JP2009060065A - Nitride semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new group III-V nitride semiconductor device of a high withstand voltage and a low on-voltage. <P>SOLUTION: A first nitride semiconductor layer and a second nitride semiconductor layer not containing gallium are laminated and formed on a substrate, and by appropriately selecting electrode metals constituting a first anode electrode bonded to the first nitride semiconductor layer and a second anode electrode bonded only to the second nitride semiconductor layer, the heights of the Schottky barriers of the respective first and second anode electrodes are made different. Also, a cathode electrode is structured so as to be bonded to the first nitride semiconductor layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a group III-V nitride semiconductor device, and more particularly to a nitride semiconductor device that has a high breakdown voltage and a low on-resistance and is suitable as a switching element for high power.

  A switching element for high power made of a semiconductor device is required to have a high breakdown voltage and a low on-resistance. Therefore, a power MOSFET (Metal Oxide Semiconductor FET) or an IGBT (Insulated Gate Bipolar Transistor) in which a bipolar transistor and a MOSFET are combined is used as a switching element. As semiconductor devices with high breakdown voltage and low on-resistance, SiC semiconductor devices and III-V nitride semiconductor devices are known in addition to power MOSFETs and IGBTs. Among these, specific applications of electronic devices that take advantage of the physical properties of III-V nitrides are desired, and the development of Schottky barrier diodes has been reported (Non-Patent Document 1).

The applicant of the present application has previously proposed a new group III-V nitride semiconductor device (Patent Document 1). The nitride semiconductor device proposed by the applicant of the present application is a Schottky barrier of a junction formed between a group III-V nitride semiconductor layer and a group III-V nitride semiconductor layer formed at a low temperature. The height is different.
Yoshida et al., “Low On-Voltage Operation GaN-FESBD”, IEICE Technical Committee Materials, The Institute of Electrical Engineers of Japan, 2004, EDD-04-69, p17-21 JP 2006-237430 A

  Group III-V nitride semiconductor devices, which are being developed as semiconductor devices with high breakdown voltage and low on-resistance, are in the process of development, and there are very few reported examples that fully take advantage of their physical properties. It is also desirable to improve the characteristics of III-V nitride semiconductor devices already reported. An object of the present invention is to provide a new group III-V nitride semiconductor device having a high breakdown voltage and a low on-voltage. In particular, an object is to provide a group III-V nitride semiconductor device having a low on-voltage and a large forward current.

  To achieve the above object, the invention according to claim 1 of the present application is a III-V composed of a group III element consisting of at least one of the group consisting of gallium, aluminum, and indium, and a group V element consisting of nitrogen. In a nitride semiconductor device comprising a group nitride semiconductor layer, a first nitride semiconductor layer comprising the group III-V nitride semiconductor layer stacked on a substrate, and the group III-V nitride semiconductor not containing gallium A second nitride semiconductor layer composed of a layer, a first recess that partially lacks the second nitride semiconductor layer and exposes the first nitride semiconductor layer, and the first The first anode electrode that is Schottky-bonded on the first nitride semiconductor layer exposed in the concave portion of the first and the first anode electrode that is Schottky-connected to the second nitride semiconductor layer are the same as or different from the first anode electrode. From metal A shot of a junction formed between the second anode electrode and the second nitride semiconductor layer, comprising a second anode electrode and a cathode electrode that is in ohmic contact with the first nitride semiconductor layer The height of the key barrier is higher than the height of the Schottky barrier of the junction formed between the first anode electrode and the first nitride semiconductor layer.

  The invention according to claim 2 of the present application comprises a group III-V nitride semiconductor layer composed of a group III element consisting of at least one of the group consisting of gallium, aluminum and indium and a group V element consisting of nitrogen. In the nitride semiconductor device, a first nitride semiconductor layer made of the group III-V nitride semiconductor layer stacked on a substrate and a second nitride made of the group III-V nitride semiconductor layer not containing gallium. A third nitride semiconductor layer having a microcrystalline structure composed of the III-V nitride semiconductor layer formed at a temperature lower than that of the first nitride semiconductor layer, and the third nitride semiconductor layer A part of the nitride semiconductor layer is not formed in a concave shape, a second recess exposing the second nitride semiconductor layer inside, and one of the second nitride semiconductor layer exposed in the second recess The first nitride semiconductor is not formed in a concave shape and the first nitride semiconductor is inside. A first recess exposing the layer, a first anode electrode Schottky-bonded on the first nitride semiconductor layer exposed in the first recess, and a shot on the second nitride semiconductor layer A second anode electrode made of the same or different metal as the first anode electrode that is key-connected, and a cathode electrode that is in ohmic contact with the first nitride semiconductor layer; and the second anode electrode and the second anode electrode The height of the Schottky barrier of the junction formed between the first and second nitride semiconductor layers is equal to the height of the Schottky barrier of the junction formed between the first anode electrode and the first nitride semiconductor layer. It is characterized by being higher than the height.

