陳首部，陸 軸，蘭 椿

(中南民族大學(xué) 電子信息工程學(xué)院，武漢 430074)

氧化鋅薄膜的微觀結(jié)構(gòu)及其結(jié)晶性能研究

陳首部，陸 軸，蘭 椿

(中南民族大學(xué) 電子信息工程學(xué)院，武漢 430074)

以普通玻璃作為襯底材料，采用射頻磁控濺射方法制備了氧化鋅(ZnO)透明導(dǎo)電薄膜，通過(guò)X射線衍射(XRD)和X射線光電子能譜(XPS)測(cè)試，研究了襯底溫度對(duì)薄膜微觀結(jié)構(gòu)及其結(jié)晶性能的影響.結(jié)果表明：所制備的ZnO薄膜均為(002)晶面擇優(yōu)取向生長(zhǎng)的多晶薄膜，其微觀結(jié)構(gòu)和結(jié)晶性能與襯底溫度密切相關(guān).襯底溫度對(duì)ZnO薄膜的織構(gòu)系數(shù)TC(hkl)、平均晶粒尺寸、位錯(cuò)密度、晶格應(yīng)變和晶格常數(shù)都具有不同程度的影響，當(dāng)襯底溫度為800 K時(shí)，ZnO薄膜樣品的織構(gòu)系數(shù)TC(002)最高(4.929)、平均晶粒尺寸最大(20.91 nm)、位錯(cuò)密度最小(2.289×1015line·m-2)、晶格應(yīng)變最低(2.781×10-3)，具有最高的(002)晶面擇優(yōu)取向生長(zhǎng)性和最佳的微觀結(jié)構(gòu)性能.

氧化鋅；薄膜；微觀結(jié)構(gòu)；結(jié)晶性能

作為第三代新型半導(dǎo)體材料的主要代表之一，氧化鋅(ZnO)不僅自然儲(chǔ)量豐富、價(jià)格低廉、綠色環(huán)保，同時(shí)還具有優(yōu)異的光電、光敏、壓電和壓敏等性質(zhì).它與硫化鋅(ZnS)和氮化鎵(GaN)相比，ZnO在室溫條件下具有較寬的直接帶隙和較高的自由激子結(jié)合能，是制備光電功能器件的優(yōu)良材料，已被廣泛應(yīng)用于太陽(yáng)能電池[1-5]、發(fā)光顯示器[6-11]、半導(dǎo)體激光器[12]、紫外探測(cè)器[13]、聲表面波器件[14]以及觸摸控制面板[15]等領(lǐng)域具有廣闊的應(yīng)用前景.

目前，制備ZnO薄膜的方法多種多樣，如水熱法[16]、溶膠-凝膠法[17]、化學(xué)氣相沉積法[18]、原子層沉積法[19]、脈沖激光沉積法[20]、噴霧熱分解法[21]和磁控濺射法[22-25]等，其中磁控濺射技術(shù)具有工藝簡(jiǎn)單、成膜均勻、致密性好、成本低廉、易于大面積制備等優(yōu)點(diǎn)，因此得到了業(yè)界的廣泛應(yīng)用.ZnO薄膜的晶體質(zhì)量及其性能與其制備工藝參數(shù)密切相關(guān)，其中影響較大的工藝因素有襯底溫度、濺射功率和工作壓強(qiáng)等，因此深入研究濺射工藝參數(shù)對(duì)ZnO薄膜微觀結(jié)構(gòu)的影響具有十分重要的意義.本文以普通玻璃作為襯底材料，采用射頻磁控濺射方法制備ZnO薄膜樣品，通過(guò)X射線衍射(XRD)和X射線光電子能譜(XPS)測(cè)試表征，研究了襯底溫度對(duì)ZnO薄膜微觀結(jié)構(gòu)及其結(jié)晶性能的影響.

