JP2008303119A - HIGHLY FUNCTIONAL Ga2O3 SINGLE CRYSTAL FILM AND METHOD FOR PRODUCING THE SAME - Google Patents
HIGHLY FUNCTIONAL Ga2O3 SINGLE CRYSTAL FILM AND METHOD FOR PRODUCING THE SAME Download PDFInfo
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- QZQVBEXLDFYHSR-UHFFFAOYSA-N gallium(III) oxide Inorganic materials O=[Ga]O[Ga]=O QZQVBEXLDFYHSR-UHFFFAOYSA-N 0.000 title 1
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Abstract
Description
本発明は、表示、通信、記録機器などの分野で商品化されている発光ダイオードやレーザダイオードなどの発光素子、特に深紫外域で発光する光源として使用可能な高品質のＧａ2Ｏ3単結晶膜及びその製造方法に関する。 The present invention relates to a light emitting element such as a light emitting diode or a laser diode that has been commercialized in the fields of display, communication, recording equipment, etc., and particularly a high quality Ga 2 O 3 single crystal that can be used as a light source that emits light in the deep ultraviolet region. The present invention relates to a film and a manufacturing method thereof.
青色、紫外発光を担うIII族窒化物ＡｌＧａＩｎＮ系混晶は、蛍光体を利用することで、可視域全域の発光を実現することができるようになった。しかしながら、深紫外域の発光は未だ困難な状況にある。その原因として、単結晶基板の作製難、高温成長による熱歪、深いアクセプター準位、不純物酸素などがあげられる。
現在までに、ＡｌＮを活性層とした青色発光ダイオードが作製され、波長２１０ｎｍからの電流注入発光が実現している（非特許文献１）が、その発光効率は１％に満たない。
The group III nitride AlGaInN mixed crystal responsible for blue and ultraviolet emission can emit light in the entire visible range by using a phosphor. However, light emission in the deep ultraviolet region is still difficult. The causes include difficulty in manufacturing a single crystal substrate, thermal strain due to high temperature growth, deep acceptor levels, impurity oxygen, and the like.
To date, blue light emitting diodes using AlN as an active layer have been manufactured and current injection light emission from a wavelength of 210 nm has been realized (Non-Patent Document 1), but the light emission efficiency is less than 1%.
このIII族窒化物に対抗して、ダイヤモンドや酸化物半導体であるＺｎＭｇＯ混晶系を用いて深紫外発光を目指す研究もなされている（非特許文献２，３）。ダイヤモンドはバンドギャップが〜５.５ｅＶと深紫外発光できる特性を有するが、ダイヤモンド基板の使用など製造コスト的に実用化には課題がある。これに対し、ＺｎＭｇＯ混晶は窒化物と異なり反応性の高い酸素が不純物とならない酸化物であるため多くの成長方法が選択できるなどの利点がある。しかし、ＺｎＯ、ＭｇＯの結晶構造がそれぞれ異なり高Ｍｇ組成では相分離を引き起こし、そのためバンドギャップは最大で４.５ｅＶ(〜２８０ｎｍ)までしか拡大できない。 In contrast to this group III nitride, research aimed at deep ultraviolet emission using a ZnMgO mixed crystal system which is diamond or an oxide semiconductor has also been made (Non-Patent Documents 2 and 3). Diamond has a characteristic that it can emit deep ultraviolet light with a band gap of ˜5.5 eV, but there are problems in practical use in terms of production cost such as use of a diamond substrate. On the other hand, ZnMgO mixed crystal has an advantage that many growth methods can be selected because highly reactive oxygen is not an impurity unlike nitride. However, the crystal structures of ZnO and MgO are different, and a high Mg composition causes phase separation. Therefore, the band gap can be expanded only up to 4.5 eV (˜280 nm).
ところで、上記のような深紫外で発光する素子は、ダイヤモンド自体の広いバンドギャップを利用しようとするものであるが、ダイヤモンドの製膜には気相合成法が用いられる。気相合成法では高真空を必要とするばかりでなく、製膜に高度の制御技術を必要とするためにコスト的に高くなってしまい、結果的にダイヤモンド膜系深紫外発光素子の実用化が遅れている。また、ダイヤモンドは混晶化できないため、波長のチューニングは基本的にできない。
一方、ＺｎＭｇＯ混晶の深紫外発光素子への適用は、前記したとおり、波長チューニングにより最大で〜２８０ｎｍまでの短波長化が可能であるが、深紫外領域ではさほど期待できない。
By the way, although the element which emits light in the deep ultraviolet as described above intends to use the wide band gap of diamond itself, a vapor phase synthesis method is used for film formation of diamond. In the vapor phase synthesis method, not only high vacuum is required, but also high control technology is required for film formation, resulting in high cost. Running late. Further, since diamond cannot be mixed, wavelength tuning is basically impossible.
On the other hand, as described above, ZnMgO mixed crystal can be applied to a deep ultraviolet light emitting device, and as described above, the wavelength can be shortened up to ˜280 nm by wavelength tuning, but it cannot be expected so much in the deep ultraviolet region.
本発明は、深紫外域で発光する光源として使用可能な高機能性のＧａ2Ｏ3単結晶薄膜を、簡便な手段で提供することを目的とする。 An object of the present invention is to provide a highly functional Ga 2 O 3 single crystal thin film that can be used as a light source that emits light in the deep ultraviolet region by a simple means.
本発明の高機能性Ｇａ2Ｏ3単結晶膜は、その目的を達成するため、β‐Ｇａ2Ｏ3単結晶ウエハ上に積層して形成されていることを特徴とする。
β‐Ｇａ2Ｏ3単結晶ウエハとしては、ステップテラス構造を有するものが好ましい。そして、β‐Ｇａ2Ｏ3単結晶ウエハの（１００）面上にβ‐Ｇａ2Ｏ3単結晶膜が積層されていることが好ましい。さらに、β‐Ｇａ2Ｏ3単結晶膜は、分子線エピタキシー法（Molecular Beam Epitaxy：ＭＢＥ法）により形成されたものが好ましい。
In order to achieve the object, the highly functional Ga 2 O 3 single crystal film of the present invention is characterized by being laminated on a β-Ga 2 O 3 single crystal wafer.
The β-Ga 2 O 3 single crystal wafer preferably has a step terrace structure. A β-Ga 2 O 3 single crystal film is preferably laminated on the (100) plane of the β-Ga 2 O 3 single crystal wafer. Further, the β-Ga 2 O 3 single crystal film is preferably formed by a molecular beam epitaxy (MBE method).