  The invention according to claim 3 of the present application is the nitride semiconductor device according to claim 1 or 2, wherein the group III-V nitride semiconductor layer is formed between the substrate and the first nitride semiconductor layer. 4 nitride semiconductor layers, wherein the fourth nitride semiconductor layer has an energy band gap smaller than that of the first nitride semiconductor.

  The invention according to claim 4 of the present application is the nitride semiconductor device according to claim 3, wherein the first nitride semiconductor layer is aluminum gallium nitride, the second nitride semiconductor layer is indium aluminum nitride, The nitride semiconductor layer is made of gallium nitride, and the lattice constants of the first and second nitride semiconductor layers are the same as or smaller than the lattice constant of the fourth nitride semiconductor layer. The metal semiconductor layer and the fourth nitride semiconductor layer are aligned with each other, or the first and second nitride semiconductor layers are subjected to tensile stress.

  In the nitride semiconductor device of the present invention, under the condition of a low forward bias, a current flows through a Schottky junction with a low Schottky barrier height, resulting in a low on-voltage characteristic. A current also flows through a Schottky junction with a high barrier, and a large current can flow. In addition, under the low reverse bias condition, the reverse leakage current is small, and under the high reverse bias condition, only a high Schottky barrier junction functions and high breakdown voltage characteristics are obtained. In particular, in the nitride semiconductor device of the present invention, since the Schottky junction is formed in the second nitride semiconductor layer not containing gallium, the reverse leakage current under a low reverse bias condition can be extremely reduced. it can.

  The first nitride semiconductor layer and the second nitride semiconductor layer are formed by removing a part of the nitride semiconductor layer or forming a recess by selective growth in order to form junctions having different Schottky barrier heights. An anode electrode to be bonded to each other may be formed, and can be easily formed. In particular, in the nitride semiconductor device of the present invention, the height of the Schottky barrier formed in the second nitride semiconductor layer not containing gallium is a Schottky made of a normal III-V group nitride semiconductor layer containing gallium. It becomes higher than the height of the barrier. As a result, the difference in the height of the Schottky barrier becomes large, and higher breakdown voltage characteristics can be obtained.

  Furthermore, in order to form junctions having different Schottky barrier heights, the anode electrode to be joined to the second nitride semiconductor layer may be made of a different metal, which can be easily formed.

  In addition, when the Schottky barrier diode of the present invention is formed using a nitride semiconductor layer having a so-called HEMT structure and the two-dimensional electron gas is used as a carrier, the on-voltage is further lowered, the forward current is large, and the good A forward voltage characteristic was obtained.

  Furthermore, when the Schottky barrier diode of the present invention is formed using a nitride semiconductor layer having a so-called HEMT structure and a two-dimensional electron gas is used as a carrier, InAlN having a very large spontaneous polarization is used as the first nitride semiconductor layer. When GaN is the fourth nitride semiconductor layer, a high concentration two-dimensional electron gas is formed, which is preferable. When a tensile stress is further applied, good forward voltage characteristics with a lower ON voltage and a larger forward current can be obtained due to the effect of piezoelectric polarization.

  In the nitride semiconductor device of the present invention, a first nitride semiconductor layer and a second nitride semiconductor layer not containing gallium are stacked on a substrate, and the first nitride semiconductor layer is bonded to the first nitride semiconductor layer. The height of the Schottky barrier of each of the first and second anode electrodes is appropriately selected by appropriately selecting the anode metal and the electrode metal constituting the second anode electrode joined only to the second nitride semiconductor layer. It is configured differently. The cathode electrode is structured to be bonded to the first nitride semiconductor layer.

  In the nitride semiconductor device having such a structure, when a forward bias is applied between the first and second anode electrodes and the cathode electrode, at the initial stage where the forward bias is small, the first nitride semiconductor is used. The first anode electrode that forms a junction with a low height between the layer and the Schottky barrier mainly functions, so that a current flows with a low on-voltage. When the forward bias is further increased, a current also flows through the second anode electrode, and a large current flows.

