Crystal growing method

Abstract

The present invention includes: a first step S101 in which a buffer layer comprising a first rare earth oxide is formed on a silicon layer; a second step S102 in which a crystal layer comprising cerium oxide (CeO2) is formed by crystal growth on the buffer layer; and a third step S103 in which the crystal layer formed on the buffer layer is heated in an atmosphere containing oxygen gas under a temperature condition that is higher than the temperature condition under which the crystal growth (second step) of the crystal layer is carried out. For example, the crystal layer formed on the buffer layer is heated under a temperature condition of 850-1050°C.

Description

Crystal growth method

[0001] The present invention relates to a crystal growth method for growing crystals of rare earth oxides.

[0002] Quantum repeaters are essential technology for long-distance quantum optical communication. Quantum repeaters require a single-photon source, an optical quantum memory, and an interface between the source and memory. From the perspective of practical application, research and development is being conducted to integrate each of these elemental technologies on a chip.

[0003] Rare-earth atoms in solid crystals have 4f electron orbitals that are electrically shielded from the outside world, forming ideal energy quantum levels that are discrete and have little fluctuation, regardless of the type of host material. Among these, Er (erbium) is capable of interacting with light in the telecommunications wavelength range (1.55 μm wavelength) and has a long phase relaxation time (a parameter that determines the memory duration). For this reason, Er-doped crystals are promising materials for optical quantum memories, which are necessary to realize quantum repeaters using existing optical fiber networks.

[0004] There are three required characteristics for Er-doped crystals. First, to suppress performance degradation due to interactions between Er atoms, Er must be doped dilutely to increase the distance between Er atoms within the host crystal. Second, to suppress disturbances of the Er quantum level, the doped host crystal must have small magnetic fluctuations. Third, to achieve high device performance, it is desirable to utilize optical circuit fabrication techniques (optical resonators, waveguides, multiplexing / demultiplexing devices, etc.) developed in Si (silicon) photonics, and the doped host crystal can be formed as a thin film on a Si substrate (enabling on-chip optical quantum memory).

[0005] Cerium oxide (CeO2) is an example of an Er-doped host crystal that satisfies the above conditions. First, because Er and Ce are both rare earth elements, their ionic radii are similar, allowing Er to be diluted without generating unnecessary defects when Er is doped. Second, because Ce does not have nuclear spin, magnetic fluctuations are small, resulting in little effect on the quantum state of Er (the natural abundance of oxygen isotopes with nuclear spin is very low). Third, the lattice constant of CeO2 (111) is about twice that of Si (111), making epitaxial growth possible.

[0006] T. Inaba et al., "Epitaxial growth and optical properties of Er-doped CeO2 on Si(111)", Optical Materials Express, vol. 8, no. 9, pp. 2843-2849, 2018.

[0007] As mentioned above, Er:CeO is an ideal material for on-chip optical quantum memory. However, the current Er:CeO has the problem that its luminescence characteristics are inferior to those of Er-doped bulk crystals such as Er-doped YSiO (Er:YSO). For example, the non-uniformity width of Er:YSO is approximately 1 GHz (Non-Patent Document 1), whereas the non-uniformity width of Er:CeO is as large as approximately 50 GHz (Non-Patent Document 2).

[0008] The present invention has been made to solve the above problems, and has as its object to improve the luminescence characteristics of CeO2 doped with Er.

[0009] The crystal growth method according to the present invention comprises a first step of growing, on a silicon layer, a buffer layer made of a rare earth oxide, which is less likely than cerium oxide to form an amorphous layer at the interface with the silicon layer when the buffer layer is grown on the silicon layer and made of single crystal silicon with its main surface being a (111) plane; a second step of growing, on the buffer layer, a crystal layer made of cerium oxide; and a third step of heating the crystal layer formed on the buffer layer in an atmosphere containing oxygen gas at a temperature higher than that in the second step.

[0010] As described above, according to the present invention, after growing a crystal layer made of cerium oxide, the crystal layer is heated at a temperature higher than the temperature conditions for crystal growth, thereby improving the luminescence characteristics of CeO2 doped with Er.

[0011] FIG. 1 is a flowchart illustrating a crystal growth method according to an embodiment of the present invention. FIG. 2 is a graph showing the dependence of the XRDw scan half-width of the Er:CeO2 (444) plane on the heat treatment temperature before and after heat treatment. FIG. 3 is a graph showing the XRD2q-w scan of the Er:CeO2 (222) plane of a sample before heat treatment (dashed line) and a sample heat-treated at 950°C (solid line). FIG. 4 is a graph showing the PLE spectra of the sample before heat treatment (dashed line) and a sample heat-treated at 950°C (solid line), plotting the peak PL intensity of Er (wavelength = 1530.1 nm) while sweeping the excitation wavelength.

