JP2007214515A - Structure of nitride semiconductor - Google Patents

Structure of nitride semiconductor Download PDF

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JP2007214515A
JP2007214515A JP2006035635A JP2006035635A JP2007214515A JP 2007214515 A JP2007214515 A JP 2007214515A JP 2006035635 A JP2006035635 A JP 2006035635A JP 2006035635 A JP2006035635 A JP 2006035635A JP 2007214515 A JP2007214515 A JP 2007214515A
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JP4986472B2 (en
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Atsushi Nishikawa
敦 西川
Toshiki Makimoto
俊樹 牧本
Kazuhide Kumakura
一英 熊倉
Tetsuya Akasaka
哲也 赤坂
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Nippon Telegraph and Telephone Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a structure of a nitride semiconductor element using an AlGaN layer that obtains a high breakdown voltage while keeping the resistance of the element high. <P>SOLUTION: The adoption of the AlGaN layer for a layer for holding a voltage of the nitride semiconductor element in place of a GaN layer can increase the breakdown voltage. Concretely, an n<SP>+</SP>-type AlGaN buffer layer 404, an n<SP>+</SP>-type AlGaN layer 406, an n-type AlGaN layer 408, and a p-type InGaN layer 410 are sequentially grown on the surface of an n-type conductive SiC substrate 402 to manufacture a pn junction diode. A crystal defect can be suppressed and the breakdown voltage can be increased by selecting the Al composition of the AlGaN layer 406 to be larger than the Al composition of the AlGaN buffer layer 404. Further, the element resistance is suppressed lower by forming an ohmic electrode 430 to the rear side of the SiC substrate 402. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

本発明は、一般に窒化物半導体の構造に関し、さらに詳細には、熱伝導率の高いＳｉＣ基板上に作製した窒化物半導体において、特に素子抵抗を低く抑えたまま、降伏電圧を高くするための構造に関する。   The present invention generally relates to a structure of a nitride semiconductor, and more particularly, a structure for increasing a breakdown voltage in a nitride semiconductor manufactured on a SiC substrate having high thermal conductivity, particularly while keeping a device resistance low. About.

ＧａＮやＡｌＮなどの窒化物半導体は、ワイドバンドギャップを有するため、電子デバイスとして低損失かつ高い電圧で動作することが期待でき、パワーエレクトロニクス用の半導体材料として有望視されている。   Since nitride semiconductors such as GaN and AlN have a wide band gap, they can be expected to operate as an electronic device with low loss and high voltage, and are promising as semiconductor materials for power electronics.

一般に、降伏電圧（ブレイクダウン電圧）は、電圧を保持する層の半導体材料の絶縁破壊電界と層厚によって決まる。従って、層厚を増加することで、高い降伏電圧を得ることができる。しかし、ＧａＮは残留キャリア密度が高く、層厚を増加させていくと、ＧａＮ層が完全に空乏化する前に降伏が起こるため、層厚の増加による高耐圧化が制限されていた。そこで、半導体材料をＧａＮよりもバンドギャップが大きく、絶縁破壊電界の大きなＡｌＧａＮに変更することにより、高い降伏電圧を有する電子デバイスの作製が検討されている。   In general, the breakdown voltage (breakdown voltage) is determined by the breakdown electric field and the layer thickness of the semiconductor material of the layer holding the voltage. Therefore, a high breakdown voltage can be obtained by increasing the layer thickness. However, GaN has a high residual carrier density, and when the layer thickness is increased, breakdown occurs before the GaN layer is completely depleted, so that a high breakdown voltage due to an increase in the layer thickness is limited. Therefore, the production of an electronic device having a high breakdown voltage has been studied by changing the semiconductor material to AlGaN having a larger band gap than that of GaN and a large dielectric breakdown electric field.

図１に、電圧を保持する層としてＡｌＧａＮ層を使用し、降伏電圧を増加させた例として、ショットキーダイオードの構造を示す（非特許文献１参照）。このショットキーダイオード１００は、半絶縁性サファイア基板１０２上に２０ｎｍ厚のＡｌＮ層１０４と、不純物ドーピングを行わない（アンドープ）２．５μｍ厚のＡｌＧａＮ層１０６とを順次形成し、その表面上にＰｔ／Ｔｉ／Ａｕのショットキー電極１２０およびＴｉ／Ａｌ／Ｐｔ／Ａｕオーミック電極１３０を形成している。オーミック電極１３０を形成するために電極層の近傍１０８にはＳｉイオンを打ち込んでいる。アンドープＡｌＧａＮ層１０６のＡｌ組成を０％から２５％まで変化させることにより、降伏電圧を２．３ｋＶから４．３ｋＶまで増加させている。   FIG. 1 shows a Schottky diode structure as an example in which an AlGaN layer is used as a voltage holding layer and the breakdown voltage is increased (see Non-Patent Document 1). In this Schottky diode 100, a 20 nm thick AlN layer 104 and an undoped (undoped) 2.5 μm thick AlGaN layer 106 are sequentially formed on a semi-insulating sapphire substrate 102, and Pt is formed on the surface thereof. A / Ti / Au Schottky electrode 120 and a Ti / Al / Pt / Au ohmic electrode 130 are formed. In order to form the ohmic electrode 130, Si ions are implanted in the vicinity 108 of the electrode layer. The breakdown voltage is increased from 2.3 kV to 4.3 kV by changing the Al composition of the undoped AlGaN layer 106 from 0% to 25%.

図１に示す構造では、ＡｌＧａＮ層を用いることによって降伏電圧を増加しているが、電流はＡｌＧａＮ層の表面近傍を横方向に流れるため、表面欠陥の影響によりアンドープＡｌＧａＮ層の絶縁破壊電界は理論的に予想されている値の１／３０程度と低く、電極間距離を長くすることによって高耐圧を実現している。そのため、アンドープＡｌＧａＮ層を横方向に流れる電流の抵抗が無視できず、またＡｌＧａＮ層の表面にオーミック電極を形成するため、ＡｌＧａＮ層と電極層との接触抵抗の増大が避けられない。その結果、オン抵抗は３．２Ωｃｍ２と高くなっている。このように、図１に示す構造では、ＡｌＧａＮの持つ優れた特性が十分に発揮されておらず、低い素子抵抗と高い降伏電圧の両方を実現できていなかった。 In the structure shown in FIG. 1, the breakdown voltage is increased by using the AlGaN layer, but since the current flows laterally in the vicinity of the surface of the AlGaN layer, the breakdown electric field of the undoped AlGaN layer is theoretically affected by the surface defects. The breakdown voltage is as low as about 1/30 of the expected value, and a high breakdown voltage is realized by increasing the distance between the electrodes. For this reason, the resistance of the current flowing in the lateral direction through the undoped AlGaN layer cannot be ignored, and since an ohmic electrode is formed on the surface of the AlGaN layer, an increase in contact resistance between the AlGaN layer and the electrode layer is inevitable. As a result, the on-resistance is as high as 3.2 Ωcm 2 . As described above, in the structure shown in FIG. 1, the excellent characteristics of AlGaN are not fully exhibited, and both a low element resistance and a high breakdown voltage cannot be realized.

