JP2007081346A - Nitride semiconductor field effect transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a gate leak current and to obtain a superior high output characteristic in a nitride semiconductor field effect transistor. <P>SOLUTION: A gate insulating film 2 of the nitride semiconductor field effect transistor is constituted of a halogenide. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor field effect transistor, and more particularly to a MIS field effect transistor capable of high-temperature operation, high-speed switching, and high-power operation used for mobile base stations and satellite communications, particularly a gate of MIS-HEMT. The present invention relates to a nitride semiconductor field effect transistor characterized by a configuration for reducing a leakage current.

  With the recent development of wireless communication technology or satellite communication technology, there is a demand for higher speed and higher output of devices, and there is also a demand for operation in areas where conventional Si devices and GaAs devices cannot. .

  Group III nitride semiconductors typified by GaN, AlN, InN and mixed crystals thereof have attracted a great deal of attention as high-power electronic devices and short-wavelength light-emitting devices that replace GaAs-based devices because of their excellent material properties.

For example, in the case of the representative GaN,
a. The band gap is as high as 3.4 eV, and high temperature operation near 200 ° C is possible
b. Breakdown electric field and 2 × 10 ⁶ V · cm ^-1 a high breakdown voltage,
c. Although the saturation drift velocity of electrons is lower than that of GaAs, it has a characteristic that it is relatively high at 2.7 × 10 ⁷ cm / sec.

  As a high-power electronic device utilizing such characteristics, many reports have been made on field-effect transistors, particularly high-electron mobility transistors (HEMTs) in which a two-dimensional electron gas with little ionized impurity scattering is formed in a channel layer. Yes.

  However, since Group III nitride semiconductors such as GaN are not very crystalline, there are many dangling bonds and surface levels in the crystal, which are attributed to these dangling bonds and surface levels. There is a problem that the gate leakage current is large.

Therefore, in recent years, there has been reported a MIS-HEMT in which a SiN film is inserted between a gate electrode and a group III nitride semiconductor in order to reduce reverse and forward gate leakage current and improve breakdown voltage. (For example, refer nonpatent literature 1).
http: // www. nuee. nagoya-u. au. jp / labs / mizutakalab / thindex. html

However, since the band gap of the group III nitride semiconductor itself is large, the band gap is not sufficient in oxides and nitrides such as SiN, SiO ₂ , and Al ₂ O ₃ that are conventionally used as a gate insulating film. There is a problem that the gate leakage current cannot be suppressed when a forward voltage is applied to the gate.

  Accordingly, an object of the present invention is to reduce gate leakage current and to realize excellent high output characteristics.

FIG. 1 is a diagram illustrating the basic configuration of the present invention. Means for solving the problems in the present invention will be described with reference to FIG.
Reference numerals 3 and 4 in the figure denote a gate electrode and a source / drain electrode, respectively. See FIG. 1 In order to solve the above-mentioned problems, the present invention is characterized in that in a nitride semiconductor field effect transistor, the gate insulating film 2 is made of a halide.

Halides represented by CaF ₂ , MgF ₂ , LiF, etc. have a large band gap and a sufficient barrier height of the conduction band for nitride semiconductors such as GaN and AlGaN. Therefore, halides are used as the gate insulating film 2. As a result, the gate leakage current can be kept very low.

Alternatively, the gate insulating film 2 may be a multilayer structure film including at least one halide layer, and in particular, SiO ₂ , SiON, Al ₂ O ₃ , or polycrystalline AlN on the nitride semiconductor layer 1 side. By using a multilayer structure film provided with an insulating film made of any of the above, the interface state can be further reduced.

The halide in this case may be a chloride, but an alkali metal fluoride such as LiF, an alkaline earth metal fluoride such as CaF ₂ , a rare earth element fluoride such as CeF ₂ , or MgF ₂ Any one of them is preferred, and CaF ₂ , MgF ₂ or LiF is particularly desirable from the viewpoint of ease of handling and the like and stability.
In a broad sense, Mg is included in the alkaline earth metal, but in the present invention, Mg is not included in the alkaline earth metal.

