JP2013193918A - Self-supporting gallium nitride crystal substrate and method of manufacturing the same - Google Patents
Self-supporting gallium nitride crystal substrate and method of manufacturing the same Download PDFInfo
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Abstract
Description
本発明は、窒化ガリウム結晶自立基板、詳しくは、サファイア下地基板上に貫通転位密度（暗点密度）が小さな窒化ガリウム（ＧａＮ）結晶層を積層し、該積層基板から分離した窒化ガリウム結晶自立基板、並びにその製造方法に関する。 The present invention relates to a gallium nitride crystal free-standing substrate, and more specifically, a gallium nitride crystal free-standing substrate in which a gallium nitride (GaN) crystal layer having a low threading dislocation density (dark spot density) is stacked on a sapphire base substrate and separated from the stacked substrate. And a method for manufacturing the same.
発光ダイオード（ＬＥＤ）や半導体レーザ（ＬＤ）などの半導体発光素子として、サファイア基板上に、ｎ型ＧａＮ層、ＩｎＧａＮ層からなる量子井戸層とＧａＮ層からなる障壁層とが交互積層された多重量子井戸層（Multiple Quantum Wells：ＭＱＷｓ）、及びｐ型ＧａＮ層が順に積層形成された構造を有するものが量産化されている。このような量産化されている半導体発光素子では、いずれのＧａＮ層も、軸方向にＧａＮ結晶が結晶成長し、表面が、ｃ面（＜０００１＞面）となっている。
ところで、表面がｃ面であるＧａＮ結晶層では、Ｇａ原子のみを含むＧａ原子面が僅かにプラスに帯電する一方、Ｎ原子のみを含むＮ原子面が僅かにマイナスに帯電し、結果としてｃ軸方向（層厚さ方向）に自発分極が発生する。また、ＧａＮ結晶層上に異種半導体層をヘテロエピタキシャル成長させた場合、両者の格子定数の違いによって、ＧａＮ結晶に圧縮歪や引っ張り歪が生じ、ＧａＮ結晶内でｃ軸方向に圧電分極（ピエゾ分極）が発生する（特許文献１及び２）。
この結果、前記構成の半導体発光素子では、多重量子井戸層において、ＩｎＧａＮ量子井戸層に固定電荷に起因する自発分極に加えて、ＩｎＧａＮ量子井戸層に加わる圧縮歪により生じたピエゾ分極が重畳され、そのためｃ軸方向に大きな内部分極電場が発生することとなる。この内部分極電場の影響を受けて、量子閉じ込めシュタルク効果（Quantum−Confined Stark Effect：ＱＣＳＥ）により、発光効率の低下や必要な注入電流の増大に伴う発光のピーク波長シフトなどの問題が生じると考えられている。
As a semiconductor light emitting device such as a light emitting diode (LED) or a semiconductor laser (LD), multiple quantum devices in which a quantum well layer composed of an n-type GaN layer, an InGaN layer, and a barrier layer composed of a GaN layer are alternately stacked on a sapphire substrate. Those having a structure in which a well layer (Multiple Quantum Wells: MQWs) and a p-type GaN layer are sequentially stacked are mass-produced. In such a mass-produced semiconductor light emitting device, in any GaN layer, a GaN crystal grows in the axial direction, and the surface is a c-plane (<0001> plane).
By the way, in a GaN crystal layer having a c-plane surface, a Ga atom surface containing only Ga atoms is slightly positively charged, while an N atom surface containing only N atoms is slightly negatively charged, resulting in a c-axis. Spontaneous polarization occurs in the direction (layer thickness direction). In addition, when heterogeneous semiconductor layers are heteroepitaxially grown on the GaN crystal layer, compressive strain or tensile strain is generated in the GaN crystal due to the difference in lattice constant between the two, and piezoelectric polarization (piezo polarization) in the c-axis direction in the GaN crystal. Occurs (Patent Documents 1 and 2).
As a result, in the semiconductor light emitting device having the above-described structure, in the multiple quantum well layer, in addition to the spontaneous polarization due to the fixed charge, the piezoelectric polarization generated by the compressive strain applied to the InGaN quantum well layer is superimposed on the InGaN quantum well layer, Therefore, a large internal polarization electric field is generated in the c-axis direction. Under the influence of this internal polarization electric field, the quantum confined Stark effect (QCSE) is considered to cause problems such as peak emission wavelength shift due to decrease in luminous efficiency and increase in required injection current. It has been.
上記問題を解決するために、ＧａＮ結晶の無極性面である、ａ面（＜１１−２０＞面）やｍ面（＜１−１００＞面）を用いて、その上にＩｎＧａＮ層を形成し、自発分極とピエゾ分極の重畳された内部電界の影響を回避することが検討されている（特許文献１〜３）。更に、ｃ面が、ａ軸あるいはｍ軸方向に約６０度傾斜した半極性面といわれている面、例えば、半極性の＜１１−２２＞面上にＩｎＧａＮ量子井戸層を形成し、それによって内部電界の影響を回避することも検討されている（非特許文献１及び２）。
しかしながら、現在入手可能な上記ＧａＮ結晶の無極性面や半極性面を主面とする基板は、貫通転位密度が２〜３×１０８個／ｃｍ２程度と言われており、より貫通転位密度が低い高い結晶品質の結晶基板が望まれている。
In order to solve the above problem, an InGaN layer is formed on an a-plane (<11-20> plane) or m-plane (<1-100> plane) that is a nonpolar plane of a GaN crystal. It has been studied to avoid the influence of an internal electric field in which spontaneous polarization and piezoelectric polarization are superimposed (Patent Documents 1 to 3). In addition, an InGaN quantum well layer is formed on a plane called a semipolar plane whose c-plane is inclined about 60 degrees in the a-axis or m-axis direction, for example, a semipolar <11-22> plane, thereby It has also been studied to avoid the influence of the internal electric field (Non-patent Documents 1 and 2).
However, it is said that a substrate having a nonpolar plane or a semipolar plane as a main surface of the GaN crystal that is currently available has a threading dislocation density of about 2 to 3 × 10 8 pieces / cm 2. A crystal substrate having a low crystallinity and a high crystal quality is desired.
結晶転位は、下地基板であるサファイアと成長結晶であるＧａＮとの格子定数差などによって、サファイアとＧａＮとの界面から発生する。この転位発生を抑制するためには、ＧａＮ結晶自立基板上へＧａＮ結晶層を成長するホモエピタキシャル成長が有効である。ホモエピタキシャル成長では、下地基板と成長層とが同じ物質であるため、物性の違いによる転位の発生を抑えることが可能である。このため、ホモエピタキシャル成長を行うためにはＧａＮ結晶自立基板が必要となる。
加えて、上記の内部電界の影響を回避するため、無極性面および半極性面を主面とするＧａＮ結晶自立基板が高効率な半導体発光素子の作製に有用である。しかしながら、現在は無極性面および半極性面を主面とするＧａＮ結晶自立基板は流通しておらず、入手が困難である。
無極性面および半極性面を主面とするＧａＮ結晶自立基板の作製方法としては、バルク状のＧａＮ結晶から特定の面を主面として切り出す方法があるが、極めてコストが高くなる、また大面積の基板が得られないなどの問題があり、工業的には受け入れられない。
Crystal dislocations are generated from the interface between sapphire and GaN due to the difference in lattice constant between sapphire as the base substrate and GaN as the growth crystal. In order to suppress the occurrence of this dislocation, homoepitaxial growth in which a GaN crystal layer is grown on a GaN crystal free-standing substrate is effective. In homoepitaxial growth, since the base substrate and the growth layer are the same material, it is possible to suppress the occurrence of dislocation due to the difference in physical properties. For this reason, a GaN crystal free-standing substrate is required to perform homoepitaxial growth.
In addition, in order to avoid the influence of the internal electric field, a GaN crystal free-standing substrate having a nonpolar plane and a semipolar plane as a main surface is useful for manufacturing a highly efficient semiconductor light emitting device. However, at present, GaN crystal free-standing substrates having a nonpolar plane and a semipolar plane as a main surface are not in circulation and are difficult to obtain.
As a method for producing a GaN crystal free-standing substrate having a nonpolar plane and a semipolar plane as a main surface, there is a method of cutting a specific surface from a bulk GaN crystal as a main surface, but it is extremely expensive and has a large area. However, it is not accepted industrially.
現行の自立基板の作製方法では、下地基板とその上に成長したＧａＮ結晶層との分離が必要である。従来から下地基板とＧａＮ結晶層との分離技術はいくつか知られている。レーザーリフトオフ法は、サファイア基板上にＧａＮ結晶層を積層した積層基板のサファイア側からレーザーを照射し、サファイア界面のＧａＮ結晶層をレーザーによって分解し、サファイア基板とＧａＮ結晶層とを分離する方法であるが、分離の際、レーザー照射によるダメージがＧａＮ結晶層に発生する恐れがある。化学エッチング法は、サファイアが化学的に安定で酸溶液やアルカリ溶液に不溶の為、サファイア基板の分離には適していない。機械研磨は、サファイアが非常に硬いため困難を極め、また研磨工程においてＧａＮ結晶層にダメージが発生する恐れがある。結晶成長過程での剥離層の挿入は、成長させるＧａＮ結晶の品質が悪化する恐れがあり、また結晶成長の工程が増えるという問題がある。 In the current method for producing a free-standing substrate, it is necessary to separate the base substrate from the GaN crystal layer grown thereon. Conventionally, several techniques for separating a base substrate and a GaN crystal layer are known. The laser lift-off method is a method of irradiating a laser from the sapphire side of a laminated substrate in which a GaN crystal layer is laminated on a sapphire substrate, decomposing the GaN crystal layer at the sapphire interface with the laser, and separating the sapphire substrate and the GaN crystal layer. However, during the separation, damage due to laser irradiation may occur in the GaN crystal layer. The chemical etching method is not suitable for separation of the sapphire substrate because sapphire is chemically stable and insoluble in an acid solution or an alkali solution. Mechanical polishing is extremely difficult because sapphire is very hard, and there is a risk of damage to the GaN crystal layer during the polishing process. Insertion of a release layer during the crystal growth process has a problem that the quality of the GaN crystal to be grown may be deteriorated and the number of crystal growth steps increases.
