JP3725589B2 - Single crystal substrate product, optical component, method of manufacturing single crystal substrate product, and method of manufacturing optical component - Google Patents

Description

【００８４】
【表２】

【００８５】
表１からわかるように、種々の保持温度で成膜したところ、飽和温度が１２００℃以下の範囲内においては、本発明の組成範囲内であれば、半値幅の小さな、結晶性の良好な固溶体膜を形成できる。特に、飽和温度が１１５０℃以下である組成範囲内においては、固溶体膜の前記半値幅が、ニオブ酸リチウム単結晶基板の前記半値幅よりも小さい、即ち、基板よりも結晶性の良い固溶体膜を、形成することができた。
【００８６】
一方、表２に示す結果からわかるように、飽和温度が１２００℃を越えると、固溶体膜の前記半値幅が顕著に劣化することが分かる。
【００８７】
また、表１からわかるように、図１に示すような温度スケジュールを採用することによって、溶融体の仕込み組成がどのように変化しても、一旦固相を析出する保持段階が存在するので、極めて広い温度範囲で成膜が可能になっている。そして、溶融体の各仕込み組成について、いずれも、固溶体膜の前記半値幅が、きわめて良好になっている。
【００８８】
（実験２）
実験１と同様にして、液相エピタキシャル法によって固溶体膜を形成した。ただし、保持温度Ｔ₂ 、溶融体の液相部分の組成を表３、表４に示すように変更させた。過冷却度の影響を一定にするため、過冷却度は１５℃に統一した。得られた固溶体膜の組成ｘと前記半値幅とを測定し、表３と表４とに表示した。なお、これらの例については、溶融体の液相部分の組成を変更させることを目的としており、このために溶融体の仕込み組成を種々変更している。
【００８９】
【表３】

【００９０】
【表４】

【００９１】
表３、表４において採用した各条件は、飽和温度はいずれも１１６０℃以下であり、過冷却度は１５℃であって、表１の結果から見て問題はない。また、ＬｉＴａＯ₃ の割合も、表１の各例から見て、十分に高品質の固溶体膜を形成できるはずの範囲内である。
【００９２】
実際、液相部分の組成において溶媒の割合が９０〜６０％の範囲内であると、ニオブ酸リチウム単結晶基板の方よりも小さな前記半値幅を有する固溶体膜を形成することができる。しかし、表４に示すように、溶媒の割合が９５％以上になると、前記半値幅が顕著に、かつ急激に劣化している。また、溶媒の割合が６０％未満になると、やはり前記半値幅が顕著に、かつ急激に劣化している。
【００９３】
（実験３）
実験１と同様にして固溶体膜を形成した。ただし、保持温度Ｔ₂ 、溶融体の液相部分の組成を、表５に示すように変更させた。ただし、飽和温度は、表１の結果から見て許容可能な１２００℃以下の範囲内に保持した。また、溶媒の割合は、実験１および実験２の結果からみて問題のない８５％に保持した。
【００９４】
そして、ＬｉＮｂＯ₃ のモル数に対するＬｉＴａＯ₃ のモル数の割合を、表５に示すように種々変更した。そして、各組成について、成膜が可能な過冷却度を測定した。この結果を表５に示す。
【００９５】
【表５】

【００９６】
表５からわかるように、ＬｉＮｂＯ₃ のモル数：ＬｉＴａＯ₃ のモル数が、１２：３であると、きわめて広い範囲内で成膜が可能であった。これが１０：５になると、最低でも１５℃の過冷却度が必要になったが、やはり良好な膜が得られた。これが８：７になると、約３０℃の過冷却度が必要になった。更に、これが６：９になると、約４５℃以上の過冷却度が必要となり、成膜条件が非常に厳しく限定されてきた。これが成膜の限界であった。
【００９７】
（実験４）
実験１と同様の方法によって図９の単結晶基板品１３を作成した。
ただし、溶融体の仕込み組成を、Ｌｉ₂ Ｏ：Ｖ₂ Ｏ₅ ：Ｎｂ₂ Ｏ₅ ：Ｔａ₂ Ｏ₅ ＝５０：３６：９：５とし、液相部分の飽和温度（保持温度）Ｔ₂ を９７０℃とし、過冷却度ΔＴを３０℃とし、成膜時間を１０分間とした。この条件下で、第一層目の固溶体膜１０を形成した。この膜厚は２０μｍであり、固溶体膜の組成がｘ＝０．３５であった。波長６３３ｎｍで、固溶体膜１０の常光屈折率は２．２４８であり、異常光屈折率は、２．１９４であった。
【００９８】
次に、溶融体の仕込み組成を、Ｌｉ₂ Ｏ：Ｖ₂ Ｏ₅ ：Ｎｂ₂ Ｏ₅ ＝５０：４０：１０とし、液相部分の飽和温度（保持温度）Ｔ₂ を９１０℃とし、過冷却度ΔＴを５℃とし、成膜時間を８分間とした。この条件下で、固溶体膜１０上に第二層１１を形成した。第二層１１の膜厚は８μｍであり、第二層の組成は、ＬｉＮｂＯ₃ であった。波長６３３ｎｍで、第二層１１の常光屈折率は２．２８６であり、異常光屈折率は、２．１９３であった。
【００９９】
この第二層１１は、屈折率の関係から、十分に光導波路として使用できるものである。そして、実際に波長８３０ｎｍのレーザー光を通して、この光導波路１１の耐光損傷特性を測定したところ、２ｍＷの出射光パワーにおいても、まったく光損傷は認められなかった。
【０１００】
（実験５）
実験４と同様の方法によって図９の単結晶基板品１３を作成した。
ただし、溶融体の仕込み組成を、Ｌｉ₂ Ｏ：Ｖ₂ Ｏ₅ ：Ｎｂ₂ Ｏ₅ ：Ｔａ₂ Ｏ₅ ＝５０：４０：５：５とし、液相部分の飽和温度Ｔ₂ を１０１５℃とし、過冷却度ΔＴを４０℃とし、成膜時間を１５分間とした。この条件下で、第一層目の固溶体膜１０を形成した。この膜厚は１８μｍであり、固溶体膜の組成がｘ＝０．６３であった。波長６３３ｎｍで、固溶体膜１０の常光屈折率は２．２１７であり、異常光屈折率は、２．１８９であった。
【０１０１】
次に、溶融体の仕込み組成を、Ｌｉ₂ Ｏ：Ｖ₂ Ｏ₅ ：Ｎｂ₂ Ｏ₅ ：Ｔａ₂ Ｏ＝５０：４２．５：５：２．５とし、液相部分の飽和温度Ｔ₂ を９５０℃とし、過冷却度ΔＴを２０℃とし、成膜時間を５分間とした。この条件下で、固溶体膜１０上に第二層１１を形成した。第二層１１の膜厚は５μｍであった。波長６３３ｎｍで、第二層１１の常光屈折率は２．２６３であり、異常光屈折率は、２．１９６であった。
【０１０２】
この第二層１１は、屈折率の関係から、十分に光導波路として使用できるものである。そして、実際に波長８３０ｎｍのレーザー光を通して、この光導波路１１の耐光損傷特性を測定したところ、２ｍＷの出射光パワーにおいても、まったく光損傷は認められなかった。
【０１０３】
（実験６）
成膜温度と固溶体膜中のタンタルの置換割合との関係を検討した。
図１の温度スケジュールに従って、液相エピタキシャル法によって成膜した。ＬｉＮｂＯ₃ ─ＬｉＴａＯ₃ ─ＬｉＶＯ₃ 擬三元系を用いて、ニオブ酸リチウム単結晶基板上に固溶体膜を形成した。
【０１０４】
溶融体を、十分に高い温度Ｔ₁ （１０００°Ｃ〜１３００°Ｃ）で３時間以上攪拌し、十分均一な液相の状態とした。その後、溶融体を保持温度Ｔ₂ まで冷却した後、１２時間以上保持した。その後、溶融体３の温度を、Ｔ₂ から過冷却度ΔＴだけ低い成膜温度Ｔ₃ まで冷却し、ただちにニオブ酸リチウム単結晶基板６を液相部分４に接触させて成膜を行った。
【０１０５】
ただし、溶融体の仕込み組成は、モル比でＬｉＮｂＯ₃ ：ＬｉＴａＯ₃ ＝８０：２０となるようにし、溶融体の飽和温度Ｔ₀ が、成膜温度Ｔ₃ よりも約１００℃高くなるように、ＬｉＶＯ₃ の割合を調節した。また、過冷却度ΔＴは１０℃に設定した。成膜温度Ｔ₃ を、表６および図１１に示すように種々変更し、固溶体膜を製造し、固溶体膜中のタンタルの割合ｘを測定した。この測定結果を表６および図１１に示す。
【０１０６】
【表６】

【０１０７】
このように、成膜温度Ｔ₃ と、タンタルの置換割合との間には、非常にリニアな対応関係があることを発見した。従って、この関係を利用すれば、成膜温度Ｔ₃ を正確に設定すれば、固溶体膜中のタンタルの置換割合ｘをきわめて精密に制御、変更できることがわかった。
【０１０８】
また、成膜温度が１２５０℃を越えると、固溶体膜の形成は不可能であったが、これは、ニオブ酸リチウム単結晶基板の融点が約１２５０℃であるため、１２５０℃を越える温度で固溶体膜を形成することはできないからである。この観点からは、１２３０℃以下の成膜温度を採用することが好ましく、このときの固溶体膜のｘは約０．８となった。
【０１０９】
また、成膜温度Ｔ₃ を低下させていくと、タンタルの割合ｘも低下していくが、特に成膜温度が約７５０℃未満になると、膜中にタンタルを含有させることは困難になることが判明した。更に、固溶体膜を成膜するときに、過冷却状態の液相部分の温度を約８００℃以上、１１５０℃以下とすることによって、ｘを０．１以上、０．６以下の範囲で変化させることができることも判る。
【０１１０】
このように、前記固溶体膜中のタンタルの割合ｘを自由に制御する技術は、本発明者が初めて発見したものであり、種々の用途に応じて特性を有する単結晶基板品を製造する上できわめて有用なことは明らかである。
【０１１１】
（実験７）
実験６で製造した各組成を有する固溶体膜について、Ｘ線ロッキングカーブの半値幅を測定した。また、これらの各固溶体膜に対して波長５１４．５ｎｍのアルゴンレーザーを照射し、光損傷を生ずるしきい値を測定した。これらの結果を表７に示す。また、比較例として、タンタル酸リチウム単結晶基板上にニオブ酸リチウム単結晶薄膜を形成し、この膜について上記の測定を行った。
【０１１２】
【表７】

