Ni/Heavily-Doped 4H-SiC Schottky Contacts

In this work, we focus on the electrical characterization of Ni Schottky contact on n-type heavily doped (ND>1019 cm−3) 4H-SiC layer, achieved by P-ion implantation. In particular, the forward current–voltage characterization of Schottky diodes showed a reduced turn-on voltage for the Ni/heavily-doped 4H-SiC if compared to a reference Ni/4H-SiC Schottky contact fabricated under similar conditions but without implant. Moreover, it was observed the predominance of a thermionic-field-emission (TFE) mechanism for the current transport through the interface. From a current-voltage-temperature (I-V-T) study, the temperature-dependence of the Schottky barrier and doping concentration were evaluated, obtaining a reduction of the barrier (from 1.77 to 1.66 eV), while the doping concentration maintains constant around 1.96×1019 cm-3. This study provides useful insights for a deeper comprehension of the electrical behavior of Ni contacts and can have possible applications in 4H-SiC Schottky diode technology.


Introduction
Nowadays, 4H-SiC-based devices such as Schottky barrier diodes (SBDs) and metal-oxidesemiconductor field effect transistors (MOSFETs), have reached a high technological maturity and performances (low on-state voltage drop, high switching speed, high breakdown voltage, high temperature operation, etc.), and they can be largely employed in several applications [1]. In spite of the huge progresses, there are still margins of performance improvement associated to the optimization of the metal/semiconductor contact characteristics. Generally, the Schottky barrier properties are driven by an appropriate choice of metal (e.g., low work-function metals, tunable compositions, etc.), suitable semiconductor surface treatments or intentionally changing of the electric field distribution below the interface [2][3][4]. In this context, many efforts have been dedicated to the basic understanding of the current transport in metal/semiconductor contacts in both lightly and heavily doped epitaxial 4H-SiC layers, to achieve a good control of the properties of this system. While several literature works are devoted to the characterization of Schottky contacts on lightly doped 4H-SiC, basic studies on metal/heavily-doped SiC interfaces have been rather limited so far, in spite of the importance of these systems for the development of SiC devices (e.g. in Ohmic contact regions). In a recent work, Hara et al. [5] characterized Schottky diodes fabricated on 4H-SiC epitaxial layers with a doping concentration varying from 6.8 ×10 15 up to 1.8 ×10 19 cm −3 , observing a reduction of the barrier height and an increasing dominance of a thermionic field emission (TFE) mechanism for the current transport with increasing the epilayer doping.
Specifically, heavily doped 4H-SiC layers can be obtained by ion-implantation, which is an essential process for both n-type and p-type selective doping [6]. For instance, metal/semiconductor contacts with Ohmic properties can be obtained on heavily-doped 4H-SiC implanted regions by Nibased films annealed at high temperature, owing to the formation of nickel silicide (Ni2Si), as occurring in junction barrier Schottky (JBS) rectifiers technology, where ion-implantation is typically used for selective doping of p + -type regions [7].
In this work, we focused on the electrical characterization of Ni Schottky contacts onto heavily doped (ND>10 19 cm −3 ) n-type phosphorus-implanted 4H-SiC layer, under forward bias. Specifically, the Schottky properties and the dominant current transport mechanism were highlighted by a detailed electrical characterization of the contact.

Experimental Details
N-type 4H-SiC epitaxial layer with nitrogen doping of 1 ×10 16 cm −3 , grown onto a heavily doped 4H-SiC (0001) substrate, was the starting material of our work. The upper part of the epitaxial layer was heavily doped by phosphorus (P)-ion-implantation at 400°C, at energies ranging from 30 to 200 keV and with ion doses between 7.5×10 13 and 5 ×10 14 cm −2 . The resulting implantation profile extended over 200 nm and with a concentration peak of 1×10 20 cm 3 , as confirmed by previous secondary ion mass spectrometry (SIMS) measurements [8]. A postimplantation thermal annealing treatment, performed at 1675 °C in Ar-ambient and with a carbon capping layer for protecting the sample surface, was able to activate at least 80% of implanted P-ion, as highlighted in Fig.1, which reports the comparison between the SIMS profile and that derived by a depth resolved scanning capacitance microscopy (SCM) analysis (more details in [9]). Moreover, van der Pauw and Hall measurements performed by varying the measurement temperature in the range 300-500K [9], demonstrated that the sheet resistance RSH of the implanted 4H-SiC layer increased with temperature, while an almost constant value was observed for the electron density nel (1.44×10 15 cm -2 at room temperature). These characteristics can be associated to the behavior of a degenerate semiconductor [10] On this material, Schottky barrier diodes were fabricated by depositing 100 nm thick Ni-layer, using a direct current magnetron sputter, and defined by optical lithography and lift-off process in circular structures with radius of 250 μm. A large-area back-side contact was fabricated by Ni-deposition followed by rapid thermal annealing at 950 °C in N2-atmosphere before the front-side processing [11]. To avoid any consumption of the SiC layer, these Ni-Schottky contacts were deliberately not subjected to annealing treatments. The Schottky contacts were characterized by current-voltage-temperature (I-V-T) measurements (in the range 25°C-115°C), under forward bias, in a Karl-Suss MicroTec probe station equipped with a parameter analyzer.

