JP2016207752A - CdTe-BASED COMPOUND SEMICONDUCTOR AND RADIATION DETECTION ELEMENT USING THE SAME - Google Patents
CdTe-BASED COMPOUND SEMICONDUCTOR AND RADIATION DETECTION ELEMENT USING THE SAME Download PDFInfo
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Abstract
Description
本発明は、ＣｄＴｅ系化合物半導体及びそれを用いた放射線検出素子に関する。 The present invention relates to a CdTe compound semiconductor and a radiation detection element using the same.
従来より、放射線検出素子用途で高効率、高分解能、小型化に優れた直接変換型化合物半導体の開発が進められている。その中でＩＩ−ＶＩ族化合物半導体であるテルル化カドミウム（ＣｄＴｅ）、セレンテルル化カドミウム（ＣｄＳｅＴｅ）、テルル化亜鉛カドミウム（ＣｄＺｎＴｅ）、セレンテルル化亜鉛カドミウム（ＣｄＺｎＳｅＴｅ）などのＣｄＴｅ系化合物半導体は、近年、放射線検出素子用途の有力な材料として注目されている。これらは他の半導体に比べ、原子番号が比較的大きいことから放射線の吸収率が高く検出効率が高い、バンドギャップエネルギーが大きいので熱による漏れ電流の影響が少なく冷却装置が不要（室温で動作可能）という利点がある。 2. Description of the Related Art Conventionally, direct conversion type compound semiconductors excellent in high efficiency, high resolution, and miniaturization have been developed for use in radiation detection elements. Among them, CdTe-based compound semiconductors such as cadmium telluride (CdTe), cadmium selenite telluride (CdSeTe), zinc cadmium telluride (CdZnTe) and zinc cadmium telluride tellurium (CdZnSeTe), which are II-VI group compound semiconductors, It is attracting attention as a promising material for radiation detection element applications. Compared to other semiconductors, the atomic number is relatively large, so the radiation absorption rate is high, the detection efficiency is high, and the band gap energy is large, so there is little influence of leakage current due to heat, and no cooling device is required (operable at room temperature ).
こうした放射線検出用直接変換型半導体において、放射線検出特性の良し悪しを決める半導体の重要なパラメータにキャリアの移動度μとキャリア寿命τの積μτが挙げられる。直接変換型半導体を用いた放射線検出器は、半導体の対向する２面に電極を形成し、電極を介して電場が印加された状態の半導体内に放射線が入射することで発生するキャリア（電子・正孔）を電極を介して電気信号の形で取り出すことによって放射線が入ったことを感知する。すなわち、電場が印加された状態の半導体内では、放射線を吸収して発生したキャリア（電子・正孔）が、印加された電場に沿ってそれぞれ相反する電位の方向に進み、最終的に電極まで到達する。電極に到達したキャリア（電子または正孔）が電気信号の形で取り出されるため、放射線が入ったことを電気信号の形で感知できるというのが、直接変換型半導体を用いた放射線検出素子の原理である。 In such a direct detection semiconductor for radiation detection, an important parameter of a semiconductor that determines the quality of radiation detection characteristics is the product μτ of carrier mobility μ and carrier lifetime τ. A radiation detector using a direct conversion type semiconductor has electrodes formed on two opposing surfaces of the semiconductor, and carriers (electron and electron) generated when radiation enters the semiconductor in a state where an electric field is applied through the electrodes. The presence of radiation is sensed by extracting holes) through the electrodes in the form of electrical signals. In other words, in a semiconductor in a state where an electric field is applied, carriers (electrons and holes) generated by absorbing radiation advance in opposite directions along the applied electric field and finally reach the electrodes. To reach. Since the carriers (electrons or holes) that reach the electrode are taken out in the form of an electrical signal, the principle of the radiation detection element using a direct conversion type semiconductor is that radiation can be detected in the form of an electrical signal. It is.
この際、半導体のキャリア移動度μが大きい程、半導体内をキャリアは早く移動し、電極まで到達し易い。また、キャリア寿命τが大きい程、キャリアは再結合で失われず長寿命なため電極まで到達し易くなる。従って、この二つの積μτは放射線入射で発生したキャリアが途中で失われずに電極まで届く尺度を表し、この値が大きい程、放射線検出特性に優れている。また、キャリアの種類（電子・正孔）によって、異なるμτ値を有している。正孔の移動度μは、電子の移動度μに比べて小さく、正孔における積μτ（ｈ）（正孔のμτ（ｈ））は電子における積μτ（ｅ）（電子のμτ（ｅ））より１〜２桁低い値になることが多く、放射線検出特性を下げる原因となっている。そこで、正孔のμτ（ｈ）を高めることが放射線検出特性の向上に繋がる。 At this time, the larger the carrier mobility μ of the semiconductor, the faster the carriers move in the semiconductor and reach the electrodes. Also, the longer the carrier lifetime τ, the easier it is to reach the electrode because the carriers are not lost by recombination and have a longer lifetime. Therefore, these two products μτ represent a scale by which carriers generated by radiation incidence reach the electrode without being lost, and the larger this value, the better the radiation detection characteristics. Moreover, it has a different μτ value depending on the type of carrier (electron / hole). The hole mobility μ is smaller than the electron mobility μ, and the product μτ (h) (hole μτ (h)) in the hole is the product μτ (e) in the electron (μτ (e) in the electron). ) Is often 1 to 2 orders of magnitude lower, which is a cause of lowering radiation detection characteristics. Therefore, increasing the μτ (h) of holes leads to improvement of radiation detection characteristics.