図２に、ｐｉｎダイオードの降伏電圧を増大させることを目的として、ＡｌＧａＮ層を利用した例を示す（非特許文献２）。このｐｉｎダイオード２００は、サファイア基板２０２上にＡｌＮバッファ層２０４と、０．５μｍ厚のｎ型ＡｌＧａＮ層２０６と、１．５μｍ厚のアンドープＡｌＧａＮ層２０８と、０．５μｍ厚のＭｇドーピングＧａＮ層２１０とを順次形成している。ｎ型ＡｌＧａＮ層２０６およびアンドープＡｌＧａＮ層２０８のＡｌ組成は同一であり、それぞれ０％、２０％、４０％の３通りである。サファイア基板２０２は半絶縁性であるため、ｎ型のオーミック電極２２０は、ｎ型ＡｌＧａＮ層２０６の表面に形成する必要がある。電流は、図２に示したｎ型ＡｌＧａＮ層２０６中を横方向に流れることになる。ここで、ＡｌＧａＮはＧａＮよりも抵抗が高いので、ｎ型ＧａＮ層を用いた場合に比べてｎ型ＡｌＧａＮ層２０６を用いた図２の構造では素子抵抗が増大する。さらに、ｎ型ＡｌＧａＮ層と電極層との接触抵抗も高くなるため、この構造では低抵抗を実現できない。また、アンドープＡｌＧａＮ層のＡｌ組成を増大させるにつれて結晶欠陥が増大しており、逆方向電流−電圧特性（Ｉ−Ｖ特性）におけるリーク電流が増大するため、Ａｌ組成を増加させると降伏電圧の低下も起きている。   FIG. 2 shows an example in which an AlGaN layer is used for the purpose of increasing the breakdown voltage of the pin diode (Non-Patent Document 2). The pin diode 200 includes an AlN buffer layer 204, a 0.5 μm thick n-type AlGaN layer 206, a 1.5 μm thick undoped AlGaN layer 208, and a 0.5 μm thick Mg doped GaN layer 210 on a sapphire substrate 202. Are sequentially formed. The Al composition of the n-type AlGaN layer 206 and the undoped AlGaN layer 208 is the same, and there are three ways of 0%, 20%, and 40%, respectively. Since the sapphire substrate 202 is semi-insulating, the n-type ohmic electrode 220 needs to be formed on the surface of the n-type AlGaN layer 206. The current flows in the lateral direction in the n-type AlGaN layer 206 shown in FIG. Here, since AlGaN has a higher resistance than GaN, the device resistance increases in the structure of FIG. 2 using the n-type AlGaN layer 206 as compared with the case where the n-type GaN layer is used. Furthermore, since the contact resistance between the n-type AlGaN layer and the electrode layer also increases, this structure cannot achieve a low resistance. Further, as the Al composition of the undoped AlGaN layer increases, the crystal defects increase, and the leakage current in the reverse current-voltage characteristics (IV characteristics) increases. Therefore, when the Al composition is increased, the breakdown voltage decreases. Is also happening.

A. P. Zhang, et al. “Al composition dependence of breakdown voltage in AlXGa1-XN Schottky rectifiers,” Appl. Phys. Lett., Vol.76, No.13, pp.1767-1769, March 2000.A. P. Zhang, et al. “Al composition dependence of breakdown voltage in AlXGa1-XN Schottky rectifiers,” Appl. Phys. Lett., Vol.76, No.13, pp.1767-1769, March 2000. T. G. Zhu, et al. “GaN and AlXGa1-XNp-i-n High-Voltage Rectifiers Grown by Metalorganic Chemical Vapor Deposition,” Phys. Stat. Sol. A, vol.188, No.1, pp.301-305, 2001.T. G. Zhu, et al. “GaN and AlXGa1-XNp-i-n High-Voltage Rectifiers Grown by Metalorganic Chemical Vapor Deposition,” Phys. Stat. Sol. A, vol.188, No.1, pp.301-305, 2001.

以上、従来の構造として、電圧を保持する層としてＡｌＧａＮ層を利用した例を示した。しかしながら、ＧａＮ層の代わりにＡｌＧａＮ層を用いることによって、高い降伏電圧を実現することはできるが、ＡｌＧａＮ層を横方向に流れる電流の抵抗および電極層との接触抵抗が高くなり、ショットキーダイオードまたはｐｎ接合ダイオード全体の抵抗が高くなるという問題が発生する。また、ＡｌＧａＮ層は混晶層であることから結晶欠陥が混入しやすく、リーク電流が増大すると降伏電圧の低下を招く。   As described above, an example in which an AlGaN layer is used as a layer for holding a voltage has been shown as a conventional structure. However, by using an AlGaN layer instead of the GaN layer, a high breakdown voltage can be realized, but the resistance of the current flowing in the AlGaN layer in the lateral direction and the contact resistance with the electrode layer are increased, and a Schottky diode or There arises a problem that the resistance of the entire pn junction diode increases. In addition, since the AlGaN layer is a mixed crystal layer, crystal defects are likely to be mixed in, and when the leakage current increases, the breakdown voltage decreases.

ここで、図３に示すように、ｎ型のオーミック電極２２０を形成するためにｎ型ＡｌＧａＮ層２０６ではなく、ｎ型ＧａＮ層３０６を用いると、横方向を流れる電流の抵抗および接触抵抗は小さくなるが、ｎ型ＧａＮ層３０６とアンドープＡｌＧａＮ層２０８の間の格子定数や熱膨張係数が異なるために、アンドープＡｌＧａＮ層２０８にクラックが発生しやすくなり降伏電圧は低下する。従って、従来の構造では、素子の抵抗を低く抑えたまま、降伏電圧を高くすることは不可能であった。   Here, as shown in FIG. 3, when the n-type GaN layer 306 is used instead of the n-type AlGaN layer 206 to form the n-type ohmic electrode 220, the resistance of the current flowing in the lateral direction and the contact resistance are small. However, since the lattice constant and the thermal expansion coefficient between the n-type GaN layer 306 and the undoped AlGaN layer 208 are different, cracks are likely to occur in the undoped AlGaN layer 208 and the breakdown voltage is lowered. Therefore, in the conventional structure, it is impossible to increase the breakdown voltage while keeping the resistance of the element low.

本発明の目的は、電圧を保持する層としてＡｌＧａＮ層を使用した窒化物半導体素子において、素子の抵抗を低く保ったまま、高い降伏電圧を実現することができる構造を提供することにある。   An object of the present invention is to provide a structure capable of realizing a high breakdown voltage while keeping the resistance of an element low in a nitride semiconductor element using an AlGaN layer as a voltage holding layer.

本発明は、このような目的を達成するために、請求項１に記載の発明は、導電性のＳｉＣ基板上に作製した窒化物半導体の構造であって、前記ＳｉＣ基板上に形成され、前記ＳｉＣ基板と同じ導電型を有する第１のＡｌＧａＮ層と、前記第１のＡｌＧａＮ層上に形成され、前記ＳｉＣ基板と同じ導電型を有する第２のＡｌＧａＮ層であって、Ａｌ組成が前記第１のＡｌＧａＮ層と同じかそれ以下の第２のＡｌＧａＮ層と、前記第２のＡｌＧａＮ層上に形成され、前記ＳｉＣ基板と同じ導電型を有する第３のＡｌＧａＮ層であって、Ａｌ組成が前記第２のＡｌＧａＮ層と同じかそれ以下の第３のＡｌＧａＮ層とを備えたことを特徴とする。   In order to achieve the above object, the present invention provides a nitride semiconductor structure fabricated on a conductive SiC substrate, formed on the SiC substrate, and A first AlGaN layer having the same conductivity type as the SiC substrate; and a second AlGaN layer formed on the first AlGaN layer and having the same conductivity type as the SiC substrate, wherein the Al composition is the first AlGaN layer. A second AlGaN layer equal to or less than the AlGaN layer and a third AlGaN layer formed on the second AlGaN layer and having the same conductivity type as the SiC substrate, wherein the Al composition is the first AlGaN layer. And a third AlGaN layer equal to or less than the second AlGaN layer.