【０１１３】
成膜温度が１１５０℃以下では、非常に結晶性の良好な固溶体膜が得られるのに対して、成膜温度が１１５０℃を越えると、固溶体膜の結晶性が劣化していた。しかし、この場合にもタンタル酸リチウム単結晶基板上にニオブ酸リチウム単結晶薄膜を形成した場合とは同等以上の結晶性は得られている。このように成膜温度を１１５０℃以下とすることで固溶体膜の結晶性が向上した理由は明白ではないが、基板として使用したニオブ酸リチウムのキュリー温度が約１１５０℃であるので、１１５０℃を越える温度て成膜すると、基板が多分域化し、その結晶性が劣化してしまうからであろう。
【０１１４】
（実験８）
上記したような、固溶体膜の成膜温度を選択することによって、前記固溶体膜中のタンタルの割合ｘを精密に制御する技術は、特にシングルモード光導波路を製造する場合にきわめて有用である。なぜなら、シングルモード光導波路においては、導波される光の波長を一定値に設定し、光導波路の厚さを正確に制御できたとしても、更に光導波路の屈折率を正確に制御する必要があり、この許容範囲は一般に非常に狭い。
【０１１５】
しかし、前記固溶体膜におけるタンタルの割合が僅かに変動しても、光導波路の屈折率はかなり変動し、光の伝搬損失が顕著に増大する。このため、本発明者は、前記した発見にもとづいて、シングルモード光導波路が有する高度の伝搬特性を維持するための製造方法を研究した。
【０１１６】
２０ｍｏｌ％ＬｉＮｂ_0.8 Ｔａ_0.2 Ｏ₃ ─８０ｍｏｌ％ＬｉＶＯ₃ 擬三元系を用いて、実験６と同様にして、ニオブ酸リチウム単結晶基板上に固溶体膜を形成した。過冷却度ΔＴは１０℃に設定した。成膜温度Ｔ₃ を、表８に示すように、９００℃〜９４０℃の範囲内で変更し、各例の固溶体膜（厚さ２０μｍ）を製造し、固溶体膜中のタンタルの割合ｘを測定した。
【０１１７】
更に、各固溶体膜について、プロトン交換法によって同一の条件下で光導波路を形成した。この際には、通常のフォトリソグラフィー法によって、各固溶体膜の表面に幅２μｍのアルミニウムマスクパターンを形成し、この単結晶基板品を２００℃の安息香酸中に１５分間浸漬した。マスクをエッチングによって除去し、３５０℃で４時間光導波路をアニール処理し、プロトン交換光導波路を形成した。各例の光導波路に対して波長８３０ｎｍの光を入射させ、伝搬損失を測定した。これらの測定結果を表８に示す。
【０１１８】
【表８】

【０１１９】
この結果からわかるように、ｘが０．２４６〜０．２９１の範囲内で伝搬損失が抑制されており、特にｘが０．２５３〜０．２７２というきわめて狭い範囲内、即ち、ｘの範囲としては０．０２というきわめて狭い範囲内で、光導波路の伝搬損失が最小限に抑えられている。このように、シングルモード光導波路においては、ニオブ酸リチウム─タンタル酸リチウム固溶体膜においてタンタルの割合ｘの許容範囲がきわめて小さいことが判る。
【０１２０】
これと同時に、成膜温度Ｔ₃ とタンタルの割合ｘとはきわめて精密に、リニアに対応しており、成膜温度Ｔ₃ を９０４℃〜９３２℃の範囲内とすること、更に好ましくは９０８℃〜９２０℃の範囲内とすることによって、光導波路の伝搬損失を最小限に抑制することができた。
【０１２１】
このように、本発明によって、成膜温度Ｔ₃ の許容範囲を約１０℃以下に設定することによって、伝搬損失の低いシングルモード光導波路を量産することが可能になった。
【０１２２】
（実験９）
図１の温度スケジュールに従って、ＬｉＮｂＯ₃ ─ＬｉＴａＯ₃ ─ＬｉＶＯ₃ 擬三元系を用いて、ニオブ酸リチウム単結晶基板上に固溶体膜を形成した。溶融体を、十分に高い温度Ｔ₁ （１０００°Ｃ〜１３００°Ｃ）で３時間以上攪拌し、十分均一な液相の状態とした。その後、溶融体を保持温度Ｔ₂ まで冷却した後、１２時間以上保持した。その後、溶融体３の温度を、Ｔ₂ から過冷却度ΔＴだけ低い成膜温度Ｔ₃ まで冷却し、ただちにニオブ酸リチウム単結晶基板６を液相部分４に接触させて成膜を行った。
【０１２３】
ただし、溶融体の仕込み組成は、モル比でＬｉＮｂＯ₃ ：ＬｉＴａＯ₃ ＝８０：２０となるようにし、溶融体の飽和温度Ｔ₀ が、成膜温度Ｔ₃ よりも約１００℃高くなるように、ＬｉＶＯ₃ の割合を調節した。また、過冷却度ΔＴは１０℃に設定した。成膜温度Ｔ₃ を種々変更することによって、図１２および表９に示すようなタンタルの置換割合（ｘ）を有する固溶体膜を製造した。
【０１２４】
この単結晶基板の両面に白金の電極を設置し、電気炉内にこの単結晶基板を収容し、６００℃で加熱した後、直流電圧を印加して、単分域化処理を行った。各組成を有する固溶体膜について、位相整合波長を測定し、表９および図１２に示した。
【０１２５】
【表９】

【０１２６】
実験６等の結果からわかるように、固溶体膜を成膜するときに、過冷却状態の液相部分の温度を７５０℃以上、１２５０℃以下とすることによって、タンタルの置換割合ｘを０〜０．８の範囲で変化させることができ、かつ、過冷却状態の液相部分の温度を８００℃以上、１１５０℃以下とすることによって、ｘを０．１以上、０．６以下の範囲で変化させることができる。そして、表９および図１２の結果から判るように、ｘを０．０〜０．８の範囲内で変化させることによって、位相整合波長を１．０〜２．５μｍの範囲内で制御することができる。更には、ｘを０．０〜０．６の範囲内で変化させることによって、位相整合波長を１．０〜１．６μｍの範囲内で制御することができる。
【０１２７】
このように、固溶体膜中のタンタルの置換量を変化させることで、第二高調波の位相整合波長を前記範囲内で制御することができるので、本発明の素子はＳＨＧ素子として極めて有用である。また、前記した第二層もニオブ酸リチウム─タンタル酸リチウム固溶体膜であり、このタンタルの置換割合ｙについても、まったく同様の結果が得られる。
【０１２８】
（実験１０）
実験４で製造した、図９の単結晶基板品１３を使用し、第二高調波の発生実験を行った。即ち、固溶体膜１０の膜厚は２０μｍであり、組成はｘ＝０．３５であった。第二層１１の膜厚は８μｍであり、組成は、ＬｉＮｂＯ₃ であった。
【０１２９】
この単結晶基板品１３の端面に、波長１０６４ｎｍのＹＡＧレーザー（出力１００ｍＷ）１４を結合し、ＴＥモードでＹＡＧレーザーを第二層１１の一方の端面に入射させ、第二層１１の他方の端面から波長５３２ｎｍの第二高調波を発生させた。この結果、シングルモードで約１５ｍＷの出力を得た。この出力において、第二層１１および固溶体膜１０における光損傷は、まったく認められなかった。このように、本発明のデバイスは、第二高調波発生素子としてきわめて優れて特性を有していた。
【０１３０】
（実験１１）
実験５で製造した、図９の単結晶基板品１３を使用し、第二高調波の発生実験を行った。即ち、第一層目の固溶体膜１０の膜厚は１８μｍであり、組成はｘ＝０．６３であった。第二層１１の膜厚は５μｍであり、組成はｙ＝０．４２であった。
【０１３１】
この単結晶基板品１３の端面に、波長１３００ｎｍのＹＡＧレーザー（出力５０ｍＷ）１４を結合し、ＴＥモードでＹＡＧレーザーを第二層１１の一方の端面に入射させ、第二層１１の他方の端面から波長６５０ｎｍの第二高調波を発生させた。この結果、シングルモードで約８ｍＷの出力を得た。この出力において、第二層１１および固溶体膜１０における光損傷は、まったく認められなかった。このように、本発明のデバイスは、第二高調波発生素子としてきわめて優れて特性を有していた。
【０１３２】
（実験１２）
前記した図１の温度スケジュールに従って、実験１と同様の方法に従って、液相エピタキシャル法によって、光学グレードのニオブ酸リチウム単結晶基板のＺ面上に、ニオブ酸リチウム−タンタル酸リチウム固溶体からなるエピタキシャル膜を形成した。主としてＬｉ₂ Ｏ、Ｎｂ₂ Ｏ₅ 、Ｔａ₂ Ｏ₅ 、Ｖ₂ Ｏ₅ からなる溶融体を使用した。この溶融体の仕込み組成は、モル比で、ＬｉＮｂ_1-x Ｔａ_x Ｏ₃ とＬｉＶＯ₃ との比率が２０：８０となるように、前記の各成分を混合した。
【０１３３】
Ｌｉ₂ Ｏの仕込み量を変化させることによって、溶融体中におけるリチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を変化させ、これによって、作成する固溶体膜におけるリチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を制御した。
【０１３４】
本実験例では、表１０に示すように、タンタルの置換割合ｘを変化させ、リチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）は化学量論的組成比率である１．００に制御した。作成した膜の断面に垂直に、波長４８８ｎｍのアルゴンレーザーを入射させ、出射した光のビームパターンの変化を観察することによって、膜の耐光損傷特性を測定した。この測定によって得られた光損傷のしきい値を、表１０に示す。
【０１３５】
【表１０】

【０１３６】
この結果から判るように、膜におけるリチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を化学量論的組成とした場合、タンタルの置換割合ｘが０．０５未満になると、あるいは０．７０を越えると、光損傷のしきい値が著しく減少しており、ｘを０．１０以上、０．６０以下とすることによって、１０００Ｗ／ｃｍ² 以上のしきい値が得られ、実用的な光学素子として良好に使用できることが確認された。
【０１３７】
（実験１３）
実験１２と同様にして、光学グレードのニオブ酸リチウム単結晶基板のＺ面上に、ニオブ酸リチウム−タンタル酸リチウム固溶体からなるエピタキシャル膜を形成した。ＬｉＯ₂ の仕込み量を変化させることによって、溶融体中におけるリチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を変化させ、これによって、作成する固溶体膜におけるリチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を制御した。
【０１３８】
本実験例では、表１１に示すように、タンタルの置換割合ｘを０．３に固定し、リチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を、表１１に示すように変化させた。各例の膜について、光損傷のしきい値を、実験１２と同様にして測定した。
【０１３９】
【表１１】