このような高機能性Ｇａ2Ｏ3単結晶膜は、光学式浮遊帯域溶融法を用いて作製したβ‐Ｇａ2Ｏ3単結晶ウエハを基板とし、この基板の（１００）面上に分子線エピタキシー法により形成する光学式浮遊帯域溶融法を用いて作製したβ‐Ｇａ2Ｏ3単結晶ウエハとしては、ステップテラス構造を有するものが好ましい。
分子線エピタキシー法を用いてβ‐Ｇａ2Ｏ3単結晶膜を成長させる際、成長温度を８００℃以上とすることが好ましい。また、この際、ラジカル酸素供給量を一定にし、Ｇａ供給量をＧａ供給律速、酸素供給律速の境界±１０％以内に制御することが好ましい。
Such a high-functional Ga 2 O 3 single crystal film uses a β-Ga 2 O 3 single crystal wafer produced by an optical floating zone melting method as a substrate, and a molecular beam on the (100) plane of this substrate. As a β-Ga 2 O 3 single crystal wafer produced by using an optical floating zone melting method formed by an epitaxy method, one having a step terrace structure is preferable.
When the β-Ga 2 O 3 single crystal film is grown using the molecular beam epitaxy method, the growth temperature is preferably set to 800 ° C. or higher. At this time, it is preferable that the radical oxygen supply amount be constant and the Ga supply amount be controlled within a range of ± 10% between the Ga supply rate limiting rate and the oxygen supply rate limiting rate.
本発明により提供される高機能性Ｇａ2Ｏ3単結晶膜は、酸化ガリウム単結晶（以下、「β‐Ｇａ2Ｏ3単結晶」と記す。）基板上にβ‐Ｇａ2Ｏ3単結晶薄膜を分子線エピタキシー法により形成することにより製造されている。高真空を必要とすることなく製造されるため、深紫外で発光する固体素子を低コストで製造することが可能となる。しかも、２５０〜２７０ｎｍ付近の深紫外域で発光する発光素子用光源を低コストで供給することが可能となる。この発光波長は、水銀の輝線（２５４ｎｍ）にほぼ対応するため、現行の蛍光灯の代替励起光源として、照明分野などへの応用も期待される。このように、省エネ、ＣＯ2ガス排出抑制など、環境問題などにも貢献できると期待される。 High functional Ga 2 O 3 single crystal film provided by the present invention, gallium oxide single crystals (hereinafter, referred to as "β-Ga 2 O 3 single crystal".) On the substrate β-Ga 2 O 3 single crystal It is manufactured by forming a thin film by a molecular beam epitaxy method. Since it is manufactured without requiring a high vacuum, it is possible to manufacture a solid-state device that emits light in the deep ultraviolet at a low cost. In addition, it is possible to supply a light source for a light emitting element that emits light in the deep ultraviolet region near 250 to 270 nm at a low cost. Since this emission wavelength substantially corresponds to the mercury emission line (254 nm), it is expected to be applied to the illumination field as an alternative excitation light source for the current fluorescent lamp. Thus, it is expected to contribute to environmental problems such as energy saving and CO 2 gas emission suppression.
本発明者等は、深紫外で発光する素子を簡便な手段で得る方法について鋭意検討を重ねてきた。その過程で、従来検討されてきた窒化物、酸化物を使った深紫外域発光材料の問題を解決するため、酸化物の利点を活かし、なおかつバンドギャップが４．７〜４．９ｅＶと大きなβ‐Ｇａ2Ｏ3に着目し、β‐Ｇａ2Ｏ3を用いた深紫外域発光の実現可能性について検討した。
β‐Ｇａ2Ｏ3単結晶は、ＧａＮ系の薄膜成長用基板、酸素センサ、電界効果型トランジスタ（ＦＥＴ）、深紫外受光素子、透明導電膜、ＧａＮ系ＦＥＴのゲート材料などのデバイス応用が検討されている。また、β‐Ｇａ2Ｏ3のバンドギャップ４．８ｅＶ(〜２６０ｎｍ)は、その発光波長が水銀の輝線(２５４ｎｍ)に対応するため、発光デバイスの最大の市場である照明産業において、現行の蛍光灯の代替励起光源としての応用も考えられる。
このようなＧａ2Ｏ3材料の特性を利用し、β‐Ｇａ2Ｏ3単結晶基板上に成長させたβ‐Ｇａ2Ｏ3単結晶膜の成長条件を最適化させた結果、波長２５０〜２７０ｎｍの深紫外域で発光することを見出し、本発明に至った。
以下、その概要を詳細に説明する。
The inventors of the present invention have made extensive studies on a method for obtaining an element that emits light in the deep ultraviolet by a simple means. In this process, in order to solve the problems of light emitting materials using nitrides and oxides that have been studied in the past, the advantages of oxides are utilized, and the band gap is as large as 4.7 to 4.9 eV. Focusing on -Ga 2 O 3 , the feasibility of deep ultraviolet light emission using β-Ga 2 O 3 was investigated.
β-Ga 2 O 3 single crystal is considered for device applications such as GaN-based thin film growth substrates, oxygen sensors, field-effect transistors (FETs), deep ultraviolet light-receiving elements, transparent conductive films, and gate materials for GaN-based FETs. Has been. In addition, the band gap of 4.8 eV (˜260 nm) of β-Ga 2 O 3 corresponds to the emission line of mercury (254 nm), so that in the lighting industry, which is the largest market for light-emitting devices, Application as an alternative excitation light source for lamps is also conceivable.
As a result of optimizing the growth conditions of the β-Ga 2 O 3 single crystal film grown on the β-Ga 2 O 3 single crystal substrate by utilizing such characteristics of the Ga 2 O 3 material, wavelengths of 250 to The inventors have found that light is emitted in the deep ultraviolet region of 270 nm, and have reached the present invention.
The outline will be described in detail below.
本発明の高機能性Ｇａ2Ｏ3単結晶膜は、ステップテラス構造を有するβ‐Ｇａ2Ｏ3単結晶基板の（１００）面上に成長させていることを特徴とする。
Ｇａ2Ｏ3単結晶膜成長用基板として、β-Ｇａ2Ｏ3単結晶（１００）基板を用いた。薄膜成長はその膜と同一の物質の単結晶基板を用いることで、基板の情報を引き継いだ高品質な膜成長が実現できるためである。したがって、β−Ｇａ2Ｏ3単結晶を積層する基板として同一物質のβ-Ｇａ2Ｏ3単結晶を用いる必要がある。基板が積層する薄膜と同一であれば、格子定数のミスマッチに起因して発生する欠陥や転位が低減されると共に、熱膨張率の違いによって発生する基板と薄膜間の内部応力なども抑制されるようになる。
The highly functional Ga 2 O 3 single crystal film of the present invention is characterized in that it is grown on a (100) plane of a β-Ga 2 O 3 single crystal substrate having a step terrace structure.
A β-Ga 2 O 3 single crystal (100) substrate was used as a substrate for Ga 2 O 3 single crystal film growth. This is because thin film growth can be achieved by using a single crystal substrate made of the same material as the film, thereby achieving high quality film growth that inherits the information of the substrate. Therefore, it is necessary to use a β-Ga 2 O 3 single crystal of the same material as a substrate on which the β-Ga 2 O 3 single crystal is laminated. If the substrate is the same as the laminated thin film, defects and dislocations caused by lattice constant mismatch are reduced, and internal stress between the substrate and the thin film caused by the difference in thermal expansion coefficient is also suppressed. It becomes like this.
