Materials Science Forum, ISSN: 1662-9752, Vol. 1159, pp 37-43

doi: 10.4028/p-Dlp4Ka

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

Submitted:
2024-09-21

Revised:
2025-03-20

Accepted:
2025-03-20

Online:
2025-09-09

Analysis of Ohmic Contacts Simultaneously Formed on both -Type and -Type -SiC

The original version of this paper is available on https://www.scientific.net/MSF.1159.37.pdf

Atsushi Shimbori , Rahul Valecha and Alex Q. Huang

J.J. Pickle Research Center, The University of Texas at Austin Bldg. #160, 10100 Burnet Rd. Austin, TX 78758, USA

ashimbo@utexas.edu, rahul.valecha@utexas.edu, aqhuang@utexas.edu

Abstract

In this paper, various annealing conditions using Al-based ( ) and Ni-based ( ) metal contacts to n-type and p-type ion-implanted epi layers have been studied in the effort to optimize simultaneous ohmic contact formation with the lowest specific contact resistance (SCR) values. Values of and were achieved using Al-based Ohmic metal contacts for p-type and n-type 4H-SiC, respectively, at an annealing temperature of and under vacuum for 90 sec. Ohmic formation mechanisms were analyzed using the X-Ray Diffraction (XRD) surface analysis method, indicating alloys to be the key intermediate layer formed at interface, responsible for Ohmic properties to p-type SiC. The paper summarizes the metal process combinations possible for the formation of Ohmic contacts to both n-type and p-type , offering various options in either using the same metal materials and/or common annealing conditions.

Introduction

The original version of this paper is available on https://www.scientific.net/MSF.1159.37.pdf

Simultaneous formation of Ohmic contacts to both n-type and p-type 4 H SiC is essential to simplifying the fabrication process and optimizing the device size of SiC MOSFETs and PiN diodes. Especially for vertical power SiC MOSFETs, in order to effectively reduce the cell pitch to further increase channel density and improve device performance, simultaneous formation of ohmic contacts both on source region and p -well region with low specific contact resistance using the same contact metal and/or common annealing condition is essential. Not only device performance is dependent on drain/source contact resistance and its current conduction capability, quality p -well contact is also required to short out the base and emitter through source metallization to disable the parasitic NPN transistor and provide the substrate contact for the inversion channel in the MOSFET structure.

Fig. 1. Two-terminal differential resistance value at various annealing temperature.

Fig. 1. Two-terminal differential resistance value at various annealing temperature.

Fig. 2. Fabricated Transfer Length Method (TLM) pattern with mesa-isolation etch.

Fig. 2. Fabricated Transfer Length Method (TLM) pattern with mesa-isolation etch.

Fabrication

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In this experiment, a substrate with an epi layer in a thickness of and a nitrogen doping concentration of was used. The implantations of phosphorus ions were performed at room temperature, using different ion energies ( ) and doses of to create a box profile with a depth of roughly 280 nm and a concentration of for the formation of layer (Summarized in Table I and Fig. 3(a)). The implantations of aluminum ions were performed at , using different ion energies ( ) and doses of to create a box profile with a depth of approximately 250 nm and a concentration of for the formation of layer (Summarized in Table II and Fig. 3(b)). Both layers were formed on opposite polarity well of a epi wafer.

The activation process involves the deposition of a Diamond-Like Carbon (DLC) layer through the method of plasma-based ion implantation and deposition (PBIID) in a thickness of 70 nm as the cap layer, followed by annealing at for 30 min to electrically activate both implanted dopants. It is known that high-temperature annealing at is necessary to obtain an activation ratio higher than .

In this paper, we will be exploring two different types of Ohmic metal stacks: Al-based Ohmic metal contact stacks ( ) and Ni-based Ohmic metal contact stacks . The role of the thin Ti layer (thickness of 5 nm ) is to prevent oxidation from occurring further inward by forming titanium oxides [2]. Titanium oxide can take oxygen away from , and SiC to prevent further oxidation. Transfer Length Method (TLM) patterns were formed through photolithography and then mesa-isolated with a shallow fluorine-based ICP etch as shown in Fig. 2. The annealing temperature was optimized using the twoterminal differential resistance method shown in Fig. 1. All contacts were annealed under a vacuum of Torr using Annealsys Rapid Thermal Processor (RTP).

Table I. Phosphorus implantation series for n-type ohmic contact.

Dopant | Dose [cm⁻²] | Energy [keV] | Angle/Twist [degree]
Phosphorus | 2.7×10¹⁵ | 200 | Tilt 7, Twist 23
Phosphorus | 1.4×10¹⁵ | 100 | Tilt 7, Twist 23
Phosphorus | 9.0×10¹⁴ | 50 | Tilt 7, Twist 23

Table II. Aluminum implantation series for p-type ohmic contact.

Fig. 3. Secondary Ion Mass Spectrometry (SIMS) profiles of Phosphorus and Aluminum implantation to the epi wafers.