  Further, when a reverse bias is applied between the first and second anode electrodes and the cathode electrode, the reverse leakage current is reduced due to the high insulation of the Schottky layer 14 under the condition of a low reverse bias. When the reverse bias is further increased, only the second anode electrode that forms a junction with a high Schottky barrier functions, and high breakdown voltage characteristics are obtained.

  Hereinafter, the nitride semiconductor device of the present invention will be described in detail by taking a Schottky barrier diode that can be used as a switching element as an example.

FIG. 1 is a sectional view of a field effect Schottky barrier diode using a HEMT structure according to a first embodiment of the present invention. The field effect Schottky barrier diode shown in FIG. 1 can be formed as follows. First, a thickness of 200 nm is formed on a substrate 11 made of semi-insulating or conductive silicon carbide (SiC) by MOCVD (Metal-Organic Chemical Vapor Deposition) method, RF-MBE (RF-Molecular Beam Epitaxy) method, or the like. Buffer layer 12 made of about aluminum nitride (AlN), channel layer 13 made of non-doped gallium nitride (GaN) with a thickness of 2.5 μm (corresponding to the fourth nitride semiconductor layer), non-doped aluminum nitride with a thickness of 25 nm Schottky layer 14 (corresponding to the first nitride semiconductor layer) made of gallium (Al _0.2 Ga _0.8 N), cap layer 15 (second nitride) made of indium aluminum nitride (In _0.17 Al _0.83 N) with a thickness of 10 nm A semiconductor substrate having a structure in which a corresponding semiconductor layer is sequentially stacked is prepared. Here, the lattice constant of In _0.17 Al _0.83 N is the same as that of GaN, and the lattice constant of Al _0.2 Ga _0.8 N is smaller than that of GaN.

  Even in such a semiconductor substrate, a two-dimensional electron gas (carrier) is generated in the vicinity of the heterojunction surface between the channel layer 13 made of gallium nitride and the Schottky layer 14 made of aluminum gallium nitride.

  Next, a photoresist is patterned so that a region where the cathode electrode is to be formed is opened, and the exposed cap layer 15 is etched by dry etching such as RIE using a chlorine-based gas, so that the Schottky layer 14 is formed. Exposed. Thereafter, a cathode electrode 17 made of a laminate of titanium (Ti) / aluminum (Al) / titanium (Ti) / gold (Au) or the like is patterned on the exposed Schottky layer 14 and rapidly heated at 850 ° C. for 30 seconds. To form ohmic contact with the Schottky layer 14.

  Next, a photoresist is patterned so that a region where the first anode electrode is to be formed is opened, and all the exposed cap layer 15 is etched by dry etching such as RIE using a chlorine-based gas. A recess (corresponding to the first recess) exposing the layer 14 is formed. Thereafter, a first anode electrode and a second anode electrode made of a platinum (Pt) / gold (Au) laminate or the like are formed on the Schottky layer 14 and the cap layer 15 in the recess. The anode electrode thus formed is composed of a first anode electrode 16 (a) that is in Schottky junction with the Schottky layer 14 and a second anode electrode 16 (b) that is in Schottky junction with the cap layer 15. Become.

In the semiconductor substrate used for this field effect Schottky barrier diode, a heterojunction surface between a channel layer 13 made of GaN and a Schottky layer 14 made of Al _0.2 Ga _0.8 N having a lattice constant smaller than that of GaN and receiving tensile stress. Two-dimensional electron gas (carrier) is generated in the vicinity. Further, since the spontaneous polarization of the cap layer 15 made of In _0.17 Al _0.83 N having substantially the same lattice constant as that of GaN is extremely large and the band gap is large, an extremely high Schottky barrier is formed.

Further, in the field effect Schottky barrier diode having such a structure, the height of about _0.1 mm is obtained by the junction of Pt constituting the first anode electrode 16 (a) and Al _0.2 Ga _0.8 N constituting the Schottky layer 14. A 1.35 eV Schottky barrier is formed (FIG. 2a). On the other hand, a Schottky barrier with a height of 2.40 eV is formed by the junction of Pt constituting the second anode electrode 16 (b) and In _0.17 Al _0.83 N constituting the cap layer 15 (FIG. 2b).

The height of the Schottky barrier is about 1.0 eV higher than the Schottky barrier height of the Schottky junction formed by bonding with Al _0.2 Ga _0.8 N.

