JP2007067077A - Nitride semiconductor device and method of manufacturing same - Google Patents

Nitride semiconductor device and method of manufacturing same Download PDF

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JP2007067077A
JP2007067077A JP2005249527A JP2005249527A JP2007067077A JP 2007067077 A JP2007067077 A JP 2007067077A JP 2005249527 A JP2005249527 A JP 2005249527A JP 2005249527 A JP2005249527 A JP 2005249527A JP 2007067077 A JP2007067077 A JP 2007067077A
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buffer layer
nitride semiconductor
layer
aln
substrate
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Tetsuya Akasaka
哲也 赤坂
Yasuyuki Kobayashi
康之 小林
Toshiki Makimoto
俊樹 牧本
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Nippon Telegraph and Telephone Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To form a nitride semiconductor whose the surface is smooth at an atomic layer level, and also which has no cracks on a substrate which is greatly different from a nitride semiconductor in a lattice constant or a thermal expansion coefficient, especially on a substrate whose the thermal expansion coefficient is smaller than that of a nitride semiconductor. <P>SOLUTION: The nitride semiconductor device includes an AlN system superlattice buffer layer formed by laminating alternately two or more layers respectively having an Al composition x of a first buffer layer consisting of Al<SB>x</SB>Ga<SB>1-x</SB>N (0.5≤x≤1), and an Al composition y of a second buffer layer consisting of Al<SB>y</SB>Ga<SB>1-y</SB>N (0.01≤y≤0.2); and a nitride semiconductor layer formed on the AlN system superlattice buffer layer. Therefore, the flatness of the surface of the nitride semiconductor to be formed and the prevention of the occurrence of cracks can be satisfied at the same time. The interface of the first buffer layer and the second buffer layer is smooth, and also it is made possible to produce AlN system superlattice buffer layer having an excellent crystallinity. Thus, it is made possible to manufacture an electron device with high reliability using a nitride semiconductor, an optical device and the like. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

本発明は、窒化物半導体素子およびその製造方法に関する。より詳細には、窒化物半導体と熱膨張係数の異なる基板上に作製した、表面が平滑でクラックのない高品質な窒化物半導体素子、および、その製造方法に関する。   The present invention relates to a nitride semiconductor device and a method for manufacturing the same. More specifically, the present invention relates to a high-quality nitride semiconductor device having a smooth surface and no cracks, which is manufactured on a substrate having a thermal expansion coefficient different from that of a nitride semiconductor, and a method for manufacturing the same.

窒化物半導体は、B、Al、Ga、もしくはIn等のIII 族元素のうちの少なくとも一つ以上の元素ならびにV族元素である窒素の化合物である。一般式は、AlabGacIn1-a-b-cN(0≦a≦1,0≦b≦1,0≦c≦1)により表される。窒化物半導体は、電界効果トランジスタ等の電子デバイス、または、可視光領域から紫外光領域の短波長帯における発光デバイスの活性材料として、近年盛んに研究および技術開発が行われている。一般的に、これらの窒化物半導体は、サファイア、窒化珪素、シリコン等の異種基板の上にヘテロエピタキシー技術を使用して作製される。特にシリコン基板は、大面積である、結晶性が良い、放熱性に優れる、へき開が容易である、プロセス技術が成熟している、さらに、低価格であるなどの多くの魅力的な特徴を備えている。 The nitride semiconductor is a compound of nitrogen that is at least one element of group III elements such as B, Al, Ga, or In and a group V element. General formula is represented by Al a B b Ga c In 1 -abc N (0 ≦ a ≦ 1,0 ≦ b ≦ 1,0 ≦ c ≦ 1). Nitride semiconductors have been actively studied and developed in recent years as active materials for electronic devices such as field effect transistors or light emitting devices in the short wavelength band from the visible light region to the ultraviolet light region. Generally, these nitride semiconductors are fabricated using heteroepitaxy technology on a dissimilar substrate such as sapphire, silicon nitride, or silicon. In particular, silicon substrates have many attractive features such as large area, good crystallinity, excellent heat dissipation, easy cleavage, mature process technology, and low price. ing.

しかしながら、窒化物半導体およびシリコン基板では、格子定数や熱膨張係数が大きく異なる。このため、シリコン基板上に作製された窒化物半導体には、クラック(ミクロンサイズの幅の割れ)や貫通転位等の欠陥が多数存在するという問題があった。特に、窒化物半導体の熱膨張係数は、シリコン基板に比較して非常に大きい。このため、高温の結晶成長工程から室温にまで冷却する過程において、窒化物半導体に大きな引っ張り歪が生じ、高密度のクラックが発生することが大きな問題となっていた。
そこで、シリコン基板および窒化物半導体層の間にバッファ層を挿入し、上述したクラックや貫通転位を低減することが通例となっている。このようなバッファ層の例として、AlNから成る第1のバッファ層およびGaNから成る第2のバッファ層を交互に複数積層した超格子状の多層膜(以下、AlN系格子バッファ層とする)が提案されている(非特許文献1)。
However, the lattice constant and the thermal expansion coefficient differ greatly between the nitride semiconductor and the silicon substrate. For this reason, the nitride semiconductor fabricated on the silicon substrate has a problem that there are many defects such as cracks (micron-sized width cracks) and threading dislocations. In particular, the thermal expansion coefficient of a nitride semiconductor is very large compared to a silicon substrate. For this reason, in the process of cooling from the high-temperature crystal growth step to room temperature, a large tensile strain is generated in the nitride semiconductor, and a high-density crack is generated.
Therefore, it is customary to insert a buffer layer between the silicon substrate and the nitride semiconductor layer to reduce the above-described cracks and threading dislocations. As an example of such a buffer layer, there is a superlattice multilayer film (hereinafter referred to as an AlN-based lattice buffer layer) in which a plurality of first buffer layers made of AlN and second buffer layers made of GaN are alternately stacked. It has been proposed (Non-Patent Document 1).

