【作者】 朱振；

【导师】 马瑾；

【作者基本信息】 山东大学 ， 微电子学与固体电子学， 2012， 博士

【摘要】 宽禁带氧化物半导体材料由于具有优异的光学和电学性质,因而在发光二极管、激光器、薄膜太阳能电池、透明薄膜晶体管、紫外探测器等领域有着广泛的应用前景。SnO2薄膜是一种多功能的透明氧化物半导体材料,具有制备温度低、热稳定性高和物理化学性质稳定等优点,已经被应用于透明导电电极、太阳能电池、气敏传感器及建筑玻璃等方面。另外,SnO2薄膜由于具有较宽的带隙和较大的激子束缚能,因此还是一种很有前途的紫外发光材料。用传统方法制备的SnO2薄膜大部分为多晶,不仅结晶质量较差,含有很多缺陷,而且存在晶粒粗化现象,因此不适合制作高品质的光电器件。相比之下,SnO2外延薄膜的结构更加均匀,光电性质更加优良,物理化学性质更加稳定,不仅能用来制作高性能的半导体器件,还能提供结构均匀的表面用于对SnO2表面和界面的理论研究。本论文使用金属有机化学气相沉积方法在碳化硅和蓝宝石衬底上制备了不同取向的SnO2外延薄膜,并且系统研究了它们的结构和光电性质,不仅具有重要的科学意义还具有广阔的应用前景。本论文主要的研究工作及结果如下：1.采用高纯Sn(CH3)4为Sn的有机(MO)源,高纯O2为氧化剂,超高纯N2为载气,在500℃、600℃、700℃和750℃下6H-SiC(0001)衬底上分别制备了SnO2薄膜。测试结果表明所制备的薄膜均为金红石结构。500℃和600℃制备的SnO2薄膜沿[100]单一取向生长,当衬底温度升高到700℃和750℃时,SnO2薄膜变为多晶。其中600℃下制备的SnO2薄膜结晶质量最好。随着衬底温度的升高,SnO2薄膜内部的氧空位减少,晶格常数增大。由于Sn02和6H-SiC具有不同的晶体结构和对称性,SnO2薄膜中出现了相互扭转120°的三重畴结构。薄膜和衬底的面内外延关系为SnO2[010]//SiC<1010>和SnO2[001]//SiC<1210>,其晶格失配沿SnO2[010]方向为-11.07%,沿SnO2[001]方向为3.57%。Sn02在6H-SiC上的异质外延可看成是近似六方密堆积排列的延续,只不过是O原子占据了Si原子的位置。当衬底温度从500℃升高到750℃时,SnO2薄膜的载流子浓度从1.1×1020cm-3递减至1.6×1018cm-3,而电阻率从0.01Ωcm升高到0.57Ωcm。600℃制备样品的霍尔迁移率为12.7cm2V-1s-1。在可见光范围,6H-SiC衬底的平均透过率约为60%,而薄膜样品的平均透过率均大于60%,并且最高的达到75%,这说明SnO2薄膜对6H-SiC衬底起着增透作用。2.以R面蓝宝石为衬底,在500℃、550℃、600℃和700℃下分别制备了金红石结构的SnO2薄膜。其中,500℃和550℃下制备的SnO2薄膜为多晶。当薄膜的制备温度升高到600℃和700℃时,SnO2薄膜垂直于(101)晶面单一取向生长。随着衬底温度的升高,SnO2薄膜的结晶质量不断提高,并且晶格常数增大。700℃制备的SnO2薄膜表面致密,并且不含(101)孪晶。Sn02薄膜和R面蓝宝石的面内取向关系是SnO2[010]//Al2O3[1210]和SnO2[101]//Al2O3[1011],其晶格失配沿SnO2[010]方向为-0.42%,沿SnO2[101]方向为11.31%。当衬底温度从500℃升高至700℃时,载流子浓度从7.7×1018cm-3单调递减至1.6×1016cm-3,霍耳迁移率从7.4cm2V-1s-1单调递增至28.1cm2V-1s-1,电阻率从0.11Ωcm单调递增至14.22Ωcm。500℃,550℃.600℃及700℃制备的SnO2薄膜的光学带隙分别为3.88eV、3.80eV、3.75eV和3.96eV,可见光区薄膜样品的平均透过率为78%。室温下,500℃制备的SnO2薄膜仅在530nm附近出现了一个氧空位等缺陷引起的宽发光峰。随着衬底温度的升高,这个发光峰的强度减弱并且峰值发生红移。对于700℃制备的SnO2薄膜,缺陷发光峰移动到605nm附近,并且还在紫外区333nm附近出现了一个强的带边发光峰。低温测试时,紫外发光峰发生蓝移,但强度基本不变。我们把它归因于薄膜内部的局域激子复合。另外,在13K时480nm附近还出现了一个由氧空位深能级引起的强发光峰。3.以A面蓝宝石为衬底,在500℃、600℃和700℃下分别制备了金红石结构的SnO2薄膜。500℃制备的SnO2薄膜为多晶并且结晶较差,600℃及700℃制备的薄膜垂直于(101)晶面单一取向生长。SnO2薄膜和A面蓝宝石的面内取向关系是SnO2[010]//Al2O3[0001]和SnO2[101]//Al2O3[1100],其晶格失配沿SnO2[010]方向为9.47%,沿SnO2[101]方向为3.81%。SnO2外延薄膜中出现了三种{101}孪晶,其中(101)孪晶边界平行于衬底表面,而(101)孪晶边界与衬底表面呈68°角。这些孪晶使SnO2薄膜晶格发生扭曲,导致薄膜生长方向发生偏差并在表面形成很多倾斜的台阶。4.以M面蓝宝石为衬底,在500℃、600℃、700℃和750℃下分别制备了金红石结构的SnO2薄膜。500℃制备的Sn02薄膜是(301)择优取向生长的,薄膜结晶质量较差。当衬底温度升高到600℃和700℃时,Sn02薄膜沿[001]单一取向生长。750℃制备的SnO2薄膜又变为多晶结构。其中,700℃制备的SnO2薄膜结晶质量最好。SnO2薄膜和M面蓝宝石的面内取向关系是SnO2[010]//Al2O3[0001]和SnO2[100]//Al2O3[1210],其晶格失配沿SnO2[010]方向为9.47%,沿Sn02[100]方向为-0.42%。在可见光区,薄膜样品的平均透过率为80%。700℃制备的SnO2薄膜在600nm附近有一个宽的发光峰。在13K时,个新的宽发光峰出现在480nm附近。我们分别把这两个发光峰归因为薄膜内部氧空位形成的不同能级的发光。通过薄膜结构的测试和分析,提出了SnO2薄膜在R、A和M面蓝宝石衬底上外延生长的表面原子结构模型。SnO2薄膜能在蓝宝石衬底上外延生长的主要原因是它们具有相似的氧原子排列。5.在以上研究的基础上,我们采用高纯Sn(CH3)4为Sn的MO源,高纯Sb(CH3)3为Sb的MO源,高纯O2为氧化剂,超高纯N2为载气,在6H-SiC(0001)和R面蓝宝石衬底上600℃下制备了不同Sb掺杂浓度的SnO2薄膜。所有SnO2:Sb薄膜均具有金红石结构并保持原有取向生长。Sb在SnO2薄膜中构成替位式掺杂。SnO2:Sb薄膜均为简并半导体,低温下以电离杂质散射为主,高温下以晶格振动散射为主。对于R面蓝宝石衬底的SnO2:Sb薄膜,随着Sb掺杂浓度的增加,薄膜中(101)孪晶的含量变少。在5%掺杂浓度下,SnO2:Sb薄膜的电阻率最小,为1.3×10-3Qcm,载流子浓度最大,为2.5×1020cm-3。在可见光区,SnO2:Sb薄膜样品的平均透过率约为80%。0%、1%、3%、5%和7%Sb掺杂浓度的Sn02薄膜的带隙分别为3.77eV、3.86eV、3.96eV、4.10eV和4.11eV。随着Sb掺杂浓度的增加,SnO2:Sb薄膜的光学带隙变大。对于6H-SiC(0001)衬底的SnO2:Sb薄膜,3%掺杂浓度下,SnO2:Sb薄膜的电阻率最小,为9.3×104Ωcm,载流子浓度最大,为3.4×1020cm-3。更多还原

