JP3788037B2 - GaN single crystal substrate - Google Patents

Description

【００４５】
によって与えられる。σは内部応力、Ｅは剛性率、νはポアソン比、ｂは基板の厚さ、ｄは薄膜厚さ、Ｉは基板直径、δは撓み（Ｈに当たる）である。Ｉ＝５０ｍｍとした場合は、上の定義でδ＝Ｈに当たる。これは薄膜の内部応力を撓みから計算するＳｔｏｎｅｙの式という。薄膜だけにしてしまうので（ＧａＮ単層であるから）ｄ＝ｂとして、
【００４６】
【数２】

【００４７】
この式によって、撓みδからσを計算した。反りと、曲率半径と内部応力の関係はつぎのようである。基板厚さが一定の時、内部応力が大きくなればなるほど、反りは大きく、曲率半径は小さくなる。内部応力が一定の場合、基板厚さが厚くなればなるほど、反りは低減し、曲率半径は大きくなる。本発明者らによるＧａＮ基板については、基板上へのデバイスプロセスの容易さ、基板強度を勘案し、反り、曲率半径、内部応力の許容範囲を検討した。
【００４８】
ウエハの厚さによって適当な値が変わるのであるが、一般的にいうと、
１．曲率半径Ｒ ６００ｍｍ以上（曲率が１．６７×１０^-3ｍｍ^-1以下）
２．反りＨ（５０ｍｍ直径で） ０．５５ｍｍ以下
３．内部応力σ ７ＭＰａ以下
【００４９】
つまり、本発明者がウエハに課した条件は、Ｒ≧６００ｍｍ、Ｈ≦０．５５ｍｍ、σ≦７ＭＰａである。さらに内部応力は３ＭＰａであるとより好ましい。曲率半径は７５０ｍｍ以上であるとさらに良い。
【００５０】
本発明において、１枚のＧａＮ基板を製造する方法の他に、ＧａＮ基板を種結晶としてその上にＧａＮを厚くエピタキシャル成長させ単結晶インゴットを製造する方法も採用している。その場合は厚みを１０ミリ以上にして、数十枚のウエハを切り出すようにする。インゴットが厚いから反りは小さい。反りが少ないから精度良くスライスできる。厚さが大きいので低転位化が進んでいる。スライスして切り出したウエハも低転位である。そのため反りも少ない。
【００５１】
【実施例】
［実施例１（ＨＶＰＥ法ラテラル成長によるＧａＮ単結晶１枚の作製）］
ＧａＡｓ（１１１）Ａ基板を反応容器内に設置した。基板サイズは３０ｍｍ径の円形基板とした。常圧（大気圧）ＣＶＤ装置でＧａＡｓ基板上に、Ｓｉ_３Ｎ_４層を厚さ０．１μｍになるように形成した。これに規則的な分布をする窓をフォトリソグラフィによって開けた。窓は３種類のものを採用した。図３に示す千鳥ドット窓と、＜１１−２＞ストライプ窓、＜１−１０＞ストライプ窓の３種である。
１．千鳥ドット窓…図３〜５に示すように、ＧａＡｓ＜１１−２＞に平行な直線上に並び隣接する窓群が半ピッチずれている。ｄ＝３．５μｍ、Ｌ＝４μｍ。
２．＜１１−２＞ストライプ窓…＜１１−２＞方向に平行な長窓（ストライプ）のマスク。ストライプの幅が２μｍ、間隔２μｍ、ピッチ４μｍ。
３．＜１−１０＞ストライプ窓…＜１１−２＞方向に平行な長窓（ストライプ）のマスク。ストライプの幅が２μｍで間隔が６μｍ。ピッチは８μｍである。
【００５２】
このような窓を開けたＳｉ_３Ｎ_４をマスクとして使って、ＧａＮバッファ層とエピタキシャル層を成長させる。
（１）ＧａＮバッファ層の形成
周期的な窓を有するマスクによって覆われたＧａＡｓ基板をＨＶＰＥ装置の中に設置した。ＨＶＰＥ装置を真空に引いてＧａＡｓ基板を約５００℃に加熱した。石英のＧａ溜を８５０℃以上に加熱しＧａ融液とする。原料ガス導入口から水素ガスＨ_２と塩化水素ガスＨＣｌの混合ガスをＧａ溜に導き、塩化ガリウムＧａＣｌを合成した。別の原料ガス導入口から水素Ｈ_２とアンモニアＮＨ_３の混合ガスを導入し、５００℃に加熱された基板近傍でＧａＣｌ＋ＮＨ_３→ＧａＮの反応を起こさせＧａＡｓ基板に、ＧａＮを堆積させる。これによってＧａＡｓ基板上に約７０ｎｍのＧａＮバッファ層を形成する。Ｓｉ_３Ｎ_４はＧａＮ成長抑制作用がありＳｉ_３Ｎ_４マスクの上にはＧａＮは堆積しない。バッファ層（７０ｎｍ）はマスク（１００ｎｍ）より薄い。だから窓のＧａＡｓの部分だけにＧａＮバッファ層ができる。
【００５３】
（２）ＧａＮエピタキシャル層の形成
ＨＣｌの導入を停止した。基板温度を５００℃から約１０００℃まで上げた。再びＨＣｌをＧａ溜に向けて導入する。以前の工程と同じように、ＧａとＨＣｌの反応によって塩化ガリウムＧａＣｌを合成する。水素ガスがキャリヤとして流れているからＧａＣｌは下方へともに流れる。アンモニアＮＨ_３とＧａＣｌが加熱された基板の近傍で反応しＧａＮができる。これが窓の中のバッファ層の上にエピタキシャル成長する。マスク厚み（１００ｎｍ）を越えるとマスクの上にＧａＮ結晶が正六角形状に広がって行く。ただしマスク全面がＧａＮで覆われるまでは、ＧａＮ結晶は六角錐である。図４、図５は六角錐の底面部の状況を模式的に示したものである。窓は正三角形の頂点位置にあるからそこから正六角形状に広がったＧａＮは隣接窓から広がってきた結晶と丁度きびすを接することになる。成長速度は等しいので正六角錐の結晶は隈無く接触する。ＧａＮ結晶層がマスクの上面を隈無く覆い尽くすと、今度は上方へＧａＮが堆積してゆく。成長速度は５０μｍ／Ｈである。約１００μｍの厚みのエピタキシャル層を成長させた。このように無数の小さい窓から独立に核発生させ結晶成長させる（ラテラル成長）のでＧａＮの中の内部応力を大幅に低減することができる。表面は擦りガラス状であった。
【００５４】
（３）ＧａＡｓ基板の除去
次に試料をエッチング装置の中に設置した。王水によって約１０時間エッチングした。ＧａＡｓ基板が完全に除去された。ＧａＮだけの結晶になった。両面を研磨してＧａＮ単結晶基板とした。これは自立膜であった。マスクの窓寸法と窓ピッチＬ、隣接列との距離ｄを変えその他はほぼ同じ条件で３つの試料についてＧａＮ成長させた。試料１は千鳥ドット窓（窓２μｍ角、Ｌ＝４μｍ、ｄ＝３．５μｍ）。試料２は＜１１−２＞ストライプマスクである。試料３は＜１−１０＞ストライプマスクである。
【００５５】
【表１】

【００５６】
ノンドープなのであるがｎ型の電子伝導型である。結晶性の維持を考えるとキャリヤ濃度は低い方が良く、電子移動度は高い方が良い。しかし比抵抗は高い方が良いのである。これら電気的特性は成長条件により変化する。ストライプマスクは内部応力低減という点で不完全である。これらサンプルは透明な薄茶色である。波長４００ｎｍ〜６００ｎｍでの吸収係数は、４０ｃｍ^−１〜８０ｃｍ^−１であった。Ａｓの含有量を調べたところ、９×１０^１６ｃｍ^−３であった。
【００５７】
［実施例２（ＨＶＰＥラテラル成長ＧａＮ種結晶、ＨＶＰＥ法ＧａＮ厚付け）］
２インチ径のＧａＡｓ（１１１）Ａ面を基板とした。その上にＳｉＯ_２の絶縁膜を形成した。フォトリソグラフィによって図３のような窓を設けた。
（１）ＧａＮバッファ層の形成
【００５８】
マスクを有するＧａＡｓ基板をＨＶＰＥ装置に設置した。図８の装置を使うが、Ｇａ溜５は８００℃に加熱した。原料ガスとしては、Ｈ_２＋ＨＣｌをＧａ溜に導き、Ｈ_２＋ＮＨ_３は基板に直接に導いた。約５００℃（基板温度）の低温において、ＧａＮバッファ層を形成した。バッファ層厚みは８０ｎｍである。
（２）エピタキシャル層の形成
ついで基板温度を１０００℃に上げた。同じ原料ガスを使って、ＧａＮエピタキシャル層８０μｍを形成した。
（３）ＧａＡｓの除去
ＧａＮ／ＧａＡｓ基板をＨＶＰＥ装置から取りだした。鏡面状にＧａＮ連続膜が生成されていることを確認した。これを王水中でＧａＡｓ基板をエッチング除去した。
【００５９】
（４）ＧａＮの厚付け
これを十分に洗浄した。図６（４）のような状態になる。ＧａＮだけになったものをふたたびＨＶＰＥ装置にセットした。基板温度を１０２０℃として、ＨＶＰＥ法によってＧａＮを厚付けしＧａＮのインゴットを得た。図７（１）に示す状態である。このインゴットは中央部が少し窪んだ形状であった。最低高さは約２０ｍｍ、外径５５ｍｍのインゴットであった。
【００６０】
（５）スライサーによるウエハ−の切り出し
円周刃スライサーによってインゴットを軸方向に直角な方向に切りだした。図７（２）に示すようである。外径約５０ｍｍ、厚み３５０μｍのＧａＮ単結晶基板２０枚を得た。ＧａＮを分析したところ、Ａｓ、炭素ともにバックグランドのレベルであった。ひ素（Ａｓ）、炭素が、ＧａＮのなかに殆ど含有されていない事が分かる。
【００６１】
（６）研磨
さらにラッピング研磨、仕上げ研磨をした。図７（３）のような透明ウエハ−である。機械加工をしているため基板には反りは無かった。
【００６２】
（７）電気的特性の測定
インゴットの上端（成長終期の分）から取ったウエハ−の電気的特性を測定した。ｎ型でキャリヤ濃度は５×１０^１８ｃｍ^−３であった。電子移動度は２００ｃｍ^２／Ｖｓであった。比抵抗は０．０１７Ωｃｍであった。
【００６３】
インゴットの下端（成長初期の分）から取ったウエハ−の電気的特性はつぎのようであった。ｎ型でキャリヤ濃度は１０^１８ｃｍ^−３、電子移動度は１５０ｃｍ^２／Ｖｓであった。比抵抗は０．０１Ωｃｍであった。これは両極端の部位の電気的特性である。中間部は中間的な値になるであろう。