また、例えば、組成が同じＣｄＴｅ化合物半導体を用いて放射線検出器を作製した際、同じ条件で作製した電極を用いても放射線の検出特性にばらつきが生じていた。実際に正孔のμτ（ｈ）値を測定すると、組成が同じＣｄＴｅ系化合物半導体であっても、製造の度毎に、１０−５〜１０−６ｃｍ２／Ｖ台まで大きくばらついていた。したがって、正孔のμτ（ｈ）値をできる限り高い値とし、そのばらつきを抑えることで、再現性よく高いμτ（ｈ）を有するＣｄＴｅ系化合物半導体基板を得る技術が求められてきた。 Further, for example, when a radiation detector is manufactured using CdTe compound semiconductors having the same composition, the radiation detection characteristics vary even when electrodes manufactured under the same conditions are used. When the μτ (h) value of holes was actually measured, even if it was a CdTe-based compound semiconductor having the same composition, it varied widely from 10 −5 to 10 −6 cm 2 / V every time it was manufactured. Therefore, there has been a demand for a technique for obtaining a CdTe-based compound semiconductor substrate having a high μτ (h) with high reproducibility by setting the μτ (h) value of holes as high as possible and suppressing variations thereof.
非特許文献１には、正孔の移動度μは電子の移動度μに比べて小さく、正孔のμτ（ｈ）は電子のμτ（ｅ）に比べて値が小さく、放射線検出器の出力が放射線の入射方向によって変化する問題点を指摘している。この問題を解決するために、電極に用いる金属の組成を調整してキャリアの収集効率を改善することが記載されている。
しかしながら、正孔を電極から取り出す効率を高め、放射線検出素子としての機能向上を図ることができるとしても、正孔のμτ（ｈ）値自体の改善には繋がっていない。また、組成が同じＣｄＴｅ化合物半導体に同じ条件で作製した電極を用いても放射線の検出特性にばらつきが生じていたという問題、すなわち、組成が同じＣｄＴｅ系化合物半導体であっても個体によって正孔のμτ（ｈ）値が、製造の度毎に、１０−５〜１０−６ｃｍ２／Ｖ台まで大きくばらつく問題を解決するものでもない。 However, even if the efficiency of extracting holes from the electrode can be increased and the function of the radiation detection element can be improved, it does not lead to improvement in the μτ (h) value of the holes themselves. In addition, there is a problem in that the radiation detection characteristics vary even when an electrode manufactured under the same conditions is used for a CdTe compound semiconductor having the same composition, that is, even if a CdTe compound semiconductor having the same composition is used, the hole may vary depending on the individual. This does not solve the problem that the μτ (h) value varies greatly from 10 −5 to 10 −6 cm 2 / V every time it is manufactured.
本発明の課題は、組成が同じＣｄＴｅ系化合物半導体の正孔のμτ（ｈ）値をできる限り高い値とし、あわせてそのばらつきが抑えられたＣｄＴｅ系化合物半導体を提供することである。さらに、そのようなＣｄＴｅ系化合物半導体を用いた放射線検出素子を提供することである。 An object of the present invention is to provide a CdTe-based compound semiconductor in which the μτ (h) value of holes of CdTe-based compound semiconductors having the same composition is set as high as possible and the variation thereof is suppressed. Furthermore, it is providing the radiation detection element using such a CdTe type compound semiconductor.
我々はこうした問題点の解決のため、鋭意研究開発を進めたところ、正孔のμτ（ｈ）値（移動度−寿命積μτ（ｈ））がＣｄＴｅ系半導体に含まれる不純物のうちボロン（Ｂ）、炭素（Ｃ）、窒素（Ｎ）、及び酸素（Ｏ）の濃度に強く依存していることを見出した。傾向として、ボロン（Ｂ）、炭素（Ｃ）、窒素（Ｎ）、及び酸素（Ｏ）の濃度が少ない程、正孔のμτ（ｈ）値が高いことがわかった。特に、ボロン（Ｂ）が低い場合に、正孔のμτ（ｈ）が高くなりやすく、さらに、正孔のμτ（ｈ）値はボロン（Ｂ）及び窒素（Ｎ）の合計の濃度に強く依存していることが分かった。より具体的には、（１）ボロン（Ｂ）濃度が高い場合には、窒素（Ｎ）濃度が極めて低い場合でも、正孔のμτ（ｈ）が小さくなる（比較例２）。（２）ボロン（Ｂ）の濃度及び窒素（Ｎ）濃度のいずれも低い場合には安定して正孔のμτ（ｈ）が大きな値を得る（実施例１、２）ことができ、（３）炉材の熱処理時間を長くすることによって、該炉材を使用して育成したＣｄＴｅ系結晶中の酸素（Ｏ）濃度が減少する傾向を見出し、本発明に至った。これらの不純物濃度を抑えることで正孔のμτ（ｈ）値を高めるとともに、組成が同じＣｄＴｅ系化合物半導体における個体間の正孔のμτ（ｈ）値のばらつきを改善させることに成功した。 In order to solve these problems, we have made extensive research and development. As a result, the hole μτ (h) value (mobility-life product μτ (h)) is boron (B (B)) among impurities contained in CdTe semiconductors. ), Carbon (C), nitrogen (N), and oxygen (O). As a tendency, it was found that the smaller the concentrations of boron (B), carbon (C), nitrogen (N), and oxygen (O), the higher the μτ (h) value of holes. In particular, when boron (B) is low, the hole μτ (h) tends to be high, and the hole μτ (h) value strongly depends on the total concentration of boron (B) and nitrogen (N). I found out that More specifically, (1) When the boron (B) concentration is high, even when the nitrogen (N) concentration is extremely low, the μτ (h) of holes becomes small (Comparative Example 2). (2) When both the concentration of boron (B) and the concentration of nitrogen (N) are low, the value of μτ (h) of holes can be stably obtained (Examples 1 and 2), (3 ) By increasing the heat treatment time of the furnace material, it was found that the oxygen (O) concentration in the CdTe-based crystal grown using the furnace material tends to decrease, leading to the present invention. By suppressing these impurity concentrations, the μτ (h) value of holes was increased, and the variation in μτ (h) value of holes among individuals in CdTe compound semiconductors having the same composition was successfully improved.