【Abstract】 The wide band gap oxide semiconductors have potential applications in light-emitting diodes, laser diodes, thin-film solar cells, transparent thin-film transistors and ultraviolet (UV) detectors due to their excellent optical and electrical properties. SnO2film is a multi-functional transparent oxide semiconductor. Owing to its low preparation temperature, high thermal stability and stable chemical properties, SnO2film has been used in transparent conducting electrodes, solar cells, gas sensors and architectural glass. Moreover, SnO2film has a wide band gap and a high exciton binding energy, so it is also a promising UV luminescent material. SnO2films prepared by traditional methods are almost polycrystalline. They not only have poor crystallinity with many defects, but also suffer grain coarsening. Therefore, they are not suitable for high-performance optoelectronic devices. In contrast, epitaxial SnO2films show better structural uniformity, more outstanding optical and electrical properties, and higher physical and chemical stability. The high-quality epitaxial SnO2films can be used for fabricating high-performance semiconductor devices. Meanwhile, they can provide well-defined surfaces for theoretical research of SnO2surface and interface. In this paper, epitaxial SnO2films with different orientations have been prepared on SiC and sapphire substrates by metalorganic chemical vapor deposition. The structural, optical and electrical properties were investigated in detail. This study is important for the scientific research and applications.The key research work and results of this paper are as follows:1. SnO2films were deposited on6H-SiC (0001) substrates at temperatures of500,600,700and750℃, using high purity Sn(C2H5)4, O2and N2as Sn source, oxidant, and carrier gas, respectively. The measurements indicated all the films are rutile SnO2. SnO2films deposited at500and600℃were grown along single SnO2[100] orientation. At higher temperatures of700and750℃, SnO2films became polycrystalline. SnO2films deposited at600℃had the best crystallinity. As the substrate temperature increased, the oxygen vacancies in SnO2films were reduced and the lattice parameters were enlarged. The deposited SnO2films consisted of three domains rotated by120°due to the different crystal structure and symmetry between SnO2and6H-SiC. The in-plane orientation relationship between the film and substrate is SnO2[010]//SiC<1010> and SnO2[001]//SiC <1210>. The lattice mismatch is about-11.07%along the SnO2[010] direction, while it is about3.57%along the [001] direction of SnO2. The heteroepitaxy of SnO2on6H-SiC can be regarded