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Article

Influence of Growth Process on Suppression of Surface Morphological Defects in 4H-SiC Homoepitaxial Layers

by
Yicheng Pei
1,2,
Weilong Yuan
1,2,
Yunkai Li
1,3,
Ning Guo
1,3,
Xiuhai Zhang
2,* and
Xingfang Liu
1,3,4,*
1
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
2
School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China
3
College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
4
Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Micromachines 2024, 15(6), 665; https://doi.org/10.3390/mi15060665
Submission received: 20 April 2024 / Revised: 15 May 2024 / Accepted: 18 May 2024 / Published: 21 May 2024
(This article belongs to the Special Issue Research Progress of Advanced SiC Semiconductors)
Browse Figures
Versions Notes

Abstract

:
To address surface morphological defects that have a destructive effect on the epitaxial wafer from the aspect of 4H-SiC epitaxial growth, this study thoroughly examined many key factors that affect the density of defects in 4H-SiC epitaxial wafer, including the ratio of carbon to silicon, growth time, application of a buffer layer, hydrogen etching and other process parameters. Through systematic experimental verification and data analysis, it was verified that when the carbon–silicon ratio was accurately controlled at 0.72, the density of defects in the epitaxial wafer was the lowest, and its surface flatness showed the best state. In addition, it was found that the growth of the buffer layer under specific conditions could effectively reduce defects, especially surface morphology defects. This provides a new idea and method for improving the surface quality of epitaxial wafers. At the same time, we also studied the influence of hydrogen etching on the quality of epitaxial wafers. The experimental results show that proper hydrogen etching can optimize surface quality, but excessive etching may lead to the exposure of substrate defects. Therefore, it is necessary to carefully control the conditions of hydrogen etching in practical applications to avoid adverse effects. These findings have important guiding significance for optimizing the quality of epitaxial wafers.

1. Introduction

With the gradual advancement of silicon carbide (SiC) chemical vapor deposition (CVD) technology, the SiC growth process has approached maturity, and the primary obstacle to producing SiC semiconductor epitaxial wafers lie in the presence of epitaxial defects. These defects pose significant challenges for high-voltage and high-power SiC power electronic devices [1,2,3,4,5,6,7]. The origins of these defects are often related to many factors, such as substrate quality, growth temperature and cavity structure, and the crystal structure of these defects is usually complicated [8,9,10,11,12,13,14,15]. High-voltage and high-power SiC power electronic devices need a large active area to realize high-current applications [16,17,18,19,20]. Defects in the active region degrade the performance of devices and further lead to device failure. Especially, some fatal defects can greatly reduce a device’s breakdown voltage and may cause long-term reliability problems [21,22,23].
The epitaxial defects of silicon carbide are mainly divided into surface morphology defects and internal structure defects, and internal structure defects are mainly dislocation and some hole-like point defects. Micropipes in structural defects cause serious damage, but they have already been solved [24]. Other structural defects, such as through edge dislocation (TED) and through screw dislocation (TSD), cause surface pits and increase leakage current, which are less destructive. The main destructive defects are instant surface dislocation (BPD) defects and stacking faults (SF), which are likely to continuously increase the on-resistance of bipolar devices [25,26,27,28,29,30,31,32,33,34,35]. Surface defects, such as dump, scratch, particle, downfall (DF), triangle (TD), comet and carrot defects, are typically detrimental and easily observable, and often lead to device failure [36,37,38,39,40,41,42].
Therefore, one of the main challenges in 4H-SiC epitaxial growth is to decrease the defect density of the epitaxial layer. There is less research regarding suppressing SiC epitaxial defect formation using epitaxial process modulation. Therefore, this study is significant. In this study, a multitude of experiments were conducted on different process parameters to examine the impact of different growth conditions on the density of surface defects. Through these effects, defect control methods for growth process control were obtained.