【００６４】
（８）光吸収の測定
これらのウエハは透明であり暗灰色か無色であった。波長４００ｎｍ〜６００ｎｍにおける吸収係数は２０ｃｍ^−１〜４０ｃｍ^−１であった。
（９）ＬＥＤの作製
ＧａＮ基板ができたので、その上にＩｎＧａＮを発光層とするＬＥＤを作製した。従来のサファイヤ基板のものと比較して、発光輝度が約５倍に向上した。発光輝度が向上した理由は、転位の減少による。従来のサファイヤ基板ＬＥＤでは活性層内に多くの貫通転位が存在していたが、ＧａＮ基板の本発明のＬＥＤは貫通転位が大きく減少しているからである。
【００６５】
［実施例３（ＭＯＣラテラル成長ＧａＮ種結晶、ＨＶＰＥＧａＮ厚付け）］
ＧａＡｓ（１１１）Ｂ面を基板として用いた。ＳｉＯ_２を基板に付けフォトリソグラフィによって［１−１０］方向に延びるストライプ窓を形成した。
【００６６】
（１）ＧａＮバッファ層の形成
有機金属塩化物気相成長法（ＭＯＣ法）によって約４９０℃の低温で基板上に９０ｎｍの厚みのＧａＮバッファ層を形成した。
【００６７】
（２）ＧａＮエピタキシャル層の形成
同じ装置において、基板温度を約９７０℃に上げて、ＧａＮエピタキシャル層を２５μｍの厚さに形成した。
（３）ＧａＡｓ基板の除去
ＭＯＣ装置からＧａＮ／ＧａＡｓ試料を取りだした。鏡面のＧａＮ単結晶が成長していた。ストライプマスクの方向は、ＧａＮの［１１−２０］方向であった。つまりＧａＡｓの［１−１０］方向にＧａＮの［１１−２０］方向が成長するということである。王水によってＧａＡｓ基板を溶解除去した。
【００６８】
（４）ＧａＮの厚付け成長
２５μｍ厚みのＧａＮを種結晶として、ＨＶＰＥ装置にセットした。１０００℃に加熱しＨＶＰＥ法によってＧａＮを厚くエピタキシャル成長させた。円柱状で最低高さが約３センチのＧａＮインゴットを育成した。
【００６９】
（５）内周刃スライサーによるウエハ−切り出し
内周刃スライサーによってインゴットを軸直角方向に４００μｍの厚みに切り出した。２５枚のアズカットウエハ−を切り出すことができた。
【００７０】
（６）研磨
切り出したウエハ−をラッピング研磨、仕上げ研磨した。製品としてのＧａＮ単結晶ウエハ−を得た。
（７）電気特性の測定
ウエハ−の電気的特性を測定した。ｎ型であって、電子移動度は２５０ｃｍ^２／Ｖｓであった。比抵抗は０．０５Ωｃｍであった。
【００７１】
この実施例ではＧａＮ自体を種結晶として、ＧａＮ単結晶を厚く成長させている。厚い単結晶ＧａＮを成長させこれをスライサーで切断しているから一挙に２５枚もの基板が作製できる。製造コストは、１枚１枚ＧａＡｓから成長させる場合に比較して６４％に低下した。基板の製造を低コスト化できる。品質管理も含めた１枚当たりの製造時間も大きく短縮できた。ＧａＮを分析したところ砒素（Ａｓ）、炭素（Ｃ）ともにバックグランドのレベルであった。
【００７２】
マスクの窓は正三角形の頂点にある位置に窓を穿つマスクが最も良い。しかしストライプ（縞状）の窓をもつものであっても良い。それなりの内部応力低減の効果がある。マスク上のラテラル成長によって、結晶内の低欠陥化が進み内部応力が低減される共にＧａＡｓとＧａＮの接触面積が減り内部応力を緩和できる。ために温度変化が大きいにもかかわらず反りの発生を抑制することができる。
【００７３】
［実施例４（総流量と表面モフォロジー・内部応力の関係）］
［１−１００］ストライプマスク、［１１−２］ストライプマスク、ドットマスクを使いＨＶＰＥ法によってＧａＮウエハを作製した。原料ガスはＨ_２＋ＮＨ_３とＨ_２＋ＨＣｌである。原料ガスの総流量を増やすと表面モフォロジーが改善される。しかし内部応力は増える傾向が認められた。
【００７４】
【表２】

【００７５】
これらのうち、イ、ロ、ニ、ホ、ト、チ、の６つはＡグループであり、ハ、ヘ、リ、ヌ、ルの５つはＢグループである。
（Ａ）Ｇａ分圧は１ｋＰａ（１０^-2ａｔｍ）である。９７０℃でバッファ層・マスクの上にＧａＮを１時間成長させ、１０３０℃でさらに３時間ＧａＮを成長させた。合計４時間のエピタキシャル成長である。図１３に白丸○によってその結果をしめす。６個の試料がある。これらは表面は平坦でありモフォロジーは良好である。ところが内部応力は大きい。クラックが発生した試料もある。図１３において横軸は膜厚（μｍ）である。膜厚は５μｍ〜１２０μｍに分布している。縦軸は内部応力（ＧＰａ）である。○の試料は内部応力が２ＭＰａ〜４０ＭＰａである。ほとんどが１０ＭＰａより大きい内部応力を呈する。しかし内部応力は７ＭＰａ以下（７×１０^−３ＧＰａ）が好ましい。
（Ｂ）Ｇａ分圧は２ｋＰａである。９７０℃でバッファ層・マスクの上にＧａＮを６時間エピタキシャル成長させた。試料の数は１０個である。図１３に同じように示す。膜厚は７０μｍ〜３００μｍの間に分布する。ＧａＮ試料の表面は粗い。Ｒｍａｘは約２０μｍである。ＧａＮ基板寸法は２０ｍｍ×２０ｍｍである。表面状態は悪いが内部応力は小さい。内部応力は図１３に黒丸によって示すように１ＭＰａ〜１０ＭＰａである。目標は７ＭＰａ以下であるからこれを充たすことができる。しかし同じ条件であるのに膜厚に広いばらつき（７０μｍ〜３００μｍ）がある。反りの曲率半径ＲはＲ＝７８０ｍｍ〜１５００ｍｍである。
【００７６】
［実施例５（曲率半径の関係）］
前例と同じ（Ａ）の試料６枚と、（Ｂ）５枚の試料について膜厚と曲率半径の関係について調べた。触針法によって反りを評価した。図１４にその結果を示す。横軸は膜厚（μｍ）である。縦軸は曲率半径である。曲率半径は６００ｍｍ以上が好ましい。
（Ａ）９７０℃１時間＋１０３０℃３時間成長の試料Ａは膜厚が薄く表面は平坦であるが反りが大きい。曲率半径は２００ｍｍ以下である。曲率半径の望ましい範囲は６００ｍｍ以上である。６個の試料の全てが目標に達しない。
（Ｂ）９７０℃６時間成長の試料Ｂは膜厚が厚く、表面は粗面化しているが、内部応力が小さく、反りも小さい。５個のＢ試料のすべては６００ｍｍという望ましい範囲をこえている。マスクなし成長では、曲率半径が極めて小さくて反りが大きい。１９７０年代のＧａＡｓ基板の試みが失敗したのはそのような理由にもよる。
【００７７】
［実施例６（研磨）］
試料Ａは研磨に失敗した。試料Ｂのうち、膜厚１５０μｍ、内部応力４ＭＰａ、曲率半径１０３０ｍｍ、Ｒｍａｘ２０μｍの試料について研磨した。研磨により膜厚は８０μｍに減った。曲率半径は研磨後６５０ｍｍに減っている。研磨によって表面粗さはＲｍａｘ７．２ｎｍ、Ｒａ２ｎｍに減少した。研磨は表面を平滑にしているが、反りを増大させる場合もある。
［実施例７（ウエハ内の内部応力の異方性）］
ストライプマスクを使用してＧａＮ成長した場合と、ドットマスクを使用してＧａＮ成長した場合の内部応力を比較した。図１５に示したのは、マスクと内部応力の関係を示すグラフである。
ＧａＡｓウエハ上に、＜１−１０＞方向に伸びるストライプマスクを作製し平均膜厚（膜の厚さはバラツキがあるのでこれを平均化したもの）１２０μｍのＧａＮを成長させたもの、
実施例１と同様のドットマスクを使用し、平均膜厚１６０μｍのＧａＮを成長させたもの、
を使用した。
成長の条件はいずれも、Ｇａ分圧１×１０^−２ａｔｍ（１ｋＰa）、ＮＨ_３分圧０．２４ａｔｍ（２４ｋＰa）、成長時間９７０℃、成長時間２４０分である。成長の後、冷却してＧａＡｓ基板を除去した。ＧａＮ自立膜としＧａＮの内部応力をＸ線によって調べた。その結果を図１５に示す。ドットマスクを使用して製造したＧａＮ基板は、異方性が極めて少なく内部応力も小さい。それに反してストライプマスクを使用して製造したＧａＮ基板は、ストライプ方向とそれに直交する方向に異方性がある。とくにストライプに平行な方向に大きい反りが現れる。垂直方向に比較して１０倍もある。図１５の方位はＧａＮ結晶に関する方位である。ＧａＡｓの＜１−１０＞はＧａＮの＜１１−２０＞に、ＧａＡｓの＜１１−２＞は、ＧａＮの＜１−１００＞に対応する。ＧａＮの＜１１−２０＞と＜１−１００＞は直交する。
【００７８】
【発明の効果】
本発明は大型のＧａＮ単結晶ウエハを提供する。ラテラル成長法によるからＧａＮ結晶中の転位等の欠陥が少ない。炭素を使わない手法によってＧａＮを厚付けするから透明度の高いウエハとなる。ラテラル成長であるから内部応力が小さく抑えられ反りも少ない。反りが僅かでフォトリソグラフィなどの既存のウエハプロセスを使うことができる。このＧａＮ単結晶ウエハをＧａＮ発光素子の基板として利用することができる。薄膜部分と基板が同一であるから反りが出ない。内部応力も小さい。したがって転位密度も少なくなる。発光効率が上がる。ＧａＮＬＥＤ、ＧａＮＬＤの寿命が伸びる。
【図面の簡単な説明】
【図１】ＧａＮ結晶を成長させるための基板材料とＧａＮとの熱膨張係数、格子定数の差を、ｘ、ｙ座標に示すグラフ。
【図２】サファイヤ基板上にＧａＮエピタキシャル成長させた場合に、ＧａＮ膜厚が変化することによって格子定数が滑らかに変化することを示すグラフ。
