APL Materia RESEARCH ARTICLE |  NOVEMBER 20 2023 Energy levels of carbon dangling-bond center (P cc center) at 4H-SiC(0001)/SiO₂ interface Mitsuru Sometani 😊 😊 ; Yusuke Nishiya © ; Ren Kondo © ; Rei Inohana; Hongyu Zeng © ; Hirohisa Hirai © ; Dai Okamoto © ; Yu-ichiro Matsushita © ; Takahide Umeda © ( )  Check for updates APL Mater. 11, 11119 (2023) https://doi.org/10.1063/5.0171143 CrossMark
Cite as: APL Mater. 11, 111119 (2023); doi: 10.1063/5.0171143 Submitted: 7 August 2023 • Accepted: 27 October 2023 • Published Online: 20 November 2023 Mitsuru Sometani,¹ ♂  Yusuke Nishiya,²  ©  Ren Kondo,³    Rei Inohana,³  Hongyu Zeng,³    R  Hirohisa Hirai,¹ Dai Okamoto,³ © Yu-ichiro Matsushita,².4 © and Takahide Umeda³ ©
AFFILIATIONS
¹ Advanced Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ² Institute for Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo IS2-8S50, Japan ³Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan 4 Quantum Material and Applications Research Center, National Institutes for Quantum Science and Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan a)Author to whom correspondence should be addressed: m.sometani@aist.go.jp
ABSTRACT
The electric properties of the carbon dangling-bond (P b C ) center at a thermally oxidized 4H-SiC(0001)/SiO 2 interface are investigated. We experimentally and theoretically determine the energy levels of the associated interface states to estimate the impacts of the P c center on power device operations. By combining electrically detected magnetic resonance spectroscopy and capacitance–voltage measurements, the two P cc electronic levels [(0/−) and (+/0)] are determined as ~1.2 and 0.6 eV from the valence band maximum, respectively. The effective correlation energy of the P bc center is 0.6 eV, which is 1.5 times larger than that of the silicon dangling-bond (P b ) center at Si/SiO 2 interfaces. Our first-principles calculations confirm that the electronic levels of P c are similar to experimental values. Considering these energy levels, the P cc center must impact both p- and n-channel devices, which is closely related to previously reported channel features. © 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0171143 The 4H polytype of silicon carbide (SiC) has attracted consid- erable interest in high-voltage and high-temperature power devices due to its wide bandgap, high breakdown electric field, and high thermal conductivity. 1,2 SiC is preferred over other wide-bandgap semiconductors when fabricating metal–oxide–semiconductor (MOS) devices due to its ability to form thermally grown SiO 2 as a gate oxide film. 3-5 Although SiC-based MOS field-effect transistors (FETs) have become commercially available in recent years, their on-resistance is still much higher than expected, 2,6,7 which is caused by the low field-effect mobility ( μ FE ) in n-MOS. One reason for the decrease in μ FE is the inversion charge trapping by interface defects, resulting in a decreased mobile carrier density in the channel. 8-11 Furthermore, the threshold voltage (V th ) varies from its ideal value due to fewer mobile carriers. 12 Although various defect structures have been suggested at t the 4H-SiC(0001)/SiO 2 interface, such as carbon dimers, 13 details of the microscopic origins are unclear. Electron-spin-resonance (ESR) and electrically detected-magnetic-resonance (EDMR) spectroscopy are powerful tools for identifying the microscopic origins of such interface defects. 14- We previously reported ESR observations of carbon-related paramagnetic defects at t the 4H-SiC(0001)/SiO 2 interface, called the P bc center. The P bc center is a carbon version of the well-known Pb center. 19,20 The Pb center belongs to the family of Si dan- gling bonds (DBs) intrinsically formed at Si/SiO 2 interfaces. In the Si-MOS system, the P b center is one of the primary interface defects. Thus, it is also considered the primary interface defect in the SiC- MOS system. A reduced P C center density was evaluated by ESR to closely correlate with μ FE improvements in n-MOS when employing modified oxidation processes, such as NO post oxidation annealing (POA), POCl 3 POA, and ultra-high-temperature (UHT) oxidation processes. 21-25 Our study uses EDMR and first-principles calcula- tions to reveal that the P c center consists of a carbon adatom on the 4H-SiC(0001) honeycomb-like structure. 