[0012] A crystal growth method according to an embodiment of the present invention will be described below with reference to FIG. 1 . First, in step S101, a buffer layer made of a first rare earth oxide is formed on a silicon layer. The silicon layer is made of single-crystal silicon and has a (111) plane as its main surface. The silicon layer can be, for example, a silicon substrate. The silicon layer can also be the surface silicon layer of a well-known SOI (Silicon on Insulator) substrate. When using a crystal layer (described later) to form an optical waveguide, it is effective to use an SOI substrate with a refractive index distribution in the stacking direction. The buried insulating layer of the SOI substrate can be used as a cladding layer.

[0013] The buffer layer is made of a rare earth oxide that is less likely to form an amorphous layer at the interface with the silicon layer than cerium oxide when the buffer layer is grown on top of the silicon layer. This rare earth oxide can be gadolinium oxide (Gd2O3). Gd2O3, which is the same rare earth oxide as CeO2, is not as chemically active as CeO2, so it is less likely to form an amorphous layer at the interface with the silicon layer.

[0014] Furthermore, it is believed that similar effects can be expected by using other rare earth oxides, excluding La2O3 and Pr2O3, which are prone to oxygen defects, as the buffer layer material instead of Gd2O3. For example, erbium oxide (Er2O3; see Reference 1) and europium oxide (EuO; see Reference 2) are promising buffer layer materials, based on previous research showing that no interfacial amorphous layer is formed when grown on a Si layer.

[0015] Next, in the second step S102, a crystal layer made of cerium oxide (CeO2) is grown on the buffer layer. Alternatively, if the crystal layer is an Er:CeO2 layer doped with erbium, an undoped layer can be grown on the buffer layer before the crystal layer is grown on the buffer layer. The undoped layer is a CeO2 crystal layer not doped with erbium. After the undoped layer is grown, a crystal layer (Er:CeO2) can be grown on the undoped layer (second step).

[0016] Next, in the third step S103, the crystal layer formed on the buffer layer is heated in an atmosphere containing oxygen gas at a temperature higher than the temperature used in the crystal growth of the crystal layer (the second step). For example, the crystal layer formed on the buffer layer can be heated at a temperature of 850 to 1050°C. The crystal layer can also be heated in an atmosphere containing only oxygen gas. The crystal layer can also be heated in an atmosphere containing a mixed gas of oxygen gas and an inert gas such as nitrogen gas or helium gas.

[0017] According to the above-described embodiment, the third step can improve the crystallinity of the crystal layer to a higher quality, and as a result, the light-emitting properties of CeO2 doped with Er can be improved.

[0018] As mentioned above, the luminescence characteristics of conventional Er:CeO2 are inferior to those of Er-doped bulk crystals such as Er-doped YSiO5 (Er:YSO). This is partly due to differences in the quality of the Er-doped host crystal. Generally, when determining the growth conditions for rare-earth-doped compounds, parameters are adjusted to optimize the luminescence characteristics of the rare-earth ions, but this growth condition does not necessarily result in the best quality doped host crystal. In other words, it is believed that by improving the crystallinity of CeO2, the Er-doped host, it is possible to bring out the excellent optical properties inherent in Er:CeO2.

[0019] According to the embodiment, the crystallinity of CeO2, which is the base material for adding Er, can be improved, so that the excellent optical properties inherent to Er:CeO2 can be brought out, and the luminescence properties of CeO2 doped with Er can be improved.

[0020] The characteristics of the crystal layer that was actually fabricated are described below. First, a 5-nm-thick buffer layer made of GdO was formed on an SOI substrate. Then, an 85-nm-thick undoped layer (CeO) was formed on this buffer layer. Then, a crystal layer (Er:CeO) was formed on this undoped layer to form a laminated structure. The nonuniform width of the crystal layer in this laminated structure before heat treatment was approximately 10 GHz.

[0021] Next, the fabricated stacked structure was divided into chip-shaped samples using a dicing machine. The samples were then heat-treated in oxygen gas for 30 minutes at temperatures of 850, 950, and 1050°C. These temperature conditions were higher than the temperature (750°C) used during crystal growth of the crystalline layer (Er:CeO).

[0022] Figure 2 shows the heat treatment temperature dependence of the XRD w scan half-width of the Er:CeO (444) plane before and after heat treatment. The width was 868 arcsec before heat treatment (as grown), but improved to 611 arcsec at a heat treatment temperature of 850°C and 569 arcsec at a heat treatment temperature of 950°C, confirming the homogenization of the CeO crystals. Therefore, it is considered more preferable to perform the heat treatment in the third step at a temperature of 850°C or higher.

[0023] On the other hand, the sample at 1050°C showed a small value of 574 arcsec, but small cracks occurred in parts of the periphery of the sample piece (wafer) due to the difference in thermal expansion coefficients between Si and CeO2 (References 3 and 4). Small cracks of this magnitude are not considered to pose a major problem, but heat treatment at temperatures above 1050°C is likely to result in larger cracks, which may cause problems during device fabrication. Therefore, it is considered more preferable to perform the heat treatment in the third step at a temperature not exceeding 1050°C, with 1050°C as the upper limit.