【図３】千鳥型点状窓マスクをＧａＡｓ（１１１）Ａ面に固定したものの平面図。
【図４】マスク窓から露呈した部分にＧａＮバッファ層をエピタキシャル成長させた状態の平面図。
【図５】ＧａＮをマスク、バッファ層の上にさらにエピタキシャル成長させ隣接窓からの結晶が相会した時に状態を示す平面図。
【図６】ＧａＡｓ基板の上にマスクを載せてＧａＮバッファ層、ＧａＮエピタキシャル層を成長させ、ＧａＡｓ基板をエッチング除去する工程を示す工程図。（１）はＧａＡｓ（１１１）基板上にマスクを形成した工程の図。（２）はマスクによって覆われていない部分にバッファ層を成長させた工程の図。（３）はバッファ層、マスクの上にＧａＮエピタキシャル層を成長させた工程の図。（４）はＧａＡｓ基板を除去しＧａＮの自立膜となった状態を示す図。
【図７】ＧａＮ基板の上にさらにＧａＮを厚く成長させてＧａＮインゴットを作りこれを切断してウエハにする工程を示す図。（１）はＧａＮ基板に厚付けしたＧａＮインゴットの図。（２）はインゴットを内周刃スライサーでアズカットウエハに切断している状況を示す図。（３）は切り出されたウエハの図。
【図８】ＨＶＰＥ装置の概略断面図。
【図９】ＧａＡｓ基板の上にＧａＮを成長させた複合基板が熱応力のために反っている状態を示す断面図。
【図１０】もしも内部応力が０であれば、ＧａＡｓを除去した後のＧａＮは平坦になることを示す断面図。
【図１１】もしもＧａＮ自体のなかに内部応力が存在するならばＧａＡｓを除去しても尚歪みが残ることを示す断面図。
【図１２】 ＧａＮウエハの反りの定義を示す図。５０ｍｍ直径のウエハの中央部の盛り上がりＨによって反りを表現する。
【図１３】ガス流量を（Ａ）１８００ｓｃｃｍ〜９００ｓｃｃｍの一群と（Ｂ）９００ｓｃｃｍの一群について、膜厚と内部応力の測定値の分布を示す図。黒丸が表面粗くてＢ群である。白丸が表面平滑でＡ群である。
【図１４】同じＡ群試料（３０枚）とＢ群試料（９枚）について、膜厚と曲率半径の分布を示す図。黒丸がＢ群、白丸がＡ群である。
【図１５】ストライプマスクとドットマスク使用時の内部応力の異方性を測定した結果をしめすグラフ。
【符号の説明】
１ 反応炉
２ ヒ−タ
３ 原料ガス導入口
４ 原料ガス導入口
５ Ｇａ溜
６ Ｇａ融液
７ サセプタ
８ シャフト
９ ＧａＡｓ基板またはＧａＮ基板
１０ ガス排出口[0001]
The present invention relates to a GaN single crystal substrate for a blue light emitting element such as a light emitting diode (LED) or a laser diode (LD) using a III-V nitride compound semiconductor (GaN-based).
[0002]
[Prior art]
FIG. 1 shows the ratio of the lattice constant to the thermal expansion coefficient of GaN, which can be a substrate for GaN growth. Sapphire (Al ₂ O ₃ ), SiC, Si, GaAs, ZnO, and the like are weighed. A nitride-based semiconductor light-emitting device or a GaN-based light-emitting device has been conventionally produced by epitaxially growing a GaN thin film on a sapphire substrate. A sapphire (Al ₂ O ₃ ) substrate is chemically stable and heat resistant. By forming a buffer layer having a lattice constant different from that of GaN by about 16%, GaN is epitaxially grown thereon. Because of these advantages, a sapphire substrate is used. Even after a thin film such as GaN is attached, the sapphire substrate is still attached, and it is used as an LED or LD. In other words, it is a composite element of sapphire and GaN. This is a practical element, and GaN-based LEDs on a sapphire substrate are commercially available. It is also said that GaN-based LDs will be commercially available soon.
[0003]
Sapphire and GaN have different lattice constants. Nevertheless, practical GaN elements grow on sapphire substrates. This is because the relaxation of the lattice constant occurs smoothly. FIG. 2 is a graph showing the relationship between the film thickness of GaN on sapphire and the change in lattice constant. The lattice constant changes slowly as the film thickness changes. Sapphire is still the most excellent substrate. All currently mass-produced products have a GaN / Al ₂ O ₃ structure. Such a structure is described in the following document, for example.
(1) JP-A-5-183189 (2) JP-A-6-260680
However, there are still problems with sapphire substrates. The defect density of the GaN epitaxial layer on the sapphire substrate is extremely high. Does this come from a lattice mismatch? There is a defect density as high as 10 ⁹ cm ⁻² . It may be said that it is full of defects. Nevertheless, GaN LEDs have a long life. It is a mysterious material. Therefore, it can be said that high-density defects may be a problem in crystallography, but not really a problem.
[0005]
But sapphire has another mechanical difficulty. Sapphire (Al ₂ O ₃ ) is chemically stable and has high hardness. It seems good to be chemically stable, but it is not. GaN remains, and only the substrate cannot be removed by etching. The most troublesome is the lack of cleavage. Hard to it. Dicing when dividing the GaN / sapphire substrate into LED chips is difficult. Because there is no natural cleavage, the blade is pressed and cut. Yield is low due to damage. An example in which sapphire was removed from a GaN / sapphire substrate by polishing was reported.
