表面修飾金剛石薄膜導(dǎo)電性研究進(jìn)展＊

劉峰斌，陳文彬

(北方工業(yè)大學(xué) 機(jī)械與材料工程學(xué)院，北京 100144)

表面修飾金剛石薄膜導(dǎo)電性研究進(jìn)展＊

劉峰斌1，陳文彬1

(北方工業(yè)大學(xué) 機(jī)械與材料工程學(xué)院，北京 100144)

表面經(jīng)過不同化學(xué)修飾，金剛石薄膜會(huì)表現(xiàn)出不同的表面導(dǎo)電性能。這使得其在平面微電子、微電化學(xué)器件開發(fā)方面有著廣闊的應(yīng)用前景?？偨Y(jié)國內(nèi)外的研究，結(jié)合近年來的研究成果，詳述了不同表面修飾金剛石薄膜的實(shí)現(xiàn)方法，討論了表面修飾金剛石薄膜的幾何結(jié)構(gòu)和電子結(jié)構(gòu)，并對(duì)當(dāng)前有關(guān)表面修飾金剛石薄膜導(dǎo)電機(jī)理的主要觀點(diǎn)進(jìn)行了分析。在此基礎(chǔ)上，提出了今后不同表面修飾金剛石薄膜導(dǎo)電性的研究重點(diǎn)。

金剛石；表面修飾；導(dǎo)電性

0 引 言

金剛石具有原子密度大、硬度高、彈性模量大、帶隙寬、導(dǎo)熱性好和摩擦系數(shù)小等特點(diǎn)，非常適用于制作工具涂層、大功率激光器、表面波器件以及光學(xué)窗口材料等，受到研究者的廣泛關(guān)注。同時(shí)，由于化學(xué)氣相沉積工藝的出現(xiàn)和摻雜技術(shù)的發(fā)展，導(dǎo)電性良好的金剛石薄膜實(shí)現(xiàn)了常壓低溫制備，氣體壓力一般在1～50 kPa之間，基底溫度低于1 000 ℃，為其在電子元件和電化學(xué)元件的應(yīng)用提供了可能[1-2]。

1989年，Landstrass和Ravi[3]首次發(fā)現(xiàn)經(jīng)過氫等離子濺射處理的金剛石薄膜表面表現(xiàn)出類似金屬的高導(dǎo)電性。這一獨(dú)特性質(zhì)使得表面修飾金剛石薄膜可能用于平面微電子器件的開發(fā)，立刻引起了研究人員的興趣。研究者通過大量理論計(jì)算和實(shí)驗(yàn)方法驗(yàn)證了該現(xiàn)象[4-11]，并進(jìn)一步提出表面氨基修飾的金剛石薄膜也呈現(xiàn)類似性質(zhì)，但是氧修飾不能改變金剛石薄膜絕緣性[4,10-11]。利用所發(fā)現(xiàn)的導(dǎo)電特性，基于表面修飾金剛石薄膜的平面場(chǎng)效應(yīng)晶體管(FET)[12-13]、pH值傳感器[14]等微器件的研制獲得了較快發(fā)展。同時(shí)，為了探明金剛石薄膜表面修飾后呈現(xiàn)不同導(dǎo)電性的導(dǎo)電機(jī)理，研究者在表面修飾金剛石薄膜幾何結(jié)構(gòu)與電子結(jié)構(gòu)上進(jìn)行了系統(tǒng)的研究，取得了大量有價(jià)值的結(jié)果。但是，現(xiàn)有部分研究結(jié)果存在一定的爭(zhēng)議，關(guān)于表面修飾金剛石薄膜的導(dǎo)電機(jī)理也尚不明確[15-20]。

本文旨在結(jié)合最近的研究成果，對(duì)國內(nèi)外在表面修飾金剛石薄膜導(dǎo)電機(jī)理方面的研究進(jìn)行總結(jié)分析，并提出該領(lǐng)域后續(xù)的研究重點(diǎn)。

1 金剛石薄膜表面修飾方法

自1989年Landstrass和Ravi利用氫等離子濺射獲得氫修飾金剛石薄膜以來[3]，研究者又陸續(xù)開發(fā)了多種表面修飾金剛石薄膜的制備方法。這些方法包括光化學(xué)引發(fā)[21-22]、熱化學(xué)反應(yīng)[23-24]以及電化學(xué)氧化還原等[25-27]，獲得了氫基、氨基、氧基(羰基、羧基)等大量表面修飾金剛石薄膜。

