Polarized light detector and method of manufacturing the same

Abstract

The application provides a polarized light detector and a preparation method thereof, and relates to the technical field of semiconductors, wherein the polarized light detector comprises a substrate, a gallium oxide layer located on the substrate, a cathode electrode and a nickel oxide layer located on the gallium oxide layer, an anode electrode located on the nickel oxide layer, and a nano photosensitive structure formed on the gallium oxide layer; wherein the cathode electrode is a first ring structure, the nickel oxide layer is a second ring structure, the anode electrode is a third ring structure, the first ring structure, the second ring structure and the third ring structure are concentrically arranged, the second ring structure and the third ring structure are located in a first containing space formed by the first ring structure, and the nano photosensitive structure is located in a second containing space formed by the second ring structure and the third ring structure. The application at least realizes the function of polarization detection.

Description

Technical Field

[0001] This invention relates to semiconductor technology, and more particularly to a polarization detector and its fabrication method. Background Technology

[0002] Because the ozone layer completely absorbs ultraviolet radiation in the 200–280 nm wavelength range, this band of ultraviolet radiation is virtually non-existent in the atmosphere, hence it is known as the "solar-blind" band. Ultraviolet detectors developed based on this band, with their significant advantages of low background noise and low false alarm rate, have broad application prospects in ultraviolet guidance, ultraviolet space early warning, high-response fire early warning, corona detection, and atmospheric environmental monitoring, thus becoming a research hotspot and attracting widespread attention.

[0003] β-Ga₂O₃, with its monoclinic crystal system and high-temperature chemical stability, possesses a very suitable bandgap (4.5-5.1 eV), making it a promising candidate material for direct polarization detection in the solar-blind ultraviolet band, as it is expected to cover the entire solar-blind band. Furthermore, thanks to the continuous maturation of single-crystal gallium oxide fabrication technology, large-size single-crystal β-Ga₂O₃ can now be obtained, as well as high-quality epitaxial β-Ga₂O₃ layers can be obtained through doping techniques and bandgap engineering. This has facilitated its rapid research and application in ultraviolet photodetector applications.

[0004] In recent years, although significant progress has been made in the research of β-Ga2O3 photodetectors, the development of high-performance polarization solar-blind ultraviolet detectors based on gallium oxide materials still requires solving the key problem of their small anisotropic photocurrent for linearly polarized light.

[0005] Therefore, there is an urgent need to provide a polarization detector capable of polarization detection and its fabrication method. Summary of the Invention

[0006] In view of the above problems, the present invention is proposed to provide a polarization detector and its preparation method that overcome or at least partially solve the above problems.

[0007] According to one aspect of the present invention, a polarization detector is provided, characterized in that it comprises:

[0008] Substrate;

[0009] A gallium oxide layer located on the substrate;

[0010] A cathode electrode and a nickel oxide layer located on top of the gallium oxide layer;

[0011] An anode electrode located above the nickel oxide layer; and

[0012] Nanoscale photosensitive structures formed in the gallium oxide layer;

[0013] The cathode electrode has a first annular structure, the nickel oxide layer has a second annular structure, and the anode electrode has a third annular structure. The first annular structure, the second annular structure, and the third annular structure are concentrically arranged. The second annular structure and the third annular structure are located in a first accommodating space formed by the first annular structure, and the nano-photosensitive structure is located in a second accommodating space formed by the second annular structure and the third annular structure.

[0014] Optionally, in the polarization detector according to the invention, the substrate is made of one of gallium oxide, gallium nitride, and silicon carbide.

[0015] Optionally, in the polarization detector according to the present invention, the gallium oxide layer is made of Si-doped N-type Ga2O3 with a first doping concentration of 2 × 10⁻⁶. 17 cm -3 .

[0016] Optionally, in the polarization detector according to the present invention, the nickel oxide layer is made of Li-doped P-type NiO with a corresponding second doping concentration, wherein the second doping concentration is 1×10⁻⁶. 18 cm -3 .

[0017] Optionally, in the polarization detector according to the present invention, the nanophotosensitive structure includes multiple nanowire gratings formed in the gallium oxide layer.

