MP  >> Vol. 9 No. 1 (January 2019)

    SnSex薄膜中SnSe与SnSe2两相调控机理研究
    The Regulation Mechanism Study of SnSe and SnSe2 in SnSex Thin Films

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作者:  

陈 娜,牛逸潇,唐家豪,张凯凯,陈俏宇,朱 希,唐曙锋,潘媛媛:绍兴文理学院物理系,浙江 绍兴

关键词:
SnSeSnSe2X射线光电子能谱拉曼光谱分子束外延SnSe SnSe2 X-Ray Photoemission Spectroscopy (XPS) Raman Spectra Molecular Beam Epitaxy (MBE)

摘要:

本论文利用分子束外延技术在BaF2 (111)衬底上制备了SnSex薄膜,并利用拉曼光谱和X光电子能谱(XPS)对SnSex薄膜中SnSe、SnSe2两组分的调控机理进行了系统研究,研究结果表明当锡硒原子比率Sn/Se ≈ 1时,以250℃和300℃衬底温度制备的SnSex薄膜均存在SnSe和SnSe2两种组分,提高衬底温度将有利于薄膜中SnSe的形成,当衬底温度由250℃提高到300℃时,XPS中与SnSe、SnSe2相对应的Sn 3d5/2组分峰的强度比ISnSe/SnSe2由0.23提高到了0.54。进一步的研究结果表明,与提高衬底温度相比,通过增大生长过程中锡硒原子比率可更有效地提升薄膜中SnSe的比例,XPS数据显示当锡硒原子比率由Sn/Se ≈ 1提高到Sn/Se ≈ 1.02时,与SnSe、SnSe2相对应的Sn 3d5/2组分峰的强度比ISnSe/SnSe2由0.23提高到了2.38。本论文的研究将为今后纯SnSe相的可控制备提供理论依据。

In this paper, SnSex thin films were epitaxially grown on BaF2 (111) substrate using molecular beam epitaxy (MBE), and the regulation mechanism of SnSe and SnSe2 components in SnSex thin films with substrate temperature and the Sn-to-Se atomic ratio is systematically studied by means of Raman spectra and X-ray photoemission spectroscopy (XPS). The results indicated that with the atomic ratio of Sn/Se ≈ 1, both SnSe and SnSe2 components are present in SnSex thin films at the substrate temperature of 250˚C and 300˚C, the enhancement of the substrate temperature will fa-cilitate the formation of SnSe in the film. When the substrate temperature is increased from 250˚C to 300˚C, the intensity ratio (ISnSe/SnSe2) of the Sn 3d5/2 component peak corresponding to SnSe, SnSe2 in the XPS is increased from 0.23 to 0.54. Further research results show that the ratio of SnSe in the film can be more effectively enhanced by increasing the Sn-to-Se atomic ratio in the growth process compared with increasing the substrate temperature. The XPS data showed that when the Sn-to-Se atomic ratio was increased from Sn/Se ≈ 1 to Sn/Se ≈ 1.02, the intensity ratio (ISnSe/SnSe2) of the Sn 3d5/2 component peak corresponding to SnSe and SnSe2 was increased from 0.23 to 2.38. The researches in this paper will provide theoretical references for future controllable preparation of pure SnSe phase.

1. 引言

自从在实验上实现石墨烯(graphene)的制备以来,二维材料的研究受到了极其广泛的关注 [1] [2] [3] [4] 。SnSe为典型的二维层状半导体材料。室温下,SnSe属于Pnma群的GeS型斜方晶系(orthorhombic),呈现p型导电特征,晶格常数分别为a = 11.49、b = 4.44、c = 4.135,各向异性,在单层内,Sn和Se原子间以共价键的方式形成类黑磷结构,层与层间与过渡金属硫化物、石墨一样,靠弱的范德瓦耳斯力维系 [5] [6] ,Sn2+的5s2孤对电子在结构中进行非简谐成键(anharmonic bonding)产生声子散射的增强效应,使得SnSe的热导率很低,加上无毒、便宜、储量丰富和化学性质稳定等特点,SnSe已被公认为最具应用潜力的新兴绿色环保型热电材料 [7] [8] [9] 。然而SnSe在合成过程中时常伴随SnSe2相,致使SnSe性能大打折扣 [10] [11] ,且目前国际上对SnSe与SnSe2间组分的调控条件存在一定的争议,其相互转换机理尚不明确,如Guillen 等人提出低温有利于SnSe相的形成 [12] ,而Fernandes等人却提出高温有利于SnSe相的形成 [13] 。为此,本论文将利用分子束外延技术制备SnSex薄膜,系统研究不同的生长温度和锡硒原子比率对SnSe、SnSe2两相转换的调控机理,为纯SnSe相的可控制备提供理论依据。

