A vertical liquid phase epitaxy graphite boat and a method for epitaxy using the same

Abstract

The present application relates to the technical field of liquid phase epitaxy growth of thin film materials, and particularly to a vertical liquid phase epitaxy graphite boat and a method for epitaxy using the same. The graphite boat comprises a sample holder, a stirring rod, a stirring base, and a crucible. The graphite boat can improve the uniformity of epitaxial materials, reduce surface defects, reduce the epitaxial composition and thickness difference caused by the inhomogeneity of the mother liquor, and avoid the melting of the surface layer of the substrate at high temperature in the mother liquor, which causes the change of the composition of the mother liquor. While ensuring the uniformity of the epitaxial thickness and composition, the epitaxial carbon contamination defects can be effectively reduced, and the batch yield can be effectively improved.

Description

Technical Field

[0001] This invention relates to the technical field of liquid phase epitaxial growth of thin film materials, specifically to a vertical liquid phase epitaxial graphite boat and a method for epitaxy using the same. Background Technology

[0002] Mercury cadmium telluride (MCH) is a pseudobinary alloy semiconductor with a zincblende structure. Its bandgap can be adjusted by controlling the cadmium element, allowing detectors to be used in the short-wave infrared (SWIR: 1–3 μm), mid-wave infrared (MWIR: 3–5 μm), long-wave infrared (LWIR: 8–14 μm), and very long-wave infrared (VLWIR: 14–30 μm) spectral regions. Due to its wide tunable bandgap and high quantum efficiency, MCH has become the most widely used infrared photosensitive semiconductor material, finding extensive applications in meteorological detection, aerospace, and military reconnaissance.

[0003] Liquid phase epitaxy is currently the most mature method for preparing mercury cadmium telluride (HCd) single-crystal thin films. Among these methods, tellurium-rich vertical liquid phase epitaxy, employing a "semi-infinite mother liquor" growth approach, results in a surface free of macroscopic ripples compared to horizontal epitaxy. Consequently, the fabricated detectors exhibit no "background lines" or "ghosting" phenomena. Furthermore, compared to horizontal epitaxy, production capacity is at least 2-4 times higher, and the HCd mother liquor can be reused hundreds of times, significantly reducing process costs. Therefore, it has become the preferred choice for mass production lines of HCd single-crystal thin films.

[0004] Liquid phase epitaxy employs a solid-liquid phase change growth method. The uniformity and cleanliness of the mother liquor directly determine the control of the thickness composition and defect density of the epitaxial single crystal film. During cooling, the segregation of the mercury cadmium telluride ternary alloy leads to a deterioration in the uniformity of the mother liquor. Therefore, before each growth cycle, the mother liquor needs to be heated to 30-50°C above the crystallization point, and simultaneously stirred to ensure thorough homogenization. Chinese patent CN203768483U discloses a sample holder for immersion-type mercury cadmium telluride liquid phase epitaxy. Although this sample holder structure allows the bottom stirring plate to achieve stirring through vertical movement, the substrate must be immersed in the mother liquor during homogenization. The high temperature of the mother liquor causes the substrate to remelt back into the mother liquor, altering its composition and severely affecting the control of the thickness composition consistency of the epitaxial film. Simultaneously, substrate remelting reduces the substrate surface flatness, further affecting the flatness of the epitaxial surface and increasing the difficulty of flip-chip interconnection.

[0005] Due to its excellent machinability and chemical stability, graphite is the primary material used in the processing of complex structural parts requiring high purity. However, graphite is prone to carbon shedding during operation. Because the density of this carbon is lower than that of the mercury cadmium telluride mother liquor, it floats on the surface of the mother liquor. During epitaxial growth, these carbon particles adhere to the substrate surface, causing epitaxial growth defects. If these defects are not controlled during the design and fabrication of the graphite boat, carbon contamination will severely impact the yield rate.

[0006] Therefore, a solution is needed to address the technical problems in the existing technology. Summary of the Invention

[0007] This invention provides a vertical liquid phase epitaxial graphite boat for improving the uniformity of epitaxial materials and reducing surface defects, as well as a method for epitaxy using the same, which can at least solve some of the problems existing in the prior art.

[0008] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0009] A vertical liquid phase epitaxy graphite boat includes: a sample holder, a stirring rod, a stirring base, and a crucible; the sample holder is detachably connected to the stirring rod and the stirring base, and the stirring rod is detachably connected to the stirring base. Both the sample holder and the stirring base are located within the crucible, and the lower end of the stirring rod is connected to the stirring base. The sample holder is detachably connected to the stirring rod or the stirring base.