  The gate insulating film 2 may be provided between the source and the gate and between the gate and the drain via a protective insulating film, thereby exposing the exposed portion between the source and the gate and between the gate and the drain. Since it is covered with a protective insulating film, the surface state can be reduced and stable characteristics can be obtained.

The film thickness of the gate insulating film 2 is preferably in the range of 1 nm to 50 nm, and if it is 1 nm or less, it becomes difficult to ensure a stable film quality, and if it exceeds 50 nm, the gate insulating film 2 becomes too thick. As a result, the driving ability is greatly reduced.
From a practical viewpoint, the range of 5 nm to 30 nm is more preferable.

  In the present invention, since a halide having a large band gap is used as the gate insulating film, the gate leakage current can be greatly reduced, and an MIS field effect transistor using a nitride semiconductor having excellent high output characteristics is realized. can do.

The present invention provides a halide having a large band gap such as CaF ₂ , MgF ₂ or LiF directly on a carrier supply layer such as an AlGaN layer for forming a two-dimensional carrier gas layer or via a surface protective semiconductor layer such as GaN. A gate insulating film is provided, and a gate electrode is provided thereon.

Here, with reference to FIG.2 and FIG.3, the manufacturing process of the AlGaN / GaN-type MIS-HEMT of Example 1 of this invention is demonstrated.
See Figure 2
First, an i-type GaN electron transit layer 12 having a thickness of, for example, 3 μm is formed on a SiC substrate 11 having a (0001) plane as a main surface by using a normal MOCVD method (metal organic chemical vapor deposition method). For example, an i-type AlGaN layer 13 with a thickness of 5 nm, an Si doping concentration of, for example, 5 × 10 ¹⁸ cm ⁻³ , and an n-type AlGaN electron supply layer 14 with a thickness of, for example, 30 nm, and an Si doping concentration of, for example, The n-type GaN protective layer 15 having a thickness of 5 × 10 ¹⁸ cm ⁻³ and a thickness of, for example, 10 nm is sequentially deposited.

  Next, a SiN film 16 serving as a passivation film having a thickness of 10 to 200 nm, for example, 100 nm is deposited on the entire surface.

Next, a resist pattern having openings in the source electrode / drain electrode formation region is provided by photolithography, and the SiN film 16 in the source / drain electrode formation region is formed by dry etching using fluorine-based and chlorine-based gas. The n-type GaN protective layer 15 is removed.
In this case, the distance between the source electrode formation region and the drain electrode formation region is, for example, 10 μm.

  Next, after sequentially depositing a Ti film and an Al film on the removal portion by using an evaporation / lift-off method, the source electrode 17 and the drain electrode 18 having ohmic characteristics are formed by performing heat treatment at, for example, 600 ° C. in a nitrogen atmosphere. Form.

See Figure 3
Next, a resist pattern having an opening with a length of, for example, 1.0 μm along the channel direction is provided in the gate electrode formation region using photolithography, and SiN in the gate electrode formation region is formed by dry etching using a fluorine-based gas. The film 16 is removed to expose the n-type GaN protective layer 15.

Next, an AlGaN / GaN system is formed by sequentially depositing a CaF ₂ film having a thickness of 5 to 20 nm, for example, 10 nm as a gate insulating film 19 and a Ni film and an Au film as a gate electrode 20 by using an evaporation / lift-off method. The basic structure of MIS-HEMT is completed.

In this AlGaN / GaN-based MIS-HEMT, since the gate insulating film 19 is made of CaF ₂ having a large band gap, the barrier height of the conduction band against a nitride semiconductor such as GaN or AlGaN can be made sufficiently high. Thereby, the gate leakage current is sufficiently small as compared with the AlGaN / GaN-based MIS-HEMT using the conventional SiN film as the gate insulating film, and good high output characteristics can be obtained.