  When a forward bias is applied between the first anode electrode 16 (a) and the second anode electrode 16 (b) and the cathode electrode 17, the height of the Schottky barrier varies depending on the contact layer, and 0.7 to A rising edge in which the forward current rapidly increased at an on-voltage of 1.0 V was observed. When a reverse bias was applied, a large breakdown voltage of about 600 V was observed, confirming that the Schottky barrier diode had a high breakdown voltage and low on-resistance.

In the Schottky barrier diode having the above structure, the on-voltage required for the rising of the forward current at the junction of Pt and Al _0.2 Ga _0.8 N is generally about 0.7 to 1.0V. On the other hand, the ON voltage of the junction between Pt and In _0.17 Al _0.83 N is about 2.0 to 2.5V. In the Schottky barrier diode according to the present embodiment, the first anode electrode 16 (a) that mainly forms a Schottky junction with the Schottky layer 14 (Al _0.2 Ga _0.8 N) is mainly used in the first stage of the forward current rising. By functioning, it is considered that the ON voltage of the Schottky barrier diode is close to the ON resistance of the Schottky layer 14 and the first anode electrode 16. Furthermore, it is considered that the two-dimensional electron gas generated in the vicinity of the heterojunction surface between the channel layer 13 and the Schottky layer 14 serves as a carrier and contributes to an increase in forward current.

  Thereafter, when the forward bias reaches about 2.0 to 2.5 V, the second anode electrode 16 (b) contacting the cap layer 15 in addition to the first anode electrode 16 (a) contacting the Schottky layer 14. ) Function and a larger current can be passed.

When a reverse bias was applied to the first anode electrode 16 (a) and the second anode electrode 16 (b), a large breakdown voltage of about 600 V was observed. Generally, when a reverse bias of −10 V is applied between a Pt electrode and an Al _0.2 Ga _0.8 N layer that are Schottky junctions, a reverse leakage current of about 10 ⁻⁶ to 10 ⁻⁴ A is generated. To do. Further, the reverse leakage current when the Pt electrode and the In _0.17 Al _0.83 N layer are joined is two orders of magnitude smaller than that, and a breakdown voltage of about 600 V is obtained.

  In the Schottky barrier diode having the above structure, in the first stage where the reverse bias is about −10 V or less, the Schottky layer 14 has a high insulating property, so that almost no reverse leakage current is generated. When the reverse bias becomes larger than about −10 V, the two-dimensional electron gas serving as a carrier becomes almost non-existent due to the depletion layer formed by the junction between the second anode electrode 16 (b) and the cap layer 15, A slight reverse leakage current is prevented from flowing out. When the reverse bias is further increased, a depletion layer formed by the junction between the second anode electrode 16 (b) and the cap layer 15 is expanded, and a withstand voltage of about 600 V is obtained.

FIG. 3 shows a sectional view of a field effect Schottky barrier diode using the HEMT structure of the second embodiment of the present invention. The field effect Schottky barrier diode shown in FIG. 3 can be formed as follows. First, a thickness of 200 nm is formed on a substrate 11 made of semi-insulating or conductive silicon carbide (SiC) by MOCVD (Metal-Organic Chemical Vapor Deposition) method, RF-MBE (RF-Molecular Beam Epitaxy) method or the like. Buffer layer 12 made of about aluminum nitride (AlN), channel layer 13 made of non-doped gallium nitride (GaN) with a thickness of 2.5 μm (corresponding to the fourth nitride semiconductor layer), non-doped aluminum nitride with a thickness of 25 nm Schottky layer 14 (corresponding to the first nitride semiconductor layer) made of gallium (Al _0.2 Ga _0.8 N), cap layer 15 (second nitride) made of indium aluminum nitride (In _0.17 Al _0.83 N) with a thickness of 10 nm Equivalent to a physical semiconductor layer), which is formed at a temperature about 600 ° C. lower than the film formation temperature of the Schottky layer 14 and has a microcrystalline structure and has an insulating property. There low temperature growth cap layer 18 (corresponding to the third nitride semiconductor layer) is a semiconductor substrate stacked sequentially. Here, the lattice constant of In _0.17 Al _0.83 N is the same as that of GaN, and the lattice constant of Al _0.2 Ga _0.8 N is smaller than that of GaN.

  Even in such a semiconductor substrate, a two-dimensional electron gas (carrier) is generated in the vicinity of the heterojunction surface between the channel layer 13 made of gallium nitride and the Schottky layer 14 made of aluminum gallium nitride.

