JP4327114B2 - Nitride semiconductor device - Google Patents

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. In addition, SiC semiconductor devices and III-V nitride semiconductor devices are known as semiconductor devices with high breakdown voltage and low on-resistance. Among these, specific applications of electronic devices that take advantage of the physical properties of III-V nitride semiconductors are desired, and the development of Schottky barrier diodes has been reported (Non-Patent Document 1).
Yoshida et al., “Low On-Voltage Operation GaN-FESBD”, IEICE Technical Report, The Institute of Electrical Engineers of Japan, 2004, EDD-04-69, p17-21

  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. 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 order to achieve the above object, the invention according to claim 1 of the present application provides at least one group III element consisting of at least one of the group consisting of gallium, aluminum, boron and indium, and at least one of the group consisting of nitrogen, phosphorus and arsenic. In a nitride semiconductor device including a group III-V nitride semiconductor layer composed of a group V element containing nitrogen, a first nitride semiconductor layer including the group III-V nitride semiconductor layer stacked on a substrate; A second nitride semiconductor layer having a microcrystalline structure made of the III-V nitride semiconductor layer formed at a temperature lower than that of the first nitride semiconductor layer, and a second nitride semiconductor layer A first anode electrode that is partially recessed and exposed to the first nitride semiconductor layer exposed in the recess, and is connected to the first anode electrode, and the second nitride is connected to the first anode electrode. Schottky on semiconductor layer A second anode electrode made of the same or different metal as the first anode electrode, and a cathode electrode in ohmic contact with the first or second nitride semiconductor layer, the second anode electrode Of the junction formed between the first anode electrode and the second nitride semiconductor layer is equal to the height of the Schottky barrier formed between the first anode electrode and the first nitride semiconductor layer. It is characterized by being higher than the height of the Schottky barrier.

The invention according to claim 2 comprises a group III element consisting of at least one of the group consisting of gallium, aluminum, boron and indium and a group V element containing at least nitrogen from the group consisting of nitrogen, phosphorus and arsenic. In the nitride semiconductor device composed of a group III-V nitride semiconductor layer, a first nitride semiconductor layer composed of the group III-V nitride semiconductor layer stacked on a substrate, and the first nitride semiconductor A second nitride semiconductor layer having a microcrystalline structure formed of the group III-V nitride semiconductor layer formed at a temperature lower than that of the layer, and a first anode electrode that is in Schottky junction with the second nitride semiconductor layer And a second anode electrode made of a metal different from the first anode electrode connected to the first anode electrode and Schottky joined to the second nitride semiconductor layer, and the first or the second A cathode electrode that is in ohmic contact with the nitride semiconductor layer, and a height of a Schottky barrier of a junction formed between the second anode electrode and the second nitride semiconductor layer is set to The height of the Schottky barrier of the junction formed between the anode electrode and the second nitride semiconductor layer is higher.

According to a third aspect of the present invention, in the nitride semiconductor device according to the first or second aspect , the energy gap of the first nitride semiconductor is between the substrate and the first nitride semiconductor layer. A third nitride semiconductor layer made of the group III-V nitride semiconductor having a small energy gap is provided.

  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, a Schottky junction is formed in the second nitride semiconductor layer having a high insulating microcrystalline structure, so that the reverse leakage current under a low reverse bias condition is very high. Can be reduced.

  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 having a high insulating microcrystalline structure is formed in the nitride semiconductor layer formed at a normal growth temperature. Higher than the Schottky barrier height. 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.

  In the nitride semiconductor device of the present invention, a first nitride semiconductor layer and a second nitride semiconductor layer having a highly insulating microcrystalline structure are stacked on a substrate, and the first and second nitrides are stacked. The first and second anode electrodes are selected by appropriately selecting the first anode electrode and the electrode metal constituting the second anode electrode that are bonded to each of the semiconductor layers or only to the second nitride semiconductor layer. The height of the Schottky barrier is different. The cathode electrode has a structure that is bonded to the first or second 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.

  In addition, when a reverse bias is applied between the first and second anode electrodes and the cathode electrode, the second nitride semiconductor layer has a microcrystalline structure with high insulating properties under low reverse bias conditions. Therefore, the reverse leakage current is reduced. 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.