  Further, since the surface of the n-type GaN protective layer 15 between the source and gate and between the drain and gate is covered with the SiN film 16 having excellent passivation properties, the device characteristics can be kept stable.

  In addition, since the n-type GaN protective layer 15 is provided on the n-type AlGaN electron supply layer, it is possible to effectively suppress a decrease in the effective potential barrier due to the interface state and the like.

Next, with reference to FIG.4 and FIG.5, the manufacturing process of the AlGaN / GaN-type MIS-HEMT of Example 2 of this invention is demonstrated.
See Figure 4
First, an i-type GaN electron transit layer 12 having a thickness of, for example, 3 μm is formed on a SiC substrate 11 having a (0001) plane as a main surface by using a normal MOCVD method (metal organic chemical vapor deposition method). For example, an i-type AlGaN layer 13 with a thickness of 5 nm, an Si doping concentration of, for example, 5 × 10 ¹⁸ cm ⁻³ , and an n-type AlGaN electron supply layer 14 with a thickness of, for example, 30 nm, and an Si doping concentration of, for example, The n-type GaN protective layer 15 having a thickness of 5 × 10 ¹⁸ cm ⁻³ and a thickness of, for example, 10 nm is sequentially deposited.

Next, a CaF ₂ film 21 serving as a gate insulating film having a thickness of 5 to 20 nm, for example, 10 nm is deposited on the entire surface by vapor deposition.

Then, a resist pattern having a respective opening in the source electrode and drain electrode formation region by using photolithography is provided, by dry etching, removing the CaF ₂ film 21 and the n-type GaN protective layer 15 of the source electrode and drain electrode formation region To do.
In this case, the distance between the source electrode formation region and the drain electrode formation region is, for example, 10 μm.

See Figure 5
Next, after sequentially depositing a Ti film and an Al film on the removal portion by using an evaporation / lift-off method, the source electrode 17 and the drain electrode 18 having ohmic characteristics are formed by performing heat treatment at, for example, 600 ° C. in a nitrogen atmosphere. Form.

  Next, a resist pattern having an opening with a length of, for example, 1.0 μm along the channel direction is provided, and an Ni film and an Au film as the gate electrode 22 are sequentially deposited by using an evaporation / lift-off method, thereby AlGaN. / Basic structure of GaN-based MIS-HEMT is completed.

  In the AlGaN / GaN-based MIS-HEMT according to the second embodiment of the present invention, the SiN film deposition step and the formation step of the recess for forming the gate electrode are omitted, and therefore the manufacturing process is simpler than the first embodiment. It becomes.

Next, with reference to FIG.6 and FIG.7, the manufacturing process of the AlGaN / GaN-type MIS-HEMT of Example 3 of this invention is demonstrated.
See FIG.
First, an i-type GaN electron transit layer 12 having a thickness of, for example, 3 μm is formed on a SiC substrate 11 having a (0001) plane as a main surface by using a normal MOCVD method (metal organic chemical vapor deposition method). For example, an i-type AlGaN layer 13 with a thickness of 5 nm, an Si doping concentration of, for example, 5 × 10 ¹⁸ cm ⁻³ , and an n-type AlGaN electron supply layer 14 with a thickness of, for example, 30 nm, and an Si doping concentration of, for example, The n-type GaN protective layer 15 having a thickness of 5 × 10 ¹⁸ cm ⁻³ and a thickness of, for example, 10 nm is sequentially deposited.

Next, a SiN film 23 having a thickness of 5 to 20 nm, for example, 10 nm, and a CaF ₂ film 24 having a thickness of 5 to 20 nm, for example, 10 nm, are deposited on the entire surface by vapor deposition.