  Next, the photoresist is patterned so that the region where the cathode electrode is to be formed is opened, and the exposed low temperature growth cap layer 18 and the cap layer 15 are etched by dry etching such as RIE using a chlorine-based gas. The Schottky layer 14 is exposed. Thereafter, a cathode electrode 17 made of a laminate of titanium (Ti) / aluminum (Al) / titanium (Ti) / gold (Au) or the like is patterned on the exposed Schottky layer 14 and rapidly heated at 850 ° C. for 30 seconds. To form ohmic contact with the Schottky layer 14.

  Next, a photoresist is patterned so that a region where the second anode electrode is to be formed is opened, and all the low-temperature growth cap layer 18 exposed by dry etching such as RIE using a chlorine-based gas is etched to form a cap layer. 15 is formed (corresponding to the second recess). Further, a photoresist is patterned so that a region where the first anode electrode is to be formed is opened, and the cap layer 15 exposed by dry etching such as RIE using a chlorine-based gas is entirely etched to form the Schottky layer 14. An exposed recess (corresponding to the first recess) is formed. Thereafter, a first anode electrode and a second anode electrode made of a platinum (Pt) / gold (Au) laminate or the like are formed on the Schottky layer 14 and the cap layer 15 in the first recess. The anode electrode formed in this way is composed of a first anode electrode 16 (a) in Schottky contact with the Schottky layer 14 and a second anode electrode 16 (b) in Schottky contact with the cap layer 15. Become.

  In the field effect Schottky barrier diode having such a structure, a low-temperature growth cap layer 18 is provided on the cap layer 15 between the anode electrode 16 and the cathode electrode 17 as compared with the first embodiment described above. The point is different. By providing the low temperature growth cap layer 18, an effect of suppressing a current collapse in which a desired current does not flow when switching from the off time to the on time can be obtained.

Here, the cathode electrode 17 is formed by removing the low temperature growth cap layer 18 and the cap layer 15 by etching and forming a structure in ohmic contact with the Schottky layer 14 on the low temperature growth cap layer 18 having a microcrystalline structure after film formation. The cathode electrode 17 can also be formed. In this case, it is possible to obtain a cathode electrode having a low contact resistivity (10 ⁻⁶ Ωcm ² units) and in ohmic contact with the first nitride semiconductor layer by the metal constituting the cathode electrode entering the crystal grain boundary. ,preferable.

Next, a third embodiment in which the Schottky barrier diode having the structure shown in FIG. 1 is formed by another manufacturing method will be described. A thickness of about 200 nm is formed on a substrate 11 made of semi-insulating or conductive silicon carbide (SiC) by MOCVD (Metal-Organic Chemical Vapor Deposition) method, RF-MBE (RF-Molecular Beam Epitaxy) method or the like. Buffer layer 12 made of aluminum nitride (AlN), channel layer 13 made of non-doped gallium nitride (GaN) with a thickness of 2.5 μm (corresponding to the fourth nitride semiconductor layer), non-doped aluminum gallium nitride with a thickness of 25 nm ( A semiconductor substrate having a structure in which a Schottky layer 14 (corresponding to a first nitride semiconductor layer) made of Al _0.2 Ga _0.8 N) is sequentially stacked is prepared.

Next, a mask material made of silicon oxide (SiO ₂ ) is formed in the first anode electrode formation planned region on the Schottky layer 14 in order to selectively grow the cap layer. Thereafter, a cap layer 15 (corresponding to the second nitride semiconductor layer) made of indium aluminum nitride (In _0.17 Al _0.83 N) having a thickness of 10 nm is selectively grown. Thereafter, by removing the mask material, the first recess described in the first embodiment can be formed. Hereinafter, according to the steps described in the first embodiment, the cathode electrode 17 and the first anode electrode 16 can be formed to form the Schottky barrier diode of the present invention.

  As described above, even when the cap layer 15 is selectively grown without using etching, a nitride semiconductor device having a low on-voltage characteristic and a high breakdown voltage characteristic can be formed as in the first embodiment.

Next, a fourth embodiment will be described. Similar to the first embodiment, an MOCVD (Metal-Organic Chemical Vapor Deposition) method, an RF-MBE (RF-Molecular Beam Epitaxy) method, etc. are formed on a substrate 11 made of semi-insulating or conductive silicon carbide (SiC). Thus, a buffer layer 12 made of aluminum nitride (AlN) having a thickness of about 200 nm, a channel layer 13 made of non-doped gallium nitride (GaN) having a thickness of 2.5 μm (corresponding to the fourth nitride semiconductor layer), and a thickness of 25 nm Schottky layer 14 (corresponding to the first nitride semiconductor layer) made of non-doped aluminum gallium nitride (Al _0.2 Ga _0.8 N), and a cap layer 15 made of indium aluminum nitride (In _0.17 Al _0.83 N) having a thickness of 10 nm. A semiconductor substrate having a structure in which (corresponding to the second nitride semiconductor layer) is sequentially stacked is prepared.