26 Another important insight obtained by EDMR is that the Рьс center signal is strong when applying a certain negative gate voltage (V g ). Thus, a high density of interface states near the valence band maximum is the primary product of the P b C center. This suggests that the P c center should have a minor impact on n-MOS characteristics. To identify the relationship between the MOS characteristics and P bc center, it is essential to quantitatively estimate the energy level. It has been reported that the combination of ESR and capacitance–voltage (C – V) measurements accurately evaluates the defect levels in Si MOS systems. 27 This paper develops an analytical method to apply their combination for 4H-SiC MOS systems to determine the energy level of the interface states caused by the P c  center. For the highly sensitive EDMR measurements, we pre- pared n-channel lateral 4H-SiC MOSFETs with wide dimen- sions (gate length/width = 5/2000 μm) on a 4° off-cut Al-doped p-type 4H-SiC(0001) epitaxial layer (NA - ND = 3 × 10 15 cm -3 ). The source/drain regions were formed via P ion implantation, and the body region was formed using Al ion implantation, both at 600 °C. This process was followed by activation annealing at 1650 °C for 5 min under an Ar atmosphere with a carbon cap- ping layer. 28 After removing the carbon capping layer, the surface was cleaned using RCA cleaning, followed by sacrificial oxida- tion and hydrofluoric acid (HF) dipping. Thermal oxidation at 1200 "C provided a 30-nm thick gate oxide using dry O 2 . After ther- mal oxidation, post-oxidation annealing was conducted at 1200 °C under an argon atmosphere for 30 min. Next, chemical vapor deposition (CVD) deposited n-type polycrystalline Si, which was patterned using photolithography techniques to form the gate elec- trodes. The source/drain and body contacts were formed by deposit- ing Al and Ni stacks, followed by annealing at 900 °C under an N 2 atmosphere. We conducted bipolar amplification-effect (BAE) EDMR mea- surements using this sample to amplify the EDMR signals of inter- face defects. 29 In this regime, we biased a constant current ( I a ) from the drain to the body and monitored the ESR-induced current changes in the drain–source current ( Iedmr ), as shown in Fig. 1(a) . We used the standard lock-in amplification technique synchronized to magnetic-field modulations at 1.56 kHz. 15,23 The ESR transitions were excited by a 200-mW 9.45 GHz microwave at room temper- ature (297 K). We also investigated the C – V characteristics of a MOS capacitor fabricated on the same p-type epitaxial layers. The frequency of the C – V measurements was 5 Hz. Figure 1(b) shows the typical EDMR spectra of the Si-face 4H-SiC MOSFET. The sin- gle strong EDMR peak is attributed to the P c center (electron spin of S = 1/2). We believe that the origin of the EDMR signal is the same as for the ESR signal (spin density of 3–4 × 10 12 cm −2 ) observed for as-oxidized 4H-SiC(0001) epitaxial layers under similar conditions. 16,25 Figure 2 shows the gate-bias dependencies of the EDMR sig- nals. The V g was swept toward the negative direction, changing the Fermi energy of SiC at t the interface ( ξ ) from the mid-gap to the valence band maximum. The EDMR signal intensity ( ΔI ) increases at V g = −3 V and decreases at V g = −10 V for all I a conditions. This indicates that the spin state of the P bc center changes from dou- bly occupied (S = 0) to singly occupied (S = 1/2) at V g = −3 V and then changes to the empty state (S = 0) at Vg = -10 V. These volt- ages reflect the electric properties of the P b c center. When V g less FIG. 1. (a) Schematic cross-section of n-channel lateral 4H–SiC MOSFETs with a wide dimension (gate length/width = 5/2000 μm) for highly sensitive EDMR mea- surements and the BAE EDMR setup. (b) Typical EDMR spectra show only one strong EDMR peak attributed to the P bc center (electron spin of S = 1/2). than −10 V is applied, a small ΔI is observed. This signal does not show hyperfine structures due to the P bc center, suggesting it orig- inated from other minor structures. 30 I EDMR in the figure is used to normalize the signals. –3, –5, –10 μA, and the upward V a was swept from –3 to –20 V. We then measured the C – V characteristics to estimate E f dur- ing the upward V g sweep. To estimate the relationship between the capacitance and E ξ , the surface potential (ψ ε ) is calculated from the low-frequency capacitance ( C LF ) and oxide capacitance (C ox ) as