(3) JJAppl. Phys. Vol. 37, Part 2, No. 3B (1988), ppL309-L312
Still at the laboratory level.
[0006]
In order to easily perform dicing, it was considered to use a material having a cleavage property such as SiC as a substrate. For example, (4) Appl. Phys. Lett. Vol. 71, No. 17 (1997).
Has been proposed. However, there is a problem with SiC. It is chemically stable and the processing temperature for production is 1500 ° C. or higher. It is difficult to manufacture the SiC substrate itself. Therefore, it becomes an expensive substrate, and the cost of the GaN light emitting device becomes high. Actually, SiC is not used as a substrate for a GaN light emitting device. SiC / GaN devices are not manufactured on a mass production scale.
[0007]
In any case, the conventional GaN element is formed by growing GaN on a heterogeneous substrate and does not remove the substrate, so that the sapphire remains attached. It is a composite device.
[0008]
In order to epitaxially grow GaN on the substrate, the substrate must be heated to a high temperature of 1000 ° C. or higher. A gas phase reaction does not occur unless the temperature is high. When the temperature is lowered after growing an epitaxial layer such as GaN, an influence due to the difference in thermal expansion coefficient between the thin film and the substrate appears. The coefficient of thermal expansion is a function of temperature and is not constant. So it's not easy to compare, but a quick comparison is as follows. Assuming that the thermal expansion coefficient of GaN is 1, GaAs has a thermal expansion coefficient of approximately 1.08 times, SiC has a thermal expansion coefficient of 0.87 times, and sapphire has a thermal expansion coefficient of 1.36 times.
[0009]
The first problem due to the difference in the thermal expansion coefficient between the thin film and the substrate is that thermal stress is generated in the GaN thin film and defects or microcracks enter the GaN thin film. The second problem due to the difference in thermal expansion coefficient is that warpage occurs during cooling. The entire wafer is deformed by warping. The third problem is that a large composite GaN substrate is not possible. A composite in which a GaN thin film is placed on a sapphire substrate is not necessarily a GaN substrate. However, since the thermal stress and warpage due to the difference in thermal expansion coefficient between the thin film and the substrate are large, a large composite substrate cannot be obtained. Only a few mm square GaN / sapphire GaN composites have been reported. It is not a size that can be used industrially.
[0010]
In retrospect, attempts to grow GaN using GaAs as a substrate were vigorously promoted in the 1970s. The difference in coefficient of thermal expansion between GaAs and GaN is the smallest of these candidates. Since the difference in thermal expansion coefficient is small, there should be little warpage and defects. But all these brave attempts were unsuccessful. GaN did not attach well to GaAs. A high-quality composite GaN substrate could not be produced because GaN was not a single crystal, the defect density was too high, or GaN was peeled off. The reason is not clear. There may have been several causes. For one thing, a good GaN crystal could not be grown due to the evaporation of As of GaAs at a high temperature during growth and reaction with NH _3, and GaN growth on a GaAs substrate was hardly promising. Is it because the lattice constant is quite different between GaAs and GaN? If the lattice constant of GaN is 1, GaAs is 1.25. About 5/4 is different. The lattice constant of SiC is 1.03 and is close to GaN. The lattice constant of sapphire is 0.86 times that of GaN. In terms of lattice constant, GaAs is farthest from GaN. The GaN / GaAs composite device has already been tried and praised 20 years ago.
[0011]
Only the GaN / sapphire elements still survive today. Therefore, one way of development would be to further refine the sapphire substrate method. Even if the dislocation density is high, the LED has a long life, but if the dislocation density is low, it may have a longer life. And blue LD is still not a satisfactory life. It may be due to the high density of defects. Further attempts are made to grow GaN with lower defects on sapphire substrates.
(5) IEICE Transactions C-II, vol. J81-C-II, p58-64 This is a sapphire substrate with a striped (stripe) mask and a GaN film grown on it. GaN grows from the surfaces separated in the horizontal direction by the vertical stripes (stripes) and eventually coalesces beyond the stripes. It is reported that the defect density is greatly reduced by such stripe growth. If the defect density is reduced, it is one result. However, the stripe growth method on the sapphire substrate is silent against other problems. It is only a growth on sapphire, and the sapphire substrate remains attached. It does not solve the stubborn uncanny problem. Since it is not cleaved, the dicing process is difficult and the yield is poor. Since sapphire remains attached, many dislocations and microcracks are introduced into the GaN single crystal due to the difference in thermal expansion coefficient. Also, warping cannot be ignored. Another problem is that it is unsuitable for wafer processing due to warpage.
[0012]
The difference in coefficient of thermal expansion and the difference in lattice constant will always follow as long as different materials are used. The most ideal substrate is a GaN substrate. After all, it is a GaN substrate. However, there is no wide GaN substrate. A wafer suitable for the semiconductor manufacturing process must have a diameter of 1 inch or more, preferably 2 inches or more. But there is no such a large GaN substrate.
[0013]
For growing large crystals, there are a chocolate ski method and a Bridgman method, both of which solidify the solid from the raw material melt. Large single crystals can be produced because they can start from the melt. However, GaN cannot be melted only by heating. Sublimates and turns into gas. A small amount of GaN can be added to Ga and heated by applying an ultrahigh pressure of tens of thousands of atmospheres to obtain a Ga-GaN melt. However, the space where ultra-high pressure can be achieved is extremely narrow. Large crystals cannot be made in a small space. Manufacturing a large ultra-high pressure device is not practical because it is too costly. Since a method for producing a large crystal cannot be applied, a large GaN crystal has not been formed so far, and a GaN substrate has not existed.
[0014]
The GaN thin film is made by a thin film growth method. These are all reactions from the gas phase to the solid phase. The following four methods are known for growing a GaN thin film on a sapphire substrate.
1. HVPE (Hydride Vapor Phase Epitaxy)
2. MOC method (metallorganic chloride method)
3. MOCVD (metallorganic chemical vapor deposition)
4). Sublimation method [0015]
In the MOC method, an organometallic metal such as trimethylgallium TMG and HCl gas are reacted in a hot wall type furnace to synthesize GaCl once and react with ammonia NH ₃ flowing in the vicinity of the substrate. A GaN thin film is grown thereon. In actuality, organometallic gas and HCl gas are transported using hydrogen as a carrier gas. Since an organic metal is used as a Ga raw material, carbon is mixed into GaN as an impurity. Yellow due to carbon. It is difficult to obtain a colorless and transparent GaN crystal. The carrier concentration (free electrons) increases due to carbon, and the electron mobility decreases. Due to the carbon, the electrical properties also deteriorate. The organometallic chloride vapor phase production method is an excellent method, but still has such drawbacks.
[0016]
The MOCVD method is most frequently used as a GaN thin film growth method. In a cold-wall type reactor, an organometallic metal such as TMG and ammonia NH ₃ are sprayed onto a heated substrate together with hydrogen gas. TMG and NH ₃ react on the substrate to form a GaN thin film. Since this method uses a large amount of gas, the raw material gas yield is low. When a thin layer such as a light emitting layer is formed, it may not be a big problem. However, this problem becomes a serious difficulty when trying to make a thick GaN substrate as in the present application. In addition, there is a problem of carbon contamination as in the MOC method. Color yellow. The electrical characteristics of the HVPE method use metallic Ga as a Ga raw material. A Ga reservoir is provided in a hot wall type reactor and Ga metal is placed therein. Since Ga has a low melting point, it becomes a Ga melt at 30 ° C. or higher. When hydrogen gas or HCl gas is sprayed there, gallium chloride GaCl is produced. GaCl is carried to the vicinity of the substrate by the carrier gas H ₂ and reacts with ammonia to deposit GaN on the substrate surface. This method uses metal Ga and does not contain carbon in the raw material. It does not color because carbon does not enter the thin film. There is an advantage that the electron mobility is not lowered.
[0017]
[Problems to be solved by the invention]
A GaN single crystal is most suitable as a substrate for producing a GaN light emitting device. Large GaN substrates have not existed so far. It is a first object of the present invention to provide a GaN crystal that has a large area that can be used as a substrate and can be self-supporting. It is a second object of the present invention to provide a GaN substrate that does not warp. It is a third object of the present invention to provide a colorless and transparent GaN substrate that does not contain carbon. However, depending on the growth conditions, As having a high vapor pressure may be taken into the GaN single crystal and light brown. It is a fourth object of the present invention to provide a method of manufacturing a plurality of wafers by manufacturing a GaN single crystal ingot and cutting it into thin pieces.
[0018]
[Means for Solving the Problems]
A mask having windows arranged at equal intervals in the [11-2] direction and distributed at equal intervals in the [−110] direction is attached on a GaAs (111) single crystal substrate, and a GaN buffer is formed at a low temperature in the window portion of the mask. The layer is grown, and then the temperature is raised, and the GaN layer is epitaxially grown on the buffer layer and the mask by the HVPE method, and the GaAs substrate is removed to produce a GaN single crystal substrate. This is a method of making one substrate. Alternatively, this single crystal substrate is used as a seed crystal, and a GaN epitaxial layer is formed thickly thereon to form a GaN ingot having a thickness of at least 10 mm, which is cut or cleaved to form a plurality of GaN substrates. This is the GaN substrate manufacturing method of the present invention. The GaAs substrate can be removed by etching with aqua regia. Further, the surface of GaN is polished and smoothed. In this way, a large crystal is produced by using an epitaxial growth method which is a method for producing a thin film.
