Improvement of high temperature stability of nickel contacts on n-type 6H–SiC

doi:10.1016/S0169-4332(01)00509-8

Applied Surface Science

Volume 184, Issues 1–4, 12 December 2001, Pages 295-298

https://doi.org/10.1016/S0169-4332(01)00509-8 Get rights and content

Abstract

The structural and electrical characterisation of nickel contacts on n-type silicon carbide was performed to improve the ohmic behaviour at high temperatures. The formation of nickel silicide (Ni₂Si) was observed after annealing in vacuum of Ni/SiC samples at 600 °C, as well as after rapid thermal annealing (RTA) in N₂ at 700 for 60 s. The carbon was almost uniformly distributed inside the Ni₂Si layer, as monitored by energy dispersion X-ray analysis (EDX).

The specific contact resistance ρ_c was determined by the transmission line method (TLM) for different values of the substrate carrier concentration N_D. Ohmic contacts with $ρ_{c} =3.9×10^{−5} Ω cm^{2}$ were obtained for substrates with $N_{D} =7.4×10^{18} cm^{−3}$ after RTA in N₂ at 950 °C. The optimised contacts maintain their electrical stability even after annealing in N₂ up to 1000 °C.

Introduction

Silicon carbide (SiC) is a potential candidate for applications in high power electronics because of its unique properties like wide band gap (2.3–3.3 eV), high critical electric field (3–4 MV cm⁻¹), good thermal conductivity (4.9 Ω cm⁻¹ K⁻¹), etc. [1], [2]. Since metallisation represents one of the most important steps in the fabrication of electronic devices, the knowledge of the metal/SiC interaction after annealing as well as the electrical properties of metal silicide–SiC contacts is of primary importance for understanding and optimising the devices performances. Indeed, obtaining ohmic contacts with low values of specific resistance ρ_c is a basic requirement for reducing the serial resistance of a device.

Although different metallic contacts on SiC have been object of study in the last decade (Al, Ti, Co, Pd, NiTi, TiC, etc. [3], [4]), in many works nickel has been proposed as the most suitable candidate for the fabrication of ohmic and rectifying contacts on n-type SiC [5], [6], [7], and already used for high voltage SiC Schottky rectifiers [8], [9]. For fabricating ohmic contacts on SiC using nickel, annealing processes of Ni/SiC are generally performed under different atmospheres (vacuum, N₂, Ar, forming gas) at 950–1000 °C [5], [6], [10], [11], all resulting in low values of specific contact resistance (∼10⁻⁴–10⁻⁶ W cm²). However, the difficulty of controlling the metal contact properties (e.g. interface roughness, uniformity of the Schottky barrier height, thermal stability), as well as the need of optimal annealing conditions to reduce the risk of nickel oxidation, remains the limitation factors for many industrial applications. The thermal stability of nickel ohmic contacts on SiC was already demonstrated under operation for several hours at 500–600 °C [5], [10]. However, the authors are not aware of recent investigations on the reliability of this kind of contacts after post-annealing processes in a higher temperatures range (800–1000 °C), which may be required for sensors in automotive applications.

In this paper, the structural and electrical characterisation of nickel contacts on n-type 6H–SiC is presented. The aim of this work is to show that rapid thermal annealing is a suitable method for producing low resistance Ni₂Si ohmic contacts, which in turn preserve their electrical stability under annealing in N₂ up to 1000 °C.

Section snippets

Experimental

n-Type single crystalline 6H–SiC (0 0 0 1) wafers from CREE were used in the experiments. The deposition of 100 nm thick nickel contacts was performed by e-gun evaporation at a pressure of $1×10^{−8} mbar$ and an average rate of 5 nm/s. Subsequent thermal treatments of the Ni/SiC samples were carried both in a vacuum furnace at a pressure of $8×10^{−7} mbar$ and undertaking the samples to rapid thermal annealing (RTA) in N₂ atmosphere between 600 and 950 °C for 60 s.

X-ray diffraction (XRD) analysis and Rutherford

Results and discussion

The structural characterisation of the contacts was performed at different annealing temperatures. However, the optimal conditions for complete thermal reaction of the deposited nickel films were found to be above 600 °C.

Fig. 1 shows the XRD spectra of two samples annealed in vacuum at 600 °C for 25 min or undertaken to RTA at 700 °C in N₂ for 60 s. The thermal treatments of the Ni/SiC samples result in both cases (vacuum annealing and RTA in N₂) in the formation of polycrystalline nickel silicide

Conclusions

Low resistance ohmic contacts $(3–4×10^{−5} Ω cm^{2})$ in n-type SiC were fabricated by performing RTA of Ni/SiC samples in N₂ at 900 °C.

The structural characterisation of the samples showed the formation of the same phase (Ni₂Si) both in RTA and under vacuum. This kind of thermal process leads to rectifying contacts for substrate carrier concentration lower than $5×10^{−17} cm^{−3}$ .