  Next, as in the first embodiment, a photoresist is patterned so that the region where the cathode electrode is to be formed is opened, and all the exposed cap layer 15 is etched by dry etching such as RIE using a chlorine-based gas. Thus, the Schottky layer 14 is exposed. Thereafter, a cathode electrode 17 made of a laminate of titanium (Ti) / aluminum (Al) / titanium (Ti) / gold (Au) or the like is patterned on the exposed Schottky layer 14 and rapidly heated at 850 ° C. for 30 seconds. To form ohmic contact with the Schottky layer 14.

  Thereafter, a recess (corresponding to the first recess) exposing the Schottky layer 14 is formed. Then, a first anode electrode 16 (a) made of a titanium (Ti) / aluminum (Al) laminate or the like is patterned on the Schottky layer 14 in the recess. Further, the first anode electrode 16 (a) and the cap layer 15 are made of a platinum (Pt) / gold (Au) laminate or the like so as to be electrically connected to the first anode electrode 16 (a). The second anode electrode 16 (b) is patterned and a Schottky junction is formed with the cap layer 15. Note that the first recess can also be formed by a selective growth method.

In the field-effect Schottky barrier diode having such a structure, a Schottky having a height of about 0.3 eV is formed by the junction of Ti constituting the first anode electrode 16 (a) and AlGaN constituting the Schottky layer 14. A barrier is formed. On the other hand, a Schottky barrier with a height of 2.40 eV is formed by the junction of Pt constituting the second anode electrode 16 (b) and In _0.17 Al _0.83 N constituting the cap layer 15.

  When a forward bias is applied between the first anode electrode 16 (a) and the second anode electrode 16 (b) having different Schottky barrier heights and the cathode electrode 17, 0.1 to 0.3V is applied. A good rise was observed in which the forward current increased rapidly at the on-voltage. When a reverse bias was applied, a large breakdown voltage of about 600 V was observed, confirming that the Schottky barrier diode had a high breakdown voltage and a lower on-resistance than that of the first example.

In the Schottky barrier diode having the above structure, the on-voltage required for the rising of the forward current at the junction of Ti and Al _0.2 Ga _0.8 N is generally about 0.3 to 0.5V. On the other hand, the ON voltage of the junction between Pt and In _0.17 Al _0.83 N is about 2.0 to 2.5V. In the Schottky barrier diode according to the present embodiment, the first anode electrode 16 (a) that mainly forms a Schottky junction with the Schottky layer 14 (Al _0.2 Ga _0.8 N) is mainly used in the first stage of the forward current rising. By functioning, it is considered that the ON voltage of the Schottky barrier diode is close to the ON resistance of the Schottky layer 14 and the first anode electrode 16. Furthermore, it is considered that the two-dimensional electron gas generated in the vicinity of the heterojunction surface between the channel layer 13 and the Schottky layer 14 serves as a carrier and contributes to an increase in forward current.

Next, a fifth embodiment will be described. Similar to the fourth embodiment, in the second embodiment described above, the first anode electrode and the second anode electrode can be formed of different metals. Similar to the second embodiment, on a substrate 11 made of semi-insulating or conductive silicon carbide (SiC), MOCVD (Metal-Organic Chemical Vapor Deposition) method, RF-MBE (RF-Molecular Beam Epitaxy) method, etc. Thus, a buffer layer 12 made of aluminum nitride (AlN) having a thickness of about 200 nm, a channel layer 13 made of non-doped gallium nitride (GaN) having a thickness of 2.5 μm (corresponding to the fourth nitride semiconductor layer), and a thickness of 25 nm Schottky layer 14 (corresponding to the first nitride semiconductor layer) made of non-doped aluminum gallium nitride (Al _0.2 Ga _0.8 N), and a cap layer 15 made of indium aluminum nitride (In _0.17 Al _0.83 N) having a thickness of 10 nm. A semiconductor substrate having a structure in which (corresponding to the second nitride semiconductor layer) is sequentially stacked is prepared.