where A is an integration constant. 31,32 The flat-band capaci- tance (C FB ) and flat-band voltage (V FB ) are used to determine A. Figure 3(a) shows the C – V characteristics of a fresh MOS capacitor fabricated on the same chip as the MOSFET for the EDMR measure- ments. The C – V characteristic in the upward measurement showed stretching due to charge trapping at various interface traps and/or near interface traps (NIT). 33 The unusual shape of the C – V curve on the upward Vg sweep makes it difficult to accurately determine the C FB and V FB . To reliably estimate E f at V g, the impact of hole traps on the C – V characteristics should be carefully considered. Associated with the high density of interface defects, there is a trade- off between the observations of strong EDMR signals and distortion in the C – V characteristics, prohibiting accurate E f estimations. C – V characteristics using the C – y, method. To minimize E ε estimation errors due to interface defects, we note that the reverse Vg sweep shows little stretching. The differ- ence in the curves between the sweep directions is attributed to hole traps, which are easily filled during the upward Vg sweep and kept filled during the reverse sweep because the de-trapping time is longer than the C – V measurement duration. The relationship between the capacitance and E f of the MOS is less affected by filled traps. We determined the E f from the reverse sweep C – V characteristics. The C – ψ , method has been proposed to evaluate the accuracy of the ψ , at t the SiC-MOS interface with a large density of interface traps (D it ). 34.35 In this method, the integration constant A in Eq. (1) is determined from the linear relationship between the depletion capacitance (Cdep) and ψ s , while C B and V FB are not required. Using Cdep in the analysis cancels the effect of interface traps. Figure 3(b) shows the relationship between the MOS capacitance and E ε estimated from the reverse side C – V characteristics using the C – ψ , method. Assuming that the relationship between Cdep and ψ , is identical irrespective of the sweep direction allows estimating E f at a given V g for the upward sweep using the relationship in Fig. 3 (b). V g in Fig. 2 is then converted into E f to give Fig. 4 . Here, the normalized signals, ΔI/Iedmr, were employed for the vertical axis. The EDMR signals were observed for V g from −3 to −10 V during the upward sweep. E f in this voltage region shifts from ~1.2 to 0.6 eV from the valence band maximum of the 4H-SiC(0001) at the inter- face. This means that the (0/-) and (+/0) levels of the P b c center are ~1.2 and 0.6 eV from the valence band maximum ( Ev ), respec- tively. Therefore, the effective correlation energy (U eff ) is derived from the difference between the doubly occupied level [(0/-) level] and the singly occupied level [(+/0) level] and is estimated as 0.6 eV for the P cc center. This energy value is 1.5 times larger than that for Pb center in Si-MOS systems (~0.4 eV). 36 As s spin localizations for both the P ь c center and P ь center are ~80% on each dangling-bond site, 20,26 the larger U eff of P cc is ascribed primarily to the smaller spa- tial extension of carbon atomic orbitals, causing greater Coulomb repulsion for the (0/−) level of the P cc center. We also performed first-principles calculations based on den- sity functional theory for the (0/-) level of the P b c center using FIG. 4. EDMR signals at various E f from the valence band maximum of the 4H-SiC(0001) interface. RSDFT 37,38 with norm-conserving pseudopotentials generated by the Troullier–Martins scheme. Our calculation model includes 170 atoms H-terminated in the 4H-SiC(0001)/SiO 2 interface with a single P c center. The single k-point (Γ point) is sampled, and spin-polarization is considered. The 80 × 80 × 180 real-space grids corresponding to 1.3 × 10 3 eV are adopted in the hexagonal unit cell where a = 12.38 Å and c = 30.38 Å. The lattice parameters are set at the values from previous research. 39,40 The atom positions in the simulation cell are optimized using the Perdew–Burke– Ernzerhof functional (PBE) 41 until the remaining forces go below 5 × 10¯4 hartree/bohr. To examine the in-gap state generated by the dangling bond of the C adatom, the calculations with the Heyd–Scuseria–Ernzerhof hybrid functional (HSE06) 42 for the structure obtained by PBE followed as HSE06 reproduces the experimental bandgap of SiC. The calculated bandgap of perfect 4H-SiC bulk is 3.21 eV, while the experimental bandgap is 3.26 eV at RT. 43 The results Fig. 5 suggest that the greatest occupied level in the negatively charged system by adding an extra electron is 1.24 eV above the valence band maximum. Visualization of the crystal structure and wave function was performed in VESTA. 