Schottky barrier diode

Abstract

To provide a Schottky barrier diode which includes a semiconductor layer made of gallium oxide and a cobalt layer forming a Schottky junction with the semiconductor layer, and in which generation of a leakage current in the cobalt layer is suppressed when a reverse voltage is applied.SOLUTION: A Schottky barrier diode 1 is provided that includes an n-type semiconductor layer 10 made of gallium oxide, and a cobalt layer 111 stacked on the n-type semiconductor layer 10 and forming a Schottky junction with the n-type semiconductor layer 10. The cobalt layer 111 is made of polycrystalline grains 111b having a face-centered cubic lattice structure and polycrystalline grains 111a of cobalt containing a hexagonal close-packed lattice structure. No region occupied by the polycrystalline grains 111b having the face-centered cubic lattice structure is continuously present between both surfaces of the cobalt layer 111.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a Schottky barrier diode. [Background technology]

[0002] Schottky barrier diodes are well-known electronic devices that have a rectifying effect by contacting a semiconductor with an anode electrode made of a metal material that meets certain conditions. Gallium oxide is also a semiconductor, with a band gap of 4.8 eV, which is larger than common semiconductor materials such as silicon (1.1 eV) and GaAs (1.4 eV), and is also larger than SiC (3.3 eV) and gallium nitride (3.4 eV), which have recently been used for power semiconductors (high-voltage semiconductors). For this reason, using gallium oxide in Schottky barrier diodes may make it easier to achieve high-voltage performance.

[0003] On the other hand, examples of anode electrode materials applicable to Schottky barrier diodes using gallium oxide are disclosed in Non-Patent Documents 1 and 2 and Patent Documents 1 and 2. The Schottky barrier diode described in Non-Patent Document 1 uses cobalt or nickel with a thickness of 20 nm, while the Schottky barrier diode described in Non-Patent Document 2 uses cobalt, nickel, titanium, molybdenum, palladium, or gold with a thickness of 30 nm. Furthermore, Patent Document 1 discloses nickel, platinum, palladium, gold, molybdenum, or copper, while Patent Document 2 discloses nickel, platinum, gold, or palladium. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-150519 [Patent Document 2] International Publication No. 2019 / 155768 [Non-patent literature]

[0005] [Non-Patent Document 1] Elizabeth V. Favela et al., “Effects of Annealing on Co / Au and Ni / Au Schottky Contacts on β-Ga2O3”, Journal of Electronic Materials, (2023) 52:1927-1936 [Non-patent document 2] Luke AM Lyle et al., “Effect of metal contacts on (100) β-Ga2O3 Schottky barriers”, J. Vac. Sci. Technol. A 39(3) May / Jun 2021; doi: 10.1116 / 6.0000877 Summary of the Invention [Problem to be solved by the invention]

[0006] In Schottky barrier diodes, leakage current during reverse operation (the direction in which current does not flow easily) depends on the work function of the anode electrode and the electric affinity of the semiconductor, but it can also increase or vary due to specific effects that occur only when two materials come into contact, such as the generation of interface states and trap states at the electrode-semiconductor interface. This undesirable effect is more likely to occur the higher the maximum reverse voltage of the diode being designed, so when designing a high-voltage diode, it is necessary to select the anode electrode material and its film formation method to prevent the increase in leakage current specific to the interface, which is not necessarily easy to achieve.

[0007] As a result of experiments conducted by the present inventors using cobalt as the anode electrode material, it was confirmed that a large leakage current occurs when a large reverse voltage (for example, 400 V) is applied to a Schottky barrier diode using a normal cobalt layer as the anode electrode, as described in Non-Patent Documents 1 and 2. Note that Non-Patent Documents 1 and 2 only describe the results of applying a low reverse voltage of 5 V or less to the Schottky barrier diode.

[0008] An object of the present invention is to provide a Schottky barrier diode including a semiconductor layer made of gallium oxide and a cobalt layer that forms a Schottky junction with the semiconductor layer, in which the generation of leakage current in the cobalt layer when a reverse voltage is applied is suppressed. [Means for solving the problem]

[0009] In order to achieve the above object, one aspect of the present invention provides the following Schottky barrier diode.