ＧａＮの、貫通転位密度が低く高い結晶品質のａ面やｍ面を主面とする自立基板、或いは＜１１−２２＞面を主面とする自立基板など、無極性面や半極性面を主面としたＧａＮ結晶のみからなるＧａＮ結晶自立基板、並びにその製造方法を提供することを目的とする。 Mainly non-polar or semipolar surfaces such as free standing substrates with a high crystal quality a-plane and m-plane with low threading dislocation density, or self-standing substrates with <11-22> plane as the main plane. An object of the present invention is to provide a GaN crystal free-standing substrate made of only GaN crystal as a surface, and a method for manufacturing the same.
本発明者らは、複数本の凹状の溝部を有するサファイア下地基板を用いて、該下地基板の溝部の側壁面を起点として所望の結晶面を有するＧａＮ結晶を作製する方法について研究を進め、いくつかの提案をした。この研究を更に推し進める中で、側壁面から成長したＧａＮ結晶においては、ＧａＮとサファイアとの熱膨張率の違い、それにより生じる両者界面付近の熱応力によって、下地基板から自然剥離する現象を確認し、本願発明を完成するに至った。 The present inventors have advanced research on a method for producing a GaN crystal having a desired crystal plane starting from the side wall surface of the groove portion of the base substrate using a sapphire base substrate having a plurality of concave groove portions. I made a suggestion. As we continue this research, we confirmed that the GaN crystal grown from the side wall surface spontaneously peels from the underlying substrate due to the difference in thermal expansion coefficient between GaN and sapphire and the resulting thermal stress near the interface. The present invention has been completed.
即ち、本発明により、下地基板の主面に対して傾斜した側壁を有する複数本の溝部を形成したサファイア下地基板を用いて、前記側壁から横方向結晶成長させて窒化ガリウム結晶層を形成し、該結晶層の膜厚を１００μｍ以上に成長させた後、前記サファイア下地基板と窒化ガリウム結晶層とを同時に冷却することにより、サファイア下地基板と窒化ガリウム結晶層とを剥離せしめることを特徴とする窒化ガリウム結晶自立基板の製造方法が提供される。
上記発明において、
１）下地基板の窒化ガリウム結晶層を形成する側の全表面積に対して、結晶成長の起点となる側壁の総面積の割合が、１〜２０％であること、
２）結晶成長の起点となる側壁が、サファイア単結晶のｃ面であること
が好適である。
本発明により、また、暗点密度が２×１０８個／ｃｍ２未満であり、表面が無極性または半極性の面方位を有することを特徴とする窒化ガリウム結晶自立基板が提供される。
That is, according to the present invention, a gallium nitride crystal layer is formed by laterally crystal growth from the side wall using a sapphire base substrate having a plurality of grooves having side walls inclined with respect to the main surface of the base substrate, After the film thickness of the crystal layer is grown to 100 μm or more, the sapphire base substrate and the gallium nitride crystal layer are simultaneously cooled to separate the sapphire base substrate and the gallium nitride crystal layer. A method of manufacturing a gallium crystal free-standing substrate is provided.
In the above invention,
1) The ratio of the total area of the side wall that is the starting point of crystal growth is 1 to 20% with respect to the total surface area of the base substrate on the side on which the gallium nitride crystal layer is formed,
2) It is preferable that the side wall from which crystal growth starts is the c-plane of a sapphire single crystal.
The present invention also provides a gallium nitride crystal free-standing substrate having a dark spot density of less than 2 × 10 8 pieces / cm 2 and a surface having a nonpolar or semipolar plane orientation.
本発明により、暗点密度が２×１０８個／ｃｍ２未満の無極性或いは半極性の面方位を有するＧａＮ結晶自立基板を、低コストで、簡便な操作で、且つ結晶品質を悪化させることなく提供することができる。
該自立基板を用いてＬＥＤやＬＤなどの半導体発光素子構造を形成した場合、サファイア基板上へのヘテロエピタキシャル成長とは異なり、下地基板と成長層との物質が同じであるため、基板と成長層との間での転位の発生を抑えることができ、高発光効率の半導体発光素子が作製できる。
更に、ＧａＮ結晶層表面の面方位は無極性面または半極性面であるため、従来のｃ面を主面とする窒化ガリウム結晶層基板に比べて、量子閉じ込めシュタルク効果による発光効率の低下の影響が小さい。
According to the present invention, a GaN crystal free-standing substrate having a nonpolar or semipolar plane orientation with a dark spot density of less than 2 × 10 8 pieces / cm 2 can be produced at low cost with simple operation and deterioration of crystal quality. Can be offered without.
Unlike the heteroepitaxial growth on the sapphire substrate, when the semiconductor light emitting device structure such as LED or LD is formed using the self-standing substrate, the substrate and the growth layer are the same because the materials of the base substrate and the growth layer are the same. Generation of dislocations can be suppressed, and a semiconductor light emitting device with high luminous efficiency can be manufactured.
Furthermore, since the surface orientation of the surface of the GaN crystal layer is a nonpolar plane or a semipolar plane, the effect of a decrease in light emission efficiency due to the quantum confined Stark effect compared to a conventional gallium nitride crystal layer substrate having a c-plane as a main surface Is small.
本発明の自立基板の製造方法は、複数本の溝部を有するサファイアからなる下地基板を使用し、且つ、該基板上に特定量の膜厚のＧａＮ結晶を成長させることに特徴がある。
上記下地基板は、サファイアからなる基板であって、該基板の主面に対して傾斜した側壁を有する溝部を複数本有し、該側壁の一部が結晶成長起点となる。
暗点密度とは、結晶の転位欠陥である貫通転位の密度を示すための指標となる物性値であり、走査型電子顕微鏡／カソードルミネッセンス（ＳＥＭ・ＣＬ）装置を用いて測定される。測定時の加速電圧は５ｋＶとし、観察範囲は２０μm×２０μmとする。このとき、観察範囲内に観察された暗点の総数より暗点密度を算出する。
The self-supporting substrate manufacturing method of the present invention is characterized in that a base substrate made of sapphire having a plurality of grooves is used, and a GaN crystal having a specific thickness is grown on the substrate.
The base substrate is a substrate made of sapphire, and has a plurality of groove portions having side walls inclined with respect to the main surface of the substrate, and a part of the side walls serves as a crystal growth starting point.
The dark spot density is a physical property value that serves as an index for indicating the density of threading dislocations, which are dislocation defects in the crystal, and is measured using a scanning electron microscope / cathode luminescence (SEM / CL) apparatus. The acceleration voltage during measurement is 5 kV, and the observation range is 20 μm × 20 μm. At this time, the dark spot density is calculated from the total number of dark spots observed within the observation range.
サファイア下地基板の主面は、目的とするＧａＮ結晶の結晶面に合わせて任意の面方位が選択される。例えば、＜１１−２２＞面を表面に有するＧａＮ結晶を成長させたい場合は、サファイア下地基板の主面は＜１０−１２＞とする。＜１０−１１＞面を表面に有するＧａＮ結晶を成長させたい場合は、サファイア下地基板の主面は＜１１−２３＞とする。その他、＜１０−１０＞面、＜１１−２０＞面、＜２０−２１＞面等を主面することができる。この主面は、所望のＧａＮ結晶を得るために、結晶軸に対して所定の角度傾斜したミスカット面であってもよい。
該下地基板は、通常、厚みが０．３〜３．０ｍｍ、直径が５０〜３００ｍｍの円盤状のものが使用される。
As the main surface of the sapphire base substrate, an arbitrary plane orientation is selected according to the crystal plane of the target GaN crystal. For example, when it is desired to grow a GaN crystal having a <11-22> plane, the main surface of the sapphire base substrate is set to <10-12>. When it is desired to grow a GaN crystal having a <10-11> plane on the surface, the main surface of the sapphire base substrate is set to <11-23>. In addition, the <10-10> plane, the <11-20> plane, the <20-21> plane, and the like can be main. This main surface may be a miscut surface inclined at a predetermined angle with respect to the crystal axis in order to obtain a desired GaN crystal.
As the base substrate, a disk-shaped substrate having a thickness of 0.3 to 3.0 mm and a diameter of 50 to 300 mm is usually used.
上記のとおり、下地基板の主面は任意に選択できるが、該下地基板は複数の溝部を有し、且つ、溝部側壁の一部が結晶成長の起点となりうることが必要である。例えば、下地基板主面が＜１０−１２＞面、溝部の延びる方向が＜１１−２０＞面の面方位、即ち、ａ軸方向である場合に、溝部の一方の側壁にｃ面が形成される。或いは、下地基板主面が＜１１−２３＞面、溝部の延びる方向が＜１０−１０＞面の面方位、即ち、ｍ軸方向である場合に、溝部の一方の側壁にｃ面が形成される。
主面が上記何れの面方位であっても、成長起点となる側壁を起点として選択的横方向成長（Epitaxial Lateral Overgrowth：ＥＬＯ）法により結晶成長させて厚みを特定量以上とすれば、本発明が達成される。後述する自然剥離を容易にするためには、側壁は、その総面積が下地基板の窒化ガリウム結晶層を形成する側の全表面積に対して、１〜２０％であることが好ましい。
As described above, the main surface of the base substrate can be arbitrarily selected. However, the base substrate needs to have a plurality of groove portions, and a part of the side walls of the groove portions can be a starting point of crystal growth. For example, when the base substrate main surface is the <10-12> plane and the direction in which the groove extends is the <11-20> plane orientation, that is, the a-axis direction, the c-plane is formed on one side wall of the groove. The Alternatively, when the base substrate main surface is the <11-23> plane and the extending direction of the groove is the <10-10> plane orientation, that is, the m-axis direction, a c-plane is formed on one side wall of the groove. The
Regardless of the orientation of the main surface, if the thickness is increased to a specific amount or more by crystal growth by a selective lateral growth (ELO) method using a side wall as a growth starting point as a starting point, the present invention Is achieved. In order to facilitate natural peeling described later, the side wall preferably has a total area of 1 to 20% with respect to the total surface area of the base substrate on the side where the gallium nitride crystal layer is formed.