2. Experimental

To determine the influence of some process parameters on the density of defects, we used a 4° inclination angle to grow a 6-inch 4H silicon carbide epitaxial wafer. Trichlorosilane (TCS, SiHCl3) was used as the silicon source. TCS ensured that the ratio of silicon to chlorine was 1/3, ethylene was used as the carbon source, hydrogen was used as the carrier gas, and the ratio of silicon to hydrogen was 0.05%, which is around 50 sccm TCS and 100 slm H2. The conventional conditions were a C/Si ratio of 0.72, temperature of about 1570 °C, pressure of 40 torr, and epitaxial growth for 30 min. The experimental conditions of the target test items were adjusted, while ensuring that other experimental conditions remained unchanged. SICA examination equipment for distinguishing defects using photoluminescence defect morphology was used to detect and count the density of defects to investigate how the density of defects changes under different conditions. The main defects found using this examination are presented in Figure 1. Among them, defects with PL preceding the defect name are those that have not been found under ordinary optical inspection, but only found under photoluminescence examination. PL.white and PL.black are defects such as TED and TSD, and this examination method could not accurately distinguish between these two defects.

3. Results

Through comparative experiments, the effects of process parameters, such as the C/Si ratio, temperature, growth time, buffer layer and etching conditions, on the density of epitaxial defects of 4H-SiC were identified. A scanning electron microscope (SEM) provides high-resolution images of the surface topography and can be used to analyze the morphology, composition, and crystal structure of materials. The surface of the grown epitaxial wafer was observed by SEM, and the main defects observed are shown in Figure 2. The two defect types are comet defects and triangle defects; comet defects are caused by downfall defects, while triangle defects may be caused by structural defects inherited from the substrate in addition to dropouts [1,27].
Figure 2 shows the Raman examination of two common surface defects. The one with a peak at 777 cm−1 is a 4H crystal form, and the one with a high peak at 798 cm−1 is a 3C crystal form. Therefore, the head and body of comet defects were both the second phase of the 3C structure, the head of triangular defects was a mixed crystal form dominated by a 3C structure, and the body parts were all 4H crystal forms, like epitaxial layers.
(1)
Defect suppression by the C/Si ratio
First, the mainstream process parameter is the ratio of carbon to silicon, which can reflect the growth source. In the experiment, we mainly explored the relationship between the ratio of carbon to silicon and the density of defects in epitaxial wafers, in which the ratio of carbon to silicon was changed by changing the ratio of TCS to ethylene, keeping the amount of TCS unchanged, and reducing the amount of ethylene to reduce the ratio of carbon to silicon. In addition, to explore the influence of the C/Si ratio on the density of defects, the TCS flux was set at 50 sccm, and the ethylene flux was adjusted at 25, 20, 18 and 13, so that the C/Si ratios were 1, 0.8, 0.72 and 0.52, respectively. Under these conditions, the epitaxy was carried out for 30 min.
Only cases in which C/Si ratios were less than 1 were selected because TCS contains chlorine, which generates SiCl4 with a part of Si. To balance the actual C/Si ratio participating in the reaction, a small amount more of the Si source was more suitable for epitaxial growth. Only conditions when the C/Si ratio was 1, 0.8, 0.72 and 0.52 were selected for epitaxial experiments.
In Figure 3a, boxes show the overall situations of defect density under different C/Si ratios, which could obtain the density of epitaxial defects under different C/Si ratios, reflect the discrete nature of defect density data to a certain extent, and obtain the stability of the defect density under these C/Si ratios, i.e., it was not easy to obtain epitaxial wafers with particularly high defects. The effect of C/Si on roughness is shown in Figure 3b, and the results for roughness were close to those for defect density. The optimum value was obtained around C/Si = 0.72, and it can be concluded that the defect density can be better suppressed at this C/Si ratio. According to the analysis of obtained data, the defect density did not increase gradually with the increase in the C/Si ratio, but was obtained when the C/Si ratio was maintained at a low level, and after the C/Si ratio reached 0.8, the defect density greatly improved. It is inferred from the experimental results that an increase in the C/Si ratio led to an increase in defect density to some extent. When the C/Si ratio was 0.72, the sample with the smallest defect density was obtained. The overall defect density was low [42]. On the contrary, samples with a higher defect density than those in the same group appeared in the other three groups of C/Si ratio tests, which may have been caused by the temperature change during the heating process.
Figure 4 shows atomic force microscope (AFM) images taken at different C/Si ratios; an AFM was used to test a plane of 10 μm × 10 μm in tapping mode. Roughness, as shown in Figure 4, was generally used to express the surface quality of epitaxial wafers. However, in the same batch of epitaxy, the main reasons for roughness were pits and steps, as shown in the following atomic force images in Figure 4, which seriously increased the surface roughness. The pits were formed by BPD defects extending to the epitaxial surface. When the C/Si ratio was 0.72, both the roughness and the density of defects were relatively low, which showed that a C/Si ratio of 0.72 is an excellent condition to keep the surface smooth, with fewer defects in process growth.
The reason for this phenomenon may be that the C/Si ratio was affected by the Cl element in the growth source TCS, which hindered the Si elements combining to form silicon drops during SiC epitaxy. On the other hand, under the condition of rich silicon, element C is also difficult to polymerize into carbon particles, and silicon droplets and carbon falling particles are the main generation sites of surface defects, which means that the higher the C content, the easier it is to produce defects. We can control the generation of epitaxial defects to some extent by controlling the ratio of carbon to silicon. After subsequent reference to defect types, defect density, defect distribution, surface roughness, roughness distribution uniformity and various tests, we think that when TCS is used as a Si source for silicon carbide epitaxial wafers growth, there are fewer defects when the C/Si ratio is about 0.72.
(2)
Effect of growth time on the density of defects