ＣｄＴｅ系結晶（結晶には、単結晶及び多結晶を含む。）の育成工程において、混入している酸素（Ｏ）はそれ自体、ＣｄＴｅ系半導体内でμτ低下に関与する等電位トラップ準位を形成する。さらに、炉材に付着した水分の酸素（Ｏ）成分は炉材に使われる窒化ほう素材（ＢＮやｐＢＮ）から、窒素（Ｎ）、ボロン（Ｂ）をＮＯｘ、ＢＯｘの形で炉材から解離させ、結晶内に取り込ませてしまう性質があると考えられる。加えて、同じく炉材に使われる石英は炭素酸化物（ＣＯｘ）を取り込む性質があり、これも汚染源となる。そのため、まずはＣｄＴｅ系半導体の結晶製造装置内に含まれる酸素（Ｏ）濃度及び炭素酸化物（ＣＯｘ）濃度を下げることで、ＣｄＴｅ系半導体結晶中の不純物濃度を下げることが可能となるかを試みた。 In the growth process of CdTe-based crystals (crystals include single crystals and polycrystals), oxygen (O) mixed in itself has an equipotential trap level that is involved in the μτ decrease in the CdTe-based semiconductor. Form. Furthermore, the oxygen (O) component of the moisture adhering to the furnace material is dissociated from the boron nitride material (BN or pBN) used in the furnace material, nitrogen (N) and boron (B) in the form of NOx and BOx from the furnace material. It is considered that there is a property of being incorporated into the crystal. In addition, quartz, which is also used for furnace materials, has the property of taking in carbon oxides (COx), which is also a pollution source. For this reason, first, it is attempted to reduce the impurity concentration in the CdTe semiconductor crystal by reducing the oxygen (O) concentration and the carbon oxide (COx) concentration contained in the CdTe semiconductor crystal manufacturing apparatus. It was.
より具体的には、ＣｄＴｅ系化合物半導体の結晶成長は、その原料を坩堝に充填して所定時間加熱した後、冷却することにより実施され、その結果、ＣｄＴｅ系化合物半導体のインゴットが生成される。そこで、Ｏ、ＣＯｘの混入源として最も大きいと思われる坩堝や坩堝を保持するサセプター、石英製アンプルなどの炉材（窒化ホウ素、炭素、炉材に含有する水分など）を不活性雰囲気中で加熱することにより、残留水分量を減らして、残留水分に起因する酸素（Ｏ）を減らすことで炉材中の酸素（Ｏ）濃度を下げ、加えて不活性雰囲気中で加熱することで炭素酸化物（ＣＯｘ）濃度も下がるので、ひいてはＣｄＴｅ系化合物半導体の中の不純物濃度を減らすことを試みた。 More specifically, the crystal growth of the CdTe-based compound semiconductor is performed by filling the raw material in a crucible, heating for a predetermined time, and then cooling, and as a result, an ingot of the CdTe-based compound semiconductor is generated. Heating furnace materials (boron nitride, carbon, moisture contained in furnace materials, etc.) such as crucibles that are considered to be the largest sources of O and COx, susceptors that hold crucibles, and quartz ampoules in an inert atmosphere By reducing the amount of residual moisture and reducing oxygen (O) due to residual moisture, the oxygen (O) concentration in the furnace material is lowered, and in addition, the carbon oxide is heated by heating in an inert atmosphere. Since the (COx) concentration also decreases, an attempt was made to reduce the impurity concentration in the CdTe-based compound semiconductor.
また、坩堝などの炉材の加熱処理時間とＣｄＴｅ系化合物半導体不純物濃度の関係を調べるため、坩堝等の炉材の加熱は、加熱温度を同一とし、加熱時間を１か月、２週間、１週間、１日、及び１２時間と変えて、不活性雰囲気下で行ない、加熱時間の異なる坩堝等の炉材を用意した。加熱処理された種々の炉材を用いて、ＣｄＴｅ系化合物半導体のインゴットを育成し、さらに育成された各インゴットから基板表面の結晶方位が（１１１）面となるように円盤状の基板を切り出した。各ＣｄＴｅ系化合物半導体基板中の不純物元素の濃度は、グロー放電質量分析装置（ＧＤＭＳ：Glow Discharge Mass Spectrometry）を用いて評価した。また、正孔のμτ（ｈ）値は、該円盤状の基板をダイシングにより矩形に加工し、基板の表裏面を鏡面研磨処理を施した後、当該基板の一方の主面にＰｔオーミック電極、もう一方の主（裏）面にＩｎショットキー電極を形成して素子基板を作製し、所定値の電圧を印加しながら、標準放射線源より放射線を素子基板に照射し、生成された正孔キャリアを電気信号として取り出し、多チャンネル波高分析装置（ＭＣＡ）を介して測定した。このようにして、測定された不純物の濃度と正孔のμτ（ｈ）値との相関を調べた。 In order to investigate the relationship between the heat treatment time of the furnace material such as the crucible and the CdTe compound semiconductor impurity concentration, the heating of the furnace material such as the crucible is performed at the same heating temperature for 1 month, 2 weeks, 1 It changed in week, 1 day, and 12 hours, and performed in inert atmosphere and prepared furnace materials, such as a crucible with different heating time. Using various heat-treated furnace materials, CdTe-based compound semiconductor ingots were grown, and a disk-shaped substrate was cut out from each of the grown ingots so that the crystal orientation of the substrate surface was the (111) plane. . The concentration of the impurity element in each CdTe-based compound semiconductor substrate was evaluated using a glow discharge mass spectrometer (GDMS). In addition, the μτ (h) value of the holes is obtained by processing the disk-shaped substrate into a rectangle by dicing, performing a mirror polishing process on the front and back surfaces of the substrate, and then forming a Pt ohmic electrode on one main surface of the substrate, Forming an element substrate by forming an In Schottky electrode on the other main (back) surface, irradiating the element substrate with radiation from a standard radiation source while applying a predetermined voltage, and generating hole carriers Was taken out as an electrical signal and measured via a multichannel wave height analyzer (MCA). In this way, the correlation between the measured impurity concentration and the hole μτ (h) value was examined.