圖1 兩種修飾金剛石薄膜表面XPS譜[10]

Fig 1 XPS spectra of hydrogen-terminated and oxygen-terminated diamond films[10]

相比于氫、氧修飾工藝來說，氨基的修飾方法相對(duì)復(fù)雜，傳統(tǒng)的氨基修飾方式是先在薄膜表面修飾上氯元素，再通過元素置換的方式修飾氨基。由于薄膜表面被氯元素活化，修飾氨基時(shí)反應(yīng)相對(duì)比較容易進(jìn)行。然而，這種兩步走的修飾方法有一定的局限性，因?yàn)楸∧け砻媛仍氐男揎椔什桓?，?dǎo)致了氨基修飾率也受到限制[21,23]。所以，研究者們提出一步到位的氨基修飾工藝。目前最常用氨基化方法有以下幾種：(1) 離子體處理法，即將氫修飾金剛石薄膜放置在13.65 MHz的等離子體容器中，通入He和NH3混合氣體進(jìn)行反應(yīng)，Szunerits等利用此方法制備出氨修飾金剛石薄膜，并從XPS表征結(jié)果中看到薄膜在400 eV處出現(xiàn)N1s特征峰[35]；(2) 光化學(xué)反應(yīng)法，即將氫修飾金剛石薄膜放置在氨氣連續(xù)鼓泡的反應(yīng)器中，用紫外線燈(波長(zhǎng)254 nm)照射進(jìn)行反應(yīng)實(shí)現(xiàn)氨基化[36]。安云玲等[21]用光化學(xué)方法實(shí)現(xiàn)氨基修飾，從XPS結(jié)果得出氨修飾后金剛石薄膜的C1s特征峰強(qiáng)度有所下降，O1s特征峰強(qiáng)度有所增強(qiáng)，并且在400 eV處出現(xiàn)N1s特征峰；(3) 陽極氧化法，即將金剛石薄膜進(jìn)行超聲清洗后，放入濃度為0.1 mol/L的硫酸溶液中施加+2.4 V的電位進(jìn)行電化學(xué)陽極氧化30 min。將完成電化學(xué)陽極氧化的BDD薄膜放入含有2%(體積分?jǐn)?shù))APTES的乙醇溶液中2 h，取出后超聲清洗，即完成金剛石薄膜表面的氨基化處理[37]。

通過大量實(shí)驗(yàn)發(fā)現(xiàn)，不同修飾的金剛石薄膜表面表現(xiàn)出完全不同的電子親和性[7,38-40]、親疏水性[41-44]等。同時(shí)，也可通過這些性能的測(cè)試來對(duì)不同修飾金剛石薄膜進(jìn)行表征。目前，研究者通過檢測(cè)不同修飾金剛石薄膜表面的潤(rùn)濕角發(fā)現(xiàn)，氫修飾金剛石薄膜表面的潤(rùn)濕角超過90°，表現(xiàn)出疏水性質(zhì)[41]；而氧修飾金剛薄膜表面的潤(rùn)濕角為0.6～64.7°，表現(xiàn)出親水性[42-44]。氧修飾金剛石薄膜較大的潤(rùn)濕角差異結(jié)果與金剛石表面氧化處理方法、薄膜表面形貌特征以及sp2/sp3比值大小有很大關(guān)系。本課題組測(cè)試的氫修飾薄膜表面潤(rùn)濕角為92.5°，而氧修飾金剛石薄膜表面潤(rùn)濕角為1.6°[10,28,31]。另外，研究者還發(fā)現(xiàn)氫修飾薄膜表面表現(xiàn)出負(fù)的電子親和勢(shì)(NEA)，而氧修飾薄膜表面呈現(xiàn)正的電子親和勢(shì)(PEA)[7,38-40]。