[0018] According to another aspect of the present invention, a method for fabricating the aforementioned polarization detector is provided, comprising the following steps:

[0019] A gallium oxide layer is formed on the substrate;

[0020] A first patterning process is performed on the side of the gallium oxide layer away from the substrate to form a nano-photosensitive structure located in the gallium oxide layer;

[0021] A second patterning process is performed on the side of the gallium oxide layer away from the substrate to form a cathode electrode with a first ring structure located on the gallium oxide layer;

[0022] A third patterning process is performed on the side of the gallium oxide layer away from the substrate to form a nickel oxide layer with a second ring structure located on the gallium oxide layer;

[0023] A fourth patterning process is performed on the side of the nickel oxide layer away from the substrate to form an anode electrode with a third ring structure located on the nickel oxide layer. The first ring structure, the second ring structure, and the third ring structure are concentrically arranged. The second ring structure and the third ring structure are located in a first accommodating space formed by the first ring structure. The nano-photosensitive structure is located in a second accommodating space formed by the second ring structure and the third ring structure.

[0024] Optionally, in the method according to the invention, a first patterning process is performed on the side of the gallium oxide layer away from the substrate to form a nanoscale photosensitive structure located in the gallium oxide layer, including:

[0025] A first photoresist layer is formed on the side of the gallium oxide layer away from the substrate, and the first photoresist layer is subjected to a first patterning process to form a first photolithographic pattern;

[0026] Based on the first photolithography pattern, the gallium oxide layer is etched to form a nanoscale photosensitive structure located on the gallium oxide layer.

[0027] Optionally, in the method according to the invention, a second patterning process is performed on the side of the gallium oxide layer away from the substrate to form a cathode electrode with a first annular structure located on the gallium oxide layer, including:

[0028] A second photoresist layer is formed on the side of the gallium oxide layer away from the substrate, and the second photoresist layer is subjected to a second patterning process to form a second photolithographic pattern;

[0029] Based on the second photolithographic pattern, a corresponding titanium deposition operation is performed on the gallium oxide layer to form a cathode electrode with a first ring structure located on the gallium oxide layer.

[0030] Optionally, in the method according to the invention, a third patterning process is performed on the side of the gallium oxide layer away from the substrate to form a nickel oxide layer with a second annular structure located on the gallium oxide layer, including:

[0031] A third photoresist layer is formed on the side of the gallium oxide layer away from the substrate, and the third photoresist layer is subjected to a third patterning process to form a third photolithographic pattern;

[0032] Based on the third photolithography pattern, a corresponding nickel deposition operation is performed on the gallium oxide layer to form a nickel oxide layer with a second ring structure on the gallium oxide layer.

[0033] Optionally, in the method according to the invention, a fourth patterning process is performed on the side of the nickel oxide layer away from the substrate to form an anode electrode with a third annular structure located on the nickel oxide layer, including:

[0034] A fourth photoresist layer is formed on the side of the nickel oxide layer away from the substrate, and the fourth photoresist layer is subjected to a fourth patterning process to form a fourth photolithographic pattern.

[0035] Based on the fourth photolithographic pattern, a corresponding gold deposition operation is performed on the nickel oxide layer to form an anode electrode with a third ring structure located on the nickel oxide layer.

[0036] According to the present invention, the crystal plane of the β-phase gallium oxide monoclinic crystal system has a high degree of anisotropy, and the crystal plane of this crystal system is selected as the core material of the polarization detection photosensitive area; at the same time, the gallium oxide material in the photosensitive area is etched into multiple parallel nanowire gratings, and the absorption of polarized light in the short axis direction of gallium oxide nanowires is suppressed by means of size confinement effect, thereby improving the anisotropic photocurrent of the solar-blind ultraviolet polarization detector, and finally realizing the efficient polarization detection function. Attached Figure Description

[0037] Figure 1 A schematic diagram of a polarization detector according to an embodiment of the present invention is shown;

[0038] Figure 2 A flowchart illustrating a method for fabricating the polarization detector mentioned in the foregoing embodiments, according to another embodiment of the present invention, is shown. Detailed Implementation

[0039] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0040] Figure 1 A schematic diagram of a polarization detector according to an embodiment of the present invention is shown, as follows: Figure 1 As shown, the polarization detector proposed in this embodiment specifically consists of the following structure:

[0041] Substrate, gallium oxide layer, cathode electrode, nickel oxide layer, anode electrode, and nanophotosensitive structure.

