Thermal annealing studies in epitaxial 4H-SiC Schottky barrier diodes over wide temperature range
Introduction
Epitaxial 4H‑silicon carbide (4H-SiC) Schottky barrier diodes (SBDs) are recommended for high-power electronic systems [1, 2], high-temperature applications [3], and radiation detection in hostile environments [4, 5]. For the possible use in these applications, the electrical properties of the 4H-SiC SBDs need to be optimized and improved. As deposited 4H-SiC SBDs may exhibit non-ideal electrical characteristics [[6], [7], [8], [9]]. The electrical parameters of the SBDs can be improved by thermal annealing [[10], [11], [12], [13], [14], [15], [16]]. Among the metals suitable for contacts, Ni and Ti are preferred for Schottky contact on n-type epitaxial 4H-SiC [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]]. The low work function of the Ti (unlike Ni) allows the formation of Ohmic contact on highly doped (>1018 cm−3) n-type 4H-SiC substrate with a low specific contact resistivity (SCR) of 2.25 × 10−3 Ω-cm2 [18] without any heat treatment [10, 19]. Hence, two types of SBDs such as Ni/4H-SiC SBDs and Ti/4H-SiC SBDs (Ti/Au bimetal layer Schottky contact) are fabricated in this work, with the Ti/Au bilayer as an Ohmic contact for both the SBDs.
Many authors investigated the thermal annealing induced changes in the electrical characteristics of Ni/4H-SiC and Ti/4H-SiC SBDs with Ni as back Ohmic contact [11, 12, 15, 16, [20], [21], [22], [23], [24], [25], [26], [27]]. Whereas, limited reports are available for heat treatment effects on 4H-SiC SBDs with the Ti Ohmic contact [10, 13, 14, 17, 19]. Kestle et al. [10] obtained an improved electrical performance at an annealing temperature of 500 °C for Ni/4H-SiC/Ti SBDs; but the authors observed poor rectifying nature at 600 °C. Vacuum annealed Ni/4H-SiC/(Ti/Ni/Ti) SBDs have shown [13] better Schottky barrier properties at the temperature of 500 °C. Zaman et al. [19] investigated the thermal annealing impacts on Ni/4H-SiC/(Ti/Ni/Ag) SBDs in the temperature range of 600 °C to 800 °C; the authors identified degradation in the diode rectifying behavior even at the annealing temperature of 600 °C. The Ni/4H-SiC SBDs and Ti/4H-SiC SBDs used in this work have the physical structure of Ni/4H-SiC/(Ti/Au) and (Ti/Au)/4H-SiC/(Ti/Au). Gupta et al. [14, 17] reported an improvement in the electrical characteristics of Ni/4H-SiC/(Ti/Pt/Au) SBDs upon annealing at 400 °C for 30 min in Ar ambient. Since our Ni/4H-SiC SBD structure is similar to Gupta et al. [14, 17], the same annealing parameters (30 min annealing in Ar ambient) are considered and the 400 °C is chosen as the lower limit for the annealing study. It is reported that Ni can form an Ohmic contact even with the lightly doped (~4 × 1015 cm−3) n-type 4H-SiC around the annealing temperature of 950 °C [[28], [29], [30]]. Hence, annealing studies are carried out up to an elevated temperature of 1100 °C. The current work may be helpful in determining the optimal annealing temperature of similar kinds of SBD structures and to understand the annealing effects on their electrical characteristics over wide temperature range than studied so far.
The SBD characteristics not only depend on the metal/semiconductor interface properties, but also on the electrically active defects present in the SBDs [1, 4, 5, 8]. The thermal evolution of electrically active defects in the 4H-SiC SBDs has been studied by deep-level transient spectroscopy (DLTS) [31, 32]. Storasta et al. [31] analyzed the high-temperature annealing (1600–1800 °C) effect on Z1/2 defect in Ni/4H-SiC/Al SBDs. Recently, Mannan et al. [32] reported the thermal annealing (100 °C to 800 °C) of deep level defects in Ni/4H-SiC/Ni SBDs for the annealing time of 30 min. In the above works, the contacts were formed only after the annealing process. Therefore, the current work is focused to analyze the thermal evolution of the trap signatures in the fabricated SBDs (i.e. after the contact formations) for the further improvement of the SBD characteristics. Thermally stimulated capacitance (TSCAP) spectroscopy [[33], [34], [35], [36]], an irreversible single-shot capacitance transient technique, has been used [37] to identify the traps in the 4H-SiC SBDs. The attractive feature of the TSCAP compared to DLTS is the simple measurement setup. Furthermore, alike DLTS, the TSCAP is suitable to determine the type of defects (electron/hole trap) in the sample and the signal sensitivity is independent of the device leakage current. The changes in the trap concentrations are examined after annealing by using TSCAP.
Section snippets
Experiment
The SBDs were fabricated on 30 μm thick n-type epitaxial 4H-SiC substrate (epilayer doping 5 × 1014 cm−3) from CREE Inc. The 4H-SiC SBD fabrication process steps are explained elsewhere [4, 5, 37]. In Ni/4H-SiC SBDs, Ni was considered as Schottky contact on Si-face and bimetal layer Ti (50 nm)/Au (150 nm) was used as Ohmic contact on C-face. On the other hand, in Ti/4H-SiC SBDs, the Ti (50 nm)/Au (150 nm) bilayer was chosen for the Schottky and Ohmic contacts. Both the SBDs were unterminated
As-deposited SBD characteristics
Electrically active defects (traps) in the SBDs identified by the TSCAP spectroscopy are reported elsewhere [37] and are summarized: Two deep traps such as P1 (EC-0.63 eV, Z1/2) and P2 (EC-1.13 eV, EH5) are identified in the Ni/4H-SiC SBDs with a trap concentration of ~7 × 1012 cm−3 and ~1.3 × 1013 cm−3. It should be noted that the trap P2 (EH5) is detected in the TSCAP spectrum for the DUT cooling procedure carried out with and without bias voltage. Thus, the trap P2 does not exhibit the
Conclusion
Annealing induced effects in the Ni/4H-SiC and Ti/4H-SiC SBDs are investigated in the temperature range of 400–1100 °C. As-deposited Ni/4H-SiC SBDs have shown non-ideal electrical characteristics might be due to the inhomogeneous SBH at the interface associated with the two distinct SBHs. On the other hand, high and stable SBH (1.3 eV), good ideality factor (1.14), low forward voltage drop (1.1 V at 1 mA) and low leakage current (~55 pA at −100 V) are obtained for the Ti/4H-SiC SBDs before
Acknowledgement
The authors are grateful to Dr. Jamil Akhtar, Chief Scientist and Head, SNTG, and all the members of CSIR-CEERI for the support and help during the SBD fabrication.
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