We studied the influence of growth time on the density of epitaxial defects of silicon carbide, and we thought that growth time was also an important factor affecting the density of defects. At the same time, under conditions of 1570 °C and 40 torr, we controlled the C/Si ratio to be 1 to perform multiple groups of epitaxial growth experiments for 10 min, 20 min, 25 min, 30 min and 40 min, calculated the average density of defects in each group, and plotted the relationship between the density of defects and growth time.
It can be observed from Figure 5 that the density of defects decreased when epitaxial growth time increased from 10 min to 25 min, and reached a minimum at 25 min, but when epitaxial growth time was adjusted to 40 min, the density of defects increased abnormally compared with the previous states.
The reason for this phenomenon may be that the types of defects were related to the differences in nucleation methods. In the initial stage of growth, defects were mainly structural defects inherited from the substrate or surface defects formed with substrate defects as nucleation sites. During the growth process of 10–25 min, some structural defects inherited from the substrate gradually decreased due to their own transformation and annihilation mechanisms, until they reached a low level at 25 min, during which the transformation from BPD to TED occurred. When the epitaxial time continued to increase to 30 min, the surface morphology defects of downfalls and nucleation of falling particles may have become the mainstream of defects. With the continuous accumulation of defects, the density of defects increased sharply. Therefore, controlling the growth time can be an effective method to reduce the density of defects. This method requires numerous attempts during actual production, focusing on identifying the lowest density of defects growing with time, and then devising production plans accordingly.
(3)
Restrain the inheritance of substrate defects via buffer layer
We conducted a control experiment on whether to grow the buffer layer. The growth of the buffer layer occurred under conditions of a C/Si ratio of 0.72, temperature of 1570 °C and pressure of 40 torr. The buffer and epitaxial layers were homogeneous, both 4H-SiC, but the doping concentration of buffer layer was higher than that of the epitaxial layer and lower than that of the substrate. One experiment directly grew the buffer layer for 25 min, and the other experiment grew the buffer layer for 5 min before the same 25 min epitaxy, with a H2 flow rate of 100 slm, nitrogen flow rate of 50 sccm, C/Si ratio of 0.53, growth pressure of 40 torr and growth temperature of 1570 °C.
According to the literature [43], the buffer layer can impede the growth of structural defects into the epitaxial layer with the stress difference brought about by the difference in doping concentration, which can reduce structural defects in the epitaxial layer and surface morphology defects caused by structural defects. Statistical results from experimental data in Figure 6 show that the defect density of epitaxial wafers with a buffer layer was slightly higher than that of epitaxial wafers without a buffer layer, which is contrary to the literature reports [43]. This was because the homogeneous buffer layer was less effective in hindering defects. Instead, the need to grow the buffer layer prolonged the overall epitaxial growth time, making the number of downfall defects higher.
However, the epitaxial layer shown in Figure 6b with a growing buffer layer had a lower roughness, which may mean that the buffer layer reduced defects that have a large impact on the surface roughness, such as triangle defects.
To verify the effectiveness of the buffer layer on epitaxial defects, we studied serious defects, such as triangular defects, separately. The buffer layer was grown under specific conditions, with a H2 flow rate of 100 slm, TCS flow rate of 50 sccm, ethylene flow rate of 25 sccm, growth pressure of 40 torr, growth temperature of 1570 °C and growth time of 4 min. During the experiment, two types of buffer layers were grown; one was a buffer layer with a nitrogen flow rate of 50 sccm, and the other was a buffer layer with a nitrogen flow rate of 300 sccm.
Obviously, the buffer layer grown on the epitaxial wafer plays an important role in triangular defects. We observed more triangular defects on epitaxial wafers without buffer layer growth, and the density of defects decreased significantly after buffer layer growth. This was because the buffer layer prevented some substrate defects from extending to the epitaxial layer and transforming into nucleation sites of surface morphology defects. In addition, the stress and interface defects caused by the doping concentration difference between the substrate and the epitaxial layer can become the nucleation sites of triangular defects, and the nucleation of triangular defects can be inhibited by adding the buffer layer. Therefore, the difference in the effect of buffer layers grown at different nitrogen flow rates, as shown in Figure 7, was due to the difference in doping concentration. Therefore, it can be determined that the buffer layer is extremely effective in controlling the density of defects, and this effect can be achieved by blocking the extension of defects and reducing the concentration difference stress.
(4)
Influence of hydrogen etching
Hydrogen etching is the initial stage of the epitaxial process. Its purpose is to expose the high-density atomic steps with narrow mesa width on the (0001) surface of the 4H-SiC epitaxial wafer cut off-axis at 4°, so that 4H-SiC can grow in step flow, which can effectively maintain the crystal form of the epitaxial layer as a 4H structure and improve the uniformity of the epitaxial wafer in all aspects.
Hydrogen etching can enhance the surface thickness uniformity of the epitaxial wafer and reduce the density of defects to some extent. In the experiment, before the epitaxial layers were formally grown, substrates were etched with hydrogen, and etching times were as follows: 0 min, 2 min, 6 min and 12 min.
It can be seen in Figure 8 that under the condition of high hydrogen flow etching, the density of defects first decreased and then rapidly increased. The reason for this may be that under the condition of short-time etching, the effect of exposing steps to optimize the growth of surface step flow was achieved, leading to a reduction in the density of defects. However, the hydrogen flow was too large. With the increase in hydrogen etching time, the original defect position of the substrate was not easy to etch because of greater stress, which exposed all substrate defects, and the density of defects inherited by the epitaxial layer from the substrate increased rapidly, resulting in an abnormal increase in the final density of defects. Therefore, hydrogen etching was used to improve the surface energy of the substrate and reduce the density of surface morphology defects. However, care was taken to prevent excessive etching that could expose substrate defects.