H. Ishikawa et al., Physical Status Solidi, vol. 0 (2003) pp. 2177-2180.H. Ishikawa et al., Physical Status Solidi, vol. 0 (2003) pp. 2177-2180.

上記の非特許文献1においては、バッファ層の第1のバッファ層としてはAlNを用い、第2のバッファ層としてGaN を用いることが開示されている。しかしながら、上記の非特許文献1においては、形成される窒化物半導体の表面の平坦性およびクラック発生の防止を両立する最適な条件については、十分に開示されていない。   Non-Patent Document 1 discloses that AlN is used as the first buffer layer of the buffer layer and GaN is used as the second buffer layer. However, the above-mentioned Non-Patent Document 1 does not sufficiently disclose the optimum conditions for achieving both the flatness of the surface of the formed nitride semiconductor and the prevention of cracks.

図1は、従来技術による窒化物半導体素子の断面図である。この本窒化物半導体素子において、AlN系超格子バッファ層の組成は、第1のバッファ層はAlxGa1-xNとし、第2のバッファ層はAlyGa1-yNと表す。図1に示すように、(111)の主方位面を有する3インチ径のシリコン基板11上に、AlN(すなわち、x=1)から成り5nmの厚さを持つ第1のバッファ層12aが形成されている。さらに、第1のバッファ層の上にはGaN(すなわち、y=0)から成り20nmの厚さを持つ第2のバッファ層12bが形成されている。第1のバッファ層および第2のバッファ層を各々20層ずつ交互に積層して、AlN系超格子バッファ層12が構成される。さらに、このバッファ層12の上に、1μmの厚さのGaNから成る窒化物半導体層13を成長させた。 FIG. 1 is a cross-sectional view of a conventional nitride semiconductor device. In this nitride semiconductor device, the composition of the AlN-based superlattice buffer layer is expressed as Al x Ga 1-x N for the first buffer layer and Al y Ga 1-y N for the second buffer layer. As shown in FIG. 1, a first buffer layer 12a made of AlN (ie, x = 1) and having a thickness of 5 nm is formed on a 3-inch diameter silicon substrate 11 having a (111) main orientation plane. Has been. Further, a second buffer layer 12b made of GaN (ie, y = 0) and having a thickness of 20 nm is formed on the first buffer layer. The AlN-based superlattice buffer layer 12 is configured by alternately stacking 20 first buffer layers and 20 second buffer layers. Further, a nitride semiconductor layer 13 made of GaN having a thickness of 1 μm was grown on the buffer layer 12.

図2は、上述の窒化物半導体素子の高分解能X線回折装置(HR−XRD)によるθ−2θスキャンチャートを示す。当業者にはよく知られているように、このスキャンチャートによって、バッファ層12における超格子層の界面状態を把握することが出来る。バッファ層12内の各界面が原子層レベルで平滑な超格子である場合には、サテライトピークが観測される。図2においては、0次ピークのみしか観察されず、サテライトピークはほとんど見られない(後述する図5と比較することにより理解される)。すなわち、AlN系超格子バッファ層12内における第1のバッファ層12a(AlN)および第2のバッファ層12b(GaN)の界面が荒れていることが分かった。一方、GaNから成る窒化物半導体層13の表面を原子間力顕微鏡(AFM)で観察したところ、確かにクラックは全くなかったが、平均二乗根(RMS)粗さは15nmもあった。   FIG. 2 shows a θ-2θ scan chart by the high-resolution X-ray diffractometer (HR-XRD) for the nitride semiconductor device described above. As is well known to those skilled in the art, the interface state of the superlattice layer in the buffer layer 12 can be grasped by this scan chart. When each interface in the buffer layer 12 is a smooth superlattice at the atomic layer level, satellite peaks are observed. In FIG. 2, only the 0th order peak is observed, and the satellite peak is hardly observed (understandable by comparing with FIG. 5 described later). That is, it was found that the interface between the first buffer layer 12a (AlN) and the second buffer layer 12b (GaN) in the AlN-based superlattice buffer layer 12 was rough. On the other hand, when the surface of the nitride semiconductor layer 13 made of GaN was observed with an atomic force microscope (AFM), there was certainly no crack, but the mean square root (RMS) roughness was 15 nm.

次に、上述の図1と同じ構成において、バッファ層12である第1のバッファ層および第2のバッファ層の組成をそれぞれ変化させた。すなわち、(111)の主方位面を有する3インチ径のシリコン基板11上に、5nmの厚さのAlN(すなわち、x=1)から成る第1のバッファ層12aを形成し、さらに20nmの厚さのAl0.7Ga0.3N(すなわち、y=0.7)から成る第2のバッファ層12bを形成した。この第1のバッファ層および第2のバッファ層を、各々20層ずつ交互に積層してAlN系超格子バッファ層12を構成した。さらに、このバッファ層12の上に1μmの厚さのGaNから成る窒化物半導体層13を成長させた。この窒化物半導体層13の表面を原子間力顕微鏡(AFM)で観察したところ、平均二乗根(RMS)粗さは0.25nmであり、原子層レベルにおいて平滑であった。ところが、この窒化物半導体層13表面には高密度のクラックが発生していた。 Next, in the same configuration as in FIG. 1 described above, the compositions of the first buffer layer and the second buffer layer, which are the buffer layers 12, were changed. That is, a first buffer layer 12a made of AlN (ie, x = 1) having a thickness of 5 nm is formed on a 3-inch diameter silicon substrate 11 having a (111) main orientation plane, and further a thickness of 20 nm. A second buffer layer 12b made of Al 0.7 Ga 0.3 N (ie, y = 0.7) was formed. The first buffer layer and the second buffer layer were alternately stacked to form an AlN-based superlattice buffer layer 12. Further, a nitride semiconductor layer 13 made of GaN having a thickness of 1 μm was grown on the buffer layer 12. When the surface of the nitride semiconductor layer 13 was observed with an atomic force microscope (AFM), the mean square root (RMS) roughness was 0.25 nm, which was smooth at the atomic layer level. However, high-density cracks occurred on the surface of the nitride semiconductor layer 13.