半導体膜成長用の基板は一般に鏡面状態に仕上げしたものが使用される。ところが、素子に高機能化や高密度化が進むにつれて、さらに原子レベルで平坦な基板が求められるようになっている。すなわち、鏡面仕上げした基板の表面は微視的にみれば依然として凹凸が残っており、この表面凹凸の斜面に沿って特定の結晶面以外の結晶方位をもつ異種結晶面が存在する。そして、この異種結晶面が半導体膜とのより完全な接合界面を形成する上での障害となる。 As a substrate for growing a semiconductor film, a mirror-finished substrate is generally used. However, as devices have higher functionality and higher density, a flat substrate at the atomic level is required. That is, the surface of the mirror-finished substrate is still uneven when viewed microscopically, and different crystal planes having crystal orientations other than a specific crystal plane exist along the slope of the surface unevenness. This heterogeneous crystal plane becomes an obstacle to forming a more complete junction interface with the semiconductor film.
そこで、表面に存在する凹凸を除去すると共に、表面の原子を再配列化させることで、原子レベルの高さを持つステップと原子レベルの平坦な面を持つテラスとを備えた基板が重要になっており、例えばすでに実用化されているサファイア基板については、９００℃以上の温度の熱処理で上記ステップとテラスを備えた基板を得る方法が提案されている。このようにして得られた基板のテラスは実質的に同一結晶方位を有するため、高性能の半導体素子を得ることが可能となる。 Therefore, by removing irregularities present on the surface and rearranging the atoms on the surface, a substrate having steps with an atomic level height and a terrace with an atomic level flat surface becomes important. For example, for a sapphire substrate that has already been put into practical use, a method of obtaining a substrate having the above steps and a terrace by heat treatment at a temperature of 900 ° C. or higher has been proposed. Since the terraces of the substrate thus obtained have substantially the same crystal orientation, a high-performance semiconductor element can be obtained.
β-Ｇａ2Ｏ3単結晶はへき開性が強く、２つのへき開面が存在することが知られている。（１００）面が最も強いへき開面で、(００１)面がこれに次ぐ。また面方位によって異方性があり、結晶方位の違いによる切断性や研磨性などβ-Ｇａ2Ｏ3ウエハ製造の観点からみると、（１００）面に平行にスライスした場合が最も研磨が容易で、工業的にも生産しやすい。従って、（１００）面でデバイスができれば、他の面を使用するよりコスト的に有利となる。
ＭＢＥ法で成長させるβ−Ｇａ2Ｏ3膜の膜厚は１００〜４００ｎｍとすることが好ましい。１００ｎｍ以下だとデバイス化する膜厚としては薄く、逆に４００ｎｍを超えると厚くなる上、ＭＢＥ法の成長時間が長くなり生産性の観点からも好ましくない。
It is known that the β-Ga 2 O 3 single crystal has a strong cleavage and has two cleavage planes. The (100) plane is the strongest cleavage plane, followed by the (001) plane. Also, there is anisotropy depending on the plane orientation, and from the viewpoint of β-Ga 2 O 3 wafer production such as cutting ability and polishing ability due to the difference in crystal orientation, polishing is easiest when sliced parallel to the (100) plane It is easy to produce industrially. Therefore, if a device can be formed on the (100) plane, it is more cost-effective than using the other plane.
The thickness of the β-Ga 2 O 3 film grown by the MBE method is preferably 100 to 400 nm. If it is 100 nm or less, the film thickness for device formation is thin. Conversely, if it exceeds 400 nm, the film thickness becomes thick, and the growth time of the MBE method becomes long, which is not preferable from the viewpoint of productivity.
次に、本発明の高機能性Ｇａ2Ｏ3単結晶薄膜の製造方法について説明する。
まず、基板として用いるβ‐Ｇａ2Ｏ3単結晶の製造方法について説明する。
β‐Ｇａ2Ｏ3単結晶は現在までに、ベルヌーイ法、フラックス法、化学輸送法、チョクラルスキー（ＣＺ）法、フローティングゾーン（ＦＺ）法などにより作製されている。このなかで、ＦＺ法が最も品質に優れているため、β‐Ｇａ2Ｏ3単結晶の作製にはＦＺ法を採用した。
ＦＺ法では溶融帯を形成する際に容器を使わないため、育成して得られるＧａ2Ｏ3単結晶は容器に由来する汚染の心配がなく、高品質な単結晶を得ることができる。ＦＺ法で用いる装置については特に制限されず、例えば、加熱手段としては、必要によりサセプターを併用した高周波による電磁誘導加熱や電気抵抗加熱、赤外線、電子ビーム、アーク、又はランプを用いた集光加熱、あるいはレーザーや火炎による加熱等を用いることができるが、安定した加熱条件が確保できると共に加熱に際しての不純物導入のおそれがないランプを用いた集光加熱であるのが好ましい。
Next, a description will be given highly functional Ga 2 O 3 method for producing a single crystal thin film of the present invention.
First, a method for producing a β-Ga 2 O 3 single crystal used as a substrate will be described.
To date, β-Ga 2 O 3 single crystals have been produced by the Bernoulli method, flux method, chemical transport method, Czochralski (CZ) method, floating zone (FZ) method, and the like. Among these, since the FZ method has the highest quality, the FZ method was adopted for the production of a β-Ga 2 O 3 single crystal.
In the FZ method, since a container is not used when forming a melt zone, the Ga 2 O 3 single crystal obtained by growing does not have to worry about contamination derived from the container, and a high-quality single crystal can be obtained. The apparatus used in the FZ method is not particularly limited. For example, as a heating means, if necessary, electromagnetic induction heating or electric resistance heating using a susceptor together, electric resistance heating, infrared heating, electron beam, arc, or condensing heating using a lamp Alternatively, heating by a laser or a flame can be used, but it is preferable to perform condensing heating using a lamp that can ensure stable heating conditions and does not cause the introduction of impurities during heating.
