Schottky barrier diode

Abstract

To provide a Schottky barrier diode that comprises a semiconductor layer made of a gallium oxide-based semiconductor, and that effectively suppresses surface leakage by a passivation film made of SiO2 and effectively improves the withstand voltage.SOLUTION: Provided is a Schottky barrier diode 1 that comprises: an n-type semiconductor layer 10 made of a gallium oxide-based semiconductor; an insulating film 11 made of SiO2 and covering a part of a top face 101 of the n-type semiconductor layer 10; and an anode electrode 14 connected with the top face 101 of the n-type semiconductor layer 10 to form a Schottky junction with the n-type semiconductor layer 10, and having at least a part of its edge on the insulating film 11. The insulating film 11 includes a first layer 12 in contact with the n-type semiconductor layer 10, and a second layer 13 on the first layer 12. The refractive index of the first layer 12 is lower than the refractive index of the second layer 13. The n-type semiconductor layer 10 includes a guard ring 16 surrounding the junction with the anode electrode 14.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, Schottky barrier diodes having a semiconductor layer made of Ga2O3-based single crystals are known, which have a field plate structure to mitigate electric field concentration at the ends of the anode electrode (see Patent Document 1).

[0003] In the Schottky barrier diode described in Patent Document 1, an insulating layer made of an insulating material such as SiO2 is provided on a semiconductor layer, and the anode electrode has a field plate that is in Schottky contact with the semiconductor layer within an opening in the insulating layer and is raised in the region surrounding the opening of the insulating layer. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-45969 [Overview of the project] [Problems that the invention aims to solve]

[0005] An insulating film provided on a semiconductor layer, such as the insulating layer in the Schottky barrier diode described in Patent Document 1, also functions as a passivation film that suppresses surface leakage current flowing on the upper surface of the semiconductor layer. However, as a result of diligent research, the inventors of the present invention have found that, particularly when the semiconductor layer is made of a gallium oxide-based semiconductor, when an SiO2 film is used as an insulating film on the semiconductor layer, increasing the density of the insulating film leads to increased surface leakage due to damage to the semiconductor layer during film formation, while decreasing the density of the insulating film results in poor film quality and thus a lower dielectric breakdown voltage.

[0006] The object of the present invention is to provide a Schottky barrier diode comprising a semiconductor layer made of a gallium oxide-based semiconductor, wherein surface leakage is effectively suppressed and dielectric breakdown voltage is effectively improved by a passivation film made of SiO2. [Means for solving the problem]

[0007] One aspect of the present invention provides the following Schottky barrier diode [1] to achieve the above objective.

[0008] [1] A Schottky barrier diode comprising: an n-type semiconductor layer made of a gallium oxide semiconductor; an insulating film made of SiO2 covering a portion of the upper surface of the n-type semiconductor layer; and an anode electrode connected to the upper surface of the n-type semiconductor layer, forming a Schottky junction with the n-type semiconductor layer, with at least a portion of its edge on the insulating film, wherein the insulating film includes a first layer in contact with the n-type semiconductor layer and a second layer on the first layer, the refractive index of the first layer being lower than that of the second layer, and the n-type semiconductor layer including a guard ring surrounding the junction with the anode electrode. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a Schottky barrier diode having a semiconductor layer made of a gallium oxide-based semiconductor, wherein surface leakage is effectively suppressed and the dielectric breakdown voltage is effectively improved by a passivation film made of SiO2. [Brief explanation of the drawing]

[0010] [Figure 1] Figures 1(a) and 1(b) are vertical cross-sectional views of a Schottky barrier diode according to an embodiment of the present invention. [Figure 2] Figure 2 is a vertical cross-sectional view of a Schottky barrier diode according to an embodiment of the present invention. [Figure 3]Figures 3(a), (b), and (c) are vertical cross-sectional views of a Schottky barrier diode used to examine the electrical resistance of a guard ring. [Figure 4] Figure 4 is a graph showing the box profile of the N-injection region contained in a Schottky barrier diode. [Figure 5] Figures 5(a), (b), and (c) are graphs showing the change in current density when a forward voltage is applied to a Schottky barrier diode. [Figure 6] Figures 6(a), (b), and (c) are graphs showing the change in current when a forward voltage is applied to a Schottky barrier diode. [Figure 7] Figures 7(a), (b), and (c) are vertical cross-sectional views of a Schottky barrier diode to investigate the effects of the insulating film and guard ring. [Figure 8] Figures 8(a), (b), and (c) are graphs showing the change in current density when a reverse voltage is applied to a Schottky barrier diode. [Figure 9] Figure 9 is a graph showing the change in current when a reverse voltage is applied to a Schottky barrier diode. [Modes for carrying out the invention]

