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锰钴镍氧化物(Mn-Co-Ni-O)薄膜的光学电学性质研究
马超
学位类型博士
导师徐金宝
2016-05-27
学位授予单位中国科学院大学
学位授予地点北京
学位专业微电子学与固体电子学
关键词薄膜 尖晶石 光学性质 电学性质 负温度系数热敏电阻
摘要

尖晶石结构锰钴镍氧化物(Mn-Co-Ni-O)作为一类重要的过渡金属氧化物材料,具有优异的负温度电阻系数(NTC)特性、较宽范围的光谱响应以及适当的电阻值,被广泛用来制造负温度系数热敏电阻器和非制冷红外探测器。目前,对Mn-Co-Ni-O材料的基础研究主要集中在结构、阻温特性等方面,对材料光学性质的研究工作则相对较少报道。然而光学性质对于深入理解材料的光、电响应机制及光电器件的性能设计都具有重要意义。另外,传统的Mn-Co-Ni-O器件主要由高温烧结的体材料制成,其器件在成品率,可重复性,长期稳定性和与半导体器件制备工艺兼容等方面存在一定问题。因此,还需要对Mn-Co-Ni-O薄膜材料进行深入研究。针对这些问题,本论文选择Mn-Co-Ni-O尖晶石薄膜材料为主要研究对象,以化学溶液沉积(CSD)法进行材料制备,并对其光学和电学性质进行了以下几方面的探讨:(1) 通过CSD法在Si(100)衬底上制备的Mn1.56Co0.96Ni0.48O4薄膜材料具有尖晶石结构。对薄膜材料在不同温度下进行紫外-可见-近红外光区的椭圆偏振光谱测量,并利用Tauc-Lorentz方程拟合椭偏数据,获得不同温度下薄膜材料的光学常数。结果表明,折射率n在480 ~ 620 nm范围随波长的增大而减小(正常色散),在其他区间为反常色散,即薄膜产生吸收共振的波长范围;消光系数k表明其吸收结构分别位于紫外和近红外区,它们之间是弱吸收区。随温度升高,折射率在短波长区减小,在长波长区增大,消光系数在可见光区随温度升高而增大。分析认为,折射率的温度依赖性取决于热膨胀和晶格振动的平衡,消光系数的改变主要是由温度引起的材料中阳离子浓度的变化造成。(2) 以MnxCo2.52?xNi0.48O4 (x=1.56, 1.88和2.20)系列薄膜为研究对象,讨论离子浓度对薄膜光学常数的影响和作用机制。通过拟合椭偏测试结果后得到的样品光学常数显示,折射率随着Mn含量的增加而增大,其主要影响因素是样品的晶粒大小不同;消光系数在1.5 ~ 2.3 eV之间随Mn含量的增加而降低,而在大于2.3 eV范围内增大。通过进一步分析薄膜样品的复介电函数和拉曼光谱,可得知1.7 eV处的吸收结构主要是由Co2+/Co3+之间的d-d电子转移造成的,而其余两个吸收结构是由Mn3+和O2-之间的电荷转移跃迁引起的。同时,该研究也提供了一种无损检测MCN样品中Mn3+/Mn4+离子浓度的方法。(3) 利用开尔文探针力显微镜(KPFM)测得MCN薄膜在不同温度下的接触电势差图谱,从而得到了样品功函数随温度变化曲线。为探索薄膜表面电子和离子的具体变化,通过椭圆偏振光谱测量得到MCN薄膜在相应温度下的复介电函数的虚部。分别在2.7和4.1 eV处产生吸收结构。此处的吸收结构可以归结为O 2p和Mn 3d之间的电荷转移跃迁,这些吸收结构的强度与Mn3+浓度引起的电子结构的变化相对应。同时,发现这些吸收峰的强度与功函数值随温度的变化趋势呈负相关性。即最小吸收峰强度对应最大的功函数值,反之亦然。其主要原因可归结为过渡金属氧化物的离子其功函数的值随氧化态的升高而增大。对于材料中含有一定量的Mn而言,如果低氧化态离子(Mn3+)增多/减少,而高氧化态离子(Mn4+)减少/增多,材料的功函数就会降低/升高。(4) 通过对(Mn1.56Co0.96Ni0.48O4)1-x(LaMnO3)x (MCN-LM, x =0, 0.1, 0.3和0.5)系列复合薄膜的结构及光学电学性质的研究,发现随La含量的增多,薄膜的结构从尖晶石纯相向钙钛矿和尖晶石的混合相过渡。同时,薄膜的致密性和表面粗糙度也随La含量的增多而改善。椭偏测试表明,复合薄膜的光学常数和纯相MCN薄膜有较大差别。折射率在大于550 nm的波长范围有明显的增大。这主要是受到薄膜的致密度和形貌的影响,以及较强离子键结合的钙钛矿中的激子相互作用。消光系数随La含量的增多迅速增大,并且在可见光区有两个强吸收结构。当x = 0.5时,薄膜材料的吸收系数在396和561 nm处分别达到了1.87×105和1.43×105 cm-1。阻温测试表明,所有薄膜样品都具有明显的负温度系数热敏特性,当x = 0.5时,该组分薄膜材料的电阻值最小,而其B25/50值最大,约为4022 K,表明该组分的薄膜温度灵敏度最高。本研究有望为热敏型红外探测器提供一种吸收覆盖范围更宽,灵敏度更高的新材料。