ＦＺ法を適用する源素材としては、高純度のＧａ2Ｏ3焼結体を使用することが好ましい。例えば純度が４Ｎ以上のＧａ2Ｏ3粉末をラバーチューブ等に封じ、静水圧５０〜６００ＭＰａ、好ましくは１００〜５００ＭＰａで５分間程度ラバープレスし、円柱状に成型した後、１４００〜１７００℃、好ましくは１５００〜１６００℃の焼結温度で１０〜２０時間、好ましくは１２〜１５時間焼結させて得た焼結体がよい。この焼結温度が１４００℃より低いと焼結が不足して十分なかさ密度のＧａ2Ｏ3焼結体を得ることが困難になり、反対に１７００℃より高温になるとＧａ2Ｏ3の融点（〜１７４０℃）に近づいてしまい好ましくない。また、焼結時間が１０時間より短いと焼結が十分に行えないおそれがあり、反対に２０時間を越えると効果が飽和する。一方、焼結雰囲気については特に制限はされず、大気中で行ってもよい。このようにして得られたＧａ2Ｏ3焼結体は円柱状の形状となり、好ましくは得られたＧａ2Ｏ3焼結体のかさ密度が５．８〜５．９ｇ／ｃｍ3となるようにするがのよい。 As a source material to which the FZ method is applied, it is preferable to use a high-purity Ga 2 O 3 sintered body. For example, a Ga 2 O 3 powder having a purity of 4N or more is sealed in a rubber tube or the like, and is rubber-pressed at a hydrostatic pressure of 50 to 600 MPa, preferably 100 to 500 MPa for about 5 minutes. Is a sintered body obtained by sintering at a sintering temperature of 1500 to 1600 ° C. for 10 to 20 hours, preferably 12 to 15 hours. When the sintering temperature is lower than 1400 ° C., the sintering is insufficient and it becomes difficult to obtain a Ga 2 O 3 sintered body having a sufficient bulk density. Conversely, when the sintering temperature is higher than 1700 ° C., the melting point of Ga 2 O 3 ( ˜1740 ° C.), which is not preferable. On the other hand, if the sintering time is shorter than 10 hours, the sintering may not be sufficiently performed. Conversely, if the sintering time exceeds 20 hours, the effect is saturated. On the other hand, the sintering atmosphere is not particularly limited and may be performed in the air. The Ga 2 O 3 sintered body thus obtained has a cylindrical shape, and preferably the bulk density of the obtained Ga 2 O 3 sintered body is 5.8 to 5.9 g / cm 3. It is good to do.
上記で得たＧａ2Ｏ3焼結体を原料棒としてＦＺ法で用いる加熱炉の上軸に設置し、下軸には種結晶としてＧａ2Ｏ3単結晶を取り付けて、本発明におけるＧａ2Ｏ3単結晶を育成する。この際、種結晶については、好ましくは予めＧａ2Ｏ3粉末を焼成して得たＧａ2Ｏ3焼結体を原料としてＦＺ法により製造したＧａ2Ｏ3単結晶であることが好ましい。また、原料棒及び種結晶の回転速度については、それぞれ１０〜３０ｒｐｍ、好ましくは１５〜２０ｒｐｍであるのがよく、互いに逆向きに回転させるのが好ましい。 The Ga 2 O 3 sintered body obtained above is installed as a raw material rod on the upper axis of a heating furnace used in the FZ method, and a Ga 2 O 3 single crystal as a seed crystal is attached to the lower axis, and Ga 2 in the present invention is attached. Grow O 3 single crystals. At this time, the seed crystal is preferably a Ga 2 O 3 single crystal produced by a FZ method using a Ga 2 O 3 sintered body obtained by firing a Ga 2 O 3 powder in advance as a raw material. Moreover, about the rotational speed of a raw material stick | rod and a seed crystal, it is 10-30 rpm, respectively, Preferably it may be 15-20 rpm, and it is preferable to make it rotate mutually reverse.
Ｇａ2Ｏ3単結晶を育成して得る育成雰囲気については、窒素、アルゴン、ヘリウム等の不活性ガスの１種以上と酸素との混合ガスを用いて、不活性ガスの総量に対する酸素の流量比（Ｏ2／不活性ガス総量）が１〜２０ｖｏｌ％、好ましくは２〜５ｖｏｌ％となるように加熱炉に供給するのがよい。不活性ガスの総量に対する酸素の流量比が１ｖｏｌ％より小さいと酸素の比率が少な過ぎて原料棒からの蒸発が顕著となり、Ｇａ2Ｏ3単結晶が十分に成長しなくなる。反対にこの流量比が２０ｖｏｌ％より大きくなると融液内にバブリングが発生して得られる単結晶に閉じ込められクラック発生の誘因となるおそれがある。また、加熱炉内に長さ：１８０ｍｍ，外径：４０ｍｍ，内径：３２ｍｍの石英管を配している場合、石英管内が上記の好適な育成雰囲気となるように、石英管内に酸素と不活性ガスとの混合ガスを２００〜６００ｍｌ／ｍｉｎで供給することが好ましい。 About the growth atmosphere obtained by growing a Ga 2 O 3 single crystal, the flow rate ratio of oxygen to the total amount of inert gas using a mixed gas of one or more inert gases such as nitrogen, argon, helium and oxygen. It is preferable to supply to the heating furnace so that (O 2 / total amount of inert gas) is 1 to 20 vol%, preferably 2 to 5 vol%. If the flow rate ratio of oxygen to the total amount of inert gas is less than 1 vol%, the ratio of oxygen is too small and evaporation from the raw material rod becomes remarkable, and the Ga 2 O 3 single crystal does not grow sufficiently. On the contrary, if this flow rate ratio is larger than 20 vol%, there is a possibility that it will be confined in the single crystal obtained by bubbling in the melt and cause cracking. In addition, when a quartz tube having a length of 180 mm, an outer diameter of 40 mm, and an inner diameter of 32 mm is arranged in the heating furnace, oxygen and inertness are contained in the quartz tube so that the inside of the quartz tube has the above-described preferable growth atmosphere. It is preferable to supply a mixed gas with gas at 200 to 600 ml / min.
Ｇａ2Ｏ3単結晶の結晶成長速度については、２．５〜２０ｍｍ／ｈ、好ましくは５〜１０ｍｍ／ｈであるのがよい。ＦＺ法では、一般には、成長速度が比較的遅いほうが得られる結晶の品質が良いとされている。しかしながら、本発明に用いられるＧａ2Ｏ3単結晶としては、従来の方法で採用されるように結晶成長速度よりも速い速度を採用しても、Ｇａ2Ｏ3薄膜を育成するに十分な品質のＧａ2Ｏ3単結晶を得ることができる。また、結晶成長の際の圧力については、大気圧であってもよく、加圧した状態で行ってもよい。加圧する効果としては、単結晶育成中の原料棒からの蒸発を抑制することができて、雰囲気ガスを流す透明石英管の内壁のくもりを抑えて石英管外部からの加熱の効率を低下させることなく操作できる等が考えられる。 The crystal growth rate of the Ga 2 O 3 single crystal is 2.5 to 20 mm / h, preferably 5 to 10 mm / h. In the FZ method, it is generally considered that a crystal having a relatively slow growth rate has a good crystal quality. However, the Ga 2 O 3 single crystal used in the present invention has a quality sufficient to grow a Ga 2 O 3 thin film even if a rate higher than the crystal growth rate is adopted as employed in the conventional method. Ga 2 O 3 single crystal can be obtained. Moreover, about the pressure in the case of crystal growth, atmospheric pressure may be sufficient and you may carry out in the pressurized state. As an effect of pressurization, it is possible to suppress evaporation from the raw material rod during single crystal growth, to suppress the clouding of the inner wall of the transparent quartz tube through which atmospheric gas flows, and to reduce the efficiency of heating from the outside of the quartz tube It is possible to operate without any problems.