2 不同修飾金剛石薄膜表面幾何構(gòu)型

對(duì)于氫修飾金剛石薄膜表面幾何結(jié)構(gòu)，研究者們進(jìn)行了廣泛的研究。Lurie等[45]利用低能電子衍射(LEED)觀察了金剛石(100)表面在退火過程中表面幾何構(gòu)型的變化，發(fā)現(xiàn)隨著溫度的升高，其幾何構(gòu)型從(1×1)結(jié)構(gòu)轉(zhuǎn)變?yōu)?2×1)結(jié)構(gòu)。Hamza等[46]指出轉(zhuǎn)變會(huì)以兩步實(shí)現(xiàn)：首先，從C(1×1)∶nH轉(zhuǎn)變?yōu)镃(2×1)∶H再構(gòu)形式；然后，隨著溫度的升高，氫完全脫附，進(jìn)一步轉(zhuǎn)變?yōu)镃(2×1)清潔表面。其中1×1∶nH中n的數(shù)值則可能為對(duì)稱結(jié)構(gòu)的C(1×1)∶2H[47]、傾斜型C(1×1)∶2H[48]以及C(1×1)∶1.5H[49]，見圖2所示。Bobrov等[50]利用掃描隧道顯微鏡(STM)直接在原子尺度下觀察到了金剛石(100)表面單層氫吸附(2×1)結(jié)構(gòu)，并測(cè)量了C—C二聚體的間距。不同金剛石晶面氫修飾后表面幾何結(jié)構(gòu)不同。對(duì)于氫修飾金剛石(100)晶面，實(shí)驗(yàn)和理論計(jì)算都表明單原子層覆蓋的(2×1)結(jié)構(gòu)是其最穩(wěn)定結(jié)構(gòu)(見圖3所示)。針對(duì)C(111)表面，Aizawa等[51]利用LEED和高分辨電子能量損失譜(HREELS)發(fā)現(xiàn)表面除了存在CH基團(tuán)(361 meV)外，還存在CH3功能團(tuán)(352 meV)。

圖2 亞穩(wěn)態(tài)氫修飾金剛石(100)表面幾何結(jié)構(gòu)[47-49]

圖3 氫修飾金剛石(100)表面C(2×1)幾何結(jié)構(gòu)

Fig 3 Geometry structure C (2×1) of hydrogen modified diamond (100) surface

圖4 亞穩(wěn)態(tài)氫修飾金剛石(100)表面幾何結(jié)構(gòu)[58]

圖5 氧修飾金剛石(100)表面幾何結(jié)構(gòu)

Fig 5 Geometric structure of oxygen modified diamond (100) surface

Pehrsson等[34,63]利用HREELS、俄歇譜(AES)、能量損失譜(ELS)和低能電子衍射，分析了不同氧覆蓋程度的金剛石(100)表面的表面組分，發(fā)現(xiàn)開始吸附

對(duì)于氨基的表面幾何結(jié)構(gòu)的研究，是近幾年才被研究者所關(guān)注的。氨基在金剛石薄膜表面的幾何構(gòu)型與氧基在金剛石薄膜表面的構(gòu)型類似，如圖6所示。Hassan等[11]利用基于密度泛函理論的第一原理計(jì)算指出，橋接結(jié)構(gòu)與頂接結(jié)構(gòu)的氨基修飾金剛石薄膜總能量相近，頂接結(jié)構(gòu)的氨基金剛石薄膜更為穩(wěn)定。

圖6 氨修飾金剛石(100)表面幾何結(jié)構(gòu)

Fig 6 Geometric structure of ammonia modified diamond (100) surface

3 不同修飾金剛石薄膜的導(dǎo)電機(jī)理

氫修飾金剛石薄膜表現(xiàn)出高導(dǎo)電性主要決定于其表面電子結(jié)構(gòu)。目前，研究者提出兩種模型試圖對(duì)其進(jìn)行解釋。一種是能帶彎曲模型，Sugino等[64]認(rèn)為氫修飾金剛石薄膜費(fèi)米能級(jí)附近的淺受主能級(jí)引起了能帶向上彎曲，從而表現(xiàn)出P型導(dǎo)電性。Kern等[52-55]用從頭算的方法分析了金剛石不同晶面上的電荷態(tài)密度，發(fā)現(xiàn)(100)和(110)表面帶隙中表面態(tài)存在與否跟氫的覆蓋度有關(guān)，而(111)面，1×1構(gòu)型氫修飾金剛石表面始終存在表面態(tài)。Davidson等[47]通過緊約束分子動(dòng)力學(xué)的方法得出相同結(jié)論，并且Davidson與Yang[65]都認(rèn)為是由于懸掛鍵的存在而誘發(fā)表面態(tài)的產(chǎn)生。本課題組[10,28]借助XPS價(jià)帶譜以及掃描隧道譜分析表明，氫修飾金剛石薄膜表面能帶向上彎曲，在高于價(jià)帶頂位置存在淺受主能級(jí)；氧修飾表面能帶向下彎曲，帶隙較寬，帶隙中不存在表面態(tài)，如圖7所示。