また、請求項２に記載の発明は、請求項１に記載の構造であって、前記ＳｉＣ基板の裏面に形成されたオーミック電極をさらに備えたことを特徴とする。   The invention according to claim 2 is the structure according to claim 1, further comprising an ohmic electrode formed on a back surface of the SiC substrate.

また、請求項３に記載の発明は、請求項１または２に記載の構造であって、前記第３のＡｌＧａＮ層よりも上の層に形成され、前記導電性基板と異なる導電性を有するＩｎＧａＮ層（Ｉｎ組成≧０）をさらに備えたことを特徴とする。   The invention according to claim 3 is the structure according to claim 1 or 2, wherein the InGaN is formed in a layer above the third AlGaN layer and has conductivity different from that of the conductive substrate. A layer (In composition ≧ 0) is further provided.

また、請求項４に記載の発明は、請求項１ないし３のいずれかに記載の構造であって、前記第２のＡｌＧａＮ層のＡｌ組成を前記導電性ＳｉＣ基板側から前記第２のＡｌＧａＮ層側へ漸次減少させたことを特徴とする。   According to a fourth aspect of the present invention, there is provided the structure according to any one of the first to third aspects, wherein the Al composition of the second AlGaN layer is changed from the conductive SiC substrate side to the second AlGaN layer. It is characterized by being gradually reduced to the side.

また、請求項５に記載の発明は、請求項１ないし４のいずれかに記載の構造であって、前記第１のＡｌＧａＮ層の不純物ドーピング濃度は、１×１０１８ｃｍ−３以上であることを特徴とする。 The invention according to claim 5 is the structure according to any one of claims 1 to 4, wherein the impurity doping concentration of the first AlGaN layer is 1 × 10 18 cm −3 or more. It is characterized by.

また、請求項６に記載の発明は、請求項１ないし５のいずれかに記載の構造であって、前記第３のＡｌＧａＮ層の不純物ドーピング濃度は、１×１０１８ｃｍ−３未満であることを特徴とする。 The invention according to claim 6 is the structure according to any one of claims 1 to 5, wherein the impurity doping concentration of the third AlGaN layer is less than 1 × 10 18 cm −3. It is characterized by.

また、請求項７に記載の発明は、請求項１ないし６のいずれかに記載の構造であって、前記第３のＡｌＧａＮ層は、アンドープ層であることを特徴とする。   The invention according to claim 7 is the structure according to any one of claims 1 to 6, wherein the third AlGaN layer is an undoped layer.

また、請求項８に記載の発明は、請求項１ないし７のいずれかに記載の構造であって、前記ＳｉＣ基板の導電型は、ｎ型であることを特徴とする。   The invention according to claim 8 is the structure according to any one of claims 1 to 7, wherein the conductivity type of the SiC substrate is n-type.

本発明によれば、オーミック電極層を導電性ＳｉＣ基板の裏面に形成することによって、電極を形成するための窒化物半導体層が必要なくなり、素子抵抗を小さくできる。また、ＡｌＧａＮ層の結晶欠陥によるリーク電流の増大に対して、ＡｌＧａＮ層のＡｌ組成以上のＡｌ組成を有するＡｌＧａＮバッファ層をＡｌＧａＮ層とＳｉＣ基板の間に挿入することで、両者の格子定数差および熱膨張係数差により発生する結晶欠陥を低減でき、ＡｌＧａＮ層のクラックおよび結晶欠陥によるリーク電流を低減し、高い降伏電圧を実現することができる。これにより、素子の抵抗を低く保ったまま、降伏電圧を高くすることができる。   According to the present invention, by forming the ohmic electrode layer on the back surface of the conductive SiC substrate, the nitride semiconductor layer for forming the electrode is not necessary, and the element resistance can be reduced. Also, in response to an increase in leakage current due to crystal defects in the AlGaN layer, an AlGaN buffer layer having an Al composition equal to or higher than the Al composition of the AlGaN layer is inserted between the AlGaN layer and the SiC substrate, so that the difference in both lattice constants and Crystal defects generated due to the difference in thermal expansion coefficient can be reduced, leakage current due to cracks and crystal defects in the AlGaN layer can be reduced, and a high breakdown voltage can be realized. Thereby, the breakdown voltage can be increased while keeping the resistance of the element low.

本発明は、導電性ＳｉＣ基板に導電性のＡｌＧａＮバッファ層を形成し、その上にＡｌＧａＮ層よりも不純物濃度およびＡｌ組成の低いＡｌＧａＮ層を形成するとともに、導電性ＳｉＣ基板の裏面にオーミック電極を形成するように構成する。ＡｌＧａＮバッファ層を挿入することにより、ＡｌＧａＮ層の結晶欠陥を低減し、高い降伏電圧を実現することができる。また、基板に半絶縁性のサファイアではなく、導電性のＳｉＣ基板を用い、さらにその裏面にオーミック電極を形成することで素子の抵抗を低く保つことができる。   In the present invention, a conductive AlGaN buffer layer is formed on a conductive SiC substrate, an AlGaN layer having a lower impurity concentration and Al composition than the AlGaN layer is formed thereon, and an ohmic electrode is formed on the back surface of the conductive SiC substrate. Configure to form. By inserting the AlGaN buffer layer, crystal defects in the AlGaN layer can be reduced and a high breakdown voltage can be realized. Further, the resistance of the element can be kept low by using a conductive SiC substrate instead of semi-insulating sapphire for the substrate and further forming an ohmic electrode on the back surface thereof.

以下、図面を参照しながら本発明の実施形態について詳細に説明する。   Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

図４に、本発明によるｐｎ接合ダイオードの構造の一例を示す。このｐｎ接合ダイオード４００は、有機金属気相成長（ＭＯＶＰＥ）法により、ｎ型導電性ＳｉＣ基板４０２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層４０４と、５００ｎｍ厚のｎ＋型ＡｌＧａＮ層４０６と、２２５ｎｍ厚のｎ型ＡｌＧａＮ層４０８と、１４０ｎｍ厚のｐ型ＩｎＧａＮ層４１０とを順次成長させて作製した。ｎ型およびｐ型の不純物にはそれぞれＳｉおよびＭｇを用いた。また、ｎ＋型ＡｌＧａＮバッファ層４０４、ｎ＋型ＡｌＧａＮ層４０６およびｎ型ＡｌＧａＮ層４０８のＳｉ不純物ドーピング濃度は、それぞれ２×１０１９ｃｍ−３、２×１０１８ｃｍ−３、１×１０１７ｃｍ−３であり、ｐ型ＩｎＧａＮ層のＭｇ不純物濃度は、４×１０１９ｃｍ−３である。ここで、高濃度Ｓｉ不純物ドーピング層と低濃度Ｓｉ不純物ドーピング層を意図的に区別するために、高濃度のＳｉ不純物層には「ｎ＋」の記号を、低濃度の不純物層には「ｎ」の記号を用いることにする。 FIG. 4 shows an example of the structure of a pn junction diode according to the present invention. The pn junction diode 400 includes an n + type AlGaN buffer layer 404 having a thickness of 100 nm and an n + type AlGaN layer 406 having a thickness of 500 nm on the surface of the n type conductive SiC substrate 402 by metal organic vapor phase epitaxy (MOVPE). Then, an n-type AlGaN layer 408 having a thickness of 225 nm and a p-type InGaN layer 410 having a thickness of 140 nm were sequentially grown. Si and Mg were used for n-type and p-type impurities, respectively. In addition, the Si impurity doping concentrations of the n + -type AlGaN buffer layer 404, the n + -type AlGaN layer 406, and the n-type AlGaN layer 408 are 2 × 10 19 cm −3 , 2 × 10 18 cm −3 , and 1 × 10 17, respectively. cm −3 , and the Mg impurity concentration of the p-type InGaN layer is 4 × 10 19 cm −3 . Here, in order to intentionally distinguish the high-concentration Si impurity doped layer from the low-concentration Si impurity-doped layer, the symbol “n + ” is used for the high-concentration Si impurity layer, and “n + ” is used for the low-concentration impurity layer. "Is used.

