氧化锌薄膜及其与钙钛矿氧化物异质结特性研究
摘要
ZnO是一种直接宽带隙半导体，在蓝紫外发光器件、光电转换器件、稀磁半导体、气敏传感器、透明导电材料、透明薄膜晶体管等方面都有着广阔的应用前景。纤锌矿结构的ZnO晶体沿着c轴方向具有极性，自发极化对ZnO的光电性能、化学性质等都有重要影响。本文成功制备了ZnO外延膜及其与钙钛矿氧化物的外延异质结，重点研究了极性对ZnO薄膜及其异质结性质的影响。我们的研究对新型多功能ZnO器件的未来应用具有重要的指导意义。
利用脉冲激光沉积的方法，选用不同取向的SrTiO3（STO）单晶衬底，通过控制生长条件，我们能够能同时制备出表面为非极性面的Zn(110)(称为a取向)和表面为极性面的ZnO(001)(称为c取向)两种取向的外延薄膜。尽管两者的生长温度、氧气压强、激光能量等镀膜条件完全一样，但是我们发现a取向ZnO(110)薄膜的电阻率要远远大于c取向ZnO(001)薄膜的电阻率。通过深入分析，我们提出a取向ZnO(110)薄膜由于面内的自发极化在晶粒界面形成势垒，增加了对电子的散射，从而会导致很高的电阻率。实验结果与理论模型符合的非常好，通过计算得到室温下自发极化引起的能带弯曲大小约为0.22eV。
Nb掺杂的SrTiO3（NSTO）衬底具有良好的导电性，我们在NSTO衬底上成功制备了具有良好整流特性的ZnO/NSTO外延异质结。不同极性取向的ZnO异质结的I-V特性差别很明显。ZnO(110)/NSTO(001)异质结的电流要明显大于ZnO(001)/NSTO(110)异质结的电流。我们认为，由于NSTO具有很强的热电效应，c取向的ZnO薄膜在沉积的过程中带正电的Zn离子会优先沉积到基片上面，所以制备的ZnO是以O离子为表面的面。由于ZnO的极性效应，ZnO(001)/NSTO(110)异质结界面处ZnO一侧的能带会向上弯曲，抬高了界面处的势垒高度，从而增加了结电阻。将ZnO与p型材料LSMO、LCMO制备成异质结，也能得到良好的整流特性。在相同的温度和电压下，ZnO/LCMO/NSTO(001)异质结的电阻要小于ZnO/LCMO/NSTO(110)异质结的电阻。
另外，我们还制备了LCMO/NSTO异质结。在ZnO/LCMO/NSTO(001)和LCMO/NSTO异质结中，都观察到了良好的整流特性和显著的负磁阻现象。而且，LCMO/NSTO(110)异质结的磁阻数值要明显大于LCMO/NSTO(001)异质结。
Wurtize ZnO is a direct wide-band gap semiconductor. It is attractive because of itspotential applications in many fields including blue-ultraviolet light emitting diodes,optoelectronic devices, diluted magnetic semiconductors, gas sensors, transparentconducting oxides as well as transparent thin film transistors. Due to its lack ofinversion symmetry, wurtzite ZnO is a polar oxide with a polar direction along the caxis. The spontaneous polarization along the c-axis will affect the structural, optical,electrical, and chemical properties of ZnO thin films and nano-structures. In this thesis,we succeeded in fabricating the epitaxial ZnO thin films and heterojunctions withperovskite oxides. Our focus is mainly on the influence of polarity on the properties ofthe ZnO thin film and heterostructures. Our investigations are of importance fordeveloping new functional ZnO related devices.
With the method of pulsed laser deposition, nonpolar (110)(a plane) and polar(001)(c-plane) oriented ZnO epitaxial thin films can be grown simultaneously on the(001) and (110) oriented SrTiO3substrates, respectively. Although all the growthparameters are the same, the resistivity of the a-plane ZnO film is much higher than thatof the c-plane film. We attribute the larger resistivity of a-plane ZnO thin films to thebarrier height at the grain boundaries induced by the in-plane spontaneous polarization,which enhances the scattering of electrons passing through the grain boundaries. Ourexperiment data can be well fitted by the theoretical model, yielding a barrier height ofabout0.22eV at room temperature.
Nb-doped SrTiO3(NSTO) is conductive. Both nonpolar ZnO(110)/NSTO(001) andpolar ZnO(001)/NSTO(110) epitaxial heterostructures with good rectifyingcharacteristics are fabricated. We found that at the same voltage bias, the current ofnonpolar ZnO(110)/NSTO(001) heterostructure is larger than that of the polarZnO(001)/NSTO(110) heterostructure. We proposed that during the growth of the polarZnO thin films on the NSTO substrates, the inevitable temperature gradient across theNSTO substrate generates a layer of excess negative charges on the substrate surfacedue to the significant pyroelectric effect of NSTO. Consequently, the positively chargedZn ions should be deposited first on the NSTO substrate. The as-prepared ZnO film is terminated by O layer on the free surface with the spontaneous polarization pointing tothe surface. The negative bound polarization charges at the interface would bend theenergy band upward, and increase the barrier height of the heterostructure. So thec-ZnO/NSTO(110) heterostructure showed larger resistance than the other. Theheterostructures of ZnO with p-type perovskite oxides, LSMO and LCMO, also showgood rectifying property. The current of ZnO/LCMO/NSTO(001) heterostructure islarger than that of the ZnO/LCMO/NSTO(110) heterostructure, which can be explainedby the influence of polarity of ZnO as well.