[0019]
The greatest characteristic of the GaN crystal of the present invention is its size. In the present invention, the diameter of the GaN substrate is 1 inch or more, preferably 2 inches or more. In order to manufacture light emitting elements such as LEDs at low cost industrially, a wider GaN substrate is better. Therefore, it is 20 mm square or more, preferably 1 inch (25 mm) or more, more preferably 2 inches or more. If the starting material GaAs substrate is wide, a large-area GaN crystal can be manufactured.
[0020]
The thickness of the GaN substrate must be such that it can be self-supporting and has no inconvenience in handling. For that purpose, about 50 μm to 1 mm is preferable. Even if it exceeds 1 mm, a semiconductor element can be manufactured, but the price of the wafer itself is increased. If the thickness is 50 μm or less, the shape cannot be maintained. More preferably, the thickness is 70 μm to 1 mm.
[0021]
The HVPE method is adopted so that carbon is not included in the raw material. Since carbon is not contained in GaN, it hardly colors yellow. Carbon does not add electrons as carriers and lower the electron mobility. Since carbon does not enter, GaN becomes a colorless and transparent wafer. Actually, when the GaN wafer of the present invention is placed on the letters, the letters on the base can be seen through. It looks like glass. There is no contamination from raw materials. However, there may be a light yellow, light brown, or dark gray color due to contamination of As or the like evaporated from the GaAs substrate side. However, since the mask layer and the buffer layer prevent evaporation of As from the GaAs substrate, the amount of As mixed into GaN is extremely small. The amount is 10 < ¹⁷ > cm <^-3> or less. If the amount of As is large, the crystallinity of GaN is disturbed. In the case of an LED, As degrades the characteristics. However, according to the HVPE method, there is no fear.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
The GaN production of the present invention starts from a GaAs substrate. Not sapphire. The sapphire substrate cannot be removed later. However, GaAs can be removed over time with aqua regia. As described above, the GaAs substrate was a method that was actively tried in the 1970s and was once abandoned as a prospect. Why do you pick it up now? This is because the present inventor newly found a powerful thin film growth method called a lateral growth method. This is a technique in which a GaAs substrate is covered with a mask having island-shaped windows, and GaN is grown independently from the isolated windows. This technique developed by the present inventors made GaN on a GaAs substrate practically meaningful (Japanese Patent Application No. 10-078333).
[0023]
GaN is hexagonal. The (0001) plane has sixfold symmetry. Since GaAs is cubic, the (100) and (110) planes do not have a three-fold symmetry. Therefore, the GaAs (111) A surface or B surface is used as a substrate. This is a plane orthogonal to an axis having threefold symmetry. The A plane is a plane where Ga atoms are exposed. The B surface is a surface where As atoms are exposed.
[0024]
FIG. 3 shows a part of a mask used for lateral growth. The mask is preferably Si ₃ N ₄ or SiO ₂ that does not directly attach GaN. The mask thickness is 100 nm to several 100 nm. It is a mask having windows at equal intervals. The window is a small square. It is a small window with a diameter of several μm. This may be round, triangular, elliptical, hexagonal, etc. Since it is a fine process, it is easier to process a circle or ellipse. Sequence is important. [11-2] Lined up in a direction. Let L be the interval. The columns adjacent to the [−110] direction perpendicular to the same are shifted by a half pitch. Let d be the distance to the adjacent row. Preferably, d = 3 ^1/2 L / 2. In other words, it is best to arrange windows at the vertices of equilateral triangles. For example, the window may be a square with a side of 2 μm, the window pitch L may be 6 μm, and the column spacing d may be 5 μm. The reason why such an equilateral triangular distribution window is preferable is that GaN grown from adjacent windows simultaneously touches the boundary as shown in FIG. However, d and L may slightly deviate from the above formula.
Further, GaN can be grown even with a mask having a striped window. The use of a striped window has the advantage that the time until the entire mask surface is covered with GaN is short. However, there are still many defects and tend to be warped.
[0025]
The lateral growth method of growing GaN through a mask with a window has the following meaning. Since the mask and GaN are not directly bonded, the underlying GaAs and thin film GaN are bonded only to the window portion. In the case of normal GaN growth, a large number of nuclei are formed on the buffer layer, and they grow by striking each other. In doing so, many defects are introduced. When there is a mask as in the present invention, there is nothing that hinders the portion that protrudes from the mask and grows laterally. It is thought that it grows almost without defects because there is no interference. Since the bonding area is small, thermal stress is relaxed even if the temperature is lowered after growth at a high temperature. The lateral growth layer connected only by the window is much smaller in thermal stress than in the case where the entire area is dense. With that alone, any array distribution window can be used. Instead, it is desirable to have a window distribution in which regular hexagons contact at the same time as shown in FIG. 5 and may grow to a uniform thickness thereafter. 4 and 5 show the shape of the bottom of the hexagonal pyramid.
[0026]
To attach the mask, the entire GaAs substrate is coated with a mask material, and windows are opened at equal intervals by photolithography. The same state is shown in cross section in FIG.
[0027]
Thereafter, a thin GaN buffer layer of about several tens of nm to 100 nm is formed by HVPE at a relatively low temperature of about 450 ° C. to 500 ° C. Since it is thinner than the mask, the buffer layer is isolated in the window. FIG. 6 (2) shows this state.
[0028]
A GaN epitaxial layer is formed by HVPE at a high temperature of about 800 ° C. to 1050 ° C. At this time, the buffer layer crystallizes. As shown in FIG. 4, a GaN crystal nucleated in an isolated window usually forms a hexagonal pyramid. After nucleation, hexagonal pyramids gradually grow in the height direction and the bottom side. The bottom spreads in a hexagonal shape and fills the window. Eventually GaN spreads beyond the mask. It still retains the hexagonal pyramid shape. As shown in FIG. 5, the crystal grows in contact with the crystal from the adjacent window. The size of the substrate crystal is determined by the thickness of the epitaxial growth layer. Since one wafer has a thickness of 70 μm to 1 mm, it is sufficient if the thickness is about that level. This is the state of FIG. Since the growth process as described above is taken, the growth surface is rough and rubbed. To be transparent, it must be polished.
[0029]
Further, the GaAs portion is removed by etching with aqua regia. The mask portion is removed by polishing. The state shown in FIG. This is a single GaN crystal. Transparent and independent. If only one wafer is to be produced, this is the end.
[0030]
Further, if it is desired to manufacture a plurality of wafers, this substrate is used as a seed crystal for further epitaxial growth. FIG. 7 illustrates this. FIG. 7 (1) shows a GaN substrate epitaxially grown thicker by HVPE on a GaN substrate. It becomes a cylinder-shaped GaN ingot. The thickness is 10 mm or more. A support member is fixed to the side surface, and the wafer is cut into wafers one by one with an inner peripheral slicer or the like. FIG. 7 (2) shows this. The as-cut wafer is polished to obtain a transparent and smooth GaN wafer as shown in FIG. In this case, As is not mixed into the GaN crystal.
[0031]
The HVPE method used for epitaxial growth in the present invention will be described with reference to FIG. A cylindrical heater 2 surrounds a vertically long reactor 1. There are source gas inlets 3 and 4 at the top of the reactor 1. A source gas of HCl + H ₂ is introduced from the source gas inlet 3. H ₂ is a carrier gas. Directly there is a Ga reservoir 5. Here, metal Ga is accommodated. Since it has a low melting point, it is heated by the heater 2 to become a Ga melt 6. Since HCl is sprayed onto the Ga melt, a reaction of Ga + HCl → GaCl occurs and gallium chloride GaCl is produced. The mixed gas of GaCl and carrier gas H ₂ is conveyed downward through the space in the reaction furnace. The raw material gas inlet 4 opens further downward. A mixed gas of ammonia NH ₃ + hydrogen H ₂ is introduced into the reactor from here. The reaction of GaCl + NH ₃ → GaN occurs due to GaCl and NH ₃ .
[0032]
The susceptor 7 is provided so as to be rotatable up and down by a shaft 8. On the susceptor 7, a GaAs substrate 9 or a GaN substrate is attached. Since the substrate is heated, the product GaN subjected to the gas phase reaction adheres to the substrate. The exhaust gas is discharged from the exhaust gas outlet 10. The HVPE method uses Ga metal as a raw material. GaCl is then made as an intermediate product. This is a feature.
[0033]
In epitaxial growth, the raw material must be gas, but there is no gas containing Ga. Ga itself is a liquid at 30 ° C. or higher. The MOC method and the MOCVD method use an organic metal to form a gas. Although it is a gas, it contains carbon, so carbon is mixed as an impurity in the GaN crystal. In the HVPE method, Ga is heated to a liquid and reacted with HCl to form GaCl. It is carried as a gas by the dominant hydrogen gas. Since no organic metal is used, there is an advantage that carbon does not enter the crystal as an impurity.