ｎ＋型ＡｌＧａＮバッファ層４０４のドーピング濃度を２×１０１９ｃｍ−３と高くした理由は、ｎ型ＳｉＣ基板との接合面における結晶性を損なわない範囲でキャリア濃度を高くして、接合面の異種接合によるバンドオフセットやバリアに対する導電性を確保するためである。また、ｎ型ＡｌＧａＮ層４０８のドーピング濃度を１×１０１７ｃｍ−３と低くした理由は、キャリア濃度を低くしてこの層における耐圧を確保するためである。 The reason why the doping concentration of the n + -type AlGaN buffer layer 404 is increased to 2 × 10 19 cm −3 is that the carrier concentration is increased within a range that does not impair the crystallinity at the bonding surface with the n-type SiC substrate. This is to ensure conductivity with respect to the band offset and barrier due to the heterogeneous bonding. The reason why the doping concentration of the n-type AlGaN layer 408 is lowered to 1 × 10 17 cm −3 is to ensure the breakdown voltage in this layer by lowering the carrier concentration.

ｎ＋型ＡｌＧａＮバッファ層４０４のＡｌ組成を１５％とした。ｎ＋型ＡｌＧａＮ層４０６およびｎ型ＡｌＧａＮ層４０８のＡｌ組成は同一であり、０〜２５％とし、ｐ型ＩｎＧａＮ層４１０のＩｎ組成は１０％とした。なお、メサ構造の作製にはＥＣＲエッチング法を用いた。また、電子ビーム蒸着により、ｐ型ＩｎＧａＮ４１０上にはＰｄ／Ａｕのオーミック電極４２０を、導電性ｎ型ＳｉＣ基板４０２の裏面にはＴｉ／Ａｕのオーミック電極４３０を形成した。ｐ型ＩｎＧａＮ上に形成したＰｄ／Ａｕのオーミック電極の大きさは１００μｍ×１００μｍである。 The Al composition of the n + -type AlGaN buffer layer 404 was 15%. The Al composition of the n + -type AlGaN layer 406 and the n-type AlGaN layer 408 is the same, 0 to 25%, and the In composition of the p-type InGaN layer 410 is 10%. Note that the ECR etching method was used to fabricate the mesa structure. Also, a Pd / Au ohmic electrode 420 was formed on the p-type InGaN 410 and a Ti / Au ohmic electrode 430 was formed on the back surface of the conductive n-type SiC substrate 402 by electron beam evaporation. The size of the Pd / Au ohmic electrode formed on the p-type InGaN is 100 μm × 100 μm.

本発明では、ｎ型ＳｉＣ基板の裏面にｎ型のオーミック電極を形成し、電流を基板の表面から基板の裏面へ流しているため、図１または図２のように、ｎ型ＡｌＧａＮ層の横方向の抵抗および電極層との接触抵抗がなく、素子抵抗を低くできる。また、ＡｌＧａＮ層のリーク電流をサファイア基板ではなく、格子定数差の小さいＳｉＣ基板を用い、さらにＡｌ組成がＡｌＧａＮ層のＡｌ組成以上であるＡｌＧａＮバッファ層を挿入することで、格子定数および熱膨張係数差によるＡｌＧａＮ層内のクラックおよび結晶欠陥を低減している。   In the present invention, an n-type ohmic electrode is formed on the back surface of the n-type SiC substrate, and a current is passed from the front surface of the substrate to the back surface of the substrate. Therefore, as shown in FIG. There is no resistance in the direction and no contact resistance with the electrode layer, and the element resistance can be lowered. Also, the leakage constant of the AlGaN layer is not a sapphire substrate, but a SiC substrate with a small difference in lattice constant is used, and an AlGaN buffer layer whose Al composition is greater than or equal to the Al composition of the AlGaN layer is inserted, so that the lattice constant and thermal expansion coefficient are Cracks and crystal defects in the AlGaN layer due to the difference are reduced.

図５に、図４の構造におけるｐｎ接合ダイオードの降伏電圧とｎ型ＡｌＧａＮ層のＡｌ組成の関係を示す。降伏電圧は、逆方向Ｉ−Ｖ特性において１μＡの電流が流れる電圧と定義した。図４に示した構造では、降伏電圧はｎ型ＡｌＧａＮ層４０８によって決まる。ｐｎ接合ダイオードの降伏電圧はｎ型ＡｌＧａＮ層のＡｌ組成が１０％以下のときには、膜厚が２２５ｎｍと小さいためＧａＮ層を用いた場合との顕著な差は現れない。また、ｎ＋型ＡｌＧａＮバッファ層４０４のＡｌ組成よりもｎ型ＡｌＧａＮ層４０７のＡｌ組成が大きくなるとＡｌＧａＮのバンドギャップが大きくなるにも関わらず、降伏電圧は急激に低下する。この理由は次の通りである。まず、ｎ＋型ＡｌＧａＮバッファ層のＡｌ組成に対してｎ型ＡｌＧａＮ層のＡｌ組成が大きくなるとｎ型ＡｌＧａＮ層の格子定数がｎ＋型ＡｌＧａＮバッファ層の格子定数より小さくなるため、ｎ型ＡｌＧａＮ層内に引っ張り応力がかかることになる。一般にクラックや結晶欠陥の混入する臨界膜厚は、圧縮応力よりも引っ張り応力がかかるときのほうが小さい。よって、ｎ型ＡｌＧａＮ層内の引っ張り応力によって、クラックや結晶欠陥が混入しやすく、ｎ型ＡｌＧａＮ層内のリーク電流が増加するため、降伏電圧の低下が起こると考えられる。従って、ｎ＋型ＡｌＧａＮバッファ層のＡｌ組成は、ｎ型ＡｌＧａＮ層のＡｌ組成以上であることが必要である。図５を参照すると、ｎ型ＡｌＧａＮ層のＡｌ組成がｎ＋型ＡｌＧａＮバッファ層のＡｌ組成（１５％）以上になる点線の右側の領域で、降伏電圧が急速に低下していることが分かる。しかし、ＡｌＧａＮはＡｌ組成が高くなるほど、Ｓｉ不純物が活性化しにくくなり、かつ電子の移動度が減少するために抵抗率が高くなるので、素子抵抗を低く保ったまま降伏電圧を高くするためには設計に注意が必要である。 FIG. 5 shows the relationship between the breakdown voltage of the pn junction diode and the Al composition of the n-type AlGaN layer in the structure of FIG. The breakdown voltage was defined as a voltage through which a current of 1 μA flows in the reverse IV characteristic. In the structure shown in FIG. 4, the breakdown voltage is determined by the n-type AlGaN layer 408. When the Al composition of the n-type AlGaN layer is 10% or less, the breakdown voltage of the pn junction diode is as small as 225 nm, so that there is no significant difference from the case where the GaN layer is used. In addition, when the Al composition of the n-type AlGaN layer 407 is larger than the Al composition of the n + -type AlGaN buffer layer 404, the breakdown voltage is drastically lowered although the band gap of AlGaN is increased. The reason is as follows. First, since the lattice constant of the n-type AlGaN layer becomes smaller than the lattice constant of the n + -type AlGaN buffer layer when the Al composition of the n-type AlGaN layer becomes larger than the Al composition of the n + -type AlGaN buffer layer, the n-type AlGaN layer A tensile stress is applied inside. In general, the critical film thickness mixed with cracks and crystal defects is smaller when tensile stress is applied than compressive stress. Therefore, it is considered that cracks and crystal defects are likely to be mixed due to tensile stress in the n-type AlGaN layer, and the leakage current in the n-type AlGaN layer is increased, so that the breakdown voltage is lowered. Therefore, the Al composition of the n + -type AlGaN buffer layer needs to be greater than or equal to the Al composition of the n-type AlGaN layer. Referring to FIG. 5, it can be seen that the breakdown voltage rapidly decreases in the region on the right side of the dotted line where the Al composition of the n-type AlGaN layer is greater than or equal to the Al composition (15%) of the n + -type AlGaN buffer layer. However, the higher the Al composition of AlGaN, the more difficult the Si impurities are activated and the higher the resistivity because the mobility of electrons decreases, so in order to increase the breakdown voltage while keeping the device resistance low. Careful design is required.