サファイア下地基板の主面には、複数本の溝部が並行に設けられる。溝部の開口部幅は特に制限されず、通常０．５〜１０μｍの範囲から設定される。溝部の間隔、即ち、相互に隣接する溝部と溝部の下地基板主面線上の間隔は、１〜１００μｍである。溝部底面の横方向の幅、即ち溝部の延びる方向に直角な方向の距離（ｗ）も特に限定されず、１〜１０００００μｍが一般的である。主面上の溝部の数は、形成されるＧａＮ結晶の所望する面積に応じて任意に設けることができるが、上記開口幅、溝部の間隔、底面の幅を勘案して、通常、１ｍｍ当り、１０〜５００本程度設ければ良い。図１に、代表的な下地基板を示す。 A plurality of groove portions are provided in parallel on the main surface of the sapphire base substrate. The opening width of the groove is not particularly limited, and is usually set from a range of 0.5 to 10 μm. The interval between the groove portions, that is, the interval between the adjacent groove portions and the groove portion on the base substrate main surface line is 1 to 100 μm. The lateral width of the bottom surface of the groove, that is, the distance (w) in the direction perpendicular to the extending direction of the groove is not particularly limited, and is generally 1 to 100000 μm. The number of grooves on the main surface can be arbitrarily set according to the desired area of the GaN crystal to be formed, but usually taking into account the opening width, the interval between the grooves, and the width of the bottom surface, About 10 to 500 may be provided. FIG. 1 shows a typical base substrate.
上記溝部は、下地基板主面に対して所定の角度で傾斜した側壁を有しており、図２に示すように、その断面形状は、溝開口部から溝底部に向かって溝幅を狭めるように外向きに傾斜したテ―パー状になっている。傾斜角度とは、図２に示すように、下地基板主面と溝部側壁の延長面とがなす角度（Θ）を意味する。該角度は、下地基板主面の面方位に対応して形成される側壁面の面方位を勘案して決定される。 The groove has a side wall inclined at a predetermined angle with respect to the main surface of the base substrate. As shown in FIG. 2, the cross-sectional shape narrows the groove width from the groove opening toward the groove bottom. The taper is inclined outward. As shown in FIG. 2, the inclination angle means an angle (Θ) formed by the base substrate main surface and the extended surface of the groove side wall. The angle is determined in consideration of the surface orientation of the side wall surface formed corresponding to the surface orientation of the base substrate main surface.
例えば、サファイア下地基板主面の面方位が＜１０−１２＞であり、所望するＧａＮ結晶の面方位が＜１１−２２＞面である場合は、この角度（Θ）を５８．４度として、この側壁から、ＧａＮ結晶を、サファイア下地基板のｃ軸にＧａＮ結晶のｃ軸が同一の方向となるように成長させて所望の結晶を得る。
このときの角度５８．４度は、所望するＧａＮ結晶の主面である＜１１−２２＞面と、成長方向であるＧａＮ結晶のｃ軸に対して垂直となるＧａＮ結晶のｃ面とがなす角度が、５８．４度であることから決定される。しかし、用いるサファイア下地基板の主面である＜１０−１２＞面と、溝部の側壁に現れるサファイアｃ面とがなす角度は５７．６度であるため、下地基板主面と溝部側壁とがなす角度（Θ）は５７．６度となり、その上に成長したＧａＮ結晶層の表面は、サファイア下地基板の主面に対し、約０．８度傾斜する。そこで、この角度を相殺するように、基板主面部分がサファイア＜１０−１２＞面にオフ角をつけた面であるミスカット基板を用いることにより、ＧａＮ結晶の＜１１−２２＞面がサファイア下地基板主面に対して平行となるように成長したＧａＮ結晶層を得ることができる。
For example, when the surface orientation of the main surface of the sapphire base substrate is <10-12> and the surface orientation of the desired GaN crystal is the <11-22> plane, this angle (Θ) is set to 58.4 degrees, From this side wall, a GaN crystal is grown so that the c-axis of the GaN crystal is in the same direction as the c-axis of the sapphire base substrate to obtain a desired crystal.
The angle 58.4 degrees at this time is formed by the <11-22> plane that is the principal surface of the desired GaN crystal and the c-plane of the GaN crystal that is perpendicular to the c-axis of the GaN crystal that is the growth direction. The angle is determined from being 58.4 degrees. However, the angle formed between the <10-12> plane, which is the main surface of the sapphire base substrate to be used, and the sapphire c-plane appearing on the side wall of the groove is 57.6 degrees, so the main surface of the base substrate and the side wall of the groove are formed. The angle (Θ) is 57.6 degrees, and the surface of the GaN crystal layer grown thereon is inclined by about 0.8 degrees with respect to the main surface of the sapphire base substrate. Therefore, by using a miscut substrate in which the main surface portion of the substrate has an off-angle with respect to the sapphire <10-12> plane so as to cancel out this angle, the <11-22> plane of the GaN crystal becomes sapphire. A GaN crystal layer grown so as to be parallel to the main surface of the base substrate can be obtained.
サファイア下地基板主面の面方位が＜１１−２３＞であり、所望するＧａＮ結晶の面方位が＜１０−１１＞面である場合は、この角度を６２．０度とする。しかしながら、上記理由と同じ理由により生じる、約０．８度の下地基板主面とＧａＮ結晶表面との傾斜角度を相殺するように、基板主面部分がサファイア＜１１−２３＞面にオフ角をつけた面であるミスカット基板を用いることにより、ＧａＮ結晶の＜１０−１１＞面がサファイア下地基板主面に対して平行となるように成長したＧａＮ結晶層を得ることができる。 When the surface orientation of the main surface of the sapphire base substrate is <11-23> and the surface orientation of the desired GaN crystal is the <10-11> plane, this angle is set to 62.0 degrees. However, the main surface portion of the substrate has an off angle with respect to the sapphire <11-23> surface so as to cancel out the inclination angle between the base substrate main surface and the GaN crystal surface of about 0.8 degrees, which is caused by the same reason as described above. By using the miscut substrate which is the attached surface, it is possible to obtain a GaN crystal layer grown so that the <10-11> plane of the GaN crystal is parallel to the main surface of the sapphire base substrate.
サファイア下地基板主面の面方位が＜１１−２０＞であり、所望するＧａＮ結晶の面方位が＜１０−１０＞面である場合、或いは、サファイア下地基板主面の面方位が＜１０−１０＞であり、所望するＧａＮ結晶の面方位が＜１１−２０＞面である場合は、この角度を９０度として、この側壁から、ＧａＮ結晶を、サファイア下地基板のｃ軸とＧａＮ結晶のｃ軸とが同一の方向となるように成長させて所望の結晶を得る。
しかし、下地基板主面と溝部側壁とがなす角度（Θ）が９０度となる溝部を形成することはエッチング技術上困難であるが、下地基板主面と溝部側壁とがなす角度（Θ）が９０度に近い溝部を有するサファイア下地基板を用いることにより、＜１１−２０＞面を主面とするサファイア下地基板上に、ＧａＮ結晶の＜１０−１０＞面がサファイア下地基板主面に対して平行となるように成長したＧａＮ結晶層を、或いは、＜１０−１０＞面を主面とするサファイア下地基板上に、ＧａＮ結晶の＜１１−２０＞面がサファイア下地基板主面に対して平行となるように成長したＧａＮ結晶層を得ることができる。
When the surface orientation of the main surface of the sapphire base substrate is <11-20> and the surface orientation of the desired GaN crystal is the <10-10> plane, or the surface orientation of the main surface of the sapphire base substrate is <10-10 When the plane orientation of the desired GaN crystal is the <11-20> plane, this angle is set to 90 degrees, and the GaN crystal is separated from this sidewall by the c-axis of the sapphire base substrate and the c-axis of the GaN crystal. Are grown in the same direction to obtain a desired crystal.
However, although it is difficult to form a groove with an angle (Θ) formed by the base substrate main surface and the groove side wall of 90 degrees, the angle (Θ) formed by the base substrate main surface and the groove side wall is By using a sapphire base substrate having a groove portion close to 90 degrees, a <10-10> plane of a GaN crystal is on the sapphire base substrate main surface on a sapphire base substrate having a <11-20> plane as a main surface. A GaN crystal layer grown so as to be parallel or on a sapphire base substrate having a <10-10> plane as a main surface, and a <11-20> plane of the GaN crystal is parallel to the main surface of the sapphire base substrate A GaN crystal layer grown so as to be obtained can be obtained.