1 實(shí)驗(yàn)部分

1.1 襯底處理

采用普通玻璃作為襯底材料，切割成大小為30 mm×30 mm的方塊，實(shí)驗(yàn)時(shí)按照如下程序?qū)ΣＡбr底進(jìn)行處理：(1)采用丙酮擦拭襯底表面，并用清水沖洗干凈；(2)依次使用丙酮、無(wú)水乙醇和純凈水對(duì)襯底進(jìn)行超聲清洗13 min，以去除襯底表面的微粒和有機(jī)污染物；(3)在無(wú)水乙醇中煮沸，吹干待用.

1.2 樣品制備

利用射頻磁控濺射方法在玻璃襯底上制備ZnO薄膜樣品，所用實(shí)驗(yàn)設(shè)備為KDJ-567型高真空復(fù)合鍍膜系統(tǒng)，濺射源為直徑50 mm、厚度4 mm的ZnO陶瓷靶材，它以ZnO粉體(999.99%)為原料通過(guò)常壓固相燒結(jié)工藝制成.濺射制備ZnO薄膜樣品之前，將濺射室的真空度抽至5×10-4Pa后通入99.999%的高純氬氣作為工作氣體，并先采用氬等離子體對(duì)玻璃襯底表面清洗7 min，然后再預(yù)濺射10 min以清潔靶材表面和穩(wěn)定系統(tǒng)，提高沉積ZnO薄膜樣品的質(zhì)量.實(shí)驗(yàn)時(shí)，襯底與靶材之間的距離為75 mm、濺射功率為200 W、工作氣壓為0.5 Pa、沉積時(shí)間為25 min、襯底溫度為600～800 K.

1.3 樣品表征

通過(guò)X射線衍射儀(Bruker advance D8型，德國(guó)Bruker公司)對(duì)ZnO薄膜樣品進(jìn)行晶體結(jié)構(gòu)表征，測(cè)試時(shí)使用Cu Kα射線源(波長(zhǎng)λ=0.1541 nm)，采用θ-2θ連續(xù)掃描方式，掃描速度為10°/min，掃描步長(zhǎng)為0.0164 ?，掃描范圍為20°≤2θ≤70°，工作電壓為40 kV，工作電流為40 mA.利用X射線光電子能譜儀(VG Multilab 2000型，美國(guó)Thermo Electron公司)對(duì)ZnO薄膜樣品進(jìn)行XPS分析，測(cè)試時(shí)本底真空度為2.0×10-6Pa，X射線源為單色Al Kα射線源(hv=1486.60 eV)，采用C 1s結(jié)合能(284.60 eV)作為內(nèi)標(biāo)，對(duì)所有測(cè)試譜峰進(jìn)行荷電校正.所的測(cè)試均在室溫條件下完成.

2 結(jié)果與討論

圖1為不同襯底溫度時(shí)ZnO薄膜樣品的XRD圖譜，由圖可見(jiàn)，在2θ為20 °～70°的掃描范圍內(nèi)，所有ZnO薄膜樣品在峰位2θ為30.9°和34.1°附近都出現(xiàn)了2個(gè)特征峰，比對(duì)ZnO的標(biāo)準(zhǔn)PDF卡片(JCPDS #36-1451，見(jiàn)圖1)可以看出，這2個(gè)衍射峰分別與ZnO的(100)和(002)晶向相吻合.

圖1 ZnO薄膜樣品的XRD圖譜Fig.1 XRD patterns of the ZnO samples

另外從圖1中還可看到，襯底溫度不同時(shí)，ZnO薄膜樣品還存在有其它晶向的特征峰，如襯底溫度為600和800 K時(shí)，分別顯示有(110)和(103)晶面的衍射峰，而襯底溫度為700 K時(shí)，則顯示有(110)、(102)和(103)等多個(gè)晶面的衍射峰.上述XRD圖譜結(jié)果表明，所制備的ZnO樣品均為多晶薄膜，并具有六角纖鋅礦結(jié)構(gòu).觀察圖1的XRD圖譜還可以看出，襯底溫度對(duì)衍射峰位2θ的影響較小，而對(duì)各個(gè)晶向的衍射峰強(qiáng)度的影響較大，為了評(píng)估ZnO薄膜樣品沿某一晶面(hkl)的擇優(yōu)取向程度，本文采用織構(gòu)系數(shù)(TC(hkl))來(lái)定量表征樣品沿不同晶面生長(zhǎng)的取向程度.織構(gòu)系數(shù)TC(hkl)定義如下[26]：