図６に、本発明によるｐｎ接合ダイオードの構造の一例を示す。このｐｎ接合ダイオード６００は、実施例１と同様に、ＭＯＶＰＥ法により、導電性ｎ型ＳｉＣ基板６０２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層６０４と、５００ｎｍ厚のｎ＋型ＡｌＧａＮ層６０６と、２２５ｎｍ厚のｎ型ＡｌＧａＮ層６０８と、１４０ｎｍ厚のｐ型ＩｎＧａＮ層６１０とを順次成長させて作製した。ｎ＋型ＡｌＧａＮバッファ層のＡｌ組成は２５％とした。ｎ＋型ＡｌＧａＮ層およびｎ型ＡｌＧａＮ層のＡｌ組成はそれぞれ１５％とし、ｐ型ＩｎＧａＮ層のＩｎ組成は１０％とした。各ＡｌＧａＮ層のＡｌ組成以外は実施例１と同一である。 FIG. 6 shows an example of the structure of a pn junction diode according to the present invention. Similar to the first embodiment, the pn junction diode 600 includes a 100 nm thick n + -type AlGaN buffer layer 604 and a 500 nm thick n + -type AlGaN layer 606 on the surface of the conductive n-type SiC substrate 602 by MOVPE. Then, an n-type AlGaN layer 608 having a thickness of 225 nm and a p-type InGaN layer 610 having a thickness of 140 nm were sequentially grown. The Al composition of the n + -type AlGaN buffer layer was 25%. The Al composition of the n + -type AlGaN layer and the n-type AlGaN layer was 15%, and the In composition of the p-type InGaN layer was 10%. Except for the Al composition of each AlGaN layer, it is the same as Example 1.

この構造における降伏電圧は６０Ｖである。このとき、ｎ型ＡｌＧａＮ層の絶縁破壊電界は２．７ＭＶ／ｃｍとなり、ｎ型ＧａＮ層を用いたときに得られた絶縁破壊電界２．１ＭＶ／ｃｍから増加している。この降伏電圧の増加は、ｎ型ＡｌＧａＮ層６０８内のリーク電流を抑えるためにｎ＋型ＡｌＧａＮバッファ層６０４のＡｌ組成をｎ＋型ＡｌＧａＮ層６０６のＡｌ組成よりも１０％高くしたことによる。順方向Ｉ−Ｖ特性における１００ｍＡでの微分抵抗をオン抵抗と定義すると、１．３ｍΩｃｍ２と十分に低い値を示した。同様の構造で、ｎ＋型ＡｌＧａＮ層およびｎ型ＡｌＧａＮ層をそれぞれｎ＋型ＧａＮ層およびｎ型ＧａＮ層としたときのオン抵抗は１．２ｍΩｃｍ２であるから、ＡｌＧａＮ層を用いたことによる大きな抵抗の増加は見られない。従って、ＡｌＧａＮ層による降伏電圧の増加と低抵抗を同時に実現している。 The breakdown voltage in this structure is 60V. At this time, the breakdown electric field of the n-type AlGaN layer is 2.7 MV / cm, which is increased from the breakdown electric field 2.1 MV / cm obtained when the n-type GaN layer is used. This increase in breakdown voltage is due to the fact that the Al composition of the n + -type AlGaN buffer layer 604 is made 10% higher than the Al composition of the n + -type AlGaN layer 606 in order to suppress the leakage current in the n-type AlGaN layer 608. When the differential resistance at 100 mA in the forward IV characteristics was defined as the on-resistance, a sufficiently low value of 1.3 mΩcm 2 was shown. With the same structure, when the n + -type AlGaN layer and the n-type AlGaN layer are an n + -type GaN layer and an n-type GaN layer, respectively, the on-resistance is 1.2 mΩcm 2. There is no increase in resistance. Therefore, an increase in breakdown voltage and low resistance are simultaneously realized by the AlGaN layer.

図７に、本発明によるｐｎ接合ダイオードの構造の一例を示す。このｐｎ接合ダイオード７００は、実施例２と同様に、ＭＯＶＰＥ法により、導電性ｎ型ＳｉＣ基板７０２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層７０４と、５００ｎｍ厚のｎ＋型ＡｌＧａＮ層７０６と、２２５ｎｍ厚のｎ型ＡｌＧａＮ層７０８と、１４０ｎｍ厚のｐ型ＩｎＧａＮ層７１０とを順次成長させて作製した。ｎ＋型ＡｌＧａＮバッファ層７０４のＡｌ組成は、ＳｉＣ基板との界面では２５％とし、ｎ＋型ＡｌＧａＮ層７０６へ向かって漸次減少させ、ｎ＋型ＡｌＧａＮ層７０６との界面では１５％とした。ｎ＋型ＡｌＧａＮ層７０６およびｎ型ＡｌＧａＮ層７０８のＡｌ組成は、ともに１５％とし、ｐ型ＩｎＧａＮ層７１０のＩｎ組成は、１０％とした。ｎ＋型ＡｌＧａＮバッファ層のＡｌ組成がＳｉＣ基板界面からｎ＋型ＡｌＧａＮ層界面に向かって漸次減少している以外は実施例２と同一である。 FIG. 7 shows an example of the structure of a pn junction diode according to the present invention. Similar to the second embodiment, the pn junction diode 700 includes a 100 nm thick n + -type AlGaN buffer layer 704 and a 500 nm thick n + -type AlGaN layer 706 on the surface of the conductive n-type SiC substrate 702 by MOVPE. Then, an n-type AlGaN layer 708 having a thickness of 225 nm and a p-type InGaN layer 710 having a thickness of 140 nm were sequentially grown. Al composition of the n + -type AlGaN buffer layer 704 at the interface between the SiC substrate of 25% towards n + -type AlGaN layer 706 gradually decreases, was 15% in the interface between the n + -type AlGaN layer 706. Both the Al composition of the n + -type AlGaN layer 706 and the n-type AlGaN layer 708 were 15%, and the In composition of the p-type InGaN layer 710 was 10%. The same as Example 2, except that the Al composition of the n + -type AlGaN buffer layer gradually decreases from the SiC substrate interface toward the n + -type AlGaN layer interface.

