JP2011082392A - Schottky barrier diode - Google Patents

Abstract

A Schottky barrier diode capable of improving withstand voltage is provided.
A Schottky barrier diode includes a substrate, a GaN layer formed on the substrate, and a Schottky electrode formed on and in contact with the GaN layer. The Schottky electrode 4 is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more. In the Schottky barrier diode 10, preferably, the peak concentration of carbon at the interface between the GaN layer 3 and the Schottky electrode 4 is 1 × 10 ¹⁹ cm ⁻³ or more.
[Selection] Figure 2

Description

  The present invention relates to a Schottky barrier diode.

  Gallium nitride (GaN) has various excellent characteristics such as a band gap that is about three times that of silicon (Si), a breakdown electric field strength that is about ten times higher, and a larger saturation electron velocity. Since GaN can be expected to achieve both high breakdown voltage, which is difficult with conventional Si power devices, and low loss, that is, low on-resistance, power devices (power semiconductor elements) such as Schottky barrier diodes (SBD) Application to is expected.

  As such a Schottky barrier diode, a Schottky barrier diode including a GaN support base, a GaN epitaxial layer formed on the GaN support base, and a Schottky electrode formed on the GaN epitaxial layer is disclosed. (For example, Patent Document 1).

JP 2006-100801 A

  In the Schottky barrier diode as disclosed in Patent Document 1, it is desired to improve the barrier height of the Schottky electrode in order to improve the breakdown voltage.

  The present invention has been made in view of the above-described problems, and provides a Schottky barrier diode capable of improving the breakdown voltage.

As a result of intensive studies on the factors that improve the barrier height (barrier height) in the Schottky barrier diode, the present inventor has found that the barrier height is caused by the oxygen (O) concentration in the Schottky electrode. It was. Therefore, as a result of intensive studies on the conditions under which the barrier height is effectively improved, the present inventor has found that the barrier height can be effectively improved when the Schottky electrode has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more.

That is, the Schottky barrier diode of the present invention includes a substrate, a gallium nitride (GaN) layer formed on the substrate, and a Schottky electrode formed on and in contact with the GaN layer. And an oxygen (O) concentration of 4 × 10 ¹⁹ cm ⁻³ or more.

According to the Schottky barrier diode of the present invention, the Schottky electrode is a base metal that easily binds to oxygen. Using this base metal as a Schottky electrode material, it has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more. For this reason, the barrier height of the Schottky electrode can be effectively improved. Therefore, the breakdown voltage can be improved.

In the Schottky barrier diode, preferably, the peak concentration of carbon (C) at the interface between the GaN layer and the Schottky electrode is 1 × 10 ¹⁹ cm ⁻³ or more.

As a result of further intensive research on the factors that improve the barrier height (barrier height) in the Schottky barrier diode, the present inventors have also found that the barrier height is caused by the carbon concentration at the interface between the GaN layer and the Schottky electrode. I found out. Therefore, as a result of diligent research on the conditions under which the barrier height is effectively improved, the present inventor has determined that the barrier height is increased when the carbon peak concentration at the interface between the GaN layer and the Schottky electrode is 1 × 10 ¹⁹ cm ⁻³ or more. I found that it could be improved further. Therefore, when the carbon peak concentration at the interface between the GaN layer and the Schottky electrode is 1 × 10 ¹⁹ cm ⁻³ or more, the breakdown voltage can be further improved.

  As described above, according to the Schottky barrier diode of the present invention, the breakdown voltage can be improved.

1 is a perspective view schematically showing a Schottky barrier diode in an embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line II-II in FIG. It is a flowchart which shows the manufacturing method of the Schottky barrier diode in embodiment of this invention. 10 is a flowchart showing a method for manufacturing the Schottky barrier diode of Comparative Example 1. It is a figure which shows the analysis result by SIMS of the Schottky barrier diode of the example 1 of this invention. It is a figure which shows the analysis result by SIMS of the Schottky barrier diode of the comparative example 1. It is a figure which shows the relationship between a voltage and a current density about the Schottky barrier diode of this invention example 1 and the comparative example 1 before heat processing process (S72). It is a figure which shows the relationship between a voltage and a current density about the Schottky barrier diode of this invention example 1 and the comparative example 1 after heat processing process (S72). It is a figure which shows the relationship between annealing time and barrier height in case a Schottky electrode is Pd (palladium). It is a figure which shows the relationship between annealing time and barrier height in case a Schottky electrode is Pt (platinum). It is a figure which shows the relationship between the annealing temperature and barrier height in case a Schottky electrode is Ti (titanium). It is a figure which shows the relationship between the annealing temperature in case a Schottky electrode is W (tungsten), and barrier height. It is a figure which shows the relationship between annealing temperature and barrier height in case a Schottky electrode is Al (aluminum).