[0010] [1] A Schottky barrier diode comprising: an n-type semiconductor layer made of gallium oxide; and a cobalt layer laminated on the n-type semiconductor layer and forming a Schottky junction with the n-type semiconductor layer, wherein the cobalt layer is made of a polycrystalline body of cobalt including polycrystalline grains having a face-centered cubic lattice structure and polycrystalline grains having a hexagonal close-packed lattice structure, and the region occupied by the polycrystalline grains having the face-centered cubic lattice structure is not continuous between both surfaces of the cobalt layer. [2] A Schottky barrier diode comprising: an n-type semiconductor layer made of gallium oxide; and a cobalt layer laminated on the n-type semiconductor layer and forming a Schottky junction with the n-type semiconductor layer, wherein the cobalt layer is made of a polycrystalline body of cobalt including polycrystalline grains having a face-centered cubic lattice structure and polycrystalline grains having a hexagonal close-packed lattice structure, and the volume ratio of an area of ​​the cobalt layer occupied by the polycrystalline grains having the face-centered cubic lattice structure to the entire area is 23% or less. [3] The Schottky barrier diode according to [1] or [2] above, wherein the cobalt layer has a thickness of 60 nm or more. [4] The Schottky barrier diode according to [1] or [2] above, wherein a p-type semiconductor layer or a semi-insulating layer is provided in the n-type semiconductor layer in contact with the cobalt layer, thereby forming a JBS structure. [5] A first p-type semiconductor layer or semi-insulating layer in contact with the cobalt layer is formed in the n-type semiconductor layer, and an acceptor concentration of 3×10 18 cm -3 The Schottky barrier diode according to [1] or [2] above, wherein a second p-type semiconductor layer as described above is provided, and an ohmic electrode that forms an ohmic junction with the second p-type semiconductor layer is provided, thereby forming an MPS structure. [6] The Schottky barrier diode according to [1] or [2] above, wherein the n-type semiconductor layer has a trench forming a mesa portion that opens to a surface on the cobalt layer side, the inner surface of the trench is covered with an insulating film, and a part of the cobalt layer is provided in the trench while being covered with the insulating film. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a Schottky barrier diode including a semiconductor layer made of gallium oxide and a cobalt layer that forms a Schottky junction with the semiconductor layer, in which the generation of leakage current in the cobalt layer when a reverse voltage is applied can be suppressed. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a vertical cross-sectional view of a Schottky barrier diode having a planar structure according to an embodiment of the present invention. [Figure 2] 2(a) and 2(b) are enlarged views of a vertical cross section of the cobalt layer, each showing a schematic view of the cobalt polycrystalline grains that constitute the cobalt layer. [Figure 3] FIG. 3 is a vertical cross-sectional view of a Schottky barrier diode having a JBS structure according to an embodiment of the present invention. [Figure 4] FIG. 4 is a vertical cross-sectional view of a Schottky barrier diode having an MPS structure according to an embodiment of the present invention. [Figure 5] FIG. 5 is a vertical cross-sectional view of a Schottky barrier diode having a MOS structure according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] (Configuration of Schottky barrier diode) 1 is a vertical cross-sectional view of a Schottky barrier diode 1 according to an embodiment of the present invention. The Schottky barrier diode 1 is a vertical Schottky barrier diode having a planar structure with a semiconductor layer made of gallium oxide.

[0014] The Schottky barrier diode 1 includes an n-type semiconductor layer 10 made of gallium oxide, and a cobalt layer 111 laminated on the n-type semiconductor layer 10 and forming a Schottky junction with the n-type semiconductor layer 10. A low-resistance layer 112 is laminated on the cobalt layer 111, and the laminate of the cobalt layer 111 and the low-resistance layer 112 constitutes an anode electrode 11 of the Schottky barrier diode 1. A cathode electrode 12 is connected to the surface of the n-type semiconductor layer 10 opposite the cobalt layer 111 (the lower surface in FIG. 1 ).