以上述べたように、シリコン基板上に、表面が平滑で、しかも、クラックのない窒化物半導体を得るためには、AlN系超格子バッファ層において、AlxGa1-xNから成る第1のバッファ層のAl組成xおよびAlyGa1-yNから成る第2のバッファ層のAl組成yが大きな影響を与えるにも関わらず、その詳細は明らかではなかった。 As described above, in order to obtain a nitride semiconductor having a smooth surface and no cracks on a silicon substrate, the AlN-based superlattice buffer layer includes the first Al x Ga 1-x N layer. Although the Al composition x of the buffer layer and the Al composition y of the second buffer layer made of Al y Ga 1-y N have a great influence, the details are not clear.

本発明においては、これら第1のバッファ層および第2のバッファ層のAl組成xおよびyに関して、その値を厳密に規定することにより、窒化物半導体のクラック発生を抑えながら、バッファ層の超格子の界面および窒化物半導体の表面を原子層レベルで平坦化することが可能であることを見出した。逆に、xおよびyの値によっては、超格子の界面や窒化物半導体の表面が荒れたり、窒化物半導体にクラックが発生したりしてしまうことも分かった。   In the present invention, the values of the Al compositions x and y of the first buffer layer and the second buffer layer are strictly defined, thereby suppressing the generation of cracks in the nitride semiconductor and the superlattice of the buffer layer. It has been found that the interface and the surface of the nitride semiconductor can be planarized at the atomic layer level. Conversely, it has been found that depending on the values of x and y, the interface of the superlattice and the surface of the nitride semiconductor are roughened, or cracks are generated in the nitride semiconductor.

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、基板と、前記基板上に形成され、AlxGa1-xN(0.5≦x≦1)から成る第1のバッファ層と、AlyGa1-yN(0.01≦y≦0.2)から成る第2のバッファ層とを交互に各々複数層を積層して形成されたAlN系超格子バッファ層と、前記AlN系超格子バッファ層上に形成された窒化物半導体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 substrate according to claim 1, formed on the substrate, and made of Al x Ga 1-x N (0.5 ≦ x ≦ 1). A first buffer layer and a second buffer layer made of Al y Ga 1-y N (0.01 ≦ y ≦ 0.2) are formed by alternately laminating a plurality of layers. A lattice buffer layer and a nitride semiconductor layer formed on the AlN-based superlattice buffer layer are provided.

請求項2に記載の発明は、請求項1に記載の発明であって、前記基板の熱膨張係数は、前記窒化物半導体層の熱膨張係数より小さいことを特徴とする。   The invention according to claim 2 is the invention according to claim 1, characterized in that a thermal expansion coefficient of the substrate is smaller than a thermal expansion coefficient of the nitride semiconductor layer.

請求項3に記載の発明は、請求項1または請求項2に記載の発明であって、前記第1のバッファ層の厚さは0.5nm以上100nm以下であり、前記第2のバッファ層の厚さは2nm以上80nm以下であることを特徴とする。   Invention of Claim 3 is invention of Claim 1 or Claim 2, Comprising: The thickness of the said 1st buffer layer is 0.5 nm or more and 100 nm or less, The said 2nd buffer layer of The thickness is 2 nm or more and 80 nm or less.

請求項4に記載の発明は、請求項1から請求項3のいずれかに記載の発明であって、前記窒化物半導体層の格子定数は、前記AlN系超格子バッファ層の格子定数よりも大きいことを特徴とする。   The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein a lattice constant of the nitride semiconductor layer is larger than a lattice constant of the AlN-based superlattice buffer layer. It is characterized by that.

請求項5に記載の発明は、基板と、前記基板上に形成されたAlN系超格子バッファ層と、前記AlN系超格子バッファ層の上に形成された窒化物半導体層とからなる窒化物半導体素子の製造方法であって、前記基板上に、AlxGa1-xN(0.5≦x≦1)の組成を持つ第1のバッファ層を形成するステップと、前記第1のバッファ層の上に、AlyGa1-yN(0.01≦y≦0.2)の組成を持つ第2のバッファ層を形成するステップと、前記第1のバッファ層を形成するステップと前記第2のバッファ層を形成するステップを交互に繰り返して前記AlN系超格子バッファ層を形成するステップとを備えたことを特徴とする。 The invention according to claim 5 is a nitride semiconductor comprising a substrate, an AlN-based superlattice buffer layer formed on the substrate, and a nitride semiconductor layer formed on the AlN-based superlattice buffer layer. A method for manufacturing an element, comprising: forming a first buffer layer having a composition of Al x Ga 1-x N (0.5 ≦ x ≦ 1) on the substrate; and the first buffer layer Forming a second buffer layer having a composition of Al y Ga 1-y N (0.01 ≦ y ≦ 0.2), forming the first buffer layer, and the first Forming the AlN-based superlattice buffer layer by alternately repeating the steps of forming the second buffer layer.