このようにして作製した単結晶を、へき開性が最も強い（１００）面に平行な面をワイヤソーでスライスし、（１００）面を化学的機械研磨（Chemical Mechanical Polishing：ＣＭＰ）で原子レベルの表面粗さに鏡面研磨する。この場合、β‐Ｇａ2Ｏ3単結晶表面にはＣＭＰ研磨条件によりステップテラス構造を形成させることができる。このようなステップテラス構造を有するβ‐Ｇａ2Ｏ3基板を用いることで、高品質なβ‐Ｇａ2Ｏ3単結晶膜の成長が可能になる。 The single crystal produced in this way is sliced with a wire saw on a plane parallel to the (100) plane having the strongest cleavage, and the (100) plane is subjected to chemical mechanical polishing (CMP) at the atomic level. Mirror finish to roughness. In this case, a step terrace structure can be formed on the surface of the β-Ga 2 O 3 single crystal under the CMP polishing conditions. By using a β-Ga 2 O 3 substrate having such a step terrace structure, a high-quality β-Ga 2 O 3 single crystal film can be grown.
なお、研磨後のβ‐Ｇａ2Ｏ3単結晶基板を用いてＭＢＥ法によりβ‐Ｇａ2Ｏ3膜成長を行なうに当たり、成長前の前処理として基板洗浄を行うことが好ましい。
基板は超音波洗浄器を用いて、アセトン、メタノール、超純水の順で各５分間程度の有機洗浄を行なう。アセトンは基板に付着した有機物の汚れを、メタノールはアセトンを、超純水はメタノールを溶かして落すために用いた。最後に窒素ブローを行い、基板表面の水分をすべて吹き飛ばす。
Note that, when the β-Ga 2 O 3 single crystal substrate after polishing is subjected to the β-Ga 2 O 3 film growth by the MBE method, it is preferable to perform substrate cleaning as a pretreatment before the growth.
The substrate is subjected to organic cleaning for about 5 minutes in the order of acetone, methanol, and ultrapure water using an ultrasonic cleaner. Acetone was used to remove organic matter adhering to the substrate, methanol was used to dissolve acetone, and ultrapure water was used to dissolve and remove methanol. Finally, nitrogen blowing is performed to blow off all moisture on the substrate surface.
上記の方法で得られたＧａ2Ｏ3単結晶を基板とし、その表面にβ‐Ｇａ2Ｏ3単結晶薄膜を成長させる。
β-Ｇａ2Ｏ3膜の製造方法として、スパッタ法、ＰＬＤ法、ＣＶＤ法、ＭＢＥ法、スプレー法、ゾルゲル法などが想定される。しかしながら、発光デバイスに用いる薄膜は単結晶膜であることが必要であるため、本発明ではＭＢＥ法を用いた薄膜成長法を採用した。
Using the Ga 2 O 3 single crystal obtained by the above method as a substrate, a β-Ga 2 O 3 single crystal thin film is grown on the surface.
As a method for producing the β-Ga 2 O 3 film, a sputtering method, a PLD method, a CVD method, an MBE method, a spray method, a sol-gel method, and the like are assumed. However, since the thin film used for the light-emitting device needs to be a single crystal film, the present invention employs a thin film growth method using the MBE method.
ＭＢＥ法は、単体あるいは化合物の固体をセルと呼ばれる蒸発源で加熱し、超高真空中において分子線として基板表面に供給する結晶成長方法である。ＭＢＥ法の長所としては、
１．極端に低蒸気圧の物質、例えば単体の金属などの固体原料が使用可能である。
２．機械的シャッタで分子線を遮ることで急峻なヘテロ界面の作製が可能である。
３．不純物の混入を極力避けることができる。
４．反射高速電子線回折により、表面形状のその場観察が可能である、などが挙げられる。
The MBE method is a crystal growth method in which a simple substance or a compound solid is heated by an evaporation source called a cell and is supplied to a substrate surface as a molecular beam in an ultrahigh vacuum. The advantages of the MBE method are:
1. A material having an extremely low vapor pressure, for example, a solid material such as a single metal can be used.
2. A steep hetero interface can be produced by blocking the molecular beam with a mechanical shutter.
3. Impurities can be avoided as much as possible.
4). It is possible to observe the surface shape in situ by reflection high-energy electron diffraction.
本発明の成膜に用いたＭＢＥ装置の概要を図１に示す。成長室はターボ分子ポンプおよび油回転ポンプによって排気され、到達圧力は１０−１０Ｔｏｒｒ台後半から１０−9Ｔｏｒｒ台前半になる。基板や成長膜表面のその場観察するためにＲＨＥＥＤ(Reflection
High-energy electron diffraction)装置も備えている。基板の加熱は、ＳｉＣコート黒鉛ヒータからの輻射熱がホルダを伝わり、ホルダからの熱伝導により行なわれる。セルやラジカルの出口、基板の下部には原料フラックスを遮るためのシャッタが取り付けられており、便宜シャッタを開閉することで成長を制御する。
An outline of the MBE apparatus used for film formation of the present invention is shown in FIG. The growth chamber is evacuated by a turbo molecular pump and an oil rotary pump, and the ultimate pressure is from the lower 10 −10 Torr level to the lower 10 −9 Torr level. RHEED (Reflection for in-situ observation of substrate and growth film surface
High-energy electron diffraction) equipment is also provided. The substrate is heated by radiant heat from the SiC-coated graphite heater transmitted through the holder and heat conduction from the holder. A shutter for blocking the raw material flux is attached to the outlet of the cell and radical, and the lower part of the substrate, and the growth is controlled by opening and closing the shutter for convenience.
このＭＢＥ装置を用いたβ-Ｇａ2Ｏ3膜製造の原料として、金属源として純度の高いＧａを用いる。３Ｎ以上のＧａを用いることが好ましい。また、酸素源には金属との反応性を高めるためラジカル酸素を用いた。ラジカル酸素は、１３.５６ＭＨｚの高周波が印加された石英製励起室内に、純度３Ｎの酸素ガスを導入してプラズマを発生させて得る。生成したラジカルは、中心に直径１ｍｍの穴をもつオリフィスを通したのち基板上に供給される。しかしながら、プラズマ中にはラジカルと比較してエネルギーの高い酸素イオンも含まれ、膜への損傷を防ぐため、オリフィス後段に設置した６００Ｖの直流電圧を印加した電極により酸素イオンを除去した。この場合の酸素プラズマ生成条件は、入力電力３００Ｗ、導入酸素量０.６ｓｃｃｍとした。 As a raw material for producing a β-Ga 2 O 3 film using this MBE apparatus, high purity Ga is used as a metal source. It is preferable to use 3N or more Ga. Further, radical oxygen was used as the oxygen source in order to increase the reactivity with the metal. Radical oxygen is obtained by introducing a 3N purity oxygen gas into a quartz excitation chamber to which a high frequency of 13.56 MHz is applied to generate plasma. The generated radicals are supplied onto the substrate after passing through an orifice having a hole with a diameter of 1 mm in the center. However, oxygen ions having higher energy than radicals are contained in the plasma, and in order to prevent damage to the film, oxygen ions were removed with an electrode to which a DC voltage of 600 V applied at the subsequent stage of the orifice was applied. The oxygen plasma generation conditions in this case were an input power of 300 W and an introduced oxygen amount of 0.6 sccm.
