The origin of the integral barrier height in inhomogeneous Au/Co/GaAs₆₇P₃₃-Schottky contacts: A ballistic electron emission microscopy study

In this work we investigated the relationship between the integral Schottky barrier height (SBH) obtained from conventional current-voltage (I-V) measurement and the distribution of the local SBH measured by ballistic electron emission microscopy (BEEM) on a nanometer scale length. For this purpose, we investigated inhomogeneous Au/Co/GaAs₆₇/P₃₃-Schottky contacts. The samples were prepared by the deposition of a discontinuous Co film on the semiconductor followed by the deposition of a continuous Au film. This provided regions with local presence of one or the other metal (Au or Co) at the metal-semiconductor interface, resulting in mesoscopically extended SBH inhomogeneities. The local SBH distribution as well as the integral SBH depended on the preparation parameter of the Co layer, i.e., on the combination of the substrate temperature (300 or 500 K) and the nominal Co thickness (0, 0.25, 0.5, 0.8, 1.0 nm). For the different preparation parameters, statistical distributions of the local SBH were measured by BEEM. Treating these SBH distributions in terms of a parallel conduction model for the electron transport across the MS interface, we calculated for each preparation parameter an integral SBH and compared it with the measured integral SBH obtained from conventional I-V measurement. The calculated and measured integral SBH's were in very good agreement, demonstrating clearly the strong influence of the low SBH regions on the electron transport across the interface and therefore on the integral SBH. The SBH values for homogeneous Au/GaAs₆₇-P₃₃- and Co/CaAs₆₇P₃₃-Schottky contacts, i.e., with only one sort of metal at the interface, were determined to be Φ^Au_SB=1180±10 meV and Φ^Co_SB=1030±10 meV. As with regard to the inhomogeneous Schottky contacts the fraction of area of the MS interface covered by Co increased, the local SBH distributions as well as the integral SBH's decreased gradually from the value of Φ^Au_SB to Φ^Co_SB.