以上に記述したように、本発明によれば、窒化物半導体と格子定数や熱膨張係数が大きく異なる基板、特に窒化物半導体よりも熱膨張係数が小さい基板上に、表面が原子層レベルで平滑で、しかも、クラックのない窒化物半導体を形成することができる。また、本発明によれば、界面が平滑で、しかも、結晶性の良いAlN系超格子バッファ層を作製することができる。窒化物半導体を使用した信頼性の高い電子デバイス・光デバイス等を作成することができる。   As described above, according to the present invention, the surface is smoothed at the atomic layer level on a substrate having a lattice constant or thermal expansion coefficient significantly different from that of a nitride semiconductor, particularly on a substrate having a smaller thermal expansion coefficient than that of a nitride semiconductor. In addition, a nitride semiconductor having no cracks can be formed. In addition, according to the present invention, an AlN superlattice buffer layer having a smooth interface and good crystallinity can be produced. A highly reliable electronic device / optical device using a nitride semiconductor can be produced.

以下、図面を参照しながら本発明の実施の形態を詳細に説明する。
[実施例1]
図3は、本発明にかかる窒化物半導体素子の断面図を示す。この窒化物半導体素子においては、直径が3インチで、(111)から±5度以内の主方位面を持つシリコン基板31上に、AlN系超格子バッファ層12が形成されている。このバッファ層32は、厚さが5nmのAlNから成る第1のバッファ層32aおよび厚さが20nmのAl0.04Ga0.96Nから成る第2のバッファ層32bを、各々交互に20層積ずつ積層して構成されている。さらに、このバッファ層32の上に、厚さが1μmのGaNから成る窒化物半導体層33を形成した。
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[Example 1]
FIG. 3 shows a cross-sectional view of a nitride semiconductor device according to the present invention. In this nitride semiconductor device, an AlN-based superlattice buffer layer 12 is formed on a silicon substrate 31 having a diameter of 3 inches and a main orientation plane within ± 5 degrees from (111). In this buffer layer 32, a first buffer layer 32a made of AlN having a thickness of 5 nm and a second buffer layer 32b made of Al 0.04 Ga 0.96 N having a thickness of 20 nm are alternately laminated in 20 layers. Configured. Further, a nitride semiconductor layer 33 made of GaN having a thickness of 1 μm was formed on the buffer layer 32.

図4は、上述の窒化物半導体層の表面の原子間力顕微鏡(AFM)による像を示す。観察した表面は極めて滑らかで、単原子層からなるしま模様状のステップが明瞭に観察され、RMS粗さは0.1nmであった。また、光学顕微鏡により窒化物半導体層33の表面を観察したところ、直径3インチのシリコン基板の全面にわたってクラックは存在しなかった。   FIG. 4 shows an image of the surface of the nitride semiconductor layer described above by an atomic force microscope (AFM). The observed surface was very smooth, and a stripe pattern step consisting of a monoatomic layer was clearly observed, and the RMS roughness was 0.1 nm. Further, when the surface of the nitride semiconductor layer 33 was observed with an optical microscope, no cracks existed on the entire surface of the silicon substrate having a diameter of 3 inches.

図5は、この窒化物半導体層33のHR−XRDによるθ−2θスキャンチャートを示す。窒化物半導体33のGaNから発生する鋭いピークと共に、AlN系超格子バッファ層32から発生する5〜6次までにわたるサテライトピークが明瞭に観察されている。これは、第1のバッファ層32aおよび第2のバッファ層32bの結晶性が高く、しかもこの2つの層の界面が非常に平坦であることを示している。第2のバッファ層32bをGaNではなく、Alを4%含むAl0.04Ga0.96Nとすることにより、AlNから成る第1のバッファ層32aに対する第2のバッファ層の濡れ性が良くなったために、このような界面の平坦性を向上させる効果を得ることができる。 FIG. 5 shows a θ-2θ scan chart of the nitride semiconductor layer 33 by HR-XRD. Along with a sharp peak generated from GaN of the nitride semiconductor 33, a satellite peak extending from the 5th to 6th order generated from the AlN-based superlattice buffer layer 32 is clearly observed. This indicates that the crystallinity of the first buffer layer 32a and the second buffer layer 32b is high, and the interface between the two layers is very flat. Since the second buffer layer 32b is not GaN but Al 0.04 Ga 0.96 N containing 4% Al, the wettability of the second buffer layer with respect to the first buffer layer 32a made of AlN is improved. Such an effect of improving the flatness of the interface can be obtained.

[実施例2]
本実施例においては、図3と同様な構造の窒化物半導体素子において、AlyGa1-yNから成る第2のバッファ層32bのAl組成yを様々に変化させた。そして、GaNから成る窒化物半導体層33の表面のRMS粗さをAFMによって、また、クラックの密度を光学顕微鏡によって、それぞれ評価をした。
[Example 2]
In the present embodiment, in the nitride semiconductor device having the same structure as that of FIG. 3, the Al composition y of the second buffer layer 32b made of Al y Ga 1-y N was changed variously. Then, the RMS roughness of the surface of the nitride semiconductor layer 33 made of GaN was evaluated by AFM, and the density of cracks was evaluated by an optical microscope.

図6は、窒化物半導体層の表面のRMS粗さと第2のバッファ層のAl組成yとの関係を示す。yの値が0.01以上の場合おいて、RMS粗さは0.3nm以下となっており、原子層レベルで滑らかな窒化物半導体33の表面が得られていることが分かる。実施例1において記述したように、第2のバッファ層32bが十分な組成(0.01以上)のAlを含むことによって、第1のバッファ層32aおよび第2のバッファ層32bの間の濡れ性がよくなり、お互いの界面が平滑になっている。その平滑さがそのままバッファ層32の上の窒化物半導体層33に受け継がれ、原子層レベルで滑らかな窒化物半導体層33の表面が得られる。   FIG. 6 shows the relationship between the RMS roughness of the surface of the nitride semiconductor layer and the Al composition y of the second buffer layer. When the value of y is 0.01 or more, the RMS roughness is 0.3 nm or less, and it can be seen that a smooth surface of the nitride semiconductor 33 is obtained at the atomic layer level. As described in Example 1, when the second buffer layer 32b contains Al having a sufficient composition (0.01 or more), the wettability between the first buffer layer 32a and the second buffer layer 32b. And the interface between each other is smooth. The smoothness is directly inherited by the nitride semiconductor layer 33 on the buffer layer 32, and a smooth surface of the nitride semiconductor layer 33 is obtained at the atomic layer level.

