Abstract
Cubic boron nitride (cBN) single crystals are synthesized under high temperature and high pressure in the Li-based system. The growth defects on hexagonal and triangular (111) surfaces of cBN single crystals after rapid cooling are discussed systemically for the first time using the atomic force microscope. Some impurity particles, triangle cone hole defects, lamellar-fault structures, and big steps are obvious on the surfaces of cBN single crystals. The formation mechanism of these defects is analyzed briefly at the synthetic process of cBN single crystals, and the growth mechanism of cBN single crystals transform from the two-dimensional growth to dislocation growth mechanism under high temperature and high pressure.
1. Introduction
Cubic boron nitride (cBN) single crystals have a high hardness, high melting point, high thermal conductivity, wide energy gap, and low dielectric constant, which make them highly promising materials [1–4]. At present, cBN single crystals are synthesized using hexagonal boron nitride (hBN) as the source material and lithium nitride (Li3N) as the catalyst by the static high temperature and high pressure (HPHT) catalytic method [5–7]. The further development of electronic and optical applications need large cBN single crystals with high quality, while the size of present cBN single crystals cannot meet the demand for the study of advanced functional products [8, 9]. In a previous experimental study, the cBN nucleation was discussed, which indicates that cBN crystal nuclei are formed directly by the transition of hBN [10]. The growth process of cBN single crystals determines their quality after cBN nucleation under HPHT. The growth defects could affect the crystal quality; thus, the importance of growth defects in cBN single crystals growth should be clearly recognized [11]. It is almost impossible to study in situ the growth process of cBN single crystals under HPHT; however, much information about cBN growth under HPHT is preserved on the cBN crystal surfaces and the interface layer of cBN single crystals at room temperature and ambient pressure when the synthetic cell assembly was cooled rapidly. The morphology of crystals surfaces and the constituent of the interface layer are closely associated with the cBN single crystals growth under HPHT, and it may be of great significance to explain the growth mechanism of cBN single crystals.
At present, the study of crystal growth at nanometer sale has attracted much interest, and some new growth theories have been established with the help of the AFM [11]. The surface topography of synthetic cBN single crystals was reported [4, 10, 12]; however, thus far, there is almost no report on the analysis of cBN single crystal surfaces by atomic force microscopy (AFM). The factors that affect the formation of defects on (111) surfaces of cBN single crystals and the growth mechanism of cBN single crystals are discussed on the basis of the AFM observations. The present study provides a better understanding of cBN single crystals growth mechanism under HPHT.
2. Materials and Methods
cBN single crystals were synthesized from high purity hBN (>98%) with Li3N as catalyst, and the weight ratio of the two was 9 : 1 by the static HPHT catalyst method. HPHT experiments were performed using the cubic anvil apparatus HTDS-034 hm. In these experiments, the synthetic pressure, temperature, and time were 4.5 GPa, 1700 K, and 10 min, respectively. After rapid cooling, the synthesized cBN bulk was removed from the cubic anvil and prepared for further examination. As shown in Figure 1(a), the cBN single crystal was surrounded by powders. The samples were cleaned, and then, the images of cBN single crystals were taken using a JSM-6380 LA-type scanning electron microscope (SEM). The morphology of HPHT as-grown crystal surfaces was obtained using a Bruker Multimode 3D AFM in contact mode under ambient atmosphere and room temperature. The powders around the HPHT as-grown cBN single crystals were carefully collected under the view of an optic microscope. A JEOLJEM-2010F-type high resolution transmission electron microscope (HRTEM) was used to examine their phases in powders with an operating voltage of 200 kV.
(a)
(b)
(c)
(d)
3. Results and Discussion
Since the facets are in direct contact with the molten catalyst under HPHT in the cBN crystal growth process, the surface morphology provides much information to analyze the crystal growth. Figures 1(b) and 1(c) shows the SEM images of cBN single crystals with triangular and hexagonal facets (111) and their plane projection drawing. The sizes of cBN single crystals are about 0.5 mm. The synthesized cBN single crystals have truncated tetrahedron structures, which are composed of four coordination B-N4 or N-B4 growth units. There exist solid structures in short or medium range order in the molten catalyst when cBN single crystals are synthesized under HPHT [10, 12]; thus, much information about the molten catalyst can be remained in the powders surrounded by cBN single crystals at ambient condition after the cell assembly is cooled rapidly.
The powders in the interface layer (in Figure 1(a)) were collected, and their compositions were determined with HRTEM. Figures 2(b) and 2(d) show the SAD patterns of powders surrounded cBN single crystals in Figures 2(a) and 2(c), respectively. The corresponding SAD pattern in Figure 2(b) indicates that the white powders in Figure 2(a) are cBN and hBN, from which cBN can be identified by (111), (200), (220), (311), and (222) diffraction ring, while hBN can be identified by (100), (014), and () diffraction pattern. This means many microcubic grains are formed under HPHT in the growth process of cBN crystals. However, the corresponding SAD pattern in Figure 2(d) indicates that the white powders in Figure 2(c) are all hBN, from which hBN can be identified by (100), (010), and () diffraction pattern. Figures 2(a) and 2(c) are the morphologies of the same zone of the same powders before and after electron beam irradiation. It is obvious that the black material in Figure 2(a) disappears.