この構造における降伏電圧はｎ型ＡｌＧａＮ層７０８によって決まるため、６０Ｖと実施例２と変わらないが、オン抵抗は１．０ｍΩｃｍ２と減少する。この抵抗の減少は、ｎ＋型ＡｌＧａＮバッファ層のＡｌ組成をＳｉＣ基板界面からｎ＋型ＡｌＧａＮ層界面へと漸次減少させることによって、ｎ＋型ＡｌＧａＮバッファ層とｎ＋型ＡｌＧａＮ層との界面でのバンド不連続の影響を小さくしているためである。 Since the breakdown voltage in this structure is determined by the n-type AlGaN layer 708, it is 60 V, which is the same as in Example 2, but the on-resistance is reduced to 1.0 mΩcm 2 . This decrease in resistance is caused by gradually decreasing the Al composition of the n + -type AlGaN buffer layer from the SiC substrate interface to the n + -type AlGaN layer interface, and thereby at the interface between the n + -type AlGaN buffer layer and the n + -type AlGaN layer. This is because the influence of the band discontinuity is reduced.

図８に、本発明によるｐｎ接合ダイオードの構造の一例を示す。このｐｎ接合ダイオード８００は、実施例２と同様に、ＭＯＶＰＥ法により、導電性ｎ型ＳｉＣ基板８０２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層８０４と、５００ｎｍ厚のｎ＋型ＡｌＧａＮ層８０６と、２２５ｎｍ厚のｎ型ＡｌＧａＮ層８０８と、１４０ｎｍ厚のｐ型ＩｎＧａＮ層８１０とを順次成長させて作製した。ｎ＋型ＡｌＧａＮバッファ層８０４のＡｌ組成は２５％とした。ｎ＋型ＡｌＧａＮ層８０６のＡｌ組成は、ｎ＋型ＡｌＧａＮバッファ層８０４との界面では２５％とし、ｎ型ＡｌＧａＮ層８０８へ向かって漸次減少させ、ｎ型ＡｌＧａＮ層８０８との界面では１５％とした。ｎ型ＡｌＧａＮ層８０８のＡｌ組成は１５％とし、ｐ型ＩｎＧａＮ層８１０のＩｎ組成は１０％とした。ｎ＋型ＡｌＧａＮ層のＡｌ組成がｎ＋型ＡｌＧａＮバッファ層界面からｎ型ＡｌＧａＮ層界面に向かって漸次減少している以外は実施例２と同一である。 FIG. 8 shows an example of the structure of a pn junction diode according to the present invention. As in the second embodiment, the pn junction diode 800 includes a 100 nm thick n + -type AlGaN buffer layer 804 and a 500 nm thick n + -type AlGaN layer 806 on the surface of the conductive n-type SiC substrate 802 by MOVPE. Then, an n-type AlGaN layer 808 having a thickness of 225 nm and a p-type InGaN layer 810 having a thickness of 140 nm were sequentially grown. The Al composition of the n + -type AlGaN buffer layer 804 was 25%. The Al composition of the n + -type AlGaN layer 806 is 25% at the interface with the n + -type AlGaN buffer layer 804, gradually decreases toward the n-type AlGaN layer 808, and 15% at the interface with the n-type AlGaN layer 808. did. The Al composition of the n-type AlGaN layer 808 was 15%, and the In composition of the p-type InGaN layer 810 was 10%. except that the n + -type AlGaN layer of Al composition is gradually reduced toward the n-type AlGaN layer interface from the n + -type AlGaN buffer layer interface is the same as in Example 2.

この構造における降伏電圧はｎ型ＡｌＧａＮ層８０８によって決まるため、６０Ｖと実施例２と変わらない。オン抵抗は１．１ｍΩｃｍ２と実施例２と比べて減少するが、実施例３よりも高くなる。この理由は次の通りである。ｎ＋型ＡｌＧａＮ層のＡｌ組成をｎ＋型ＡｌＧａＮバッファ層界面からｎ型ＡｌＧａＮ層界面に向かって漸次減少させることによって、ｎ＋型ＡｌＧａＮ層とｎ型ＡｌＧａＮ層の界面でのバンド不連続の影響を小さくなり、実施例２と比べて抵抗は低くなるが、実施例３と比べるとＡｌ組成が高いＡｌＧａＮ層の部分が長くなるため抵抗が高くなる。 Since the breakdown voltage in this structure is determined by the n-type AlGaN layer 808, it is 60 V, which is the same as in the second embodiment. The on-resistance is 1.1 mΩcm 2, which is lower than that in Example 2, but higher than that in Example 3. The reason is as follows. By the Al composition of the n + -type AlGaN layer toward the n + -type AlGaN buffer layer interface to the n-type AlGaN layer interface decreases gradually, the band discontinuity effects at the interface of the n + -type AlGaN layer and the n-type AlGaN layer However, the resistance is lower than that of the second embodiment, but the resistance is higher because the portion of the AlGaN layer having a higher Al composition is longer than that of the third embodiment.

図９に、本発明によるｐｎ接合ダイオードの構造の一例を示す。このｐｎ接合ダイオード９００は、実施例３と同様に、ＭＯＶＰＥ法により、導電性ｎ型ＳｉＣ基板９０２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層９０４と、５００ｎｍ厚のｎ＋型ＡｌＧａＮ層９０６と、２２５ｎｍ厚のアンドープＡｌＧａＮ層９０８と、１４０ｎｍ厚のｐ型ＩｎＧａＮ層９１０とを順次成長させて作製した。アンドープＡｌＧａＮ層には意図的に不純物ドーピングをしていないこと以外は実施例３と同一である。 FIG. 9 shows an example of the structure of a pn junction diode according to the present invention. Similar to the third embodiment, the pn junction diode 900 includes an n + type AlGaN buffer layer 904 having a thickness of 100 nm and an n + type AlGaN layer 906 having a thickness of 500 nm on the surface of the conductive n type SiC substrate 902 by the MOVPE method. Then, an undoped AlGaN layer 908 having a thickness of 225 nm and a p-type InGaN layer 910 having a thickness of 140 nm were sequentially grown. Example 3 is the same as Example 3 except that the undoped AlGaN layer is not intentionally doped with impurities.

この構造における降伏電圧は７０Ｖと高くなる。これは、アンドープＡｌＧａＮ層に不純物ドーピングを行わないことによってキャリア濃度が下がり、空乏層幅が大きくなるため、ＡｌＧａＮ層９０８により均一に電界がかかるためである。このときの絶縁破壊電圧は、３．１ＭＶ／ｃｍである。オン抵抗は、ＡｌＧａＮ層のキャリア濃度が下がることにより抵抗が高くなるため、１．２ｍΩｃｍ２となった。しかし、依然として素子抵抗を低く保ったまま、降伏電圧を高くすることができる。 The breakdown voltage in this structure is as high as 70V. This is because an electric field is uniformly applied to the AlGaN layer 908 because the carrier concentration is lowered and the depletion layer width is increased by not doping the undoped AlGaN layer. The dielectric breakdown voltage at this time is 3.1 MV / cm. The on-resistance was 1.2 mΩcm 2 because the resistance increased as the carrier concentration of the AlGaN layer decreased. However, the breakdown voltage can be increased while the device resistance is still kept low.

