Crystal growing method

Abstract

A columnar Si pattern (102) is formed on a substrate (111) by patterning an Si layer (101), and a rare earth oxide layer (103) composed of a rare earth oxide is grown in crystal on the Si pattern (102). A buffer layer (104) configured from a rare earth oxide different from that of the rare earth oxide layer (103) is grown in crystal on the Si pattern (102), and a rare earth oxide layer (103) is grown in crystal on the buffer layer (104). Furthermore, before an erbium-added rare earth oxide layer (103) is grown in crystal on the buffer layer (104), a no-additive layer (105) configured from only the rare earth oxide constituting the rare earth oxide layer (103) is grown in crystal on the buffer layer (104), and then the rare earth oxide layer (103) is grown in crystal on the no-additive layer (105).

Description

Crystal growth method

[0001] The present invention relates to a crystal growth method.

[0002] In recent years, active research has been conducted into Si photonics, which realizes high-performance chips by integrating optical elements such as lasers, detectors, and modulators on a Si layer made of a Si substrate. This technology is expected to be applied to various fields, including AI, biosensing, LiDAR, and quantum optics. High-performance detectors and modulators have been realized using Si as a material, but because Si is an indirect transition semiconductor, it is not possible to fabricate highly efficient light-emitting elements using Si. Therefore, light-emitting elements that operate on a Si layer are required. III-V semiconductors and SiGe have been researched as materials for this purpose, but their large lattice mismatch with Si requires costly processing processes such as wafer bonding.

[0003] On the other hand, many rare earth oxides have a lattice match with the Si(111) surface, allowing crystal growth on a Si(111) substrate, eliminating the need for wafer bonding and reducing manufacturing costs. Furthermore, by adding rare earth ions as luminescent centers, it is possible to fabricate light-emitting devices that operate in a variety of wavelength ranges.

[0004] S. Sameshima, et al., "Thermal Expansion of Rare-Earth-Doped Ceria Ceramics", Journal of the Ceramic Society of Japan, vol. 110, no. 7, pp. 597-600, 2002.H. Watanabe et al., "Linear Thermal Expansion Coefficient of Silicon from 293 to 1000 K", International Journal of Thermophysics, vol. 25, no. 1, pp. 221-236, 2004.S. Stecura and WJ Campbell, "THERMAL EXPANSION AND PHASE INVERSION OF RARE-EARTH OXIDES", Technical Report, 1960, [Retrieved August 15, 2020], (https: / / doi.org / 10.2172 / 4840970).

[0005] However, since the thermal expansion coefficients of Si and rare earth oxides are significantly different, cracks occur when the critical thickness is exceeded, resulting in a problem of deterioration of the light-emitting characteristics. For example, the lattice constant difference between CeO2 and Si(111) is small at 0.36%. On the other hand, the thermal expansion coefficient of CeO2 at the crystal growth temperature (730°C) is 12.8[10 -6 / K] (Non-Patent Document 1), Si is 4.4 [10 -6 / K] (Non-Patent Document 2). Therefore, when the thickness of the rare earth oxide layer formed on the Si layer exceeds 180 nm, cracks are observed.

[0006] If the critical thickness of the rare-earth oxide layer on the Si layer can be increased, the intensity of light emitted from the rare-earth ions can be increased, enabling the device to be used as a light-emitting device for Si photonics. Furthermore, thick oxides on Si layers are also in demand as materials for the BOX layer when fabricating SOI (Silicon-On-Insulator) substrates by crystal growth. Therefore, how to increase the critical thickness of the rare-earth oxide layer formed on the Si layer has been an important issue.

[0007] The present invention has been made to solve the above problems, and has as its object to form a thicker layer of rare earth oxide on a Si layer without generating cracks or the like.

[0008] The crystal growth method according to the present invention comprises a first step of patterning a Si layer made of single-crystal silicon formed on a substrate and having a (111) plane as its main surface to form a columnar Si pattern on the substrate, and a second step of crystal-growing a rare-earth oxide layer made of a rare-earth oxide on the Si pattern.

[0009] As described above, according to the present invention, a rare earth oxide layer is grown by crystal growth on a columnar Si pattern made of single crystal silicon and having a (111) plane as its main surface. This makes it possible to form a thicker rare earth oxide layer on the Si layer without generating cracks or the like.

