Date of Award
Open Access Dissertation
The metal-semiconductor (MS) Schottky barrier junction, formed by putting a metal in contact with a semiconductor crystal, is the simplest form of electronic rectifier. Despite the simple structure, the MS junction shows a variety of anomalous electrical characteristics. The non-ideality is generally described as a linear bias-dependence of the energy barrier height at the MS junction, quantified by an ideality factor. As the physical origin of this bias-dependent barrier height, the presence of various interface effects, such as the Schottky barrier inhomogeneity, interface trap states and morphological defects, have been proposed. However, there is no consensus among the researchers about the extent to which each of these interface anomalies effect the ideality of the junction.
Another intriguing aspect of the Schottky junction is its ability to inject minority carriers under certain conditions, as was demonstrated by the early works in the 1940s (e.g., the point contact transistor). However, the lack of physical understanding of this phenomenon, combined with poor reproducibility and the development of the p-n junction, inhibited technological progress of Schottky bipolar emitters. In recent years, the development of new material technologies, such as epitaxial graphene, has opened up possibilities for novel bipolar mode Schottky devices, reviving the interest in the theory of minority carrier injection in Schottky junctions.
In this study, the role of non-ideal interface effects and minority carrier injection on the transport properties of the Schottky junction interface are explored in relation to experimental observations made in silicon carbide Schottky interfaces. Silicon carbide (SiC) is an indirect wide band gap material with electronic and thermal properties suitable for high power, high temperature and high frequency electronic applications. The electronic applications of SiC electronics include high power systems such a hybrid/electric vehicle and smart grid systems as well as high sensitivity sensors, such as nuclear radiation detectors. Many of these applications require large barrier Schottky junctions, which are obtained by using large work function metals, such as nickel (Ni) and platinum (Pt).
As the Schottky junctions are formed on the surface of the semiconductor crystal, the crystal quality, and especially the surface characteristics are important regulators of the Schottky device performance. In this work, the epitaxial growth of 4H-SiC by CVD was optimized using dichlorosilane, a halogenated reactant gas as the silicon precursor. Large barrier (> 1.6 eV) Ni/4H-SiC Schottky contacts were fabricated on lightly doped n-type SiC epitaxial layers. The as-deposited diodes showed non-ideal characteristics, Rapid thermal annealing of the contacts at > 650oC improved the diode ideality.
In this dissertation, the Schottky barrier inhomogeneity in the as-deposited diodes is studied using Tung’s inter-acting barrier model. It is shown that the Tung model was not applicable for the highly non-ideal (n > 1.2) Schottky junctions. Rather, it is argued that interface trap states are responsible for the high level of non-ideality based on the observation of hysteresis patterns in the I-V and C-V characteristics. The trap density is estimated at 108~1010 cm-2 from the hysteresis results.
In a parallel effort, the very large barrier (Фp ~2.6 eV) Schottky heterojunction between epitaxial graphene (EG) and p-doped SiC was studied in this work for its potential in sensing applications. Surprisingly, the junction showed the capability of high efficiency ( > 99%) minority carrier injection. The theories of minority carrier injection in MS junctions are re-visited in this dissertation for explaining this result. It is shown analytically that highly efficient minority carrier injection is possible in large barrier Schottky junctions under a high injection level. An EG/p-SiC/n-SiC photo-transistor structure was developed that showed a bipolar gain in the order of 102 and a responsivity of 101~102 A/W under UV illumination. The bipolar EG/SiC Schottky junction, therefore, opens up unique possibilities in radiation detection and power switching applications.
Omar, S. U.(2014). The Role of Interface Effects and Minority Carriers in the Metal-Semiconductor Schottky Junction. (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/3582