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

  FIG. 1 is a perspective view schematically showing a Schottky barrier diode in the present embodiment, and FIG. 2 is a cross-sectional view. 2 is a cross-sectional view taken along line II-II in FIG. As shown in FIGS. 1 and 2, the Schottky barrier diode (SBD) 10 includes a GaN substrate 2, a GaN layer 3, a Schottky electrode 4, a field plate (FP) electrode 16, an insulating layer 17, And an ohmic electrode 6. The GaN layer 3 is formed on the surface 2 a of the GaN substrate 2. The insulating layer 17 is formed on the surface 3 a of the GaN layer 3. The Schottky electrode 4 is formed on the surface 3 a of the GaN layer 3 and inside the opening of the insulating layer 17. The field plate electrode 16 is connected to the Schottky electrode 4 located in the opening of the insulating layer 17 and is formed so as to overlap the insulating layer 17. The ohmic electrode 6 is formed under the back surface 2 b of the GaN substrate 2.

The GaN substrate 2 has a front surface 2a and a back surface 2b. The GaN substrate 2 is a free-standing substrate, for example, and has a thickness of 100 μm or more, for example. The carrier concentration of the GaN substrate 2 is, for example, about 1 × 10 ¹⁶ cm ⁻³ . In this embodiment, a GaN substrate is used as the substrate, but the substrate to be used is not limited to GaN, and a sapphire substrate or the like may be used.

  The GaN layer 3 has a surface 3 a opposite to the surface in contact with the surface 2 a of the GaN substrate 2. The GaN layer 3 has a thickness of about 5 μm, for example. The conductivity type of the GaN layer 3 is not particularly limited, but is preferably n-type from the viewpoint that it can be easily formed.

The insulating layer 17 has an opening for arranging the Schottky electrode 4 therein. The insulating layer 17 is made of, for example, a silicon nitride film (SiN _x ).

  The Schottky electrode 4 is in a position in contact with the GaN layer 3 and is formed in the opening of the insulating layer 17. The Schottky electrode 4 forms a Schottky junction with the GaN layer 3.

  The Schottky electrode 4 is a base metal, such as an alkali metal, an alkaline earth metal, Ni, Ti, Al, Zn, or W. Base metal means a metal that has a higher ionization tendency than hydrogen (H). Since the base metal easily binds to oxygen, the oxygen concentration in the Schottky electrode 4 can be increased. Note that the Schottky electrode may include a plurality of layers.

The Schottky electrode 4 has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, preferably 5 × 10 ¹⁹ cm ⁻³ or more. When the oxygen concentration in the Schottky electrode 4 is 4 × 10 ¹⁹ cm ⁻³ or more, the present inventor can effectively improve the barrier height by performing the heat treatment after forming the metal layer to be the Schottky electrode 4. I found out. For this reason, the breakdown voltage of the Schottky barrier diode 10 can be improved. When the oxygen concentration in the Schottky electrode 4 is 5 × 10 ¹⁹ cm ⁻³ or more, the barrier height can be further improved. The upper limit of the oxygen concentration in the Schottky electrode 4 is, for example, 5 × 10 ²¹ cm ^{−3 because} of easy handling of the forward voltage Vf.

  Here, the “oxygen concentration” means the lowest oxygen concentration when the oxygen concentration of the Schottky electrode 4 is measured by SIMS in the thickness direction.

The Schottky electrode 4 preferably has a carbon concentration of 4.5 × 10 ¹⁸ cm ⁻³ or more. In this case, the barrier height can be further improved. Here, the “carbon concentration” means the lowest carbon concentration when the carbon concentration of the Schottky electrode 4 is measured by SIMS in the thickness direction.

The peak carbon concentration at the interface 11 between the GaN layer 3 and the Schottky electrode 4 is preferably 1 × 10 ¹⁹ cm ⁻³ or more, and more preferably 3 × 10 ¹⁹ cm ⁻³ or more. When the carbon peak concentration at the interface 11 between the GaN layer 3 and the Schottky electrode 4 is 1 × 10 ¹⁹ cm ⁻³ or more, the present inventor performs a heat treatment after forming a metal layer to be the Schottky electrode 4. And found that the barrier height can be improved more effectively. When the carbon peak concentration at the interface 11 between the GaN layer 3 and the Schottky electrode 4 is 3 × 10 ¹⁹ cm ⁻³ or more, the barrier height can be further improved. The upper limit of the carbon concentration in the Schottky electrode 4 is, for example, 1 × 10 ²¹ cm ⁻³ from the viewpoint of suppressing the Schottky electrode 4 from expanding.