2. 实验方法

SnSex薄膜的制备是在固体源分子束外延(SSMBE)超高真空系统中完成的,系统的真空度好于1.5 × 10−10 Torr,该系统的具体描述见文献 [14] 。Sn、SnSe固体源纯度均为99.999%,衬底采用大气下新解理的BaF2 (111)解理面。外延生长前,BaF2 (111)衬底在进样室以200℃的温度烘烤40分钟左右以去除吸附的水分子,当BaF2 (111)衬底转移到生长室后,继续将衬底加热到550℃保持10 min,以对BaF2 (111)衬底进行进一步的清洁处理,其表面清洁程度可通过系统配备的RHEED进行原位监测。生长过程中,通过束流规监控SnSe和Sn的束流大小来控制锡硒原子比率,生长完成后,通过Dektak-XT台阶仪检测薄膜厚度,制备的SnSex薄膜厚度均为15 nm。薄膜组份及成键方式通过PHI5000VersaProbe X射线光电子能谱(XPS)及Horiba HR Evolution激光拉曼光谱检测,XPS使用的是Mg的Kα (1253.6 eV),激光拉曼光谱检测中使用的激光波长为532 nm。

3. 结果与讨论

为研究衬底温度和锡硒原子比率对SnSex薄膜组分的影响规律,我们对不同衬底温度及不同锡硒原子比率下制备的SnSex薄膜进行了XPS的测试,每个样品进行测试前都用氩离子对薄膜表面进行了轰击,以去除薄膜表面的氧化层,其测试结果如图1所示。图1(a)、图1(b)分别是锡硒原子比率Sn/Se ≈ 1下,以250℃和300℃衬底温度外延生长的SnSex薄膜的Sn 3d5/2和Se 3d5/2芯态的XPS谱图,图1(a)中我们用高斯型曲线对不同衬底温度下的Sn 3d5/2芯态进行了拟合,均得到了结合能位于485.8 eV和486.5 eV的两条子谱线,这两条子谱线的结合能分别与SnSe和SnSe2的Sn 3d5/2芯态结合能相对应 [15] [16] ,当衬底温度从250℃升高到300℃时,与SnSe、SnSe2相对应的Sn 3d5/2组分峰的强度比ISnSe/SnSe2从0.23提高到了0.54。该结果表明,当Sn/Se ≈ 1时,以250℃和300℃衬底温度制备的SnSex薄膜均以SnSe、SnSe2两相共存的方式存在,衬底温度的升高可提升薄膜中SnSe的比例。图1(b)中给出的Se 3d5/2芯态峰随衬底温度的变化趋势可进一步验证此结果。如图1(b)所示,当衬底温度从250℃升高到300℃时,Se 3d5/2的芯态峰向低结合能端偏移了约0.31 eV,这是由于SnSe2中的Se为−1价,SnSe中的Se为−2价,在Se 3d5/2芯态的XPS谱图上,与SnSe组分相对应的结合能应低于与SnSe2组分相对应的结合能,Se 3d5/2的芯态峰向低结合能端的偏移进一步说明了高温时SnSex薄膜中SnSe组分的增加。

(a) (b) (c) (d)

Figure 1. XPS spectra of 3d5/2; (a) Se 3d5/2; (b) core-levels of SnSex thin films grown at different temperatures, and XPS spectra of 3d5/2; (a) Se 3d5/2; (b) core-levels of SnSex thin films grown at different Sn-to-Se atomic ratio

图1. 不同程度温度下生长的SnSex薄膜Sn 3d5/2;(a) Se 3d5/2;(b) 芯态的XPS谱图,及不同锡硒原子比率下生长的SnSex薄膜Sn 3d5/2;(c) Se 3d5/2;(d) 芯态的XPS谱图

图1(c)、图1(d)为250℃衬底温度下,以锡硒原子比率Sn/Se ≈ 1和Sn/Se = 1.02外延生长的SnSex薄膜的Sn 3d5/2和Se 3d5/2芯态的XPS谱图。我们同样用高斯型曲线对Sn 3d5/2芯态进行了拟合,与Sn/Se ≈ 1相比,Sn/Se ≈ 1.02时,Sn 3d5/2芯态共拟合出了结合能位于485.03 eV、485.8 eV和486.5 eV的三条子谱线,这三条子谱线的结合能分别与金属Sn、SnSe和SnSe2的Sn 3d5/2结合能相对应 [15] [16] [17] 。此外,与SnSe、SnSe2相对应的Sn 3d5/2组分峰的强度比ISnSe/SnSe2从0.23提高到了2.38,即组分比上出现了明显的反转,同时,Se 3d5/2的芯态峰向低结合能端偏移了约0.51 eV,可见,外延生长过程中通过增加锡硒原子比率的方式可大幅度提升SnSex薄膜中SnSe组分的含量,而过量的Sn不再继续与SnSe2结合转化成SnSe,而是以金属Sn的方式存在于SnSex薄膜中。