分子線エピタキシー法を用いてβ‐Ｇａ2Ｏ3単結晶膜を成長させる際、ラジカル酸素供給量を一定にし、Ｇａ供給量をＧａ供給律速、酸素供給律速の境界±１０％以内に制御する。すなわち、ＭＢＥ装置へのＣａ及びラジカル酸素の供給量は、Ｇａ供給量をＰＧａ[atoms/cm2/s]，ラジカル酸素供給量をＰＯ[atoms/cm2/s]とし、その等価供給比ＸをＸ＝ＰＧａ／（ｋ×ＰＯ）とするとき、０．９＜Ｘ＜１．１の範囲に制御することが好ましい。ただし、比例定数ｋは，成長速度曲線の酸素リッチとＧａリッチの境界点、すなわち等価的な化学量論比において、ｋ＝ＰＯ／ＰＧａとなるように定めたものである。
この場合、Ｇａフラックス圧力２．４×１０−７Torrは酸素供給律速、Ｇａフラックス圧力１．１×１０−７Torrは境界領域に当たると考えられる。
When the β-Ga 2 O 3 single crystal film is grown using the molecular beam epitaxy method, the radical oxygen supply amount is made constant, and the Ga supply amount is controlled within ± 10% of the boundary between Ga supply rate control and oxygen supply rate control. That is, the supply amount of Ca and radical oxygen to the MBE apparatus is such that the Ga supply amount is P Ga [atoms / cm 2 / s] and the radical oxygen supply amount is P 2 O [atoms / cm 2 / s]. When the ratio X is X = P Ga / (k × P O ), it is preferable to control the ratio in the range of 0.9 <X <1.1. However, the proportional constant k, a boundary point of the oxygen-rich and Ga-rich growth rate curve, i.e. the equivalent stoichiometric ratio, are as previously defined such that k = P O / P Ga.
In this case, it is considered that the Ga flux pressure of 2.4 × 10 −7 Torr corresponds to the oxygen supply rate limiting, and the Ga flux pressure of 1.1 × 10 −7 Torr corresponds to the boundary region.
ＭＢＥ法によりβ-Ｇａ2Ｏ3膜を製造しようとするとき、ＧａとＯとは、化学式Ｇａ2Ｏ3の通り、Ｇａ：Ｏ＝２：３の等価化学量論比となった状態で、最も良好な結晶が得られ、デバイスの特性も良好であると考えられる。その化学量論比となる原料Ｇａ及びラジカル酸素の供給量を直接的に調べるは困難であるため、成長速度の原料供給比依存性より探索する。
ところで、二元化合物であるＧａ2Ｏ3は、Ｇａ及びＯの供給量によってその成長速度が決定される。そこで、横軸にラジカル酸素供給量をＰＯ[atoms/cm2/s]をとり、パラメーターにＧａ供給量をＰＧａ[atoms/cm2/s]を設定すると、Ｇａ供給量が一定の時，ラジカル酸素の供給量が少ない領域では，ラジカル酸素の量によって膜厚は制御できる。言い換えれば、ラジカル酸素が無ければ、Ｇａ2Ｏ3はできないからである。
When a β-Ga 2 O 3 film is to be produced by the MBE method, Ga and O are in the state of an equivalent stoichiometric ratio of Ga: O = 2: 3 as represented by the chemical formula Ga 2 O 3 . The best crystal is obtained and the device characteristics are also considered to be good. Since it is difficult to directly check the supply amounts of the raw material Ga and radical oxygen, which are the stoichiometric ratio, the search is made based on the dependency of the growth rate on the raw material supply ratio.
By the way, the growth rate of Ga 2 O 3 which is a binary compound is determined by the supply amount of Ga and O. Therefore, when the radical oxygen supply amount is set to P O [atoms / cm 2 / s] on the horizontal axis and the Ga supply amount is set to P Ga [atoms / cm 2 / s] on the parameter, the Ga supply amount is constant. In the region where the supply amount of radical oxygen is small, the film thickness can be controlled by the amount of radical oxygen. In other words, Ga 2 O 3 cannot be made without radical oxygen.
この領域をＧａリッチ(図２参照)と呼び、ラジカル酸素供給量に対して線形に成長速度が増加する。しかし、ラジカル酸素供給量が過剰になると、今度はＧａ供給量が足りないためＧａ2Ｏ3の成長速度は飽和する。この領域を酸素リッチと呼ぶ。したがって、Ｇａ：Ｏが２：３におおよそなっていると思われる供給比は、図２の黒丸の位置であり、この条件で成長することが望ましい。
この条件を完全に合致させることは、現実的には困難を極める。そこで、上記のようなＸなる等価供給比を導入したとき、そのＸが０．９以上１．１以下の範囲内で、良好なβ-Ｇａ2Ｏ3膜が効率的に生成される。Ｘが０．９を下回ると成膜速度がおそくなり、Ｘが１．１を上回ると良好な成膜はできない。
This region is called Ga-rich (see FIG. 2), and the growth rate increases linearly with the amount of radical oxygen supplied. However, when the radical oxygen supply amount becomes excessive, the Ga 2 O 3 growth rate is saturated because the Ga supply amount is insufficient. This region is called oxygen rich. Therefore, the supply ratio at which Ga: O is assumed to be approximately 2: 3 is the position of the black circle in FIG. 2, and it is desirable to grow under this condition.
In reality, it is extremely difficult to perfectly meet this condition. Therefore, when the above-described equivalent supply ratio of X is introduced, a good β-Ga 2 O 3 film is efficiently generated when X is in the range of 0.9 to 1.1. When X is less than 0.9, the deposition rate is slow, and when X is greater than 1.1, satisfactory film formation cannot be achieved.
洗浄を行ったβ-Ｇａ2Ｏ3単結晶基板を成長室へ搬入後、基板表面に吸着したガス等を除去する目的で、８００℃、１０分間程度の加熱処理（サーマルクリーニング）を行なう。
サーマルクリーニング終了後、β-Ｇａ2Ｏ3単結晶基板上へのβ-Ｇａ2Ｏ3単結晶膜の成長を行うが、効率の優れた発光素子を得るには、β-Ｇａ2Ｏ3単結晶基板とβ-Ｇａ2Ｏ3単結晶膜との積層構造の調整と成長させたβ-Ｇａ2Ｏ3単結晶膜の品質向上が重要となる。前記したように、ステップテラス構造を有するβ-Ｇａ2Ｏ3単結晶上に、品質の優れたβ-Ｇａ2Ｏ3単結晶膜を成長させるためには、ホモエピタキシーの成長条件を適正化する必要がある。
After the washed β-Ga 2 O 3 single crystal substrate is carried into the growth chamber, heat treatment (thermal cleaning) is performed at 800 ° C. for about 10 minutes for the purpose of removing gas adsorbed on the substrate surface.