[0042] It can be explained that the gallium oxide layer is located on the substrate, the cathode electrode and the nickel oxide layer are located on the gallium oxide layer, and the anode electrode is located on the nickel oxide layer. Based on... Figure 1As can be seen from the content, the cathode electrode is a first ring structure, the nickel oxide layer is a second ring structure, and the anode electrode is a third ring structure. The first ring structure, the second ring structure, and the third ring structure are all concentrically arranged. Furthermore, the second ring structure and the third ring structure are located in the first accommodating space formed by the first ring structure, while the corresponding nano-photosensitive structure is located on the gallium oxide layer and is located in the second accommodating space formed by the second ring structure and the third ring structure.

[0043] Here, the substrate is made of one of gallium oxide, gallium nitride, or silicon carbide. The choice of substrate is to ensure ideal device performance when fabricating gallium oxide ultraviolet detectors with integrated metasurface nanophotosensitive structures, including but not limited to good photoelectric conversion efficiency, high crystal quality, reliable device stability, and acceptable manufacturing cost. Each substrate material has its unique advantages, and the choice can be made based on a trade-off between the actual application scenario and the maturity of the technology.

[0044] Furthermore, the gallium oxide layer is made of Si-doped N-type Ga2O3 material with a first doping concentration of 2 × 10⁻⁶. 17 cm -3 .

[0045] For example, a gallium oxide layer, located on the upper surface of the substrate, is Si-doped N-type β-Ga₂O₃, and the first doping concentration can be 2 × 10⁻⁶. 17 cm -3 Preferably, the thickness of the gallium oxide layer can be 200nm~300nm.

[0046] Furthermore, the nickel oxide layer is made of Li-doped P-type NiO with a corresponding second doping concentration, wherein the second doping concentration is 1×10⁻⁶. 18 cm -3 .

[0047] For example, in this embodiment, the nickel oxide layer is Li-doped p-type NiO, and the second doping concentration is 1×10⁻⁶. 18 cm -3 It is located in the gallium oxide layer, forming a heterojunction.

[0048] Furthermore, the nanophotosensitive structure includes multiple nanowire gratings formed on the gallium oxide layer.

[0049] For example, in this embodiment, the nanophotosensitive structure can be either platinum or aluminum, with a length of 1-3 μm, a width of 200-300 nm, and a thickness of 700-900 nm. The nanophotosensitive structure may include multiple nanowire gratings, each with its major axis oriented as follows: <100> Crystal orientation or <001> Crystal orientation, short axis direction is <010> The crystal orientation is such that each nanowire grating has a length of 1~3 μm, a width of 100~200 nm, and a depth of 600~800 nm, and each nanowire grating has an aspect ratio of less than 50%, and the spacing between adjacent nanowire gratings is greater than the width of the nanowire grating.

[0050] based on Figure 1 As can be seen from the content, the cathode electrode is located in a ring shape on the upper surface of the gallium oxide layer, and the anode electrode is located on the upper surface of the nickel oxide layer. The cathode electrode and the gallium oxide layer form an ohmic contact, and the anode electrode and the nickel oxide layer form an ohmic contact. The gallium oxide layer and the nickel oxide layer are fully covered by the space charge region formed by the PN junction. Preferably, both the cathode electrode and the anode electrode include a bottom contact metal and a top metal, and the bottom contact metal forms an ohmic contact with the gallium oxide layer. The bottom contact metal can be made of titanium, nickel, or palladium, and the top metal of the anode electrode is gold.

