping was kept constant and the electron concentration
Mat. Res. Soc. Symp. P
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