然而，究竟氫修飾金剛石薄膜表面的表面態(tài)是由什么因素引起的，尚存在廣泛爭(zhēng)議。Kawarada等[66]認(rèn)為是由于表面吸附氫原子誘發(fā)的。Hayashi[15]則提出該表面態(tài)是由次表面的氫誘發(fā)產(chǎn)生。另外，雖然部分研究支持能帶彎曲模型，但是，也存在不少質(zhì)疑。Maire等[67]和Weide等[38]用UPS研究金剛石(100)表面并無發(fā)現(xiàn)表面態(tài)。Bobrov等[68]用STM研究金剛石表面也沒有發(fā)現(xiàn)表面態(tài)，但是氫脫附后表面態(tài)出現(xiàn)。此外，研究者還發(fā)現(xiàn)次表面氫原子所誘發(fā)能級(jí)較深，難以起到淺受主能級(jí)的作用[69-70]。

圖7 氫修飾和氧修飾金剛石薄膜表面I-V隧道譜分析[10]

圖8 氫修飾金剛石薄膜表面電化學(xué)導(dǎo)電機(jī)理[16]

Fig 8 Electrochemical conducting mechanism of hydrogen modified diamond film surface[16]

圖9 氫修飾金剛石薄膜與不同化學(xué)勢(shì)離子溶液固液界面能帶圖[72]

Fig 9 The solid liquid interfacial energy band diagram of hydrogen modified diamond film with different chemical potential and ionic solution[72]

然而，上述的模型也并非是無懈可擊。Chakrapani發(fā)表在Science上的論文中提到，由于氫修飾金剛石表面是疏水的，所以水層在該表面導(dǎo)電性中所起的作用大小有待商榷，另外，所提出的電化學(xué)電偶的熱力學(xué)和動(dòng)力學(xué)機(jī)制也存在疑問[19]。Chakrapani深入指出，表面高導(dǎo)電性有可能與水層中提供的離子有關(guān)，通過調(diào)節(jié)離子種類與濃度，可以控制氫修飾金剛石薄膜表面導(dǎo)電性。后續(xù)研究更是發(fā)現(xiàn)僅僅暴露在純凈NO2氣氛中就會(huì)導(dǎo)致電導(dǎo)率上升，但在氣氛中通入水蒸氣后電導(dǎo)率卻下降了[79]。甚至，其它研究者發(fā)現(xiàn)將金剛石薄膜暴露在干燥的NO2、O3、SO2等氣氛中時(shí)，薄膜次表面空穴濃度增加，表面電導(dǎo)率將顯著提高[80-81]。所以，之前Maier等發(fā)展起來的模型并不完善。

Yoshiteru等[82]在前人研究的基礎(chǔ)上，進(jìn)一步提出修正的電荷轉(zhuǎn)移模型。將吸附氣體分為兩組：一組包括NO2、O3、SO2、NO。這些氣體分子的最低未占據(jù)分子軌道(LUMO)能級(jí)或單占據(jù)分子軌道(SOMO)能級(jí)低于氫修飾金剛石薄膜表面價(jià)帶頂(VBM)；第2組包括水分子、N2O、CO2等，這些分子最低未占據(jù)分子軌道(LUMO)能級(jí)或單占據(jù)分子軌道(SOMO)能級(jí)都高于氫修飾金剛石薄膜表面價(jià)帶頂(VBM)，如圖10所示。通過實(shí)驗(yàn)結(jié)果分析，氫修飾金剛石薄膜通過吸附第1組氣體分子可以增加空穴濃度，而吸附第2組氣體分子則對(duì)于空穴沒有影響。他們認(rèn)為這是由于第1組氣體分子的最低未占據(jù)分子軌道(LUMO)能級(jí)低于金剛石的價(jià)帶頂，則氫修飾金剛石薄膜表面的價(jià)電子轉(zhuǎn)移至氣體分子上的未占據(jù)電子軌道，從而使氫修飾金剛石薄膜次表面集聚空穴，使其表面呈現(xiàn)p型高導(dǎo)電性。

圖10 金剛石不同晶面能帶及各類氣體分子最低未占據(jù)分子軌道能級(jí)[82]

Fig 10 The lowest molecular orbital energy levels of different crystal planes of diamond and all kinds of gas molecules[82]