Giant negative magnetoresistance was observed in the heterostructure of ZnO(110)/LCMO/NSTO(001). In order to investigate the mechanism of magetoresistance,we also made LCMO/NSTO heterostructures with different orientations. Both LCMO/NSTO(001) and LCMO/NSTO(110) heterostructure showed good rectifying propertiesand significant magnetoresistance. We also find that the magnetoresisatnce of theformer heterostructure is much larger than that of the latter. The underlying mechanismneeds further investigations.
利用脉冲激光沉积的方法，选用不同取向的SrTiO3（STO）单晶衬底，通过控制生长条件，我们能够能同时制备出表面为非极性面的Zn(110)(称为a取向)和表面为极性面的ZnO(001)(称为c取向)两种取向的外延薄膜。尽管两者的生长温度、氧气压强、激光能量等镀膜条件完全一样，但是我们发现a取向ZnO(110)薄膜的电阻率要远远大于c取向ZnO(001)薄膜的电阻率。通过深入分析，我们提出a取向ZnO(110)薄膜由于面内的自发极化在晶粒界面形成势垒，增加了对电子的散射，从而会导致很高的电阻率。实验结果与理论模型符合的非常好，通过计算得到室温下自发极化引起的能带弯曲大小约为0.22eV。
Nb掺杂的SrTiO3（NSTO）衬底具有良好的导电性，我们在NSTO衬底上成功制备了具有良好整流特性的ZnO/NSTO外延异质结。不同极性取向的ZnO异质结的I-V特性差别很明显。ZnO(110)/NSTO(001)异质结的电流要明显大于ZnO(001)/NSTO(110)异质结的电流。我们认为，由于NSTO具有很强的热电效应，c取向的ZnO薄膜在沉积的过程中带正电的Zn离子会优先沉积到基片上面，所以制备的ZnO是以O离子为表面的面。由于ZnO的极性效应，ZnO(001)/NSTO(110)异质结界面处ZnO一侧的能带会向上弯曲，抬高了界面处的势垒高度，从而增加了结电阻。将ZnO与p型材料LSMO、LCMO制备成异质结，也能得到良好的整流特性。在相同的温度和电压下，ZnO/LCMO/NSTO(001)异质结的电阻要小于ZnO/LCMO/NSTO(110)异质结的电阻。
另外，我们还制备了LCMO/NSTO异质结。在ZnO/LCMO/NSTO(001)和LCMO/NSTO异质结中，都观察到了良好的整流特性和显著的负磁阻现象。而且，LCMO/NSTO(110)异质结的磁阻数值要明显大于LCMO/NSTO(001)异质结。
Wurtize ZnO is a direct wide-band gap semiconductor. It is attractive because of itspotential applications in many fields including blue-ultraviolet light emitting diodes,optoelectronic devices, diluted magnetic semiconductors, gas sensors, transparentconducting oxides as well as transparent thin film transistors. Due to its lack ofinversion symmetry, wurtzite ZnO is a polar oxide with a polar direction along the caxis. The spontaneous polarization along the c-axis will affect the structural, optical,electrical, and chemical properties of ZnO thin films and nano-structures. In this thesis,we succeeded in fabricating the epitaxial ZnO thin films and heterojunctions withperovskite oxides. Our focus is mainly on the influence of polarity on the properties ofthe ZnO thin film and heterostructures. Our investigations are of importance fordeveloping new functional ZnO related devices.
With the method of pulsed laser deposition, nonpolar (110)(a plane) and polar(001)(c-plane) oriented ZnO epitaxial thin films can be grown simultaneously on the(001) and (110) oriented SrTiO3substrates, respectively. Although all the growthparameters are the same, the resistivity of the a-plane ZnO film is much higher than thatof the c-plane film. We attribute the larger resistivity of a-plane ZnO thin films to thebarrier height at the grain boundaries induced by the in-plane spontaneous polarization,which enhances the scattering of electrons passing through the grain boundaries. Ourexperiment data can be well fitted by the theoretical model, yielding a barrier height ofabout0.22eV at room temperature.
Nb-doped SrTiO3(NSTO) is conductive. Both nonpolar ZnO(110)/NSTO(001) andpolar ZnO(001)/NSTO(110) epitaxial heterostructures with good rectifyingcharacteristics are fabricated. We found that at the same voltage bias, the current ofnonpolar ZnO(110)/NSTO(001) heterostructure is larger than that of the polarZnO(001)/NSTO(110) heterostructure. We proposed that during the growth of the polarZnO thin films on the NSTO substrates, the inevitable temperature gradient across theNSTO substrate generates a layer of excess negative charges on the substrate surfacedue to the significant pyroelectric effect of NSTO. Consequently, the positively chargedZn ions should be deposited first on the NSTO substrate. The as-prepared ZnO film is terminated by O layer on the free surface with the spontaneous polarization pointing tothe surface. The negative bound polarization charges at the interface would bend theenergy band upward, and increase the barrier height of the heterostructure. So thec-ZnO/NSTO(110) heterostructure showed larger resistance than the other. Theheterostructures of ZnO with p-type perovskite oxides, LSMO and LCMO, also showgood rectifying property. The current of ZnO/LCMO/NSTO(001) heterostructure islarger than that of the ZnO/LCMO/NSTO(110) heterostructure, which can be explainedby the influence of polarity of ZnO as well.
Giant negative magnetoresistance was observed in the heterostructure of ZnO(110)/LCMO/NSTO(001). In order to investigate the mechanism of magetoresistance,we also made LCMO/NSTO heterostructures with different orientations. Both LCMO/NSTO(001) and LCMO/NSTO(110) heterostructure showed good rectifying propertiesand significant magnetoresistance. We also find that the magnetoresisatnce of theformer heterostructure is much larger than that of the latter. The underlying mechanismneeds further investigations.
引文
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