(1)

(1)式中，下標(biāo)h、k、l表示密勒指數(shù)，TC(hkl)表示(hkl)晶面的織構(gòu)系數(shù)，I(hkl)為ZnO薄膜樣品在(hkl)晶面的衍射強(qiáng)度，Ir(hkl)為標(biāo)準(zhǔn)ZnO粉未試樣(JCPDS #36-1451)在(hkl)晶面的衍射強(qiáng)度，n為計(jì)算時(shí)所取的衍射峰數(shù)目.TC(hkl)的數(shù)值越大，說(shuō)明薄膜中有更多的晶粒沿(hkl)晶面生長(zhǎng)，即薄膜在(hkl)晶面的擇優(yōu)取向性越好.表1列出了不同襯底溫度時(shí)ZnO薄膜樣品的織構(gòu)系數(shù)TC(hkl)，由表1可見(jiàn)，當(dāng)襯底溫度為600、700和800 K時(shí)，ZnO薄膜樣品的TC(002)值分別為4.916、4.363和4.929，均遠(yuǎn)遠(yuǎn)高于其它晶面的TC(hkl)數(shù)值，這說(shuō)明所制備的ZnO樣品都表現(xiàn)出明顯的(002)晶面擇優(yōu)取向生長(zhǎng)特征，并且襯底溫度升高時(shí)，TC(002)的數(shù)值呈現(xiàn)出先減小后增大的變化趨勢(shì).可見(jiàn)，襯底溫度從600 K增加到800 K時(shí)，雖然沒(méi)有改變ZnO薄膜(002)擇優(yōu)取向生長(zhǎng)特征，但是對(duì)其擇優(yōu)取向程度有一定的影響，當(dāng)襯底溫度為800 K時(shí)所制備的ZnO樣品具有最高的(002)擇優(yōu)取向程度.其原因是：ZnO薄膜在(002)晶面的表面自由能密度是最小的，因此晶粒沿(002)晶面具有生長(zhǎng)優(yōu)勢(shì)，在生長(zhǎng)過(guò)程中晶粒極易沿c軸即(002)晶面平行于襯底的方向生長(zhǎng)[27,28].

表1 ZnO薄膜樣品的織構(gòu)系數(shù)Tab.1 The values of TC(hkl) for the ZnO samples

圖2為襯底溫度800 K時(shí)所制備ZnO薄膜樣品的XPS能譜圖，由圖2可見(jiàn)，XPS圖譜上除了Zn和O原子的光電子特征峰之外，在284.6 eV處還存在有C 1s特征峰，這可能是由于濺射鍍膜時(shí)油擴(kuò)散泵污染或者ZnO薄膜樣品暴露在大氣中吸附了CO2所造成的[29].

圖2 襯底溫度為700 K時(shí)所制備ZnO樣品的XPS能譜圖Fig.2 XPS spectra of the ZnO sample prepared at substrate temperature of 700 K

圖3(a)為不同襯底溫度時(shí)ZnO薄膜樣品的(002)衍射峰半高寬(B)數(shù)值，可見(jiàn)半高寬B的值與襯底溫度密切相關(guān)，襯底溫度增加時(shí)，半高寬B單調(diào)減小，當(dāng)襯底溫度為800 K時(shí)，ZnO薄膜樣品(002)衍射峰的半高寬B最小值為0.392°，說(shuō)明襯底溫度為800 K時(shí)制備的ZnO薄膜樣品具有最大的晶粒尺寸和最佳的結(jié)晶性能.ZnO薄膜樣品的平均晶粒尺寸(D)可以根據(jù)謝樂(lè)公式[30]計(jì)算：

(2)