サファイア下地基板主面の面方位が＜１０−１０＞であり、所望するＧａＮ結晶の面方位が＜１１−２０＞面である場合は、この角度を９０度として、この側壁から、ＧａＮを、サファイア下地基板のｃ軸にＧａＮ結晶のｃ軸が同一の方向となるように成長させて所望の結晶を得る。
このときの角度９０度は、所望するＧａＮ結晶の主面である＜１０−１０＞面と、成長方向であるＧａＮ結晶のｃ軸に対して垂直となるＧａＮ結晶のｃ面とがなす角度が、９０度であることから決定される。しかし、前記のように下地基板主面と溝部側壁とがなす角度（Θ）が９０度となる溝部を形成することは技術上困難である。しかしながら、下地基板主面と溝部側壁とがなす角度（Θ）が９０度に近い溝部を有するサファイア基板を用いることにより、＜１０−１０＞面を主面とするサファイア下地基板上に、ＧａＮの＜１１−２０＞面がサファイア下地基板主面に対して平行となるように成長したＧａＮ層を得ることができる。
When the surface orientation of the main surface of the sapphire base substrate is <10-10> and the surface orientation of the desired GaN crystal is the <11-20> plane, this angle is set to 90 degrees, A desired crystal is obtained by growing so that the c-axis of the GaN crystal is in the same direction as the c-axis of the sapphire base substrate.
In this case, the angle of 90 degrees is the angle formed between the <10-10> plane that is the principal surface of the desired GaN crystal and the c-plane of the GaN crystal that is perpendicular to the c-axis of the GaN crystal that is the growth direction. , 90 degrees. However, as described above, it is technically difficult to form a groove portion where the angle (Θ) formed by the base substrate main surface and the groove side wall is 90 degrees. However, by using a sapphire substrate having a groove portion where the angle (Θ) formed between the base substrate main surface and the groove side wall is close to 90 degrees, a GaN sapphire substrate having a <10-10> surface as a main surface is formed. A GaN layer grown so that the <11-20> plane is parallel to the main surface of the sapphire base substrate can be obtained.
サファイア下地基板主面の面方位が＜０００１＞であり、所望するＧａＮ結晶の面方位が＜１０−１０＞面である場合は、この角度を９０度として、この側壁から、ＧａＮを、サファイア下地基板のa軸にＧａＮ結晶のｃ軸が同一の方向となるように成長させて所望の結晶を得る。
このときの角度９０度は、所望するＧａＮ結晶の主面である＜１０−１０＞面と、成長方向であるＧａＮ結晶のｃ軸に対して垂直となるＧａＮ結晶のｃ面とがなす角度が、９０度であることから決定される。しかし、前記のように下地基板主面と溝部側壁とがなす角度（Θ）が９０度となる溝部を形成することは技術上困難である。しかしながら、下地基板主面と溝部側壁とがなす角度（Θ）が９０度に近い溝部を有するサファイア基板を用いることにより、＜０００１＞面を主面とするサファイア下地基板上に、ＧａＮの＜１０−１０＞面がサファイア下地基板主面に対して平行となるように成長したＧａＮ層を得ることができる。
When the plane orientation of the main surface of the sapphire base substrate is <0001> and the plane orientation of the desired GaN crystal is the <10-10> plane, this angle is set to 90 degrees and GaN is applied to the sapphire base from this side wall. A desired crystal is obtained by growing so that the c-axis of the GaN crystal is in the same direction as the a-axis of the substrate.
In this case, the angle of 90 degrees is the angle formed between the <10-10> plane that is the principal surface of the desired GaN crystal and the c-plane of the GaN crystal that is perpendicular to the c-axis of the GaN crystal that is the growth direction. , 90 degrees. However, as described above, it is technically difficult to form a groove portion where the angle (Θ) formed by the base substrate main surface and the groove side wall is 90 degrees. However, by using a sapphire substrate having a groove portion where the angle (Θ) formed between the main surface of the base substrate and the side wall of the groove portion is close to 90 degrees, the GaN <10 is formed on the sapphire base substrate having the <0001> plane as the main surface. A GaN layer grown so that the −10> plane is parallel to the main surface of the sapphire base substrate can be obtained.
上記溝部側壁における、ＧａＮ結晶を成長させる領域（以下、結晶成長領域という）の幅（ｄ）は特に制限されないが、暗点密度を低減するためには、１０〜３０００ｎｍとすることが好ましく、１００〜１０００ｎｍとすることが特に好ましい。結晶成長領域の幅（ｄ）の下限は小さいほど良いが、下記に述べる溝部作製の際の技術上の制約から決定される。
結晶成長領域の幅（ｄ）とは、図２に示す如く、側壁全てが結晶成長領域である場合は、下地基板主面と側壁が交わる辺と、側壁と溝部底面が交わる辺との間の、側壁上の最短距離（間隔）を云う。図３に示す如く、側壁の一部がマスキングされ結晶成長領域が制限されている場合は、上記最短距離（間隔）から、マスキング部分の幅を除いた距離（ｄ）を云う。
The width (d) of the region where the GaN crystal is grown (hereinafter referred to as crystal growth region) on the side wall of the groove is not particularly limited, but is preferably 10 to 3000 nm in order to reduce the dark spot density. It is especially preferable to set it to -1000 nm. The smaller the lower limit of the width (d) of the crystal growth region is, the better.
As shown in FIG. 2, the width (d) of the crystal growth region is defined between the side where the base substrate main surface and the side wall intersect and the side where the side wall and the groove bottom face intersect when all the side walls are crystal growth regions. The shortest distance (interval) on the side wall. As shown in FIG. 3, when a part of the side wall is masked and the crystal growth region is limited, the distance (d) is obtained by subtracting the width of the masking portion from the shortest distance (interval).
下地基板の側壁を起点としてＧａＮ結晶がＥＬＯ成長し、最終的には下地基板を覆って、下地基板主面に平行な表面を有する種々の面方位のＧａＮ結晶層が形成される。
本発明においては、上記側壁を起点として成長したＧａＮの結晶層を、下地基板上に１００μｍ以上の厚みで形成させることが必須である。結晶層の膜厚を１００μｍ以上とすることにより、成長後の冷却工程において、下地基板とＧａＮ結晶層との境界面に応力が集中して、両者が別途剥離の為の操作をする必要が無く自然剥離する（図４参照）。膜厚が１００μｍ未満では、境界でのスムースな自然剥離は起こらず、ＧａＮ結晶層が破壊される（図５参照）。
The GaN crystal grows ELO starting from the side wall of the base substrate, and finally, the base substrate is covered, and GaN crystal layers having various plane orientations having surfaces parallel to the main surface of the base substrate are formed.
In the present invention, it is essential to form a GaN crystal layer grown from the side wall as a starting point with a thickness of 100 μm or more on the base substrate. By setting the film thickness of the crystal layer to 100 μm or more, stress concentrates on the boundary surface between the base substrate and the GaN crystal layer in the cooling process after the growth, so that it is not necessary for the both to perform a separate peeling operation. Naturally peels (see FIG. 4). When the film thickness is less than 100 μm, smooth spontaneous peeling at the boundary does not occur, and the GaN crystal layer is destroyed (see FIG. 5).
この自然剥離が生じる理由としては、第一に、下地基板のサファイアとＧａＮとの熱膨張係数の差異にあると考えられる。サファイア下地基板とＧａＮの熱膨張係数の違いにより、ＧａＮ結晶成長後の冷却工程において、サファイアとＧａＮ結晶層との界面に熱応力が掛かり、自然剥離を引き起こす原動力となる。第二の理由として、サファイアとＧａＮ結晶とは、結晶成長領域であるサファイアの側壁面上では化学的に結合しているものの、溝部の他の面や主面とは単に接触して積層されていることが考えられる。サファイアに形成した溝部の側壁のみからＧａＮ結晶を成長させた場合、下地基板であるサファイアとその上に成長したＧａＮ結晶とが化学的に結合しているのは成長起点となった側壁部のみであり、その他の部分は単に接触しているのみである。また側壁からのＥＬＯ成長によって空隙が形成され、当然ながらこの空隙部もサファイアとＧａＮとは結合していない。 The reason why this natural peeling occurs is considered to be mainly due to the difference in thermal expansion coefficient between sapphire and GaN of the base substrate. Due to the difference in thermal expansion coefficient between the sapphire base substrate and the GaN, thermal stress is applied to the interface between the sapphire and the GaN crystal layer in the cooling step after the GaN crystal growth, which becomes a driving force for causing natural separation. The second reason is that sapphire and GaN crystal are chemically bonded on the side wall surface of sapphire, which is the crystal growth region, but are simply in contact with and stacked on the other surface and main surface of the groove. It is possible that When a GaN crystal is grown only from the side wall of the groove formed in sapphire, the underlying substrate sapphire and the GaN crystal grown on it are chemically bonded only at the side wall from which the growth starts. Yes, the other parts are just touching. Also, voids are formed by ELO growth from the side walls, and naturally, the voids are not bonded to sapphire and GaN.
用いるサファイア下地基板の側壁は、その総面積が下地基板のＧａＮ結晶層を形成する側の全表面積に対して、１〜２０％であることが好適である。すなわち、ＧａＮ結晶層が横方向成長して下地基板表面全域を覆った時に、ＧａＮ結晶の成長起点となる領域が１〜２０％である場合は、サファイアとＧａＮ結晶層との結合力は１/１００〜１/５に低減していると考えられる。
図６、７に、剥離後のＧａＮ結晶の剥離面の光学顕微鏡写真（断面、表面ノマルスキー像）を示す。この写真から、サファイアから剥離したＧａＮ結晶層の剥離表面形状は、図１や図２に示した溝部を形成したサファイア下地基板上に成長したＧａＮ結晶層の形状と同一であることが確認され、剥離がサファイア下地基板とＧａＮ結晶層との界面で生じていることがわかる。
結晶層の膜厚は、自然剥離時にＧａＮ結晶層も一部破壊される場合があるので、１００μｍ以上とする必要があり、特に３００μｍ以上とすることが好適である。
The side wall of the sapphire base substrate used preferably has a total area of 1 to 20% with respect to the total surface area of the base substrate on the side where the GaN crystal layer is formed. That is, when the GaN crystal layer grows in the lateral direction and covers the entire surface of the underlying substrate, if the region that is the growth starting point of the GaN crystal is 1 to 20%, the bonding force between the sapphire and the GaN crystal layer is 1 / It is thought that it is reduced to 100 to 1/5.
