Surface-State and Interface Effects in Schottky Barriers at n-Type Silicon Surfaces

In an attempt to develop a Schottky barrier sufficiently low to be Ohmic at high current densities, Ca and Mg have been evaporated on freshly cleaved Si surfaces and on cleaved surfaces given controlled exposure to oxygen and water vapor. The results are explained in terms of the metal work function, the density of semiconductor surface (or interface) states and the thickness of the interface layer between metal and semiconductor. For clean metal-Si interfaces, measurements of barrier height as a function of the metal work function and as a function of the electric field in the Si are shown to determine an upper limit of 0.9 Å for the (thickness)/(dielectric constant) of the interface layer, and a lower limit of 2×1014 states/cm2/eV for the surface-state density. The Thomas-Fermi shielding length (≈0.5 Å for most metals) is proposed as the width of the interface layer. The resulting surface-state density is in good agreement with that for a free Si surface. This relationship appears to be a fairly general pattern for a wide variety of metal-semiconductor contacts. When a monolayer of oxygen separates the metal and the Si, the surface-state density relative to that of clean Si decreases appreciably, but the thickness of the interface layer also increases. With a given metal, the resulting dipole moment associated with the interface layer is only slightly changed. Thus only a small decrease in barrier height occurs. In both clean contacts and contacts with an interface composed of approximately one monolayer of oxygen, approximately ¼ of any change in the metal-Si contact potential is effective in changing the Schottky barrier height. The surface-state density and barrier height are very greatly reduced if the silicon surface is exposed to water vapor before deposition of the metal, but the resulting barrier is not stable in time.