趙盼盼,許　崗,馮亞西,邱海蔡,李俊英

(西安工業(yè)大學(xué) 材料與化工學(xué)院,西安 710021)

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HgCl2-KI-H2O溶液體系中碘化汞(α-HgI2)籽晶層生長(zhǎng)研究

趙盼盼,許崗,馮亞西,邱海蔡,李俊英

(西安工業(yè)大學(xué) 材料與化工學(xué)院,西安 710021)

摘要：為提高氣相沉積碘化汞(α-HgI2)多晶薄膜定向生長(zhǎng)效果,研究了HgCl2和KI溶液化合反應(yīng)法制備用于同質(zhì)外延的HgI2籽晶層的生長(zhǎng)工藝.建立了HgCl2/KI=1∶1～1∶6(摩爾比)的反應(yīng)體系,通過改變體系中碘離子([I]-)濃度,研究了液相體系的反應(yīng)過程.通過紫外-可見分光光度計(jì)研究了反應(yīng)體系中組元轉(zhuǎn)變特征,采用偏光顯微鏡分析了不同濃度下晶體結(jié)晶形貌和分布特征.UV光譜表明：隨著[I]-濃度增大,體系中出現(xiàn)[HgI2]n聚體并導(dǎo)致吸收峰紅移,以及K[HgI3]·H2O吸收峰出現(xiàn)藍(lán)移.宏觀形貌分析表明:籽晶層粒度和覆蓋度隨[I]-濃度增大而增大,晶粒逐漸發(fā)育充分.[HgI2]n聚體的形成導(dǎo)致電子能級(jí)密度增加,躍遷能量降低,形成紅移；孤對(duì)電子濃度增大在K[HgI3]·H2O出現(xiàn)電子從非鍵軌道向反鍵軌道的躍遷現(xiàn)象,形成藍(lán)移.當(dāng)HgCl2/KI=1∶6時(shí),覆蓋度最佳.分析認(rèn)為,控制高濃度體系中反應(yīng)速率可降低晶粒堆垛現(xiàn)象.

關(guān)鍵詞：α-HgI2；定向生長(zhǎng)；溶液法；覆蓋度

碘化汞(α-HgI2)晶體是直接躍遷寬帶隙的Ⅱ-Ⅶ族化合物半導(dǎo)體.該晶體原子序數(shù)高(ZHg=80,ZI=53),禁帶寬度大(300 K,2.13 eV),光電吸收系數(shù)大,探測(cè)效率高,能量分辨率好,對(duì)X、γ射線有較高的靈敏度等特性,是優(yōu)秀的室溫半導(dǎo)體探測(cè)器的材料之一[1-2].

近20年來,作為醫(yī)學(xué)成像用的多晶薄膜成為碘化汞晶體研發(fā)的重要商業(yè)目標(biāo)[3-6].文獻(xiàn)[7]研究發(fā)現(xiàn), HgI2薄膜[001]朝向性可以獲得高達(dá)1 014 Ω·cm的電阻率,接近HgI2單晶的電阻率.高朝向性([001])的晶粒可以產(chǎn)生良好輻射吸收和形成均勻電場(chǎng),進(jìn)一步優(yōu)化了探測(cè)器電場(chǎng)均勻性和成像儀空間分辨率[8].文獻(xiàn)[9-10]通過溫控技術(shù)獲得了高朝向性的多晶薄膜,晶粒粒度可以控制到3 μm以下.但是其生長(zhǎng)工藝復(fù)雜、重復(fù)性較差且對(duì)薄膜襯底要求較高[7].文獻(xiàn)[11]利用超聲輔助定向形核技術(shù)成果獲得了高朝向性的碘化汞籽晶層.文獻(xiàn)[12]利用液相外延技術(shù)生長(zhǎng)了碘化汞籽晶層,獲得了近乎100% [001]晶向的多晶薄膜.文獻(xiàn)[13]也研究了不同溶液體系下多晶碘化汞薄膜的生長(zhǎng)特征.同時(shí)對(duì)于機(jī)理性的問題各國學(xué)者也展開了深入的研究.文獻(xiàn)[14]指出了薄膜形核初期納米形貌形成的影響因素；文獻(xiàn)[15]以Hg(NO3)2·H2O和I2為原料,通過有機(jī)物體系中的懸浮法研究了碘化汞納米顆粒的形成機(jī)理,并指出該方法可能是碘化汞籽晶層的定性生長(zhǎng)機(jī)理的重大突破.因此用于同質(zhì)外延的籽晶層工藝研究是獲得高朝向性碘化汞多晶薄膜的重要技術(shù).