6 and 7 show optical micrographs (cross section, surface Nomarski image) of the peeled surface of the GaN crystal after peeling. From this photograph, it is confirmed that the peeled surface shape of the GaN crystal layer peeled from sapphire is the same as the shape of the GaN crystal layer grown on the sapphire base substrate in which the groove portion shown in FIG. 1 and FIG. 2 is formed, It can be seen that peeling occurs at the interface between the sapphire base substrate and the GaN crystal layer.
The film thickness of the crystal layer needs to be 100 μm or more, and is preferably 300 μm or more, because the GaN crystal layer may be partially destroyed during natural peeling.
上記所定の傾斜角度の側壁を有する溝部は、溝部形成予定部分だけが開ロ部となるようにフォトレジストのパターニングを形成し、フォトレジストをエッチングレジストとし、サファイア下地基板を反応性イオンエッチング（Reactive Ion Etching：ＲＩＥ）等のドライエッチング或いはウエットエッチングをすることにより形成することができる。
更に、側壁の幅、溝部開口部幅、溝部間隔、底面幅などの制御手段としては、フォトレジストのパターニングを形成する段階において、フォトレジストの塗布量、ベーク温度、ベーク時間、ＵＶ照射量、ＵＶ照射する際のフォトマスクの形状などが挙げられる。また、エッチングの段階において、エッチングガス種、エッチングガス濃度、エッチングガス混合比、アンテナパワー、バイアスパワー、エッチング時間などによっても制御できる。
これら種々の条件を組み合わせることにより、所定の形状である溝部を有したサファイア下地基板を得ることができる。前記側壁の幅は、単位時間あたりにサファイアがエッチングされる速度であるエッチングレートを求め、エッチング時間を変更することで制御が可能である。
The groove portion having the sidewall having the predetermined inclination angle is formed by patterning a photoresist so that only a portion where the groove portion is to be formed becomes an open portion, the photoresist is used as an etching resist, and the sapphire base substrate is subjected to reactive ion etching (Reactive It can be formed by dry etching such as Ion Etching (RIE) or wet etching.
Further, as control means such as the width of the side wall, the width of the groove opening, the space between the grooves, and the width of the bottom surface, the photoresist coating amount, baking temperature, baking time, UV irradiation amount, UV, Examples include the shape of a photomask when irradiating. In the etching stage, it can be controlled by the etching gas type, etching gas concentration, etching gas mixture ratio, antenna power, bias power, etching time, and the like.
By combining these various conditions, a sapphire base substrate having a groove having a predetermined shape can be obtained. The width of the side wall can be controlled by obtaining an etching rate, which is the rate at which sapphire is etched per unit time, and changing the etching time.
上記方法において、サファイア下地基板主面の選定、並びに溝部の延びる方向の設定により、種々の面方位の側壁を有する下地基板を作成することができる。
具体的には、下地基板主面が＜１０−１２＞面、溝部の延びる方向が＜１１−２０＞面の面方位、即ち、ａ軸方向である場合は、結晶成長面である側壁にはｃ面が露出する。或いは、下地基板主面が＜１１−２３＞面、溝部の延びる方向が＜１０−１０＞面の面方位、即ち、ｍ軸方向である場合は、結晶成長面である側壁にはｃ面が露出する。或いは、下地基板主面が＜１１−２０＞面、溝部の延びる方向が＜１０−１０＞面の面方位、即ち、ｍ軸方向である場合は、結晶成長面である側壁にはｃ面が露出する。或いは、下地基板主面が＜１０−１０＞面、溝部の延びる方向が＜１１−２０＞面の面方位、即ち、ａ軸方向である場合は、結晶成長面である側壁にはｃ面が露出する。或いは、下地基板主面が＜０００２＞面、溝部の延びる方向が＜１０−１０＞面の面方位、即ち、ｍ軸方向である場合は、結晶成長面である側壁にはａ面が露出する。
上記の通り、サファイア下地基板は、その主面の面方位並びに結晶成長の起点の面となる側壁の面方位を任意に設計することができる。種々の面方位を有する側壁の中で、ｃ面側壁を起点とした横方向成長が優先的に起こり易いし制御し易い。従って、溝部を構成する側壁の少なくとも一部にｃ面からなる側壁を形成しておくことは、好ましい態様である。
In the above method, a base substrate having side walls of various plane orientations can be created by selecting the main surface of the sapphire base substrate and setting the direction in which the groove extends.
Specifically, when the base substrate main surface is the <10-12> plane and the extending direction of the groove is the <11-20> plane orientation, that is, the a-axis direction, The c-plane is exposed. Alternatively, when the underlying substrate main surface is the <11-23> plane and the extending direction of the groove is the <10-10> plane orientation, that is, the m-axis direction, the c-plane is formed on the side wall which is the crystal growth plane. Exposed. Alternatively, when the base substrate main surface is the <11-20> plane and the extending direction of the groove is the <10-10> plane orientation, that is, the m-axis direction, the c-plane is formed on the side wall which is the crystal growth plane. Exposed. Alternatively, when the base substrate main surface is the <10-10> plane and the extending direction of the groove is the <11-20> plane orientation, that is, the a-axis direction, the c-plane is formed on the side wall which is the crystal growth plane. Exposed. Alternatively, if the base substrate main surface is the <0002> plane and the extending direction of the groove is the <10-10> plane orientation, that is, the m-axis direction, the a-plane is exposed on the side wall that is the crystal growth plane. .
As described above, the sapphire base substrate can be arbitrarily designed with respect to the surface orientation of its main surface and the surface orientation of the side wall that becomes the crystal growth starting surface. Of the sidewalls having various plane orientations, lateral growth starting from the c-plane sidewall is likely to occur preferentially and easily controlled. Therefore, it is a preferable aspect to form a side wall composed of the c-plane on at least a part of the side wall constituting the groove.
また、側壁の一部をマスキングする場合、その手段としては、真空蒸着、スパッタリング、ＣＶＤ（Chemical Vapor Deposition）等の方法により、結晶成長領域以外の領域に、ＳｉＯ２膜、ＳｉＮｘ、膜、ＴｉＯ２膜、ＺｒＯ２膜等を形成してマスキングする方法が挙げられる。該マスキング層の厚さは、通常０．０１〜３μｍ程度である。 Further, when part of the side wall is masked, as a means thereof, a SiO 2 film, a SiN x film, a TiO 2 is formed in a region other than the crystal growth region by a method such as vacuum deposition, sputtering, or CVD (Chemical Vapor Deposition). Examples of the method include masking by forming two films, ZrO 2 films, and the like. The thickness of the masking layer is usually about 0.01 to 3 μm.
本発明においては、前記下地基板を用いて、その側壁を起点として、ＧａＮ結晶層を、ＥＬＯ法により横方向に結晶成長させることを特徴とする。得られるＧａＮ結晶層の結晶表面の面方位は、前記の通りサファイ下地基板主面に対応し、＜１１−２２＞面、＜１０−１１＞面、＜２０−２１＞面などからなる。 The present invention is characterized in that the GaN crystal layer is grown in the lateral direction by the ELO method using the base substrate as a starting point. The plane orientation of the crystal surface of the obtained GaN crystal layer corresponds to the main surface of the sapphire base substrate as described above, and is composed of <11-22> plane, <10-11> plane, <20-21> plane, and the like.
ＧａＮ結晶の成長方法は、特に限定されず、有機金属気相成長法（Metal Organic Vapor Phase Epitaxy：ＭＯＶＰＥ）、分子線エピタキシ法（Molecular Beam Epitaxy：ＭＢＥ）、ハイドライド気相成長法（Hydridｅ Vapor Phase Epitaxy：ＨＶＰＥ）が採用される。
本発明においては、結晶の成長速度が速いことから、ＨＶＰＥ法が好ましく採用される。特に、第一段の成長法にＭＯＶＰＥ法を採用して先ず高品質の結晶を生成させ、続いてＨＶＰＥ法でその上に高速で結晶成長させる二段法は特に好適な方法である。
以下では、ＨＶＰＥ法を利用した成長方法について説明する。また、本願発明の発明者等によって提案されたＷＯ２０１０／０２３８４６号公報に記載の溝部を有する下地基板に関する技術を何ら制限なく準用できる。
The growth method of the GaN crystal is not particularly limited, and metal organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (Hydrid Vapor Phase Epitaxy). : HVPE) is adopted.
In the present invention, the HVPE method is preferably employed because the crystal growth rate is high. In particular, the two-stage method in which the MOVPE method is adopted as the first-stage growth method to first produce high-quality crystals and then the crystals are grown at a high speed by the HVPE method is a particularly preferable method.
Below, the growth method using HVPE method is demonstrated. Further, the technology relating to the base substrate having a groove described in WO2010 / 023846 proposed by the inventors of the present invention can be applied mutatis mutandis.
ＨＶＰＥ法とは、塩化ガリウムと、窒素源となるアンモニア等を、気相反応させて下地基板上にＧａＮ結晶をエピタキシャル成長させる方法であり、それ自体公知の方法である。
結晶成長に用いられるＨＶＰＥ装置は、大きくは反応管、加熱系、ガス供給系、及びガス排気系から構成される。ガス供給系はマスフローコントローラーによって、ガス供給量の精密な制御が可能である。加熱系は、石英製の反応管を抵抗加熱式のヒーターで覆い、反応管、及びその中に設置される基板やサセプタ、金属原料を加熱するホットウォール法が用いられている。ホットウォール法を用いることにより、原料ガスを充分に加熱して基板表面に供給することができ、供給原料の飽和蒸気圧を高くすることができる。その結果、多量の原料供給が可能となり高速成長を実現できる。
The HVPE method is a method in which GaN crystal is epitaxially grown on a base substrate by reacting gallium chloride with ammonia as a nitrogen source in a gas phase, and is a method known per se.