Next, a resist pattern having openings in the source electrode / drain electrode formation region is provided using photolithography, and the CaF ₂ film 24 to the n-type GaN protective layer 15 in the source electrode / drain electrode formation region are removed by dry etching. To do.
In this case, the distance between the source electrode formation region and the drain electrode formation region is, for example, 10 μm.

See FIG.
Next, after sequentially depositing a Ti film and an Al film on the removal portion by using an evaporation / lift-off method, the source electrode 17 and the drain electrode 18 having ohmic characteristics are formed by performing heat treatment at, for example, 600 ° C. in a nitrogen atmosphere. Form.

  Next, a resist pattern having an opening with a length of, for example, 1.0 μm along the channel direction is provided, and an Ni film and an Au film as the gate electrode 25 are sequentially deposited by using an evaporation / lift-off method to thereby obtain AlGaN. / Basic structure of GaN-based MIS-HEMT is completed.

In the AlGaN / GaN-based MIS-HEMT of Example 3 of the present invention, the gate insulating film is composed of a multilayer structure film of SiN film / CaF ₂ film with a SiN film having excellent passivation properties with respect to GaN as a base. Therefore, the interface state can be further reduced.

Next, with reference to FIG.8 and FIG.9, the manufacturing process of the AlGaN / GaN-type MIS-HEMT of Example 4 of this invention is demonstrated.
See FIG.
First, an i-type GaN electron transit layer 12 having a thickness of, for example, 3 μm is formed on a SiC substrate 11 having a (0001) plane as a main surface by using a normal MOCVD method (metal organic chemical vapor deposition method). For example, an i-type AlGaN layer 13 with a thickness of 5 nm, an Si doping concentration of, for example, 5 × 10 ¹⁸ cm ⁻³ , and an n-type AlGaN electron supply layer 14 with a thickness of, for example, 30 nm, and an Si doping concentration of, for example, The n-type GaN protective layer 15 having a thickness of 5 × 10 ¹⁸ cm ⁻³ and a thickness of, for example, 10 nm is sequentially deposited.

  Next, a SiN film 23 having a thickness of 5 to 20 nm, for example, 10 nm is deposited on the entire surface by vapor deposition.

Next, a resist pattern having openings in the source electrode / drain electrode formation region is provided using photolithography, and dry etching using a fluorine-based gas and a chlorine-based gas is performed to form the SiN films 23 to 23 in the source / drain electrode formation region. The n-type GaN protective layer 15 is removed.
In this case, the distance between the source electrode formation region and the drain electrode formation region is, for example, 10 μm.

See FIG.
Next, after sequentially depositing a Ti film and an Al film on the removal portion by using an evaporation / lift-off method, the source electrode 17 and the drain electrode 18 having ohmic characteristics are formed by performing heat treatment at, for example, 600 ° C. in a nitrogen atmosphere. Form.

Next, a resist pattern having an opening having a length of, for example, 1.0 μm along the channel direction is provided, and a CaF ₂ film 26 and a gate having a thickness of 5 to 20 nm, for example, 10 nm, are formed using a vapor deposition / lift-off method. The basic structure of the AlGaN / GaN-based MIS-HEMT is completed by sequentially depositing the Ni film and the Au film as the electrode 27.

Also in the AlGaN / GaN MIS-HEMT of Example 4 of the present invention, the gate insulating film is composed of a multilayer structure film of SiN film / CaF ₂ film based on a SiN film having excellent passivation properties with respect to GaN. Therefore, the interface state can be further reduced.

The embodiments of the present invention have been described above. However, the present invention is not limited to the configurations and conditions described in the embodiments, and various modifications are possible. For example, in each embodiment, a substrate is used. However, the present invention is not limited to the SiC substrate. For example, a sapphire substrate or a ZnO substrate excellent in lattice matching with a GaN-based semiconductor may be used as in the case of SiC.
When a sapphire substrate is used, an i-type GaN carrier traveling layer may be provided via an AlN low-temperature buffer layer or a GaN low-temperature buffer layer.