[0011] (Configuration of Schottky barrier diodes) Figure 1(a) is a vertical cross-sectional view of a Schottky barrier diode 1 according to an embodiment. The Schottky barrier diode 1 is a vertical Schottky barrier diode equipped with a semiconductor layer made of a gallium oxide semiconductor.

[0012] The Schottky barrier diode 1 includes an n-type semiconductor layer 10 made of a gallium oxide-based semiconductor, an insulating film 11 made of SiO2 that covers a part of the upper surface 101 of the n-type semiconductor layer 10, and an anode electrode 14 that is connected to the upper surface 101 of the n-type semiconductor layer 10, forms a Schottky junction with the n-type semiconductor layer 10, and at least a part of the edge is on the insulating film 11. The insulating film 11 includes a first layer 12 that contacts the n-type semiconductor layer 10 and a second layer 13 on the first layer 12. The refractive index of the first layer 12 is lower than the refractive index of the second layer 13. The n-type semiconductor layer 10 includes a guard ring 16 that surrounds the junction with the anode electrode 14. Also, a cathode electrode 15 is connected to the lower surface 102, which is the surface opposite to the upper surface 101 of the n-type semiconductor layer 10.

[0013] In the Schottky barrier diode 1, by applying a forward voltage between the anode electrode 14 and the cathode electrode 15, the energy barrier at the interface between the anode electrode 14 and the n-type semiconductor layer 10 as seen from the n-type semiconductor layer 10 decreases, and current flows from the anode electrode 14 to the cathode electrode 15. On the other hand, when a reverse voltage is applied between the anode electrode 14 and the cathode electrode 15, no current flows due to the Schottky barrier.

[0014] The n-type semiconductor layer 10 is made of a single crystal of a gallium oxide-based semiconductor having a β-type crystal structure. Here, the gallium oxide-based semiconductor refers to Ga2O3 or Ga2O3 to which elements such as Al and In are added. For example, the gallium oxide-based semiconductor has a composition represented by (Ga x Al y In (1-x-y) )2O3 (0 < x ≤ 1, 0 ≤ y ≤ 1, 0 < x + y ≤ 1). When Al is added to Ga2O3, the bandgap widens, and when In is added, the bandgap narrows.

[0015] The n-type semiconductor layer 10 contains donor impurities such as Si and Sn. The donor concentration of the n-type semiconductor layer 10 is, for example, 1×10 15 cm -3 or more and 1×10 17 cm -3The following applies: The thickness of the n-type semiconductor layer 10 is, for example, 2 μm or more and 100 μm or less. The n-type semiconductor layer 10 is made from a substrate cut from a single crystal grown by liquid phase growth, for example.

[0016] The n-type semiconductor layer 10 may be a stack of multiple semiconductor layers, for example, it may consist of a substrate and an epitaxially grown epitaxial layer thereon.

[0017] In the Schottky barrier diode 1, as described above, at least a portion of the edge of the anode electrode 14 rests on the insulating film 11. This portion 141 of the edge of the anode electrode 14 that rests on the insulating film 11 is called a field plate, and a structure equipped with a field plate is called a field plate structure. By providing such a field plate structure, electric field concentration near the edge of the anode electrode 14 can be mitigated, and the dielectric strength of the Schottky barrier diode 1 can be improved.

[0018] In order to effectively improve the dielectric strength using a field plate structure, it is preferable that the edge of the anode electrode 14 rests on the insulating film 11 over its entire circumference. For example, if the planar shape of the insulating film 11 is an annular shape surrounding the junction between the n-type semiconductor layer 10 and the anode electrode 14, then the planar shape of the portion 141 resting on the insulating film 11 will also be annular.