【０１４０】
この結果から判るように、膜におけるリチウムの含有割合／（タンタルの含有割合とニオブの含有割合との合計）を０．９８以上、１．０２以下とすることによって、光損傷のしきい値が著しく増大し、１０００Ｗ／ｃｍ² 以上のしきい値が得られ、実用的な光学素子として良好に使用できることが確認された。このように顕著な作用効果が得られた理由は明確ではないが、化学量論的組成に近いために、結晶の完全性が高いためであろうかと思われる。
【０１４１】
【発明の効果】
以上述べてきたように、本発明は、ニオブ酸リチウム単結晶基板上に、特定組成のニオブ酸リチウム─タンタル酸リチウム固溶体膜を有する新規な単結晶基板品を提供するものである。
【図面の簡単な説明】
【図１】液相エピタキシャル法において、固相と液相とを共存させる方法における、溶融体の温度スケジュールを模式的に示すグラフである。
【図２】（ａ）、（ｂ）は、ルツボ１内における溶融体の状態を模式的に示す断面図である。
【図３】液相エピタキシャル法において、固相と液相とを共存させる方法における、溶融体の温度スケジュールの他の例を模式的に示すグラフである。
【図４】ＬｉＮｂ_1-x Ｔａ_x Ｏ₃ ─ＬｉＶＯ₃ の擬二元系組成の相図である。
【図５】図４に示す相図を、ＬｉＮｂＯ₃ ─ＬｉＴａＯ₃ ─ＬｉＶＯ₃ の擬三元系組成について表示しなおした、三角図である。
【図６】本発明において固溶体膜の作成が可能な溶融体の組成の範囲を表示する、ＬｉＮｂＯ₃ ─ＬｉＴａＯ₃ ─ＬｉＶＯ₃ の擬三元系組成についての三角図である。
【図７】（ａ）、（ｂ）および（ｃ）は、上記の擬三元系組成において、ＬｉＴａＯ₃ の割合と、基板からのニオブ酸リチウムの溶出速度と、基板上への固溶体膜の析出速度との関係を模式的に示すグラフである。
【図８】ニオブ酸リチウム単結晶基板６上に固溶体膜１０を有する単結晶基板品１２を模式的に示す正面図である。
【図９】ニオブ酸リチウム単結晶基板６、固溶体膜１０および光導波路として機能する第二相を有する単結晶基板品１３を、模式的に示す正面図である。
【図１０】（ａ）は、本発明の単結晶基板品１３を使用した第二高調波発生素子１８を示す模式図であり、（ｂ）は、リブ型の光導波路を備えた単結晶基板品１９を示す斜視図であり、（ｃ）は、単結晶基板品１９を使用した第二高調波発生素子２５を示す模式図である。
【図１１】成膜温度Ｔ₃ とタンタルの割合ｘとの関係を示すグラフである。
【図１２】ニオブ酸リチウム─タンタル酸リチウム固溶体膜におけるタンタルの置換割合ｘ（またはｙ）と、位相整合波長との関係を示すグラフである。
【符号の説明】
１ ルツボ，２ 均一に溶融している溶融体，３ 固相部分と液相部分とが共存している溶融体，４ 液相部分，５ 固相部分，６ ニオブ酸リチウム単結晶基板，１０ 固溶体膜，１１ 第二層，１２、１３ 単結晶基板品，Ａ （９５、５、０）の組成を示す点，Ｂ （９５、２、３）の組成を示す点，Ｃ （６０、４０、０）の組成を示す点，Ｄ（６０、０、４０）の組成を示す点，Ｅ（０、４０、６０）の組成を示す点，Ｇ 点Ａ（９５、５、０）と点Ｃ（６０、４０、０）とを結ぶ直線，Ｈ 点Ｃ（６０、４０、０）と点Ｄ（６０、０、４０）とを結ぶ直線，Ｉ 飽和温度１２００℃以下の組成範囲を示す曲線，Ｊ 点Ｂ（９５、２、３）と点Ｅ（０、４０、６０）とを結ぶ直線，Ｋ 点Ａ（９５、５、０）と点Ｂ（９５、２、３）とを結ぶ直線[0001]
[Industrial application fields]
The present invention relates to a single crystal substrate product (particularly an optical waveguide substrate) including a lithium niobate single crystal substrate, an optical component using the same, a method for manufacturing a single crystal substrate product, and a method for manufacturing an optical component.
[0002]
[Prior art]
Lithium niobate (LiNbO _Three ) Single crystal, lithium tantalate (LiTaO) _Three ) Single crystals are expected as materials for optoelectronics such as second harmonic generation elements, optical modulators, and optical switch substrates. The liquid phase epitaxial method is expected as a method for forming a high-quality thin film with good crystallinity because a single crystal film can be formed at a lower temperature than the pulling method.
[0003]
For example, according to the description on pages 314 to 322 of "Journal of Crystal Growth" 46 (1979), a lithium niobate-lithium tantalate solid solution film is formed on a lithium tantalate single crystal substrate by liquid phase epitaxy. are doing. According to the description on pages 8 to 10 of "Applied Physics Letters" Vol.26 No.1 (1975), a lithium niobate single crystal thin film is formed on a lithium tantalate single crystal substrate by liquid phase epitaxy. . According to the description on pages 291 to 298 of “Journal of Crystal Growth” 50 (1980), a lithium niobate single crystal thin film is formed on a lithium tantalate single crystal substrate by the “Capillary Liquid Epitaxial” method. . However, this is fundamentally different from a normal liquid phase epitaxial method, and the formation of a lithium niobate-lithium tantalate solid solution film is not described.
[0004]
An outline of a film forming method in the liquid phase epitaxial method will be described. First, for example, lithium niobate (solute) and LiVO _Three (Melting medium) is charged and mixed. The saturation temperature corresponding to the charged composition of the melt is T ₀ And The temperature of this melt is expressed as the saturation temperature T ₀ Higher than T ₁ And hold lithium niobate and LiVO _Three And uniformly melt. The temperature of the melt is then changed to the saturation temperature T ₀ To a lower temperature to bring the melt into a supercooled state. The substrate is brought into contact with the supercooled melt.
[0005]
In particular, a method of manufacturing an optical waveguide substrate, in which a single crystal film having a higher refractive index than that of a single crystal substrate is formed by liquid phase epitaxy and this single crystal film is used as an optical waveguide, has recently been developed. It is attracting attention as a method for manufacturing devices.
[0006]
Here, the refractive index of lithium tantalate is smaller than that of lithium niobate. Further, lithium niobate and lithium tantalate produce a solid solution at an arbitrary ratio, and the refractive index of the solid solution decreases as the ratio of tantalum increases. Therefore, a lithium tantalate single crystal substrate has been mainly used as a substrate for forming a lithium niobate-lithium tantalate solid solution film or a lithium niobate thin film by a liquid phase epitaxial method. This is because lithium tantalate has a lower refractive index than lithium niobate or lithium niobate-lithium tantalate solid solution. This is because it can be used.
[0007]
[Problems to be solved by the invention]
However, the Curie temperature of the lithium tantalate single crystal is about 600 ° C., and the film forming temperature in the liquid phase epitaxial method is 700 ° C. or higher. Therefore, even if a single-domain treated lithium tantalate single crystal substrate is used, the single crystal substrate becomes multidomain in the process of film formation, and the crystallinity deteriorates. Since it becomes multi-domain, it was difficult to produce a high-quality film. Moreover, in order to use such a multi-domain thin film as an optical device, it is necessary to perform a single domain process again. However, the crystallinity of the thin film is further damaged at the stage of the single domain process. I will give it.
[0008]
Furthermore, as for the lithium niobate single crystal substrate, an optical grade single crystal substrate with good crystallinity has been obtained. However, at the present stage, the lithium tantalate single crystal substrate manufactured by the pulling method has poor crystallinity as compared with the lithium niobate single crystal substrate. Even if a single crystal film is originally formed on a lithium tantalate single crystal substrate having poor crystallinity, a single crystal film having crystallinity superior to a film fabricated on an optical grade lithium niobate single crystal substrate is obtained. It is difficult.
[0009]
On the other hand, the Curie temperature of the lithium niobate single crystal substrate is about 1200 ° C., and the crystallinity of the substrate is sufficiently stable even at the film forming temperature in the liquid phase epitaxial method. . In addition, as described above, since a high-quality single crystal substrate is manufactured by a pulling method, it is easy to obtain.
[0010]
However, the melting point of lithium niobate is higher than that of the lithium niobate-lithium tantalate solid solution. As a result, the saturation temperature of the melt for liquid phase epitaxy increases as the tantalum ratio increases. Therefore, when the lithium niobate single crystal substrate is brought into contact with the melt, the solid solution film does not grow and the substrate dissolves. Therefore, the lithium niobate-tantalate is formed on the lithium niobate single crystal substrate. It has been considered difficult to form a lithium solid solution film.
[0011]
An object of the present invention is to form a film made of a lithium niobate-lithium tantalate solid solution having a melting point higher than that of a substrate on a lithium niobate single crystal substrate or the like by a liquid phase epitaxial method.
[0012]
[Means for Solving the Problems]
The present invention relates to LiNb _1-z Ta _z O _Three A single crystal substrate having a composition of (0 ≦ z <0.8) and a lithium niobate-lithium tantalate solid solution film formed on the single crystal substrate by a liquid phase epitaxial method The composition of the lithium niobate-lithium tantalate solid solution film is LiNb _1-x Ta _x O _Three (0 <x ≦ 0.8), and x and z satisfy the relationship of z <x. The present invention relates to a single crystal substrate product.
[0013]
The present invention also relates to a single crystal substrate product comprising a single crystal substrate and a single crystal film epitaxially grown on the single crystal substrate, wherein the composition of the single crystal film is LiNb. _1-x Ta _x O _Three (0 ≦ x ≦ 0.8), and the half width of the X-ray rocking curve of the single crystal film is smaller than the half width of the X-ray rocking curve of the single crystal substrate Is.
[0014]
The present invention also relates to a single crystal substrate product comprising a single crystal substrate and a single crystal film epitaxially grown on the single crystal substrate, wherein the composition of the single crystal film is LiNb. _1-x Ta _x O _Three (0.ltoreq.x.ltoreq.0.8), wherein the molar concentration of tantalum in the single crystal film is larger than the molar concentration of tantalum in the single crystal substrate. .
[0015]
The present invention also relates to a method for producing a single crystal substrate product as described above, wherein when the single crystal substrate is brought into contact with the supercooled liquid phase portion of the melt, the melt is mainly Li. ₂ O _Three , Nb ₂ O _Five , Ta ₂ O _Five And the composition of the liquid phase part is LiNbO as shown in FIG. _Three ─LiTaO _Three -In the triangular diagram of the melting medium pseudo ternary system, a straight line connecting A (95, 5, 0) and B (95, 2, 3), A (95, 5, 0) and C (60, 40, 0) ), A straight line connecting C (60, 40, 0) and D (60, 0, 40), a straight line connecting B (95, 2, 3) and E (0, 40, 60), and It is a composition ratio belonging to a region surrounded by a curve satisfying a saturation temperature of 1200 ° C. or lower.
[0016]
In addition, the present invention applies the above-described single crystal substrate product and its manufacturing method to various optical components, and particularly applies to a second harmonic generation element.
[0017]
Hereinafter, the background to the present invention will be described in detail.
The present inventor has reviewed the method of creating a supercooled state in the course of studying the liquid phase epitaxial method. Conventionally, the solute and the melting medium are first sufficiently melted at a sufficiently high temperature of 1000 to 1300 ° C., and then the temperature is lower than the saturation temperature corresponding to the charged composition, thereby creating a supercooled state. That is, it was common knowledge that it is necessary to create a supercooled state from a sufficiently high-temperature liquid phase.
[0018]
However, as a result of re-examination of the liquid phase epitaxial method by the present inventors, it has been found that a high-quality single crystal film cannot be formed with good reproducibility. The reason for this will be described. First, since the liquid phase epitaxial method can be formed at a lower temperature than the pulling method, a single crystal film with good crystallinity should be formed.
[0019]
However, as the concentration of solute increases, the saturation temperature increases and approaches the growth temperature in the pulling method. Therefore, in order to form a film with good crystallinity, it is necessary to form the film at as low a temperature as possible. From this viewpoint, it is preferable to form the film at a low temperature of 1000 ° C. or lower.
[0020]
However, on the other hand, when the solute concentration is lowered, particularly when the saturation temperature is 1000 ° C. or lower, the inclination of the liquidus becomes very large. Therefore, even when the concentration of the solute in the melt varies slightly, the saturation temperature varies greatly. In the liquid phase epitaxial method, the melt is first maintained at a temperature equal to or higher than the saturation temperature, and then the film is formed in a supercooled state at a film forming temperature lower than the saturation temperature. The crystallinity of the film is determined by this supercooled state, and this supercooled state is determined by the saturation temperature and the film forming temperature.
[0021]
Therefore, if the concentration of the solute in the melt varies slightly, it becomes impossible to form a film with good crystallinity. In particular, in the actual film forming process, when the film formation on the substrate is repeated, the composition of the melt changes immediately, and a constant solute concentration cannot be maintained. Therefore, it is difficult to form a film with good reproducibility.
[0022]
The inventor has focused on this point and has come up with a method that is essentially different from the conventional one. This method will be described with reference to FIGS. 1 and 3 are graphs schematically showing a temperature schedule of a melt in a novel liquid phase epitaxial method developed by the present inventors. 2A and 2B are cross-sectional views schematically showing the state of the melt in the crucible 1.
[0023]
First, the solute and the melting medium are charged into the crucible 1 and mixed. Saturation temperature T of this melt ₀ Is determined to be a constant value corresponding to the concentration of the solute in the melt, that is, the charged composition. This saturation temperature can be calculated from the liquidus.
[0024]
First, the temperature of the melt is changed to the saturation temperature T ₀ Higher than T ₁ To melt the solute and the melting medium uniformly. In FIG. 1, “A” corresponds to this molten state. Moreover, as shown to Fig.2 (a), all the melts 2 are a liquid phase.
[0025]
The temperature of the melt is then changed to the saturation temperature T ₀ Lower solid-phase deposition temperature T ₂ Allow to cool. In this state, the melt is initially in a supercooled state, but if it is kept at this temperature for a sufficiently long time, a solid phase is precipitated from the melt. In FIG. 1, “B” corresponds to the holding state for this solid phase precipitation. At this time, the melt 3 is separated into the liquid phase portion 4 and the solid phase portion 5 as shown in FIG. This solid phase 5 portion is deposited mainly along the wall surface of the crucible 1.
[0026]
Next, the temperature of the melt is lowered to bring the liquid phase portion 4 into a supercooled state. In FIG. 1, “C” corresponds to this supercooled state. The substrate 6 is lowered and brought into contact with the supercooled liquid phase portion 4 as shown by an arrow 7 to epitaxially grow a single crystal film.
[0027]
The following method was also examined. That is, as shown in FIG. 3, first, the temperature of the melt is changed to the saturation temperature T ₀ Higher than T ₁ To melt the solute and the melting medium uniformly. In FIG. 3, “A” corresponds to this molten state. The temperature of the melt is then changed to the saturation temperature T ₀ Higher holding temperature T ₂ Allow to cool. Of course, no solid phase precipitates at this stage. Therefore, a predetermined amount of solute is newly added to the melt. At this time, the saturation temperature of the melt is T ₂ Higher than T _Five To rise. This temperature T ₂ And holding for a sufficiently long time, the state of the solid phase and the liquid phase is stabilized. In FIG. 3, “D” corresponds to a holding state in which the solid phase and the liquid phase coexist.
[0028]
Then the temperature of the melt is T _Three Until the liquid phase portion 4 is supercooled. In FIG. 3, “C” corresponds to this supercooled state. The substrate 6 is lowered and brought into contact with the supercooled liquid phase portion 4 as shown by an arrow 7 to epitaxially grow a single crystal film.
In order to bring the liquid phase of the melt into a supercooled state, the temperature T ₂ The temperature T ₂ Lower temperature T _Three A cooled substrate may be brought into contact with the substrate. As a result, the melt in the vicinity of the substrate surface has the temperature of the entire melt T ₂ In the same manner as in the case of cooling, the film is overcooled and a film is formed on the substrate.
[0029]
Thus, in this method, states B and D (temperature T ₂ ) As the starting point, from this stable state, the film formation temperature T _Three The liquid phase portion is in a supercooled state by lowering the temperature to. As described above, in the state where the solid phase and the liquid phase coexist stably, the concentration of the solute in the liquid phase portion is maintained at the holding temperature T unless the saturation temperature of the entire system is exceeded. ₂ At a saturated concentration.
[0030]
For example, when the concentration of the solute in the melt decreases, the holding temperature T ₂ , The amount of the solid phase portion decreases correspondingly, and when the concentration of the solute increases, the amount of the solid phase portion increases accordingly. Therefore, the temperature and concentration of the liquid phase part are always kept constant. And the film formation temperature T _Three Of course, since it is set to a constant value, T ₂ And T _Three (Supercooling degree) is also kept constant, and it becomes possible for the first time to completely control the supercooling state.
[0031]
As a result, the supercooled state is maintained almost completely constant even when the composition of the melt changes due to repeated film formation on the substrate in the actual film forming process. Therefore, a single crystal film with good crystallinity can be formed with high reproducibility.
[0032]
Moreover, according to this method, not only a single crystal film having a constant quality can be formed with good reproducibility, but also the crystallinity of the single crystal film itself has been remarkably improved. In particular, the present inventors have succeeded in forming a single crystal film in which the half-value width of the X-ray rocking curve is smaller than that of a single crystal substrate, which could not be produced conventionally.
[0033]
The reason for this is not clear, but is probably due to the following reasons. In the conventional method, when the substrate is brought into contact with the melt, the entire melt is in a uniform liquid phase. Therefore, at the moment when the substrate comes into contact with the melt, solid phase precipitation occurs for the first time in the entire liquid phase on the surface of the substrate. For this reason, it can be estimated that a relatively large nucleation energy is required to start the growth of the single crystal film. Therefore, since the nucleation energy is large when film growth is started at the interface between the substrate and the film, the crystallinity of the film is disturbed at this interface, and the crystallinity of the film deposited on this interface is It seems to reflect the disturbance.
[0034]
On the other hand, in this method, as shown in FIG. 2 (b), the solid phase portion 5 coexists in the melt 3 in advance before the substrate 6 contacts the melt 3. In this state, at the interface between the solid phase portion 5 and the liquid phase portion 4, melting and precipitation occur from the microscopic viewpoint. Therefore, even if the substrate 6 is newly brought into contact with the liquid phase portion, it is considered that the film growth starts smoothly and a single crystal film having excellent crystallinity can be produced.
[0035]
The present inventor has realized a method for controlling the supercooling state in this way, and tried to produce a lithium niobate-lithium tantalate solid solution film on a lithium niobate single crystal substrate using this method. I tried. This is a combination that has conventionally been considered difficult due to solubility.
[0036]
As a result, it was surprisingly discovered that the above-mentioned solid solution film can be formed on a lithium niobate single crystal substrate in a considerably wide composition range, and the present invention has been achieved.
[0037]
In practice, the composition of the lithium niobate-lithium tantalate solid solution film is LiNb. _1-x Ta _x O _Three It was found that a solid solution film can be formed within the range of 0 <x ≦ 0.8.
[0038]
The inventor also doped a tantalum component into lithium niobate constituting the single crystal substrate to produce a single crystal substrate composed of a lithium niobate-lithium tantalate solid solution, on which lithium niobate is formed. ─ Tried to form a lithium tantalate solid solution film. As a result, it was confirmed that the solid solution film could be formed in a wide composition range as described above. The composition of this lithium niobate-lithium tantalate single crystal substrate is LiNb _1-z Ta _z O _Three (0 <z ≦ 0.8), where x and z satisfy the relationship z <x, so that (0 <z <0.8). That is, the solid solution film has a larger replacement ratio of tantalum than the substrate.
[0039]
This will be described more specifically. The melt for contacting the substrate is mainly Li ₂ O, Nb ₂ O _Five , Ta ₂ O _Five Made of flux. Li ₂ The amount of O charged is Nb ₂ O _Five , Ta ₂ O _Five And the total amount (number of moles) of the flux charged. The melt is LiNbO which is a solute component. _Three And LiTaO _Three And a solvent component (flux), LiNbO _Three ─LiTaO _Three ─ It can be considered as a quasi-ternary composition of the melting medium. The melt is LiNb which is a solute component. _1-x Ta _x O _Three LiNb consisting of a solvent component (flux) _1-x Ta _x O _Three ─ It can be considered as a quasi-binary composition of the molten medium. Such flux includes V ₂ O _Five , B ₂ O _Three , MoO _Three , WO _Three Can be illustrated as suitable.