不純物濃度と正孔のμτ（ｈ）値との相関によれば、酸素（Ｏ）のみならず、ボロン（Ｂ）、炭素（Ｃ）、及び窒素（Ｎ）もトラップ準位を形成し、正孔のμτ（ｈ）の低下に寄与すると考えられる。特に、ボロン（Ｂ）及び窒素（Ｎ）は、他の不純物に比べて、正孔のμτ（ｈ）値を低下させる効果が大きいことが分かった。 According to the correlation between the impurity concentration and the μτ (h) value of holes, not only oxygen (O) but also boron (B), carbon (C), and nitrogen (N) form trap levels and are positive. It is thought that it contributes to the decrease of μτ (h) of the hole. In particular, it was found that boron (B) and nitrogen (N) have a greater effect of reducing the μτ (h) value of holes than other impurities.
そこで、本発明によれば、正孔のμτ（ｈ）値を下げる主たる原因となるボロン（Ｂ）の濃度が２０ａｔｏｍ ｐｐｂ以下であることを特徴としたＣｄＴｅ系化合物半導体基板（１）であり、さらに、ボロン（Ｂ）の濃度が２０ａｔｏｍ ｐｐｂ以下であり、且つ、窒素（Ｎ）の濃度が１５ａｔｏｍ ｐｐｂ以下であることを特徴としたＣｄＴｅ系化合物半導体基板（２）が提供される。 Therefore, according to the present invention, there is provided a CdTe-based compound semiconductor substrate (1) characterized in that the concentration of boron (B), which is a main cause of lowering the μτ (h) value of holes, is 20 atom ppb or less, Furthermore, a CdTe-based compound semiconductor substrate (2) is provided in which the concentration of boron (B) is 20 atom ppb or less and the concentration of nitrogen (N) is 15 atom ppb or less.
本発明によれば、ボロン（Ｂ）濃度と窒素（Ｎ）濃度との和が３０ａｔｏｍ ｐｐｂ以下である、上記（１）及び（２）に記載のＣｄＴｅ系化合物半導体基板（３）が提供される。 According to the present invention, there is provided the CdTe-based compound semiconductor substrate (3) according to the above (1) and (2), wherein the sum of the boron (B) concentration and the nitrogen (N) concentration is 30 atom ppb or less. .
本発明によれば、正孔の移動度μと該正孔の寿命τの積であるμτ積（ｈ）が５．０×１０−５ｃｍ２／Ｖ以上であることを特徴とする、上記（１）〜（３）のいずれかに記載のＣｄＴｅ系化合物半導体基板（４）が提供される。
本発明によれば、上記（１）〜（４）のいずれか１項に記載のＣｄＴｅ系化合物半導体基板を用いて作製されたことを特徴とするＣｄＴｅ系半導体直接検出型放射線検出素子が提供される。
According to the present invention, the μτ product (h), which is the product of the hole mobility μ and the lifetime τ of the holes, is 5.0 × 10 −5 cm 2 / V or more, A CdTe-based compound semiconductor substrate (4) according to any one of (1) to (3) is provided.
According to the present invention, there is provided a CdTe-based semiconductor direct detection type radiation detecting element manufactured using the CdTe-based compound semiconductor substrate according to any one of (1) to (4) above. The
本発明によれば、前記ＣｄＴｅ系化合物半導体の対向する面には、ＩｎまたはＰｔからなる電極が設けられていることを特徴する、上記（１）〜（５）のいずれか１項に記載の半導体直接検出型放射線検出素子が提供される。 According to the present invention, in any one of the above (1) to (5), an electrode made of In or Pt is provided on the opposing surface of the CdTe-based compound semiconductor. A semiconductor direct detection type radiation detection element is provided.
本発明にかかる半導体直接検出型放射線検出素子は、ショットキー素子であり、正孔が移動する側の電極がインジウム（Ｉｎ）であり、電子が移動する側の電極がプラチナ（Ｐｔ）であることが望ましいが、本発明の本質はＣｄＴｅ系半導体材料を対象とし、素子の種類、電極金属の種類によらず適用できるものである。 The semiconductor direct detection type radiation detection element according to the present invention is a Schottky element, the electrode on the side where holes move is indium (In), and the electrode on the side where electrons move is platinum (Pt). However, the essence of the present invention is intended for CdTe-based semiconductor materials and can be applied regardless of the type of element and the type of electrode metal.
本発明によれば、ＣｄＴｅ系半導体に含まれる不純物のうちボロン（Ｂ）、炭素（Ｃ）、窒素（Ｎ）、及び酸素（Ｏ）の濃度を所定濃度以下に抑えることが可能となった。さらに、ボロン（Ｂ）、炭素（Ｃ）、窒素（Ｎ）、及び酸素（Ｏ）の濃度を所定濃度以下とすることにより、正孔のμτ（ｈ）値を高めるのみならず、正孔のμτ（ｈ）値が大きくばらつくことを抑制でき、再現性よく、高いμτ（ｈ）値を有するＣｄＴｅ系化合物半導体基板を作製することが可能となった。特に、ボロン（Ｂ）濃度を所定の濃度以下に抑えることによって、さらには、ボロン（Ｂ）濃度と窒素（Ｎ）濃度を各々所定の濃度以下に抑えることによって、組成が同じＣｄＴｅ系化合物半導体であっても制御しきれなかった正孔のμτ（ｈ）値のばらつきを再現性よく抑えることが可能となった。 According to the present invention, the concentration of boron (B), carbon (C), nitrogen (N), and oxygen (O) among impurities contained in the CdTe-based semiconductor can be suppressed to a predetermined concentration or less. Furthermore, by making the concentrations of boron (B), carbon (C), nitrogen (N), and oxygen (O) not more than a predetermined concentration, not only the μτ (h) value of holes is increased, but also A large variation in μτ (h) value can be suppressed, and a CdTe-based compound semiconductor substrate having a high μτ (h) value can be manufactured with good reproducibility. In particular, by suppressing the boron (B) concentration to a predetermined concentration or less, and further suppressing each of the boron (B) concentration and the nitrogen (N) concentration to a predetermined concentration or less, the CdTe-based compound semiconductor having the same composition can be used. It was possible to suppress the variation in the μτ (h) value of holes that could not be controlled even with good reproducibility.