In addition, the stacked structure of the semiconductor in each of the above embodiments is merely an example, and the number of stacked layers and the material composition are completely arbitrary as long as they function as a field effect transistor.
For example, the n-type GaN protective layer in each of the above embodiments may be omitted, and the composition of the i-type AlGaN layer and the n-type AlGaN layer is premised on Al _0.3 Ga _0.7 N, but Al _0.25 Other composition ratios such as Ga _0.75 N may be used.

  Furthermore, the present invention is not limited to the MIS-HEMT, and can be applied to, for example, a MISFET using a bulk GaN semiconductor layer.

  In addition, the layer structure / formation method of the source / drain electrode and the gate electrode in each of the above embodiments is an example, and other layer structures / materials may be used, and the manufacturing method is not limited to the lift-off method. The source / drain electrode or the gate electrode may be formed by depositing the entire surface and then selectively removing it by etching.

Further, in the above-mentioned embodiments, although using CaF ₂ as a large insulator band gap of the gate insulating film is not limited to the CaF _2, similar to the CaF ₂ alkaline earth metal Fluoride, alkali metal fluoride such as MgF ₂ or LiF, or rare earth element fluoride such as CeF ₂ may be used. In any case, an insulator having a larger band gap than SiN good.

Further, the halogen compounds are not limited to fluorides, but halogen compounds using other halogen elements such as chlorides such as CaCl ₂ , that is, alkali metal halogen compounds, alkaline earth metal halogen compounds, rare earth element halogens. A compound may be used.
However, CaF ₂ , MgF _2, or LiF is desirable from the viewpoints of ease of handling, availability, and stability.

In each of the above embodiments, the vapor deposition method is used when forming the CaF ₂ film. However, it is not limited to the vapor deposition method, and other film formation methods such as sputtering and molecular beam epitaxy are used. Is also good.

  In each of the above embodiments, the source / drain electrode formation process is described on the premise of just etching. However, it is not always necessary to perform just etching, and even if a little n-type GaN protective layer is left, Alternatively, the AlGaN layer may be slightly etched by excessive etching.

In the second embodiment, the surface of the n-type GaN protective layer 15 between the source and gate and between the drain and gate is only covered with the CaF ₂ film having the same thickness as the gate insulating film. In order to enhance the stability, the CaF ₂ film in this region may be thicker than the gate insulating film.

For example, the CaF ₂ film is first formed to be sufficiently thick, and the gate electrode forming region is made to have a CaF ₂ film thickness of about 10 nm using a resist pattern used in the lift-off method in the gate electrode formation process. The gate electrode may be deposited after etching.

Further, in the above-described Example 3 and Example 4, the thickness of the SiN film serving as the base gate insulating film is, for example, 10 nm. However, in this case, it is desirable that the SiN film as the gate insulating film is thinner. For example, after forming the SiN film to about 10 nm, the gate electrode forming portion is selectively etched to 5 nm or less, and then the CaF ₂ film is deposited.

Further, in the above-described Example 3 and Example 4, the gate insulating film has a two-layer structure of SiN / CaF ₂ , but the underlying layer does not necessarily need to be a SiN film, and is a group III from a halide such as CaF _2. Any insulator having excellent passivation properties for nitride semiconductors may be used. For example, SiO ₂ , SiON, Al ₂ O ₃ , polycrystalline AlN, or the like may be used.