  Here, the “carbon peak concentration” means the value of the peak concentration of carbon when the interface 11 between the GaN layer 3 and the Schottky electrode 4 is measured by SIMS.

The oxygen peak concentration at the interface 11 between the GaN layer 3 and the Schottky electrode 4 is preferably 4.5 × 10 ²⁰ cm ⁻³ or more. In this case, the barrier height can be further effectively improved. Here, the above “oxygen peak concentration” means the value of the oxygen peak concentration when the interface 11 between the GaN layer 3 and the Schottky electrode 4 is measured by SIMS.

  The Schottky electrode may be a single layer, or may have a configuration in which another layer is formed on the Schottky electrode 4 described above, that is, two or more layers.

  The field plate electrode 16 is formed to alleviate electric field concentration, and has, for example, a ring shape with a planar shape having a diameter of about 220 μm.

  The field plate electrode 16 and the Schottky electrode 4 constitute an electrode 15. That is, the electrode 15 includes the Schottky electrode 4 that is a portion in contact with the surface 3 a of the GaN layer 3 inside the opening of the insulating layer 17 and the field plate electrode 16 that is a portion overlapping the insulating layer 17.

  The field plate electrode 16 and the insulating layer 17 form a field plate structure. By reducing the electric field concentration by this field plate structure, the breakdown voltage of the Schottky barrier diode 10 can be further improved. Hereinafter, the field plate structure will be described.

  The thickness t of the insulating layer 17 is, for example, not less than 10 nm and not more than 5 μm. When the thickness t of the insulating layer 17 is 10 nm or more, it is possible to suppress the resistance of the insulating layer 17 from being lowered, and the effect of the field plate structure is exhibited without the insulating layer 17 being destroyed first. On the other hand, if the thickness of the insulating layer 17 is 5 μm or less, electric field relaxation by the field plate structure can be obtained.

  The field plate length L is, for example, not less than 1 μm and not more than 1 mm. When the field plate length L is 1 μm or more, the field plate structure can be easily manufactured, and the effect of the field plate structure can be stably obtained. On the other hand, when the field plate length L is 1 mm or less, electric field relaxation by the field plate structure is obtained.

  Here, the field plate length L is a length in which the field plate electrode 16 overlaps the insulating layer 17. In the present embodiment, the field plate length L is a length in which the field plate electrode 16 overlaps the insulating layer 17 in a cross section of the Schottky barrier diode 10 passing through the center of the electrode 15 having a circular planar shape. is there. That is, when the planar shape of the opening of the insulating layer 17 is circular and the planar shape of the Schottky electrode 4 which is a part of the electrode 15 is circular, the field plate length L is the radial direction of the electrode 15. The length of the field plate electrode 16 overlaps the insulating layer 17 in FIG. In other words, the field plate length L means that the field plate electrode 16 has the insulating layer 17 in a linear direction connecting the center of gravity with respect to the planar shape of the Schottky electrode 4 and a certain point on the outer periphery of the planar shape. It is the length that overlaps.

  Further, as shown in FIG. 2, the insulating layer 17 has an end face 17 a that faces an opening that is a part where the electrode 15 contacts the GaN layer 3. The end face 17a is inclined with respect to the surface 3a of the GaN layer 3 so as to form an angle θ. The field plate electrode 16, which is a portion overlapping the insulating layer 17 in the electrode 15, is overlaid on the insulating layer 17 so as to adhere to the end face 17 a.

  Since the end face 17a is inclined with respect to the surface 3a, the effect of electric field relaxation by the field plate structure can be increased. As a result, the breakdown voltage of the Schottky barrier diode 10 can be further improved. Such an inclination of the end surface 17a of the insulating layer 17 can be formed by wet etching, dry etching, or the like. End face 17a is formed such that angle θ is in the range of, for example, not less than 0.1 ° and not more than 60 °. When the angle of inclination is 0.1 ° or more, the angle reproducibility is easily obtained. On the other hand, when the inclination angle is 60 ° or less, the effect of electric field relaxation is increased.

  Then, with reference to FIGS. 1-3, the manufacturing method of the Schottky barrier diode 10 in this Embodiment is demonstrated. FIG. 3 is a flowchart showing the manufacturing method of the Schottky barrier diode in the present embodiment in the order of steps.