[0010] FIG. 1A is a cross-sectional view showing the state of a Si layer in an intermediate process to explain a crystal growth method according to an embodiment of the present invention. FIG. 1B is a cross-sectional view showing the state of a Si pattern in an intermediate process to explain a crystal growth method according to an embodiment of the present invention. FIG. 1C is a cross-sectional view showing the state of a rare earth oxide layer grown by a crystal growth method according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing the configuration of a comparative sample prepared for comparison. FIG. 3A is a micrograph of the comparative sample. FIG. 3B is a micrograph of a sample prepared by a crystal growth method according to an embodiment. FIG. 3C is a micrograph of a sample prepared by a crystal growth method according to an embodiment. FIG. 4 is a characteristic diagram showing the relationship between Si pattern size (mesa size) and crack density.

[0011] 1A to 1C, a crystal growth method according to an embodiment of the present invention will be described below. This crystal growth method is a method for growing a crystal of a rare earth oxide layer.

[0012] 1A, a substrate is prepared that is made of single-crystal silicon and has a Si layer 101 whose main surface is a (111) plane. The substrate is, for example, an SOI (Silicon on Insulator) substrate, and has a buried oxide layer 112 on the Si substrate 111, and a Si layer 101 as a surface Si layer on the buried oxide layer 112.

[0013] Next, the Si layer 101 is patterned to form columnar Si patterns 102 on the substrate 111 as shown in FIG. 1B (first step). The Si patterns 102 have, for example, a rectangular shape on a surface parallel to the substrate plane. Alternatively, multiple stripe-shaped Si patterns 102 can be formed on the substrate 111. While forming the Si patterns 102, the buried oxide layer 112 can also be patterned to form support patterns 112a. In this example, the Si patterns 102 are formed on the substrate 111 and supported by the support patterns 112a.

[0014] 1C , a rare earth oxide layer 103 made of a rare earth oxide is crystal-grown on the Si pattern 102 (step 2). In this example, a buffer layer 104 made of a different rare earth oxide from the rare earth oxide layer 103 is crystal-grown on the Si pattern 102, and the rare earth oxide layer 103 is crystal-grown on the buffer layer 104. Furthermore, in this example, the rare earth oxide layer 103 is doped with erbium, and before the rare earth oxide layer 103 is crystal-grown on the buffer layer 104, an undoped layer 105 made only of the rare earth oxide that constitutes the rare earth oxide layer 103 is crystal-grown on the buffer layer 104, and then the rare earth oxide layer 103 is crystal-grown on the undoped layer 105.

[0015] The buffer layer 104 can be made of a rare earth oxide that is less likely to form an amorphous layer at the interface with the Si pattern 102 than the second rare earth oxide when the buffer layer 104 is grown on the Si layer 101. For example, the rare earth oxide constituting the buffer layer 104 can be any one of europium oxide, gadolinium oxide, and erbium oxide. For example, the buffer layer 104 can be made of gadolinium oxide.

[0016] The rare earth oxide layer 103 can be made of any of the rare earth oxides, for example, lanthanum oxide, cerium oxide, and praseodymium oxide. The rare earth oxide layer 103 can be made of cerium oxide.

[0017] The results of actually preparing samples will be described below.

[0018] First, samples were fabricated in which the Si pattern 102 had a square pattern shape with sides of 50, 100, 150, 200, or 300 mm parallel to the substrate plane. Additionally, samples were fabricated in which the Si pattern 102 had a rectangular pattern shape with short sides of 50 μm and long sides of 15 mm parallel to the substrate plane. The rectangular pattern shape can be used to fabricate a waveguide device.

[0019] The rare earth oxide layer 103 was made of erbium-doped cerium oxide (Er:CeO), the buffer layer 104 was made of gadolinium oxide (GdO) to suppress the interface reaction between Si and cerium oxide, and the non-doped layer 105 was made of CeO to suppress the diffusion of impurities such as Si and Gd from the Si pattern 102 and the buffer layer.

[0020] In sample group 1, the buffer layer 104 was formed to a thickness of 5 nm, the undoped layer 105 was formed to a thickness of 90 nm, and the rare earth oxide layer 103 was formed to a thickness of 145 nm (total thickness: 240 nm).In sample group 2, the buffer layer 104 was formed to a thickness of 5 nm, the undoped layer 105 was formed to a thickness of 90 nm, and the rare earth oxide layer 103 was formed to a thickness of 405 nm (total thickness: 500 nm).