[0019] The insulating film 11 is formed using plasma CVD, which allows for relatively good film deposition at low temperatures. The first layer 12 and the second layer 13, which constitute the insulating film 11 and have different refractive indices, can be fabricated by controlling the plasma output. After the insulating film 11 is deposited over the entire upper surface 101 of the n-type semiconductor layer 10, it is patterned to expose the region on the upper surface 101 to which the anode electrode 14 is connected.

[0020] Generally, the higher the density of the insulating film on which the field plate is mounted, the greater the improvement in the dielectric breakdown voltage of the Schottky barrier diode. Increasing the plasma output of plasma CVD can increase the density of the insulating film, but this increases the damage to the upper surface of the semiconductor layer during deposition. When the damage increases, the density of interface states on the upper surface of the semiconductor layer increases, which increases the leakage current flowing at the interface between the semiconductor layer and the insulating film (hereinafter referred to as interface leakage current). On the other hand, reducing the plasma output of plasma CVD can suppress damage to the upper surface of the semiconductor layer and suppress the increase in interface leakage current, but the density of the insulating film decreases, making it difficult to effectively improve the dielectric breakdown voltage of the Schottky barrier diode.

[0021] In the Schottky barrier diode 1 according to an embodiment of the present invention, as described above, the insulating film 11 includes a first layer 12 in contact with the n-type semiconductor layer 10 and a second layer 13 on the first layer 12, wherein the refractive index of the first layer 12 is lower than the refractive index of the second layer 13. Here, there is a correlation between the density of the insulating film 11 and the refractive index, with higher density resulting in a higher refractive index.

[0022] Therefore, the first layer 12 is formed under conditions of lower plasma output than the second layer 13. By forming the first layer 12, which is in contact with the n-type semiconductor layer 10, under conditions of lower plasma output, damage to the n-type semiconductor layer 10 can be suppressed and interfacial leakage current can be reduced.

[0023] On the other hand, the second layer 13, which has a higher refractive index than the first layer 12, is denser than the first layer 12. By including the denser second layer 13 in the insulating film 11, the dielectric breakdown voltage of the Schottky barrier diode 1 can be increased. The denser second layer 13 is deposited under conditions of high plasma output in plasma CVD, but since the first layer 12 is located beneath the second layer 13, damage to the n-type semiconductor layer 10 during the deposition of the second layer 13 can be suppressed.

[0024] That is, by using the insulating film 11 including the first layer 12 and the second layer 13, it is possible to achieve both an improvement in the breakdown voltage and a suppression of the interface leakage current of the Schottky barrier diode 1.

[0025] In order to effectively suppress the damage to the n-type semiconductor layer 10 during the film formation of the first layer 12, the refractive index of the first layer 12 is preferably 1.44 or less. Further, in order to effectively suppress the damage to the n-type semiconductor layer 10 during the film formation of the second layer 13, the thickness of the first layer 12 is preferably 450 nm or more.

[0026] In order to effectively improve the breakdown voltage of the Schottky barrier diode 1, the refractive index of the second layer 13 is preferably 1.46 or more, and the thickness of the second layer 13 is preferably 20 nm or more. Further, in order to suppress the generation of stress, the thickness of the second layer 13 is preferably 2000 nm or less. Furthermore, although the cause has not been clarified, it has been confirmed that when the thickness of the second layer 13 exceeds 100 nm, the interface leakage current tends to increase. Therefore, the thickness of the second layer 13 is particularly preferably 100 nm or less.

[0027] The guard ring 16 is a region containing acceptor impurities provided so as to surround the junction of the anode electrode 14 and the n-type semiconductor layer 10. By using the guard ring 16, the concentration of the electric field at the end of the anode electrode 14 can be alleviated, and the breakdown voltage of the Schottky barrier diode 1 can be improved.

[0028] The guard ring 16 is formed, for example, by ion-implanting acceptor impurities such as N (nitrogen) into the upper surface 101 of the n-type semiconductor layer 10, and then performing an annealing treatment at about 900 ° C in a nitrogen atmosphere for damage recovery.