以下、図面を参照して本発明に係るＣｄＴｅ系半導体及びその製造方法について説明する。但し、本発明に係るＣｄＴｅ系半導体及びその製造方法は多くの異なる態様で実施することが可能であり、以下に示す実施の形態の記載内容に限定して解釈されるものではない。なお、本実施の形態で参照する図面において、同一部分または同様な機能を有する部分には同一の符号を付し、その繰り返しの説明は省略する。 Hereinafter, a CdTe semiconductor according to the present invention and a manufacturing method thereof will be described with reference to the drawings. However, the CdTe-based semiconductor and the manufacturing method thereof according to the present invention can be implemented in many different modes, and are not construed as being limited to the description of the embodiments described below. Note that in the drawings referred to in this embodiment, the same portions or portions having similar functions are denoted by the same reference numerals, and repetitive description thereof is omitted.
図１は、ＣｄＴｅ系半導体を、垂直温度勾配凝固法（ＶＧＦ法）により成長させるための結晶成長装置の概略構成図である。坩堝内で融解した原料融液の一端から徐々に固化を行い、結晶（単結晶及び多結晶を含む。）を育成する方法である。この成長方法は、結晶成長方向の温度勾配が小さいため低転位密度の結晶が容易に得られるという長所を有する。 FIG. 1 is a schematic configuration diagram of a crystal growth apparatus for growing a CdTe-based semiconductor by a vertical temperature gradient solidification method (VGF method). In this method, crystals (including single crystals and polycrystals) are grown by gradually solidifying from one end of the raw material melt melted in the crucible. This growth method has an advantage that a crystal having a low dislocation density can be easily obtained because the temperature gradient in the crystal growth direction is small.
図１中、符号２００は常圧容器を示し、この常圧容器２００の中心にはＣｄを充填するリザーバ部２０１ａを有する石英アンプル２０１が配置されている。また、石英アンプル２０１内にはｐＢＮ（ｐｙｒｏｌｙｔｉｃ Ｂｏｒｏｎ Ｎｉｔｒｉｄｅ）製坩堝２０３が配置され、石英アンプル２０１を包囲するようにヒータ２０２が設けられている。ヒータ２０２は、図１に示すように坩堝２０３に対応する部分とリザーバ部２０１ａに対応する部分とを別々の温度に加熱でき、かつ常圧容器２００内の温度分布を細かく制御できる３段の多段型構造を有する。
In FIG. 1,
まず、結晶成長装置内のリザーバ部２０１ａを有する石英アンプル２０１及び坩堝２０３の炉材を、王水を用いて洗浄した後、さらに超純水で洗浄した。次に、窒素ガス雰囲気下で、ヒータ２０２を用いて１５０℃にて加熱する。加熱時間は、１か月、２週間、１週間、１日、及び１２時間と変えて、加熱温度は同じであるが、加熱時間の異なる坩堝等の炉材を用意した。なお、本実施例では、不活性ガスとして窒素ガスを用いたが、あくまで一例であって、例えばアルゴンガスでも構わない。
First, the
加熱時間が各々異なる炉材を用いて、垂直温度勾配凝固法（ＶＧＦ）によりＣｄ０．９Ｚｎ０．１Ｔｅ結晶を育成した。具体的には、まず、石英アンプル２０１のリザーバ部２０１ａに易揮発性元素であるＣｄ単体２０４を入れるとともに、ｐＢＮ製坩堝２０３にＣｄＺｎＴｅの結晶原料２０５を入れて石英アンプル２０１内に配置した後、石英アンプル２０１を真空封止した。
Cd 0.9 Zn 0.1 Te crystals were grown by vertical temperature gradient solidification (VGF) using furnace materials having different heating times. Specifically, first, the Cd
次に、ヒータ２０２で加熱昇温して坩堝２０３内のＣｄＴｅ原料２０５を融解した後、ヒータ２０２でリザーバ部２０１ａを７８０℃に加熱して、Ｃｄ蒸気圧を０．１１６ＭＰａに制御を行うとともに、坩堝２０３を１１００℃に加熱した。さらに、常圧容器２００内に所望の温度分布が生じるように各ヒータへの供給電力量を制御装置（図示しない）で制御しながら加熱炉内の温度を０．１℃／ｈｒの降温速度で徐々に下げて、約２００時間かけて原料融液の表面から下方に向かってＣｄＺｎＴｅ結晶を成長させた。
Next, after heating and heating the
各々異なる時間で加熱処理された炉材を用いて育成されたＣｄＺｎＴｅ結晶中の不純物濃度はグロー放電質量分析装置（ＧＤＭＳ）を用いて評価された。また、各々異なる時間で加熱処理された炉材を用いて育成されたＣｄＴｅ結晶の正孔のμτ（ｈ）値の測定は、以下の方法で実施した。すなわち、育成されたＣｄＴｅ結晶のインゴットから基板表面の結晶方位が（１１１）面となるように円盤状に基板を切り出し、さらに円盤状の基板を矩形に加工して、基板サイズが４×４×１．４ｍｍ３であるＣｄＺｎＴｅ基板を作製した。次に、基板の表裏面を鏡面研磨した後、メタノール、アセトン等の有機溶剤に浸漬し、室温で超音波洗浄することで、基板に付着した異物を除去し、さらに、臭化水素、臭素及び水を混合したエッチング液に基板を浸漬して、室温で基板の研磨面の加工変質層を除去した。このようにして、洗浄処理されたＣｄＺｎＴｅ基板の（１１１）面の一方の面には無電解めっきにてＰｔ膜を５０ｎｍ堆積し、（−１−１−１）面の他方の面には真空蒸着にてＩｎ膜を３００ｎｍ堆積してショットキー素子を作製した（図２）。 Impurity concentrations in CdZnTe crystals grown using furnace materials heated at different times were evaluated using a glow discharge mass spectrometer (GDMS). Moreover, the measurement of the μτ (h) value of the holes of the CdTe crystal grown using the furnace materials heat-treated at different times was performed by the following method. That is, a substrate is cut out from the grown CdTe crystal ingot so that the crystal orientation of the substrate surface is the (111) plane, and the disk-shaped substrate is processed into a rectangle, so that the substrate size is 4 × 4 ×. A CdZnTe substrate having a thickness of 1.4 mm 3 was produced. Next, the front and back surfaces of the substrate are mirror-polished, then immersed in an organic solvent such as methanol and acetone, and ultrasonically cleaned at room temperature to remove foreign substances attached to the substrate. Further, hydrogen bromide, bromine and The substrate was immersed in an etching solution mixed with water, and the work-affected layer on the polished surface of the substrate was removed at room temperature. In this way, a Pt film of 50 nm is deposited by electroless plating on one surface of the (111) surface of the cleaned CdZnTe substrate, and a vacuum is applied on the other surface of the (-1-1-1) surface. A Schottky element was manufactured by depositing an In film by 300 nm by vapor deposition (FIG. 2).