[0024] Figure 3 shows the XRD2q-w scans of the Er:CeO2 (222) plane of a sample before heat treatment and a sample heat-treated at 950°C. In Figure 3, the dashed line indicates the data before heat treatment, and the solid line indicates the data after heat treatment. The heat treatment reduced the half-width of the CeO2 (222) peak, confirming an improvement in the lattice constant fluctuation of CeO2. Furthermore, the X-ray diffraction intensity of the CeO2 (222) peak also increased, demonstrating that heat treatment can improve the quality of CeO2.

[0025] Figure 4 shows the PLE spectrum, plotting the peak PL intensity of Er (wavelength = 1530.1 nm) while sweeping the excitation wavelength. In Figure 4, the dashed line indicates the intensity before heat treatment, and the solid line indicates the intensity after heat treatment. Heat treatment at 950 °C increased the peak PL intensity by 1.4 times, and the Er inhomogeneity width, estimated from the half-width of the PLE spectrum, decreased from 9.69 GHz to 7.03 GHz. This is superior to the inhomogeneity width of approximately 43.5 GHz for Er:YO / Si(111) of similar thickness reported in a previous study (Reference 5), suggesting that Er:CeO is a promising material for photonic quantum memories.

[0026] In addition, the peak wavelength in FIG. 4 was red-shifted. This is because the lattice constant of CeO2 was changed by the heat treatment, as can be seen from FIG. 3+ This is thought to be due to a change in the local structure around the ions.

[0027] From the above results, it was confirmed that by performing heat treatment after Er:CeO2 crystal growth at a temperature higher than the temperature conditions during Er:CeO2 crystal growth, the quality of the CeO2 crystal is improved and the Er luminescence characteristics are improved.

[0028] As described above, according to the present invention, after growing a crystalline layer made of cerium oxide, the crystalline layer is heated at a temperature higher than the temperature condition for the crystal growth, thereby improving the luminescence characteristics of CeO doped with Er. The present invention further improves the quality of CeO, making it possible to realize an on-chip optical quantum memory using Er:CeO.

[0029] Some or all of the above-described embodiments may also be described as, but are not limited to, the following supplementary notes.

[0030] [Supplementary Note 1] A crystal growth method comprising: a first step of growing, on a silicon layer made of single-crystal silicon and having a main surface in a (111) plane, a buffer layer made of a rare-earth oxide that is less likely to form an amorphous layer at an interface with the silicon layer than cerium oxide when crystal is grown on the silicon layer; a second step of growing, on the buffer layer, a crystal layer made of cerium oxide; and a third step of heating the crystal layer formed on the buffer layer in an atmosphere containing oxygen gas at a temperature higher than that in the second step.

[0031] [Supplementary Note 2] In the crystal growth method according to Supplementary Note 1, the crystal layer is a layer doped with erbium, and the second step includes a step of growing an undoped layer made of cerium oxide not doped with erbium on the buffer layer before growing the crystal layer on the buffer layer, and the second step is a crystal growth method in which the crystal layer is grown on the undoped layer.

[0032] [Supplementary Note 3] The crystal growth method according to Supplementary Note 1 or 2, wherein the rare earth oxide is any one of europium oxide, gadolinium oxide, and erbium oxide.

[0033] [Appendix 4] The crystal growth method according to appendix 3, wherein the rare earth oxide is gadolinium oxide.

[0034] [Supplementary Note 5] The crystal growth method according to any one of Supplementary Notes 1 to 4, wherein the third step heats the crystal layer at a temperature of 850 to 1050°C.

[0035] 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.

[0036] [References] [References 1] T. Tawara et al., "Population dynamics in epitaxial Er2O3 thin films grown on Si(111)", Applied Physics Letters, vol. 102, 241918, 2013. [References 2] R. Ohsugi et al., "Spin splitting in EuO(111) / Si(111) spin-filter tunnel junctions with 57, 110304, 2018. [Reference 3] 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. [Reference 4] 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. [Reference 5] MK Singh et al., "Epitaxial Er-doped Y2O3 on silicon for quantum coherent devices", APL Materials, vol. 8, 031111, 2020.

Claims

1. A crystal growth method comprising: a first step of growing a buffer layer made of a rare earth oxide on a silicon layer, the buffer layer being less likely than cerium oxide to form an amorphous layer at the interface with the silicon layer when the buffer layer is grown on the silicon layer and made of single crystal silicon with a main surface having a (111) plane; a second step of growing a crystal layer made of cerium oxide on the buffer layer; and a third step of heating the crystal layer formed on the buffer layer in an atmosphere containing oxygen gas at a temperature higher than that of the second step.

2. A crystal growth method according to claim 1, wherein the crystal layer is a layer doped with erbium, and the second step includes a step of growing an undoped layer made of cerium oxide not doped with erbium on the buffer layer before growing the crystal layer on the buffer layer, and the second step is a crystal growth method in which the crystal layer is grown on the undoped layer.

3. A crystal growth method according to claim 1, wherein the rare earth oxide is any one of europium oxide, gadolinium oxide, and erbium oxide.

4. A method for growing crystals according to claim 3, wherein said rare earth oxide is gadolinium oxide.

5. A crystal growth method according to any one of claims 1 to 4, wherein the third step heats the crystal layer at a temperature of 850 to 1050°C.

Images