【作者】 吴兆丰；

【导师】 吴雪梅； 诸葛兰剑；

【作者基本信息】 苏州大学 ， 凝聚态物理， 2010， 博士

【摘要】 近年来,稀磁半导体由于可能在同种材料中同时实现对电子电荷和自旋的调控而吸引了众多研究者的兴趣。2000年,Dietl等人从理论上预言了ZnO、GaN等宽带隙半导体材料有可能成为居里温度高于室温的稀磁半导体材料,该预言使得过渡金属掺杂宽带隙半导体成为国际材料界的研究热点,随后在过渡金属掺杂ZnO材料中发现室温铁磁性的实验报道更是引起了科学界的巨大热情。ZnO基稀磁半导体材料相对其它候选半导体材料有许多独特的优点,如很宽的带隙(约3.4 eV)、室温下很高的激子结合能(约60 meV)、很高的光学增益(300 cm–1)和极短的发光驰豫时间等等,这些特点使得ZnO基稀磁半导体材料不仅可能成为未来自旋电子器件的候选材料,还有可能在未来的光电子器件和磁光器件等领域得到广泛的应用。本文紧扣ZnO材料研究中的热点,以验证ZnO基薄膜材料是否具有室温铁磁性并理解其铁磁耦合机理为目的,利用射频磁控溅射方法选择制备了Co、Cr、Cu、Mn等一系列过渡金属掺杂ZnO薄膜,并详细研究了其微结构、光学性质和磁学性质,分析了其磁性耦合机理,得出的主要结果如下:1、微结构测试的结果表明:在掺杂浓度较低时,Co、Cr、Cu、Mn等过渡金属掺杂ZnO薄膜是单一相的,这些过渡金属掺杂原子成功地占据了ZnO晶格中的Zn位,实现了替位式掺杂而且没有改变ZnO的纤锌矿结构。然而,在高掺杂浓度的样品中,比如掺杂浓度大于等于31.3 atom%的Co掺杂ZnO薄膜中出现了少量纳米尺寸的Co3O4团簇,在掺杂浓度达到9.8 atom%的Cr掺杂ZnO薄膜中则出现了ZnCr2O4杂质相。值得注意的是:在本文制备的Cu掺杂和Mn掺杂ZnO薄膜中存在有排列整齐紧凑的纳米尺寸柱状晶粒阵列,该现象引起了我们的极大兴趣,相关工作内容目前已在Appl. Phys. Lett上发表。由于磁控溅射方法可以经济方便地大面积沉积薄膜,采取适当的工艺,磁控溅射有望在将来可用于大规模生产Mn掺杂或Cu掺杂ZnO纳米尺寸柱状晶粒阵列或纳米棒阵列。2、光致发光的测试结果表明Mn掺杂ZnO薄膜和低浓度Co掺杂ZnO薄膜的光致发光谱中都存在中心位于375 nm附近的近带边发光,表明其有可能在短波长发光器件中得到应用。有趣的是,在Co掺杂ZnO薄膜中,当Co掺杂浓度增加到31.3 atom%和37.7 atom%时,其光致发光谱中出现了中心位于332 nm处的强烈深紫外发光峰,该发光应源于包裹在Zn1-xCoxO中的Co3O4团簇内部从O2-→Co2+的电荷转移过程。Co3O4/ Zn1-xCoxO复合结构中强烈的深紫外发射表明其将来有望在短波长磁光器件、发光二极管及激光二极管等领域得到应用。3、实验中制备的过渡金属掺杂ZnO薄膜在掺杂浓度合适时都具有铁磁性,尤其是在Cr、Cu和Mn掺杂ZnO薄膜中发现了居里温度高于室温的铁磁性,结构测试分析的结果表明这些薄膜中发现的铁磁性来源于过渡金属掺杂原子溶入ZnO晶格之后产生的本征属性,而并非源于杂质沉淀。进一步的电学性质测试结果和分析表明薄膜中形成这些铁磁性的可能机理为束缚磁极化子(BMP)模型。这些过渡金属掺杂ZnO薄膜表现出的铁磁行为表明了其在未来自旋电子器件领域的潜在应用价值。另外,实验中还发现,当过渡金属元素掺杂的浓度足以在ZnO薄膜中形成铁磁性之后,进一步增加过渡金属元素的掺杂浓度,反而导致每个过渡金属原子的平均饱和磁矩减小。这可能是因为掺杂浓度增加以后,更多过渡金属原子占据了ZnO晶格中的相邻位置并在相互间发生反铁磁耦合而引起了铁磁耦合效率的降低。更多还原