  1 and 2 show a field effect Schottky barrier diode using a HEMT structure according to a first embodiment of the present invention. FIG. 1 shows a sectional view thereof and FIG. 2 shows a manufacturing process thereof. As shown in FIG. 2, aluminum nitride (AlN) having a thickness of about 200 nm is formed on a substrate 11 made of semi-insulating or insulating silicon carbide (SiC) by MOCVD (Metal-Organic Chemical Vapor Deposition) method or the like. A buffer layer 12, a channel layer 13 made of non-doped gallium nitride (GaN) with a thickness of 2.5 μm, and a Schottky layer 14 made of non-doped aluminum gallium nitride (AlGaN) with a thickness of 25 nm (on the first nitride semiconductor layer) Equivalent), a cap layer 15 (second film) made of gallium nitride (GaN) having a thickness of about 500 ° C. lower than the film formation temperature of the Schottky layer 14 and having a microcrystalline structure and a high insulation thickness of 10 nm. A semiconductor substrate having a structure in which nitride semiconductor layers are sequentially stacked is prepared (FIG. 2a). In such a semiconductor substrate, a two-dimensional electron gas (carrier) is shown 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, as schematically shown by a broken line in the figure. Will occur.

Next, the cathode electrode 16 made of a laminate of titanium (Ti) / aluminum (Al) or the like is patterned on the cap layer 15 and rapidly heated at 850 ° C. for 30 seconds to form ohmic contact with the cap layer 15. (Figure 2b). Here, the cathode electrode 16 may have a structure in which the cap layer 15 is removed by etching and is in ohmic contact with the Schottky layer 14, but the ohmic electrode is formed on the cap layer 15 in the microcrystalline structure after the film formation. By forming the metal, the metal constituting the ohmic electrode penetrates into the crystal grain boundary, and an ohmic electrode having a low contact resistivity (10 ⁻⁶ Ω · cm ² ) can be obtained, which is preferable.

  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 17 exposing the layer 14 is formed (FIG. 2b).

  Thereafter, a first anode electrode 18 made of a titanium (Ti) / aluminum (Al) laminate or the like is patterned on the Schottky layer 14 in the recess 17 (FIG. 2c). Further, a second anode electrode 19 made of a laminate of platinum (Pt) / gold (Au) or the like is electrically connected to the first anode electrode 18 on the first anode electrode 18 and the cap layer 15. Then, a Schottky junction is formed with the cap layer 15 (FIG. 2d).

  In the field effect Schottky barrier diode having such a structure, a Schottky barrier having a height of about 0.3 eV is formed by the junction of Ti constituting the first anode electrode 18 and AlGaN constituting the Schottky layer 14. Is done. On the other hand, a Schottky barrier having a height of about 2.4 eV is formed by the junction of Pt constituting the second anode electrode and low-temperature grown GaN constituting the cap layer 17. The Schottky barrier height is about 0.8 eV higher than the Schottky barrier height of the same Schottky junction formed by the junction of GaN and Pt grown at a normal temperature.

  When a forward bias is applied between the first anode electrode 18 and the second anode electrode 19 and the cathode electrode 16 having different Schottky barrier heights as described above, the ON voltage is 0.1 to 0.3 V. A good rise was observed in which the forward current increased rapidly. When a reverse bias was applied, a large breakdown voltage of about 500 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 forward current rise of the junction between Ti and AlGaN is generally about 0.3 to 0.5V. On the other hand, the ON voltage of the junction between Pt and the low-temperature grown GaN of the present invention is about 2.0 to 2.5V.

  In the Schottky barrier diode 10 according to the present embodiment, the first anode electrode 18 (Ti) that functions as a Schottky junction with the Schottky layer 14 (AlGaN) mainly functions at the initial stage of rising of the forward current. Therefore, it is considered that the ON voltage of the Schottky barrier diode 10 is close to the ON resistance of the Schottky layer 14 and the first anode electrode 18. Further, 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, both the first anode electrode 18 and the second anode electrode 19 function as anode electrodes so that a larger current can flow. Become.

When a reverse bias was applied between the first anode electrode 18 and the second anode electrode 19, a large breakdown voltage of about 500 V was observed. In general, when a reverse bias of −10 V is applied between the Ti electrode and the AlGaN layer that are Schottky-bonded to each other, a reverse leakage current of about 10 ⁻⁵ to 10 ⁻³ A is generated. Further, the reverse leakage current when the Pt electrode and the low-temperature grown GaN layer are joined is two orders of magnitude smaller than that, and a breakdown voltage of about 500 V is obtained.

  In the Schottky barrier diode 10 according to the present embodiment, in the initial 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 19 and the Schottky layer 15, and slightly The reverse leakage current that is generated is prevented from flowing out. When the reverse bias is further increased, a depletion layer formed by the junction of the second anode electrode 19 and the Schottky layer 15 spreads, and only the second anode electrode 19 functions as an anode electrode, and approximately 500 V The withstand voltage is obtained.