[0010] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat of the present invention, the sample holder is sleeved on the stirring rod, and the lower end of the sample holder is detachably connected to the lower end of the stirring rod.

[0011] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat of the present invention, the sample holder includes a quartz inner sleeve and a graphite frame sleeved outside the quartz inner sleeve. The inner wall of the quartz inner sleeve has a smooth surface, and the quartz inner sleeve and the graphite frame are fixed by threads.

[0012] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat of the present invention, the graphite boat further includes a sample holder fixing cover, which is disposed at the upper opening of the crucible and is detachably connected to the crucible.

[0013] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat of the present invention, a clamp for fixing and assembling the substrate is provided on the periphery of the sample holder.

[0014] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat of the present invention, a hole is provided at the top center of the sample holder fixing cover, the upper end of the stirring rod passes through the hole and is connected to the drive shaft, and the upper end of the sample holder is detachably connected to the bottom of the sample holder fixing cover.

[0015] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat of the present invention, the lower end of the sample holder is detachably connected to the lower end of the stirring rod through a first threaded connection structure, and the upper end of the sample holder is detachably connected to the bottom of the sample holder fixing cover through a second threaded connection structure, wherein the thread directions of the first threaded connection structure and the second threaded connection structure are opposite.

[0016] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat described in this invention, the bottom surface of the stirring base has a serrated structure.

[0017] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat described in this invention, the stirring rod is made of carbon or SiC fiber composite material.

[0018] As a preferred embodiment of the vertical liquid phase epitaxial graphite boat described in this invention, the outer surface of the sample holder is coated with a pyrolytic carbon coating.

[0019] To solve the above-mentioned technical problems, according to another aspect of the present invention, the present invention provides the following technical solution:

[0020] A method for vertical liquid phase epitaxy, employing the aforementioned vertical liquid phase epitaxy graphite boat, includes a mother liquor high-temperature homogenization step and an epitaxial thin film growth step, characterized in that:

[0021] In the high-temperature homogenization step of the mother liquor, the sample holder is separated from the stirring base, the sample holder is located above the mother liquor, and the stirring rod is located in the mother liquor to homogenize and stir the mother liquor.

[0022] In the epitaxial film growth step, the sample holder is connected to the stirring base, and both the sample holder and the stirring base are located in the mother liquor. The sample holder and the stirring base move simultaneously to achieve the growth of the epitaxial film.

[0023] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the vertical liquid phase epitaxy method specifically includes the following steps:

[0024] S1: Grinding and polishing the graphite boat;

[0025] S2: Clean the graphite boat;

[0026] S3: Graphite boat assembly and substrate loading;

[0027] S4: Polishing of floating carbon on the mother liquor surface and compensation of mother liquor;

[0028] S5: Epitaxial growth;

[0029] S6: Degassing and cleaning of graphite boat.

[0030] In a preferred embodiment of the vertical liquid phase epitaxy method described in this invention, during the vertical epitaxial growth process, the stirring rod is pre-stirred and rotated by the drive shaft of the epitaxy equipment to stir the mother liquor vertically and rotationally.

[0031] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the grinding and polishing process in step S1 is as follows: the graphite boat is ground with metallographic sandpaper to remove machining marks. After grinding the stirring rod, it is confirmed that its diameter is 0.5-1mm lower than the inner diameter of the sample holder to reduce the relative friction between the two. After removing the machining marks, the graphite boat is further polished with metallographic sandpaper with a higher mesh size until the surface is bright.

[0032] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the cleaning process in step S2 is as follows: the graphite boat is placed in water and ultrasonically cleaned to remove the embedded carbon particles until there are no obvious floating carbon particles in the water. Then, oil stain cleaning and metal ion cleaning are performed, and the surface is thoroughly cleaned with deionized water and dried.

[0033] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the steps of graphite boat assembly and substrate loading in step S3 are as follows: tighten the threads of the quartz inner sleeve and the sample holder, fix the cleaned substrate on the assembled sample holder with a clamp, tighten the sample holder on the sample holder fixing cover, connect the stirring base and the stirring rod with the stirring rod screw, insert the stirring rod into the sample holder and tighten the connecting thread, and finally tighten the sample holder fixing cover and the crucible. The graphite boat assembly is completed.

[0034] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the steps of grinding the floating carbon on the surface of the mother liquor and compensating the mother liquor in step S4 are as follows: After each round of epitaxial growth, there are carbon particles on the surface after cooling and solidification. All the black floating carbon particles on the surface of the mother liquor are ground clean, and then a compensating mother liquor with a composition similar to that of the mother liquor is added to ensure that the total weight of the mother liquor is similar.