(2)式中，K為謝樂(lè)常數(shù)(這里取K=0.89)，θ為所(002)晶面的布拉格角，B為(002)衍射峰的半高寬數(shù)值，λ為XRD測(cè)試時(shí)的X射線波長(zhǎng)[31].圖3(b)為不同襯底溫度時(shí)ZnO薄膜樣品的平均晶粒尺寸D，從圖中3(b)看出，襯底溫度對(duì)ZnO樣品的平均晶粒尺寸D具有明顯的影響.當(dāng)襯底溫度為600～800 K時(shí)，ZnO樣品的平均晶粒尺寸D為9.73～20.91 nm，平均晶粒尺寸D隨襯底溫度增加而增大，當(dāng)襯底溫度為800 K時(shí)，ZnO薄膜樣品的D值最大(20.91 nm).

圖3 不同襯底溫度時(shí)ZnO樣品的B和DFig.3 The B and D values of the ZnO sample prepared at different substrate temperatures

ZnO薄膜樣品的位錯(cuò)密度(δ)[31]利用公式(3)計(jì)算獲得：

(3)

(3)式中，D為ZnO薄膜樣品的平均晶粒尺寸.ZnO薄膜樣品的位錯(cuò)密度δ隨襯底溫度變化的曲線如圖4所示，可以看出，隨著襯底溫度的增加，δ呈現(xiàn)出單調(diào)減小的變化趨勢(shì)，當(dāng)襯底溫度為800 K時(shí)，ZnO薄膜樣品的位錯(cuò)密度δ最小為2.289×1015line·m-2.

圖4 不同襯底溫度時(shí)ZnO樣品的δ值Fig.4 The δ values of the ZnO sample prepared at different substrate temperatures

ZnO薄膜樣品的晶格應(yīng)變(ε)可由下式[32]計(jì)算：

(4)

(4)式中，K為由謝樂(lè)常數(shù)，θ為所(002)晶面的布拉格角，B為(002)衍射峰的半高寬數(shù)值.不同襯底溫度時(shí)ZnO薄膜樣品的ε值如圖5所示，從圖5看出，襯底溫度對(duì)ZnO薄膜ε值具有明顯的影響，ε值隨著襯底溫度的增加而逐漸減小，當(dāng)襯底溫度為800 K時(shí)，ZnO薄膜樣品具有最小的晶格應(yīng)變?chǔ)?，其值?.781×10-3.

圖5 不同襯底溫度時(shí)ZnO樣品的ε值Fig.5 The ε values of the ZnO sample prepared at different substrate temperatures

ZnO薄膜樣品為六角纖鋅礦結(jié)構(gòu)，其晶格常數(shù)由公式(5)確定[33,34]：

(5)

(5)式中，a和c為ZnO樣品的晶格常數(shù).對(duì)于(002)晶面，由(5)式可得：

(6)

對(duì)于(100)晶面，(5)式可簡(jiǎn)化為：

(7)

圖6為不同襯底溫度時(shí)ZnO薄膜樣品的晶格常數(shù)a、c和c/a的數(shù)值，從圖6看出，襯底溫度增大時(shí)，a先減后增、c單調(diào)增加、c/a先增后減，在襯底溫度的變化范圍為600～800 K時(shí)，a、c和c/a的數(shù)值范圍分別為0.32845～0.33608 nm、0.52259～0.52857 nm和1.57275～1.59411，這些結(jié)果與標(biāo)準(zhǔn)ZnO試樣(JCPDS #36-1451)數(shù)據(jù)(a=0.32498 nm、c=0.52066 nm、c/a=1.60213)是一致的.文獻(xiàn)[35,36]在研究摻釔ZnO和摻鋰ZnO薄膜時(shí)也有類似的報(bào)道.