  Thereafter, a photoresist is patterned so that a region where the second anode electrode is to be formed is opened, and all the low-temperature growth cap layer 18 exposed by dry etching such as RIE using a chlorine-based gas is etched to form the cap layer 15. A recess (corresponding to a second recess) is formed. Further, a photoresist is patterned so that a region where the first anode electrode is to be formed is opened, and the cap layer 15 exposed by dry etching such as RIE using a chlorine-based gas is entirely etched to form the Schottky layer 14. An exposed recess (corresponding to the first recess) is formed. Thereafter, a first anode electrode 16 (a) made of a titanium (Ti) / aluminum (Al) laminate or the like is patterned on the Schottky layer 14 in the first recess. Further, platinum (Pt) / gold (Au) is electrically connected to the first anode electrode 16 (a) on the first anode electrode 16 (a) and the cap layer 15 in the second recess. The second anode electrode 16 (b) made of the laminated body is patterned and a Schottky junction is formed with the cap layer 15 to complete. Note that the second recess can also be formed by a selective growth method.

  In the field effect Schottky barrier diode having such a structure, a low temperature growth cap layer 18 is provided on the cap layer 15 between the anode electrode 16 and the cathode electrode 17 as compared with the fourth embodiment described above. The point is different. By providing the low temperature growth cap layer 18, an effect of suppressing a current collapse in which a desired current does not flow when switching from the off time to the on time can be obtained.

  When a forward bias is applied between the first anode electrode 16 (a) and the second anode electrode 16 (b) having different Schottky barrier heights and the cathode electrode 17, 0.1 to 0.3V is applied. A good rise was observed in which the forward current increased rapidly at the on-voltage. When a reverse bias was applied, a large breakdown voltage of about 600 V was observed, confirming that the Schottky barrier diode had a high breakdown voltage and a lower on-resistance than that of the second example.

Although the embodiments of the present invention have been described above, the present invention is not limited thereto. For example, the Schottky layer 14 made of Al _0.2 Ga _0.8 N and the cap layer 15 made of In _0.17 Al _0.83 N have an aluminum composition. Is increased, the lattice constant is reduced in each layer, and a tensile stress can be imparted. Thereby, it is possible to form a higher Schottky barrier and more two-dimensional electron gas (carrier).

Further, the nitride semiconductor layer of the present invention is not limited to the heterostructure of GaN / Al _0.2 Ga _0.8 N / In _0.17 Al _0.83 N, but from at least one of the group consisting of gallium, aluminum, and indium. The lattice constants of the group III-V nitride semiconductor layers of the Schottky layer 14 and the cap layer 15 are formed by a group III-V nitride semiconductor layer composed of a group III element and a group V element consisting of nitrogen. The first and second nitride semiconductor layers may be the same as or smaller than gallium nitride, and the first and second nitride semiconductor layers may be aligned with the fourth nitride semiconductor layer or subjected to tensile stress. In this case, the thickness of each layer is preferably not more than the critical film thickness.

  The metal material constituting the anode electrode may be appropriately selected according to the type of the III-V nitride semiconductor layer that forms the junction. For example, in the case of the nitride semiconductor layer described in the above embodiment, the metal material constituting the first anode electrode is not limited to Ti. For example, aluminum (Al), molybdenum (Mo), tantalum (Ta), tungsten Any metal that forms a Schottky barrier, such as (W) or silver (Ag), that is relatively lower than the second anode electrode may be used. The metal material constituting the second anode electrode is not limited to Pt. For example, a relatively high Schottky barrier such as nickel (Ni), palladium (Pd), gold (Au), etc., is used. What is necessary is just to select the metal to form. For use as a high-speed switching element, a combination in which the Schottky barrier height of the first anode electrode is lower than 0.8 eV and the Schottky barrier height of the second anode electrode is higher than 0.8 eV. When selected, a high withstand voltage characteristic can be obtained with a low on-resistance, which is preferable.

  The substrate 11 may be a sapphire substrate or a silicon substrate instead of the silicon carbide substrate. In this case, the buffer layer 12 is preferably made of gallium nitride (GaN) grown at a low temperature in the case of a sapphire substrate and aluminum nitride (AlN) in the case of a silicon substrate.

  Although the low-temperature growth cap layer 18 of the present invention has been described as having a microcrystalline structure, it is an aggregate of microcrystalline grains or a rearranged structure thereof, and includes a growth temperature, an atmosphere gas composition during growth, and a substrate to be grown. Depending on the type of crystal, the size and arrangement of crystal grains vary and can be obtained by controlling the growth temperature within a range where desired insulation characteristics can be obtained. If the growth temperature of the second nitride semiconductor layer is set to a temperature lower by about 600 ° C. than the growth temperature of the first nitride semiconductor layer, it is suitable for suppressing current collapse.