The HVPE apparatus used for crystal growth is mainly composed of a reaction tube, a heating system, a gas supply system, and a gas exhaust system. The gas supply system can precisely control the gas supply amount by a mass flow controller. As the heating system, a hot wall method is used in which a reaction tube made of quartz is covered with a resistance heating heater, and the reaction tube, a substrate and a susceptor installed therein, and a metal material are heated. By using the hot wall method, the source gas can be sufficiently heated and supplied to the substrate surface, and the saturated vapor pressure of the supplied source can be increased. As a result, a large amount of raw material can be supplied and high-speed growth can be realized.
反応管の中は大きく金属原料部と基板加熱部に分けられる。金属原料部にはＧａ金属が置かれており、高温下にて、ＨＣｌガスを供給することにより、Ｇａ金属とＨＣｌガスとが反応し、ＧａＣｌが生成する。生成したＧａＣｌガスは、石英製の配管を通り、基板加熱部へと運ばれる。基板加熱部には、ガスの流れに対して垂直に炭素製或いはＳｉＣのサセプタが配置され、サセプタは自転機構を有している。そして、そのサセプタ上にサファイア下地基板、もしくは、サファイア下地基板上にＧａＮ結晶を成長させた積層基板がセットされ、金属原料部で生成したＧａＣｌガス、及び、ＮＨ３ガスとが基板上で反応することで、ＧａＮ結晶の成長が進行する。
結晶成長には原料ガスとして窒素原料であるＮＨ３ガス、Ｇａ源であるＧａＣｌを生成するためのＨＣｌガスが用いられる。ＧａＣｌ生成には、ＨＣｌガスの代わりにＣｌ２ガスを使用しても良い。またＧａ原料であるＧａ金属は装置内に設置される。原料ガスであるＮＨ３とＨＣｌガスの他に、Ｈ２やＮ２などのキャリアガスが用いられる。
The reaction tube is roughly divided into a metal raw material part and a substrate heating part. Ga metal is placed in the metal raw material portion, and by supplying HCl gas at a high temperature, Ga metal reacts with HCl gas to generate GaCl. The generated GaCl gas passes through a quartz pipe and is carried to the substrate heating unit. In the substrate heating unit, a susceptor made of carbon or SiC is arranged perpendicular to the gas flow, and the susceptor has a rotation mechanism. Then, a sapphire base substrate or a laminated substrate in which a GaN crystal is grown on the sapphire base substrate is set on the susceptor, and the GaCl gas generated in the metal raw material part and the NH 3 gas react on the substrate. As a result, the growth of the GaN crystal proceeds.
For crystal growth, NH 3 gas as a nitrogen source and HCl gas for generating GaCl as a Ga source are used as source gases. For the generation of GaCl, Cl 2 gas may be used instead of HCl gas. Moreover, Ga metal which is a Ga raw material is installed in the apparatus. Carrier gases such as H 2 and N 2 are used in addition to the source gas NH 3 and HCl gas.
本発明のサファイア下地基板を用いてＧａＮの結晶成長を行う際、該基板主面からの成長を起こさずに、溝部の側壁から優先的に成長がおこるように制御するためには、成長温度、成長圧力、原料ガス供給量、原料ガス供給比、キャリアガス種、キャリアガス量等の種々の条件を最適化する必要があり、使用する成長方法や反応装置や原料等を決定した上で、予め、予備的な実験でその条件を決定しておけば良い。また、本発明に用いるサファイア下地基板は、結晶成長させる領域以外がＳｉＯ２からなる結晶成長阻害層で被覆されたものでも良く、結晶成長阻害層の付与によっても該基板主面からの成長を抑制し、溝部の側壁から優先的に成長が起こるよう制御することができる。
具体的には、下地基板主面が＜１０−１２＞面、或いは＜１１−２３＞面である場合、ＧａＮ結晶は下地基板主面、サファイアｃ面が露出した溝部側壁、もう一方の溝部側壁から結晶成長する可能性がある。この場合、サファイアｃ面が露出した溝部側壁から優先的に成長が起こるように制御するには、上記種々の成長条件の最適化が必要である。また、下地基板主面からの成長は、結晶成長阻害層の付与によっても抑制が可能である。
下地基板主面が＜１１−２０＞面、或いは＜１０−１０＞面、或いは＜０００２＞面である場合、ＧａＮ結晶は下地基板主面、及び溝部側壁から結晶成長する可能性がある。このとき、両側の溝部側壁は同じ面方位を有しているため、どちらからも同じ面方位を有したＧａＮ結晶が成長し、どちらか一方の溝部側壁から結晶が成長するように制御する必要はなく、下地基板主面からの成長を抑制すればよい。下地基板主面からの成長を抑制するには結晶成長阻害層の付与が効果的であるが、上記種々の成長条件の最適化のみでも制御は可能である。
When performing crystal growth of GaN using the sapphire base substrate of the present invention, in order to control the growth so as to preferentially occur from the sidewall of the groove without causing growth from the main surface of the substrate, the growth temperature, It is necessary to optimize various conditions such as growth pressure, raw material gas supply amount, raw material gas supply ratio, carrier gas type, carrier gas amount, etc. The conditions may be determined by preliminary experiments. In addition, the sapphire base substrate used in the present invention may be coated with a crystal growth inhibition layer made of SiO 2 except for a region for crystal growth, and the growth from the main surface of the substrate is also suppressed by the provision of the crystal growth inhibition layer. In addition, it is possible to control the growth so as to preferentially occur from the side wall of the groove.
Specifically, when the base substrate main surface is a <10-12> plane or a <11-23> plane, the GaN crystal has a base substrate main surface, a groove side wall from which the sapphire c surface is exposed, and another groove side wall. There is a possibility of crystal growth from. In this case, it is necessary to optimize the above various growth conditions in order to control the growth so as to preferentially occur from the groove side wall where the sapphire c-plane is exposed. Further, the growth from the main surface of the base substrate can be suppressed by providing a crystal growth inhibiting layer.
When the base substrate main surface is the <11-20> plane, the <10-10> plane, or the <0002> plane, the GaN crystal may grow from the base substrate main surface and the groove side wall. At this time, since the groove side walls on both sides have the same plane orientation, it is necessary to control so that the GaN crystal having the same plane orientation grows from either side and the crystal grows from either side wall of the trench. However, the growth from the main surface of the base substrate may be suppressed. In order to suppress the growth from the main surface of the base substrate, it is effective to provide a crystal growth inhibiting layer, but control is possible only by optimizing the above various growth conditions.
以下具体的にＧａＮ結晶層の成長とサファイア下地基板からの剥離について説明する。
まず、サファイア下地基板、或いはサファイア下地基板上にＧａＮ結晶が積層された積層基板を基板主面が上向きになるようにサセプタにセットした後、反応管を結晶成長の温度まで加熱する。ヒーターは金属原料部、基板加熱部それぞれを個別に制御することができ、金属原料部は８００〜９００℃、基板加熱部は９００℃〜１１５０℃に加熱する。このとき、基板加熱部が５００℃以上ではＧａＮ結晶の熱分解が生じるため、それを防ぐ目的でＨ２ガス、及びＮＨ３ガスを流通させる。また反応管が設定温度に達した後、その状態を数分間保持することにより、基板表面のサーマルクリーニングを行う。
次いで、反応管内に設置されたＧａ金属にＨＣｌガスを０．１〜２．０Ｌ／ｍｉｎの流量で流通させることで、Ｇａ金属とＨＣｌガスとが反応し、ＧａＣｌが生成する。また、ＮＨ３を１〜４０Ｌ／ｍｉｎの流量で流通させる。このとき、キャリアガスはＨ２やＮ２、或いはそれらの混合ガスで、流量は１〜１００Ｌ／ｍｉｎである。このとき、サファイア下地基板、或いはサファイア下地基板上にＧａＮ結晶が積層された積層基板の上にＧａＮ結晶が成長する。
ＧａＮ結晶の成長を終えるには、ＨＣｌガスの流通を止める。このとき、ＧａＮ結晶の熱分解を防ぐために、ＮＨ３ガスは基板部温度が６００℃以下になるまで流し続けておく。
Hereinafter, the growth of the GaN crystal layer and the peeling from the sapphire base substrate will be described in detail.
First, a sapphire base substrate or a multilayer substrate in which a GaN crystal is stacked on a sapphire base substrate is set on a susceptor so that the main surface of the sapphire faces upward, and then the reaction tube is heated to a crystal growth temperature. The heater can individually control the metal raw material part and the substrate heating part, the metal raw material part is heated to 800 to 900 ° C., and the substrate heating part is heated to 900 ° C. to 1150 ° C. At this time, since the thermal decomposition of the GaN crystal occurs when the substrate heating part is 500 ° C. or higher, H 2 gas and NH 3 gas are circulated for the purpose of preventing this. Further, after the reaction tube reaches the set temperature, the state is maintained for several minutes to perform thermal cleaning of the substrate surface.
Next, by allowing HCl gas to flow through the Ga metal installed in the reaction tube at a flow rate of 0.1 to 2.0 L / min, the Ga metal and the HCl gas react to generate GaCl. Further, NH 3 is circulated at a flow rate of 1 to 40 L / min. At this time, the carrier gas is H 2 or N 2, or in a mixed gas thereof, the flow rate is 1~100L / min. At this time, the GaN crystal grows on the sapphire base substrate or a laminated substrate in which the GaN crystal is laminated on the sapphire base substrate.
To finish the growth of the GaN crystal, the flow of HCl gas is stopped. At this time, in order to prevent thermal decomposition of the GaN crystal, the NH 3 gas is kept flowing until the substrate part temperature becomes 600 ° C. or lower.