  First, a substrate preparation step (S10) is performed. In this substrate preparation step (S10), the GaN substrate 2 is prepared. As this GaN substrate 2, a substrate formed by an arbitrary manufacturing method can be used. For example, a (0001) plane surface 2a produced by HVPE (Hydride Vapor Phase Epitaxy) method is used. A GaN substrate 2 is prepared.

  Next, a GaN layer forming step (S20) is performed. In this GaN layer forming step (S20), the GaN layer 3 is formed on the surface 2a of the GaN substrate 2.

  Specifically, the GaN layer 3 is grown on the GaN substrate 2 by a vapor phase growth method such as an OMVPE (Organo-Metallic Vapor Phase Epitaxy) method.

  Note that heat treatment may be performed after the GaN layer 3 is formed. In this case, the heat treatment temperature is, for example, 400 ° C. or higher and 800 ° C. or lower, preferably 600 ° C. or higher and 700 ° C. or lower. By performing the heat treatment on the GaN layer 3, the barrier height of the metal layer formed in the metal layer forming step (S71) described later can be improved, so that the barrier height of the Schottky electrode 4 can be further improved.

  Next, an insulating layer forming step (S30) is performed. In this insulating layer forming step (S30), the insulating layer 17 is formed on the surface 3a of the GaN layer 3.

Specifically, the insulating layer 17 made of SiN _x is formed on the GaN layer 3 by, for example, plasma CVD (Chemical Vapor Deposition). Although the formation method of the insulating layer 17 is not specifically limited, It is preferable to form by the plasma CVD method. In this case, the insulating layer 17 is formed on the GaN layer 3 by plasma CVD prior to the step of forming the metal layer. In the step of forming the insulating layer 17 by plasma CVD, the insulating layer 17 is formed at a temperature of 300 ° C. or higher and 400 ° C. or lower, for example. In this case, since the film quality of the insulating layer 17 can be improved and the barrier height of the metal layer formed in the metal layer forming step (S71) described later can be improved, the barrier height of the Schottky electrode 4 can be improved.

The film thickness t of the insulating layer 17 is, for example, about 1 μm. At this time, SiN _x is formed using, for example, ₃ , SiH ₄ (monosilane), NH ₃ (ammonia), H ₂ (hydrogen), nitrogen (N ₂ ), or the like as a source gas. Note that it is preferable to form a SiN _x film from SiH ₄ and N ₂ without using NH ₃ because the hydrogen concentration in the insulating layer 17 can be lowered.

  Next, an ohmic electrode formation step (S40) is performed. In the ohmic electrode forming step (S40), the ohmic electrode 6 is formed on the back surface 2b of the GaN substrate 2.

  Specifically, for example, the following steps are performed. First, the back surface 2b of the GaN substrate 2 is cleaned with organic cleaning and hydrochloric acid. Thereafter, a metal material such as Ti, Al, Au or the like is formed on the entire back surface 2b by, for example, EB (Electron Beam) vapor deposition or resistance heating vapor deposition. Thereafter, for example, heat treatment is performed at 600 ° C. for about 2 minutes in a nitrogen atmosphere to alloy the metal material, and the ohmic electrode 6 is formed.

  Next, an insulating layer etching step (S50) is performed. In this insulating layer etching step (S50), an opening is formed in a region where the Schottky electrode 4 is to be formed in the insulating layer 17 formed in the insulating layer forming step (S30).

  Specifically, a resist having an opening in which a region to be the Schottky electrode 4 is opened is formed on the insulating layer 17 by photolithography. Thereafter, the insulating layer 17 exposed from the opening of the resist is wet-etched by, for example, BHF (Buffered Hydrogen Fluoride). Further, after organic cleaning, the resist is removed by a remover. As the remover, for example, a trade name stripper 104 or a stripper 105 manufactured by Tokyo Ohka Co., Ltd. can be used. In this way, the insulating layer 17 is etched to form an opening in the insulating layer 17. At this point, the GaN layer 3 is exposed in the opening. The side surface of the opening can be formed to have a truncated cone shape having a maximum diameter of 200 μm, for example.

  Next, a carbon supply step (S60) is performed. In this carbon supply step (S60), carbon is supplied to the opening of the insulating layer 17.

  Specifically, a resist is formed in the opening of the laminate including the insulating layer 17 having the opening after the insulating layer etching step (S50). Thereafter, the entire resist surface is exposed. Next, development processing is performed to remove the entire resist exposed to light. That is, a resist is formed, and then the resist is removed. Thereby, a part of carbon constituting the resist remains in the opening. That is, carbon can be supplied to the opening of the insulating layer 17 (the surface 3a of the GaN layer 3).