(a)
(b)
(c)
(d)
This means the bunching electron beam by HRTEM induces the occurrence of the reaction from cBN to hBN. From Figure 2(b), it can be inferred that it is a good corresponding relation between (200) of cBN and (100) of hBN crystal planes. The cBN growth process suggests that when hBN transform into micro-cBN grain under HPHT, various micro-cBN grains are delivered to the growing single crystal surfaces, and the collision or coalescence of micro-cBN grains in the molten catalyst induces the growth of cBN single crystals under HPHT. Meanwhile, it is possible that the recrystallization of hBN exists in HPHT experiments. This suggests are consistent with the previous experimental results that cBN with higher quality is synthesized under 4.5 GPa and 1800 K [13]. In our earlier work, Li3BN2 are found in the interface by meaning of XRD, and it is the real catalyst [10, 12]. The crystal surface energy can directly affect the nucleation and growth of the crystal. Li3BN2 has higher surface energy [14]; thus, the Li3BN2 phase can restrain the crystals’ nucleation at the initial stage of the reaction and then promote the maximum growth of the nucleus under high temperature and low pressure.
Figure 3 shows different growth defects on the surfaces of cBN single crystals. Although cBN crystal surfaces were cleaned, some microscopic particles were still covered on the single crystal surfaces, as shown in Figure 3(a). Microscopic particles were probably derived from impurities adhering on the crystal surfaces or remaining powders surrounding cBN single crystals. Meanwhile, triangular hole defects, lamellar-fault structures, and big step structures are found in Figure 3(a). Figures 3(c) and 3(d) show the 2D AFM images of lamellar-fault structures and big step structures on cBN crystal (111) facets in an area of 5 μm × 5 μm, respectively. From section analysis in Figure 3(e), it was found that the height of a lamellar demonstrated by an arrow in Figure 3(c) is about 27 nm, and the height of the lamellar-fault structures has relatively homogeneous distribution. The height of a big step in Figure 3(d) is about 28 nm, which is basically the same as the height of the lamellar-fault structures. Compared to the height of lamellar-fault structures in Figure 3(c), the big step structures in Figure 3(d) have a heterogeneous distribution. The lamellar-fault structures derive from two-dimensional crystal nuclei.
The impurity in raw materials and the residual gas in working cells could produce a certain amount of impurities in the synthesis process of cBN crystals. The impurities adsorbed on the growing crystal surface prompt the lamellar-fault structures to form a big step under HPHT [10, 11]. When the parallel steps were disturbed during the HPHT step growth process, the growth steps merged locally, as shown in Figure 3(b). The big steps on single crystal surfaces probably derive from dislocations. Because of the presence of some twisting defects and steps, the surface energy barrier of nucleation decreases; furthermore, cubic BN crystal nuclei have a higher possibility to grow around the twisting defects and big steps. The nucleation and growth mechanism transform the two-dimensional growth to dislocation growth mechanism.
The direction of all triangle-hole defects on (111) facets are pretty consistent relative to the edge of cBN single crystals, as shown in Figure 3(d). The reasons for the formation of triangle-hole defects are probably inclusions, vacancies, or dislocations. For (111)-B facets of cBN single crystals, B atoms have an sp3 empty orbit that can be stable; thus, (111)-B facets should be smooth surfaces. For (111)-N facets of cBN single crystals, N atoms have an sp3 hybridization orbit with a lone pair electron. Thus, it can easily bind with other atoms and form inclusions inside the cBN single crystals. The triangle-hole defects can be produced on the (111)-N facets of cBN single crystals when those inclusions cleaned out the surfaces of cBN single crystals effectively. When inclusions can exist in cBN single crystals under HPHT, some big vacancy loops were produced to relieve the internal stress of cBN single crystals [15]. The triangle-hole defects probably are the etch pits of edge dislocations exist on the surfaces of cBN single crystals when cBN single crystals were cleaned using the alkaline solutions.
4. Conclusion
The growth defects on (111) surface of cBN single crystals after rapid cooling are discussed for the first time by AFM technique. Some impurity particles, triangle-hole defects, lamellar-fault structures, and big steps were found on single crystal surfaces. A possible formation mechanism of several types of defects is revealed briefly. The analysis provides a better understanding for the growth mechanism of cBN single crystals under HPHT. The growth mechanism of cBN single crystals transforms the two-dimensional growth to dislocation growth mechanism under HPHT.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare no conflicts of interest.
Authors’ Contributions
Lichao Cai conceptualized the study. Feng Jia and Xing-Dong Yuan formally analyzed the manuscript. Lichao Cai was responsible for investigation. Bin Xu and Meizhe Lv were responsible for methodology. Bin Xu supervised the work. Lichao Cai wrote the original draft. Bin Xu and Meizhe Lv wrote, reviewed, and edited the manuscript.
Acknowledgments
This study was funded by the Natural Science Foundation of China (grant no. 51272139).