Dislocation-related leakage-current paths of 4H silicon carbide

Gao, Wandong; Yang, Guang; Qian, Yixiao; Han, Xuefeng; Cui, Can; Pi, Xiaodong; Yang, Deren; Wang, Rong

doi:10.3389/fmats.2023.1022878

ORIGINAL RESEARCH article

Front. Mater., 12 January 2023

Sec. Semiconducting Materials and Devices

Volume 10 - 2023 | https://doi.org/10.3389/fmats.2023.1022878

Dislocation-related leakage-current paths of 4H silicon carbide

Wandong Gao^1,2

Guang Yang³

Yixiao Qian^1,2

Xuefeng Han^1,2

Can Cui³

Xiaodong Pi^1,2*

Deren Yang^1,2

Rong Wang^1,2*

¹State Key Laboratory of Silicon and Advanced Semiconductor Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, China
²Institute of Advanced Semiconductors and Zhejiang Provincial Key Laboratory of Power Semiconductor Materials and Devices, ZJU-Hangzhou Global Scientific and Technological Innovation Center, Hangzhou, Zhejiang, China
³Key Laboratory of Optical Field Manipulation of Zhejiang Province, Department of Physics, Zhejiang Sci-Tech University, Hangzhou, Zhejiang, China

Improving the quality of 4H silicon carbide (4H-SiC) epitaxial layers to reduce the leakage current of 4H-SiC based high-power devices is a long-standing issue in the development of 4H-SiC homoepitaxy. In this work, we compare the effect of different type of dislocations, and discriminate the effect of dislocation lines and dislocation-related pits on the leakage current of 4H-SiC by combining molten-KOH etching and the tunneling atomic force microscopy (TUNA) measurements. It is found that both the dislocation lines of threading dislocations (TDs) and the TD-related pits increase the reverse leakage current of 4H-SiC. The dislocation lines of TDs exert more significant effect on the reverse leakage current of 4H-SiC, which gives rise to the nonuniform distribution of reverse leakage current throughout the TD-related pits. Due to the different Burgers vectors of TDs, the effect of TDs on the reverse leakage current of 4H-SiC increases in the order to threading edge dislocation (TED), threading screw dislocation (TSD) and threading mixed dislocation (TMD). Basal plane dislocations (BPDs) are also found to slightly increase the reverse leakage current, with the leakage current mainly concentrated at the core of the BPD. Compared to the effect of TDs, the effect of BPDs on the reverse leakage current of 4H-SiC is negligible. Our work indicates that reducing the density of TDs, especially TMDs and TSDs, is key to improve the quality of 4H-SiC epitaxial layers and reduce the reverse leakage current of 4H-SiC based high -power devices.

1 Introduction

The superior properties of 4H silicon carbide (4H-SiC) such as wide bandgap, high breakdown electric field strength, and high thermal conductivity, have endowed 4H-SiC based high-power electronics great success in industrial motor drives, electric transportations and new-energy converters (Cooper and Agarwal, 2002; Kimoto and Cooper, 2014; Wang et al., 2020). Compared with other wide-bandgap semiconductors, owing to the great success of the single-crystal growth and wafer processing of 4H-SiC substrates, the homoepitaxy of 4H-SiC has the advantages of low dislocation density and high reliability, which facilitate the industrialization of 4H-SiC based high-power devices. Furthermore, the high thermal conductivity of 4H-SiC ensures the convenient heat dissipation of 4H-SiC based high-power devices (Lidow et al., 2019; Higashiwaki et al., 2017). The unique features of 4H-SiC based high-power devices include high breakdown voltages, low power losses, and high switching speeds (Matsunami, 2004; Kimoto, 2015; Luo et al., 2019; Kimoto and Watanabe, 2020). Even though the defect control of 4H-SiC is mostly well-developed, the defects in 4H-SiC are found to significantly degrade the performance of 4H-SiC power devices. After decades of development, the device-killing micropipes and carrot defects have been successfully eliminated (Neudeck and Powell, 1994; Kimoto et al., 1999; Kimoto, 2014). While dislocations still act as dominant recombination centers, and are found to increase the leakage current and lower the breakdown voltage of 4H-SiC based high-power devices (Ewing et al., 2007; Friedrichs, 2008; Hamada et al., 2015; Łażewski et al., 2019).

