Materials Science Forum, ISSN: 1662-9752, Vol. 1158, pp 27-32

doi: 10.4028/p-S2Zh3j

© 2025 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2024-09-10

Revised:
2025-08-18

Accepted:
2025-08-18

Online:
2025-09-08

Evolution of the Electrical and Microstructural Properties of Mo/4H-SiC Contact with the Annealing Temperature

The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

Marilena Vivona , Patrick Fiorenza , Giuseppe Greco , Salvatore Di Franco , Gabriele Bellocchi , Paola Mancuso , Simone Rascunà , Antonio Massimiliano Mio , Giuseppe Nicotra , Filippo Giannazzo and F. Roccaforte

CNR-IMM, Strada VIII n.5, Zona Industriale, I-95121, Catania, Italy

STMicroelectronics, Stradale Primosole 50, I-95121, Catania, Italy

marilena.vivona@imm.cnr.it, patrick.fiorenza@imm.cnr.it, giuseppe.greco@imm.cnr.it, salvatore.difranco@imm.cnr.it, gabriele.bellocchi@st.com, paola.mancuso@st.com, simone.rascuna@st.com, antonio.mio@imm.cnr.it, giuseppe.nicotra@imm.cnr.it, jfilippo.giannazzo@imm.cnr.it, kfabrizio.roccaforte@imm.cnr.it

* corresponding author

Abstract

In this work, we investigated the electrical properties evolution of Schottky contacts following thermal annealing treatments at temperature up to . The electrical characterization under forward and reverse bias revealed a reduction of the barrier height from 1.45 eV (as-deposited contact) to 1.30 eV ( -annealed contact), with the presence of inhomogeneity in the contact, while the leakage current followed a thermionic-field emission (TFE) model after annealing at and presented a significant increase for the -annealed contact. The electrical characterization was associated with microstructural analyses, which highlighted an enlargement of the grains forming the structure of the Mo-film and the presence of voids near the interface. These observations can be at the base of the variation in the electrical behavior of the contact.

Introduction

The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

Nowadays, Schottky-barrier diode (SBD) on is an established technology used in several real-world applications [1]. Although this large use, additional improvement is still possible for a fully exploitation of the potentialities. Essentially, the properties of the metal/ contact are at the base of the SBD performance and the achievement of a superior control on this interface drives an optimization of the use of the SBD. Over the last two decades, various approaches have been considered for gaining control in this system. Particular attention was paid to the choice of the metal and its evolution with thermal annealing in the Schottky contact formation [2,3]. Recently, the exploration of low-work function materials, such as W and Mo-based contacts, has demonstrated promising results for minimizing the power dissipation of Schottky diodes and offering good thermal stability [3,4]. In particular, Mo showed a large variability of the Schottky barrier height ( ) value dependent on the passivation treatment of the surface [5] or the temperature of the metal deposition [6], with the varying between 1.0 and 1.5 eV . From a structural point of view, Mo can readily form carbides and silicides and for that reason Mo-film deposited with Si or C element can be useful in preventing the reaction with SiC surface. For example, Mo-based contacts containing C were recently investigated, either deposited in laminated layers or from Mo-C alloyed targets [7,8], demonstrated stability of the electrical characteristics even at high annealing temperature. However, the origin of this electrical behavior is not clear and additional investigation on the interface is necessary: on the one hand to correlate the electrical properties of the contact with the process, on the other hand to link the electrical behavior also to the microstructural properties, for better understanding the role of the and C in the contact formation.

In this study, we follow the evolution of the electrical properties of contacts with increasing the annealing temperature up to . The electrical characterization is combined with a microstructural analysis to shed light on the interface behavior and possible reactions to finally explain the electrical behavior.

Experimental Details

The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

The starting material for this study consisted in a wafer with a n -type epitaxial layer (doping concentration of ) grown onto a -doped substrate. Schottky diodes were fabricated starting from the deposition of a large-area back-side Ohmic contact by sputtering 100 nm thick Ni layer, followed by a rapid thermal annealing (RTA) treatment in a furnace at in for 60 s [9]. Then, for the front-side Schottky contact, 80 nm -thick Mo film was sputtered with the contact area defined by optical photolithography, lift-off processing steps and RTA treatments for 10 min in at temperatures of 700 and . On these annealed samples, the electrical characterization under forward and reverse bias was carried out for a set of equivalent diodes by I-V measurements in a Karl-Suss MicroTec probe station equipped with a parameter analyzer. For comparison, the as-deposited contact was also investigated. The temperature dependence of both forward and reverse I-V characteristics (I-V-T) was studied for selected samples. A microstructural analysis was associated to the electrical characterization, with lamellae for the annealed samples prepared by a Dual Beal Thermofisher Helios 5 focused ion beam (FIB) and analyzed by probe Cs corrected JEOLARM200F Scanning Transmission Electronic Microscope (S/TEM). A Gatan GIF Quantum ER system was also used for electron energy loss spectroscopy (EELS) measurements.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

Firstly, the investigation on the electrical behavior of the contacts under forward and reverse bias is reported in Fig. 1. Specifically, Fig.1a shows the forward current density-voltage (J-V) characteristics, representative of a set of 10 diodes, for the as-deposited, and annealed contacts, while the significant parameters of the contact (i.e. the Schottky barrier and ideality factor n ) extrapolated from the linear part of the curves in Fig. 1a according to the thermionic emission (TE) model [10], are reported in Fig.1b. A preliminary statistics study demonstrated the possible occurrence of double-barrier curves mostly after annealing at . This aspect requires however further investigation and will be the object of a future focused work. The J-V characteristics under reverse bias are plotted in Fig.1c.