[0021] 2, a comparative sample was fabricated by growing a GdO layer 121, a GdO layer 123, and an Er:CeO layer 122 on the Si layer 101 of an SOI substrate including a substrate 111, a buried oxide layer 112, and a Si layer 101. The GdO layer 121 corresponds to the buffer layer 104, the GdO layer 123 corresponds to the undoped layer 105, and the Er:CeO layer 122 corresponds to the rare earth oxide layer 103. In the comparative sample, the planar shape of the Si layer 101 is the same as that of the SOI substrate, and can be, for example, a circle with a diameter of 6 inches.

[0022] The Gd2O3 layer 121 was formed to a thickness of 5 nm, the Gd2O3 layer 123 was formed to a thickness of 90 nm, and the Er:CeO2 layer 122 was formed to a thickness of 145 nm.

[0023] As shown in the microscope image of Fig. 3A, numerous cracks (indicated by arrows) occurred on the surface of the comparative sample. These cracks are believed to have been generated by residual strain resulting from the difference in thermal expansion coefficient between the SOI substrate (composed of substrate 111, buried oxide layer 112, and Si layer 101) and CeO2.

[0024] On the other hand, in sample group 1 produced by the crystal growth method according to the embodiment, no cracks were observed regardless of the size of the pattern shape, as shown in Figures 3B and 3C, and crack occurrence was completely suppressed.

[0025] In sample group 2, cracks were observed in some areas, but a crack-free rare earth oxide layer was obtained even in a rectangular pattern shape.

[0026] Figure 4 shows the dependence of crack density on the size of the pattern shape when the thickness of each layer is set to sample group 2 (total thickness 500 nm). The smaller the Si pattern, the lower the crack density, confirming that growing a rare earth oxide layer on a columnar Si pattern is effective in suppressing cracks. Because CeO2 and other rare earth oxides have similar crystal structures and thermal expansion coefficients, it is believed that similar results will be obtained even when the rare earth oxide layer is made of a rare earth oxide other than CeO2.

[0027] As described above, according to the embodiment of the present invention, a rare earth oxide layer is grown by crystal growth on a columnar Si pattern made of single crystal silicon and having a (111) plane as its main surface. This makes it possible to form a thicker rare earth oxide layer on the Si layer without generating cracks or the like.

[0028] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0029] 101...Si layer, 102...Si pattern, 103...rare earth oxide layer, 104...buffer layer, 105...non-doped layer, 111...substrate, 112...buried oxide layer, 112a...support pattern.

Claims

1. A crystal growth method comprising: a first step of patterning a Si layer made of single crystal silicon formed on a substrate, the Si layer having a (111) plane as its main surface, to form a columnar Si pattern on the substrate; and a second step of crystal-growing a rare earth oxide layer made of a rare earth oxide on the Si pattern.

2. A crystal growth method according to claim 1, wherein the second step comprises growing a buffer layer made of a rare earth oxide different from the rare earth oxide layer on the Si pattern, and then growing the rare earth oxide layer on the buffer layer.

3. A crystal growth method according to claim 2, wherein the rare earth oxide layer is doped with erbium, and the second step comprises, before growing the rare earth oxide layer on the buffer layer, growing a non-doped layer composed only of rare earth oxides that make up the rare earth oxide layer on the buffer layer, and then growing the rare earth oxide layer on the non-doped layer.

4. A crystal growth method according to claim 2 or 3, wherein the buffer layer is made of a rare earth oxide that is less likely to form an amorphous layer at the interface with the Si pattern than the rare earth oxide layer when the buffer layer is grown on top of the Si layer.

5. A crystal growth method according to claim 4, wherein the buffer layer is made of a rare earth oxide selected from the group consisting of europium oxide, gadolinium oxide, and erbium oxide, and the rare earth oxide layer is made of a rare earth oxide selected from the group consisting of lanthanum oxide, cerium oxide, and praseodymium oxide.

6. A crystal growth method according to claim 5, wherein the buffer layer is made of gadolinium oxide, and the rare earth oxide layer is made of cerium oxide.

Images