The ohmic Ni₂Si/SiC contacts preserve their electrical stability even after longer annealing time in flowing N₂ in a conventional

Acknowledgements

The authors are grateful to C. Bongiorno for help during EDX measurements and to G. Di Benedetto and A. La Mantı̀a for their assistance during plasma etching processes.

References (19)

W. Wesch
Nucl. Instr. Meth. B
(1996)
L.M. Porter et al.
Mater. Sci. Eng. B
(1995)
A. Kakanakova-Georgieva et al.
Thin Solid Films
(1999)
H.H. Berger
Solid State Electron.
(1972)
L.R. Doolittle
Nucl. Instr. Meth. B
(1985)
H. Morkoç et al.
J. Appl. Phys.
(1994)
M. Levit et al.
J. Appl. Phys.
(1996)
J. Crofton et al.
J. Appl. Phys.
(1995)
T. Uemoto
Jpn. J. Appl. Phys.
(1995)

There are more references available in the full text version of this article.

Cited by (61)

Tunneling current through non-alloyed metal/heavily-doped SiC interfaces
2024, Materials Science in Semiconductor Processing
In this paper, tunneling phenomena at metal/heavily-doped SiC Schottky (non-sintered) interfaces are reviewed, and a design guideline for low-resistance non-alloyed SiC ohmic contacts is proposed. Carrier transport at Schottky contacts on heavily-doped SiC epitaxial layers is quantitatively described by direct tunneling (DT) including both of the thermionic field emission (TFE) and field emission (FE) when the doping density ( $N_{d}$ ) is higher than mid- $1 0^{17} {c m}^{- 3}$ . As for p-type SiC, tunneling of holes in the split-off band is the dominant conduction mechanism due to its light effective mass ( $0.21 m_{0}$ ). Phosphorus ion (P $^{+}$ ) implantation makes the interface tunneling current larger by several orders of magnitude compared with that at contacts on epitaxial layers with almost the same $N_{d}$ , which is plausibly explained by trap-assisted tunneling (TAT) through implantation-induced defect levels. The contact resistivity ( $ρ_{c}$ ) at non-alloyed ohmic contacts formed on heavily P $^{+}$ -implanted SiC ( $N_{d} > 4 \times 1 0^{18} {c m}^{- 3}$ ) is significantly reduced thanks to the contribution of TAT especially when $N_{d}$ is lower than about $1 0^{20} {c m}^{- 3}$ , while $ρ_{c}$ decreases with increasing $N_{d}$ according to the DT theory in a higher $N_{d}$ range ( $> 1 0^{20} {c m}^{- 3}$ ). At a very high $N_{d}$ of $2 \times 1 0^{20} {c m}^{- 3}$ , an extremely low $ρ_{c}$ of 1– $2 \times 1 0^{- 7} {Ω c m}^{2}$ is demonstrated for non-alloyed Mg and Ti contacts. Through the experiment and calculation of tunneling current, a model to predict $ρ_{c}$ at non-alloyed ohmic contacts taking account of the contributions of TAT and DT is proposed, and a design guideline for low-resistance non-alloyed ohmic contacts is presented as follows; a very low $ρ_{c}$ ( $< 1 0^{- 6} {Ω c m}^{2}$ ) is achievable without sintering by lowering the interface barrier height ( $\leq 1 e V$ ) and utilizing high-dose ion implantation ( $\geq 3 \times 1 0^{19} {c m}^{- 3}$ ).
Improved performance of SiC radiation detectors due to optimized ohmic contact electrode by graphene insertion
2021, Diamond and Related Materials
Citation Excerpt :
For the formation mechanism of ohmic contact between SiC and metal, numerous studies have shown that the graphite transition layer with high carrier concentration and narrow band gap was formed at the metal contact interface, which can reduce the contact barrier [30,31]. For example, the ohmic contact between Ni and n-SiC could be formed after annealing between 900 and 1000 °C [32,33], and M. Said [34] et al. confirmed that a graphite transition layer is formed between Ni and SiC by Raman spectroscopy and X-ray diffraction. To our best knowledge, post-annealing is still necessary for electrode fabrication till now and there is no alternatively effective method has been reported.
SiC as a typical wide bandgap semiconductor has exhibited potential application in alpha particle detectors. However, due to the comparatively high annealing temperature required for traditional fabrication technique of ohmic electrode, it is easy to excite impurities in the SiC epilayer, which act as scattering centers result in the declined device performance. In present work, a graphene layer is inserted between metal ohmic electrode and SiC. The graphene promotes the formation of carbon compounds and thus decreases ohmic contact resistance. Meanwhile, due to the Fermi level adjustable of graphene by external bias, the barrier of interface at graphene insert layer is enlarged at reverse bias. The built-in field accelerates the separation of photo-generated electron-hole pairs. As a result, the graphene inserted device annealed at 400 °C achieved a 3.9%@-40 V energy resolution, and the device even without annealing achieved a 4.4%@-40 V energy resolution for ²³⁹Pu alpha source, which are both better than that of traditional device without graphene layer annealed at 880 °C (6%@-40 V). The detailed mechanism has been discussed and suggested. Present work provides an effective strategy to improve device performance.