According to the Burgers vectors and dislocation-line directions, dislocations in 4H-SiC can be classified as threading edge dislocations (TEDs), threading edge dislocations (TSDs), threading edge dislocations (TMDs), and basal plane dislocations (BPDs) (Nakamura et al., 2007; Li et al., 2022; Luo et al., 2022). It has been found that all the dislocations increase the leakage current of 4H-SiC power devices, and the effect of TSDs is more prominent than those of TEDs (Wahab et al., 2000; Berechman et al., 2010). As evidenced by synchrotron X-ray-topography (XRT) and transmission-electron-microscopy (TEM) investigations, TMDs dominate the configurations of the TSD/TMDs in 4H-SiC, while the effect of TMDs on the leakage current of 4H-SiC based high-power devices is still ambiguous (Onda et al., 2013; Konishi et al., 2019; Shinagawa et al., 2020). Meanwhile, the leakage current originating from BPDs was found in 4H-SiC p-n diodes and bipolar junction transistors (BJTs) (Muzykov et al., 2009; Skowronski and Ha, 2006; Ota et al., 2021). Because of the different Burgers vectors, different types of dislocations would generate different atomic disorders in 4H-SiC, and thus different degrees of the leakage current in 4H-SiC based high-power devices (Berechman et al., 2010; Nishio et al., 2022; Senzaki et al., 2006). However, the comparison of the type of dislocations on the leakage current of 4H-SiC is rarely understood. Furthermore, it is argued that the leakage current of threading dislocations, mainly TSDs and TEDs, is dominated by the surface pits formed by the outcrops of the threading dislocations, rather than the threading dislocations themselves (Fiorenza et al., 2020; Fujiwara et al., 2012a; Fujiwara et al., 2012b; Ohtani et al., 2012). However, Huang et al. found that the dislocation line of the TED in 4H-SiC Schottky barrier diodes (SBDs) controversially contributes to the leakage current (Huang et al., 2022). Therefore, identifying the degree of the leakage current induced by different types of dislocations, and discriminating the role of dislocations and dislocation-induced pits on the leakage current of 4H-SiC is critical to the optimization of 4H-SiC epilayers.

In this work, we investigate the effect of dislocations on the leakage current of homoepitaxial 4H-SiC by combining molten-KOH etching and the tunneling atomic force microscopy (TUNA) measurements. It turns out that the effect of dislocations on the reverse leakage current of 4H-SiC increases in the order to BPDs, TEDs, TSDs, and TMDs. The effect of dislocation line and the dislocation-related pit is also discriminated. We find that both the dislocation lines of TDs and the TD-related pits increase the reverse leakage current of 4H-SiC. The dislocation lines of TDs exert a more significant effect on the reverse leakage current of 4H-SiC, which gives rise to the nonuniform distribution of reverse leakage current throughout the TD-related pits. BPDs are also found to slightly increases the reverse leakage current, with the leakage current mainly concentrated at the core of the BPD. Compared to the effect of TDs, the effect of BPDs on the reverse leakage current of 4H-SiC is negligible.

2 Experimental methods

2.1 Epitaxy and molten-KOH etching of 4H-SiC thin film

The 30 μm 4H-SiC thin film was epitaxially grown by a hot-wall chemical vapor deposition (CVD) reactor on a 4° off-axis-sliced N-doped 4H-SiC (Skowronski and Kimoto, 2015). The homoepitaxy of 4H-SiC was carried out utilizing trichlorosilane (SiHCl₃) and ethylene (C₂H₄) as the growth sources of Si and C, respectively. The C/Si ratio was optimized to be 0.85, and hydrogen gas (H₂) was selected as the carrier gas. The flow rate of H₂ is 100 L/min. The growth temperature and pressure were 1,650°C and 100 mbar, respectively. Molten-KOH etching was performed at 550°C for 2 min in a Ni crucible to reveal the dislocation-related pits.

