Schottky barriers on GaAs: Screened pinning at defect levels

Abstract

Schottky barrier heights on n-type GaAs can be made to vary from 0.30 to 1.20 eV by adjusting the conditions under which the Schottky contact is fabricated. To explain the observed trends it is asserted (a) that there are several defect pinning points associated with GaAs surfaces, (b) that an appropriate interfacial permittivity may be defined to predict the barrier height from the metal work function and the position of the appropriate pinning point, and (c) the pinning point is sensitive to the metal-GaAs reaction chemistry. In a thermodynamic context the present analysis is an electronegativity model. The work function of a polycrystalline metal and the charge neutrality level of the semiconductor are interpreted as the bulk chemical potentials of the two materials. When the two materials are brought into contact electronegativity equalization takes place as charge is transferred across the interface in the formation of metal-semiconductor bonds. Proper derivation of the equilibrium bond charge dipole requires a knowledge of the chemical ``softness'' of the two surfaces being brought into contact as well as the electronegativity difference of the two materials. The equilibrium dipole determines the barrier height of an ``ideal interface Schottky contact.'' A description is given of how the softness of metal and semiconductor surfaces may be defined and how near surface defects in the semiconductor modify the interfacial dipole. The strength of the model is in its descriptive power as it can identify the specific native defects at which pinning is observed in GaAs Schottky diodes. It provides a framework for the development of a predictive model based on identifying the particular native defects produced by the reaction of specific metals with a semiconductor as a function of the process history.