[0051] Figure 2 A flowchart illustrating a fabrication method for the polarization detector mentioned in the foregoing embodiments, as proposed in another embodiment of the present invention, is shown below. Figure 2 As shown, the preparation method includes the following steps:

[0052] S1. A gallium oxide layer is formed on the substrate;

[0053] S2. Perform a first patterning process on the side of the gallium oxide layer away from the substrate to form a nano-photosensitive structure located on the gallium oxide layer;

[0054] S3. Perform a second patterning process on the side of the gallium oxide layer away from the substrate to form a cathode electrode with a first ring structure located on the gallium oxide layer;

[0055] S4. Perform a third patterning process on the side of the gallium oxide layer away from the substrate to form a nickel oxide layer with a second ring structure located on the gallium oxide layer;

[0056] S5. A fourth patterning process is performed on the side of the nickel oxide layer away from the substrate to form an anode electrode with a third ring structure located on the nickel oxide layer, wherein the first ring structure, the second ring structure, and the third ring structure are concentrically arranged, the second ring structure and the third ring structure are located in the first accommodating space formed by the first ring structure, and the nano-photosensitive structure is located in the second accommodating space formed by the second ring structure and the third ring structure.

[0057] The following is a detailed introduction to S1-S5 respectively:

[0058] For S1, the substrate material can be selected from gallium oxide, gallium nitride, and silicon carbide. Under preferred conditions, the lattice matching, thermal expansion coefficient, optical transmittance, and convenience of subsequent epitaxial growth with gallium oxide should be considered. After the substrate is selected, the selected substrate can be pretreated by cleaning, deoxidation, planarization, etc. Hydride vapor phase epitaxy (HVPE) or metal-organic chemical vapor deposition (MOCVD) process to epitaxially grow a 200nm~300nm N-type gallium oxide epitaxial layer on the top of the substrate.

[0059] For S2, after the formation of the gallium oxide layer is completed, photoresist can be spin-coated on the surface of the N-type gallium oxide layer away from the substrate, and a first patterning process corresponding to the first photolithography pattern can be formed using a standard photolithography process. Then, the N-type gallium oxide layer is etched using a reactive ion etching process to form multiple parallel nanowire gratings to obtain a nanophotosensitive structure.

[0060] That is, in this embodiment, the above-mentioned "performing a first patterning process on the side of the gallium oxide layer away from the substrate to form a nanophotosensitive structure located on the gallium oxide layer" may further include the following steps:

[0061] A first photoresist layer is formed on the side of the gallium oxide layer away from the substrate, and the first photoresist layer is subjected to a first patterning process to form a first photolithographic pattern;

[0062] Based on the first photolithography pattern, the gallium oxide layer is etched to form a nanoscale photosensitive structure located on the gallium oxide layer.

[0063] After the formation of the nano-photosensitive structure is completed, it can be soaked in piranha solution for 15 minutes to remove dry etching damage; then, it can be soaked in piranha solution for 15 minutes in turn, and then soaked in buffer oxide etching BOE solution for 30 minutes to repair etching damage.

[0064] For S3, after removing the dry etching damage, photoresist can be spin-coated again on the surface of the N-type gallium oxide layer away from the substrate. A second patterning process corresponding to the second photolithography pattern is formed using standard photolithography. Then, 40nm titanium is grown sequentially using magnetron sputtering to form a cathode electrode with the first ring structure.

[0065] That is, in this embodiment, the above-mentioned "performing a second patterning process on the side of the gallium oxide layer away from the substrate to form a cathode electrode with a first ring structure located on the gallium oxide layer" may further include the following steps:

[0066] A second photoresist layer is formed on the side of the gallium oxide layer away from the substrate, and the second photoresist layer is subjected to a second patterning process to form a second photolithographic pattern;

[0067] Based on the second photolithographic pattern, a corresponding titanium deposition operation is performed on the gallium oxide layer to form a cathode electrode with a first ring structure located on the gallium oxide layer.

[0068] For S4, after the cathode electrode is formed, photoresist can be spin-coated on the surface of the N-type gallium oxide layer away from the substrate, and a third patterning process corresponding to the third photolithography pattern can be formed using standard photolithography. A 200nm~300nm P-type nickel oxide layer is deposited on the surface of the N-type gallium oxide layer using processes including magnetron sputtering or electron beam evaporation, thereby forming a nickel oxide layer with a second ring structure.