Fig. 3. Secondary Ion Mass Spectrometry (SIMS) profiles of Phosphorus and Aluminum implantation to the epi wafers.

Results and Discussion

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Fig. 4. TLM evaluation of Al-based ohmic contact to p-type SiC annealed at .

Fig. 4. TLM evaluation of Al-based ohmic contact to p-type SiC annealed at .

Fig. 5. TLM evaluation of Al-based ohmic contact to n-type SiC annealed at .

Fig. 5. TLM evaluation of Al-based ohmic contact to n-type SiC annealed at .

Fig. 6. Comparison of Ni-based ohmic contact to n-type SiC at various annealing temperature.

Fig. 6. Comparison of Ni-based ohmic contact to n-type SiC at various annealing temperature.

The fabricated TLM structures were electrically characterized and the sheet resistance Rs, the contact resistance , as well as the specific contact resistance were extracted. The nickel silicide layers formed from Ni-based metal contacts, at an annealing temperature above , exhibited good ohmic behavior to heavily doped n-type layer and continued to show lower specific contact resistance with respect to the increasing annealing temperature (Fig. 6). This is because the outdiffusion of C atoms during high temperature annealing ( ) creates carbon vacancies in SiC , which acts as donors, thus contributing to even lower specific contact resistance [3]. On the other hand, this is not the same against p-type layer, as Ni-based ohmic metal shows slightly non-linear - characteristics. A different system is necessary for forming quality ohmic contacts to p-type and Ohmic formation mechanism will be explained in the following XRD spectra results. The obtained specific contact resistance against p -type SiC layer using the alternative Al -based metal contacts was at an annealing temperature of (Fig. 4). Additionally, the Al-based metal contact was also effective against n-type SiC layer, showing specific contact resistance value of achieving simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of Ohmic contacts to both n-type and p-type 4H-SiC (Fig.5). Table III below summarizes past reports of specific contact resistance for ohmic contacts simultaneously formed on , indicating the competitiveness of the values obtained from our current work.

Table III. Reported specific contact resistance for ohmic contacts simultaneously formed on .

Fig. 7. XRD spectra of Al-based ( ) Ohmic contact to p-type SiC annealed at , and .

Fig. 7. XRD spectra of Al-based ( ) Ohmic contact to p-type SiC annealed at , and .

Fig. 7 illustrates the XRD spectra of Al-based Ohmic contacts formed on p-type at various annealing temperatures. Rapid thermal annealing (RTA) at , under vacuum, forms alloys, which grow at the interface responsible for Ohmic properties to p-type SiC . These alloys have a work function of 5.07 eV , which corresponds to an intermediate value between Ti and p-type SiC , dividing the large barrier height into two smaller ones as shown in Fig. 8 [9]. As a result, the height of the original Schottky barrier is lowered and the carrier transport is enhanced through thermionic emission for Ohmic conduction.

Fig. 8. Schematic of band offsets for metal/ alloy/p-type SiC sequence.

Fig. 8. Schematic of band offsets for metal/ alloy/p-type SiC sequence.

Consequently, it becomes easier for holes, whose movement is represented by the red arrows in Fig. 8, to transport from the metal electrode to the valence band of p-type . The peak of is only clearly visible in the sample (blue spectrum in Fig. 7), and this alloy contributes to the decrease of differential resistance value observed in Fig. 1. In addition, the main role of the Al mid layer in the Al-based Ohmic metal stacks ( ) is to assist the formation of a liquid alloy that both facilitates and accelerates the reaction between the SiC and the Ti to form alloys [10]. The intense reflection peaks, observed around degrees, correspond to the pure Al , which increases with annealing temperature. Al starts to melt to the surface at in vacuum and this phenomenon accelerates as the annealing temperature increases, which results in intensifying peaks observed through XRD surface analysis. Large droplets of Al can also be confirmed with the microscope image in Fig. 9. These surface morphology transitions only take place under annealing at a high temperature, high vacuum condition due to the evaporation of the liquid phase, such as the Al-based liquid alloy with a low melting point and high vapor pressure.

Fig. 9. Microscope image of Al based electrodes .

Fig. 9. Microscope image of Al based electrodes .

Table IV. Summary of various Ohmic metal process to -type and -type 4 H -SiC and extracted parameters.

Table IV summarizes the metal process combinations possible in the formation of ohmic contacts to both and -type 4 H -SiC, offering various options in either using the same metal materials and/or common annealing conditions. Although Al-based metal contact forms quality Ohmic contact with a relatively low specific contact resistance (SCR) value, system alone may not be ideal for the ntype Ohmic contact formation (Metal Process 3 and Fig. 5 comparison). The adaptation of an additional Ni layer within the system to form nickel silicide is expected to further reduce the SCR values close to that of Metal Process 4 or 5. On the other hand, for p-type SiC, the Ni atoms added to the system will react with SiC to form , leaving more unreacted C atoms to react with Ti and Si atoms to form . Thus, the combined blend of Al -based and Ni -based metal contact through thickness optimization may offer further benefits (lower SCR values, lower annealing temperature, etc.) in achieving simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of Ohmic contacts to both n-type and p-type 4H-SiC.