図7は、窒化物半導体層表面のクラック密度と第2のバッファ層のAl組成yとの関係を示す。yの値が0から0.2の範囲の場合、3インチ径の全面積にわたって窒化物半導体層表面には、クラックは存在しなかった。ところが、yの値が0.2よりも大きくなると、クラック密度は急激に大きくなった。この理由は、第1のバッファ層32aの格子定数と第2のバッファ層32bの格子定数の差が小さくなり過ぎて、GaNから成る窒化物半導体層33およびシリコン基板31の間の熱膨張係数差による歪を、効果的に分散することができなくなるためである。   FIG. 7 shows the relationship between the crack density on the surface of the nitride semiconductor layer and the Al composition y of the second buffer layer. When the value of y was in the range of 0 to 0.2, no crack was present on the surface of the nitride semiconductor layer over the entire area of 3 inches diameter. However, when the value of y was larger than 0.2, the crack density rapidly increased. This is because the difference between the lattice constant of the first buffer layer 32 a and the lattice constant of the second buffer layer 32 b becomes too small, and the difference in thermal expansion coefficient between the nitride semiconductor layer 33 made of GaN and the silicon substrate 31. This is because the distortion caused by the above cannot be effectively dispersed.

以上述べたように、原子層レベルで、平坦で、しかも、クラックのない窒化物半導体層を得るためには、第2のバッファ層32bのAlyGa1-yNのAl組成yが、少なくとも0.01≦y≦0.2の範囲にあることが望ましい。 As described above, in order to obtain a nitride semiconductor layer that is flat at the atomic layer level and has no cracks, the Al composition y of Al y Ga 1-y N in the second buffer layer 32b is at least It is desirable that the range is 0.01 ≦ y ≦ 0.2.

なお、GaNから成る窒化物半導体33は、AlN系超格子バッファ層32よりも格子定数が大きい。この場合、窒化物半導体層33の成長中には、この窒化物半導体層内に圧縮歪が生じる。成長工程を終了した後の冷却過程においては、前述したようにシリコン基板との間の熱膨張係数差により、GaN窒化物半導体には引っ張り歪が生じる。成長中に生じた前述の圧縮歪はこの引っ張り歪の一部を相殺し、クラック発生を抑止するように作用する。同様な歪の相殺効果は、格子定数がAlN系超格子バッファ層32よりも大きくなるような組成範囲を持つAlabGacIn1-a-b-cNを窒化物半導体層33として用いる場合でも得られる。この場合も、図6、図7に示したような、表面のRMS粗さおよびクラック密度と、第2のバッファ層32bのAl組成yとの関係が得られた。 The nitride semiconductor 33 made of GaN has a larger lattice constant than the AlN-based superlattice buffer layer 32. In this case, during the growth of the nitride semiconductor layer 33, compressive strain is generated in the nitride semiconductor layer. In the cooling process after finishing the growth process, tensile strain occurs in the GaN nitride semiconductor due to the difference in thermal expansion coefficient with the silicon substrate as described above. The aforementioned compressive strain generated during the growth acts to cancel a part of the tensile strain and suppress the generation of cracks. A similar strain canceling effect can be obtained even when Al a B b Ga c In 1-abc N having a composition range in which the lattice constant is larger than that of the AlN-based superlattice buffer layer 32 is used as the nitride semiconductor layer 33. It is done. Also in this case, the relationship between the RMS roughness and crack density of the surface and the Al composition y of the second buffer layer 32b as shown in FIGS. 6 and 7 was obtained.

また、第1のバッファ層32aと第2のバッファ層32bの厚さについて考えると、薄すぎる場合には熱膨張係数差に起因する歪のエネルギーをAlN系超格子バッファ層32中で効果的に分散することが出来ず、クラックが発生してしまう。逆に、厚過ぎる場合には成長中に格子緩和が起こり、貫通転位が増える結果となってしまう。詳細な検討を行った結果、第1のバッファ層32aの厚さは0.5nm以上100nm以下、第2のバッファ層32bの厚さは2nm以上80nm以下であることが望ましいことが分かった。上記の厚さの範囲内では、窒化物半導体33の表面にはクラックは全くなく、貫通転位密度は6×107cm-2以下であった。したがって、窒化物半導体を用いた電子素子や発光素子の高品質な構成材料として使用することができる。 Considering the thicknesses of the first buffer layer 32a and the second buffer layer 32b, when the thickness is too thin, the strain energy caused by the difference in thermal expansion coefficient is effectively increased in the AlN-based superlattice buffer layer 32. It cannot be dispersed and cracks occur. On the other hand, if it is too thick, lattice relaxation occurs during growth, resulting in an increase in threading dislocations. As a result of detailed examination, it has been found that the thickness of the first buffer layer 32a is preferably 0.5 nm to 100 nm and the thickness of the second buffer layer 32b is preferably 2 nm to 80 nm. Within the above thickness range, there was no crack on the surface of the nitride semiconductor 33, and the threading dislocation density was 6 × 10 7 cm −2 or less. Therefore, it can be used as a high-quality constituent material for electronic devices and light-emitting devices using nitride semiconductors.