本文利用HgCl2和KI在水溶液中的反應(yīng),在錫氧化銦(Indium Tin Oxide,ITO)玻璃上沉積形成籽晶層的機(jī)理進(jìn)行了研究.分析了溶液體系中的生長(zhǎng)單元,反應(yīng)平衡移動(dòng)特征.通過改變KI濃度建立了系列配比,討論了濃度對(duì)籽晶生長(zhǎng)過程、籽晶層密度、覆蓋度的影響,為籽晶層生長(zhǎng)工藝優(yōu)化提供重要參照.

1實(shí)驗(yàn)材料和方法

實(shí)驗(yàn)用原料為HgCl2(4N)和KI(4N),控制總量為5.5 g.在25 ℃環(huán)境下用30 mL的去離子水(18 MΩ)將KI配置成溶液,在磁力攪拌下加入HgCl2,持續(xù)攪拌30 min后靜置30 min,配制出摩爾比[Hg2+]∶[I-]=1∶1～1∶6六組溶液,定義編號(hào)為a～f.取出部分溶液,用去離子水將配置的溶液稀釋并采用島津UV-2550紫外可見分光光度計(jì)研究反應(yīng)液中離子及配合物的存在形式.

實(shí)驗(yàn)選用10×10 mm2ITO玻璃作為生長(zhǎng)襯底,對(duì)其表面進(jìn)行潔凈處理.在25 ℃環(huán)境下,分別將6個(gè)ITO玻璃水平放入6個(gè)反應(yīng)溶液中,生長(zhǎng)時(shí)間為30 min.生長(zhǎng)結(jié)束后,使用移液管將剩余液體移出,在N2氣氛下對(duì)生長(zhǎng)襯底進(jìn)行干燥處理.采用LEICA-DM2500P偏光顯微鏡分析薄膜宏觀生長(zhǎng)形貌.六組溶液配制參數(shù)見表1.

表1　不同濃度體系的KI+HgCl2溶液配比

2結(jié)果與討論

在HgCl2和KI的水溶液體系中,存在如下反應(yīng)：

HgCl2+2KI→HgI2+2KCl

(1)

反應(yīng)產(chǎn)物HgI2不溶于水,因此結(jié)晶析出.從圖1可知,所有曲線均出現(xiàn)兩個(gè)吸收峰,分別為220 nm和265 nm.在220 nm是HgI2分子的吸收峰[16-17].隨著濃度的增大,反應(yīng)形成的HgI2含量增多,吸收峰強(qiáng)度增加,符合比爾定律.但是在濃度超過1∶5時(shí),吸收峰位發(fā)生輕微紅移,表明HgI2分子結(jié)構(gòu)/聚集狀態(tài)可能發(fā)生變化.隨著析出的HgI2濃度逐漸增大,分子間通過范德華力作用而形成[HgI2]n[18].[HgI2]n與晶體相同的結(jié)構(gòu),其分子簇中的電子軌道相互重疊,能級(jí)密度增大,導(dǎo)致電子躍遷能量降低,因此吸收峰出現(xiàn)紅移現(xiàn)象.在波長(zhǎng)為265 nm間出現(xiàn)了隨濃度上升而增加并發(fā)生藍(lán)移的吸收峰.推測(cè)其為K[HgI3]H2O的配合物,其反應(yīng)過程為

HgI2+[I]-→[HgI3]-

(2)

[K]++[HgI3]-+H2O=K[HgI3]·H2O

(3)