as a nearly continuous extension of the hcp atomic arrangements with substituting O atoms for Si atoms. As the substrate temperature increased from500to750℃, the carrier concentration of SnO2films decreased monotonously from1.l×1020to1.6×1018cm-3, while the resistivity increased from0.01to0.57Qcm. The Hall mobility was12.7cm2V-1s-1for the film grown at600℃. In the visible range, the average transmittance of6H-SiC substrate was about60%. The average transmittance of SnO2samples was more than60%with the highest transparency of75%. This result indicated that SnO2films showed anti-reflectivity on6H-SiC substrate.2. Rutile SnO2films were deposited on r-cut sapphire substrates at temperatures of500,550,600and700℃. SnO2films grown at500and550℃were polycrystalline. As the substrate temperature increased to600and700℃, SnO2films were grown along a single orientation which is perpendicular to SnO2(101) plane. As the substrate temperature increased, the crystallinity of SnO2films improved and the lattice parameters increased. SnO2film deposited at700℃had a dense surface and contained no (101) twin. The in-plane orientation relationship between SnO2film and r-cut sapphire substrate is SnO2[010]//Al2O3[1210] and SnO2[101]//Al2O3[1011]. The lattice mismatch is-0.42%along the SnO2[010] direction and11.31%along the SnO2[101] direction. As the substrate temperature increased from500to700℃, the carrier concentration, Hall mobility and resistivity changed from7.7×1018to1.6×1016cm-3,7.4to28.1cm2V-1s-1and0.11to14.22Ωcm, respectively. The optical band gaps of SnO2films prepared at500,550,600and700℃were3.88,3.80,3.75and3.96eV, respectively. The average transmittance of SnO2samples was78%in the visible range. At room temperature, SnO2film grown at500℃only showed a broad defect-related luminescence peak near530nm. As the substrate temperature increased, the intensity of this peak decreased and the peak position had red shift. For SnO2film grown at700℃, the defect-related luminescence peak shifted to605nm. In addition, a intense band edge luminescence peak appeared at333nm in the UV region. At low temperatures, the UV peak had blue shift while the intensity did not increase remarkably. This UV emission was ascribed to the recombination of local excitons in SnO2film. Moreover, an intense luminescence peak associated with oxygen vacancies in deep levels appeared near480nm at13K.3. Rutile SnO2films were deposited on a-cut sapphire substrates at temperatures of500,600and700℃. The SnO2film grown at500℃was polycrystalline with low crystallinity. At600and700℃, SnO2films were grown along a single orientation which is perpendicular to SnO2(101) plane. The in-plane orientation relationship between SnO2film and a-cut sapphire substrate is SnO2[010]//Al2O3[0001] and SnO2[101]//Al2O3[1100].The lattice mismatch is calculated to be9.47%along the SnO2[010] direction, and3.81%along the