Metal work-function-dependent barrier height of Ni contacts with metal-embedded nanoparticles to 4H-SiC
© Kang et al; licensee Springer. 2012
Received: 15 July 2011
Accepted: 13 January 2012
Published: 13 January 2012
Metal, typically gold [Au], nanoparticles [NPs] embedded in a capping metal contact layer onto silicon carbide [SiC] are considered to have practical applications in changing the barrier height of the original contacts. Here, we demonstrate the use of silver [Ag] NPs to effectively lower the barrier height of the electrical contacts to 4H-SiC. It has been shown that the barrier height of the fabricated SiC diode structures (Ni with embedded Ag-NPs) has significantly reduced by 0.11 eV and 0.18 eV with respect to the samples with Au-NPs and the reference samples, respectively. The experimental results have also been compared with both an analytic model based on Tung's theory and physics-based two-dimensional numerical simulations.
Recently, silicon carbide [SiC] has been proposed as the material of choice especially for power electronic and sensing devices operating under high temperature, fast switching, and high-power conditions mainly due to its wide bandgap (3.26 eV), high critical electric field (2.2 × 106 V/cm), superior thermal conductivity (4.9 W/Kcm), and high bulk electron mobility (900 cm2/Vs) of the 4H polytype [1, 2]. For stable operations at high power densities and elevated temperatures, SiC diodes, including Schottky barrier diodes and junction barrier Schottky diodes, as well as SiC transistors, have been under extensive exploration with great improvements in wafer growth technology and device process.
In order to realize stable SiC devices, metal contacts to SiC with suitable physical and electrical characteristics are required. For example, Ohmic contacts with low contact resistances and Schottky contacts with controlled barrier height (ΦB) between SiC and metal are among the most important factors for determining the performance of SiC devices [3–5]. Furthermore, electrical characteristics of devices, such as voltage drop and switching speed of such devices, are dependent on the current transport behavior through the structure of the metal/4H-SiC interface. It is, therefore, of critical importance to reduce the barrier height of the metal/4H-SiC interface in order to improve the on-state voltage drop in 4H-SiC devices.
To date, extensive studies have been carried out on the properties of barrier height of various metals on n- and p-types for SiC [6, 7], and many attempts have been made to modify the contact barrier height on SiC. The effect of inhomogeneities and Fermi-level pinning on Schottky contact properties has been known to be minimal, and the barrier height depends mostly on the metal work function without strong Fermi-level pinning for SiC [4, 5]. Recent work on the electrical contacts to SiC includes the implementation of nanostructures such as metal nanoparticles [NPs] to modify the barrier height at metal-SiC interfaces and to alter fundamental SiC device properties by controlling the size of the metal NPs. Previous results in the literature have been primarily focused on the effect of size reduction of NPs on the characteristics of diode structures with embedded NPs, which experimentally investigates the change in transport properties of metal/semiconductor interfaces in SiC depending on the size of NPs [5–10]. However, so far, the focus has been mainly on the scaling effect of the NPs rather than on altering the electrical barrier of the NPs.
In this work, we demonstrate that the work function change in the embedded metal NPs can effectively control the barrier height change of the SiC diode structures. Our results show that incorporating NPs with a larger work function difference to the capping metal layer results in an improved barrier lowering by further enhancing the local electric field. The experimental results have also been compared with both an analytic model based on Tung's theory [11–13] and physics-based two-dimensional numerical simulations.
Summary of all the different sets of fabricated samples and process conditions
500°C, 20 min
500°C, 20 min
The barrier height and ideality factor were compared with the physical distribution condition of the NPs as determined by field emission scanning electron microscopy [FE-SEM]. To investigate the effect of the NPs at the Ni/SiC interface on the electrical properties, current-voltage [I-V] and capacitance-voltage [C-V] characteristics of the devices were measured by using a Keithley 4200 semiconductor parameter analyzer (Keithley Instruments Inc., Cleveland, OH, USA). The experimental results have also been compared with an analytic model based on Tung's theory [11–13] and further verified by considering band diagram and electric field distribution using a physics-based two-dimensional numerical simulator Atlas (Silvaco Inc., Santa Clara, CA, USA) .
Results and discussion
where Js is the saturation current density, ΦB is the effective barrier height [ΦB = kT/e ln(A*T2/Js)], A* is the Richard constant (for 4H-SiC, 146 A/cm2 K2) , T is the absolute temperature, k is the Boltzman constant, q is the electron charge, and n is the ideality factor [n = kT/e(dV/d(lnJ))]. The values of the effective ideality factor and barrier height were calculated from the ln (J) versus forward voltage V characteristics. Under forward voltage conditions, it clearly shows that the current value of sample NP-2 was about one order of magnitude higher than that of reference samples (10-3 A/cm2), due to the smaller barrier height of NP-2 (0.87 eV) compared with that of Ref (1.04 eV).
where Vi is the voltage intercept, Vn is the energy difference between the minimum of the conduction band and Fermi level in the bulk of n-type SiC [Vn = kT/e ln(NC/ND)], and NC is the conduction band density of states for 4H-SiC at 300 K (approximately 1.66 × 1019 cm-3) . As observed from both I-V and C-V measurement results, all the samples exhibit excellent rectifying behavior with stable ideality factors.
The results, however, clearly suggest that the barrier height difference between the Ni/SiC contacts (Ref) and samples with embedded NPs significantly increases and that the enhancement becomes greater for Ag particles (NP-2) than for Au particles (NP-1). The values of barrier height lowering are 0.06 eV and 0.07 eV for NP-1, whereas the values are clearly increased to 0.17 eV and 0.18 eV for NP-2 as obtained from I-V and C-V measurements, respectively. Note that the reduced barrier height and improved ideality factor are attributed to the the larger difference in the metal work function of Ag than that of Au with respect to the capping metal of Ni.
where z is the distance from the surface of the semiconductor, w is the depletion width, R0 is the radius of the circular patch, and ΔΦ is the difference of the barrier height between the capping metal and NPs.
In summary, we demostrate that the work function change in the embedded metal NPs can effectively lower the barrier height of the SiC diode structures. It has been experimentally shown that incorporating NPs (Ag) with a larger work function difference to the capping metal layer (Ni) results in an improved barrier lowering by further enhancing the local electric field. The barrier height of the fabricated SiC diode structures (NP-1; Ni with embedded Ag-NPs) has significantly reduced by 0.11 eV and 0.18 eV with respect to the samples with Au-NPs (NP-2) and the reference samples, respectively. The experimental results are in agreement with both analytic calulations based on Tung's model and physics-based two-dimensional numerical simulations, which confirm that the increased electric field of the samples with NPs is mainly attributed to the reduction of barrier height as the effective barrier of the conduction band at the depletion region of the surface decreases.
This work was supported by the National Research Foundation Grants 2011-0017942 and 2011-0003298 through a research grant from Kwangwoon University in 2011, and Korea-Sweden Collaboration Project.
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