4. Discussion

This study mainly focused on how to solve the problem of 4H-SiC defects using the epitaxial process. We identified the effects of process parameters, for epitaxial growth, including the C/Si, growth temperature, growth time, buffer layer, and etching conditions, on the density of 4H-SiC epitaxial defects through many experiments.
Various factors affecting the epitaxial defects of 4H-SiC were considered. Combined with the actual defect distribution, we think that the key factors affecting defect density are the C/Si ratio, growth temperature, growth duration, buffer layer and etching strip. The C/Si ratio and growth temperature mainly affected the proportion of elements to control defects. Silicon droplets can become surface defects under silicon-rich conditions, and the proportion of elements affects deep-level defects such as holes. During the growth time, structural defects were the main defects in the early stage, and surface morphology defects were the main defects in the later stage. Controlling the growth time balanced the density of the two defects at a low level. The buffer layer suppressed structural defects and lattice differences caused by excessive variation in doping concentration. Etching can optimize surface quality, but excessive etching exposed the defects of the substrate and greatly increased the density of defects.
However, in fact, the influence of these process parameters on SiC epitaxial defects was interactive and synergistic. For example, shortening epitaxial time inhibited surface morphology defects, but increased structural defects. The proportion of elements affected the extension of structural defects and the concentration of vacancies, and produced silicon droplets or carbon falling objects, resulting in surface morphology defects. It is quite difficult to discuss these composite effects, so we only studied the total defect density.

5. Conclusions

In summary, we conducted systematic research on the influence of process control on SiC epitaxial defects. For defect density, the C/Si ratio was the most influential factor. With an increase in the C/Si ratio from 0.52 to 1, the defect density first decreased and then increased, reaching a lowest level of around 0.72. It reached the lowest defect density when the growth time was about 25 min. The buffer layer effectively reduced the density of surface defects such as triangular defects, but it may have had the opposite effect on other defects. In addition, 100 slm high-flow H2 etching for 2–6 min effectively reduced the defect density. Based on the above factors, the defect density of the epitaxial layer decreased by around 60%. Thus, high-flow H2 etching played a good role in defect control, especially regarding the density of the serious defects, which was significantly reduced. This defect control method is expected to be beneficial for studies regarding solving SiC defects.