[0040]
FIG. 4 shows LiNb _1-x Ta _x O _Three ─LiVO _Three It is a phase diagram of a pseudo binary system composition. The horizontal axis is LiNb _1-x Ta _x O _Three The ratio is shown in mol%. The vertical axis represents temperature. FIG. 5 shows the phase diagram shown in FIG. 4 as LiNbO. _Three ─LiTaO _Three ─LiVO _Three It is re-displayed as a triangular diagram of the quasi-ternary composition. Each numerical value from “900” to “1300” in FIG. 5 is a numerical value indicating the saturation temperature at each composition point.
[0041]
First, the composition of the liquid phase in the saturated state needs to be a portion having a saturation temperature of 1200 ° C. or lower in this triangular diagram. Because the Curie temperature of the lithium niobate single crystal substrate is about 1200 ° C., if the substrate is brought into contact with the liquid phase portion having a temperature exceeding 1200 ° C., the substrate becomes multi-domain, and the crystallinity of the substrate is reduced. This is because it causes damage and makes it difficult to produce a high-quality film. This composition range is indicated by curve I in the triangular diagram of FIG.
[0042]
From this viewpoint, in order to produce a high-quality film, it is preferable that the temperature of the supercooled liquid phase portion be 1150 ° C. or lower. This is because by adopting a film forming temperature of 1150 ° C. or lower, the crystallinity of the film is further improved, and when an optical waveguide is formed in the film, optical damage in the optical waveguide is remarkably reduced. The film forming temperature is preferably 750 ° C. or higher, and it is difficult to form the solid solution film at a film forming temperature lower than 750 ° C.
[0043]
The inventor tried to form films on the melts having various composition ratios in FIG. 5 by using the film forming method described above. As a result, it has been surprisingly discovered that the solid solution film can be stably formed within a considerably wide composition range.
[0044]
However, it was also confirmed that the composition range of the melt was within the composition range shown in the triangular diagram of the pseudo ternary composition in FIG. Here, the numbers in parentheses are (LiVO _Three Number of moles of LiNbO _Three Number of moles of LiTaO _Three Number of moles).
[0045]
LiNbO _Three Number of moles: LiTaO _Three It has been found that the molar ratio of must be 40% or more and 60% or less. The reason for this is not clear, but was estimated as follows. This will be described with reference to the schematic graphs of FIGS. In each graph of FIG. 7, the vertical axis indicates the rate of elution or precipitation, and the horizontal axis indicates LiTaO. _Three The number of moles of LiNbO _Three The ratio with respect to the number of moles is shown.
[0046]
In order to deposit the solid solution film on the lithium niobate single crystal substrate, the deposition rate at which solid precipitates from the liquid phase is faster than the elution rate at which lithium niobate elutes into the supercooled liquid phase. Must-have. LiNbO _Three The smaller the number of moles, that is, the more toward the right in the graph, the higher the solubility of lithium niobate in the melt, and as a result, the elution rate of lithium niobate increases. On the other hand, LiNbO _Three The smaller the number of moles, the smaller the precipitation rate. As a result, at a certain point F, the elution rate of lithium niobate exceeds the deposition rate of the solid solution film, and film formation becomes impossible.
[0047]
Also, the degree of supercooling of the liquid phase seems to be important. FIG. 7 (a) is a graph when the degree of supercooling is large, FIG. 7 (b) is a graph when the degree of supercooling is smaller than that of FIG. 7 (a), and FIG. It is a graph in case a supercooling degree is smaller than b). As the degree of supercooling of the liquid phase decreases, the solid solution film deposition rate decreases and the temperature of the liquid phase increases, so that the elution rate of lithium niobate increases. As a result, LiTaO capable of film formation _Three The molar ratio of decreases.
[0048]
Looking only at this graph, LiTaO _Three In order to increase the molar ratio, the degree of supercooling should be increased. However, if the degree of supercooling is increased too much, the supercooled state cannot be stably controlled, and if it exceeds 50 ° C., the supercooled state cannot be maintained.
[0049]
For the above reasons, LiTaO in the liquid phase can be formed. _Three The content ratio of is limited. Specifically, LiNbO _Three When the number of moles of was less than 40%, film formation could not be carried out even if the degree of supercooling was precisely controlled so as to lower it to near 50 ° C. This composition range is indicated by a straight line J connecting point B (95, 2, 3) and point E (0, 40, 60) in the triangular diagram of FIG.
[0050]
In addition, the ratio of the solvent to the solute has a range in which a film can be formed. That is, LiVO _Three In the case where the ratio is 60% or less, a high-quality film could not be formed. The reason for this is also not clear, but it is probably because the proportion of the solute in the liquid phase increases because the proportion of the solvent decreases and the proportion of the solute component increases, and the viscosity of the liquid phase portion increases. It is considered that the growth of the film is not performed smoothly and the crystallinity of the film is deteriorated. This composition range is indicated by a straight line H connecting the point C (60, 40, 0) and the point D (60, 0, 40) in the triangular diagram of FIG.
[0051]
In addition, when the solute ratio is 5% or less, the solute concentration becomes too thin, so that the solute concentration in the melt easily changes greatly as the film grows, so that the film cannot be formed. It is done. This composition range is indicated by a straight line K connecting point A (95, 5, 0) and point B (95, 2, 3) in the triangular diagram of FIG. This composition range is naturally surrounded by a straight line G connecting the point A (95, 5, 0) and the point C (60, 40, 0).
[0052]
The specific temperature control method of the liquid phase epitaxial method described with reference to FIGS. 1 to 3 was carried out in order to accurately and stably control the supercooled state for the above-described pseudo ternary composition. It is. However, if the supercooled state can be precisely and stably controlled by other methods, it is considered that a solid solution film within the scope of the present invention can be formed.
[0053]
In the above, the case of the lithium niobate single crystal substrate has been described. However, it has been found that these relations similarly apply to the case of a lithium niobate-lithium tantalate solid solution single crystal.
[0054]
In the above example, the flux is V ₂ O _Five We have explained the case of using. However, B as flux ₂ O _Three , MoO _Three , WO _Three When using, basically the same results were obtained.
[0055]
FIG. 8 is a front view schematically showing the single crystal substrate product 12 formed as described above. A solid solution film 10 is formed on a single crystal substrate 6 having the above composition. The composition of the solid solution film 10 is LiNb. _1-x Ta _x O _Three However, x was 0.8 at the maximum. This x is preferably 0.02 or more.
[0056]
Here, when the material of the single crystal substrate is a lithium niobate-lithium tantalate solid solution single crystal, the tantalum ratio z is 0.8 or less, but z is 0.2 or less. Is preferred. This is because if the z is large, the Curie temperature of the single crystal substrate is lowered, but if this Curie temperature is lower than the film formation temperature, the single crystal substrate and the single crystal film are divided into multiple domains in the film formation process. This is because the property is impaired and a single-domain treatment process is required again. In addition, in order to form a film having particularly excellent crystallinity, it is necessary to raise the deposition temperature to some extent. From this point of view, if z is in the range of 0.2 or less, there is no possibility that the film forming temperature is lower than the Curie temperature of the single crystal substrate at the normal film forming temperature.
[0057]
Further, as schematically shown in FIG. 9, a single crystal substrate product 13 can be manufactured by further forming a second layer 11 made of a lithium niobate-lithium tantalate solid solution film on the solid solution film 10. it can. Here, the composition of the second layer 11 is LiNb. _1-y Ta _y O _Three (0 ≦ y ≦ 0.8), and x and y satisfy the relationship y <x.
[0058]
As a result, since the refractive index of the second layer 11 is smaller than the refractive index of the solid solution film 10, the second layer 11 can be used as an optical waveguide. Thus, it was possible to provide for the first time a substrate in which an optical waveguide mainly composed of a lithium niobate-lithium tantalate solid solution film was formed on a lithium niobate single crystal substrate or a single crystal substrate doped with tantalum.
[0059]
The single crystal substrate product according to the present invention can be widely used as a base material for various optical components. Examples of such optical components include optical external modulators used in optical communications, optical switches, optical amplifiers, optical pickups, SHG elements (second harmonic generation elements) used in optical pickups, and optical fiber gyroscope light used in optical measuring devices. An integrated circuit etc. can be mentioned. In this case, as shown in FIG. 9, the single crystal substrate product 13 having the second layer 11 having a refractive index smaller than that of the solid solution film 10 can be used as a low-loss optical waveguide substrate. Is preferred.
[0060]
In the single crystal substrate product of FIG. 9, the second layer 11 is generally used as an optical waveguide. From this viewpoint, it is necessary to increase the difference in refractive index between the solid solution film 10 and the second layer 11. . For this reason, the difference between x and y is particularly preferably 0.1 or more.
[0061]
Each composition formula (LiNb _1-z Ta _z O _Three , LiNb _1-x Ta _x O _Three , LiNb _1-y Ta _y O _Three ) Is an expression method commonly used in this technical field, and as usual, in each composition formula, the ratio of atoms (Li) constituting the A site and atoms (Nb and Ta) constituting the B site Is not expressed.
[0062]
The solid solution film or single crystal substrate of the present invention can contain a rare earth element as a doping component. This rare earth element acts as an additive element for laser oscillation. As this rare earth element, Nd, Er, Tm, Ho, Dy, and Pr are particularly preferable.
[0063]
One or more metal atoms selected from the group consisting of zinc, scandium, magnesium, indium, titanium and vanadium can be contained in the solid solution film or the single crystal substrate. These elements have the effect of improving the light damage resistance.
[0064]
Furthermore, by adopting a film formation method as described with reference to FIGS. 1 to 3 in particular, to form the solid solution film having a smaller half-value width of the X-ray rocking curve than the optical grade lithium niobate single crystal substrate, An optical waveguide was formed on this, and the optical characteristics were measured. As a result, it was confirmed that the light damage resistance of the optical waveguide was remarkably improved. As a result, the single crystal substrate product of the present invention can be effectively used as various optical components.
[0065]
Here, the half width of the X-ray rocking curve will be described. The crystallinity of the single crystal substrate and the solid solution film can be evaluated by the half width of the X-ray rocking curve. In general, it can be determined that the smaller the half width, the better the crystallinity of the single crystal. Since this value itself varies depending on a reference crystal or the like used in the X-ray measuring apparatus, an absolute value cannot be specified.
[0066]
However, the crystallinity of the solid solution film produced by the liquid phase epitaxial method is strongly influenced by the crystallinity of the single crystal substrate. Therefore, in order to determine the superiority or inferiority of the crystallinity of the produced solid solution film, the half-value width of the X-ray rocking curve of the substrate used must be used as a reference. In particular, since the optical grade lithium niobate single crystal substrate is currently produced by the pulling method, the half width of the X-ray rocking curve of the solid solution film is smaller than that of the optical grade lithium niobate single crystal substrate. Is preferred.
[0067]
The difference between the holding temperature of the saturated liquid phase and the deposition temperature (degree of supercooling) is set to 20 ° C. or less from the viewpoint of further reducing the half-value width of the X-ray rocking curve of the single crystal film. More preferably.
[0068]
Next, an embodiment when the single crystal substrate product of the present invention is used as a second harmonic generation element will be described. FIG. 10A is a schematic diagram showing a second harmonic generation element using the single crystal substrate product 13 of FIG. Such an element can be used in a wide range of applications such as optical disk memory, medical use, photochemistry use, and various optical measurement uses.
[0069]
In the second harmonic generation element 18, the laser light source 14, the optical system 16, and the single crystal substrate product 13 are aligned. Incident light is emitted from the laser light source 14 to the optical system 16 as indicated by an arrow 15, and laser light from the optical system 16 is incident from one end face of the second layer (optical waveguide) 13 of the single crystal substrate product 13. From the other end face of the second layer 13, the second harmonic is emitted as indicated by an arrow 17.
[0070]
As shown in FIG. 10B, a rib-type optical waveguide is formed by processing the second layer 11 by etching or the like to form a rib, and this is used for generating a second harmonic. Can do. This single crystal substrate product 19 has a chip shape, a solid solution film 21 is formed on the main surface 20 a of the single crystal substrate 20, and a second layer 22 is formed on the solid solution film 21. In the second layer 22, an elongated rib-shaped optical waveguide 24 is formed so as to extend straight. Such a rib-type optical waveguide 24 can be formed by a method such as etching.
[0071]
In this case, for example, the laser light source 14, the optical system 16, and the single crystal substrate product 19 are aligned as in the second harmonic generation element 25 schematically shown in FIG. Incident light is emitted from the laser light source 14 to the optical system 16 as indicated by an arrow 15, and laser light from the optical system 16 is incident from one end face of the rib-type optical waveguide 24 of the single crystal substrate product 19. From the other end face of the optical waveguide 24, the second harmonic is emitted as indicated by an arrow 17.
[0072]
When the single crystal substrate product of the present invention is applied to a practical device, strong light is often incident. However, when strong light is incident, a phenomenon in which characteristics such as a refractive index are changed, so-called optical damage phenomenon is a problem. It has become. For example, according to the description of “Appl. Phys. Lett.” 30, 1977, pp. 238 to 239, the lithium tantalate single crystal has better light damage resistance than the lithium niobate single crystal. Yes. Lithium niobate single crystals and lithium tantalate single crystals are usually manufactured by the pulling method, but lithium tantalate has a higher melting point than lithium niobate and is difficult to grow. It is difficult to obtain a single crystal.
[0073]
Both the lithium niobate and lithium tantalate single crystals grown by the normal pulling method have a harmonic melting composition (congruent composition). For example, in the lithium niobate single crystal, Li / Nb = 0. 946 (“J. Chem. Phys.” 56, 1972, pp. 4848-4851). Further, in lithium tantalate, for example, according to the description of “J. Crystal growth” 10, 1971, pp. 276 to 278, it was known that Li / Ta = 0.951. For example, “J. Crystal growth” 116, 1992, pp. 327 to 332 describes an example in which a lithium niobate single crystal having a stoichiometric composition is produced by a pulling method using a double crucible. However, the crystallinity of the crystal produced by this method is inferior to the congruent composition lithium niobate single crystal produced by the ordinary pulling method, and the light damage resistance is also inferior. As for lithium tantalate single crystals, there is no example in which a single crystal having a stoichiometric composition is produced by a pulling method, and the characteristics thereof have not been studied.
[0074]
However, the present inventor confirmed that the composition of the lithium niobate-lithium tantalate solid solution film in the single crystal substrate product of the present invention is LiNb. _1-x Ta _x O _Three (Within 0.05 ≦ x ≦ 0.70) Lithium content (A-site metal content) / (Total of tantalum and niobium content) (B-site metal) If the single crystal film having a composition close to the stoichiometric composition is used, the photodamage property of the film is remarkably improved. As a result, the present invention has been reached. In addition, the present inventor also sets the composition of the second layer to LiNb. _1-y Ta _y O _Three (0.05 ≦ y ≦ 0.70) and the content ratio of lithium / (total of the content ratio of tantalum and the content ratio of niobium) is 0.98 or more and 1.02 or less, and the same as above It has been found that the light damage resistance can be remarkably improved.
[0075]
Here, it has been found that the light damage resistance is remarkably improved by setting the substitution ratio x or y by tantalum to 0.05 or more. This is considered to be an effect by the addition of tantalum. In this sense, it is even more preferable that x or y is 0.1 or more. It was also confirmed that when x or y exceeded 0.7, the threshold value for photodamage was remarkably lowered. This is considered to be because when the content ratio of tantalum is larger than 0.70, the difference in lattice constant between the film and the substrate (lattice mismatch) increases, and the crystallinity deteriorates. From this viewpoint, it was even more preferable that x or y was 0.6 or less.
[0076]
Further, the lithium content ratio (A-site metal content ratio) / (total of tantalum content ratio and niobium content ratio) (B-site metal content ratio) is 0.98 or more and 1.02 or less. The reason why the light damage resistance is remarkably improved is not clear, but if the crystal composition is close to the stoichiometric composition, non-stoichiometric crystal defects are reduced and the crystal integrity is increased. it is conceivable that.
[0077]
【Example】
Hereinafter, more specific experimental results will be described.
(Experiment 1)
The film was formed by liquid phase epitaxy according to the temperature schedule shown in FIG. LiNbO _Three ─LiTaO _Three ─LiVO _Three In the pseudo ternary system, the present invention was carried out under the conditions shown in Table 1.
[0078]
Each melt 2 has a sufficiently high temperature T ₁ The mixture was stirred at (1000 ° C to 1300 ° C) for 3 hours or more to obtain a sufficiently uniform liquid phase. Thereafter, the melt is held at a holding temperature T ₂ After being cooled to 10 ° C., the mixture was held for 12 hours or more and waited until the solid solution 5 was nucleated and the solid phase 5 was precipitated. At this time, the liquid phase portion 4 of the melt has a temperature T ₂ The melt 3 is in a state where the liquid phase portion 4 and the solid phase portion 5 made of a solid solution coexist. Holding temperature T ₂ And the composition of the liquid phase part 4 at this time was changed as shown in Table 1 and Table 2.
[0079]
Thereafter, the temperature of the melt 3 is changed to T ₂ Deposition temperature T lower than the degree of supercooling ΔT _Three Then, the lithium niobate single crystal substrate 6 was immediately brought into contact with the liquid phase portion 4 to form a film. The degree of supercooling ΔT was also changed as shown in Tables 1 and 2.
[0080]
The obtained solid solution films are all LiNb. _1-x Ta _x O _Three The composition was The compositions of these solid solution films were measured, and the values of x are shown in Tables 1 and 2. Moreover, the half width of the X-ray rocking curve of each solid solution film was measured and shown in Tables 1 and 2.
[0081]
The half-value width of the X-ray rocking curve was measured using reflection on the (001) plane by a double crystal method. CuKα1 was used as the incident X-ray, and the (422) plane of GaAs single crystal was used as the monochromator.
[0082]
The half-value width of the X-ray rocking curve of the optical grade lithium niobate single crystal substrate used by the present inventors was 6.8 to 6.9 [arc sec]. This was used as a standard for crystallinity of the substrate.
[0083]
[Table 1]