Si ohmic contacts on N-type SiC studied by XPS
2013, Microelectronic Engineering
Citation Excerpt :
This feature is unsatisfactory for ohmic contacts. A contact should be mechanically stable and reliable [9,10]. This insufficiency of the Si contact on SiC is however irrelevant from the point of view of the secondary contacts.
Si ohmic contacts on N-type 4H– and 6H–SiC with contact resistivity comparable with Ni metallizations were prepared. After etching-off of the Si contacts and deposition of new, unannealed ones, good electrical parameters of the original contacts remain preserved. Such unannealed secondary contacts were already successfully created with original Ni or Ni silicide metallizations. An advantage offered by the secondary contacts prepared after the original Si contacts is that they are prepared on a high-quality SiC surface as the original Si does not react with SiC. XPS and AFM analyses were carried out. Under the original Si contacts when they are etched-off there is observed a shift of Si and C peaks to higher binding energies in comparison with a clean, bare SiC surface. After less than 2 nm of the surface layer is sputtered-off the shift disappears. We suggest that preserved electrical parameters after the etching-off of the Si contacts stem from a SiC surface modification.
Role of carbon in the formation of ohmic contact in Ni/4HSiC and Ni/Ti/4HSiC
2012, Applied Surface Science
In this work, we focus on the role of carbon in the Ni and Ni/Ti contacts on n-type 4HSiC. The contacts, formed on the backside of the wafers C-face by electron gun evaporation and annealed at 950 °C, were studied by Raman spectroscopy (RS), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The results show that titanium acts as a diffusion barrier for Si and C preventing the formation of the unfavourable phase NiSi and interacts with carbon to form TiC. The transformation of carbon to graphitic structure (in Ni/Ti/SiC) considerably lowers the sheet resistance and greatly improves the ohmic contact.
Structural characterization of Ni and Ni/Ti ohmic contact on n-type 4H-SiC
2011, Applied Surface Science
In this study, we report on the structural characterization of Ni layer and Ni/Ti bilayer contacts on n-type 4H–SiC. The resulting Ni-silicides and the redistribution of carbon, after annealing at 950 °C, in the Ni/SiC and the Ni/Ti/SiC contacts are particularly studied by Rutherford Backscattering Spectrometry (RBS) at E_α = 3.2 MeV, nuclear reaction analysis (NRA) at E_d = 1 MeV, scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) techniques.
Thermal degradation of Ni-based Schottky contacts on 6H-SiC
2011, Applied Surface Science
Citation Excerpt :
Similar results (Schottky behavior of Ni contacts annealed at 950 °C) were reported by Calcagno et al. for lightly doped 6H–SiC [7]. In [2], rectifying behavior of 950 °C annealed Ni contacts was observed even on 6H–SiC with a doping level of 5.2 × 1017 cm−3. In our experiments with Ni contacts on 6H–SiC doped to 3.5–5.5 × 1017 cm−3 [8,9], we obtained ohmic contacts with low values of contact resistivity after annealing at approximately 960 °C.
We prepared Ni and Ni₂Si Schottky contacts on lightly doped (5.5 × 10¹⁵ cm⁻³) n-type 6H–SiC and evaluated their thermal degradation after annealing in the temperature range of 750–1150 °C. Ni contacts had Schottky behavior after annealing at 750 and 850 °C; they gradually degraded at 960 and 1065 °C, and achieved ohmic behavior at 1150 °C. Ni₂Si contacts had higher values of saturation current density and lower Schottky barrier heights than Ni contacts after annealing at 750 and 850 °C, and an abrupt transition from Schottky behavior at 850 °C to ohmic behavior at 960 °C. We assume that the abrupt transition from Schottky to ohmic behavior in the case of Ni₂Si metallization is caused by its low reactivity with silicon carbide, which was verified by XPS depth profiling. The results indicate that the discrepancy in the behavior of Ni contacts on lightly doped SiC reported in the literature might have been caused by non-equal processing or by inaccurate determination of the annealing temperature.

View all citing articles on Scopus

View full text

Improvement of high temperature stability of nickel contacts on n-type 6H–SiC

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Nucl. Instr. Meth. B

Mater. Sci. Eng. B

Thin Solid Films

Solid State Electron.

Nucl. Instr. Meth. B

J. Appl. Phys.

J. Appl. Phys.

J. Appl. Phys.

Jpn. J. Appl. Phys.