これに、アメリシウム−２４１（Ａｍ２４１）を核種とした標準放射線源（（社）日本アイソトープ協会）を１０ｍｍの間隔を置いてＩｎ膜側に配置し、Ａｍ２４１から出る放射線をショットキー素子が検出できるようにした。この状態でショットキー素子に２５０、５００、７００、９００Ｖの電圧を印加すると、ショットキー素子内に入射したＡｍ２４１からの放射線により素子内部で電子、正孔のキャリアが生成される。生成されたキャリアは印加されている電場に沿って、互いに相反する電位の方向に進むが、Ａｍ２４１線源の位置がＩｎ電極側に近いことから素子から取り出されるキャリアは正孔のみで、電子はＰｔ側に移動中に再結合で消失する。 To this, a standard radiation source (American Society of Isotopes) using Americium-241 (Am241) as a nuclide is arranged on the In film side with an interval of 10 mm so that the Schottky element can detect the radiation emitted from Am241. I made it. When a voltage of 250, 500, 700, or 900 V is applied to the Schottky element in this state, electrons and hole carriers are generated inside the element by radiation from Am241 incident on the Schottky element. The generated carriers proceed in the direction of the opposite electric potential along the applied electric field, but since the position of the Am241 radiation source is close to the In electrode side, the carriers extracted from the element are only holes, and the electrons are It disappears by recombination while moving to the Pt side.
Ｉｎ電極を介して取り出された電気信号は多チャンネル波高分析装置（ＭＣＡ）により信号処理される。ピーク位置はμτ（ｈ）×（結晶内電場）に単調増加で依存するため、電圧を変えて結晶内電場の値を変化させた上でピーク位置の変化を調べることでμτ（ｈ）の値を知ることができる。 The electric signal taken out through the In electrode is subjected to signal processing by a multichannel wave height analyzer (MCA). Since the peak position depends monotonically on μτ (h) × (electric field in the crystal), the value of μτ (h) can be determined by examining the change in the peak position after changing the value of the electric field in the crystal by changing the voltage. Can know.
各々異なる時間で育成前に加熱処理された炉材を用いて育成させたＣｄ０．９Ｚｎ０．１Ｔｅ単結晶について、ＧＤＭＳによる不純物濃度の評価結果、及び作製したショットキー素子から評価した正孔のμτ（ｈ）値を表１に示す。 For Cd 0.9 Zn 0.1 Te single crystals grown using furnace materials that were heat-treated before growth at different times, the evaluation results of impurity concentration by GDMS and the positive evaluation evaluated from the fabricated Schottky element Table 1 shows the μτ (h) values of the holes.
表１によれば、結晶育成前の炉材の加熱時間が長い程、酸素（Ｏ）濃度が低くなっていく傾向がわかる。他の不純物（Ｂ、Ｃ、Ｎ）については、ばらつきが見られるが、特に、加熱時間が１か月になると、どの不純物についても低くなっている。したがって、育成前に炉材を加熱することによって、結晶成長装置内の残留水分量を減らし、ひいてはＣｄＴｅ系半導体内の酸素（Ｏ）濃度を減らすことが可能となることが分かる。また、比較例２のデータに着目すると、窒素（Ｎ）濃度が２．０ａｔｏｍ ｐｐｂ程度と極めて低くなっているが、そのような場合でも正孔のμτ（ｈ）値は２．３×１０−５ｃｍ２／Ｖと低い値となっていた。したがって、単純に、１つの不純物濃度を低減するだけでは、正孔のμτ（ｈ）が増大化する傾向を得ることができないと言える。 According to Table 1, it can be seen that the longer the heating time of the furnace material before crystal growth, the lower the oxygen (O) concentration. As for other impurities (B, C, N), a variation is observed, but in particular, when the heating time is one month, all the impurities are low. Therefore, it can be seen that by heating the furnace material before the growth, it is possible to reduce the amount of residual moisture in the crystal growth apparatus, and thus to reduce the oxygen (O) concentration in the CdTe-based semiconductor. Further, when attention is paid to the data of Comparative Example 2, the nitrogen (N) concentration is as extremely low as about 2.0 atom ppb, but even in such a case, the μτ (h) value of the hole is 2.3 × 10 −. The value was as low as 5 cm 2 / V. Therefore, it can be said that the tendency to increase the μτ (h) of holes cannot be obtained simply by reducing one impurity concentration.