[0015] In the Schottky barrier diode 1, when a forward voltage is applied between the anode electrode 11 and the cathode electrode 12, the energy barrier at the interface between the anode electrode 11 and the n-type semiconductor layer 10 as viewed from the n-type semiconductor layer 10 is lowered, and a current flows from the anode electrode 11 to the cathode electrode 12. On the other hand, when a reverse voltage is applied between the anode electrode 11 and the cathode electrode 12, almost no current flows due to the Schottky barrier, but a small reverse current, called leakage current, flows that is three to six orders of magnitude lower than that during forward operation.

[0016] The n-type semiconductor layer 10 contains donor impurities such as Si and Sn. The n-type semiconductor layer 10 may be a laminate of multiple semiconductor layers, and may be composed of, for example, a substrate and an epitaxial layer epitaxially grown thereon. The n-type semiconductor layer 10 may have a donor concentration of, for example, 1×10 18 cm -3 A substrate with a low resistance of 1×10 or more and a donor concentration of 1×10 14 ~1×10 18 cm -3 and consists of an epitaxial layer with a thickness of 3 to 50 μm.

[0017] The cobalt layer 111 is made of a polycrystalline body of cobalt (Co) containing polycrystalline grains having a face-centered cubic lattice structure and polycrystalline grains having a hexagonal close-packed lattice structure. In the cobalt layer 111, it is preferable that the region occupied by the polycrystalline grains having the face-centered cubic lattice structure does not exist continuously between both surfaces of the cobalt layer 111 (the upper and lower surfaces in FIG. 1).

[0018] This makes it possible to effectively suppress the occurrence of leakage current in the cobalt layer 111 even when a high reverse voltage (for example, 400 V or more) is applied to the Schottky barrier diode 1. The cobalt layer 111 will be described in detail later.

[0019] The low-resistance layer 112 is a layer having a lower electrical resistance than the cobalt layer 111, and is made of, for example, an Al film, an Au film, or a Cu film having a thickness of 2 μm or more, or a Ti film or a TiN film having a thickness of 1 μm or less, or a laminated film obtained by laminating an Al film, an Au film, or a Cu film having a thickness of 2 μm or more on top of a laminated film of these.

[0020] 1, an insulating layer 13 is provided around the region of the upper surface of the n-type semiconductor layer 10 that is in contact with the anode electrode 11, and at least a part of the end (edge) of the anode electrode 11 rests on the insulating layer 13. By providing such a field plate structure, it is possible to alleviate the electric field concentration near the end of the anode electrode 11 and improve the breakdown voltage of the Schottky barrier diode 1.

[0021] The insulating layer 13 is an insulating layer, and is made of, for example, an SiO2 film, an Si3N4 film, or a laminated film thereof.

[0022] 1, a high-resistance layer 15 is provided near the end of the anode electrode 11 in the n-type semiconductor layer 10. By providing the high-resistance layer 15, electric field concentration near the end of the anode electrode 11 can be alleviated, and the breakdown voltage of the Schottky barrier diode 1 can be improved.

[0023] The high resistance layer 15 is formed by ion-implanting impurities such as N and Mg, which act as acceptors in gallium oxide, into the n-type semiconductor layer 10.

[0024] 1, a passivation film 14 that suppresses leakage current flowing on the upper surface of the n-type semiconductor layer 10 is provided so as to cover the upper surface of the n-type semiconductor layer 10 around the anode electrode 11. The passivation film 14 is an insulating film, and is made of, for example, a SiN film, a polyimide film, or a laminated film thereof.

[0025] (Details of the cobalt layer) 2(a) and 2(b) are enlarged views of a vertical cross section of the cobalt layer 111, each showing a schematic view of cobalt polycrystalline grains 111a and 111b that constitute the cobalt layer 111. The polycrystalline grains 111a have a hexagonal close-packed lattice structure, and the polycrystalline grains 111b have a face-centered cubic lattice structure.

[0026] Figure 2(a) shows an example of a case where the region occupied by the polycrystalline grains 111b having a face-centered cubic lattice structure is not continuous between both surfaces of the cobalt layer 111. Figure 2(b) shows an example of a case where the region occupied by the polycrystalline grains 111b having a face-centered cubic lattice structure is continuous between both surfaces of the cobalt layer 111. The four downward arrows in Figure 2(b) indicate the portions where the region occupied by the polycrystalline grains 111b is continuous between both surfaces of the cobalt layer 111.