  Next, a Schottky electrode formation step (S70) including a metal layer formation step (S71) and a heat treatment step (S72) is performed. In the Schottky electrode formation step (S70), first, a metal layer formation step (S71) is performed. In this metal layer forming step (S71), a metal layer is formed in contact with the GaN layer 3 inside the opening of the insulating layer 17 formed in the insulating layer etching step (S50).

  Specifically, for example, the following steps are performed. First, the surfaces of the GaN layer 3 and the insulating layer 17 are cleaned. Next, a metal layer to be the Schottky electrode 4 is formed. In this metal layer forming step (S71), a metal layer made of a base metal to be the Schottky electrode 4 is formed. This metal layer can be formed by an arbitrary method, for example, an EB method, a resistance heating vapor deposition method, or the like. Thereafter, a resist is formed on the region to be the Schottky electrode 4 using photolithography. In this state, the metal layer other than the region to be the Schottky electrode 4 is removed by etching. Next, the resist is removed. Thereby, a metal layer to be the Schottky electrode 4 is formed. The shape of the metal layer can be formed, for example, so that the planar shape is circular.

  The method for forming the metal layer in the metal layer forming step (S71) is not limited to etching, and may be formed by lift-off or the like.

  In this metal layer forming step (S71), since the base metal layer is formed, oxygen is easily taken in by the carbon supplied in the carbon supplying step (S60). For this reason, a large amount of oxygen is taken into the formed metal layer. When the metal layer forming step (S71) is performed, carbon is supplied to the opening of the insulating layer 17, that is, the surface 3a of the GaN layer 3 by the carbon supplying step (S60). There is a lot of carbon at the interface.

  Thereafter, the metal layer is heat-treated (heat treatment step (S72)). By performing the heat treatment step (S72), the Schottky electrode 4 can be formed from the metal layer. At this time, since the metal layer is a base metal, it is easier to take in oxygen. The inventor has found that the barrier height of the Schottky electrode 4 is increased by the heat treatment step (S72) because the metal layer contains a large amount of oxygen. For this reason, the barrier height of the Schottky electrode 4 is improved.

The Schottky electrode 4 formed by the Schottky electrode formation step (S70) is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, preferably 5 × 10 ¹⁹ cm ⁻³ or more. The peak concentration of carbon at the interface 11 between the GaN layer 3 and the Schottky electrode 4 is preferably 1 × 10 ¹⁹ cm ⁻³ or more.

  In the metal layer forming step (S71), two or more metal layers may be formed. In this case, the metal layer in contact with the GaN layer 3 may be a base metal.

  Next, a field plate electrode forming step (S80) is performed. In the field plate electrode formation step (S80), the field plate electrode 16 is formed so as to be connected to the Schottky electrode 4 and to overlap the insulating layer 17.

  Specifically, for example, the following steps are performed. An electrode material to be the field plate electrode 16 is formed. This electrode material can be formed by an arbitrary method, for example, an EB method, a resistance heating vapor deposition method, or the like. Thereafter, a resist is formed on the region to be the field plate electrode 16 using photolithography. In this state, the electrode material other than the region to be the field plate electrode 16 is removed by etching. Thereafter, the resist is removed. Thereby, the field plate electrode 16 can be formed.

  The method for forming the field plate electrode is not limited to etching, and may be formed by lift-off or the like.

  Further, the field plate electrode 16 may be formed of the same material as the Schottky electrode 4. Alternatively, the field plate electrode 16 may be formed using a material different from the material of the Schottky electrode 4, such as a material having good adhesion to the insulating layer 17.

  By performing the above steps (S10 to S80), the Schottky barrier diode 10 shown in FIGS. 1 and 2 can be manufactured.

  In the above manufacturing method, the field plate electrode 16 is formed after the Schottky electrode 4 is formed. However, the Schottky electrode 4 and the field plate electrode 16 may be formed simultaneously.

  In this case, for example, the following steps are performed. A metal layer to be the Schottky electrode 4 and an electrode material to be the field plate electrode 16 are formed by, for example, vapor deposition. Thereafter, a resist is formed on regions to be the Schottky electrode 4 and the field plate electrode 16 using photolithography. In this state, the electrode material other than the region to be the field plate electrode 16 is simultaneously removed by etching. Thereafter, the resist is removed.