[0035] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the epitaxial growth steps in step S5 are as follows: the upper end of the stirring rod is connected to the drive shaft of the epitaxial equipment, the drive shaft can realize the up-and-down movement and rotation of the stirring rod; the epitaxial furnace is heated to the homogenization temperature, the stirring rod is loosened and moved up and down to fully stir the mother liquor, the temperature is lowered to the growth temperature, the stirring rod is rotated in the opposite direction to tighten the lower thread and loosen the upper thread, the sample holder and the stirring rod are lowered at the same time to carry out epitaxial growth, after reaching the end temperature, the sample holder is lifted and the mother liquor is rapidly cooled.

[0036] As a preferred embodiment of the vertical liquid phase epitaxy method of the present invention, the steps of degassing and cleaning the graphite boat in step S6 are as follows: after growth is completed, the epitaxial wafer is removed, and the mother liquor adhering to the sample holder and stirring rod surface is removed by high temperature degassing; after degassing, the surface of the graphite boat is thoroughly wiped with lint-free paper to clean the residual mother liquor powder and floating carbon.

[0037] The beneficial effects of this invention are as follows:

[0038] 1. This invention provides a vertical liquid phase epitaxial graphite boat for improving epitaxial uniformity and reducing surface defects, and a method for epitaxy using the same. The up-and-down movement of a stirring base on the graphite boat agitates the mother liquor, ensuring uniform composition and reducing variations in epitaxial composition and thickness caused by mother liquor inhomogeneity. The separate design of the sample holder and stirring device prevents the sample holder containing the substrate from entering the mother liquor during the homogenization process, thus avoiding the substrate surface layer melting into the mother liquor at high temperatures and altering the mother liquor composition. Therefore, it facilitates control over the composition and thickness of the epitaxial film, and is particularly suitable for the preparation of liquid phase epitaxial single-crystal films with high mother liquor segregation coefficients and stringent requirements for mother liquor uniformity. It is especially suitable for the large-scale mass production of immersion-type mercury cadmium telluride liquid phase epitaxy.

[0039] 2. The design of the graphite boat and the liquid phase epitaxy method of the present invention can effectively reduce epitaxial carbon contamination defects and effectively improve batch production yield while ensuring epitaxial thickness and composition uniformity. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the cross-sectional structure of the graphite boat of the present invention;

[0042] Figure 2 This is a schematic diagram of the graphite boat epitaxial growth process of the present invention;

[0043] Figure 3 Microscopic image of unmodified graphite boat epitaxial growth material;

[0044] Figure 4 This is a microscopic image of the graphite boat epitaxial growth material of the present invention.

[0045] Explanation of icon numbers:

[0046] 1-Sample rack fixing cover, 2-Quartz inner sleeve, 3-Sample rack, 4-Stirring rod, 5-Stirring base, 6-Stirring rod screw, 7-Crucible, 8-Clamping fixture, 9-Substrate.

[0047] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0048] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicators will also change accordingly.

[0050] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0051] This invention provides a vertical liquid phase epitaxial graphite boat for improving the uniformity of epitaxial materials and reducing surface defects, as well as a method for epitaxy using the same. It can reduce differences in epitaxial composition and thickness caused by mother liquor inhomogeneity, and prevent the surface layer of the substrate from melting into the mother liquor at high temperatures, thus avoiding changes in the mother liquor composition. While ensuring the uniformity of epitaxial thickness and composition, it can effectively reduce epitaxial carbon contamination defects and significantly improve batch production yield.

[0052] Example 1

[0053] With attachment Figure 1-2For example, a vertical liquid phase epitaxial graphite boat includes: a sample holder fixing cover 1, a quartz inner sleeve 2, a sample holder 3, a stirring rod 4, a stirring base 5, a stirring rod screw 6, a crucible 7, and a clamp 8; the sample holder 3 and the stirring base 5 are both located in the crucible 7, the lower end of the stirring rod 4 is connected to the stirring base 5, and the sample holder 3 and the stirring rod 4 or the sample holder 3 and the stirring base 5 are detachably connected.