Fig. 1. a) Forward J-V curves of the as-deposited, 700 and -annealed Schottky contacts. b) Ideality factor and Schottky barrier height for the as-deposited and annealed contacts. c) Reverse J-V characteristics of the as-deposited, and -annealed Mo/4H-SiC Schottky contacts.

Fig. 1. a) Forward J-V curves of the as-deposited, 700 and -annealed Schottky contacts. b) Ideality factor and Schottky barrier height for the as-deposited and annealed contacts. c) Reverse J-V characteristics of the as-deposited, and -annealed Mo/4H-SiC Schottky contacts.

We observed that the as-deposited and -annealed contacts have similar electrical characteristics, whereas a slight shift towards lower voltage appeared for the J-V curves of the -annealed contact. As shown in Fig.1b, up to annealing at only a slight variation of the , from 1.45 eV to 1.40 eV occurred while is 1.30 eV for the -annealed contact. The ideality factor n is low, including for the -annealed sample. Regarding the reverse characteristics reported in Fig.1c, the -annealed contact featured an increase of the leakage current from the as-deposited, suggesting the predominance of a thermionic field emission mechanism (TFE) [11] for the current transport. Instead, a different electrical behavior under reverse bias was observed for the -annealed sample, with an increase of the leakage current mainly over 15 V . More quantitative analyses are ongoing to better understand the physical mechanisms ruling the transport under reverse bias. In order to highlight the variation of the forward and reverse characteristics for the set of measured diodes in the two cases ( and -annealed contacts), Fig. 2 reports the forward J value at 0.25 V against the reverse J value at 40 V .

Fig. 2. Forward current density at 0.25 V versus reverse current density at 40 V for a set of diodes annealed at and .

Fig. 2. Forward current density at 0.25 V versus reverse current density at 40 V for a set of diodes annealed at and .

Clearly, the -annealed contact presented a larger statistical distribution than that related to the -annealed contact, supposed related to a certain level of inhomogeneity of the metal/semiconductor interface.
Then, a current-voltage-temperature (I-V-T) study was performed on the as-deposited, and -annealed contacts with measurements temperature varying between and . The J-V curves acquired at varying temperature for the three cases are reported in Fig.3, respectively.

Fig. 3. a) Forward J-V characteristics acquired in a range of temperature varying between and (step of ) for a) as-deposited -SiC contact annealed, b) and c) annealed contacts.

Fig. 3. a) Forward J-V characteristics acquired in a range of temperature varying between and (step of ) for a) as-deposited -SiC contact annealed, b) and c) annealed contacts.

The temperature-dependence of the ideality factor n and Schottky barrier , derived from the analysis of the forward characteristics, is plotted in Fig.4. A similar behavior is observed by the nand temperature-dependence for the three cases.

Fig. 4. a) Forward J-V characteristics acquired in a range of temperature varying between and (step of ) for a) as-deposited -SiC contact annealed, b) and c) annealed contacts.

Fig. 4. a) Forward J-V characteristics acquired in a range of temperature varying between and (step of ) for a) as-deposited -SiC contact annealed, b) and c) annealed contacts.

Finally, we combined the electrical characterization of the and -annealed contacts to a microstructural investigation by bright-field S/TEM analysis. The micrographs of the two annealed contacts are shown in Figs. 5a and 5b.

Fig. 5. Bright-field S/TEM of a) and b) thermal annealed Schottky contacts.

Fig. 5. Bright-field S/TEM of a) and b) thermal annealed Schottky contacts.

This analysis demonstrated a different situation occurred after and annealing: in the first case, an unreacted surface is still present, keeping the Mo layer in the metallic form with grain sizes in the range of few tens of nanometers and providing continuous and homogeneous interface. Instead, at we observed the presence of columnar grain in the Mo-film, therefore with a vertical size of about 75 nm . Noteworthy, even upon treatment at high temperature ( ), it was not observed a reaction between the Mo-film and the semiconductor interface. In addition to the variation of the grain size, the Mo-films in the contact annealed at highest temperature revealed the presence of less dense (brighter) areas near the interface, as consequence of the re-organization of this film, as confirmed by the associated High-Angle annular Dark-Field (HADDF) imaging (not shown here). Chemical analyses performed by EELS in the interfacial region demonstrated that no reaction occurred between the Mo and , while the brighter regions observed at can be
associated to voids formed as a consequence of the grains' enlargement. The different structure of the Mo film and interface morphology can explain the evolution of the electrical characteristics of the contacts annealed at .

Summary

The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

In this paper, we performed an electrical characterization on Schottky diodes. In particular, the electrical properties of the contacts following thermal annealing treatments at temperature up to were studied under forward and reverse bias. Under forward bias, a reduction of the barrier height from 1.45 eV (as-deposited contact) to 1.30 eV ( -annelaed contact) was observed, with the presence of a certain level of inhomogeneity for all the contacts. Under reverse bias, leakage current followed a thermionic-field emission (TFE) model after annealing at , whereas it presented a significant increase after 15 V for the -annealed contact. Microstructural analyses were carried out by S/TEM analyses, highlighted an enlargement of the grains forming the structure of the Mo-film and the presence of voids near the Mo/ interface. These observations occurring at the interface can explain the variation in the electrical behavior of the contact.

Acknowledgements

The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

This work is supported by the European Union Next Generation EU program through the MURPNRR project SAMOTHRACE (No. ECS00000022). This paper has been partially supported by AdvanSiC. The AdvanSiC project has received funding from the European Union's Horizon Europe programme under grant agreement No 101075709. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.

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The original version of this paper is available on https://www.scientific.net/MSF.1158.27.pdf

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