[0084]
[Table 2]

[0085]
As can be seen from Table 1, when the film was formed at various holding temperatures, a solid solution having a small half-value width and a good crystallinity was within the composition range of the present invention when the saturation temperature was 1200 ° C. or lower. A film can be formed. In particular, in the composition range where the saturation temperature is 1150 ° C. or less, the solid solution film has a half width smaller than the half width of the lithium niobate single crystal substrate, that is, a solid solution film having better crystallinity than the substrate. Could be formed.
[0086]
On the other hand, as can be seen from the results shown in Table 2, it can be seen that when the saturation temperature exceeds 1200 ° C., the half width of the solid solution film is significantly deteriorated.
[0087]
Further, as can be seen from Table 1, by adopting a temperature schedule as shown in FIG. 1, no matter how the charged composition of the melt changes, there is a holding stage to precipitate the solid phase once, Film formation is possible in a very wide temperature range. And about each preparation composition of a melt, the said half value width of a solid solution film | membrane is very favorable in all.
[0088]
(Experiment 2)
In the same manner as in Experiment 1, a solid solution film was formed by a liquid phase epitaxial method. However, holding temperature T ₂ The composition of the liquid phase part of the melt was changed as shown in Tables 3 and 4. In order to make the influence of the degree of supercooling constant, the degree of supercooling was unified at 15 ° C. The composition x of the obtained solid solution film and the full width at half maximum were measured and displayed in Tables 3 and 4. In addition, about these examples, it aims at changing the composition of the liquid phase part of a melt, and various preparation compositions of the melt are changed for this purpose.
[0089]
[Table 3]