[実施例3]
次に、本実施例においては、図3と同様な構成の窒化物半導体素子において、第1のバッファ層のAlの組成xについて検討する。直径が3インチで、(111)から±5度以内の主方位面を持つシリコン基板31上に、AlN系超格子バッファ層32が形成された。バッファ層32は、厚さが5nmのAlxGa1-xNから成る第1のバッファ層32aおよび厚さが20nmのAl0.01Ga0.99Nから成る第2のバッファ層32bを、各々20層ずつ交互に積層し構成した。さらに、バッファ層32に上に、厚さが1μmのGaNから成る窒化物半導体層33を形成した。本実施例においては、第1のバッファ層32aのAl組成xを様々に変化させた。
[Example 3]
Next, in the present example, the Al composition x of the first buffer layer in the nitride semiconductor device having the same configuration as in FIG. 3 will be examined. An AlN-based superlattice buffer layer 32 was formed on a silicon substrate 31 having a diameter of 3 inches and a main orientation plane within ± 5 degrees from (111). The buffer layer 32 includes a first buffer layer 32a made of Al x Ga 1-x N having a thickness of 5 nm and a second buffer layer 32b made of Al 0.01 Ga 0.99 N having a thickness of 20 nm. Layered alternately. Further, a nitride semiconductor layer 33 made of GaN having a thickness of 1 μm was formed on the buffer layer 32. In the present embodiment, the Al composition x of the first buffer layer 32a was variously changed.

図8は、窒化物半導体層の表面のRMS粗さおよび第1のバッファ層32aのAl組成xの関係を示す。xの値に関係なく、RMS粗さが0.3nm以下となっており、原子層レベルで滑らかなGaN窒化物半導体層33の表面が得られている。すなわち、Al0.01Ga0.99Nから成る第2のバッファ層32bのAl組成は0.01であり、実施例2において述べた0.01≦y≦0.2の好適な範囲内にあるため、第1のバッファ層32aのAlxGa1-xNのAl組成xに関係なく、平滑な表面が得られる。 FIG. 8 shows the relationship between the RMS roughness of the surface of the nitride semiconductor layer and the Al composition x of the first buffer layer 32a. Regardless of the value of x, the RMS roughness is 0.3 nm or less, and a smooth surface of the GaN nitride semiconductor layer 33 is obtained at the atomic layer level. That is, since the Al composition of the second buffer layer 32b made of Al 0.01 Ga 0.99 N is 0.01 and is within the preferred range of 0.01 ≦ y ≦ 0.2 described in the second embodiment, A smooth surface can be obtained regardless of the Al composition x of Al x Ga 1-x N in one buffer layer 32a.

図9は、窒化物半導体層の表面のクラック密度および第1のバッファ層32aのAl組成xの関係を示す。xの値が0.5から1の範囲において、3インチ径のシリコン基板全面積にわたってクラックは存在しなかった。ところが、xの値が0.5よりも小さくなると、クラック密度が急激に大きくなっている。この理由は、実施例2と同様に、xの値が0.5よりも小さくなると、第1のバッファ層32aの格子定数および第2のバッファ層32bの格子定数の差が小さくなり過ぎて、窒化物半導体層33およびシリコン基板31の間の熱膨張係数差による歪を、効果的に分散することができなくなるためである。   FIG. 9 shows the relationship between the crack density on the surface of the nitride semiconductor layer and the Al composition x of the first buffer layer 32a. In the range of x from 0.5 to 1, no cracks existed over the entire area of the 3-inch diameter silicon substrate. However, when the value of x is smaller than 0.5, the crack density is rapidly increased. The reason for this is that, as in Example 2, when the value of x is smaller than 0.5, the difference between the lattice constant of the first buffer layer 32a and the lattice constant of the second buffer layer 32b becomes too small. This is because the strain due to the difference in thermal expansion coefficient between the nitride semiconductor layer 33 and the silicon substrate 31 cannot be effectively dispersed.

以上述べたように、原子層レベルで平坦であり、しかも、クラックのない窒化物半導体を得るために、第1のバッファ層32aのAlxGa1-xNのAl組成xは、少なくとも0.5≦x≦1の範囲にあることが望ましい。 As described above, in order to obtain a nitride semiconductor that is flat at the atomic layer level and has no cracks, the Al composition x of Al x Ga 1-x N of the first buffer layer 32a is at least 0. It is desirable that the range is 5 ≦ x ≦ 1.

なお、実施例2の場合と同様に、窒化物半導体層33として、AlN系超格子バッファ層32よりも格子定数が大きな窒化物半導体AlabGacIn1-a-b-cNを用いた場合でも、図9に示したように、表面のRMS粗さおよびクラック密度と、第1のバッファ層32aのAlxGa1-xNのAl組成xとの関係が得られた。また、第1のバッファ層32aの厚さが0.5nm以上100nm以下、および、第2のバッファ層32bの厚さが2nm以上80nm以下の範囲にあることが望ましい。 Similarly to the case of Example 2, as the nitride semiconductor layer 33, even if the lattice constant of AlN-based superlattice buffer layer 32 with a large nitride semiconductor Al a B b Ga c In 1 -abc N As shown in FIG. 9, the relationship between the RMS roughness and crack density of the surface and the Al composition x of Al x Ga 1-x N of the first buffer layer 32a was obtained. Further, it is desirable that the thickness of the first buffer layer 32a be in the range of 0.5 nm to 100 nm and the thickness of the second buffer layer 32b be in the range of 2 nm to 80 nm.

実施例1から実施例3においては基板としてシリコンを用いたが、これに限るものではない。基板と窒化物半導体層との熱膨張係数に差異がある場合に、本発明の構成による窒化物半導体素子により、表面が原子層レベルで平滑であり、しかも、クラックのない窒化物半導体素子を形成することができる。特に、窒化物半導体よりも熱膨張係数が小さい基板、例えば、炭化珪素(SiC)等を用いた場合に、本発明の効果は高い。   In Examples 1 to 3, silicon is used as the substrate, but the present invention is not limited to this. When there is a difference in the thermal expansion coefficient between the substrate and the nitride semiconductor layer, the nitride semiconductor element according to the configuration of the present invention forms a nitride semiconductor element with a smooth surface at the atomic layer level and without cracks. can do. In particular, the effect of the present invention is high when a substrate having a smaller thermal expansion coefficient than that of a nitride semiconductor, such as silicon carbide (SiC), is used.