對(duì)于氧修飾和氨修飾金剛石表面電子結(jié)構(gòu)，現(xiàn)如今的理論和研究數(shù)據(jù)都還十分欠缺。Zhang等[56]利用UPS研究氧修飾金剛石薄膜表面，發(fā)現(xiàn)隨著氧覆蓋度的提高，表面態(tài)的強(qiáng)度不斷降低。本課題組[57]利用密度泛函理論的第一原理方法計(jì)算了橋接型和頂接型氧吸附金剛石(100)表面的平衡態(tài)幾何結(jié)構(gòu)和電子結(jié)構(gòu)，并利用掃描隧道顯微鏡分析了氧修飾金剛石薄膜表面的掃描隧道譜，未發(fā)現(xiàn)其帶隙結(jié)構(gòu)變化。部分文獻(xiàn)則推測(cè)氨修飾金剛石薄膜與氫修飾金剛石薄膜電子結(jié)構(gòu)相似，甚至在吸附活性基團(tuán)后將表現(xiàn)出更高的導(dǎo)電性[11,35-36]。

4 結(jié) 語

表面修飾金剛石薄膜表現(xiàn)出不同的導(dǎo)電性能，且制備簡(jiǎn)單，在平面電子元器件制備方面有潛在的應(yīng)用價(jià)值。但是，其導(dǎo)電機(jī)理尚不清楚，這不可避免地限制了其進(jìn)一步應(yīng)用。研究者針對(duì)表面不同修飾金剛石薄膜導(dǎo)電機(jī)理進(jìn)行了大量工作，取得了大量重要的研究結(jié)果，但尚需在以下兩個(gè)方面進(jìn)一步開展工作：

(1) 關(guān)于表面修飾金剛石薄膜導(dǎo)電機(jī)理，研究者們從最初歸因于金剛石表面氫原子、次表面氫原子，到后來提出的電荷轉(zhuǎn)移模型以及表面吸附的活性氣體的影響，都難以完美解釋表面修飾金剛石薄膜的導(dǎo)電性變化。實(shí)際上，發(fā)現(xiàn)次表面氫原子的作用與次表面氫原子的濃度有關(guān)，低濃度氫原子濃度雖然難以誘發(fā)淺受主能級(jí)[70,83]，但是如果次表面氫原子濃度較高時(shí)，則由次表面氫原子誘發(fā)的受主能級(jí)會(huì)向價(jià)帶方向移動(dòng)[83]。因此，氫修飾金剛石薄膜表面高導(dǎo)電性很可能是表面氫原子、次表面氫原子以及表面吸附活性基團(tuán)的協(xié)同作用引起的。研究者可以通過精心設(shè)計(jì)實(shí)驗(yàn)，結(jié)合理論計(jì)算進(jìn)一步明確金剛石薄膜的導(dǎo)電機(jī)理。

(2) 當(dāng)前研究者大多研究的為氫修飾金剛石薄膜的導(dǎo)電性，對(duì)于氧修飾、氨基以及其它基團(tuán)的修飾研究相對(duì)較少。實(shí)際上，不同表面修飾的金剛石薄膜表現(xiàn)出不同的表面性能，例如，氫修飾金剛石薄膜具有疏水性和負(fù)的電子親和勢(shì)，而氧修飾金剛石薄膜具有親水性和正的電子親和勢(shì)。因此，對(duì)不同表面修飾的金剛石薄膜進(jìn)行系統(tǒng)研究，獲得多種不同表面性能的薄膜材料，有利于推動(dòng)金剛石薄膜表面功能化。

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Progress on the conducting mechanism of the surface-modified diamond films

LIU Fengbin，CHEN Wenbin

(School of Mechanical and Material Engineering,North China University of Technology，Beijing 100144，China)

By different surface modification, the diamond films would show various surface conductivities. This makes it have a broad application prospect in the development of in-plane micro-electronics and micro-electrochemical devices. By summarizing the research at home and abroad, combined with the recent research results of our group, the preparation methods of the various surface-modified diamond films are described in detail. In addition, the equilibrium geometries and electronic structure of the surface-modified diamond films are discussed. Finally, the main opinions on the conducting mechanism of the surface-modified diamond films are also analyzed. On the basis of the above, the remarkable research points on the surface conductivity of the diamond films with different surface modification in the future are proposed.

diamond; surface modification; electrical conductivity

1001-9731(2016)12-12050-08

國家自然科學(xué)基金資助項(xiàng)目(50575004)；北京市自然科學(xué)基金資助項(xiàng)目(3162010)

2015-12-17

2016-03-31 通訊作者：劉峰斌，E-mail: fbliu@ncut.edu.cn

劉峰斌 (1974-)，男，河北徐水人，博士，副教授，研究方向?yàn)椴牧媳砻胬碚撆c控制技術(shù)。

TQ127.1+1

A

10.3969/j.issn.1001-9731.2016.12.008