[0054] The sample holder fixing cover 1 is detachably connected to the upper opening of the crucible 7 and is positioned above the crucible 7. The sample holder 3 includes a quartz inner sleeve 2 and a graphite frame, with the graphite frame fitted over the quartz inner sleeve 2. The two are fixedly connected by threads. The inner wall of the quartz inner sleeve 2 has a smooth surface, which, while ensuring strength, effectively reduces friction and carbon shedding between the sample holder 3 and the stirring rod 4. A hole is provided at the center of the top of the sample holder fixing cover 1. The upper end of the sample holder 3 is detachably connected to the lower end of the hole in the sample holder fixing cover 1 by, but not limited to, threads, or other detachable connection methods, including but not limited to snap-fit ​​connections. The sample holder 3 is fitted onto the stirring rod 4. The upper end of the sample holder 3 is detachably connected to the bottom of the sample holder fixing cover 1, and the lower end of the sample holder 3 is detachably connected to the lower end of the stirring rod 4 by, but not limited to, threads, or other detachable connection methods, including but not limited to snap-fit ​​connections. The sample holder 3 can also be detachably connected directly to the stirring base 5. The stirring rod 4 and the stirring base 5 are detachably connected by the stirring rod screw 6. The stirring rod 4 can be inserted into the quartz inner sleeve 2. The upper end of the stirring rod 4 passes through the hole and is connected to the drive shaft to realize up-down and rotational movements. The clamp 8 is used to fix the substrate 9 on the sample holder 3.

[0055] The bottom surface of the stirring base 5 is designed with a serrated structure, which can cover the floating carbon on the surface of the mother liquor when the substrate descends, thus preventing carbon contamination defects caused by the substrate passing over the surface of the mother liquor.

[0056] The stirring rod 4 is made of carbon or SiC fiber composite material, which improves its load-bearing capacity and torsional mechanical properties. The outer surface of the sample holder is coated with a pyrolytic carbon coating, which further reduces carbon shedding from the graphite boat.

[0057] Example 2

[0058] A method for improving epitaxial uniformity and reducing surface defects in vertical liquid phase epitaxy. The method uses the vertical liquid phase epitaxy graphite boat in Example 1, including a mother liquor high-temperature homogenization step and an epitaxial film growth step. In the mother liquor high-temperature homogenization step, the sample holder is separated from the stirring base, the sample holder is located above the mother liquor, and the stirring rod is located in the mother liquor to homogenize and stir the mother liquor.

[0059] In the epitaxial film growth step, the sample holder is connected to the stirring base. Both the sample holder and the stirring base are located in the mother liquor. The sample holder and the stirring base move simultaneously to achieve the growth of the epitaxial film.

[0060] The vertical liquid phase epitaxy method specifically includes the following steps:

[0061] S1: Polish the graphite boat by grinding and polishing it with metallographic sandpaper to remove machining marks. After grinding the stirring rod, confirm that its diameter is lower than the inner diameter of the sample holder to reduce the relative friction between the two. After removing the machining marks, use metallographic sandpaper with a higher grit to further polish the graphite boat until the surface is bright.

[0062] S2: Clean the graphite boat by placing it in water and ultrasonically cleaning the embedded carbon particles until there are no obvious floating carbon particles in the water. Then, clean the oil stains and metal ions, thoroughly clean with deionized water, and dry.

[0063] S3: Graphite boat assembly and substrate loading. Tighten the threads of the quartz inner sleeve and sample holder. Use a clamp to fix the cleaned substrate onto the assembled sample holder. Tighten the sample holder onto the sample holder fixing cover. Connect the stirring base to the stirring rod with the stirring rod screw. Insert the stirring rod into the sample holder and tighten the connecting thread. Finally, tighten the sample holder fixing cover onto the crucible. The graphite boat assembly is complete.

[0064] S4: Surface carbon removal and mother liquor compensation. After each round of epitaxial growth, carbon particles remain on the cooled and solidified surface. All black floating carbon particles on the surface of the mother liquor are removed. Then, a compensation mother liquor with a similar composition to the mother liquor is added to ensure a similar total weight of the mother liquor.

[0065] S5: Epitaxial growth. Connect the upper end of the stirring rod to the drive shaft of the epitaxial equipment. This drive shaft can move up and down and rotate. The epitaxial furnace is heated to the homogenization temperature. The thread between the stirring rod and the lower end of the sample holder is loosened, separating the stirring rod from the sample holder and allowing it to move up and down to thoroughly stir the mother liquor. The temperature is then lowered to the growth temperature. The stirring rod is rotated in the opposite direction, tightening the thread between the lower end of the sample holder and the stirring rod. The thread between the upper end of the sample holder and the sample holder fixing cap is loosened, separating the sample holder from the sample holder fixing cap and fixing it to the stirring rod. The sample holder and stirring rod descend simultaneously to begin epitaxial growth. After reaching the final temperature, the sample holder is lifted, and the mother liquor is rapidly cooled.