Author Contributions

Methodology, formal analysis, data curation and writing—original draft preparation, Y.P.; data curation, formal analysis and visualization, W.Y.; software, supervision, resources and validation, N.G. and Y.L.; project administration, investigation and supervision, X.Z.; conceptualization, supervision, funding acquisition and writing—review and editing, X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Key Area Research and Development Program of Guangdong Province (Grant No. 2021B0101300005), the National Key Research and Development Program of China (Grant No. 2021YFB3401603) and the National Natural Science Foundation of China (Grant No. 12175236).

Data Availability Statement

Data that support the findings of this study are available from the corresponding authors, X.Z. and X.L., upon reasonable request.

Conflicts of Interest

No potential conflicts of interest are reported by the authors of this paper.

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Figure 1. Sample of epitaxial wafer defect test: (a) defect distribution and (b) defect quantity statistics.
Figure 1. Sample of epitaxial wafer defect test: (a) defect distribution and (b) defect quantity statistics.
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Figure 2. Identification of comet defects and triangle defects by Raman spectroscopy.
Figure 2. Identification of comet defects and triangle defects by Raman spectroscopy.
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Figure 3. Effect of C/Si ratio on defect and roughness. Red, yellow, blue and green blocks represent sample data range when the C/Si ratio is 0.52, 0.72, 0.8 and 1, respectively: (a) density of defects and (b) roughness.
Figure 3. Effect of C/Si ratio on defect and roughness. Red, yellow, blue and green blocks represent sample data range when the C/Si ratio is 0.52, 0.72, 0.8 and 1, respectively: (a) density of defects and (b) roughness.
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Figure 4. AFM images of epitaxial wafer roughness sampling at different C/Si ratios: (a) C/Si = 0.53; (b) C/Si = 0.72; (c) C/Si = 0.8 and (d) C/Si = 1.
Figure 4. AFM images of epitaxial wafer roughness sampling at different C/Si ratios: (a) C/Si = 0.53; (b) C/Si = 0.72; (c) C/Si = 0.8 and (d) C/Si = 1.
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Figure 5. Relationship between the density of defects and growth time.
Figure 5. Relationship between the density of defects and growth time.
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Figure 6. Effect of buffer layer on defects and roughness. Red and green blocks represent the sample data range without buffer layer, while blue and yellow blocks represent the sample data range with buffer layer: (a) density of defects and (b) roughness.
Figure 6. Effect of buffer layer on defects and roughness. Red and green blocks represent the sample data range without buffer layer, while blue and yellow blocks represent the sample data range with buffer layer: (a) density of defects and (b) roughness.
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Figure 7. Correspondence between buffer layer and triangle defects. The red, blue and green color blocks respectively represent the sample data range of no buffer layer, with buffer layer of 50 sccm N2 flow and with buffer layer of 300 sccm N2 flow.
Figure 7. Correspondence between buffer layer and triangle defects. The red, blue and green color blocks respectively represent the sample data range of no buffer layer, with buffer layer of 50 sccm N2 flow and with buffer layer of 300 sccm N2 flow.
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Figure 8. Effect of hydrogen etching time on defect density.
Figure 8. Effect of hydrogen etching time on defect density.
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Pei, Y.; Yuan, W.; Li, Y.; Guo, N.; Zhang, X.; Liu, X. Influence of Growth Process on Suppression of Surface Morphological Defects in 4H-SiC Homoepitaxial Layers. Micromachines 2024, 15, 665. https://doi.org/10.3390/mi15060665

AMA Style

Pei Y, Yuan W, Li Y, Guo N, Zhang X, Liu X. Influence of Growth Process on Suppression of Surface Morphological Defects in 4H-SiC Homoepitaxial Layers. Micromachines. 2024; 15(6):665. https://doi.org/10.3390/mi15060665

Chicago/Turabian Style

Pei, Yicheng, Weilong Yuan, Yunkai Li, Ning Guo, Xiuhai Zhang, and Xingfang Liu. 2024. "Influence of Growth Process on Suppression of Surface Morphological Defects in 4H-SiC Homoepitaxial Layers" Micromachines 15, no. 6: 665. https://doi.org/10.3390/mi15060665

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