2.2 Characterizations

To discriminate the type of dislocations in 4H-SiC, the two-dimensional and three-dimensional morphologies of the dislocation-related pits were observed by the optical microscope (OM) (Olympus BX53M) and laser scanning confocal microscope (LSCM) (Zeiss LSM 900), respectively. The local leakage current of each type of dislocation was detected by the tunneling atomic force microscopy (TUNA) equipped with an atomic force microscope (AFM) (Dimension Icon, Bruker). TUNA is capable of imaging the local leakage current of semiconductors as low as 300 fA, with the conductive tip (PtIr-coated Si) acting as a nanoscale Schottky contact on the 4H-SiC epitaxial sample. TUNA measurements were performed by 256 × 256 points over 20 × 20 or 40 × 40 square micrometers. All images were taken at the scan speed of 0.3 Hz and a peak force of 3.5 μN.

3 Results and discussion

Figure 1A displays the OM image of the molten-KOH etched 4H-SiC epitaxial layer. The larger hexagonal pits, the smaller hexagonal pits, and the sea-shell pits correspond to the etch pits of TSDs/TMDs, TEDs, and BPDs, respectively (Konishi et al., 2019; Katsuno et al., 2011; Dong et al., 2013). By the statics of wafer-scale etch-pit density of different dislocations, the average dislocation densities of TSDs/TMDs, TEDs and BPDs are 3,770/cm², 2,352/cm², and 18/cm², respectively. To discriminate TMDs from TSDs, we take the three-dimensional morphologies of etch pits into account by LSCM. According to previous researches, the slope of etch pit of a TSD is unitarily steep and the etch pit ends by a deep point (Figures 1B, F). While the hexagonal pit of a TMD ends by a round plate, and the slope for the etch pit of a TMD is heterogeneous (Figures 1C, F).

FIGURE 1

FIGURE 1. Representative OM images of etched 4H-SiC epitaxial layer samples (A), the LSCM images of (B) TSD, (C) TMD, (D) TED, (E) BPD and depth profiles (F).

After distinguishing all types of dislocations, we investigate the local leakage current of each type of dislocation by TUNA. As shown in Figures 2A–C, the leakage current throughout the pits of all types of TDs is higher than that in the dislocation-free region under the reverse bias. This indicates that TD-related pits exert a significant effect on the leakage current of 4H-SiC. Interestingly, we find that the magnitude of the leakage current depends on the lattice plane of the etch pit. Under the reverse bias of 6 V, the maximum leakage current of the TSD, TMD, and TED approaches 2.3 pA, 10.5 pA, and 1.5 pA, respectively. Furthermore, the BPD also gives rise to the leakage current through the BPD core. Compared to what happens in TDs, the BPD exerts a minor effect on the leakage current of 4H-SiC. Even when the reverse bias increases to 8 V, the BPD-induced leakage current is only 416.9 fA.

FIGURE 2

FIGURE 2. TUNA topography images of (A) TSD, (B) TMD, (C) TED, (D) BPD in 4H-SiC epitaxial layers.

The inhomogeneity of leakage current throughout the dislocation pits of 4H-SiC can be understood as follows. It was found that the dislocation-relate pit composes six equivalent $(10 \bar{1} 1)$ planes in hexagonal GaN (Northrup et al., 1999). Given the similar crystal symmetries and Burgers vectors of dislocations, the preferential etching along TDs also creates six equivalent $(10 \bar{1} 1)$ planes. Ideally, the leakage current would be uniformly distributed throughout the TD pits. However, due to the off-axis slicing of 4H-SiC substrates and the step-controlled homoepitaxy of 4H-SiC epitaxial layers, the [0001] crystallographic axis of the 4H-SiC epitaxial layer inclines towards the $[\bar{1} \bar{1} 20]$ direction (Figure 3). This gives rise to the incline of the dislocation lines of TDs with respect to the surface. As shown in Figure 3, the gray part of the etch pit of TDs is closer to the dislocation lines of TDs than the white part. It is well established that the thermionic field emission theory dominates the reverse current of 4H-SiC-based Schottky diodes (Treu et al., 2001). The defect states of dislocation lines of TDs serve as transport paths of electrons and give rise to the increase of reverse leakage (Wang et al., 2021). Therefore, the reverse leakage current along the dislocation line of TDs would be the strongest. Since the gray part of TD-related pits locates closer to the dislocation lines of TDs, the enhanced reverse leakage current of the gray part of Figure 3 originates from the enhanced reverse leakage current dislocation lines of TDs. Therefore, the TUNA measurements verify that both the dislocation lines of TDs and TD-related pits participate in the reverse leakage current of 4H-SiC, of which the effect of dislocation lines of TDs is dominant.