After thermal cleaning completion, but to grow a β-Ga 2 O 3 β- Ga 2 O 3 single crystal film on a single crystal substrate, in order to obtain an excellent light-emitting element of efficiency, β-Ga 2 O 3 single It is important to adjust the laminated structure of the crystal substrate and the β-Ga 2 O 3 single crystal film and to improve the quality of the grown β-Ga 2 O 3 single crystal film. As described above, in order to grow a high -quality β-Ga 2 O 3 single crystal film on a β-Ga 2 O 3 single crystal having a step terrace structure, the growth conditions for homoepitaxy are optimized. There is a need.
そこで、基板温度、Ｇａフラックス圧力依存性などを調べた。詳細は実施例の記載に譲るが、基板温度は８００℃以上、Ｇａフラックス圧力は２.４×１０−７Torr未満が最適であることがわかった。
成長温度が８００℃に満たないと、吸着した原子のマイグレーションが弱いため、ステップテラス構造はあるものの数１００ｎｍオーダーのゲレインが形成される。これに対し、８００℃以上の成長させたβ-Ｇａ2Ｏ3薄膜表面ではＣＭＰ研磨後の基板表面よりも明瞭なステップとテラスが観察されステップフロー成長が確認できた。図３はその結果を示したもので、成長温度依存性を調べるために、Ｇａフラックスを１.１×１０−７Torrに固定し、５時間成長後の表面形状を原子間力顕微鏡（Atomic Force Microscopy：ＡＦＭ）で観察した結果である。
この表面形状の結果は、図４のＲＨＥＥＤ像にも対応する。すなわち、平坦性を示すストリークが観測されている。
Therefore, the substrate temperature, Ga flux pressure dependency, etc. were examined. Although details are left to the description of the examples, it was found that the substrate temperature is optimally 800 ° C. or higher and the Ga flux pressure is less than 2.4 × 10 −7 Torr.
If the growth temperature is less than 800 ° C., the migration of the adsorbed atoms is weak, and a gelin on the order of several hundred nm is formed although there is a step terrace structure. On the other hand, on the surface of the β-Ga 2 O 3 thin film grown at 800 ° C. or higher, clearer steps and terraces were observed than the substrate surface after CMP polishing, and step flow growth was confirmed. FIG. 3 shows the results. In order to investigate the growth temperature dependence, the Ga flux was fixed at 1.1 × 10 −7 Torr, and the surface shape after 5 hours of growth was measured with an atomic force microscope (Atomic Force Microscope). Microscopy: AFM).
This surface shape result also corresponds to the RHEED image of FIG. That is, a streak indicating flatness is observed.
Ｇａフラックス圧力依存性は、２.４×１０−７Torr未満であることが望ましい。Ｇａフラックス量が２.４×１０−７Torr以上の条件下での成長膜ではＧａドロップレットが発生しＧａの供給量が過剰となる。過剰な供給によりテラス上に二次元核が生成しやすくなる。したがって、最適Ｇａフラックス量は２.４×１０−７Torr未満、好ましくは１.１×１０−７Torr程度である。図５は８００℃で成長させた膜のＡＦＭ像である。Ｇａフラックス量が２.４×１０−７TorrではＧａドロップレットが発生している。
このように、ステップテラス構造を有するβ-Ｇａ2Ｏ3単結晶基板上に成長させるβ-Ｇａ2Ｏ3膜の成長温度、Ｇａフラックス量を最適化することで、高品質のβ-Ｇａ2Ｏ3膜の成長が実現できた。実際にステップフロー成長させたときの結果を図６に示す。ステップ高さ；約５.９Åでステップフロー成長していることがわかる。対応するＲＨＥＥＤ像からも菊池線が観察されている。
The Ga flux pressure dependency is desirably less than 2.4 × 10 −7 Torr. In the grown film under the condition that the Ga flux amount is 2.4 × 10 −7 Torr or more, Ga droplets are generated and the supply amount of Ga becomes excessive. Excessive supply facilitates the formation of two-dimensional nuclei on the terrace. Therefore, the optimum Ga flux amount is less than 2.4 × 10 −7 Torr, preferably about 1.1 × 10 −7 Torr. FIG. 5 is an AFM image of a film grown at 800 ° C. Ga droplets are generated when the Ga flux amount is 2.4 × 10 −7 Torr.
Thus, by optimizing the growth temperature and the amount of Ga flux of the β-Ga 2 O 3 film grown on the β-Ga 2 O 3 single crystal substrate having the step terrace structure, high-quality β-Ga 2 The growth of O 3 film was realized. FIG. 6 shows the result of actual step flow growth. It can be seen that the step flow is growing at a step height of about 5.9 mm. The Kikuchi line is also observed from the corresponding RHEED image.
実施例１：
酸化ガリウム粉末（純度４Ｎ）をラバーチューブに封入しこれを静水圧プレス成形し、大気中１５００℃、１０時間で焼結した。この焼結体を原料棒として光ＦＺ装置を用いて単結晶育成を行った。成長速度は７．５ｍｍ／ｈとし、雰囲気ガスとしてドライエアを用いた。得られた単結晶の（１００）面を切り出し、ＣＭＰで鏡面研磨してウエハ状の基板とした。この基板を、アセトン、メタノール、超純水の順で各５分間ずつ有機洗浄を行なった後、８００℃×１０分間のサーマルクリーニングを行った。
Example 1:
Gallium oxide powder (purity 4N) was sealed in a rubber tube, subjected to isostatic pressing, and sintered in the atmosphere at 1500 ° C. for 10 hours. Using this sintered body as a raw material rod, single crystal growth was performed using an optical FZ apparatus. The growth rate was 7.5 mm / h, and dry air was used as the atmospheric gas. The (100) plane of the obtained single crystal was cut out and mirror-polished by CMP to obtain a wafer-like substrate. This substrate was subjected to organic cleaning for 5 minutes each in the order of acetone, methanol, and ultrapure water, and then subjected to thermal cleaning at 800 ° C. for 10 minutes.
このβ‐Ｇａ2Ｏ3（１００）基板上にＭＢＥ法でβ‐Ｇａ2Ｏ3膜を作製した。成長条件は、成長温度８００℃、Ｇａフラックスを１.１×１０−7Torr、成長時間５時間で膜厚は約１５０ｎｍであった。この膜に対して、ＣＬ（Cathodoluminescence：ＣＬ）測定を行なった。測定条件は室温、電子ビームの加速電圧は５ｋＶ、電流値は５ｎＡである。この結果、図７に示すように電子線の励起によりβ‐Ｇａ2Ｏ3のバンド端の波長２５０〜２７０ｎｍから強い発光を確認できた。この結果は、β‐Ｇａ2Ｏ3膜による深紫外発光の実現性を示唆するもので、β‐Ｇａ2Ｏ3単結晶基板を用いることで欠陥のない高品質の膜成長したためと考えられる。
比較例として、サファイア基板上に同様の条件で成長させたβ‐Ｇａ2Ｏ3膜について、ＣＬ発光を測定した。その結果、図８に示すようにβ‐Ｇａ2Ｏ3基板上の膜のような波長２５０〜２７０ｎｍからの強い発光は確認できなかった。
To prepare a β-Ga 2 O 3 film in the β-Ga 2 O 3 (100 ) MBE method on a substrate. The growth conditions were a growth temperature of 800 ° C., a Ga flux of 1.1 × 10 −7 Torr, a growth time of 5 hours, and a film thickness of about 150 nm. CL (Cathodoluminescence: CL) measurement was performed on this film. The measurement conditions are room temperature, the acceleration voltage of the electron beam is 5 kV, and the current value is 5 nA. As a result, as shown in FIG. 7, strong emission was confirmed from the wavelength of 250 to 270 nm at the band edge of β-Ga 2 O 3 by excitation of the electron beam. This result suggest the deep ultraviolet emission of feasibility by β-Ga 2 O 3 film, presumably because the high-quality film growth of defect-free by using a β-Ga 2 O 3 single crystal substrate.