[0034]
A GaN single crystal substrate made according to the present invention is non-doped but n-type. The carrier concentration is about 1 × 10 ¹⁶ cm ⁻³ . The present inventor has found that what gives n-type conductivity is oxygen contained in a trace amount in the source gas. By controlling the oxygen partial pressure in the HVPE furnace, the carrier concentration can be controlled in the range of 1 × 10 ¹⁶ cm ⁻³ to 1 × 10 ²⁰ cm ⁻³ . By controlling the oxygen partial pressure, the electron mobility can be adjusted in the range of 80cm ² / Vs~800cm ² / Vs. The specific resistance can be controlled in the range of 1 × 10 ⁻⁴ Ωcm to 1 × 10 Ωcm. The carrier concentration is preferably in the range of 5 × 10 ¹⁷ cm ⁻³ to 10 ¹⁹ cm ⁻³ , and the specific resistance is more preferably in the range of 5 × 10 ⁻² Ωcm or less.
[0035]
The GaN substrate thus produced has excellent characteristics. wide. It is a self-supporting membrane. It is transparent. It is a property such as being colorless. However, it is yellow, light brown, or dark gray depending on the growth conditions. The substrate for an optical device is required to have little light absorption. Therefore, it is required to be colorless and transparent. This is important as a GaN substrate. The transparency of the substrate is described by the Lambert-Beer rule and is given by
I = I ₀ exp (−αx)
Here, I ₀ is the intensity of the incident light, I is the intensity of the observation light, α is the absorption coefficient, and x is the thickness of the substrate. The smaller α is, the higher the light transmittance and the more transparent. The absorption coefficient of the GaN substrate obtained by the present invention is low over 400 to 600 nm, and the GaN substrate has high transparency to light of these wavelengths. The absorption coefficient was measured for the GaN substrate whose both surfaces were polished. According to the present invention, an absorption coefficient of 120 cm ⁻¹ or less can be easily obtained in this wavelength band. Furthermore, the thing with an absorption coefficient of 80 cm < ^-1 > or less was also producible. As an optical device substrate, the smaller the absorption coefficient, the better. &#9;&#9;&#9;&#9;&#9;
However, that is not enough. There are still more problems. What is it? It is a problem of strain and internal stress. If the internal stress is large, the warpage becomes severe and the wafer process such as photolithography is hindered.
[0036]
GaN is grown on the heated GaAs substrate and cooled to room temperature, and the composite is taken out from the apparatus. Since the coefficient of thermal expansion is different, the strain varies with decreasing temperature. The GaN / GaAs composite is bent as shown in FIG. Stress is generated in GaN. Stress in the opposite direction is also generated in GaAs. There are two types of stress. Thermal stress and intrinsic stress. Thermal stress is generated when there is a temperature change when two different materials having different thermal expansion coefficients are bonded together.
[0037]
If only the thermal stress is present, the thermal stress disappears when the GaAs substrate is removed. Therefore, GaN should be flat as shown in FIG. This is not the case with intrinsic stress. Even when the GaAs substrate is removed, there is still residual stress in the GaN. Therefore, GaN itself is distorted as shown in FIG. This warp appears due to the difference in stress between the front and back surfaces and the gradient of stress in the thickness direction regardless of GaAs.
[0038]
The reason why GaN could not be skillfully grown on a GaAs substrate in the 1970s is also due to the large intrinsic stress. The internal stress including thermal stress was too large, and GaN had a lot of defects and was peeled off. The device for reducing the intrinsic internal stress is actually the lateral growth method using the mask described above. A large number of isolated windows are formed as masks, from which a GaN buffer layer is grown and an epitaxial layer is further grown. We believe that the internal stress is caused by defects such as dislocations. Since they are separated from the dislocations, the portion grown on the mask has a reduced defect, thereby reducing the internal stress.
[0039]
That's fine, but some internal stress still remains. Therefore, the GaN substrate is warped. When warping is large, the wafer process is not affected. The warpage must be evaluated to determine the upper limit of allowable warpage.
[0040]
FIG. 12 shows the definition of the measurement method or expression method of warpage. A central ridge H is measured on a flat table on a wafer of constant diameter. For example, the center lift H is calculated in terms of a 2 inch diameter wafer. H is one measurement method and expression method.
[0041]
The warpage can be defined and expressed by the curvature ξ or the curvature radius R of the curvature of the wafer. It can be converted by R = D ² / 8H or ξ = 8H / D ² . D is the diameter of the wafer. In the case of a 2-inch wafer, D = 50 mm.
[0042]
Since warping or bending is a phenomenon that appears outside, it can be measured directly. Internal stress cannot be easily measured because it is an intrinsic potential.
[0043]
The internal stress when the disk bends with a curvature δ is [0044]
[Expression 1]

[0045]
Given by. σ is the internal stress, E is the rigidity, ν is the Poisson's ratio, b is the thickness of the substrate, d is the thickness of the thin film, I is the substrate diameter, and δ is the deflection (corresponds to H). When I = 50 mm, δ = H in the above definition. This is called Stoney's formula for calculating the internal stress of the thin film from the deflection. Since it is only a thin film (since it is a GaN single layer), d = b,
[0046]
[Expression 2]

[0047]
From this equation, σ was calculated from the deflection δ. The relationship between warpage, radius of curvature and internal stress is as follows. When the substrate thickness is constant, the greater the internal stress, the greater the warp and the smaller the radius of curvature. When the internal stress is constant, the warpage decreases and the radius of curvature increases as the substrate thickness increases. Regarding the GaN substrate by the present inventors, considering the ease of device process on the substrate and the strength of the substrate, the allowable range of warpage, radius of curvature, and internal stress was studied.
[0048]
The appropriate value varies depending on the thickness of the wafer, but generally speaking,
1. Curvature radius R 600 mm or more (curvature is 1.67 × 10 ⁻³ mm ⁻¹ or less)
2. Warpage H (with 50 mm diameter) 0.55 mm or less Internal stress σ 7 MPa or less
That is, the conditions imposed on the wafer by the present inventors are R ≧ 600 mm, H ≦ 0.55 mm, and σ ≦ 7 MPa. Furthermore, the internal stress is more preferably 3 MPa. More preferably, the radius of curvature is 750 mm or more.
[0050]
In the present invention, in addition to a method of manufacturing a single GaN substrate, a method of manufacturing a single crystal ingot by using a GaN substrate as a seed crystal and epitaxially growing GaN thickly thereon is also employed. In that case, the thickness is 10 mm or more, and several tens of wafers are cut out. Warpage is small because the ingot is thick. Slicing with high accuracy because there is little warping. Lower dislocations are progressing because of the large thickness. Wafers sliced and cut out also have low dislocations. Therefore, there is little warp.
[0051]
【Example】
[Example 1 (Preparation of one GaN single crystal by HVPE lateral growth)]
A GaAs (111) A substrate was placed in the reaction vessel. The substrate size was a 30 mm diameter circular substrate. An Si ₃ N ₄ layer was formed to a thickness of 0.1 μm on a GaAs substrate with an atmospheric pressure (atmospheric pressure) CVD apparatus. A window having a regular distribution was opened by photolithography. Three types of windows were used. There are three types: a staggered dot window shown in FIG. 3, a <11-2> stripe window, and a <1-10> stripe window.
1. Staggered dot window: As shown in FIGS. 3 to 5, adjacent windows arranged on a straight line parallel to GaAs <11-2> are shifted by a half pitch. d = 3.5 μm, L = 4 μm.
2. <11-2> Striped window A mask of a long window (striped) parallel to the <11-2> direction. The stripe width is 2 μm, the interval is 2 μm, and the pitch is 4 μm.
3. <1-10> Striped window A mask of a long window (striped) parallel to the <11-2> direction. The stripe width is 2 μm and the interval is 6 μm. The pitch is 8 μm.
[0052]
A GaN buffer layer and an epitaxial layer are grown using Si ₃ N ₄ having such a window as a mask.
(1) Formation of GaN buffer layer A GaAs substrate covered with a mask having a periodic window was placed in an HVPE apparatus. The HVPE apparatus was evacuated to heat the GaAs substrate to about 500 ° C. The quartz Ga reservoir is heated to 850 ° C. or higher to form a Ga melt. A mixed gas of hydrogen gas H ₂ and hydrogen chloride gas HCl was introduced into the Ga reservoir from the source gas inlet, and gallium chloride GaCl was synthesized. A mixed gas of hydrogen H ₂ and ammonia NH ₃ is introduced from another source gas inlet, and a reaction of GaCl + NH ₃ → GaN is caused in the vicinity of the substrate heated to 500 ° C. to deposit GaN on the GaAs substrate. As a result, a GaN buffer layer of about 70 nm is formed on the GaAs substrate. Si ₃ N ₄ has a GaN growth suppressing action, and GaN is not deposited on the Si ₃ N ₄ mask. The buffer layer (70 nm) is thinner than the mask (100 nm). Therefore, a GaN buffer layer is formed only on the GaAs portion of the window.