【Abstract】 Diluted magnetic semiconductors (DMS) have been attracting much interest of almost a decade now due to their potential to manipulate charge and spin degrees of freedom in a single material. Dietl et al. predicted high-temperature ferromagnetism in transition-metal (TM) doped wide-band-gap semiconductors particularly in ZnO and GaN. This fact has motivated many researchers to study the properties of TM doped semiconductors. Recent reports on the observation of room-temperature (RT) ferromagnetism in TM-doped ZnO have been welcomed with great enthusiasm by the scientific community. ZnO-based DMS have some advantages over others because of their unique characteristics, such as having a large band gap (~3.4 eV), large exciton binding energy at RT (~60 meV), high optical gain (300 cm–1), and very short luminescence lifetime, which are required for various optoelectronic and magneto-optical devices.This thesis is focused on the hotspots and challenges in the field of ZnO materials research. The purpose of this thesis is to investigate the room ferromagnetic behavior and the orgin of ferromagnetism in ZnO based DMS films. TM (Here Co, Cr, Cu, and Mn) doped ZnO thin films were synthesized via magnetron sputtering method and its microstructure, optical properties, and magnetic properties were investigated. The following is the major results:1. Structural analyses suggest that Co, Cr, Cu, and Mn occupied the Zn sites successfully and did not change the wurtzite structure of ZnO at low doping content. However, a small amount of Co3O4 clusters could be found in Zn1-xCoxO films when the doping level x reached 0.313. In addition, ZnCr2O4 could also be found in Zn1-xCrxO films when the doping level x reached 0.098. A point worth emphasizing is that nanoscale columnar grain arrays were found in the cross-sectional images of Mn- and Cu-doped ZnO films. As the magnetron sputtering method can produce economically feasible large area films with well-controlled composition, we suggest that the method in our experiment may be applied to future large-scale manufacturing of aligned Mn- and Cu-doped ZnO nanoscale columnar grains and nanorod arrays.2. The near band edge (NBE) emissions at around 375 nm were observed in the photoluminescent spectra of the Co- and Mn-doped ZnO films, which suggest the possibility of their application in ultraviolet light-emitting devices. Interestingly, when Co concentration was further increased up to 31.3 atom% and 37.7 atom%, a broad intense emission band centered at 332 nm (deep ultraviolet emission) was observed. The deep ultraviolet emission should be related to the O2-→Co2+ charge-transfer process in Co3O4 clusters embedded in Zn1-xCoxO films. The intense deep ultraviolet emissions of the Co3O4/ Zn1-xCoxO composite structures suggest their potential applications for short wavelength magneto-optical devices, such as light-emitting diodes and laser diodes.3. The ZnO films doped with moderate TM (Co, Cr, Cu, or Mn) exhibit obvious ferromagnetic ordering, which origins from TM-ZnO matrix rather than impurities and can be ascribed to originate from the bound magnetic polarons (BMP) model. The ferromagnetism of these TM-doped ZnO films also suggests their potential applications for future spintronics. In addition, the magnetic moment per TM atom decreased as the TM concentration further increased. It is proposed that the decrease in magnetic moment per TM atom as the TM concentration increases is due to an increase in the number of TM atoms that occupy adjacent cation lattice positions with an attendant increase in antiferromagnetic interaction between those TM atoms.更多还原