[0090]
[Table 4]

[0091]
In each of the conditions employed in Tables 3 and 4, the saturation temperature is 1160 ° C. or less, the degree of supercooling is 15 ° C., and there is no problem in view of the results in Table 1. LiTaO _Three The ratio is also in a range where a sufficiently high-quality solid solution film should be able to be formed as seen from each example in Table 1.
[0092]
In fact, when the ratio of the solvent in the composition of the liquid phase portion is in the range of 90 to 60%, a solid solution film having the half width smaller than that of the lithium niobate single crystal substrate can be formed. However, as shown in Table 4, when the ratio of the solvent is 95% or more, the half width is remarkably and rapidly deteriorated. In addition, when the proportion of the solvent is less than 60%, the half width is remarkably and rapidly deteriorated.
[0093]
(Experiment 3)
A solid solution film was formed in the same manner as in Experiment 1. However, holding temperature T ₂ The composition of the liquid phase part of the melt was changed as shown in Table 5. However, the saturation temperature was kept within the allowable range of 1200 ° C. or less as seen from the results in Table 1. Further, the ratio of the solvent was maintained at 85%, which is satisfactory from the results of Experiment 1 and Experiment 2.
[0094]
And LiNbO _Three LiTaO for the number of moles _Three The ratio of the number of moles was variously changed as shown in Table 5. And about each composition, the supercooling degree in which film-forming was possible was measured. The results are shown in Table 5.
[0095]
[Table 5]

[0096]
As can be seen from Table 5, LiNbO _Three Number of moles: LiTaO _Three When the number of moles was 12: 3, film formation was possible within a very wide range. When this was 10: 5, a minimum degree of supercooling of 15 ° C. was required, but still a good film was obtained. When this was 8: 7, a degree of supercooling of about 30 ° C. was required. Furthermore, when this is 6: 9, a degree of supercooling of about 45 ° C. or higher is required, and the film forming conditions have been very severely limited. This was the limit of film formation.
[0097]
(Experiment 4)
A single crystal substrate product 13 of FIG. 9 was prepared by the same method as in Experiment 1.
However, the charge composition of the melt is Li ₂ O: V ₂ O _Five : Nb ₂ O _Five : Ta ₂ O _Five = 50: 36: 9: 5, saturation temperature (holding temperature) T of the liquid phase part ₂ Was 970 ° C., the degree of supercooling ΔT was 30 ° C., and the film formation time was 10 minutes. Under this condition, the first solid solution film 10 was formed. This film thickness was 20 μm, and the composition of the solid solution film was x = 0.35. At a wavelength of 633 nm, the solid light refractive index of the solid solution film 10 was 2.248, and the extraordinary light refractive index was 2.194.
[0098]
Next, charge composition of the melt is set to Li ₂ O: V ₂ O _Five : Nb ₂ O _Five = 50: 40: 10, saturation temperature (holding temperature) T of the liquid phase part ₂ Was 910 ° C., the degree of supercooling ΔT was 5 ° C., and the film formation time was 8 minutes. Under this condition, the second layer 11 was formed on the solid solution film 10. The film thickness of the second layer 11 is 8 μm, and the composition of the second layer is LiNbO. _Three Met. At the wavelength of 633 nm, the ordinary light refractive index of the second layer 11 was 2.286, and the extraordinary light refractive index was 2.193.
[0099]
The second layer 11 can be sufficiently used as an optical waveguide because of the refractive index. Then, when the optical damage resistance characteristic of the optical waveguide 11 was actually measured through laser light having a wavelength of 830 nm, no optical damage was observed even at an output light power of 2 mW.
[0100]
(Experiment 5)
A single crystal substrate product 13 of FIG. 9 was prepared by the same method as in Experiment 4.
However, the charge composition of the melt is Li ₂ O: V ₂ O _Five : Nb ₂ O _Five : Ta ₂ O _Five = 50: 40: 5: 5, saturation temperature T of the liquid phase part ₂ Was 1015 ° C., the degree of supercooling ΔT was 40 ° C., and the film formation time was 15 minutes. Under this condition, the first solid solution film 10 was formed. The film thickness was 18 μm and the composition of the solid solution film was x = 0.63. At a wavelength of 633 nm, the solid light refractive index of the solid solution film 10 was 2.217, and the extraordinary light refractive index was 2.189.
[0101]
Next, charge composition of the melt is set to Li ₂ O: V ₂ O _Five : Nb ₂ O _Five : Ta ₂ O = 50: 42.5: 5: 2.5, saturation temperature T of the liquid phase part ₂ Was 950 ° C., the degree of supercooling ΔT was 20 ° C., and the film formation time was 5 minutes. Under this condition, the second layer 11 was formed on the solid solution film 10. The film thickness of the second layer 11 was 5 μm. At the wavelength of 633 nm, the ordinary light refractive index of the second layer 11 was 2.263, and the extraordinary light refractive index was 2.196.
[0102]
The second layer 11 can be sufficiently used as an optical waveguide because of the refractive index. And when the optical damage resistance characteristic of this optical waveguide 11 was actually measured through the laser beam with a wavelength of 830 nm, no optical damage was recognized even at the output light power of 2 mW.
[0103]
(Experiment 6)
The relationship between the deposition temperature and the substitution ratio of tantalum in the solid solution film was examined.
The film was formed by liquid phase epitaxy according to the temperature schedule of FIG. LiNbO _Three ─LiTaO _Three ─LiVO _Three A solid solution film was formed on a lithium niobate single crystal substrate using a pseudo ternary system.
[0104]
The melt is heated to a sufficiently high temperature T ₁ The mixture was stirred at (1000 ° C to 1300 ° C) for 3 hours or more to obtain a sufficiently uniform liquid phase. Thereafter, the melt is held at a holding temperature T ₂ And cooled to 12 hours or longer. Thereafter, the temperature of the melt 3 is changed to T ₂ Deposition temperature T lower than the degree of supercooling ΔT _Three Then, the lithium niobate single crystal substrate 6 was immediately brought into contact with the liquid phase portion 4 to form a film.
[0105]
However, the charge composition of the melt is LiNbO in molar ratio. _Three : LiTaO _Three = 80:20, the saturation temperature T of the melt ₀ Is the deposition temperature T _Three LiVO to be about 100 ° C. higher than _Three The ratio of was adjusted. The degree of supercooling ΔT was set to 10 ° C. Deposition temperature T _Three As shown in Table 6 and FIG. 11, various changes were made to produce a solid solution film, and the tantalum ratio x in the solid solution film was measured. The measurement results are shown in Table 6 and FIG.
[0106]
[Table 6]