[窒化物半導体素子の製造方法]
以下、本発明の窒化物半導体素子の製造方法の一例を記述する。本発明においては、一般の結晶成長装置、すなわち、液層成長装置や気相成長装置を用いることができる。結晶品質、薄膜の膜厚制御性、大量生産性、または、大面積化への適応性等を考えると、有機金属気相成長法(Metal Organic Chemical Vapor Deposition:MOCVD)を用いることにより、本発明の効果を特に発揮できる。製造方法は以下の手順の通りである。
1.直径が3インチで、(111)から±5度以内に主方位面を持つシリコン基板を、アセトンやアルコール類のような有機溶媒中で超音波洗浄した後、MOCVD装置の反応炉内に設置する。
2.基板温度を1000〜1200℃に上げ、キャリアガスとして少なくとも水素を流しながら基板をクリーニングする。この工程において、シリコン基板の自然酸化膜が除去され、清浄なシリコンの表面が得られる。
3.引続き、基板温度を700〜1200℃に変化させ、原料ガスであるアンモニア、トリメチルガリウム(TMG)、および、トリメチルアルミニウム(TMA)を適量流し、第1のバッファ層であるAlxGa1-xN(0.5≦x≦1)を0.5nm以上100nm以下の厚さで堆積させる。なお、第1のバッファ層がAlN(すなわち、x=1)でない場合、シリコンの表面がTMGと反応しないように、5〜100nmの厚さのAlNをまず成長させてから、第1のバッファ層を堆積させるとさらに好ましい結果が得られる。
4.引続き、アンモニア、TMGおよびTMAの流量を適宜変化させ、第2のバッファ層であるAlyGa1-yN(0.01≦y≦0.2)を2nm以上80nm以下の厚さで堆積させる。
5.同様にして、第1のバッファ層と第2のバッファ層を交互に10〜100層積層し、AlN系超格子バッファ層を形成する。
6.最後に、原料ガスとして、アンモニア、TMA、TMG、トリエチルボロン、および、トリメチルインジウム等を反応炉内に流して、窒化物半導体AlabGacIn1-a-b-cNを成長させる。この時、すべての原料ガスの流量を適宜調整して、窒化物半導体AlabGacIn1-a-b-cNの格子定数が前記AlN系超格子バッファ層の格子定数よりも大きくなるように、各III族元素の組成a、b、cを調整する。
[Nitride Semiconductor Device Manufacturing Method]
Hereinafter, an example of the manufacturing method of the nitride semiconductor device of the present invention will be described. In the present invention, a general crystal growth apparatus, that is, a liquid layer growth apparatus or a vapor phase growth apparatus can be used. In view of crystal quality, thin film thickness controllability, mass productivity, or adaptability to large area, the present invention is achieved by using metal organic chemical vapor deposition (MOCVD). The effect of can be demonstrated especially. The manufacturing method is as follows.
1. A silicon substrate having a diameter of 3 inches and having a main orientation surface within ± 5 degrees from (111) is ultrasonically cleaned in an organic solvent such as acetone or alcohol, and then placed in a reactor of a MOCVD apparatus. .
2. The substrate temperature is raised to 1000 to 1200 ° C., and the substrate is cleaned while flowing at least hydrogen as a carrier gas. In this step, the natural oxide film on the silicon substrate is removed, and a clean silicon surface is obtained.
3. Subsequently, the substrate temperature is changed to 700 to 1200 ° C., and appropriate amounts of ammonia, trimethylgallium (TMG), and trimethylaluminum (TMA) as source gases are flown, and Al x Ga 1-x N as the first buffer layer. (0.5 ≦ x ≦ 1) is deposited at a thickness of 0.5 nm to 100 nm. If the first buffer layer is not AlN (ie, x = 1), an AlN film having a thickness of 5 to 100 nm is first grown so that the silicon surface does not react with TMG, and then the first buffer layer is formed. More favorable results can be obtained by depositing.
4). Subsequently, the flow rates of ammonia, TMG, and TMA are appropriately changed, and Al y Ga 1-y N (0.01 ≦ y ≦ 0.2) as the second buffer layer is deposited to a thickness of 2 nm to 80 nm. .
5. Similarly, 10 to 100 first buffer layers and second buffer layers are alternately stacked to form an AlN-based superlattice buffer layer.
6). Finally, ammonia, TMA, TMG, triethylboron, trimethylindium, or the like is flowed into the reaction furnace as a source gas to grow the nitride semiconductor Al a B b Ga c In 1-abc N. At this time, the flow rate of all the source gases is adjusted as appropriate so that the lattice constant of the nitride semiconductor Al a B b Ga c In 1-abc N is larger than the lattice constant of the AlN-based superlattice buffer layer. The composition a, b, c of each group III element is adjusted.

以上、詳細に述べたように、本発明によれば、窒化物半導体と格子定数や熱膨張係数が大きく異なる基板、特に窒化物半導体よりも熱膨張係数が小さい基板上に、表面が原子層レベルで平滑で、しかも、クラックのない窒化物半導体を形成することができる。また、本発明によれば、界面が平滑で、しかも、結晶性の良いAlN系超格子バッファ層を作製することができる。窒化物半導体を使用した信頼性の高い電子デバイス・光デバイス等を作成することができる。   As described above in detail, according to the present invention, the surface of the nitride semiconductor is greatly different from that of a nitride semiconductor, particularly on a substrate having a smaller thermal expansion coefficient than that of a nitride semiconductor. Thus, a nitride semiconductor which is smooth and free from cracks can be formed. In addition, according to the present invention, an AlN superlattice buffer layer having a smooth interface and good crystallinity can be produced. A highly reliable electronic device / optical device using a nitride semiconductor can be produced.