  As a result, the Schottky electrode 4 that is in contact with the surface 3 a of the GaN layer 3 inside the opening of the insulating layer 17 and the field plate electrode 16 that is connected to the Schottky electrode 4 and overlaps the insulating layer 17. Are formed. That is, since the diameter of the field plate electrode 16 is larger than the diameter of the opening formed in the insulating layer 17, a part of the electrode 15 overlaps with the insulating layer 17 to form the field plate electrode 16.

As described above, in this embodiment, the Schottky electrode is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, preferably 5 × 10 ¹⁹ cm ⁻³ or more. Since the base metal easily binds to oxygen, the Schottky electrode 4 having an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, preferably 5 × 10 ¹⁹ cm ⁻³ or more can be formed. Thus, by performing the heat treatment step (S72), the barrier height can be effectively improved, so that the breakdown voltage of the Schottky barrier diode 10 can be improved.

  In this embodiment, the Schottky barrier diode having the field plate structure is described as an example. However, the Schottky barrier diode of the present invention does not have to have the field plate structure.

In this example, the effect of a Schottky electrode which is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more was examined.

(Invention Example 1)
The Schottky barrier diode of Example 1 of the present invention was manufactured according to the manufacturing method of the Schottky barrier diode 10 of the above-described embodiment shown in FIGS.

Specifically, in the substrate preparation step (S10), an n-type GaN free-standing substrate prepared by the HVPE method and having a main surface of (0001) plane was prepared. This GaN substrate had a carrier concentration of 5 × 10 ¹⁸ cm ⁻³ and a thickness of 400 μm.

Next, in the GaN layer forming step (S20), the n-type GaN layer 3 was epitaxially grown on the GaN substrate by OMVPE. This GaN layer 3 had a carrier concentration of 1 × 10 ¹⁶ cm ⁻³ and a thickness of 5.0 μm.

Next, in the insulating layer forming step (S30), the insulating layer 17 made of SiN _x was formed by plasma CVD. This insulating layer 17 had a thickness of 0.5 μm.

  Next, in the ohmic electrode formation step (S40), the following steps were performed. First, the back surface 2b of the GaN substrate 2 was subjected to organic cleaning and hydrochloric acid cleaning. Thereafter, 20 nm thick Ti, 100 nm thick Al, 20 nm thick Ti, and 200 nm thick Au are formed on the back surface 2b of the GaN substrate 2 by EB vapor deposition and resistance heating vapor deposition. The layers were laminated in this order. After forming this metal layer, this metal layer was heat-treated at 600 ° C. for 2 minutes in an atmosphere containing nitrogen to form an alloy. Thereby, the ohmic electrode 6 was formed.

  Next, in the insulating layer etching step (S50), the following steps were performed. First, patterning was performed on the insulating layer 17 using photolithography. Thereafter, wet etching of the insulating layer 17 was performed by BHF. Next, after organic cleaning treatment, the resist was removed using a remover. As a result, the insulating layer 17 was etched, and an opening was formed in the insulating layer 17. The side surface of the opening was formed to have a truncated cone shape having a maximum diameter of 200 μm.

  Next, in the carbon supply step (S60), the following steps were performed. First, a resist was formed on the entire surface of the insulating layer 17 having openings. Thereafter, the entire resist surface was exposed. Next, the entire surface of the resist exposed to light was removed by developing.

  Next, the Schottky electrode formation step (S70) and the field plate electrode formation step (S80) were simultaneously performed as follows. First, the surfaces of the GaN layer 3 and the insulating layer 17 were cleaned by cleaning with 10% hydrochloric acid. Next, Ni having a thickness of 200 nm was formed on the insulating layer 17 and the position in contact with the GaN layer 3 (that is, the opening of the insulating layer 17) by EB vapor deposition. Thereafter, a resist was formed on the regions to be the Schottky electrode 4 and the field plate electrode 16 by using photolithography. In this state, a metal layer other than the regions to be the Schottky electrode 4 and the field plate electrode 16 was removed using a solution of hydrochloric acid: nitric acid = 10: 1. Next, by removing the resist using the same remover as described above, a metal layer made of Ni was formed on the position in contact with the GaN layer 3 and on the insulating layer 17 (metal layer forming step (S71)).