由于KI濃度的增大,過量[I]-和生成的HgI2形成了配合物 [HgI3]-.同時(shí),過量[K]+與[HgI3]-反應(yīng)形成了配合物K[HgI3]H2O.體系中過量[I]-向生成的HgI2提供孤對(duì)電子,增大了體系中孤對(duì)電子濃度.而孤對(duì)電子濃度升高和非鍵n電子與極性溶劑H2O形成的氫鍵會(huì)造成基態(tài)n軌道能量降低,形成非鍵軌道n向反鍵軌道π*的躍遷能量增大,因而造成藍(lán)移現(xiàn)象.因此,隨著KI濃度的增大,有利于K[HgI3]·H2O的析出,而[HgI2]分子碰撞形成[HgI2]n導(dǎo)致體系中析出[HgI2]濃度降低,因而式(2)和式(3)平衡向左移動(dòng),進(jìn)一步促使HgI2結(jié)晶.

圖1　不同濃度HgCl2-KI-H2O反應(yīng)體系

溶液中最先析出的HgI2是粒徑為20～30 ?的白色亞穩(wěn)態(tài)碘化汞(HgI2M)[19],其具有和β-HgI2相似的空間結(jié)構(gòu),屬于Cmc21[20].由于其單胞中分子均以范德華力鍵合,因此很容易發(fā)生HgI2M→α-HgI2的相變.在近平衡條件下,α-HgI2具有規(guī)則四方形狀,最大(001)晶面暴露的特征,這也是籽晶層需要的晶體朝向.

圖2是六組配比下產(chǎn)物的偏光照片.在較低的濃度下,析出產(chǎn)物粒度極小,無法分辨形貌,如圖2(a)所示.濃度過低,晶粒沒有充分發(fā)育.圖2(b)中已經(jīng)形成少量具有規(guī)則外形的紅色碘化汞晶粒,也形成了大量無色的HgI2M.由于HgI2M→α-HgI2結(jié)構(gòu)發(fā)生了變化[20],因此相變后的α-HgI2(001)朝向性遭到破壞,這不利于單一朝向的籽晶生長(zhǎng).從圖2(c)可以看出,大量紅色α-HgI2和微量HgI2M出現(xiàn)在KI/ HgCl2=3的配比中.紅色α-HgI2形狀多為四方晶,最大晶面均有不同程度的體現(xiàn),表明KI濃度的增大加速了HgI2晶體的析出,晶體發(fā)育良好.從圖2(d)～2(e)可知,隨著KI濃度的進(jìn)一步增大,籽晶顆粒度有增大趨勢(shì),密度(覆蓋度)變化不大,說明生長(zhǎng)原料的不足限制了致密連續(xù)的籽晶層的生長(zhǎng).但圖2(f)中,當(dāng)KI/HgCl2=6時(shí),籽晶層顆粒度略有減小,連續(xù)性增大但晶粒堆垛現(xiàn)象顯著.由于[I]-濃度增大,體系形核幾率增加,抑制了晶粒的進(jìn)一步生長(zhǎng),因此晶粒度有所下降.由圖2中堆垛部分和未覆蓋部分的面積可知,該濃度可能是形成單層致密籽晶層的最佳濃度,而堆垛現(xiàn)象的出現(xiàn)很可能與HgI2析出速率過快有關(guān).

分析認(rèn)為,隨著KI濃度增大,籽晶層密度和顆粒度都有上升趨勢(shì),晶體形貌也有很大變化,表明生長(zhǎng)體系中[I]-對(duì)結(jié)晶過程的影響很大.在Hg+和I-的絡(luò)合反應(yīng)體系中,會(huì)形成[HgI]+、[HgI2]、[HgI3]-和[HgI4]2-等型體結(jié)構(gòu).設(shè)Hg+分析濃度為cM,當(dāng)絡(luò)合反應(yīng)達(dá)到平衡時(shí),溶液中游離絡(luò)合劑的平衡濃度為[I]-,各型體濃度為

[HgIn]=βn[Hg][I]n

(4)

式中：β為穩(wěn)定常數(shù)；n=1,2,3和4分別對(duì)應(yīng)于[HgI]+、[HgI2]、[HgI3]-和[HgI4]2-等型體結(jié)構(gòu)的穩(wěn)定常數(shù).根據(jù)物料平衡關(guān)系可知：

cM=[Hg]+[HgI]+[HgI2]+…+[HgI4]