[0069] That is, in this embodiment, the above-mentioned "performing a third patterning process on the side of the gallium oxide layer away from the substrate to form a nickel oxide layer with a second ring structure on top of the gallium oxide layer" may further include the following steps:

[0070] A third photoresist layer is formed on the side of the gallium oxide layer away from the substrate, and the third photoresist layer is subjected to a third patterning process to form a third photolithographic pattern;

[0071] Based on the third photolithography pattern, a corresponding nickel deposition operation is performed on the gallium oxide layer to form a nickel oxide layer with a second ring structure on the gallium oxide layer.

[0072] For S5, after the formation of the nickel oxide layer is completed, photoresist can be spin-coated on the surface of the N-type gallium oxide layer away from the substrate, and a fourth patterning process corresponding to the fourth photolithography pattern can be formed using standard photolithography. Then, 100nm of gold can be grown sequentially using magnetron sputtering to form an anode electrode with a third ring structure.

[0073] That is, in this embodiment, the above-mentioned "performing a fourth patterning process on the side of the nickel oxide layer away from the substrate to form an anode electrode with a third annular structure located on the nickel oxide layer" may further include the following steps:

[0074] A fourth photoresist layer is formed on the side of the nickel oxide layer away from the substrate, and the fourth photoresist layer is subjected to a fourth patterning process to form a fourth photolithographic pattern.

[0075] Based on the fourth photolithographic pattern, a corresponding gold deposition operation is performed on the nickel oxide layer to form an anode electrode with a third ring structure located on the nickel oxide layer.

[0076] Furthermore, after selecting or forming the substrate, gallium oxide layer, cathode electrode, nickel oxide layer, anode electrode, and nanophotosensitive structure based on the aforementioned method steps, a corresponding annealing at 470°C for 1 minute can be performed to form an ohmic contact between the anode metal and the cathode metal, thereby completing the fabrication process of the polarization detector.

[0077] In the specification provided herein, the algorithms and displays are not inherently related to any particular computer, virtual system, or other device. Various general-purpose systems can also be used with the examples of this invention. The required structure for constructing such systems is apparent from the above description. Furthermore, this invention is not directed to any particular programming language. It should be understood that the contents of the invention described herein can be implemented using various programming languages, and the above description of specific languages ​​is for the purpose of disclosing preferred embodiments of the invention.

[0078] Numerous specific details are set forth in the specification provided herein. However, it will be understood that embodiments of the invention may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0079] Similarly, it should be understood that, in order to streamline this disclosure and aid in understanding one or more of the various aspects of the invention, in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof.

[0080] Those skilled in the art will understand that modules, units, or components of the devices disclosed in the examples herein can be arranged in the devices described in this embodiment, or alternatively, can be located in one or more devices different from the devices in this example. The modules in the foregoing examples can be combined into a single module or, in addition, can be divided into multiple sub-modules.

[0081] Those skilled in the art will understand that the modules in the device of the embodiment can be adaptively changed and placed in one or more devices different from that embodiment. Modules, units, or components in the embodiment can be combined into a single module, unit, or component, and further, they can be divided into multiple sub-modules, sub-units, or sub-components.

[0082] Furthermore, those skilled in the art will understand that although some embodiments described herein include certain features included in other embodiments but not others, combinations of features from different embodiments are meant to be within the scope of the invention and form different embodiments.

[0083] Furthermore, some of the embodiments described herein are methods or combinations of method elements that can be implemented by a processor of a computer system or by other means of performing the functions. Therefore, a processor having the necessary instructions for implementing the methods or method elements forms means for implementing the methods or method elements. Furthermore, the elements described herein in the apparatus embodiments are examples of means for implementing the functions performed by elements for the purposes of carrying out the invention.

[0084] As used herein, unless otherwise specified, the use of ordinal numbers such as “first,” “second,” “third,” etc., to describe ordinary objects merely indicates different instances of similar objects and is not intended to imply that the objects being described must have a given order in time, space, ordering, or any other manner.

[0085] Although the invention has been described with respect to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and edibility purposes, and not for the purpose of explaining or limiting the subject matter of the invention.