44 The C adatom is chemically bonded with the three underlying Si atoms in the SiC substrate, with one dangling bond perpendicular to the substrate. The wave func- tion of this in-gap state is the dangling bond of the adatom with a primarily p-like characteristic. The level is occupied by both up and down spins. Spin-polarization does not occur in the negatively charged system, while the S = 1/2 state is realized in a neutral sys- tem. These results agree well with the experimental fact that the EDMR signal loses intensity when E f reaches E x + 1.2 eV. This study is the first case that determines both the microscopic structure and energy levels of an interface defect for wide bandgap semiconductor MOS interfaces. The estimated levels of the P c center affect both n- and p-MOSFET characteristics. This study demonstrates that the P c centers act as bipolar trap levels [(0/-) and (+/0) levels] for both FIG. 5. Energy level diagram of the negatively charged 4H-SiC system with a P bc center, and the calculated structural model at a SiC/SiO 2 interface. The calculated squared wave function of the highest occupied level is drawn with the isosurface for 15% of the maximum amplitude, where the small white balls depict H atoms that terminate the dangling bonds in the SiOz film. The crystal structure and wave function are visualized using VESTA. electrons and holes. This indicates that P c centers can vary the V th from its ideal value by reducing the mobile carriers in both the p- and n-channel MOSFETs. Suppressing the V th deviation by NO POA or UHT oxidation processes 3,12,45 could be due to eliminating these levels. The wet oxidation process also effectively suppresses V th deviations in the p-channel MOSFETs via hydrogen termination of the P c center. 46 In n-channel MOSFETs, there is a correlation between the reduced P c center density and μ FE improvements for n-channel MOSFETs. 25 However, the P b c center should not directly decrease the mobile electron density for n-channel MOSFETs as it generates levels near the valence band maximum. Furthermore, it has been reported that electrons trapped by interface states should have minor impacts on electron scattering. 7A7 These results suggest that the correlation of the P C center density and μ FR is attributed to other interface states near the conduction band edge minimum rather than the P bc center itself. It is s speculated that the other inter- face state density is related to the density of the P bc center. The next challenge is to reveal the origins of the P ь C -center-related defects in n-MOSFET devices. In summary, we evaluated the energy levels of the P bc cen- ter through the combination of EDMR and C – V measurements. It was confirmed that the P c center energy level is within the 4H-SiC bandgap. Thus, it is considered one of the major active interface states. The (0/-) and (+/0) levels of the Pьc center are ~1.2 and 0.6 eV from the valence band maximum, respectively. The validity of the (0/-) level of the P ь c was confirmed by first- principles calculations. The levels of the P ь c center should reduce the mobile carrier density, resulting in V th deviating from ideal for p- and n-channel MOSFETs. Considering the energy levels of the P c center, it has minor impacts on the n-MOSFET channel mobility. The relationship between the P b c center elimination and µFE enhancement in n-MOSFETs is attributed to different defect types, whose densities are related to the density of the P b c center. Further reductions to the P b c center density and related defects are key to improving the SiC-MOS device performance. This work was supported by JSPS KAKENHI (Grant Nos. JP20H00340 and JP21H04553) and MEXT as "Program for Promoting Researches on the Supercomputer Fugaku" (Grant No. JP-MXP1020200205). The computation in this work has been done using (supercom- puter Fugaku provided by the RIKEN Center for Computational Sci- ence/Supercomputer Center at t he Institute for Solid State Physics at the University of Tokyo).
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Mitsuru Sometani : Conceptualization (equal); Formal analy- sis (equal); Funding acquisition (equal); Investigation (equal); Writing – original draft (equal). Yusuke Nishiya: Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Ren Kondo : Formal analysis (equal). Rei Inohana : Formal anal- ysis (equal). Hongyu Zeng : Formal analysis (equal). Hirohisa Hirai: Investigation (equal); Writing – review & editing (equal). Dai Okamoto : Investigation (equal); Writing – review & editing (equal). Yu-ichiro Matsushita : Conceptualization (equal); Funding acquisition (equal). Takahide Umeda : Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Writing – review & editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available within the article.