[0053]
(2) Formation of GaN epitaxial layer The introduction of HCl was stopped. The substrate temperature was increased from 500 ° C. to about 1000 ° C. HCl is again introduced into the Ga reservoir. As in the previous step, gallium chloride GaCl is synthesized by the reaction of Ga and HCl. Since hydrogen gas flows as a carrier, GaCl flows downward. Ammonia NH ₃ and GaCl react in the vicinity of the heated substrate to form GaN. This grows epitaxially on the buffer layer in the window. When the mask thickness (100 nm) is exceeded, the GaN crystal spreads in a regular hexagonal shape on the mask. However, the GaN crystal is a hexagonal pyramid until the entire mask is covered with GaN. 4 and 5 schematically show the state of the bottom surface of the hexagonal pyramid. Since the window is located at the apex position of the equilateral triangle, the GaN spreading in a regular hexagonal shape touches the crystal spreading from the adjacent window exactly. Since the growth rate is equal, the regular hexagonal pyramid crystals are in good contact. When the GaN crystal layer completely covers the upper surface of the mask, GaN is deposited upwards. The growth rate is 50 μm / H. An epitaxial layer having a thickness of about 100 μm was grown. As described above, nucleation is independently generated from a myriad of small windows and crystal growth is performed (lateral growth), so that the internal stress in GaN can be greatly reduced. The surface was frosted glass.
[0054]
(3) Removal of GaAs substrate Next, the sample was placed in an etching apparatus. Etched with aqua regia for about 10 hours. The GaAs substrate was completely removed. It became a crystal only of GaN. Both surfaces were polished to obtain a GaN single crystal substrate. This was a self-supporting membrane. Three samples were grown on GaN under almost the same conditions except that the window size and window pitch L of the mask and the distance d between adjacent rows were changed. Sample 1 is a staggered dot window (window 2 μm square, L = 4 μm, d = 3.5 μm). Sample 2 is a <11-2> stripe mask. Sample 3 is a <1-10> stripe mask.
[0055]
[Table 1]

[0056]
Although it is non-doped, it is an n-type electron conduction type. Considering the maintenance of crystallinity, the lower carrier concentration is better and the higher electron mobility is better. However, a higher specific resistance is better. These electrical characteristics vary depending on the growth conditions. The stripe mask is incomplete in terms of reducing internal stress. These samples are transparent light brown. Absorption coefficient at a wavelength 400nm~600nm ^was 40cm ^-1 ~80cm -1. When the content of As was examined, it was 9 × 10 ¹⁶ cm ⁻³ .
[0057]
[Example 2 (HVPE laterally grown GaN seed crystal, HVPE method GaN thickening)]
A 2-inch diameter GaAs (111) A surface was used as the substrate. An SiO ₂ insulating film was formed thereon. A window as shown in FIG. 3 was provided by photolithography.
(1) Formation of GaN buffer layer
A GaAs substrate having a mask was placed in an HVPE apparatus. The Ga reservoir 5 was heated to 800 ° C. using the apparatus of FIG. As source gases, H ₂ + HCl was introduced into the Ga reservoir, and H ₂ + NH ₃ was introduced directly to the substrate. A GaN buffer layer was formed at a low temperature of about 500 ° C. (substrate temperature). The buffer layer thickness is 80 nm.
(2) Formation of epitaxial layer Subsequently, the substrate temperature was raised to 1000 ° C. A GaN epitaxial layer of 80 μm was formed using the same source gas.
(3) Removal of GaAs The GaN / GaAs substrate was taken out of the HVPE apparatus. It was confirmed that a GaN continuous film was formed in a mirror shape. The GaAs substrate was removed by etching in aqua regia.
[0059]
(4) Thickening of GaN This was thoroughly cleaned. The state shown in FIG. What was only GaN was set again in the HVPE apparatus. The substrate temperature was 1020 ° C., and GaN was thickened by the HVPE method to obtain a GaN ingot. This is the state shown in FIG. This ingot had a shape in which the central portion was slightly depressed. The ingot had a minimum height of about 20 mm and an outer diameter of 55 mm.
[0060]
(5) Wafer cutting with a slicer An ingot was cut in a direction perpendicular to the axial direction with a circumferential blade slicer. As shown in FIG. Twenty GaN single crystal substrates having an outer diameter of about 50 mm and a thickness of 350 μm were obtained. When GaN was analyzed, both As and carbon were at the background level. It can be seen that arsenic (As) and carbon are hardly contained in GaN.
[0061]
(6) Polishing Further lapping polishing and final polishing were performed. This is a transparent wafer as shown in FIG. The substrate was not warped because it was machined.
[0062]
(7) Measurement of electrical characteristics The electrical characteristics of the wafer taken from the upper end of the ingot (at the end of growth) were measured. The n-type carrier concentration was 5 × 10 ¹⁸ cm ⁻³ . The electron mobility was 200 cm ² / Vs. The specific resistance was 0.017 Ωcm.
[0063]
The electrical characteristics of the wafer taken from the lower end of the ingot (early growth portion) were as follows. The n-type carrier concentration was 10 ¹⁸ cm ⁻³ and the electron mobility was 150 cm ² / Vs. The specific resistance was 0.01 Ωcm. This is the electrical property of the extremes. The middle part will be an intermediate value.
[0064]
(8) Measurement of light absorption These wafers were transparent and dark gray or colorless. Absorption coefficient at a wavelength 400nm~600nm was ^{20cm -1 ~40cm} -1.
(9) Production of LED Since a GaN substrate was formed, an LED having InGaN as a light emitting layer was produced thereon. Compared with the conventional sapphire substrate, the light emission luminance is improved about 5 times. The reason why the luminance is improved is due to a decrease in dislocations. This is because many threading dislocations exist in the active layer in the conventional sapphire substrate LED, but the threading dislocations of the present invention on the GaN substrate greatly decrease.
[0065]
[Example 3 (MOC laterally grown GaN seed crystal, HVPEGaN thickened)]
The GaAs (111) B surface was used as the substrate. SiO ₂ was attached to the substrate, and stripe windows extending in the [1-10] direction were formed by photolithography.
[0066]
(1) Formation of GaN buffer layer A GaN buffer layer having a thickness of 90 nm was formed on a substrate at a low temperature of about 490 ° C. by metal organic chloride vapor phase epitaxy (MOC method).
[0067]
(2) Formation of GaN epitaxial layer In the same apparatus, the substrate temperature was raised to about 970 ° C., and the GaN epitaxial layer was formed to a thickness of 25 μm.
(3) Removal of GaAs substrate A GaN / GaAs sample was taken out from the MOC apparatus. A mirror-finished GaN single crystal was growing. The direction of the stripe mask was the [11-20] direction of GaN. That is, the [11-20] direction of GaN grows in the [1-10] direction of GaAs. The GaAs substrate was dissolved and removed with aqua regia.
[0068]
(4) GaN thickening growth GaN having a thickness of 25 μm was set as a seed crystal in an HVPE apparatus. GaN was thickly epitaxially grown by heating at 1000 ° C. by the HVPE method. A GaN ingot having a cylindrical shape and a minimum height of about 3 cm was grown.
[0069]
(5) Wafer cut by inner peripheral edge slicer An ingot was cut to a thickness of 400 μm in the direction perpendicular to the axis by an inner peripheral edge slicer. 25 as-cut wafers could be cut out.
[0070]
(6) The lapped wafer was lapped and finished. A GaN single crystal wafer as a product was obtained.
(7) Measurement of electrical characteristics The electrical characteristics of the wafer were measured. It was n-type and had an electron mobility of 250 cm ² / Vs. The specific resistance was 0.05 Ωcm.
[0071]
In this embodiment, a GaN single crystal is grown thick using GaN itself as a seed crystal. Since thick single crystal GaN is grown and cut with a slicer, as many as 25 substrates can be fabricated at once. The manufacturing cost was reduced to 64% as compared with the case of growing from one piece of GaAs. The manufacturing cost of the substrate can be reduced. The production time per sheet including quality control could be greatly reduced. When GaN was analyzed, both arsenic (As) and carbon (C) were at the background level.
[0072]
As the mask window, a mask having a window at the apex of the equilateral triangle is best. However, it may have a striped window. There is a certain effect of reducing internal stress. Lateral growth on the mask reduces the internal stress by reducing defects in the crystal and reduces the contact area between GaAs and GaN, thereby relaxing the internal stress. Therefore, it is possible to suppress the occurrence of warpage despite the large temperature change.
[0073]
[Example 4 (Relationship between total flow rate and surface morphology / internal stress)]
Using a [1-100] stripe mask, a [11-2] stripe mask, and a dot mask, a GaN wafer was fabricated by the HVPE method. The source gases are H ₂ + NH ₃ and H ₂ + HCl. Increasing the total flow of source gas improves the surface morphology. However, the internal stress tended to increase.