  Thereafter, the metal layer was heat-treated in a nitrogen atmosphere at 600 ° C. for 2 minutes (S72). As a result, the Schottky electrode 4 made of Ni which is a portion in contact with the surface 3 a of the GaN layer 3 inside the opening of the insulating layer 17 and the field which is a portion connected to the Schottky electrode 4 and overlapping the insulating layer 17. An electrode 15 including a plate electrode 16 was formed. Since the diameter of the electrode 15 is larger than the diameter of the opening formed in the insulating layer 17, the field plate electrode 16 in which the electrode 15 partially overlaps the insulating layer 17 is formed. From the above, the Schottky barrier diode of Example 1 of the present invention shown in FIGS. 1 and 2 was manufactured.

(Comparative Example 1)
FIG. 4 is a flowchart showing a method for manufacturing the Schottky barrier diode of Comparative Example 1. As shown in FIG. 4, the Schottky barrier diode of Comparative Example 1 was basically manufactured in the same manner as Example 1 of the present invention, but the carbon supply step (S60) was not performed, and the insulating layer etching step ( The main difference was that the resist was removed by SPM in S50).

  Specifically, the substrate preparation step (S10), the GaN layer formation step (S20), and the insulating layer formation step (S30) were performed in the same manner as in Example 1 of the present invention.

  Next, in the insulating layer etching step (S50), after a resist was formed in the same manner as in Invention Example 1, the resist was removed using SPM, which is a mixture of sulfuric acid and hydrogen peroxide, instead of the remover.

  Next, similarly to Example 1 of the present invention, a metal layer to be a Schottky electrode and an electrode material to be a field plate electrode were formed.

  Next, the heat treatment step (S72) and the ohmic electrode formation step (S40) were performed simultaneously as follows. First, similarly to Example 1 of the present invention, a metal layer to be an ohmic electrode was formed. Thereafter, the metal layer to be the Schottky electrode and the metal layer to be the ohmic electrode were heat-treated in the same manner as in the heat treatment step (S72) of Example 1 of the present invention. Thereby, the Schottky electrode 4, the ohmic electrode 6, and the FP electrode 16 were formed. From the above, the Schottky barrier diode of Comparative Example 1 was manufactured.

(Evaluation methods)
For the Schottky barrier diodes of Invention Example 1 and Comparative Example 1, the oxygen concentration and carbon concentration were measured by SIMS from the Schottky electrode surface toward the GaN layer. The results are shown in FIGS. 5 and 6, respectively. Each of FIG. 5 and FIG. 6 is a diagram showing the analysis results by SIMS of the Schottky barrier diodes of Invention Example 1 and Comparative Example 1. 5 and 6, the horizontal axis represents the distance from the Schottky electrode surface (unit: μm), and the vertical axis represents the oxygen or carbon concentration (unit: Atoms / cm ³ ). 5 and 6, the position where the horizontal axis is 0.2 μm is the interface between the Schottky electrode and the GaN layer.

As shown in FIG. 5, in the Schottky electrode of the Schottky barrier diode of Example 1 of the present invention, when the oxygen concentration of the Schottky electrode is measured by SIMS in the thickness direction, the lowest oxygen concentration is 4 × 10 ¹⁹ cm ^−3. As described above, the peak concentration of carbon at the interface between the GaN layer and the Schottky electrode was 1 × 10 ¹⁹ cm ⁻³ or more. Further, in the Schottky electrode of the Schottky barrier diode of Comparative Example 1, when the oxygen concentration of the Schottky electrode was measured by SIMS in the thickness direction, the lowest oxygen concentration was less than 4 × 10 ¹⁹ cm ⁻³ , The peak carbon concentration at the interface between the GaN layer and the Schottky electrode was less than 1 × 10 ¹⁹ cm ⁻³ .

  Further, the barrier height was measured for the Schottky barrier diode before heat treatment of Invention Example 1 and Comparative Example 1 and the Schottky barrier diode after heat treatment of Invention Example 1 and Comparative Example 1. As a method for measuring the barrier height, a semi-auto prober was used and forward characteristics were measured. The results are shown in Table 1 below.

Further, with respect to the Schottky barrier diode before the heat treatment of Invention Example 1 and Comparative Example 1 and the Schottky barrier diode after the heat treatment of Invention Example 1 and Comparative Example 1, the breakdown voltage when a reverse bias was applied was measured. . As a method for measuring the reverse withstand voltage, a method was used in which current and voltage were measured in a state of being immersed in a fluorine-based inert liquid using a high withstand voltage prober. The results are shown in FIGS. 7 and 8, respectively. FIG. 7 is a diagram showing the relationship between voltage (reverse voltage) and current (current density) in Comparative Example 1 and Invention Example 1 in which heat treatment was not performed after the metal layer was formed. FIG. 8 is a diagram showing the relationship between voltage (reverse voltage) and current (current density) in Comparative Example 1 and Invention Example 1 in which heat treatment was performed after the metal layer was formed. 7 and 8, the horizontal axis indicates the reverse voltage (unit: V), and the vertical axis indicates the current density (unit: A / cm ² ).