  Next, a second embodiment in which the Schottky barrier diode having the structure shown in FIG. 1 is formed by another manufacturing method will be described. As in the first embodiment, a buffer layer 12 made of aluminum nitride (AlN) having a thickness of about 200 nm is formed on a substrate 11 made of semi-insulating or insulating silicon carbide (SiC) by MOCVD or the like. A channel layer 13 made of non-doped gallium nitride (GaN) with a thickness of 0.5 μm and a Schottky layer 14 made of non-doped aluminum gallium nitride (AlGaN) with a thickness of 25 nm (corresponding to the first nitride semiconductor layer) are sequentially laminated. A semiconductor substrate is prepared.

Next, a mask material made of SiO ₂ is formed in the region where the first anode electrode is to be formed on the Schottky layer 14 in order to selectively grow the cap layer. A cap layer 15 (corresponding to a second nitride semiconductor layer) made of gallium nitride (GaN) having a thickness of 10 nm and having a microcrystalline structure and a high insulating property is formed and laminated at a low temperature. Thereafter, the recess 17 described in the first embodiment can be formed by removing the mask material. Hereinafter, according to the steps described in the first embodiment, the cathode electrode 16, the first anode electrode 18, and the second anode electrode 19 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.

  FIG. 3 shows a third embodiment of the present invention. As in the first embodiment, a buffer layer 12 made of aluminum nitride (AlN) having a thickness of about 200 nm is formed on a substrate 11 made of semi-insulating or insulating silicon carbide (SiC) by MOCVD or the like. A channel layer 13 made of non-doped gallium nitride (GaN) having a thickness of 0.5 μm, a Schottky layer 14 made of non-doped aluminum gallium nitride (AlGaN) having a thickness of 25 nm (corresponding to the first nitride semiconductor layer), A cap layer 15 (corresponding to the second nitride semiconductor layer) made of gallium nitride (GaN) with a thickness of 30 nm, which is formed at a temperature lower than the film formation temperature by about 500 ° C., has a microcrystalline structure, and has high insulating properties. A semiconductor substrate having a sequentially stacked structure is prepared. Thereafter, a recess 17 is formed, and an anode electrode made of titanium (Ti) / aluminum (Al) is formed so as to form a Schottky junction between the Schottky layer 14 and the cap layer 15. As shown in FIG. 3, in the anode electrode of this embodiment, the first anode electrode 17 portion bonded to the Schottky layer 14 and the second anode electrode 18 portion bonded to the cap layer 15 are integrated. Is formed. In such a structure, the height of the Schottky barrier between the first anode electrode 17 and the Schottky layer 14 and the second anode electrode 18 and the cap layer 15 are compared with the first embodiment described above. The difference in the height of the Schottky barrier between the two is small. However, the Schottky barrier height of the Schottky junction bonded to the microcrystalline cap layer 15 is higher than the Schottky barrier height of the Schottky junction bonded to the nitride semiconductor layer formed at the normal growth temperature. Low on-voltage characteristics and high breakdown voltage characteristics can be obtained. The concave portion 17 can also be formed by a selective growth method.

  FIG. 4 shows a fourth embodiment of the present invention. As in the first embodiment, a buffer layer 12 made of aluminum nitride (AlN) having a thickness of about 200 nm is formed on a substrate 11 made of semi-insulating or insulating silicon carbide (SiC) by MOCVD or the like. A channel layer 13 made of non-doped gallium nitride (GaN) having a thickness of 0.5 μm, a Schottky layer 14 made of non-doped aluminum gallium nitride (AlGaN) having a thickness of 25 nm (corresponding to the first nitride semiconductor layer), A cap layer 15 (corresponding to a second nitride semiconductor layer) made of gallium nitride (GaN) having a thickness of about 10 ° C., which is formed at a temperature lower than the film formation temperature and has a microcrystalline structure and a high insulating property, is 10 nm. A semiconductor substrate having a sequentially stacked structure is prepared. Thereafter, the first anode electrode 18 made of Ti and the second anode electrode 19 made of Pt are formed on the cap layer 15 having a microcrystalline structure without forming the concave portion 17. In such a structure, the height of the Schottky barrier at the junction between the first anode electrode 18 and the cap layer 15 is slightly higher than that in the first embodiment. In addition, by appropriately selecting the electrode metal constituting the second anode electrodes 18 and 19 and forming junctions having different Schottky barrier heights, as in the first embodiment, a relatively low on-voltage characteristic and a high A breakdown voltage characteristic is obtained.