[0029] In order to effectively alleviate the electric field concentration by the guard ring 16, the concentration of acceptor impurities is 1×10 17 atoms / cm 3When the region is defined as the guard ring 16, it is preferable that the lateral distance D of the portion of the guard ring 16 in contact with the anode electrode 14 is 10 μm or more, and that the thickness T of the guard ring 16 is 10 nm or more.

[0030] Figure 1(b) is a vertical cross-sectional view of the Schottky barrier diode 1 when the anode electrode 14 is covered by the cover 17. The cover 17 covers the anode electrode 14 and is in contact with the upper surface of the insulating film 11 around the anode electrode 14. The cover 17 is conductive and has, for example, a Ti / Al multilayer structure in which an Al film is laminated on a Ti film.

[0031] When the anode electrode 14 is made of Ni or Pt, the adhesion to the insulating film 11 made of SiO2 is low, which may cause the field plate portion 141 of the anode electrode 14 to peel off from the insulating film 11. In such cases, using the cover 17 can suppress the peeling of portion 141 of the anode electrode 14 from the insulating film 11.

[0032] Figure 2 is a vertical cross-sectional view of a Schottky barrier diode 1 when the inner side surface of the insulating film 11 is inclined. In the Schottky barrier diode 1 shown in Figure 2, the inner side surfaces 121 and 131 of the first layer 12 and the second layer 13 of the insulating film 11, i.e., the side surfaces 121 and 131 on the anode electrode 14 side, are inclined to face diagonally upward.

[0033] When sides 121 and 131 are facing diagonally upward, the concentration of the electric field can be mitigated more effectively compared to when sides 121 and 131 are vertical or facing diagonally downward.

[0034] By controlling the etching rate of wet etching when patterning the first layer 12 and the second layer 13, the sides 121 and 131 can be tilted to face diagonally upward. However, if the first layer 12 and the second layer 13, which have different densities, are etched under the same conditions, the etching rates will be different, making it difficult to tilt both sides 121 and 131 to face diagonally upward. Therefore, it is necessary to etch the first layer 12 and the second layer 13 separately under different conditions.

[0035] Furthermore, after etching the second layer 13, etching the first layer 12 using a mask larger than the second layer 13 allows the position of side surface 121 to be shifted inward from the position of side surface 131, as shown in Figure 2, thereby creating a step. This further effectively reduces the concentration of the electric field.

[0036] Furthermore, the inclination angles of the sides 121 and 131 can also be controlled by controlling the etching rate of the wet etching process when patterning the first layer 12 and the second layer 13. To enhance the effect of mitigating electric field concentration, it is preferable that the inclination angles of the sides 121 and 131 be 10° or more from the direction perpendicular to the upper surface 101 of the n-type semiconductor layer 10.

[0037] Preferably, the Schottky barrier diode 1 has a sloping inner surface of the insulating film 11, the n-type semiconductor layer 10 includes a guard ring 16, and the anode electrode 14 is covered by a cover 17, as shown in Figure 2.

[0038] (Evaluation of Schottky barrier diodes) Figures 3(a), (b), and (c) are vertical cross-sectional views of Schottky barrier diodes 3a, 3b, and 3c for examining the electrical resistance of the guard ring 16 of Schottky barrier diode 1.

[0039] The Schottky barrier diode 3a shown in Figure 3(a) comprises an n-type semiconductor layer 30 made of β-Ga2O3, an anode electrode 31 formed on the upper surface 301 of the n-type semiconductor layer 30, and a cathode electrode 32 formed on the lower surface 302 of the n-type semiconductor layer 30. The Schottky barrier diode 3b shown in Figure 3(b) differs from the Schottky barrier diode 3a in that an N-implantation region 33 in which N is ion-implanted is provided in a ring shape on the n-type semiconductor layer 30. The Schottky barrier diode 3c shown in Figure 3(c) differs from the Schottky barrier diode 3b in that the N-implantation region 33 is provided so as to cover the entire area below the anode electrode 31.

[0040] Figure 4 is a graph showing the box profile of the N-implantation region 33 contained in Schottky barrier diodes 3b and 3c. The N-implantation region 33 was formed by multi-stage implantation of N ions under the conditions shown in Table 1.