図１０に、本発明によるｐｎ接合ダイオードの構造の一例を示す。このｐｎ接合ダイオード１０００は、実施例５と同様に、ＭＯＶＰＥ法により、導電性ｎ型ＳｉＣ基板１００２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層１００４と、２００ｎｍ厚のｎ＋型ＡｌＧａＮ層１００６と、２．０μｍ厚のアンドープＡｌＧａＮ層１００８と、１４０ｎｍ厚のｐ型ＩｎＧａＮ層１０１０とを順次成長させて作製した。ｎ＋型ＡｌＧａＮバッファ層１００４のＡｌ組成は、ＳｉＣ基板１００２との界面では３５％とし、ｎ＋型ＡｌＧａＮ層１００６へ向かって漸次減少させ、ｎ＋型ＡｌＧａＮ層１００６との界面では２５％とした。ｎ＋型ＡｌＧａＮ層１００６のＡｌ組成は２５％とし、アンドープＡｌＧａＮ層１００８には意図的なドーピングは行っていない。また、ｐ型ＩｎＧａＮ層１０１０のＩｎ組成は１０％とした。ｎ＋型ＡｌＧａＮバッファ層およびｎ＋型ＡｌＧａＮ層のＡｌ組成、そしてアンドープＡｌＧａＮ層の膜厚以外は実施例４と同一である。 FIG. 10 shows an example of the structure of a pn junction diode according to the present invention. Similar to the fifth embodiment, the pn junction diode 1000 includes an n + type AlGaN buffer layer 1004 having a thickness of 100 nm and an n + type AlGaN layer 1006 having a thickness of 200 nm on the surface of the conductive n type SiC substrate 1002 by the MOVPE method. Then, an undoped AlGaN layer 1008 having a thickness of 2.0 μm and a p-type InGaN layer 1010 having a thickness of 140 nm were sequentially grown. Al composition of the n + -type AlGaN buffer layer 1004, and 35% at the interface with the SiC substrate 1002, toward the n + -type AlGaN layer 1006 gradually decreases, was 25% in the interface between the n + -type AlGaN layer 1006 . The Al composition of the n + -type AlGaN layer 1006 is 25%, and the undoped AlGaN layer 1008 is not intentionally doped. The p-type InGaN layer 1010 had an In composition of 10%. Example 4 is the same as Example 4 except for the Al composition of the n + -type AlGaN buffer layer and the n + -type AlGaN layer and the film thickness of the undoped AlGaN layer.

この構造における降伏電圧は６４０Ｖである。このとき、Ａｌ組成が２５％のアンドープＡｌＧａＮ層の絶縁破壊電界は３．６ＭＶ／ｃｍである。Ａｌ組成の高いＡｌＧａＮ層１００８を用いることにより、ＳｉＣ基板との格子定数差、熱膨張係数差が小さくなるため、クラックが発生しにくく、１．８μｍまでアンドープＡｌＧａＮ層の膜厚を増加させることができたため、高い降伏電圧が実現された。また、このときのオン抵抗は、３．８ｍΩｃｍ２であり、アンドープＡｌＧａＮ層厚が増大することによって若干高くなったが許容範囲内である。 The breakdown voltage in this structure is 640V. At this time, the breakdown electric field of the undoped AlGaN layer having an Al composition of 25% is 3.6 MV / cm. By using the AlGaN layer 1008 having a high Al composition, the difference in lattice constant and thermal expansion coefficient from the SiC substrate is reduced, so that cracks are unlikely to occur and the film thickness of the undoped AlGaN layer can be increased to 1.8 μm. As a result, a high breakdown voltage was realized. Further, the on-resistance at this time is 3.8 mΩcm 2, which is slightly increased as the undoped AlGaN layer thickness increases, but is within an allowable range.

図１１に、本発明によるショットキーダイオードの構造の一例を示す。このショットキーダイオード１１００は、実施例５と同様に、ＭＯＶＰＥ法を用いて、導電性ｎ型ＳｉＣ基板１１０２の表面上に１００ｎｍ厚のｎ＋型ＡｌＧａＮバッファ層１１０４と、５００ｎｍ厚のｎ＋型ＡｌＧａＮ層１１０６と、２２５ｎｍ厚のアンドープＡｌＧａＮ層１１０８とを順次成長させて作製した。ｎ＋型ＡｌＧａＮバッファ層１１０４、ｎ＋型ＡｌＧａＮ層１１０６のＳｉ不純物ドーピング濃度は、それぞれ２×１０１９ｃｍ−３、２×１０１８ｃｍ−３である。実施例４と異なる点は、１４０ｎｍ厚のｐ型ＩｎＧａＮ層を成長していない点である。この後、電子ビーム蒸着により、アンドープＡｌＧａＮ層１１０８上にはＰｄ／Ａｕのショットキー電極１１２０、そして導電性ｎ型ＳｉＣ基板１１０２の裏面にはＴｉ／Ａｕのオーミック電極１１３０を形成した。メサ構造の作製にはＥＣＲエッチング法を用いた。ここで、Ｐｄ／Ａｕ電極は、ｐ型ＩｎＧａＮ層に対してはオーミック電極となり、アンドープＡｌＧａＮ層に対してはショットキー電極となる。 FIG. 11 shows an example of the structure of a Schottky diode according to the present invention. Similar to the fifth embodiment, the Schottky diode 1100 includes a 100 nm thick n + type AlGaN buffer layer 1104 and a 500 nm thick n + type AlGaN on the surface of the conductive n type SiC substrate 1102 using the MOVPE method. A layer 1106 and an undoped AlGaN layer 1108 having a thickness of 225 nm were sequentially grown. The Si impurity doping concentrations of the n + -type AlGaN buffer layer 1104 and the n + -type AlGaN layer 1106 are 2 × 10 19 cm −3 and 2 × 10 18 cm −3 , respectively. The difference from Example 4 is that a 140 nm thick p-type InGaN layer is not grown. Thereafter, a Pd / Au Schottky electrode 1120 was formed on the undoped AlGaN layer 1108 by electron beam evaporation, and a Ti / Au ohmic electrode 1130 was formed on the back surface of the conductive n-type SiC substrate 1102. An ECR etching method was used to fabricate the mesa structure. Here, the Pd / Au electrode is an ohmic electrode for the p-type InGaN layer and a Schottky electrode for the undoped AlGaN layer.

この構造における降伏電圧は６５Ｖであり、実施例５で示したように、同じ膜厚（２２５ｎｍ）のアンドープＡｌＧａＮ層を用いたｐｎ接合ダイオードの降伏電圧（７０Ｖ）とほぼ等しい値が得られた。降伏電圧の大きさは、ダイオードの種類（ｐｎ接合ダイオードあるいはショットキーダイオード）に依存せず、電圧を保持するアンドープＡｌＧａＮ層に依存するからである。また、順方向Ｉ−Ｖ特性からは、１．１ｍΩｃｍ２という低いオン抵抗が得られている。同じ膜厚（２２５ｎｍ）のアンドープＡｌＧａＮ層を用いたｐｎ接合ダイオードのオン抵抗（１．２ｍΩｃｍ２）よりも若干低い理由は、ｐ型ＩｎＧａＮ層に関連する抵抗成分が無くなったためと考えられる。 The breakdown voltage in this structure was 65 V, and as shown in Example 5, a value almost equal to the breakdown voltage (70 V) of a pn junction diode using an undoped AlGaN layer having the same film thickness (225 nm) was obtained. This is because the breakdown voltage does not depend on the type of diode (pn junction diode or Schottky diode) but depends on the undoped AlGaN layer holding the voltage. Further, from the forward IV characteristics, a low on-resistance of 1.1 mΩcm 2 is obtained. The reason why the resistance is slightly lower than the on-resistance (1.2 mΩcm 2 ) of the pn junction diode using the undoped AlGaN layer having the same film thickness (225 nm) is considered to be because the resistance component related to the p-type InGaN layer has disappeared.

