Defects in Schottky barrier structures

Abstract

The progress achieved in recent years in study of the element and phase composition, as well as the energy spectrum of electron states on semiconductor boundaries with metals and dielectrics, has shown the complexity and multifaceted nature of the physicochemical reactions-occurring on these boundaries. In the present review we have attemped to consider the question of the physical interrelationship between the phenomena of interdiffusion of elements and formation of intermetallic compounds, and the electrophysical properties of metalsemiconductor contacts. Analysis of the results of theoretical and experimental studies permits the conclusion that an important role in both potential barrier formation and charge carrier transmission is played by structural defects developed during these reactions in the contact region of the semiconductor. After chemical processing and deposition of metallic coatings stoichiometric composition defects are formed. Vacancies, antistructural defects, and defect complexes form surface electron states which together with intrinsic states ensure stabilization of the Fermi level on semiconductor boundaries with metals. In many cases structural defects are the vehicle by which physicoehemical interactions in thin film metal-semiconductor contacts affect the electrical parameters of devices and their stability. The initial stage of degradation, as a rule, is the appearance of excess currents in the forward branch of the CVC at temperatures of 150-77°K. Study of the physical nature of excess currents shows that their appearance is related to generation of structural defects during thermal processing, or mechanical or electrical testing of specimens. The defects create a system of inhomogeneously distributed deep centers in the space-charge region. The most probable mechanism for charge carrier transmission with participation of deep levels is resonant tunneling. Thus, to solve problems related to increasing reliability of Schottky barrier devices special attention should be given to conditions under which defects develop in the active region of the semiconductor diode and methods for eliminating these defects. Creation of conditions during device preparation favorable to formation of stable intermetallic compounds in the transition layer, maintenance of stoichiometric composition in the semiconductor, and inhibition of diffusion of electrically active metals, together with selection of barrier producing materials and protective coatings providing minimum mechanical stress in the contact will insure development of Schottky barrier devices having electrical parameters which are stable over a wide temperature range.