[0041] [Table 1]

[0042] Figures 5(a), (b), and (c) are graphs showing the change in current density when a forward voltage is applied to Schottky barrier diodes 3a, 3b, and 3c, respectively. Similarly, Figures 6(a), (b), and (c) are graphs showing the change in current when a forward voltage is applied to Schottky barrier diodes 3a, 3b, and 3c, respectively. Figures 5(a), (b), and (c) and Figures 6(a), (b), and (c) show the results of measurements performed multiple times under the same conditions.

[0043] From these measurement results, the on-resistances of Schottky barrier diodes 3a, 3b, and 3c are 8.15 mΩcm, respectively. 2 , 7.77 mΩcm 2 , 2.56 MΩcm 2It can be seen that this is the case. Furthermore, the volume resistivity of Schottky barrier diodes 3a, 3b, and 3c is calculated to be 135 Ωcm, 130 Ωcm, and 42.7 GΩcm, respectively. According to these results, the electrical resistance of Schottky barrier diode 3c is more than 100 million times that of Schottky barrier diodes 3a and 3b. From this, the N-injection region 33 and the corresponding guard ring 16 of Schottky barrier diode 1 can be defined as a region having an electrical resistance more than 100 million times that of the surrounding region where acceptor impurities have not been injected.

[0044] Figures 7(a), (b), and (c) are vertical cross-sectional views of Schottky barrier diodes 4a, 4b, and 4c to examine the effects of the insulating film 11 and guard ring 16 of Schottky barrier diode 1.

[0045] The Schottky barrier diode 4a shown in Figure 7(a) comprises an n-type semiconductor layer 40 made of β-Ga2O3, an anode electrode 44 formed on the upper surface 401 of the n-type semiconductor layer 40, a cathode electrode 45 formed on the lower surface 402 of the n-type semiconductor layer 40, a guard ring 46 which is a region in the n-type semiconductor layer 40 where N is ion-implanted, and an annealed altered layer 48. In addition, the Schottky barrier diode 4a is provided with a protective material made of polyimide to protect the area around the anode electrode 44.

[0046] The Schottky barrier diode 4b shown in Figure 7(b) comprises an n-type semiconductor layer 40 made of β-Ga2O3, an insulating film 41 made of SiO2 covering a portion of the upper surface 401 of the n-type semiconductor layer 40, an anode electrode 44 connected to the upper surface 401 of the n-type semiconductor layer 40 and forming a Schottky junction with the n-type semiconductor layer 40, with its edge on the insulating film 41, a cathode electrode 45 formed on the lower surface 402 of the n-type semiconductor layer 40, and an annealed altered layer 48. The insulating film 41 includes a first layer 42 in contact with the n-type semiconductor layer 40 and a second layer 43 on the first layer 42, with the refractive index of the first layer 42 being lower than that of the second layer 43. The planar shape of the insulating film 41 is annular, surrounding the junction between the n-type semiconductor layer 40 and the anode electrode 44.

[0047] The Schottky barrier diode 4c shown in Figure 7(c) corresponds to the Schottky barrier diode 1 shown in Figure 1(a), which has an insulating film 11 and a guard ring 16, and differs from Schottky barrier diodes 4a and 4b in that it has both a guard ring 46 and an insulating film 41. The annealed altered layer 48 included in Schottky barrier diodes 4a to 4c is an altered layer that is unintentionally formed on the surface of the n-type semiconductor layer 40 during the annealing process. This annealed altered layer 48 can be removed by dry etching or wet etching.

[0048] Figures 8(a), (b), and (c) are graphs showing the change in current density when a reverse voltage is applied to Schottky barrier diodes 4a, 4b, and 4c, respectively. Figures 8(a), (b), and (c) show the results of measurements performed multiple times under the same conditions.

[0049] Figures 8(a), (b), and (c) show that when both the guard ring 46 and the insulating film 41 are used, the current that flows when a reverse voltage is applied is smaller compared to when either the guard ring 46 or the insulating film 41 is used.