ＧａＮ結晶を成長した後、基板の冷却を行う。冷却は、例えば、低温ガス等を供給して強制的に行ってもよく、また、自然放冷によって行ってもよい。冷却によってサファイア下地基板、及びＧａＮ結晶層の温度を２０℃〜１５０℃まで低下させる。冷却速度は、例えば、１〜１００℃／ｍｉｎである。
この冷却の際、熱膨張係数が相対的に大きいサファイア下地基板は上に凸に反ろうとする熱応力が生じ、ＧａＮ結晶層には圧縮応力が掛かる。本発明においては、溝形状を有したサファイア下地基板の側壁のみからＧａＮ結晶を成長させているため、サファイアとＧａＮとが化学的に結合している領域が狭く、結合力が小さいため、特別な機械的応力を印加することなく、冷却時の熱応力のみにてサファイア下地基板とＧａＮ結晶層との界面において自然剥離が生じる。
上記作用効果の観点からは、サファイア下地基板のＧａＮ結晶層を形成する側の全表面積に対して、結晶成長の起点となる領域である側壁の総面積の割合が、１〜２０％であることが好ましく、より好ましくは１〜１５％、さらに好ましくは１〜１０％である。この、サファイア下地基板のＧａＮ結晶層を形成する側の全表面積に対する結晶成長の起点となる側壁の総面積の割合は、サファイア下地基板に形成した溝形状の溝深さを変えることや、溝を形成する間隔を変えることで制御される。
After growing the GaN crystal, the substrate is cooled. The cooling may be performed forcibly by supplying a low temperature gas or the like, or may be performed by natural cooling. The temperature of the sapphire base substrate and the GaN crystal layer is lowered to 20 ° C. to 150 ° C. by cooling. The cooling rate is, for example, 1 to 100 ° C./min.
During this cooling, the sapphire base substrate having a relatively large thermal expansion coefficient generates a thermal stress that tends to warp upward, and a compressive stress is applied to the GaN crystal layer. In the present invention, since the GaN crystal is grown only from the side wall of the sapphire base substrate having the groove shape, the region where the sapphire and GaN are chemically bonded is narrow and the bonding force is small. Natural exfoliation occurs at the interface between the sapphire base substrate and the GaN crystal layer only by thermal stress during cooling without applying mechanical stress.
From the viewpoint of the above-described effects, the ratio of the total area of the side wall, which is a region serving as a starting point for crystal growth, is 1 to 20% with respect to the total surface area of the sapphire base substrate on the GaN crystal layer forming side. Is preferable, more preferably 1 to 15%, still more preferably 1 to 10%. The ratio of the total area of the side wall which is the starting point of crystal growth to the total surface area of the sapphire base substrate on the side where the GaN crystal layer is formed can be changed by changing the groove depth of the groove shape formed on the sapphire base substrate, It is controlled by changing the forming interval.
上記方法によって、暗点密度が２×１０８個／ｃｍ２未満であり、表面が、無極性または半極性の面方位を有することを特徴とする窒化ガリウム結晶自立基板が製造される。得られたＧａＮ結晶積層自立基板は、必要に応じて表面研磨等を行い、各種半導体発光素子の基板として使用することができ、或いは、この自立基板をＧａＮ結晶成長用の下地基板として使用することもできる。 By the above method, a gallium nitride crystal free-standing substrate having a dark spot density of less than 2 × 10 8 pieces / cm 2 and a surface having a nonpolar or semipolar plane orientation is produced. The obtained GaN crystal laminated free-standing substrate can be used as a substrate for various semiconductor light-emitting elements by performing surface polishing or the like, if necessary, or this free-standing substrate can be used as a base substrate for GaN crystal growth. You can also.
以下、本発明を、実施例を挙げて具体的に説明するが、本発明はこれらの実施例によって何ら制限されるものではない。また、実施例の中で説明されている特徴の組み合わせすべてが本発明の解決手段に必須のものとは限らない。 EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not restrict | limited at all by these Examples. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.
実施例１
〔サファイア下地基板の作製〕
＜１０−１２＞面サファイア基板上にストライプ状にレジストをパターニングし、次いで反応性イオンエッチング（ＲＩＥ）によりドライエッチングすることで、サファイア基板上に複数本の溝部を形成した。溝部は、溝開口幅が３μｍ、溝の深さが０．１μｍ、及び隣接する溝部までの基板主面部分の幅が３μｍとなるように形成した。側壁の傾斜角度は約６０度であり、溝深さ及び側壁の傾斜角度より算出した側壁の幅（ｄ）は０．１１５μｍである。
ドライエッチングの後、レジストを洗浄除去することでサファイア下地基板を得た。このサファイア下地基板は、基板主面、８４６６本の溝部、c面側壁から構成された結晶成長領域、及び溝部底面を有する。ｃ面側壁の総面積は、サファイア下地基板の成長側の全面積を基準にして、およそ１．８９％である。
Example 1
[Production of sapphire base substrate]
A resist was patterned in a stripe shape on a <10-12> plane sapphire substrate, and then dry etching was performed by reactive ion etching (RIE) to form a plurality of grooves on the sapphire substrate. The groove portion was formed so that the groove opening width was 3 μm, the groove depth was 0.1 μm, and the width of the main surface portion of the substrate up to the adjacent groove portion was 3 μm. The inclination angle of the side wall is about 60 degrees, and the side wall width (d) calculated from the groove depth and the inclination angle of the side wall is 0.115 μm.
After dry etching, the resist was washed away to obtain a sapphire base substrate. The sapphire base substrate has a substrate main surface, 8466 groove portions, a crystal growth region composed of c-plane side walls, and a groove bottom surface. The total area of the c-plane side wall is approximately 1.89% based on the total area on the growth side of the sapphire base substrate.
〔ＭＯＶＰＥによるＧａＮ結晶層の形成〕
作製したサファイア下地基板を、ＭＯＶＰＥ装置内に、基板表面が上向きになるように石英トレイ上にセットした後、基板を１１５０℃に加熱すると共に反応容器内の圧力を１００ｋＰａとし、また、反応容器内にキャリアガスとしてＨ２を１０Ｌ／ｍｉｎで流通させ、その状態を１０分間保持することにより基板をサーマルクリーニングした。
次いで、基板の温度を４６０℃とすると共に反応容器内の圧力を１００ｋＰａとし、また、反応容器内を流通させるキャリアガスをＨ２５Ｌ／ｍｉｎの流量で流しながら、そこにＶ族元素供給源（ＮＨ３）、及びＩＩＩ族元素供給源（ＴＭＧ）を、それぞれの供給量が５Ｌ／ｍｉｎ及び５．５μｍｏｌ／ｍｉｎで基板上にアモルファス状のＧａＮを約２５ｎｍ堆積させた。続いて基板の温度を１０７５℃とすると共に反応容器内の圧力を２０ｋＰａとし、また、反応容器内を流通させるキャリアガスをＨ２として、それを５Ｌ／ｍｉｎの流量で流通させることで、基板上に堆積したＧａＮを再結晶化し、溝部側壁の結晶成長領域に選択的にＧａＮ結晶核を形成した。
[Formation of GaN crystal layer by MOVPE]
The prepared sapphire base substrate is set in a MOVPE apparatus on a quartz tray so that the substrate surface faces upward, and then the substrate is heated to 1150 ° C. and the pressure in the reaction vessel is set to 100 kPa. The substrate was thermally cleaned by circulating H 2 at 10 L / min as a carrier gas and maintaining this state for 10 minutes.
Then, the pressure in the reaction vessel was 100kPa while the temperature of the substrate and 460 ° C., while also a flow of carrier gas to circulate inside the reaction vessel at a flow rate of H 2 5L / min, there Group V element source ( NH 3 ) and a group III element supply source (TMG) were deposited at about 25 nm of amorphous GaN on the substrate with respective supply amounts of 5 L / min and 5.5 μmol / min. Subsequently, the temperature of the substrate is set to 1075 ° C., the pressure in the reaction vessel is set to 20 kPa, and the carrier gas flowing in the reaction vessel is set to H 2 , and the carrier gas is circulated at a flow rate of 5 L / min. The GaN deposited on was recrystallized to selectively form GaN crystal nuclei in the crystal growth region on the side wall of the groove.
続いて、基板の温度を１０２５℃とすると共に反応容器内の圧力を２０ｋＰａとし、また、反応容器内を流通させるキャリアガスをＨ２として、それを５Ｌ／ｍｉｎの流量で流通させながら、そこにＶ族元素供給源（ＮＨ３）、及びＩＩＩ族元素供給源（ＴＭＧ）を、それぞれの供給量が２Ｌ／ｍｉｎ及び３０μｍｏｌ／ｍｉｎとなるように３００分間流し、ＧａＮ結晶核の上にＧａＮ（アンドープＧａＮ）を結晶成長させることにより、基板の主面に形成された溝部の各側壁から横方向結晶成長するように基板上にＧａＮ結晶層を形成した。下地基板の主面に形成された溝部の各側壁から成長したＧａＮ結晶同士は、結晶同士がぶつかった時点において会合し、基板主面に対して平行に＜１１−２２＞面を主面とするＧａＮ結晶層を形成し、ＧａＮ結晶積層基板を作製した。 Subsequently, the temperature of the substrate is set to 1025 ° C., the pressure in the reaction vessel is set to 20 kPa, and the carrier gas to be circulated in the reaction vessel is set to H 2 while being circulated at a flow rate of 5 L / min. A group V element supply source (NH 3 ) and a group III element supply source (TMG) were allowed to flow for 300 minutes so that the respective supply amounts were 2 L / min and 30 μmol / min, and GaN (undoped) was formed on the GaN crystal nucleus. By growing the crystal of GaN), a GaN crystal layer was formed on the substrate so as to grow laterally from each side wall of the groove formed on the main surface of the substrate. The GaN crystals grown from the respective side walls of the groove formed on the main surface of the base substrate meet when the crystals collide with each other and have the <11-22> plane as the main surface parallel to the main surface of the substrate. A GaN crystal layer was formed to produce a GaN crystal multilayer substrate.