FIGURE 3

FIGURE 3. Schematic diagram showing the correlation between the dislocation lines and etch-pit morphologies in 4H-SiC epitaxial layers deposited on off-axis sliced substrates. The gray part of the etch pit denotes the etched surfaces near the dislocation lines.

At last, we compare the magnitude of reverse leakage current induced by different dislocations in 4H-SiC. It should be noted that the local I-V curves of the etch pits of different dislocations have been measured by 2-3 times at different etch pits, and the comparison of the leakage current is carried out at the same region of the etch pits. As shown in Figure 4, we find that the effect of dislocations on the reverse leakage current of 4H-SiC increases in the order of BPD, TED, TSD, and TMD. When the reverse bias voltage exceeds 2.55 V and 3.1 V, the leakage current along the TMD and TSD begin to sharply increases, respectively. TED exerts a minor effect on the reverse leakage current of 4H-SiC. When the reverse bias voltage is 3.55 V, the TED-induced leakage current gradually increases, with the degree of increase being gentler than what happens in TMD- and TSD-induced leakage current. The effect of TDs on the leakage current of 4H-SiC is associated with the Burgers vectors of TDs. The Burgers vectors of TMD, TSD, and TED are c $+$ a, [000c], and $\frac{[11 \bar{2} 0] a}{3}$ , respectively. The atomic distortions, and thus the strain field around dislocations increase in the order of BPD, TED, TSD, and TMD, which gives rise to the different extent of influences on the leakage current of 4H-SiC. Figure 4

FIGURE 4

FIGURE 4. Localized I-V curves of TSD, TMD, TED, and BPD in 4H-SiC epitaxial layers.

4 Conclusion

In conclusion, we have clarified the effect of dislocations on the leakage current of homoepitaxial 4H-SiC by combining molten-KOH etching and TUNA measurements. It turns out that both the dislocation lines of TDs and the TD-related pits increase the reverse leakage current of 4H-SiC. The dislocation lines of TDs exert a more significant effect on the reverse leakage current of 4H-SiC, which gives rise to the nonuniform distribution of reverse leakage current throughout the TD-related pits. Due to the different Burgers vectors of TDs, the effect of TDs on the reverse leakage current of 4H-SiC increases in the order of TED, TSD, and TMD. BPDs are also found to slightly increases the reverse leakage current, with the leakage current mainly concentrated at the core of the BPD. Compared to the effect of TDs, the effect of BPDs on the reverse leakage current of 4H-SiC is negligible. Our work indicates that reducing the density of TDs, especially TMDs and TSDs, is key to improving the quality of 4H-SiC epitaxial layers and reducing the reverse leakage current of 4H-SiC based high-power devices.

Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author contributions

WG contributes to the design and implementation of the experiment, data analysis, and manuscript writing. GY contributes to the sample preparation and discussion of results. YQ contributes to the sample preparation and characterizations. XH contributes to the draft review and editing. CC contributes to the draft review and discussion of results. XP contributes to supervision, conceptualization, and the draft review. DY contributes to supervision, conceptualization, and draft review. RW contributes to the manuscript writing, discussion of results, supervision, and conceptualization.

Funding

This work was supported by Natural Science Foundation of China (Grant Nos. 62274143, U22A2075, 62204216), “Pioneer” and “Leading Goose” R&D Program of Zhejiang (Grant No. 2022C01021), National Key Research and Development Program of China (Grant No. 2018YFB2200101) and Zhejiang University Education Foundation Global Partnership Fund.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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References

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