特に、比較例２では、上述のように、窒素（Ｎ）濃度が２．０ａｔｏｍ ｐｐｂと極めて低い濃度であるが、ボロン（Ｂ）濃度が１２０ａｔｏｍ ｐｐｂと高いことから、正孔のμτ（ｈ）値は２．３×１０−５ｃｍ２／Ｖと低い値になっていると考えられる。 In particular, in Comparative Example 2, the nitrogen (N) concentration is as extremely low as 2.0 atom ppb as described above, but the boron (B) concentration is as high as 120 atom ppb. The value is considered to be a low value of 2.3 × 10 −5 cm 2 / V.
以上のように、ボロン（Ｂ）濃度が正孔のμτ（ｈ）に大きく影響を与えていることが分かる。 As described above, it can be seen that the boron (B) concentration greatly affects the μτ (h) of holes.
さらに正孔のμτ（ｈ）の各不純物の濃度への依存性を見るために、各々異なる時間で育成前に加熱処理された炉材を用いて育成させたＣｄ０．９Ｚｎ０．１Ｔｅ結晶に含まれる各不純物の濃度とμτ（ｈ）との依存性をプロットしたグラフを作成した（図３ないし図８）。これらのグラフによって、不純物濃度と正孔のμτ（ｈ）との間には以下の傾向があることが分かった。 Further, in order to see the dependency of μτ (h) of holes on the concentration of each impurity, Cd 0.9 Zn 0.1 Te grown using furnace materials heat-treated before growth at different times. Graphs plotting the dependence of the concentration of each impurity contained in the crystal and μτ (h) were prepared (FIGS. 3 to 8). From these graphs, it was found that there is the following tendency between the impurity concentration and the μτ (h) of holes.
ボロン（Ｂ）の濃度と正孔のμτ（ｈ）との依存性をプロットしたグラフ(図３）によれば、ボロン（Ｂ）濃度は、正孔のμτ（ｈ）とかなり強い相関があることが分かる。ボロン（Ｂ）濃度が高ければ高いほど正孔のμτ（ｈ）は低くなり、ボロン（Ｂ）濃度が低ければ低いほど正孔のμτ（ｈ）は高くなる傾向があるということができる。特にボロン（Ｂ）の場合、その濃度を２０ａｔｏｍ ｐｐｂ以下に抑えると正孔のμτ（ｈ）は急激に高くなる傾向がある。 According to the graph (FIG. 3) plotting the dependence between the concentration of boron (B) and the μτ (h) of holes, the boron (B) concentration has a fairly strong correlation with the μτ (h) of holes. I understand that. It can be said that the higher the boron (B) concentration, the lower the hole μτ (h), and the lower the boron (B) concentration, the higher the hole μτ (h). In particular, in the case of boron (B), if the concentration is suppressed to 20 atom ppb or less, μτ (h) of holes tends to increase rapidly.
また、ＣｄＴｅ系半導体に含まれるボロン（Ｂ）濃度及び窒素（Ｎ）の濃度の和と正孔のμτ（ｈ）値との関係をプロットした図７によれば、ボロン（Ｂ）濃度と窒素（Ｎ）濃度の合計値は、正孔のμτ（ｈ）値との間に、より明確な強い相関が見られた（図７）。図７は、ボロン（Ｂ）濃度と窒素（Ｎ）濃度の合計値が低くなれば正孔のμτ（ｈ）値が高くなり、ボロン（Ｂ）濃度と窒素（Ｎ）濃度の合計値が高くなれば正孔のμτ（ｈ）値が低くなる傾向を示している。一方で、ボロン（Ｂ）濃度、炭素（Ｃ）濃度、窒素（Ｎ）濃度、酸素（Ｏ）濃度の合計値と正孔のμτ（ｈ）との関係は、図８からも明らかなとおり、ボロン（Ｂ）濃度、炭素（Ｃ）濃度、窒素（Ｎ）濃度、酸素（Ｏ）濃度の合計値が小さい場合には、ある程度の相関（不純物濃度の合計値が小さければ、正孔のμτ（ｈ）は高くなる）はあるが、不純物濃度の合計値が大きくなると、単純なμτ（ｈ）の減少傾向が見られず、ばらついている。 Further, according to FIG. 7 in which the relationship between the sum of boron (B) concentration and nitrogen (N) concentration contained in the CdTe-based semiconductor and the μτ (h) value of holes is plotted, the boron (B) concentration and nitrogen A clearer and stronger correlation was observed between the total value of (N) concentration and the μτ (h) value of holes (FIG. 7). FIG. 7 shows that when the total value of boron (B) concentration and nitrogen (N) concentration is lowered, the μτ (h) value of holes is increased, and the total value of boron (B) concentration and nitrogen (N) concentration is increased. If so, the μτ (h) value of holes tends to be low. On the other hand, the relationship between the total value of boron (B) concentration, carbon (C) concentration, nitrogen (N) concentration, and oxygen (O) concentration and μτ (h) of holes is also clear from FIG. When the total value of boron (B) concentration, carbon (C) concentration, nitrogen (N) concentration, and oxygen (O) concentration is small, a certain degree of correlation (if the total value of impurity concentration is small, μτ ( h) becomes high), but when the total value of the impurity concentration becomes large, a simple decrease tendency of μτ (h) is not seen and varies.
さらに、比較例１及び７に着目すると、以下の傾向が見える。すなわち、比較例１では、ボロン（Ｂ）濃度３５ａｔｏｍ ｐｐｂ、窒素（Ｎ）濃度１０ａｔｏｍ ｐｐｂであり、一方、比較例７のボロン（Ｂ）濃度は２５ａｔｏｍ ｐｐｂ、窒素（Ｎ）濃度４５ａｔｏｍ ｐｐｂとなっており、ボロン（Ｂ）濃度については比較例１の方が高く、窒素（Ｎ）濃度については比較例７の方が高い濃度となっている。正孔のμτ（ｈ）値は、比較例７の方が高いことから、ボロン濃度の低減がμτ（ｈ）を大きくするのに効果があると言える。 Further, when attention is paid to Comparative Examples 1 and 7, the following tendencies can be seen. That is, in Comparative Example 1, the boron (B) concentration is 35 atom ppb and the nitrogen (N) concentration is 10 atom ppb, while the boron (B) concentration in Comparative Example 7 is 25 atom ppb and the nitrogen (N) concentration is 45 atom ppb. The boron (B) concentration is higher in the comparative example 1, and the nitrogen (N) concentration is higher in the comparative example 7. Since the μτ (h) value of holes is higher in Comparative Example 7, it can be said that reduction of the boron concentration is effective in increasing μτ (h).
