导电氧化物LaNiO₃薄膜的生长及铁电集成性研究

【作者】 郑亮；

【导师】 张鹰；

【作者基本信息】 电子科技大学 ， 材料学， 2006， 硕士

【摘要】 近年来,铁电薄膜由于其在集成电路和其它功能器件中的应用受到了越来越多的关注。在这些应用中,制备供集成铁电薄膜的优质底电极就显得尤为重要。作为一种合适的侯选电极材料,LaNiO3 (LNO)的晶体结构是一种赝立方的钙钛矿结构。其晶格常数为3.84? ,与目前广泛研究的铁电薄膜(PZT、BST等)具有类似的晶体结构和良好的晶格匹配性。它在室温下的电阻率为225μ?.cm。同时,该材料还具有高的化学和热稳定性,因而被公认为铁电薄膜器件理想的电极材料之一。因此,开展LNO薄膜电极的制备和相应的铁电薄膜集成性的研究具有重要的应用价值。本文利用激光脉冲沉积方法(Pulsed laser deposition)在不同的工艺条件下(如:气氛、衬底温度、激光的能量等)在不同氧化物基片上制备了LNO薄膜,对其生长以及和典型铁电材料BaTiO3薄膜的集成性等方面展开了系统的探讨和研究,并取得了一些成果。首先,通过原位高能电子衍射(RHEED)及X光电子能谱(XPS)研究了氧分压对LNO导电薄膜微结构的影响,并进一步提出了氧分压对LNO薄膜微结构的影响的机理。我们发现在较低的真空度下,即氧分压处于2×10-4 Pa和3×10-3 Pa之间,LNO晶格中的一个O2-将会转移两个电子给两个Ni3+,并且移动到薄膜表面形成O2被泵抽走,从而导致钙钛矿结构的垮塌,其相应的RHEED图样呈现出清晰而明亮的点,表明表面为较为粗糙的三维岛状结构。随着膜厚增加而超过约30 nm的临界厚度时,越来越多的晶格氧会移动到了薄膜表面,此时所提供的氧将使得后续生长的LNO膜层重新形成钙钛矿结构,并以层状方式外延生长。当氧分压在临界值以上时,LNO薄膜在整个沉积过程中(膜厚小于30nm)的RHEED图样是条纹状,表明层状方式外延生长的特征。在此过程中,如将高的氧分压再次抽至临界值以下时,钙钛矿结构的LNO薄膜发生分解,其相应的RHEED图样也变成了点状。所以,氧分压在LNO薄膜的生长过程中是决定薄膜微结构的关键因素。并且由氧分压所导致的结构变化是可逆的。其次,在优化的工艺条件下,分别在MgO(100)、STO(100)及LAO(100)单晶基片上制备了高度(ll0)及(00l)取向的LNO薄膜。在和LNO薄膜具有较大晶格失配度的MgO(100)基片上,首次成功制备了具有双晶外延结构的LNO薄膜底电极,为实现铁电薄膜在微波器件上的应用提供了基础。在STO(100)及LAO(100)基片上,LNO是以“cubic-on-cubic”的方式外延生长的。同时通过电学表征发现,在不同基片上制备的外延LNO薄膜的电阻率都符合在其上集成铁电或介电材料的要求。最后,通过以不同取向的外延LNO底电极层为诱导层,分别制备得到具有更多还原

【Abstract】 In recent years, ferroelectric thin films have been receiving more and more attention for their applications in integrated circuit and other functional devices. In these applications, it is essential to fabricate high quality ferroelectric thin films on proper electrode materials, such as conductive oxides. LaNiO3 (LNO), as a desirable candidate for electrode materials, has a pseudo-cubic perovskite crystal structure with a lattice parameter of 3.84? and a resistivity of 225μ?.cm at 300K with good chemical and thermal stabilization. It has the similar crystal structure and good crystal matche with ferroelectric thin films,such as BaTiO3 . In this paper, the growth of LNO thin films on different substrates and the integration of ferroelectric thin films on the LNO electrodes have been systematically studied.Firstly, LaNiO3 (LNO) films with different thickness were deposited on STO (100) substrates in the different oxygen pressure. Effects of oxygen pressure on microstructure of LNO conductive thin film has been studied by in situ reflection high energy diffraction (RHEED) and ex situ X-ray photoelectron spectroscopy (XPS). In the relatively low oxygen pressure, LNO film displays spotty RHEED pattern. When the thickness increases up to a critical value, about 30 nm, the spotty RHEED pattern gradually changes to streaky pattern, and the RHEED oscillation curve appears. The RHEED pattern of the ultra thin LNO film deposited in the relatively high oxygen pressure is streaky pattern. With pumping the oxygen pressure to the relatively low value, the streaky RHEED pattern gradually changes to spotty one. When increasing the oxygen pressure, the RHEED pattern changes to streaky one again. This RHEED pattern transformation induced by the oxygen pressure is reversible. Ex situ XPS results indicate that the element Ni of LNO film deposited in the relatively low oxygen pressure with thickness below the critical value exists in the form as Ni2+, while as Ni3+ in the relatively high oxygen pressure. When the thickness increases to upon the critical value, the chemical valence of element Ni in the top layers of LNO film deposited in the relatively low oxygen pressure is +3. A mechanism of effects of oxygen pressure on microstructure of LaNiO3 conductive thin film is given.Then, a series of metallic LaNiO3 (LNO) thin films were deposited on (100)-oriented MgO, SrTiO3 (STO) and LaAlO3 (LAO) crystal substrates by pulsed laser deposition (PLD) under the oxygen pressure of 20Pa at different substrate temperatures from 450oC to 750oC. X-ray diffraction (XRD), ex situ reflection high更多还原