(Evaluation results)
As shown in Table 1, in the Schottky barrier diode of Example 1 of the present invention provided with a Schottky electrode which was a base metal and had an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, the barrier height was reduced to 0 by heat treatment. Improved by 70 eV. On the other hand, in the Schottky barrier diode of Comparative Example 1 including the Schottky electrode having an oxygen concentration of less than 4 × 10 ¹⁹ cm ^{−3, the} barrier height was lowered by the heat treatment. From this, it was found that the barrier height can be effectively improved by heat treatment by having an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more.

As shown in FIGS. 7 and 8, the Schottky barrier diode of Example 1 of the present invention having a Schottky electrode which is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more has a withstand voltage by heat treatment. It was found that can be improved. On the other hand, in the Schottky barrier diode of Comparative Example 1 including the Schottky electrode having an oxygen concentration of less than 4 × 10 ¹⁹ cm ⁻³ , the breakdown voltage was reduced by the heat treatment. From this, it was found that when the Schottky electrode has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, the breakdown voltage is effectively improved by the heat treatment.

From the above, it can be confirmed that by providing a Schottky electrode which is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more, the barrier height can be effectively improved by heat treatment, so that the breakdown voltage can be improved. It was.

  In this example, when the Schottky electrode is a base metal, the effect of improving the barrier height by performing the heat treatment step was examined.

  The Schottky barrier diode of each sample was basically manufactured in the same manner as in Comparative Example 1, but was different only in the metal layer forming step (S71) and the heat treatment step (S72).

  Specifically, Pd, Pt, Ti, W, and Al were used in the metal layer forming step (S71) in each sample. In the heat treatment step (S72) for each sample using Pd, Pt, Ti, W, and Al as the metal layer, heat treatment was performed in a nitrogen atmosphere under predetermined conditions shown in FIGS. Each sample using Ti, W, and Al as the metal layer shown in FIGS. 11 to 13 was heat-treated for 2 minutes. For samples using Pd and Pt as the metal layer, a Schottky barrier diode in which the heat treatment step (S72) was not performed was also produced.

  For the Schottky barrier diode using Pd, Pt, Ti, W, and Al as the metal layer, the barrier height was measured in the same manner as in Example 1. The results are shown in FIGS. 9 and 10, the horizontal axis represents annealing (heat treatment) time (unit: minutes), and the vertical axis represents barrier height (unit: eV). The Schottky barrier diode that has not been subjected to the heat treatment step (S72) has an annealing time of 0 minute. 11 to 13, the horizontal axis indicates the annealing (heat treatment) temperature (unit: ° C.), and the vertical axis indicates the barrier height (unit: eV).

  As shown in FIGS. 9 and 10, in the Schottky barrier diode provided with the Schottky electrode using the noble metal as the metal layer, the barrier height of the Schottky electrode is reduced by the heat treatment under any heat treatment conditions. On the other hand, as shown in FIGS. 11 to 13, in a Schottky barrier diode including a Schottky electrode using a base metal as a metal layer, the barrier height of the Schottky electrode is improved by heat treatment if a predetermined heat treatment condition is selected. I found out that I could do it.

  As described above, according to this example, it was found that the barrier height can be improved by heat treatment under a predetermined condition by using a base metal for the metal layer to be the Schottky electrode.

  The inventor of the present invention has found that, in order to improve the barrier height of the Schottky electrode by heat treatment, it is due to the property of being easily combined with base metal oxygen. And this invention was completed by experiment of Example 1 mentioned above.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above-described embodiment but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

  2 GaN substrate, 2a, 3a front surface, 2b back surface, 3 GaN layer, 4 Schottky electrode, 6 ohmic electrode, 10 Schottky barrier diode, 11 interface, 15 electrode, 16 field plate (FP) electrode, 17 insulating layer, 17a End face.

Claims (2)

A substrate,
A gallium nitride layer formed on the substrate;
A Schottky electrode formed on and in contact with the gallium nitride layer,
The Schottky electrode is a Schottky barrier diode which is a base metal and has an oxygen concentration of 4 × 10 ¹⁹ cm ⁻³ or more. 2. The Schottky barrier diode according to claim 1, wherein a peak concentration of carbon at an interface between the gallium nitride layer and the Schottky electrode is 1 × 10 ¹⁹ cm ⁻³ or more.

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