[0066] The lower end of the sample holder is detachably connected to the lower end of the stirring rod via a first threaded connection structure, and the upper end of the sample holder is detachably connected to the bottom of the sample holder fixing cover via a second threaded connection structure. The threads of the first and second threaded connection structures are opposite in direction. Connecting the sample holder and stirring rod can be achieved simply by rotating the stirring rod in different directions outside the sample holder fixing cover, without needing to open the sample holder fixing cover for any further operations.

[0067] S6: Degassing and cleaning of the graphite boat. After growth, remove the epitaxial wafer. A small amount of mother liquor will condense on the stirring rod and sample holder. Remove the mother liquor using high-temperature degassing. After degassing, thoroughly wipe the surface of the graphite boat with lint-free paper to remove any remaining mother liquor powder and loose carbon.

[0068] The graphite boat and vertical liquid phase epitaxy method provided by this invention, and the liquid phase epitaxial materials prepared by the graphite boat in the prior art without modification, are described in the appendix. Figure 3 As can be seen, the prepared epitaxial material has obvious black carbon material adhering to its surface, leading to a decrease in yield and production efficiency during the production process. Figure 4 The middle figure shows the epitaxial material prepared by the graphite boat of the present invention. The figure shows that the surface of the epitaxial material is clean and free of carbon material adhesion. This demonstrates that the improved graphite boat of the present invention and the epitaxial material obtained by vertical liquid phase epitaxial growth using this graphite boat overcome the defects existing in the prior art, significantly improving the quality and production efficiency of the epitaxial material.

[0069] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A vertical liquid-phase epitaxial graphite boat, characterized in that, include: Sample rack (3), stirring rod (4), stirring base (5), crucible (7); The sample holder (3) and the stirring base (5) are both located in the crucible (7). The lower end of the stirring rod (4) is connected to the stirring base (5). The sample holder (3) and the stirring rod (4) or the sample holder (3) and the stirring base (5) are detachably connected. The sample holder (3) is sleeved on the stirring rod (4). The sample holder (3) includes a quartz inner sleeve (2) and a graphite frame fitted outside the quartz inner sleeve (2). The inner wall of the quartz inner sleeve (2) has a smooth surface, and the quartz inner sleeve (2) and the graphite frame are fixed by threads.

2. The vertical liquid-phase epitaxial graphite boat according to claim 1, characterized in that, The lower end of the sample holder (3) is detachably connected to the lower end of the stirring rod (4).

3. A vertical liquid-phase epitaxial graphite boat according to claim 2, characterized in that, The graphite boat also includes a sample holder fixing cover (1), which is placed over the upper opening of the crucible (7) and is detachably connected to the crucible (7).

4. A vertical liquid-phase epitaxial graphite boat according to claim 3, characterized in that, The top center of the sample holder fixing cover (1) has a hole, the upper end of the stirring rod (4) passes through the hole and is connected to the drive shaft, and the upper end of the sample holder (3) is detachably connected to the bottom of the sample holder fixing cover (1).

5. A vertical liquid-phase epitaxial graphite boat according to claim 4, characterized in that, The lower end of the sample holder (3) is detachably connected to the lower end of the stirring rod (4) through a first threaded connection structure, and the upper end of the sample holder (3) is detachably connected to the bottom of the sample holder fixing cover (1) through a second threaded connection structure. The thread directions of the first threaded connection structure and the second threaded connection structure are opposite.

6. A vertical liquid-phase epitaxial graphite boat according to claim 1, characterized in that, The outer surface of the sample holder (3) is coated with a pyrolytic carbon coating.

7. A vertical liquid-phase epitaxial graphite boat according to claim 1, characterized in that, The bottom surface of the stirring base (5) has a serrated structure.

8. A vertical liquid-phase epitaxial graphite boat according to claim 1, characterized in that, The stirring rod (4) is made of carbon or SiC fiber composite material.

9. A method for vertical liquid phase epitaxy using a graphite boat according to any one of claims 1-8, comprising a mother liquor high-temperature homogenization step and an epitaxial thin film growth step, characterized in that: In the high-temperature homogenization step of the mother liquor, the sample holder (3) is separated from the stirring base (5), the sample holder (3) is located above the mother liquor, and the stirring rod (4) is located in the mother liquor to homogenize and stir the mother liquor; In the epitaxial film growth step, the sample holder (3) is connected to the stirring base (5). Both the sample holder (3) and the stirring base (5) are located in the mother liquor. The sample holder (3) and the stirring base (5) move simultaneously to achieve the growth of the epitaxial film.

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