To summarize the obtained results, the formation of nickel silicide ( ) layers is necessary in forming quality ohmic contacts to n-type SiC. Ni-based metal contacts continue to show lower specific contact resistance with respect to increasing the annealing temperature up to and possibly even at a higher temperature until surface deterioration cannot be ignored any further. Ohmic contacts to p-type SiC requires an system (Al-based), and the optimal annealing temperature
for the formation of necessary alloys is at . Additionally, the Al-based metal contact is also effective against the n-type SiC layer, achieving simultaneous formation of Ohmic contact to both n-type and p-type 4H-SiC.

Summary

The original version of this paper is available on https://www.scientific.net/MSF.1159.37.pdf

In conclusion, various annealing conditions using Al-based ( ) and Ni-based ( ) metal contacts to n-type and p-type ion-implanted epi layers have been investigated in the effort to optimize simultaneous ohmic contact formation with the lowest specific contact resistance (SCR) values. Values of and were obtained using Al-based Ohmic metal contacts for p-type and ntype , respectively, at an annealing temperature of and under vacuum for 90 sec , achieving simultaneous formation. Ohmic formation mechanisms were analyzed using the X-Ray Diffraction (XRD) surface analysis method, indicating alloys to be the key intermediate layer formed at interface, responsible for Ohmic properties to p-type SiC . We have successfully investigated the metal process combinations possible in forming Ohmic contacts to both n-type and p-type , offering various options in either using the same metal materials and/or common annealing conditions.

Acknowledgements

The original version of this paper is available on https://www.scientific.net/MSF.1159.37.pdf

The authors of this paper would like to thank Nissin Ion Equipment Co. Ltd for their support throughout the implantation process.

References

The original version of this paper is available on https://www.scientific.net/MSF.1159.37.pdf

Matsunami, H., "Fundamental research on semiconductor SiC and its applications to power electronics", Proceedings of the Japan Academy. Series B, Physical and biological sciences vol. 96,7 (2020): 235-254. doi:10.2183/pjab.96.018

Shimbori, A., and Huang, A., "Cost-Effective Formation of Ti/Ni/Ti/Au Ohmic Contacts to ntype SiC Using Encapsulation Layer during Phosphorous Implant Activation", 2022 ECS Trans. 10941 doi: 10.1149/10908.0041ecst

Kuchuk, A. V., Borowicz, P., Wzorek, M., Borysiewicz, M., Ratajczak, R., Golaszewska, K., Kaminska, E., Kladko, V., and Piotrowska, A., "Ni-Based Ohmic Contacts to n-Type 4H-SiC: The Formation Mechanism and Thermal Stability", Advances in Condensed Matter Physics, 2016, 9273702, 26 pages, 2016. doi:10.1155/2016/9273702

Tsukimoto, S., Onishi, T., Ito, K., and Murakami, M., "Simultaneous Formation of Ohmic Contacts for Both N- and P-Type 4H-Sic Using Nial-Based Contact Materials", MRS Proceedings, vol. 911, pp. 0911-B11-05, 2006. doi:10.1557/PROC-0911-B11-05

Vivona, M., Greco, G., Giannazzo, F., Lo Nigro, R., Rascunà, S., Saggio, M., and Roccaforte, F., "Thermal stability of the current transport mechanisms in Ni-based Ohmic contacts on n- and pimplanted 4H-SiC", Semiconductor Science Technology, vol. 29, no. 7, Art. no. 075018, IOP, 2014. doi:10.1088/0268-1242/29/7/075018.

Ito, K., Onishi, T., Takeda, H. et al. "Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC", J. Electron. Mater. 37, 1674-1680 (2008). doi:10.1007/s11664-008-0525-1

Kragh-Buetow, K. C., Okojie, R. S., Lukco, D., and Mohney, S. E., "Characterization of tungstennickel simultaneous Ohmic contacts to p- and n-type , Semiconductor Science Technology, vol. 30, no. 10, Art. no. 105019, IOP, 2015. doi:10.1088/0268-1242/30/10/105019

Okojie, R. S., and Lukco, D., "Simultaneous ohmic contacts to p- and n-type 4H-SiC by phase segregation annealing of co-sputtered Pt-Ti", J. Appl. Phys. 7 December 2016; 120 (21): 215301. doi:10.1063/1.4968572

Johnson, B., and Capano, M., "Mechanism of ohmic behavior of Al/Ti contacts to p-type 4H-SiC after annealing", J. Appl. Phys. 15 May 2004; 95 (10): 5616-5620. doi:10.1063/1.1707215

Buchholt, K., Ghandi, R., Domeij, M., Zetterling, C.-M., Lu, J., Eklund, P., Hultman, L., and Lloyd Spetz, A., "Ohmic contact properties of magnetron sputtered on - and p-type 4Hsilicon carbide", Appl. Phys. Lett. 24 January 2011; 98 (4): 042108. doi:10.1063/1.3549198

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