[0027] Through repeated experiments, the inventors have found that when a region occupied by polycrystalline grains 111b having a face-centered cubic lattice structure exists continuously between both surfaces of the cobalt layer 111, the leakage current increases when a high reverse voltage (for example, 400 V or more) is applied to the Schottky barrier diode 1, and that when a region occupied by polycrystalline grains 111b having a face-centered cubic lattice structure does not exist continuously between both surfaces of the cobalt layer 111, the leakage current in the cobalt layer 111 is suppressed even when a high reverse voltage is applied to the Schottky barrier diode 1.

[0028] For example, by forming the cobalt layer 111 to a thickness of 60 nm or more using a DC magnetron sputtering device under conditions of a power of 50 W, an Ar flow rate of 2 sccm, a deposition pressure of 0.7 Pa, and no substrate heating, it is possible to prevent the area occupied by the polycrystalline grains 111b from being continuous between both sides of the cobalt layer 111.

[0029] The threshold value of the volume ratio of the polycrystalline grains 111b to the entire volume in the cobalt layer 111, which determines whether the region occupied by the polycrystalline grains 111b exists continuously between both surfaces of the cobalt layer 111, is approximately 23%. That is, by setting the volume ratio of the polycrystalline grains 111b to the entire volume at 23% or less, the region occupied by the polycrystalline grains 111b does not exist continuously between both surfaces of the cobalt layer 111, and leakage current can be effectively suppressed.

[0030] Whether the region occupied by the polycrystalline grains 111b exists continuously between both surfaces of the cobalt layer 111 can be confirmed by two-dimensional mapping of the crystal structure analysis by electron diffraction of a vertical cross section of the cobalt layer 111 using a TEM (Transmission Electron Microscope).

[0031] The cobalt layer 111 can also be applied to a Schottky barrier diode having a different structure from the Schottky barrier diode 1. Below, examples of application to a Schottky barrier diode having a JBS (Junction Barrier Schottky) structure, a Schottky barrier diode having an MPS (Merged PiN Schottky) structure, and a Schottky barrier diode having a MOS (Metal Oxide Semiconductor) structure will be described.

[0032] 3 is a vertical cross-sectional view of a Schottky barrier diode 2 according to an embodiment of the present invention. The Schottky barrier diode 2 is a vertical Schottky barrier diode having a JBS structure.

[0033] In the Schottky barrier diode 2, the n-type semiconductor layer 10 has a trench 22 that opens to the upper surface (the surface on the cobalt layer 111 side), and a p-type semiconductor layer 21 that is in contact with the n-type semiconductor layer 10 and the cobalt layer 111 is provided in the trench 22. The other structures are similar to those of the Schottky barrier diode 1.

[0034] The trenches 22 are provided so as to form an n-type semiconductor region 100, which is a mesa portion sandwiched from both sides by the trenches 22. The trenches 22 are typically made up of a plurality of trenches having a dot-like or linear pattern.

[0035] The p-type semiconductor layer 21 is formed by depositing a p-type semiconductor such as copper oxide or nickel oxide in the trench 22 by sputtering, CVD, or the like.

[0036] In the Schottky barrier diode 2, by applying a forward voltage (positive potential on the anode electrode 11 side) between the anode electrode 11 and the cathode electrode 12, the potential barrier at the interface between the anode electrode 11 and the n-type semiconductor layer 10 as viewed from the n-type semiconductor layer 10 decreases, and a current flows from the anode electrode 11 to the cathode electrode 12.

[0037] On the other hand, when a reverse voltage (anode electrode 11 side has a negative potential) is applied between the anode electrode 11 and the cathode electrode 12, almost no current flows due to the Schottky barrier. At this time, depletion layers extend laterally from the p-type semiconductor layers 21 on both sides into the n-type semiconductor region 100 and connect, thereby mitigating the electric field at the interface between the cobalt layer 111 and the n-type semiconductor region 100, and effectively increasing the reverse voltage at which the diode can operate.