[0050] Figure 9 is a graph showing the change in current when a reverse voltage is applied to a Schottky barrier diode 4a equipped with a guard ring 46 and a Schottky barrier diode 4c equipped with both a guard ring 46 and an insulating film 41. In Figure 9, A represents the characteristics of Schottky barrier diode 4a, and B represents the characteristics of Schottky barrier diode 4c.

[0051] Figure 9 shows that when both the guard ring 46 and the insulating film 41 are used, the dielectric breakdown voltage is higher compared to when only the guard ring 46 is used.

[0052] As shown in the measurement results in Figures 8(a), (b), (c) and 9, it can be seen that the dielectric strength of the Schottky barrier diode 1 according to the embodiment of the present invention is particularly effectively improved by using the insulating film 11 and the guard ring 16 together.

[0053] (Manufacturing method for Schottky barrier diodes) The following is an example of a manufacturing method when the Schottky barrier diode 1 has the configuration shown in Figure 2.

[0054] First, a gallium oxide-based semiconductor substrate is prepared as an n-type semiconductor layer 10, and as a pre-process cleaning, ultrasonic cleaning using organic solvents, hydrofluoric acid cleaning, and SPM acid cleaning are performed.

[0055] Next, the upper surface 101 of the n-type semiconductor layer 10 is covered with photoresist, and an alignment pattern is formed by dry etching. Then, N ion implantation is performed on the photoresist with the pattern formed on it under the conditions shown in Table 1 above. After that, heat treatment is performed in an N2 atmosphere in a lamp furnace at 900°C for 30 minutes to form a guard ring 16 near the surface of the n-type semiconductor layer 10.

[0056] Next, SiO2 films, which will form the first layer 12 and the second layer 13 of the insulating film 11, are deposited on the upper surface 101 by CVD. First, a resist is patterned on the upper SiO2 film, which will become the second layer 13, by photolithography. Then, using the resist as a mask, wet etching is performed with BHF as the etchant to form the second layer 13 with a tilted side surface 131. After that, a resist is patterned on the lower SiO2 film, which will become the first layer 12, by photolithography. Then, using the resist as a mask, wet etching is performed with BHF as the etchant to form the first layer 12 with a tilted side surface 121.

[0057] Next, after performing SPM cleaning and ultrapure water cleaning as pretreatment, a metal film made of Ni or Pt, which will become the anode electrode 14, is deposited on the upper surface 101 of the n-type semiconductor layer 10. Then, the metal film is patterned to form the anode electrode 14.

[0058] Next, a metal film having a Ti / Al stacked structure or the like, which will serve as the cover 17, is deposited on the anode electrode 14. Then, the metal film is patterned with a pattern larger than the anode electrode 14 to form the cover 17. After that, the lower surface 102 of the n-type semiconductor layer 10 is covered with a metal film having a Ti / Ni / Au stacked structure or the like to form the cathode electrode 15.

[0059] (Effects of the embodiment) According to the above embodiment, it is possible to provide a Schottky barrier diode having a semiconductor layer made of a gallium oxide-based semiconductor, wherein surface leakage is effectively suppressed by a passivation film made of SiO2 and the dielectric breakdown voltage is effectively improved.

[0060] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the invention. Furthermore, the components of the above embodiments can be arbitrarily combined without departing from the spirit of the invention. Moreover, the embodiments described above do not limit the invention as claimed. It should also be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention. [Explanation of Symbols]

[0061] 1...Schottky diode, 10...n-type semiconductor layer, 101...top surface, 11...insulating film, 12...first layer, 121...side surface, 13...second layer, 131...side surface, 14...anode electrode, 141...part, 15...cathode electrode, 16...guard ring

Claims

[Claim 1] An n-type semiconductor layer made of a gallium oxide-based semiconductor, A portion of the upper surface of the n-type semiconductor layer is covered by SiO ２ An insulating film consisting of, An anode electrode connected to the upper surface of the n-type semiconductor layer, forming a Schottky junction with the n-type semiconductor layer, with at least a portion of its edge on the insulating film, Equipped with, The insulating film includes a first layer in contact with the n-type semiconductor layer and a second layer on the first layer. The refractive index of the first layer is lower than that of the second layer. The n-type semiconductor layer includes a guard ring surrounding the junction with the anode electrode. Schottky barrier diode.

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