圖6 不同襯底溫度時(shí)ZnO樣品的a、c和c/aFig.6 The values of a, c and c/a for the ZnO sample prepared at different substrate temperatures

ZnO薄膜樣品的Zn-O鍵長(zhǎng)(L)[37]可由公式(8)計(jì)算獲得：

(8)

(8)式中，a和c為ZnO薄膜樣品的晶格常數(shù)，u與a、c之間滿足關(guān)系式[37]：

(9)

圖7為ZnO樣品薄膜Zn-O鍵長(zhǎng)L隨襯底溫度的變化曲線，從圖可知，襯底溫度對(duì)ZnO薄膜的Zn-O鍵長(zhǎng)L具有一定的影響，當(dāng)襯底溫度為600、700和800 K時(shí)，ZnO樣品的Zn-O鍵長(zhǎng)L值分別為0.2002、0.19957和0.20337 nm，其結(jié)果與標(biāo)準(zhǔn)ZnO試樣(JCPDS No. 36-1451)數(shù)據(jù)(L=0.19778 nm)基本一致.Anandan等人[35]和Srinivasan小組[36]在研究摻雜ZnO薄膜時(shí)也報(bào)道過(guò)類似的結(jié)果.

圖7 不同襯底溫度時(shí)ZnO樣品的L值Fig.7 The L values of the ZnO sample prepared at different substrate temperatures

3 結(jié)語(yǔ)

采用ZnO陶瓷靶為濺射源材料，利用射頻磁控濺射技術(shù)在普通玻璃襯底上制備了ZnO薄膜樣品，通過(guò)XRD和XPS測(cè)試表征，研究了襯底溫度對(duì)ZnO薄膜樣品微觀結(jié)構(gòu)及其結(jié)晶性能的影響.結(jié)果表明，所有ZnO薄膜樣品均為六角纖鋅礦結(jié)構(gòu)的多晶薄膜，并且襯底溫度對(duì)薄膜生長(zhǎng)特性及其微觀結(jié)構(gòu)性能具有明顯的影響.襯底溫度升高時(shí)，ZnO薄膜的織構(gòu)系數(shù)TC(002)、晶格常數(shù)a和Zn-O鍵長(zhǎng)L先減后增，平均晶粒尺寸D和晶格常數(shù)c單調(diào)增加，而位錯(cuò)密度δ和晶格應(yīng)變?chǔ)艅t單調(diào)減小，當(dāng)襯底溫度為800 K時(shí)，ZnO薄膜樣品的織構(gòu)系數(shù)TC(002)最高為4.929、平均晶粒尺寸D最大為20.91 nm、位錯(cuò)密度δ最小為2.289×1015line·m-2、晶格應(yīng)變?chǔ)淖畹蜑?.781×10-3，所制備的ZnO薄膜具有最高的(002)晶面擇優(yōu)取向生長(zhǎng)性和最好的微觀結(jié)構(gòu)性能.

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MicrosructureandCrystallineCharacteristicsofZincOxideThinFilms

ChenShoubu,LuZhou,LanChun

(College of Electronic Information Engineering, South-Central University for Nationalities, Wuhan 430074, China)

The transparent conducting oxide thin films of zinc oxide (ZnO) were deposited on glass substrates by radio-frequency magnetron sputtering method. The influence of substrate temperature on the mirostructure and crystalline characteristics of ZnO thin films was investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The results indicate that the deposited thin films with the hexagonal crystal structure are polycrystalline and have a strongly preferred orientation of (002) plane. The mirostructure and crystalline characteristics of the thin films are observed to be subjected to the substrate temperature. When the substrate temperature is 800 K, the deposited ZnO sample exhibits the best crystalline and microstructural properties, with the highest texture coefficient of (002) plane of 4.929, the largest average grain size of 20.91 nm, the minimum dislocation density of 2.289×1015line·m-2and the lowest lattice strain of 2.781×10-3.

zinc oxide; thin film; microstructure; crystalline characteristics

2017-05-28

陳首部(1964-), 男, 高級(jí)工程師, 研究方向: 等離子體應(yīng)用技術(shù), E-mail: chensb64@sohu.com

湖北省自然科學(xué)基金資助項(xiàng)目(2011CDB418)

TM914

A

1672-4321(2017)04-0067-06