[0038] The size of the trench 22 (the size of the p-type semiconductor layer 21) is, for example, 0.3 to 3 μm deep and 0.5 to 2 μm wide (the horizontal length in the cross section of FIG. 3). The width of the n-type semiconductor region 100 sandwiched between the p-type semiconductor layers 21 is, for example, 0.5 to 2 μm.

[0039] In the Schottky barrier diode 2, instead of forming the trench 22 and the p-type semiconductor layer 21, a semi-insulating layer may be formed by implanting nitrogen or magnesium into the n-type semiconductor layer 10, which similarly achieves the effect of suppressing leakage current when a reverse voltage is applied. This semi-insulating layer is a highly insulating gallium oxide region formed in the same position as the p-type semiconductor layer 21, and is formed, for example, by implanting nitrogen or magnesium into the n-type semiconductor layer 10 by ion implantation, followed by heat treatment at 800 to 1000°C for approximately 1 to 60 minutes in a nitrogen or oxygen atmosphere. In the Schottky barrier diode 2, a JBS structure is formed by providing the p-type semiconductor layer 21 or the semi-insulating layer.

[0040] 4 is a vertical cross-sectional view of a Schottky barrier diode 3 according to an embodiment of the present invention. The Schottky barrier diode 3 is a vertical Schottky barrier diode having an MPS structure.

[0041] In the Schottky barrier diode 3, the n-type semiconductor layer 10 has trenches 22 and 32 that open to the upper surface (the surface on the cobalt layer 111 side), and the p-type semiconductor layer 21 that is in contact with the n-type semiconductor layer 10 and the cobalt layer 111 is provided in the trench 22, and the p-type semiconductor layer 21 that is in contact with the n-type semiconductor layer 10 and has an acceptor concentration of 3×10 18 cm -3 The p-type semiconductor layer 31 described above is provided, and an ohmic electrode 33 is provided to form an ohmic junction with the p-type semiconductor layer 31. The other structures are the same as those of the Schottky barrier diode 1.

[0042] The trench 22 and the p-type semiconductor layer 21 are similar to those used in the above-described Schottky barrier diode 2. The p-type semiconductor layer 31 is formed by depositing a p-type semiconductor such as copper oxide or nickel oxide in the trench 32 by sputtering, CVD, or the like. At this time, the acceptor concentration of the p-type semiconductor layer 31 is set higher than the acceptor concentration of the p-type semiconductor layer 21.

[0043] Furthermore, in the Schottky barrier diode 3, similarly to the Schottky barrier diode 2, a semi-insulating layer may be formed by implanting nitrogen or magnesium into the n-type semiconductor layer 10 instead of forming the trench 22 and the p-type semiconductor layer 21. In the Schottky barrier diode 3, an MPS structure is formed by providing the p-type semiconductor layer 21 or the semi-insulating layer, the p-type semiconductor layer 31, and the ohmic electrode 33.

[0044] In the Schottky barrier diode 3, when the forward current increases instantaneously, the pn junction diode formed by the n-type semiconductor layer 10 and the p-type semiconductor layer 31 turns on, suppressing the rise in forward voltage. Therefore, the Schottky barrier diode 3 has high forward surge resistance. Other operations of the Schottky barrier diode 3 are the same as those of the Schottky barrier diode 2.

[0045] The size of the trench 32 (the size of the p-type semiconductor layer 31) is, for example, 0.3 to 3 μm in depth and 0.5 to 5 μm in width (the horizontal length in the cross section of FIG. 4).

[0046] 5 is a vertical cross-sectional view of a Schottky barrier diode 4 according to an embodiment of the present invention. The Schottky barrier diode 4 is a vertical Schottky barrier diode having a MOS structure.

[0047] In the Schottky barrier diode 4, the n-type semiconductor layer 10 has a trench 42 that opens to the upper surface (the surface on the cobalt layer 111 side), the inner surface of the trench 42 is covered with an insulating film 41, and a part 110 of the cobalt layer 111 is provided in the trench 42 while being covered with the insulating film 41. The rest of the structure is the same as that of the Schottky barrier diode 1.

[0048] Similar to trench 22, trench 42 is provided to form n-type semiconductor region 100, which is a mesa portion sandwiched between trenches 42. Trenches 42 are typically made up of multiple trenches having a dot-like or linear pattern. Insulating film 41 is made of an insulator such as silicon oxide (SiO), hafnia (HfO), or alumina (AlO).