为进一步揭示生长条件对SnSex薄膜中SnSe、SnSe2两相的调控机理,我们对不同条件下制备的SnSex薄膜进行了进一步的激光拉曼测试,其结果如图2所示。图2(a)为锡硒原子比率Sn/Se ≈ 1下,以250℃和300℃衬底温度外延生长的SnSex薄膜的拉曼光谱,为作比较,图2(a)下方给出了衬底BaF2 (111)的拉曼光谱。图中可以看到,不同衬底温度下制备的SnSex薄膜均在37 cm−1、51 cm−1、67 cm−1、108 cm−1、155 cm−1、183 cm−1、241 cm−1处出现了不同强度的拉曼散射峰,其中位于37 cm−1、51 cm−1、67 cm−1、155 cm−1处的散射峰分别来自SnSe的B3g、B1g、A1g和A3g拉曼振动模式 [18] ,位于108 cm−1和183 cm−1处的散射峰分别来自SnSe2的Eg和A1g拉曼振动模式 [19] ,位于241 cm−1的散射峰来自BaF2 (111)衬底的LO声子振动 [20] ,表明在锡硒原子比率Sn/Se ≈ 1时,不同衬底温度下制备的SnSex薄膜均包含了SnSe和SnSe2两种组分,进一步验证了XPS的结果。随着衬底温度的增加,SnSe和SnSe2的各拉曼特征散射峰的峰位并不发生移动而它们间的强度却发生了很大的变化,SnSe的特征拉曼峰强度呈现增加趋势,而SnSe2的特征拉曼峰强度呈现减弱趋势,当衬底温度由250℃增加到300℃时,SnSe位于67 cm−1处的散射峰与SnSe2位于183 cm−1处的散射峰的强度比值由0.5提高至1.3,表明衬底温度的升高有利于SnSe相的形成,与XPS得到的结果相一致。

(a) (b)

Figure 2. The raman spectra of SnSex thin films grown at; (a) different substrate temperatures and; (b) different Sn-to-Se atomic ratio, the raman spectrum of the BaF2 (111) substrate is shown on the bottom at (a)

图2. (a) 不同衬底温度下;(b) 不同Sn/Se比率下制备的SnSex薄膜的拉曼光谱;(a) 最下方为衬底BaF2 (111)的拉曼光谱

图2(b)给出了250℃衬底温度下,以不同锡硒原子比率制备的SnSex薄膜的拉曼光谱。图中明显可以看到,随着锡硒原子比率的增加,SnSe的拉曼特征峰的强度出现了显著的增加,当Sn/Se ≈ 1.02时,SnSe位于67 cm−1处的散射峰与SnSe2位于183 cm−1处的散射峰强度比由Sn/Se ≈ 1时的0.5提高到了3.3,进一步证实了XPS中揭示的结果,即Sn/Se ≈ 1.02下,SnSex薄膜中绝大部分组分为SnSe。仔细对比图2(b)中的两条拉曼光谱图,可以发现Sn/Se ≈ 1.02条件下制备的SnSex薄膜的拉曼光谱在97 cm−1处出现了新的散射峰,根据XPS的结果,在Sn/Se ≈ 1.02条件下,过量的Sn以金属Sn的方式存在于SnSex薄膜中,由于进行拉曼测试前没有对样品进行表面清洁处理,样品长期暴露于大气,金属态Sn容易被大气中的氧氧化形成SnO2,我们推断97 cm−1处的散射峰很可能来自SnO2的拉曼特征峰。

4. 结论

本论文利用分子束外延技术在BaF2 (111)衬底上制备了SnSex薄膜,并利用拉曼光谱和X光电子能谱(XPS)对SnSex薄膜中SnSe、SnSe2两组分的调控机理进行了系统研究,研究结果表明当锡硒原子比率Sn/Se ≈ 1时,以250℃和300℃衬底温度制备的SnSex薄膜均存在SnSe和SnSe2两种组分,提高衬底温度可增大SnSe在薄膜中的比例,当衬底温度由250℃提高到300℃时,与SnSe、SnSe2相对应的Sn 3d5/2组分峰的强度比由0.23提高到了0.54,且SnSe位于67 cm−1处的拉曼散射峰与SnSe2位于183 cm−1处的拉曼散射峰的强度比也由0.5提高至1.3;此外,在相同衬底温度下,提高锡硒原子比率可更有效地提高薄膜中SnSe的比例,当锡硒原子比率由Sn/Se ≈ 1提高到Sn/Se ≈ 1.02时,与SnSe、SnSe2相对应的Sn 3d5/2组分峰的强度比由0.23提高到了2.38,且SnSe位于67 cm−1处的拉曼散射峰与SnSe2位于183 cm−1处的拉曼散射峰的强度比也由0.5提高至3.3。研究结果表明,提高衬底温度和增大生长过程中锡硒原子比率都能有效地提升薄膜中SnSe的比例。

基金项目

国家级大学生创新创业训练计划项目和浙江省大学生科技创新活动计划暨新苗计划项目资助。

文章引用:
陈娜, 牛逸潇, 唐家豪, 张凯凯, 陈俏宇, 朱希, 唐曙锋, 潘媛媛. SnSex薄膜中SnSe与SnSe2两相调控机理研究[J]. 现代物理, 2019, 9(1): 48-53. https://doi.org/10.12677/MP.2019.91007

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