[0107]
Thus, the film formation temperature T _Three And the replacement ratio of tantalum has been found to have a very linear correspondence. Therefore, if this relationship is utilized, the film formation temperature T _Three It was found that the tantalum substitution ratio x in the solid solution film can be controlled and changed with high precision if the value is set accurately.
[0108]
Further, when the film formation temperature exceeded 1250 ° C., it was impossible to form a solid solution film. However, since the melting point of the lithium niobate single crystal substrate was about 1250 ° C., the solid solution film was formed at a temperature exceeding 1250 ° C. This is because a film cannot be formed. From this point of view, it is preferable to adopt a film forming temperature of 1230 ° C. or less, and x of the solid solution film at this time is about 0.8.
[0109]
Also, the film formation temperature T _Three However, it has been found that it becomes difficult to contain tantalum in the film, particularly when the film forming temperature is less than about 750 ° C. Further, when the solid solution film is formed, the temperature of the supercooled liquid phase portion is set to about 800 ° C. or higher and 1150 ° C. or lower, thereby changing x in the range of 0.1 or higher and 0.6 or lower. You can also see that you can.
[0110]
As described above, the technique for freely controlling the ratio x of tantalum in the solid solution film was first discovered by the present inventor. In producing single crystal substrate products having characteristics according to various uses. Clearly it is very useful.
[0111]
(Experiment 7)
About the solid solution film | membrane which has each composition manufactured in Experiment 6, the half value width of the X-ray rocking curve was measured. In addition, each of these solid solution films was irradiated with an argon laser having a wavelength of 514.5 nm, and a threshold value causing optical damage was measured. These results are shown in Table 7. As a comparative example, a lithium niobate single crystal thin film was formed on a lithium tantalate single crystal substrate, and the above measurement was performed on this film.
[0112]
[Table 7]

[0113]
When the film formation temperature is 1150 ° C. or lower, a solid solution film having very good crystallinity can be obtained, whereas when the film formation temperature exceeds 1150 ° C., the crystallinity of the solid solution film is deteriorated. However, in this case, crystallinity equal to or higher than that obtained when a lithium niobate single crystal thin film is formed on a lithium tantalate single crystal substrate is obtained. The reason why the crystallinity of the solid solution film is improved by setting the film formation temperature to 1150 ° C. or less is not clear, but since the Curie temperature of lithium niobate used as the substrate is about 1150 ° C., 1150 ° C. If the film is formed at a temperature higher than that, the substrate will be divided into multiple regions, and the crystallinity will deteriorate.
[0114]
(Experiment 8)
The technique for precisely controlling the tantalum ratio x in the solid solution film by selecting the film formation temperature of the solid solution film as described above is extremely useful particularly when a single mode optical waveguide is manufactured. This is because in a single-mode optical waveguide, even if the wavelength of the guided light is set to a constant value and the thickness of the optical waveguide can be controlled accurately, it is necessary to further control the refractive index of the optical waveguide. Yes, this tolerance is generally very narrow.
[0115]
However, even if the tantalum ratio in the solid solution film slightly varies, the refractive index of the optical waveguide varies considerably, and the light propagation loss increases remarkably. For this reason, the present inventor has studied a manufacturing method for maintaining the high propagation characteristics of the single mode optical waveguide based on the above-described findings.
[0116]
20 mol% LiNb _0.8 Ta _0.2 O _Three ─80mol% LiVO _Three A solid solution film was formed on a lithium niobate single crystal substrate in the same manner as in Experiment 6 using a pseudo ternary system. The degree of supercooling ΔT was set to 10 ° C. Deposition temperature T _Three As shown in Table 8, the temperature was changed within the range of 900 ° C. to 940 ° C., the solid solution film (thickness 20 μm) of each example was manufactured, and the ratio x of tantalum in the solid solution film was measured.
[0117]
Furthermore, an optical waveguide was formed on each solid solution film under the same conditions by the proton exchange method. At this time, an aluminum mask pattern having a width of 2 μm was formed on the surface of each solid solution film by a normal photolithography method, and this single crystal substrate product was immersed in benzoic acid at 200 ° C. for 15 minutes. The mask was removed by etching, and the optical waveguide was annealed at 350 ° C. for 4 hours to form a proton exchange optical waveguide. Light with a wavelength of 830 nm was incident on the optical waveguide of each example, and the propagation loss was measured. Table 8 shows the measurement results.
[0118]
[Table 8]

[0119]
As can be seen from this result, the propagation loss is suppressed when x is in the range of 0.246 to 0.291. In particular, x is within a very narrow range of 0.253 to 0.272, that is, as the range of x. Is within a very narrow range of 0.02, the propagation loss of the optical waveguide is minimized. Thus, it can be seen that in the single mode optical waveguide, the allowable range of the tantalum ratio x in the lithium niobate-lithium tantalate solid solution film is extremely small.
[0120]
At the same time, the film formation temperature T _Three The ratio x of tantalum and x corresponds to the linearity very precisely, and the film-forming temperature T _Three By setting the value in the range of 904 ° C. to 932 ° C., more preferably in the range of 908 ° C. to 920 ° C., the propagation loss of the optical waveguide could be suppressed to the minimum.
[0121]
Thus, according to the present invention, the film formation temperature T _Three By setting the allowable range of about 10 ° C. or less, it becomes possible to mass-produce single mode optical waveguides with low propagation loss.
[0122]
(Experiment 9)
According to the temperature schedule of FIG. 1, LiNbO _Three ─LiTaO _Three ─LiVO _Three A solid solution film was formed on a lithium niobate single crystal substrate using a pseudo ternary system. The melt is heated to a sufficiently high temperature T ₁ The mixture was stirred at (1000 ° C to 1300 ° C) for 3 hours or more to obtain a sufficiently uniform liquid phase. Thereafter, the melt is held at a holding temperature T ₂ And cooled to 12 hours or longer. Thereafter, the temperature of the melt 3 is changed to T ₂ Deposition temperature T lower than the degree of supercooling ΔT _Three Then, the lithium niobate single crystal substrate 6 was immediately brought into contact with the liquid phase portion 4 to form a film.
[0123]
However, the charge composition of the melt is LiNbO in molar ratio. _Three : LiTaO _Three = 80:20, the saturation temperature T of the melt ₀ Is the deposition temperature T _Three LiVO to be about 100 ° C. higher than _Three The ratio of was adjusted. The degree of supercooling ΔT was set to 10 ° C. Deposition temperature T _Three Were changed to produce a solid solution film having a tantalum substitution ratio (x) as shown in FIG. 12 and Table 9.
[0124]
Platinum electrodes were placed on both sides of the single crystal substrate, the single crystal substrate was placed in an electric furnace, heated at 600 ° C., and then subjected to a single domain treatment by applying a DC voltage. The phase matching wavelength of the solid solution film having each composition was measured and shown in Table 9 and FIG.
[0125]
[Table 9]

[0126]
As can be seen from the results of Experiment 6 and the like, when the solid solution film is formed, the temperature of the liquid phase portion in the supercooled state is set to 750 ° C. or higher and 1250 ° C. or lower, so that the substitution ratio x of tantalum is 0 to 0. .8, and by changing the temperature of the liquid phase portion in the supercooled state to 800 ° C. or more and 1150 ° C. or less, x can be changed within the range of 0.1 or more and 0.6 or less. Can be made. As can be seen from the results of Table 9 and FIG. 12, the phase matching wavelength is controlled within the range of 1.0 to 2.5 μm by changing x within the range of 0.0 to 0.8. Can do. Furthermore, the phase matching wavelength can be controlled within the range of 1.0 to 1.6 μm by changing x within the range of 0.0 to 0.6.
[0127]
As described above, since the phase matching wavelength of the second harmonic can be controlled within the above range by changing the substitution amount of tantalum in the solid solution film, the element of the present invention is extremely useful as an SHG element. . The second layer is also a lithium niobate-lithium tantalate solid solution film, and the same result can be obtained for the tantalum substitution ratio y.
[0128]
(Experiment 10)
Using the single crystal substrate product 13 of FIG. 9 manufactured in Experiment 4, a second harmonic generation experiment was performed. That is, the film thickness of the solid solution film 10 was 20 μm, and the composition was x = 0.35. The film thickness of the second layer 11 is 8 μm and the composition is LiNbO. _Three Met.
[0129]
A YAG laser (output 100 mW) 14 having a wavelength of 1064 nm is coupled to the end face of the single crystal substrate product 13, and the YAG laser is incident on one end face of the second layer 11 in the TE mode. To generate a second harmonic having a wavelength of 532 nm. As a result, an output of about 15 mW was obtained in the single mode. In this output, no optical damage was observed in the second layer 11 and the solid solution film 10. As described above, the device of the present invention was extremely excellent as a second harmonic generation element.
[0130]
(Experiment 11)
Using the single crystal substrate product 13 of FIG. 9 manufactured in Experiment 5, a second harmonic generation experiment was performed. That is, the film thickness of the solid solution film 10 of the first layer was 18 μm, and the composition was x = 0.63. The film thickness of the second layer 11 was 5 μm, and the composition was y = 0.42.
[0131]
A YAG laser (output: 50 mW) 14 having a wavelength of 1300 nm is coupled to the end face of the single crystal substrate product 13, and the YAG laser is incident on one end face of the second layer 11 in the TE mode. To generate a second harmonic wave having a wavelength of 650 nm. As a result, an output of about 8 mW was obtained in the single mode. In this output, no optical damage was observed in the second layer 11 and the solid solution film 10. Thus, the device of the present invention was extremely excellent as a second harmonic generation element.
[0132]
(Experiment 12)
In accordance with the temperature schedule of FIG. 1 described above, an epitaxial film made of a lithium niobate-lithium tantalate solid solution on the Z-plane of an optical grade lithium niobate single crystal substrate by a liquid phase epitaxial method according to the same method as in Experiment 1. Formed. Mainly Li ₂ O, Nb ₂ O _Five , Ta ₂ O _Five , V ₂ O _Five A melt consisting of The charged composition of the melt is LiNb in molar ratio. _1-x Ta _x O _Three And LiVO _Three The above-mentioned components were mixed so that the ratio of to was 20:80.
[0133]
Li ₂ By changing the charged amount of O, the content ratio of lithium in the melt / (total of the content ratio of tantalum and the content ratio of niobium) is changed, whereby the content ratio of lithium in the solid solution film to be produced / (Total of tantalum content and niobium content) was controlled.
[0134]
In this experimental example, as shown in Table 10, the tantalum substitution ratio x was changed, and the lithium content ratio / (total of the tantalum content ratio and the niobium content ratio) is a stoichiometric composition ratio. Control to .00. The optical damage resistance property of the film was measured by irradiating an argon laser having a wavelength of 488 nm perpendicular to the cross section of the formed film and observing the change in the beam pattern of the emitted light. Table 10 shows the threshold values for optical damage obtained by this measurement.
[0135]
[Table 10]