このように、ショットキーダイオードにおいても、ダイオードの抵抗を低く保ったまま、高い降伏電圧を得ることができる。   Thus, even in a Schottky diode, a high breakdown voltage can be obtained while keeping the resistance of the diode low.

以上、本発明について、具体的にいくつかの実施例について説明したが、本発明の原理を適用できる多くの実施可能な形態に鑑みて、ここに記載した実施例は、単に例示に過ぎず、本発明の範囲を限定するものではない。例えば、本発明の原理は、例示したｐｎ接合ダイオード、ショットキーダイオードに限らず、バイポーラトランジスタ、電界効果トランジスタなどの半導体デバイスに適用することができる。また、ここに例示した実施例は、本発明の趣旨から逸脱することなくその構成と詳細を変更することができる。さらに、説明のための構成要素および手順は、本発明の趣旨から逸脱することなく変更、補足、またはその順序を変えてもよい。   While the present invention has been described with respect to several embodiments, the embodiments described herein are merely illustrative in view of many possible forms to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. For example, the principle of the present invention is not limited to the illustrated pn junction diode and Schottky diode, but can be applied to semiconductor devices such as bipolar transistors and field effect transistors. Further, the configuration and details of the embodiments exemplified herein can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

降伏電圧を増加させるために、ＡｌＧａＮ層を使用したショットキーダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the Schottky diode which uses an AlGaN layer in order to increase a breakdown voltage. 降伏電圧を増加させるために、ＡｌＧａＮ層を使用したｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode which uses an AlGaN layer in order to increase a breakdown voltage. 図２において、ＡｌＧａＮ層に代えてＧａＮ層を使用したｐｎ接合ダイオードの構造の一例を示す図である。In FIG. 2, it is a figure which shows an example of the structure of the pn junction diode which replaced with the AlGaN layer and used the GaN layer. 本発明の実施例１によるｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode by Example 1 of this invention. 図４のｐｎ接合ダイオードにおける降伏電圧とｎ型ＡｌＧａＮ層のＡｌ組成の関係を示すグラフである。5 is a graph showing the relationship between the breakdown voltage and the Al composition of the n-type AlGaN layer in the pn junction diode of FIG. 本発明の実施例２によるｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode by Example 2 of this invention. 本発明の実施例３によるｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode by Example 3 of this invention. 本発明の実施例４によるｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode by Example 4 of this invention. 本発明の実施例５によるｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode by Example 5 of this invention. 本発明の実施例６によるｐｎ接合ダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the pn junction diode by Example 6 of this invention. 本発明の実施例７によるショットキーダイオードの構造の一例を示す図である。It is a figure which shows an example of the structure of the Schottky diode by Example 7 of this invention.

符号の説明Explanation of symbols

１００ ショットキーダイオード
２００ ｐｉｎダイオード
３００ ｐｉｎダイオード
４００，６００，７００，８００，９００，１０００ ｐｎ接合ダイオード
１１００ ショットキーダイオード
DESCRIPTION OF SYMBOLS 100 Schottky diode 200 Pin diode 300 Pin diode 400,600,700,800,900,1000 pn junction diode 1100 Schottky diode

Claims (8)

導電性のＳｉＣ基板上に作製した窒化物半導体の構造であって、
前記ＳｉＣ基板上に形成され、前記ＳｉＣ基板と同じ導電型を有する第１のＡｌＧａＮ層と、
前記第１のＡｌＧａＮ層上に形成され、前記ＳｉＣ基板と同じ導電型を有する第２のＡｌＧａＮ層であって、Ａｌ組成が前記第１のＡｌＧａＮ層と同じかそれ以下の第２のＡｌＧａＮ層と、
前記第２のＡｌＧａＮ層上に形成され、前記ＳｉＣ基板と同じ導電型を有する第３のＡｌＧａＮ層であって、Ａｌ組成が前記第２のＡｌＧａＮ層と同じかそれ以下の第３のＡｌＧａＮ層と
を備えたことを特徴とする構造。 A nitride semiconductor structure fabricated on a conductive SiC substrate,
A first AlGaN layer formed on the SiC substrate and having the same conductivity type as the SiC substrate;
A second AlGaN layer formed on the first AlGaN layer and having the same conductivity type as the SiC substrate, the Al composition being the same as or lower than the first AlGaN layer; ,
A third AlGaN layer formed on the second AlGaN layer and having the same conductivity type as the SiC substrate, wherein the Al composition is the same as or less than the second AlGaN layer; A structure characterized by comprising. 請求項１に記載の構造であって、
前記ＳｉＣ基板の裏面に形成されたオーミック電極をさらに備えたことを特徴とする構造。 The structure of claim 1,
A structure further comprising an ohmic electrode formed on the back surface of the SiC substrate. 請求項１または２に記載の構造であって、
前記第３のＡｌＧａＮ層よりも上の層に形成され、前記導電性基板と異なる導電性を有するＩｎＧａＮ層（Ｉｎ組成≧０）をさらに備えたことを特徴とする構造。 The structure according to claim 1 or 2,
A structure further comprising an InGaN layer (In composition ≧ 0) formed in a layer above the third AlGaN layer and having conductivity different from that of the conductive substrate. 請求項１ないし３のいずれかに記載の構造であって、
前記第２のＡｌＧａＮ層のＡｌ組成を前記導電性ＳｉＣ基板側から前記第２のＡｌＧａＮ層側へ漸次減少させたことを特徴とする構造。 A structure according to any one of claims 1 to 3,
A structure wherein the Al composition of the second AlGaN layer is gradually reduced from the conductive SiC substrate side to the second AlGaN layer side. 請求項１ないし４のいずれかに記載の構造であって、
前記第１のＡｌＧａＮ層の不純物ドーピング濃度は、１×１０１８ｃｍ−３以上であることを特徴とする構造。 A structure according to any one of claims 1 to 4,
The structure in which the impurity doping concentration of the first AlGaN layer is 1 × 10 18 cm −3 or more. 請求項１ないし５のいずれかに記載の構造であって、
前記第３のＡｌＧａＮ層の不純物ドーピング濃度は、１×１０１８ｃｍ−３未満であることを特徴とする構造。 A structure according to any one of claims 1 to 5,
The structure characterized in that the impurity doping concentration of the third AlGaN layer is less than 1 × 10 18 cm −3 . 請求項１ないし６のいずれかに記載の構造であって、
前記第３のＡｌＧａＮ層は、アンドープ層であることを特徴とする構造。 The structure according to any one of claims 1 to 6,
The third AlGaN layer is an undoped layer. 請求項１ないし７のいずれかに記載の構造であって、
前記ＳｉＣ基板の導電型は、ｎ型であることを特徴とする構造。
A structure according to any one of claims 1 to 7,
A structure characterized in that the conductivity type of the SiC substrate is n-type.
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