Inhomogeneities in Ni∕4H-SiC Schottky barriers: Localized Fermi-level pinning by defect states

Abstract

We investigated arrays of Ni, Pt, or Ti Schottky diodes on n-type 4H-SiC epitaxial layers using current-voltage (I-V) measurements, electron beam induced current (EBIC), polarized light microscopy, x-ray topography, and depth-resolved cathodoluminescence spectroscopy. A significant percentage of diodes (∼7%–30% depending on epitaxial growth method and diode size) displayed “nonideal” or inhomogeneous barrier height characteristics. We used a thermionic emission model based on two parallel diodes to determine the barrier heights and ideality factors of high- and low-barrier regions within individual nonideal diodes. Whereas high-barrier barrier heights increased with metal work function, low-barrier barrier heights remained constant at ∼0.60, 0.85, and 1.05eV. The sources of these nonidealities were investigated with a variety of spectroscopic and imaging techniques to determine the nature and energy levels of the defects. EBIC indicated that clusters of defects occurred in all inhomogeneous diodes. Cathodoluminescence spectra revealed additional peaks in the nonideal diodes at 2.65, 2.40, and 2.20eV, which complement the low-barrier barrier heights. It is proposed that defect clusters act to locally pin the Fermi level, creating localized low-barrier patches, which account for the inhomogeneous electrical characteristics.

A Comparative Study on Electrical Characteristics of Au/N-Si Schottky Diodes, with and Without Bi-Doped PVA Interfacial Layer in Dark and Under Illumination at Room Temperature

“…Also, the magnitude of leakage current in the reverse bias region for MPS type SBD is 100 times lower than that of MS type SBD in dark condition. According to Demirezen et al [1] Defives et al [21] Ewing et al [22], these two linear regions in the forward bias I-V plots in dark show two distinct barrier heights (BHs) in the parallel. Thus, the relationship between the I and V in dark can be expressed as [1,[23][24][25].…”

“…On the other hand, these high values of n for two regions can be attributed to the high density of N ss localized at M/S interface and the effect of barrier in homogeneity [1,8,11]. Also, the image-force effect, recombination-generation and tunneling may be possible mechanisms that could lead to an ideality factor value greater than unity [19][20][21][22][23][24][25][26]. On the other hand, unless specially fabricated, a SBD possesses a thin interfacial native layer at M/S interface.…”

A study of temperature-related non-linearity at the metal-silicon interface

“…Tung states 9 that at low temperature, Ohmic effects within the few conducting patches cause the dual current paths to become deconvoluted. This might explain the frequently cited and debated double bumps that can be seen variously in Si Schottky diodes 9,11 , SiC diodes [16][17][18][19][20] , GaAs diodes 23,24 , and also in heterojunctions 12,25 . The worsening of these effects in devices without guard rings or other edge protection, could futher be explained by low SBH patches at the device extremities which are not pinched off as well as those in the centre of the device, these being surrounded on all sides by the higher background patches [9][10][11] .…”

“…Other applications include carbon nanotube Schottky barrier transistors 6 , and Schottky solar cells, with materials including lead selenide nanocrystals 7 and graphene 8 . Despite over a hundred years of research and development into Schottky barriers, across all popular semiconductors and for the various applications, we still find ourselves with unanswered questions as to the nature of current flow across the barrier, especially in light of inhomogeneity at the metal-semiconductor interface 9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24] , which can result in multiple conduction paths through the non-uniform interface. Sources of interfacial inhomogeneity include processing remnants (dirt, contamination), surface roughness, native oxide, an uneven doping profile, crystal defects and grain boundaries 9,11,12 and it is generally now accepted that the surface is better represented as a random array of different patches, each of varying barrier height and area, as represented in the inset of Figure 1a.…”

“…Interfacial inhomogeneity has been cited 9-24 as the cause for many experimental results that deviate from this response. These so called non-linearities include high ideality factors 9,11,13 , the discrepancy between Schottky barrier heights (SBH) extracted via CV and IV techniques [9][10][11][12][13][14][15] , the "T 0 anomaly" 9,11,13 , double bumps within the semi-log turn-on characteristics 9,11,13,[16][17][18][19][20] , edge effects 9,11 and non-linearity within Richardson plots 22 . It was the papers written by Tung [9][10][11] however, that first introduced a model that adapted Equation 1 to fully explain the non-linearities in light of interfacial inhomogeneity and SBH fluctuation.…”