Claims

1. A polarization detector, characterized in that, include: Substrate; A β-gallium oxide layer located on the substrate; A cathode electrode and a nickel oxide layer located on top of the β-gallium oxide layer; The anode electrode is located above the nickel oxide layer; as well as A nanophotosensitive structure comprising multiple nanowire gratings is formed on the β-gallium oxide layer; The cathode electrode has a first annular structure, the nickel oxide layer has a second annular structure, and the anode electrode has a third annular structure. The first annular structure, the second annular structure, and the third annular structure are concentrically arranged. The second annular structure and the third annular structure are located in a first accommodating space formed by the first annular structure, and the nano-photosensitive structure is located in a second accommodating space formed by the second annular structure and the third annular structure.

2. The polarization detector according to claim 1, characterized in that, The substrate is made of one of gallium oxide, gallium nitride, and silicon carbide.

3. The polarization detector according to claim 1, characterized in that, The β-gallium oxide layer is made of Si-doped N-type Ga2O3 with a first doping concentration of 2 × 10⁻⁶. 17 cm -3 .

4. The polarization detector according to claim 1, characterized in that, The nickel oxide layer is made of Li-doped P-type NiO with a corresponding second doping concentration of 1×10⁻⁶. 18 cm -3 .

5. A method for fabricating a polarization detector according to any one of claims 1-4, characterized in that, Includes the following steps: A β-gallium oxide layer is formed on the substrate; A first patterning process is performed on the side of the β-gallium oxide layer away from the substrate to form a nanophotosensitive structure located on the β-gallium oxide layer, which includes multiple nanowire gratings; A second patterning process is performed on the side of the β-gallium oxide layer away from the substrate to form a cathode electrode with a first ring structure located on the β-gallium oxide layer; A third patterning process is performed on the side of the β-gallium oxide layer away from the substrate to form a nickel oxide layer with a second ring structure located on the β-gallium oxide layer; A fourth patterning process is performed on the side of the nickel oxide layer away from the substrate to form an anode electrode with a third ring structure located on the nickel oxide layer. The first ring structure, the second ring structure, and the third ring structure are concentrically arranged. The second ring structure and the third ring structure are located in a first accommodating space formed by the first ring structure. The nano-photosensitive structure is located in a second accommodating space formed by the second ring structure and the third ring structure.

6. The preparation method according to claim 5, characterized in that, A first patterning process is performed on the side of the β-gallium oxide layer away from the substrate to form a nanoscale photosensitive structure located in the β-gallium oxide layer, including: A first photoresist layer is formed on the side of the β-gallium oxide layer away from the substrate, and the first photoresist layer is subjected to a first patterning process to form a first photolithographic pattern; Based on the first photolithography pattern, the β-gallium oxide layer is etched to form a nano-photosensitive structure located in the β-gallium oxide layer.

7. The preparation method according to claim 5, characterized in that, A second patterning process is performed on the side of the β-gallium oxide layer away from the substrate to form a cathode electrode with a first ring structure located on the β-gallium oxide layer, including: A second photoresist layer is formed on the side of the β-gallium oxide layer away from the substrate, and the second photoresist layer is subjected to a second patterning process to form a second photolithographic pattern; Based on the second photolithographic pattern, a corresponding titanium deposition operation is performed on the β-gallium oxide layer to form a cathode electrode with a first ring structure located on the β-gallium oxide layer.

8. The preparation method according to claim 5, characterized in that, A third patterning process is performed on the side of the β-gallium oxide layer away from the substrate to form a nickel oxide layer with a second ring structure located above the β-gallium oxide layer, including: A third photoresist layer is formed on the side of the β-gallium oxide layer away from the substrate, and the third photoresist layer is subjected to a third patterning process to form a third photolithographic pattern; Based on the third photolithography pattern, a corresponding nickel deposition operation is performed on the β-gallium oxide layer to form a nickel oxide layer with a second ring structure on the β-gallium oxide layer.

9. The preparation method according to claim 5, characterized in that, A fourth patterning process is performed on the side of the nickel oxide layer away from the substrate to form an anode electrode with a third annular structure located on the nickel oxide layer, including: A fourth photoresist layer is formed on the side of the nickel oxide layer away from the substrate, and the fourth photoresist layer is subjected to a fourth patterning process to form a fourth photolithographic pattern; Based on the fourth photolithographic pattern, a corresponding gold deposition operation is performed on the nickel oxide layer to form an anode electrode with a third ring structure located on the nickel oxide layer.

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