[0074]
[Table 2]

[0075]
Of these, six (i), (b), (d), (d), (e), (d), (c), and (c) are group A, and (c) are five groups (c).
(A) The Ga partial pressure is 1 kPa (10 ⁻² atm). GaN was grown on the buffer layer / mask at 970 ° C. for 1 hour, and GaN was further grown at 1030 ° C. for 3 hours. Epitaxial growth for a total of 4 hours. The result is shown by white circles in FIG. There are 6 samples. These have a flat surface and good morphology. However, the internal stress is large. Some samples have cracks. In FIG. 13, the horizontal axis represents the film thickness (μm). The film thickness is distributed between 5 μm and 120 μm. The vertical axis represents internal stress (GPa). The sample of ○ has an internal stress of 2 MPa to 40 MPa. Most exhibit internal stresses greater than 10 MPa. However, the internal stress is preferably 7 MPa or less (7 × 10 ⁻³ GPa).
(B) The Ga partial pressure is 2 kPa. GaN was epitaxially grown on the buffer layer / mask at 970 ° C. for 6 hours. The number of samples is ten. The same is shown in FIG. The film thickness is distributed between 70 μm and 300 μm. The surface of the GaN sample is rough. Rmax is about 20 μm. The GaN substrate size is 20 mm × 20 mm. Although the surface condition is poor, the internal stress is small. The internal stress is 1 MPa to 10 MPa as indicated by black circles in FIG. Since the target is 7 MPa or less, this can be satisfied. However, the film thickness varies widely (70 μm to 300 μm) even under the same conditions. The curvature radius R of the warp is R = 780 mm to 1500 mm.
[0076]
[Example 5 (Relationship of radius of curvature)]
The relationship between the film thickness and the radius of curvature was examined for the six samples (A) and the five samples (B) as in the previous example. Warpage was evaluated by the stylus method. FIG. 14 shows the result. The horizontal axis is the film thickness (μm). The vertical axis is the radius of curvature. The radius of curvature is preferably 600 mm or more.
(A) Sample A grown at 970 ° C. for 1 hour + 1030 ° C. for 3 hours has a thin film thickness and a flat surface but a large warp. The curvature radius is 200 mm or less. A desirable range of the radius of curvature is 600 mm or more. All six samples do not reach the target.
(B) Sample B grown at 970 ° C. for 6 hours has a large film thickness and a roughened surface, but the internal stress is small and the warpage is small. All five B samples exceed the desirable range of 600 mm. In growth without a mask, the radius of curvature is extremely small and the warp is large. This is why the 1970s GaAs substrate attempts failed.
[0077]
[Example 6 (polishing)]
Sample A failed to polish. Among samples B, a sample having a film thickness of 150 μm, an internal stress of 4 MPa, a curvature radius of 1030 mm, and a Rmax of 20 μm was polished. The film thickness was reduced to 80 μm by polishing. The radius of curvature is reduced to 650 mm after polishing. The surface roughness was reduced to Rmax 7.2 nm and Ra 2 nm by polishing. Polishing smoothes the surface, but may increase warpage.
[Example 7 (Anisotropy of internal stress in wafer)]
The internal stress when GaN was grown using a stripe mask and when GaN was grown using a dot mask were compared. FIG. 15 is a graph showing the relationship between the mask and the internal stress.
A stripe mask extending in the <1-10> direction was produced on a GaAs wafer and GaN having a thickness of 120 μm was grown (averaged because the thickness of the film varies)
Using the same dot mask as in Example 1 and growing GaN with an average film thickness of 160 μm,
It was used.
All of the growth conditions are Ga partial pressure 1 × 10 ⁻² atm (1 kPa), NH ₃ partial pressure 0.24 atm (24 kPa), growth time 970 ° C., and growth time 240 minutes. After the growth, the GaAs substrate was removed by cooling. A GaN free-standing film was used, and the internal stress of GaN was examined by X-ray. The result is shown in FIG. A GaN substrate manufactured using a dot mask has very little anisotropy and low internal stress. On the other hand, a GaN substrate manufactured using a stripe mask has anisotropy in the stripe direction and the direction perpendicular thereto. In particular, a large warp appears in the direction parallel to the stripe. There are 10 times as much as in the vertical direction. The orientation in FIG. 15 is the orientation with respect to the GaN crystal. <1-10> of GaAs corresponds to <11-20> of GaN, and <11-2> of GaAs corresponds to <1-100> of GaN. <11-20> and <1-100> of GaN are orthogonal to each other.
[0078]
【The invention's effect】
The present invention provides a large GaN single crystal wafer. Due to the lateral growth method, there are few defects such as dislocations in the GaN crystal. Since GaN is thickened by a method that does not use carbon, the wafer becomes highly transparent. Lateral growth reduces internal stress and reduces warping. An existing wafer process such as photolithography can be used with little warping. This GaN single crystal wafer can be used as a substrate for a GaN light emitting device. Since the thin film portion and the substrate are the same, there is no warping. Small internal stress. Therefore, the dislocation density is also reduced. Luminous efficiency increases. The lifetime of GaN LED and GaN LD is extended.
[Brief description of the drawings]
FIG. 1 is a graph showing the difference in thermal expansion coefficient and lattice constant between a substrate material for growing a GaN crystal and GaN, in x and y coordinates.
FIG. 2 is a graph showing that the lattice constant changes smoothly as the GaN film thickness changes when GaN is epitaxially grown on a sapphire substrate.
FIG. 3 is a plan view of a staggered dotted window mask fixed on a GaAs (111) A surface.
FIG. 4 is a plan view showing a state in which a GaN buffer layer is epitaxially grown on a portion exposed from a mask window.
FIG. 5 is a plan view showing a state when GaN is further epitaxially grown on a mask and a buffer layer and crystals from adjacent windows meet each other.
FIG. 6 is a process diagram showing a process of growing a GaN buffer layer and a GaN epitaxial layer by placing a mask on the GaAs substrate and etching away the GaAs substrate. (1) is a diagram of a process in which a mask is formed on a GaAs (111) substrate. (2) is a diagram of a process in which a buffer layer is grown on a portion not covered with a mask. (3) is a diagram of a process in which a GaN epitaxial layer is grown on a buffer layer and a mask. (4) is a diagram showing a state in which the GaAs substrate is removed to form a free-standing film of GaN.
FIG. 7 is a diagram showing a process of further growing GaN thick on a GaN substrate to form a GaN ingot and cutting it into a wafer. (1) is a diagram of a GaN ingot thickened on a GaN substrate. (2) is a figure which shows the condition which is cutting | disconnecting an ingot into an as-cut wafer with an internal peripheral slicer. (3) is a view of a cut wafer.
FIG. 8 is a schematic sectional view of an HVPE apparatus.
FIG. 9 is a cross-sectional view showing a state in which a composite substrate obtained by growing GaN on a GaAs substrate is warped due to thermal stress.
FIG. 10 is a cross-sectional view showing that GaN after removing GaAs becomes flat if the internal stress is zero.
FIG. 11 is a cross-sectional view showing that if an internal stress is present in GaN itself, strain remains even after GaAs is removed.
FIG. 12 is a view showing the definition of warpage of a GaN wafer. Warpage is expressed by a bulge H at the center of a 50 mm diameter wafer.
FIGS. 13A and 13B are graphs showing distributions of measured values of film thickness and internal stress for a group of (A) 1800 sccm to 900 sccm and a group of (B) 900 sccm. Black circles are rough and are group B. White circles are surface smooth and group A.
FIG. 14 is a diagram showing the distribution of film thickness and radius of curvature for the same group A sample (30 sheets) and group B sample (9 sheets). Black circles are group B and white circles are group A.
FIG. 15 is a graph showing the result of measuring the anisotropy of internal stress when using a stripe mask and a dot mask.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Reactor 2 Heater 3 Source gas inlet 4 Source gas inlet 5 Ga reservoir 6 Ga melt 7 Susceptor 8 Shaft 9 GaAs substrate or GaN substrate 10 Gas outlet

Claims (8)

  A GaN unit having a diameter of 20 mm or more, a thickness of 0.07 mm or more, and being self-supporting and having a substrate deflection of 0.55 mm or less for a length of 2 inches (50 mm). Crystal substrate.   The GaN single crystal substrate according to claim 1, which is colorless and transparent.   2. The GaN single crystal substrate according to claim 1, wherein the GaN single crystal substrate is transparent but has a yellow, light brown, or dark gray color. 2. The GaN single crystal substrate according to claim 1, wherein an absorption coefficient by light transmission is 120 cm ⁻¹ or less over a wavelength range of 400 nm to 600 nm.   2. The GaN single crystal substrate according to claim 1, wherein the intrinsic internal stress is 7 MPa or less.   The GaN single crystal substrate according to claim 1, wherein the curvature radius of the substrate is 600 mm or more. 2. The GaN single crystal substrate according to claim 1, wherein As is contained in 10 ¹⁷ cm ⁻³ or less.   2. The GaN single crystal substrate according to claim 1, which contains substantially no carbon.

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