=β1[Hg][I]+β2[Hg][I]2+…+β4[Hg][I]4

(5)

根據(jù)型體結(jié)構(gòu)分布分?jǐn)?shù)δ定義可知：

(6)

可以看出,體系中[HgI2]結(jié)構(gòu)的分布分?jǐn)?shù)/比例與[I]-離子濃度有關(guān),而與Hg含量沒有直接關(guān)系,這與實(shí)驗(yàn)結(jié)果基本一致.同時(shí)在KI/HgCl2=6時(shí),可以獲得最佳的籽晶層覆蓋度,但是過高的形核速率(沉積速率)導(dǎo)致籽晶層出現(xiàn)堆垛現(xiàn)象.因此,提高生長(zhǎng)單元的擴(kuò)散能力,減緩晶體形核速率,以及增大溶液的對(duì)流將是工藝優(yōu)化的重點(diǎn).

圖2　HgCl2-KI-H2O反應(yīng)體系形成的籽晶層形貌

3結(jié) 論

1) 在HgCl2+KI+H2O體系中,隨著KI濃度的增大,出現(xiàn)[HgI2]n聚體,導(dǎo)致紫外吸收峰出現(xiàn)紅移,K[HgI3]·H2O中孤對(duì)電子濃度增大產(chǎn)生非鍵軌道n向反鍵軌道π*的躍遷,造成吸收峰藍(lán)移.

2) 在HgCl2/KI=1∶1～1∶6(摩爾比)的反應(yīng)體系中,當(dāng)HgCl2/KI=1∶ 6時(shí),籽晶覆蓋度最佳.控制體系反應(yīng)速率可減少晶粒堆垛現(xiàn)象.

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(責(zé)任編輯、校對(duì)潘秋岑)

Growth ofα-HgI2Seed Layer in HgCl2-KI-H2O

ZHAOPanpan,XUGang,FENGYaxi,QIUHaicai,LIJunying

(School of Materials and Chemical Engineering,Xi’an Technological University,Xi’an 710021,China)

Abstract:The oriented growth process of HgI2 seed layer was investigated in HgCl2-KI-H2O solution.The reaction mechanism in HgCl2+KI+H2O was studied when the mole ratio of HgCl2/KI varies from 1∶1 to 1∶6.Growth unit in system was determined by visible and UV spectrometers.The morphology of seed layer was observed using a LEICA-DM2500P polarizing microscope.The results show: Red shift occurs along with [HgI2]n polymer,and blue shift appears with complexes K[HgI3]·H2O.Through the discussions on the characterization of UV spectra,it is suggested:The size and coverage of grain increase with [I]- concentration,and the sufficient grain develops gradually.The forming polymer [HgI2]n results in increased density of electron energy levels,reduces transition energy,leading to red shift.Blue shift emerge is dictated that electron transits from nonbonding to antibonding in K[HgI3]·H2O,resulting from increment of lone pair electron concentration.The best density of seed layer forms as HgCl2/KI=1∶6.Grain stacking would be controlled by retarding the reaction in higher [I]- concentration.

Key words:α-HgI2;oriented growth;solution;coverage

DOI:10.16185/j.jxatu.edu.cn.2016.04.012

收稿日期：2015-10-10

基金資助：國家自然科學(xué)基金(51502234);陜西省教育廳科研計(jì)劃項(xiàng)目(15JS040);

作者簡(jiǎn)介：趙盼盼(1993-),女,西安工業(yè)大學(xué)助研.通訊簡(jiǎn)介：許崗(1973-),男,西安工業(yè)大學(xué)副教授,主要研究方向?yàn)榘雽?dǎo)體材料.E-mail：xxrshhuangshan@sohu.com.

文獻(xiàn)標(biāo)志碼:中圖號(hào)：O78A

文章編號(hào)：1673-9965(2016)04-0323-05

2014年國家級(jí)大學(xué)生創(chuàng)新創(chuàng)業(yè)訓(xùn)練計(jì)劃項(xiàng)目(201410702015)