As a comparative example, CL emission was measured for a β-Ga 2 O 3 film grown on a sapphire substrate under the same conditions. As a result, as shown in FIG. 8, strong light emission from a wavelength of 250 to 270 nm like a film on a β-Ga 2 O 3 substrate could not be confirmed.
実施例２：
β‐Ｇａ2Ｏ3基板上に成長させたβ‐Ｇａ2Ｏ3膜の界面および断面構造を調べるため、透過電子顕微鏡（Transmission Electron Microscopy：ＴＥＭ）による観察を行なった。比較としてサファイア基板上に成長させたβ‐Ｇａ2Ｏ3膜についても観察した。
この結果から、β‐Ｇａ2Ｏ3基板上に成長させたβ‐Ｇａ2Ｏ3膜は、サファイア基板上の膜と比べ、例えば膜中に欠陥や転位が少ないことなどがわかり、β‐Ｇａ2Ｏ3基板を使ったホモエピタキシーの効果が確認された。図９及び１０参照。
したがって、本発明のようにステップテラス構造を有するβ‐Ｇａ2Ｏ3基板を用いて、この基板上に成長させる膜の成長条件を最適化することにより、β‐Ｇａ2Ｏ3膜の品質が向上し、その結果β‐Ｇａ2Ｏ3のバンド端からの深紫外発光が実現したと考えられる。
Example 2:
To examine the surface and the cross-sectional structure of the β-Ga 2 O 3 film grown on a β-Ga 2 O 3 substrate, transmission electron microscope was performed observation by (Transmission Electron Microscopy TEM). For comparison, a β-Ga 2 O 3 film grown on a sapphire substrate was also observed.
This result, β-Ga 2 O 3 β -Ga 2 O 3 film grown on the substrate as compared to film on a sapphire substrate, to understand and be defects and dislocations is small, for example, film, beta-Ga The effect of homoepitaxy using a 2 O 3 substrate was confirmed. See FIGS. 9 and 10.
Therefore, by using a β-Ga 2 O 3 substrate having a step terrace structure as in the present invention and optimizing the growth conditions of the film grown on this substrate, the quality of the β-Ga 2 O 3 film can be improved. As a result, it is considered that deep ultraviolet light emission from the band edge of β-Ga 2 O 3 was realized.
実施例３：
（１）成長温度依存性
成長温度依存性を調べるため、Ｇａフラックスを固定し、温度を種々変更して膜を成長させた。成長条件は、Ｇａフラックスの等価ビーム圧（Beam Equivalent Pressure：ＢＥＰ）を１.１×１０−７Torr、成長時間を５時間に固定し、成長温度を７００℃，８００℃及び９００℃とした。
成長後の表面形状をＡＦＭにより観察した。その結果は前記の図３に示した通りであって、８００℃及び９００℃で成長させたβ‐Ｇａ2Ｏ3薄膜表面ではＣＭＰ後の基板表面より明瞭にステップとテラスが観察された。しかし、７００℃成長の薄膜表面は、ステップテラス構造はあるものの数１０ｎｍオーダーのグレインを形成している。
Example 3:
(1) Growth temperature dependence
In order to investigate the growth temperature dependence, a film was grown by fixing Ga flux and changing the temperature in various ways. The growth conditions were such that the equivalent beam pressure (Beam Equivalent Pressure: BEP) of Ga flux was 1.1 × 10 −7 Torr, the growth time was fixed at 5 hours, and the growth temperatures were 700 ° C., 800 ° C., and 900 ° C.
The surface shape after growth was observed by AFM. The results are as shown in FIG. 3. Steps and terraces were clearly observed on the surface of the β-Ga 2 O 3 thin film grown at 800 ° C. and 900 ° C. from the surface of the substrate after CMP. However, the surface of the thin film grown at 700 ° C. has grains of the order of several tens of nm although it has a step terrace structure.
この原因として、成長温度７００℃では、吸着した原子のマイグレーションが弱いこと、そしてテラス上を移動しキンクに取り込まれステップを完全に伸ばす前に、テラス上に新たな２次元核を形成したものと考えられる。これはステップフロー成長とは呼べない。
この表面形状の結果は前記図４で示したＲＨＥＥＤ像にも対応する。したがって、原子のマイグレーションが盛んで、ステップフロー成長が起こる９００℃付近が最適成長温度であると考えられる。
The reason for this is that at a growth temperature of 700 ° C., the migration of the adsorbed atoms is weak, and a new two-dimensional nucleus is formed on the terrace before it moves on the terrace and is taken into the kink and fully extends the step. Conceivable. This cannot be called step flow growth.
The result of this surface shape also corresponds to the RHEED image shown in FIG. Therefore, it is considered that the optimum growth temperature is around 900 ° C. where migration of atoms is active and step flow growth occurs.
（２）Ｇａフラックス依存性
ステップフロー成長が確認できた８００℃において、さらにＧａフラックス量を増大させた。成長条件は、成長温度；８００℃、Ｇａ ＢＥＰ；１.１×１０−７Torr及び２.４×１０−７Torr、成長時間；５時間、である。
表面形状をＡＦＭで観察した結果が前記図５である。Ｇａ ＢＥＰが２.４×１０−７Torrの成長膜ではＧａドロップレットが発生し、明らかにＧａ供給量が過剰であることがわかる。また、過剰な供給量を反映して場所によってはテラス上にさらに２次元核が生成していることがわかる。従って、最適Ｇａフラックスは、Ｇａ ＢＥＰが２.４×１０−７Torr未満、１.１×１０−７Torr付近にあると考えられる。
(2) The Ga flux amount was further increased at 800 ° C. at which Ga flux dependent step flow growth was confirmed. The growth conditions are: growth temperature: 800 ° C., Ga BEP; 1.1 × 10 −7 Torr and 2.4 × 10 −7 Torr, growth time: 5 hours.
FIG. 5 shows the result of observing the surface shape with AFM. It can be seen that Ga droplets are generated in the growth film having Ga BEP of 2.4 × 10 −7 Torr, and the Ga supply amount is clearly excessive. It can also be seen that two-dimensional nuclei are further generated on the terrace in some places reflecting the excessive supply amount. Therefore, it is considered that the optimum Ga flux is such that Ga BEP is less than 2.4 × 10 −7 Torr and near 1.1 × 10 −7 Torr.
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