[0049] In the Schottky barrier diode 4, when a forward voltage is applied between the anode electrode 11 and the cathode electrode 12, the potential barrier at the interface between the anode electrode 11 and the n-type semiconductor layer 10 as viewed from the n-type semiconductor layer 10 decreases, and a current flows from the anode electrode 11 to the cathode electrode 12.

[0050] On the other hand, when a reverse voltage is applied between the anode electrode 11 and the cathode electrode 12, almost no current flows due to the Schottky barrier. At this time, depletion layers extend laterally from the interfaces between the insulating films 41 on both sides and the n-type semiconductor region 100 inside the n-type semiconductor region 100 and connect to each other, thereby mitigating the electric field at the interface between the cobalt layer 111 and the n-type semiconductor region 100, and effectively increasing the reverse voltage at which the diode can operate.

[0051] The trenches 42 have a depth of 0.3 to 5 μm and a width (lateral length in the cross section of FIG. 5) of 0.5 to 2 μm, for example. The width of the n-type semiconductor region 100 between adjacent trenches 42 is 0.5 to 2 μm, for example.

[0052] (Effects of the embodiment) According to the Schottky barrier diodes 1 to 4 according to the above-described embodiments of the present invention, the occurrence of leakage current in the cobalt layer 111 when a large reverse voltage is applied is suppressed.

[0053] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and various modifications are possible within the scope of the gist of the invention. Furthermore, the components of the above embodiments can be combined as desired within the scope of the gist of the invention. Furthermore, the above-described embodiments do not limit the invention according to the claims. Furthermore, it should be noted that not all of the combinations of features described in the embodiments are necessarily essential to the means for solving the problems of the invention. [Explanation of symbols]

[0054] 1, 2, 3, 4... Schottky diode, 10... n-type semiconductor layer, 100... n-type semiconductor region, 11... anode electrode, 111... cobalt layer, 111a, 111b... polycrystalline grains, 21, 31... p-type semiconductor layer, 22, 32, 42... trench, 41... insulating film, 110... part of cobalt layer

Claims

1. an n-type semiconductor layer made of gallium oxide; a cobalt layer stacked on the n-type semiconductor layer and forming a Schottky junction with the n-type semiconductor layer; Equipped with the cobalt layer is made of a polycrystalline cobalt body including polycrystalline grains having a face-centered cubic lattice structure and polycrystalline grains having a hexagonal close-packed lattice structure; the region occupied by the polycrystalline grains having a face-centered cubic lattice structure is not continuous between both surfaces of the cobalt layer; Schottky barrier diode.

2. an n-type semiconductor layer made of gallium oxide; a cobalt layer laminated on the n-type semiconductor layer and forming a Schottky junction with the n-type semiconductor layer; Equipped with the cobalt layer is made of a polycrystalline cobalt body including polycrystalline grains having a face-centered cubic lattice structure and polycrystalline grains having a hexagonal close-packed lattice structure; a volume ratio of a region occupied by the polycrystalline grains having a face-centered cubic lattice structure to the entire region in the cobalt layer is 23% or less; Schottky barrier diode.

3. The thickness of the cobalt layer is 60 nm or more.

3. The Schottky barrier diode according to claim 1.

4. a p-type semiconductor layer or a semi-insulating layer in contact with the cobalt layer is provided within the n-type semiconductor layer; The JBS structure was constructed, 3. The Schottky barrier diode according to claim 1.

5. a first p-type semiconductor layer or semi-insulating layer in contact with the cobalt layer within the n-type semiconductor layer; and １８ cm －３ a second p-type semiconductor layer as described above is provided, an ohmic electrode that forms an ohmic junction with the second p-type semiconductor layer is provided; The MPS structure was constructed 3. The Schottky barrier diode according to claim 1.

6. the n-type semiconductor layer has a trench that forms a mesa portion and that opens to a surface on the cobalt layer side; the inner surface of the trench is covered with an insulating film; a part of the cobalt layer is provided in the trench and is covered with the insulating film; 3. The Schottky barrier diode according to claim 1.

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