以上の考察及び表１より、正孔のμτ（ｈ）は、ボロン（Ｂ）濃度に大きく依存し、窒素（Ｎ）濃度についてはボロン（Ｂ）濃度ほどではないが、正孔のμτ（ｈ）値に影響を及ぼしていることが分かった。具体的には、ボロン（Ｂ）濃度が１８．５ａｔｏｍ ｐｐｂより低く、尚且つ窒素（Ｎ）濃度も３．５ａｔｏｍ ｐｐｂ以下に抑えられている場合は、安定的にかなり高い正孔のμτ（ｈ）値となるＣｄＴｅ系半導体になるということができる。 From the above consideration and Table 1, the μτ (h) of the hole depends greatly on the boron (B) concentration, and the nitrogen (N) concentration is not as high as the boron (B) concentration. ) The value was found to be affected. Specifically, when the boron (B) concentration is lower than 18.5 atom ppb and the nitrogen (N) concentration is suppressed to 3.5 atom ppb or less, μτ (h It can be said that it becomes a CdTe-based semiconductor having a value.
また、図７に基づけば、ボロン（Ｂ）濃度と窒素（Ｎ）濃度との和が３０ａｔｏｍ ｐｐｂ以下とすると、正孔のμτ（ｈ）値を高くすることができるといえる。 Further, based on FIG. 7, it can be said that the μτ (h) value of holes can be increased when the sum of boron (B) concentration and nitrogen (N) concentration is 30 atom ppb or less.
以上のとおり、ＣｄＴｅ系半導体の育成時において、ボロン（Ｂ）及び窒素（Ｎ）等の、不純物の混入量を極力減らすことによって、ＣｄＴｅ系化合物半導体の正孔のμτ（ｈ）値を高めるとともに、同じ組成のＣｄＴｅ系化合物半導体において、製造の度毎に、正孔のμτ（ｈ）値がばらつく問題点を再現性よく抑えることが可能となる。 As described above, when the CdTe-based semiconductor is grown, the amount of impurities such as boron (B) and nitrogen (N) is reduced as much as possible to increase the μτ (h) value of the holes in the CdTe-based compound semiconductor. In the CdTe compound semiconductor having the same composition, it is possible to suppress the problem that the μτ (h) value of the hole varies every time the product is manufactured with high reproducibility.
なお、今回の一連の実験によって、ＣｄＴｅ系半導体の育成時に、単にＣｄＴｅ系半導体内に混入してくるだけでなく、他の不純物であるボロン（Ｂ）、窒素（Ｎ）及び炭素（Ｃ）を炉材から引き抜き、結晶内に取り込む原因となる酸素（Ｏ）をＣｄＴｅ系半導体の結晶製造装置内から除く方法として、坩堝などの炉材を加熱して残留水分量を減らすことは、有効な手段の１つであることが分かった。坩堝などの炉材の加熱時間に応じて、ＣｄＴｅ系半導体内に取り込まれる不純物の量が減少傾向にあるためである。 In this series of experiments, during the growth of the CdTe semiconductor, it is not only mixed into the CdTe semiconductor, but other impurities such as boron (B), nitrogen (N), and carbon (C) are added. As a method of removing oxygen (O), which is a cause of drawing out from the furnace material and taking it into the crystal, from the inside of the CdTe-based semiconductor crystal manufacturing apparatus, it is an effective means to reduce the residual moisture content by heating the furnace material such as a crucible. It turned out to be one of This is because the amount of impurities taken into the CdTe-based semiconductor tends to decrease according to the heating time of the furnace material such as a crucible.
本発明の実施例においては、ＣｄＴｅ半導体の例を用いて説明したが、II-VI族化合物半導体である、セレンテルル化カドミウム（CdSeTe）、テルル化亜鉛カドミウム（CdZnTe）、セレンテルル化亜鉛カドミウム（CdZnSeTe）についても不純物であるボロン（Ｂ）、窒素（Ｎ）、炭素（Ｃ）及び酸素（Ｏ）の存在が大きく正孔のμτ（ｈ）値に影響を与えている。したがって、ボロン（Ｂ）及び窒素（Ｎ）の各不純物が入り込む量を極力減らすことによって、正孔のμτ（ｈ）値を高めるとともに、正孔のμτ（ｈ）値のばらつきを抑えることが可能である。 In the examples of the present invention, the example of the CdTe semiconductor is described. However, the II-VI group compound semiconductors are cadmium selenite (CdSeTe), zinc cadmium telluride (CdZnTe), and zinc cadmium selenite (CdZnSeTe). The presence of impurities boron (B), nitrogen (N), carbon (C) and oxygen (O) greatly affects the μτ (h) value of holes. Therefore, by reducing the amount of boron (B) and nitrogen (N) impurities as much as possible, it is possible to increase the μτ (h) value of holes and suppress variations in the μτ (h) values of holes. It is.
上記の実施形態に限られたものではなく、趣旨を逸脱しない範囲で適宜変更することが可能である。 The present invention is not limited to the above-described embodiment, and can be appropriately changed without departing from the spirit.
２００ 炉心管
２０１ 石英アンプルの結晶育成部
２０１a 石英アンプルのＣｄ蒸気圧印加用リザーバ部
２０２ ヒーター
２０３ 坩堝
２０４ Ｃｄ
２０５ ＣｄＴｅ、又は、ＣｄＺｎＴｅ原料
200
202 heater
203 crucible
204 Cd
205 CdTe or CdZnTe raw material
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