〔ＨＶＰＥによるＧａＮ結晶層の形成、下地基板からの剥離〕
ＭＯＶＰＥにて作製したＧａＮ結晶積層基板を基板主面が上向きになるようにサセプタにセットした後、反応管内にＮ２ガスを３０ｍｉｎ流通させ、反応管内をＮ２ガス雰囲気下とした。金属原料部が８５０℃、基板加熱部が１０４０℃となるように反応管を加熱し、設定温度到達後、２５分間保持した。このとき、基板加熱部が５００℃に達するまでは反応管内にはＮ２ガスを流通させ、５００℃以上ではＨ２ガス、及び、ＮＨ３ガスを流通させた。
２５分保持の後、反応管内に設置されたＧａ金属にＨＣｌガスを０．８Ｌ／ｍｉｎ流通させた。またＮＨ３ガスを８Ｌ／ｍｉｎ、キャリアガスであるＨ２ガスを３４Ｌ／ｍｉｎ流通し、３６０分間ＧａＮ結晶を成長させた。ＧａＮ結晶の膜厚は３６０μｍであった。
その後、ＨＣｌガスの流通を止めて、成長を終了させ、基板の冷却を行った。冷却はガスを流通させながら、自然放冷にて行った。冷却時、基板温度が６００℃以下になるまではＮＨ３ガスを５Ｌ／ｍｉｎ、及びＨ２ガスを１７．１Ｌ／ｍｉｎ流通し、６００℃以下ではＮ２ガスを３７．７Ｌ／ｍｉｎ流通した。
基板温度が１５０℃以下になった時点で、装置内から基板を取り出したが、その時点においてすでにサファイア下地基板とＧａＮ結晶層とは自然に剥離していた。自然剥離後のＧａＮ結晶を図４に示す。図の左側が剥離したサファイア下地基板であり、右側が剥離したＧａＮ結晶層の剥離面である。
図６、図７に下地基板分離後のＧａＮ結晶剥離面の、ノマルスキー型微分干渉顕微鏡により観察した断面および表面像を示す。また、図８には下地基板上にＥＬＯ成長したＧａＮ結晶の断面ＳＥＭ像を示した。溝部を形成したサファイア下地基板の側壁からＧａＮ結晶を成長させると、図８のような形状で結晶が成長する。図６からはＧａＮ結晶層の剥離面がこの形状を保持していることが観察され、剥離はＧａＮ結晶層とサファイア下地基板との界面で生じたことがわかる。また、図７に示した剥離したＧａＮ結晶層の表面像からは、ストライプ状の形状が全域において観察され、サファイア下地基板とＧａＮ結晶層との界面での剥離が、一部分ではなく全域において生じていることが確認できる。
得られた結晶は、＜１１−２２＞面を主面とする、厚み３６０μｍの自立ＧａＮ結晶であった。
[Formation of GaN crystal layer by HVPE, peeling from the underlying substrate]
After setting the GaN crystal laminated substrate manufactured by MOVPE on the susceptor so that the main surface of the GaN crystal substrate faced upward, N 2 gas was passed through the reaction tube for 30 minutes, and the inside of the reaction tube was placed in an N 2 gas atmosphere. The reaction tube was heated so that the metal raw material portion was 850 ° C. and the substrate heating portion was 1040 ° C., and was held for 25 minutes after reaching the set temperature. At this time, N 2 gas was circulated in the reaction tube until the substrate heating part reached 500 ° C., and H 2 gas and NH 3 gas were circulated above 500 ° C.
After holding for 25 minutes, HCl gas was circulated through the Ga metal installed in the reaction tube at 0.8 L / min. Further, NH 3 gas was supplied at 8 L / min and carrier gas H 2 gas was supplied at 34 L / min to grow a GaN crystal for 360 minutes. The film thickness of the GaN crystal was 360 μm.
Thereafter, the flow of HCl gas was stopped, the growth was terminated, and the substrate was cooled. Cooling was performed by natural cooling while circulating gas. During cooling, NH 3 gas was passed at 5 L / min and H 2 gas at 17.1 L / min until the substrate temperature was 600 ° C. or lower, and N 2 gas was passed at 37.7 L / min at 600 ° C. or lower.
When the substrate temperature became 150 ° C. or lower, the substrate was taken out from the apparatus. At that time, the sapphire base substrate and the GaN crystal layer had already been naturally separated. FIG. 4 shows the GaN crystal after natural peeling. The left side of the figure is the peeled sapphire base substrate, and the right side is the peeled surface of the peeled GaN crystal layer.
6 and 7 show a cross section and a surface image of the GaN crystal peeling surface after separation of the base substrate, which are observed with a Nomarski differential interference microscope. FIG. 8 shows a cross-sectional SEM image of the GaN crystal grown by ELO on the base substrate. When a GaN crystal is grown from the side wall of the sapphire base substrate in which the groove is formed, the crystal grows in a shape as shown in FIG. From FIG. 6, it is observed that the peeled surface of the GaN crystal layer maintains this shape, and it can be seen that the peeling occurred at the interface between the GaN crystal layer and the sapphire base substrate. Further, from the surface image of the peeled GaN crystal layer shown in FIG. 7, a striped shape is observed in the entire region, and peeling at the interface between the sapphire base substrate and the GaN crystal layer occurs not in a part but in the entire region. It can be confirmed.
The obtained crystal was a self-supporting GaN crystal having a thickness of 360 μm and having a <11-22> plane as a main surface.
実施例２
用いたサファイア下地基板の側壁の幅（ｄ）を１．１５μｍとした以外は、実施例１と同様にしてＧａＮ結晶を成長させた。このとき、ｃ面側壁の総面積は、サファイア下地基板の成長側の全面積を基準にして、およそ１６．１％である。冷却工程を経て取り出したＧａＮ結晶は、実施例１と同様に下地基板と自然剥離し、厚み３６０μｍの自立ＧａＮ結晶となっていた。
Example 2
A GaN crystal was grown in the same manner as in Example 1 except that the side wall width (d) of the sapphire base substrate used was 1.15 μm. At this time, the total area of the c-plane side wall is approximately 16.1% based on the total area on the growth side of the sapphire base substrate. The GaN crystal taken out through the cooling process was spontaneously peeled off from the base substrate in the same manner as in Example 1 to form a self-supporting GaN crystal having a thickness of 360 μm.
比較例１
用いたサファイア下地基板の側壁の幅（ｄ）を１．１５μｍとしたこと、および、ＨＶＰＥ装置でのＧａＮ結晶の成長時間を１５分間としたこと以外は、実施例１と同様にしてＧａＮ結晶を成長させた。冷却工程を経て取り出したＧａＮ結晶は、ＧａＮ結晶層のサファイア下地基板からの剥離が生じていなかった。
成長後のＧａＮ結晶を図５に示す。図５からは、サファイア下地基板の上にＧａＮ結晶が成長しており、クラック等も発生していないことが見て取れる。ＧａＮ結晶層の厚みは２７μｍであり、ＧａＮ結晶層の膜厚が薄いため、ＧａＮ結晶層に掛かる応力が小さく、自然剥離が生じなかったと考えられる。
Comparative Example 1
The GaN crystal was formed in the same manner as in Example 1 except that the side wall width (d) of the sapphire base substrate used was 1.15 μm and the growth time of the GaN crystal in the HVPE apparatus was 15 minutes. Grown up. The GaN crystal taken out through the cooling process did not peel off the GaN crystal layer from the sapphire base substrate.
The grown GaN crystal is shown in FIG. From FIG. 5, it can be seen that a GaN crystal has grown on the sapphire base substrate and no cracks or the like have occurred. The thickness of the GaN crystal layer is 27 μm, and since the thickness of the GaN crystal layer is thin, the stress applied to the GaN crystal layer is small, and it is considered that natural peeling did not occur.
〔暗点密度評価〕
実施例１、及び実施例２のそれぞれで得られたＧａＮ結晶自立基板について、走査型電子顕微鏡／カソードルミネッセンス（ＳＥＭ・ＣＬ）装置を用いて、ＧａＮ結晶自立基板の表面の観察を行った。このときの加速電圧は５ｋＶ、観察範囲は２０μm×２０μmとし、観察範囲内に観察された暗点の総数から暗点密度を算出したところ、表１に示す結果が得られた。
[Dark spot density evaluation]
With respect to the GaN crystal free-standing substrate obtained in each of Example 1 and Example 2, the surface of the GaN crystal free-standing substrate was observed using a scanning electron microscope / cathode luminescence (SEM / CL) apparatus. At this time, the acceleration voltage was 5 kV, the observation range was 20 μm × 20 μm, and the dark spot density was calculated from the total number of dark spots observed in the observation range. The results shown in Table 1 were obtained.
実施例１および実施例２の暗点密度評価結果から、転位の少ない高品質の半極性面ＧａＮ結晶自立基板が得られることが認識できる。 From the dark spot density evaluation results of Example 1 and Example 2, it can be recognized that a high-quality semipolar plane GaN crystal free-standing substrate with few dislocations can be obtained.
１０ サファイア下地基板
１１ 下地基板主面
２０ 下地基板溝部
２１ 溝部側壁
２２ 溝部底面
２３ 側壁結晶成長領域
３０ ＧａＮ結晶層
３１ ＧａＮ結晶層表面
４０ マスキング部
DESCRIPTION OF SYMBOLS 10 Sapphire base substrate 11 Base substrate main surface 20 Base substrate groove part 21 Groove side wall 22 Groove bottom face 23 Side wall crystal growth region 30 GaN crystal layer 31 GaN crystal layer surface 40 Masking part
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