従来の窒化物半導体素子の断面図である。It is sectional drawing of the conventional nitride semiconductor element. 従来の窒化物半導体素子のHR−XRDによるθ−2θスキャンチャートである。It is theta-2theta scan chart by HR-XRD of the conventional nitride semiconductor element. 実施例1にかかる窒化物半導体素子の断面図である。1 is a cross-sectional view of a nitride semiconductor device according to Example 1. FIG. 実施例1のGaN窒化物半導体層の表面のAFM像である。2 is an AFM image of the surface of a GaN nitride semiconductor layer of Example 1. FIG. 実施例1にかかる窒化物半導体素子のHR−XRDによるθ−2θスキャンチャートである。6 is a θ-2θ scan chart by HR-XRD of the nitride semiconductor device according to Example 1; 窒化物半導体層の表面のRMS粗さおよび第2のバッファ層のAl組成yの関係を示す図である。It is a figure which shows the relationship between the RMS roughness of the surface of a nitride semiconductor layer, and Al composition y of a 2nd buffer layer. 窒化物半導体層の表面のクラック密度および第2のバッファ層のAl組成yの関係を示す図である。It is a figure which shows the relationship between the crack density of the surface of a nitride semiconductor layer, and Al composition y of a 2nd buffer layer. 窒化物半導体層の表面のRMS粗さおよび第1のバッファ層のAl組成xの関係を示す図である。It is a figure which shows the relationship between the RMS roughness of the surface of a nitride semiconductor layer, and Al composition x of a 1st buffer layer. 窒化物半導体層の表面のクラック密度および第2のバッファ層のAl組成xの関係を示す図である。It is a figure which shows the relationship between the crack density of the surface of a nitride semiconductor layer, and Al composition x of a 2nd buffer layer.

符号の説明Explanation of symbols

11、31 シリコン基板
12、32 AlN系超格子バッファ層
12a、32a 第1のバッファ層
12b、32b 第2のバッファ層
13、33 窒化物半導体層
11, 31 Silicon substrate 12, 32 AlN-based superlattice buffer layer 12a, 32a First buffer layer 12b, 32b Second buffer layer 13, 33 Nitride semiconductor layer

Claims (5)

基板と、
前記基板上に形成され、AlxGa1-xN(0.5≦x≦1)から成る第1のバッファ層と、AlyGa1-yN(0.01≦y≦0.2)から成る第2のバッファ層とを交互に各々複数層を積層して形成されたAlN系超格子バッファ層と、
前記AlN系超格子バッファ層上に形成された窒化物半導体層と、
を備えたことを特徴とする窒化物半導体素子。
A substrate,
A first buffer layer formed on the substrate and made of Al x Ga 1-x N (0.5 ≦ x ≦ 1); and Al y Ga 1-y N (0.01 ≦ y ≦ 0.2). An AlN-based superlattice buffer layer formed by alternately laminating a plurality of layers, and a second buffer layer comprising:
A nitride semiconductor layer formed on the AlN-based superlattice buffer layer;
A nitride semiconductor device comprising:
前記基板の熱膨張係数は、前記窒化物半導体層の熱膨張係数より小さいことを特徴とする請求項1に記載の窒化物半導体素子。   The nitride semiconductor device according to claim 1, wherein a thermal expansion coefficient of the substrate is smaller than a thermal expansion coefficient of the nitride semiconductor layer. 前記第1のバッファ層の厚さは0.5nm以上100nm以下であり、前記第2のバッファ層の厚さは2nm以上80nm以下であることを特徴とする請求項1または請求項2に記載の窒化物半導体素子。   The thickness of the said 1st buffer layer is 0.5 nm or more and 100 nm or less, The thickness of the said 2nd buffer layer is 2 nm or more and 80 nm or less, The Claim 1 or Claim 2 characterized by the above-mentioned. Nitride semiconductor device. 前記窒化物半導体層の格子定数は、前記AlN系超格子バッファ層の格子定数よりも大きいことを特徴とする請求項1乃至請求項3に記載の窒化物半導体。   4. The nitride semiconductor according to claim 1, wherein a lattice constant of the nitride semiconductor layer is larger than a lattice constant of the AlN-based superlattice buffer layer. 5. 基板と、前記基板上に形成されたAlN系超格子バッファ層と、前記AlN系超格子バッファ層の上に形成された窒化物半導体層とからなる窒化物半導体素子の製造方法であって、
前記基板上に、AlxGa1-xN(0.5≦x≦1)の組成を持つ第1のバッファ層を形成するステップと、
前記第1のバッファ層の上に、AlyGa1-yN(0.01≦y≦0.2)の組成を持つ第2のバッファ層を形成するステップと、
前記第1のバッファ層を形成するステップおよび前記第2のバッファ層を形成するステップを交互に繰り返して前記AlN系超格子バッファ層を形成するステップと、
を備えたことを特徴とする窒化物半導体素子の製造方法。
A method for manufacturing a nitride semiconductor device comprising a substrate, an AlN-based superlattice buffer layer formed on the substrate, and a nitride semiconductor layer formed on the AlN-based superlattice buffer layer,
Forming a first buffer layer having a composition of Al x Ga 1-x N (0.5 ≦ x ≦ 1) on the substrate;
Forming a second buffer layer having a composition of Al y Ga 1-y N (0.01 ≦ y ≦ 0.2) on the first buffer layer;
Forming the AlN-based superlattice buffer layer by alternately repeating the step of forming the first buffer layer and the step of forming the second buffer layer;
A method for manufacturing a nitride semiconductor device, comprising:
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