Results and Discussion
As starting point, in Fig.2 we compared the room-temperature forward current density-voltage (J−VF) characteristics of Ni Schottky contacts formed on n-type heavily-doped 4H-SiC epilayer and on a reference 4H-SiC epilayer without doping implantation. Evidently, the Ni/heavily doped 4H-SiC contact exhibited a lower turn-on voltage with respect to the reference Ni/4H-SiC contact without implant [12]. Typically, the forward characteristics for Schottky contact to 4H-SiC can be described by the thermionic emission (TE) model [13], with J-VF relationship expressed as where A * is the effective Richardson's constant of 4H-SiC (146 A cm −2 K −2 ) [14], q is the elementary charge, ΦB is the Schottky barrier, kB=1.38×10 -23 J/K is the Boltzmann's constant, T is the absolute temperature, VF is the voltage applied across the metal/semiconductor interface, RON is the specific on-resistance and n is the ideality factor. In particular, by analyzing the forward characteristic using the TE model, we derived a barrier height of 0.94 eV and an ideality factor of 1.8, thus showing a high discrepancy from the ideal case for Schottky contact with n=1. Hence, the current transport through the contact cannot be described by a TE regime. As matter of fact, considering the high doping concentration of the implanted region, a tunneling contribution to the current must be taken into account, as described by the thermionic field emission (TFE) model [15] (Fig.3). The TFE J-V relationship is given by where the saturation current J0,TFE (VF) can be expressed as with ∆EF the difference between the bottom of the conduction band and the semiconductor Fermi level, 0 = 00 × ℎ � 00 � and 00 = ℎ 4 × � * , where m * =0.38 m0 the effective mass (m0 is the electron mass) and εSiC =9.66 ε0 the dielectric constant of the semiconductor (ε0 is the vacuum permittivity).   4 reports in a semilog scale the experimental room-tempertaure J-VF curve related to a representative diode on heavily doped 4H-SiC layer, fitted to a simulated TFE curve. The barrier ΦB and the doping concentration ND were determined as best-fit parameters of the TFE curve, obtaining ΦB=1.77 eV and ND=1.97×10 19 cm -3 . Notably, the ratio kBT/qE00 gives an indication about the relevance of the TE process with respect to the tunneling one. Using ND=1.97×10 19 cm -3 derived from the fit, we obtained a ratio kBT/qE00 of 0.61, which confirmed the appropriateness of the TFE model [16]. By knowing the doping concentration, the depletion width WD at the Ni/4H-SiC interface can be derived, resulting of 10 nm for ND=1.97×10 19 cm -3 [17]. Such finding is in agreement with the value measured by SCM for the active dopant profile (Fig.1) [9]. Under reverse bias the contact showed a leakage current level varying from 10 -8 up to 10 1 A/cm 2 in the 0-8V range, as discussed in [12]. In order to gain insights on the Schottky barrier properties, the temperature-dependence of the forward characteristics of the diode was studied. As shown in Fig.5

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Silicon Carbide and Related Materials 2021 (1.96±0.02) ×10 19 cm -3 was found over the measurement temperature range. This latter agrees with SCM profiling measurement related to the electrically active P-ion concentration underneath the interface and with Hall measurements that in the same range of temperature gave an almost constant value for the average carrier concentration over the entire implanted thickness. Fig.6: T-dependence of ΦB and doping concentration, derived by the TFE fits of the forward J-VF curves.

Summary
In conclusion, we have electrically characterized Ni Schottky contact on n-type-implanted 4H-SiC layer, with doping concentration ND>10 19 cm −3 . This ion-implantation doping produced a reduced forward turn-on voltage in the heavily-doped 4H-SiC diode with respect to a reference one. The thermionic-field-emission was the dominant mechanism for the current transport through the interface under forward bias. The temperature analysis of the forward current-voltage curves highlighted a reduction of the Schottky barrier (from 1.77 to 1.66 eV), while the doping concentration maintained constant around 1.96×10 19 cm -3 , in the investigate temperature range 25-115°C. This study provides useful insights for a deeper comprehension of the electrical behavior of Ni contacts and can have possible applications in 4H-SiC Schottky diode technology.