It is explanatory drawing of the nitride semiconductor device which concerns on the 1st and 3rd Example of this invention. (A) is the Al _0.2 Ga _0.8 N / GaN Schottky barrier diode in a conventional example is a diagram showing a band diagram of the heterojunction, (b) is a Schottky according to a first embodiment of the present invention barrier diode in the _{0.17 Al 0.83 N / Al 0.2 Ga} 0.8 N / GaN is a diagram showing a band diagram of the heterojunction. It is explanatory drawing of the nitride semiconductor device which concerns on the 2nd Example of this invention. It is explanatory drawing of the nitride semiconductor device which concerns on the 4th Example of this invention. It is explanatory drawing of the nitride semiconductor device which concerns on the 5th Example of this invention.

Explanation of symbols

10: Schottky barrier diode, 11; substrate, 12; buffer layer, 13; channel layer, 14; Schottky layer, 15; cap layer, 16 (a); first anode electrode, 16 (b); Anode electrode, 17; cathode electrode, 18; low temperature growth cap layer

Claims (4)

In a nitride semiconductor device comprising a group III-V nitride semiconductor layer composed of a group III element consisting of at least one of the group consisting of gallium, aluminum and indium and a group V element consisting of nitrogen,
A first nitride semiconductor layer made of the group III-V nitride semiconductor layer stacked on a substrate; a second nitride semiconductor layer made of the group III-V nitride semiconductor layer not containing gallium; A portion of the second nitride semiconductor layer is recessed, a first recess that exposes the first nitride semiconductor layer therein, and the first nitride semiconductor that is exposed in the first recess A first anode electrode having a Schottky junction on the layer, a second anode electrode made of the same or different metal as the first anode electrode to be Schottky connected to the second nitride semiconductor layer, and the first A cathode electrode that is in ohmic contact with the nitride semiconductor layer of
The height of the Schottky barrier at the junction formed between the second anode electrode and the second nitride semiconductor layer is between the first anode electrode and the first nitride semiconductor layer. A nitride semiconductor device characterized by being higher than the height of the Schottky barrier of the junction formed in (1). In a nitride semiconductor device comprising a group III-V nitride semiconductor layer composed of a group III element consisting of at least one of the group consisting of gallium, aluminum and indium and a group V element consisting of nitrogen,
A first nitride semiconductor layer made of the group III-V nitride semiconductor layer stacked on a substrate; a second nitride semiconductor layer made of the group III-V nitride semiconductor layer not containing gallium; A third nitride semiconductor layer having a microcrystalline structure made of the group III-V nitride semiconductor layer formed at a temperature lower than that of the first nitride semiconductor layer; and one of the third nitride semiconductor layers Part of the second nitride semiconductor layer exposed in the second recess, and a part of the second nitride semiconductor layer exposed in the second recess, A first recess that exposes the first nitride semiconductor layer therein, a first anode electrode that forms a Schottky junction on the first nitride semiconductor layer exposed in the first recess, and Same as the first anode electrode that is Schottky connected to the second nitride semiconductor layer A second anode electrode made of different metals, with a cathode electrode in ohmic contact with the first nitride semiconductor layer,
The height of the Schottky barrier at the junction formed between the second anode electrode and the second nitride semiconductor layer is between the first anode electrode and the first nitride semiconductor layer. A nitride semiconductor device characterized by being higher than the height of the Schottky barrier of the junction formed in (1).   The nitride semiconductor device according to claim 1, further comprising a fourth nitride semiconductor layer made of the III-V nitride semiconductor layer between the substrate and the first nitride semiconductor layer, 4. The nitride semiconductor device, wherein the fourth nitride semiconductor layer has an energy band gap smaller than that of the first nitride semiconductor.   4. The nitride semiconductor device according to claim 3, wherein said first nitride semiconductor layer is made of aluminum gallium nitride, said second nitride semiconductor layer is made of indium aluminum nitride, and said fourth nitride semiconductor layer is made of gallium nitride. The lattice constants of the first and second nitride semiconductor layers are the same as or smaller than the lattice constant of the fourth nitride semiconductor layer, and the first and second nitride semiconductor layers and the fourth nitride A nitride semiconductor device characterized in that a nitride semiconductor layer is aligned or the first and second nitride semiconductor layers are subjected to tensile stress.

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