[0136]
As can be seen from this result, when the lithium content ratio / (total of the tantalum content ratio and the niobium content ratio) in the film is stoichiometric, the tantalum substitution ratio x is less than 0.05. Or exceeding 0.70, the threshold of photodamage is remarkably reduced, and by setting x to 0.10 or more and 0.60 or less, 1000 W / cm ² The above threshold values were obtained, and it was confirmed that they could be used favorably as practical optical elements.
[0137]
(Experiment 13)
In the same manner as in Experiment 12, an epitaxial film made of a lithium niobate-lithium tantalate solid solution was formed on the Z plane of an optical grade lithium niobate single crystal substrate. LiO ₂ The content ratio of lithium in the melt / (total of the content ratio of tantalum and the content ratio of niobium) is changed by changing the charging amount of lithium. The total content of tantalum and niobium) was controlled.
[0138]
In this experimental example, as shown in Table 11, the tantalum substitution ratio x is fixed at 0.3, and the lithium content ratio / (total of the tantalum content ratio and the niobium content ratio) is shown in Table 11. Was changed. For each example film, the threshold for photodamage was measured as in Experiment 12.
[0139]
[Table 11]

[0140]
As can be seen from this result, by setting the lithium content ratio / (total of the tantalum content ratio and the niobium content ratio) in the film to be 0.98 or more and 1.02 or less, the threshold of photodamage is reduced. Increased significantly, 1000 W / cm ² The above threshold values were obtained, and it was confirmed that they could be used favorably as practical optical elements. The reason why such a remarkable effect is obtained is not clear, but it seems to be because the crystal has high integrity because it is close to the stoichiometric composition.
[0141]
【The invention's effect】
As described above, the present invention provides a novel single crystal substrate product having a lithium niobate-lithium tantalate solid solution film having a specific composition on a lithium niobate single crystal substrate.
[Brief description of the drawings]
FIG. 1 is a graph schematically showing a temperature schedule of a melt in a method in which a solid phase and a liquid phase coexist in a liquid phase epitaxial method.
FIGS. 2A and 2B are cross-sectional views schematically showing the state of a melt in the crucible 1. FIGS.
FIG. 3 is a graph schematically showing another example of a temperature schedule of a melt in a method in which a solid phase and a liquid phase coexist in a liquid phase epitaxial method.
FIG. 4 LiNb _1-x Ta _x O _Three ─LiVO _Three It is a phase diagram of a pseudo binary system composition.
FIG. 5 shows the phase diagram shown in FIG. 4 as LiNbO. _Three ─LiTaO _Three ─LiVO _Three It is a triangle figure re-displayed about the pseudo ternary system composition of.
FIG. 6 shows LiNbO, which displays the range of the composition of the melt capable of producing a solid solution film in the present invention. _Three ─LiTaO _Three ─LiVO _Three It is a triangle figure about the pseudo ternary system composition of.
7 (a), (b) and (c) show LiTaO in the pseudo ternary composition described above. _Three 5 is a graph schematically showing the relationship between the ratio of the above, the elution rate of lithium niobate from the substrate, and the deposition rate of the solid solution film on the substrate.
FIG. 8 is a front view schematically showing a single crystal substrate product 12 having a solid solution film 10 on a lithium niobate single crystal substrate 6;
FIG. 9 is a front view schematically showing a single crystal substrate product 13 having a lithium niobate single crystal substrate 6, a solid solution film 10, and a second phase that functions as an optical waveguide.
10A is a schematic diagram showing a second harmonic generation element 18 using a single crystal substrate product 13 of the present invention, and FIG. 10B is a single crystal substrate provided with a rib-type optical waveguide. It is a perspective view which shows the goods 19, (c) is a schematic diagram which shows the 2nd harmonic generation element 25 using the single crystal substrate goods 19. FIG.
FIG. 11 shows film formation temperature T _Three It is a graph which shows the relationship between the ratio x of tantalum.
FIG. 12 is a graph showing the relationship between the tantalum substitution ratio x (or y) in the lithium niobate-lithium tantalate solid solution film and the phase matching wavelength.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Crucible, 2 The melt which melt | dissolved uniformly, 3 The melt which the solid phase part and the liquid phase part coexist, 4 Liquid phase part, 5 Solid phase part, 6 Lithium niobate single crystal substrate, 10 Solid solution Film, 11 second layer, 12, 13 single crystal substrate product, point indicating the composition of A (95, 5, 0), point indicating the composition of B (95, 2, 3), C (60, 40, 0 ), A point indicating the composition of D (60, 0, 40), a point indicating the composition of E (0, 40, 60), a point A (95, 5, 0) and a point C (60 , 40, 0), a straight line connecting H point C (60, 40, 0) and D (60, 0, 40), a curve indicating a composition range at a saturation temperature of 1200 ° C. or lower, J point A straight line connecting B (95, 2, 3) and point E (0, 40, 60), K A straight line connecting point A (95, 5, 0) and point B (95, 2, 3)

Claims (15)

A single crystal substrate having a composition of LiNb _1-z Ta _z O ₃ (0 ≦ z <0.8) and a lithium niobate-lithium tantalate solid solution film formed on the single crystal substrate by a liquid phase epitaxial method Wherein the composition of the lithium niobate-lithium tantalate solid solution film is LiNb _1-x Ta _x O ₃ (0 <x ≦ 0.8), and x and z are one or more metals selected from the group consisting of zinc, scandium, magnesium, indium, titanium and vanadium , satisfying the relationship of z <x, and the single crystal substrate and the lithium niobate-lithium tantalate solid solution film A single crystal substrate product characterized by containing atoms . 2. The single crystal substrate product according to claim 1, wherein x is 0.02 or more. The single crystal substrate product according to claim 1 or 2, wherein the single crystal substrate and the lithium niobate-lithium tantalate solid solution film contain rare earth elements. The composition of the lithium niobate-lithium tantalate solid solution film is LiNb _1-x Ta _x O ₃ (0.05 ≦ x ≦ 0.70), and the lithium content / (tantalum content and niobium content) The single crystal substrate product according to claim 2 , wherein the total number of the single crystal substrate product is 0.98 or more and 1.02 or less. A second layer is further formed on the lithium niobate-lithium tantalate solid solution film, and the composition of the second layer is LiNb _1-y Ta _y O ₃ (0 ≦ y <0.8), The single crystal substrate product according to claim 1, wherein x and y satisfy a relationship of y <x. The composition of the second layer is LiNb _1-y Ta _y O ₃ (0.05 ≦ y ≦ 0.70), and the lithium content ratio / (total of the tantalum content ratio and the niobium content ratio) is 0. The single crystal substrate product according to claim 5 , wherein the single crystal substrate product is .98 or more and 1.02 or less. An optical component comprising the single crystal substrate product according to any one of claims 1 to 6 as a base material. The optical component according to claim 7 , wherein the optical component is a second harmonic generation element. A single crystal substrate product comprising a single crystal substrate and a single crystal film epitaxially grown on the single crystal substrate, wherein the composition of the single crystal film is LiNb _1-x Ta _x O ₃ ( 0 < x ≦ 0 a .8), the rather smaller than the half width of the X-ray rocking curve half width of the single crystal substrate of X-ray rocking curve of the single crystal film, the single crystal substrate and the single crystal film, zinc, scandium A single crystal substrate product comprising at least one metal atom selected from the group consisting of magnesium, indium, titanium and vanadium . The single crystal substrate product according to claim 9 , wherein the composition of the single crystal substrate is LiNb _1−z Ta _z O ₃ (0 ≦ z ≦ 0.8). A single crystal substrate having a composition of LiNb _1-z Ta _z O ₃ (0 ≦ z ≦ 0.8), and a lithium niobate-lithium tantalate solid solution film formed on the single crystal substrate by a liquid phase epitaxial method A liquid crystal part and a solid phase part coexist in the melt, and then the temperature of the melt is lowered to supercool the liquid phase part. And the epitaxial growth of the lithium niobate-lithium tantalate solid solution film by bringing the single crystal substrate into contact with the liquid phase portion, wherein the single crystal substrate is in a supercooled state of the melt. When contacting the liquid phase part, the melt is mainly composed of Li ₂ O, Nb ₂ O ₅ , Ta ₂ O ₅ and a flux, and the composition of the liquid phase part is LiNbO ₃ —LiTaO ₃ —soluble. In the triangular diagram of the melting medium pseudo-ternary system, a straight line connecting A (95, 5, 0) and B (95, 2, 3), A (95, 5, 0) and C (60, 40, 0) , A straight line connecting C (60, 40, 0) and D (60, 0, 40), a straight line connecting B (95, 2, 3) and E (0, 40, 60), and saturation A method for producing a single crystal substrate product, wherein the composition ratio is within a region surrounded by a curve satisfying a temperature of 1200 ° C. or less. And the temperature at which said liquid phase and a solid phase coexist, characterized by the following 50 ° C the difference between the temperature at the time of the liquid phase in the supercooled state, according to claim 11, wherein Manufacturing method of single crystal substrate product. When the lithium niobate-lithium tantalate solid solution film is formed, the temperature of the liquid phase portion in the supercooled state is set to 750 ° C. or more and 1250 ° C. or less, whereby the lithium niobate-lithium tantalate solid solution film 13. The single crystal according to claim 11 , wherein x is changed in a range of 0.8 or less when the composition is LiNb _1-x Ta _x O ₃ (0 <x ≦ 0.8). A method for manufacturing a substrate product. When forming the lithium niobate-lithium tantalate solid solution film, the temperature of the liquid phase portion in the supercooled state is set to 800 ° C. or more and 1150 ° C. or less, whereby x is 0.1 or more and 0.6 or less. The method for producing a single crystal substrate product according to claim 11 , wherein the single crystal substrate product is varied in a range of A method for producing an optical component, wherein the single crystal substrate product according to any one of claims 11 to 14 is used as a base material for the optical component.

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