SnO2[101] direction. Three types of{101} twins were observed in SnO2films grown on a-cut sapphire. The (101) twin boundaries were parallel to the substrate surface, while the (101) twin boundaries were68°inclined to the substrate surface. These twins caused slight misorientation of the growth plane and many steps and inclinations in the film surface.4. Rutile SnO2films were deposited on m-cut sapphire substrates at temperatures of500,600,700and750℃. The film grown at500℃was (301) preferred with poor crystallization. As the substrate temperature increased to600and700℃, SnO2films were grown along single SnO2[001] orientation. At750℃, the film became polycrystalline again. SnO2film grown at700℃has the best crystallinity. The in-plane orientation relationship between SnO2film and m-cut sapphire substrate is SnO2[010]//Al2O3[0001] and SnO2[100]//Al2O3[1210].The lattice mismatch is calculated to be-0.42%along the SnO2[100] direction, and9.47%along the SnO2 [010] direction. The average transmittance of Sno2samples was80%in the visible range. Sno2film grown at700℃showed a broad luminescence peak near600nm at room temperature. At13K, a new broad luminescence peak appeared near480nm. The two luminescence peaks were attributed to the recombination of electrons and holes through oxygen vacancies with different energy levels in the band gap.According to the measurements and analysis of the structure of SnO2films, we proposed an epitaxial growth model of SnO2films on r, a and m-cut sapphire substrates. The reason why SnO2films can be epitaxially grown on sapphire substrates is that they have similar oxygen atomic arrangement.5. On the basis of the study above, we prepared SnO2:Sb films on6H-SiC (0001) and r-cut sapphire substrates at600℃using high purity Sn(C2H5)4, Sb(CH3)3, O2and N2as Sn source, Sb source, oxidant, and carrier gas, respectively. All the Sb-doped SnO2films were rutile and grown along the same orientation to undoped SnO2films. Sb was substitutionally doped in SnO2films. SnO2:Sb films were degenerate semiconductor. The ionized impurity scattering was the main scattering mechanism at low temperature, while the lattice vibration scattering became dominant at high temperature in SnO2:Sb films. As Sb concentration increased, the (101) twins in SnO2:Sb films on r-cut sapphire were reduced. The SnO2:Sb film with the lowest resistivity of1.3×10-3Ω cm and the highest carrier concentration of2.5×1020cm-3was obtained at5%Sb-doping on r-cut sapphire. The samples showed high transparency of～80%in the visible range. The band gaps were3.77,3.86,3.96,4.10and4.11eV for the0,1%,3%,5%and7%Sb-doped SnO2films on r-cut sapphire, respectively. As Sb concentration increased, the optical band gap of SnO2:Sb films were enlarged. For the SnO2:Sb film on6H-SiC (0001) substrate, it showed the lowest resistivity of9.3×10-4Ω cm and the highest carrier concentration of3.4×1020cm-3at3%Sb-doping.更多还原