The detailed features of the present invention will be described again with reference to FIG. 1 again.
Again see Figure 1
(Additional remark 1) The nitride semiconductor field effect transistor characterized by the gate insulating film 2 consisting of a halide.
(Additional remark 2) The nitride semiconductor field effect transistor characterized by the gate insulating film 2 consisting of the multilayer structure film | membrane containing at least one halide layer.
(Appendix 3) The multilayer structure film has an insulating film made of SiN, SiO ₂ , SiON, Al ₂ O ₃ , or polycrystalline AlN between the halide and the nitride semiconductor layer 1. 4. The nitride semiconductor field effect transistor according to appendix 3, wherein
(Supplementary note 4) The supplementary items 1 to 3, wherein the halide is any one of an alkali metal fluoride, an alkaline earth metal fluoride, a rare earth element fluoride, or MgF ₂ . The nitride semiconductor field effect transistor according to any one of the above.
(Supplementary note 5) The nitride semiconductor field effect transistor according to supplementary note 4, wherein the halide is one of CaF ₂ , MgF _{2, and} LiF.
(Appendix 6) The nitride according to any one of appendices 1 to 5, wherein the gate insulating film 2 is provided between a source and a gate and between a gate and a drain via a protective insulating film. Semiconductor field effect transistor.
(Supplementary note 7) The nitride semiconductor field effect transistor according to any one of supplementary notes 1 to 6, wherein the gate insulating film 2 has a thickness of 1 nm to 50 nm.
(Supplementary note 8) The nitride semiconductor field effect transistor according to supplementary note 7, wherein the gate insulating film 2 has a thickness of 5 nm to 30 nm.

  As a practical example of the present invention, a high-power transistor for a mobile base station or satellite communication is typical, but it is monolithically integrated with a blue semiconductor light emitting element as a driving transistor for a blue semiconductor light emitting element such as a blue semiconductor laser. It is possible to make it.

It is explanatory drawing of the fundamental structure of this invention. It is explanatory drawing of the manufacturing process to the middle of the AlGaN / GaN-type MIS-HEMT of Example 1 of this invention. It is explanatory drawing of the manufacturing process after FIG. 2 of AlGaN / GaN-type MIS-HEMT of Example 1 of this invention. It is explanatory drawing of the manufacturing process to the middle of the AlGaN / GaN-type MIS-HEMT of Example 2 of this invention. It is explanatory drawing of the manufacturing process after FIG. 4 of AlGaN / GaN-type MIS-HEMT of Example 2 of this invention. It is explanatory drawing of the manufacturing process to the middle of the AlGaN / GaN-type MIS-HEMT of Example 3 of this invention. It is explanatory drawing of the manufacturing process after FIG. 6 of AlGaN / GaN-type MIS-HEMT of Example 3 of this invention. It is explanatory drawing of the manufacturing process to the middle of the AlGaN / GaN-type MIS-HEMT of Example 4 of this invention. It is explanatory drawing of the manufacturing process after FIG. 8 of AlGaN / GaN-type MIS-HEMT of Example 4 of this invention.

DESCRIPTION OF SYMBOLS 1 Nitride semiconductor layer 2 Gate insulating film 3 Gate electrode 4 Source / drain electrode 11 SiC substrate 12 i-type GaN electron transit layer 13 i-type AlGaN layer 14 n-type AlGaN electron supply layer 15 n-type GaN protective layer 16 SiN film 17 Source Electrode 18 Drain electrode 19 Gate insulating film 20 Gate electrode 21 CaF ₂ film 22 Gate electrode 23 SiN film 24 CaF ₂ film 25 Gate electrode 26 CaF ₂ film 27 Gate electrode

Claims (5)

A nitride semiconductor field effect transistor, wherein the gate insulating film is made of a halide. A nitride semiconductor field effect transistor, wherein the gate insulating film is formed of a multilayer structure film including at least one halide layer. 3. The nitriding according to claim 1, wherein the halide is any one of an alkali metal fluoride, an alkaline earth metal fluoride, a rare earth element fluoride, or MgF _2. Semiconductor field effect transistor. 4. The nitride semiconductor field effect according to claim 1, wherein the gate insulating film is provided between the source and the gate and between the gate and the drain via a protective insulating film. 5. Transistor. 5. The nitride semiconductor field effect transistor according to claim 1, wherein the gate insulating film has a thickness of 1 nm to 50 nm.

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