  In this embodiment, there is no need to form the concave portion 17, so there is an advantage that the step of forming the concave portion 17 can be reduced.

  As mentioned above, although the Example of this invention was described, it cannot be overemphasized that this invention is not limited to these. For example, instead of a nitride semiconductor device having a HEMT structure, a so-called FET in which a nitride semiconductor layer to which an impurity is added is used as an active layer (channel layer) and a cap layer 15 having the above-described microcrystalline structure is formed thereon. A nitride semiconductor device having a structure can be obtained. The nitride semiconductor layer is not limited to the GaN / AlGaN system, but a group III element consisting of at least one of the group consisting of gallium, aluminum, boron and indium, and a group consisting of nitrogen, phosphorus and arsenic. Of these, a group III-V nitride semiconductor layer composed of a group V element containing at least nitrogen can be formed.

  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 relatively lower than the second anode electrode, such as (W) or Ag (silver), may be used. Further, 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), Au (gold), or the like 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 height of the Schottky barrier of the first anode electrode is lower than 0.8 eV and the height of the Schottky barrier 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 1 may be a sapphire substrate or a silicon substrate instead of the silicon carbide substrate. In that case, the buffer layer 12 is preferably made of gallium nitride (GaN) grown at a low temperature.

  Although the cap layer 15 of the present invention has been described as having a microcrystalline structure, this is an aggregate of microcrystalline grains or a rearranged structure thereof, and includes a growth temperature, an atmosphere gas composition during growth, and a type of substrate to be grown. The crystal grain size, arrangement, and the like vary depending on the above, and can be obtained by controlling the growth temperature within a range where desired insulating characteristics can be obtained. If the growth temperature of the second nitride semiconductor layer is set to a temperature lower by about 400 ° C. than the growth temperature of the first nitride semiconductor layer, it is suitable for forming a high Schottky barrier on the anode electrode.

It is explanatory drawing of the nitride semiconductor device which concerns on the 1st Example of this invention. It is a figure explaining the manufacturing method of the nitride semiconductor device concerning the 1st example of the present invention. It is explanatory drawing of the nitride semiconductor device which concerns on the 3rd Example of this invention. It is explanatory drawing of the nitride semiconductor device which concerns on the 4th 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; cathode electrode,
17; recess, 18; first anode electrode, 19; second anode electrode

Claims (3)

Group III-V nitride composed of a group III element consisting of at least one of the group consisting of gallium, aluminum, boron and indium and a group V element containing at least nitrogen from the group consisting of nitrogen, phosphorus and arsenic In a nitride semiconductor device composed of a semiconductor layer,
A first nitride semiconductor layer made of the group III-V nitride semiconductor layer stacked on a substrate, and the group III-V nitride semiconductor layer formed at a temperature lower than that of the first nitride semiconductor layer A second nitride semiconductor layer having a microcrystalline structure, and a Schottky junction formed on the first nitride semiconductor layer that is partially recessed from the second nitride semiconductor layer and exposed in the recess And a second anode electrode made of the same or different metal as the first anode electrode connected to the first anode electrode and Schottky joined to the second nitride semiconductor layer A cathode electrode in ohmic contact with the first or second 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). Group III-V nitride composed of a group III element consisting of at least one of the group consisting of gallium, aluminum, boron and indium and a group V element containing at least nitrogen from the group consisting of nitrogen, phosphorus and arsenic In a nitride semiconductor device composed of a semiconductor layer,
A first nitride semiconductor layer made of the group III-V nitride semiconductor layer stacked on a substrate, and the group III-V nitride semiconductor layer formed at a temperature lower than that of the first nitride semiconductor layer A second nitride semiconductor layer having a microcrystalline structure, a first anode electrode which is in Schottky junction with the second nitride semiconductor layer, and the second nitride connected to the first anode electrode. A second anode electrode made of a metal different from the first anode electrode that is Schottky-bonded to a semiconductor layer, and a cathode electrode that is ohmic-bonded to the first or second 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 second nitride semiconductor layer. A nitride semiconductor device characterized by being higher than the height of the Schottky barrier of the junction formed in (1). 3. The nitride semiconductor device according to claim 1 , wherein the III and III semiconductor layers have an energy gap smaller than that of the first nitride semiconductor between the substrate and the first nitride semiconductor layer. A nitride semiconductor device comprising a third nitride semiconductor layer made of a group V nitride semiconductor.

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