其他摘要

As one of the important transition metal oxides, Mn-Co-Ni-O spinel has been widely used for negative temperature coefficient (NTC) thermistors and uncooled infrared detectors due to its excellent NTC characters, high sensitivity to wide spectral ranges and the appropriate resistivity. The current studies of Mn-Co-Ni-O spinel materials mainly focus on their crystal structures, the relationship between temperature and resistant and so on. However the study of the optical properties is rarely reported. The optical properties are very important to understand the material and manufacture the optoelecreonic devices. The Mn-Co-Ni-O spinel was traditionally used in bulk form and prepared by ceramic sintering method at high temperature. There are often problems with product yield, long-term stability and reproducibility. And the preparation method is not compatible with modern semiconductor fabrication process. Based on those issues, this thesis mainly focused on the spinel Mn-Co-Ni-O thin films prepared by chemical solution deposition (CSD) method and the optical and electrical properties as the following aspects:(1) The Mn1.56Co0.96Ni0.48O4 (MCN) thin films are prepared by CSD method on Si (100) substrates can be assigned to the spinel structure. The measured ellipsometry parameters under different temperature are fitted by Tauc-Lorentz oscillator dispersion formula. We can obtain the temperature dependence of MCN optical constants in ultraviolet-visible-near infrared (UV-VIS-NIR) ranges. The results show that the refractive index n is decreasing as wavelength increases (normal dispersion) only at wavelength range of 480 to 620 nm. Anomalous dispersion occurs in other ranges, where the wavelength of the light is close to the absorption resonance of the material. The extinction coefficient k indicates the sample has two absorption structures located at UV and NIR regions, respectively. Among them there is the low absorption region. The refractive index decreases at short-wavelength region but increases at long-wavelength region with increasing temperature. The extinction coefficient increases with increasing temperature at visible range. This work shows that the changes in refractive indexes of MCN thin films are mainly due to the lattice vibration and thermal expansion of lattice, while the extinction coefficients are due to the cation distribution under different temperatures.(2) The MnxCo2.52?xNi0.48O4 (x=1.56, 1.88 and 2.20) serial thin films are chosen to study the influence of cation distribution on optical properties. The optical constants of serial thin films are presented by fitting the spectroscopic ellipsometry data. The refractive indexes of thin films increase with increasing Mn content in most ranges due to the various grain sizes. The extinction coefficients decrease in the range of 1.5 ~ 2.3 eV but increase beyond 2.3 eV with increasing Mn content. The imaginary parts of dielectric functions and Raman spectroscopy indicated that the absorption structures mainly caused by d-d transition between Co2+/Co3+ ions around 1.7 eV energy range, and the other two absorption structures are attributed to charge transfer transitions of Mn3+ and O2-. This study provide a method to identify the cation distribution of Mn3+/Mn4+ in MCN materials.(3) The high resolution images of the contact potential difference of MCN thin films are obtained through the Kelvin probe force microscope (KPFM) and the correlations between the work functions and temperatures are also obtained. In order to explore the specific variations of electrons or cations, the imaginary part of the complex dielectric function of MCN thin films were measured by spectroscopic ellipsometry at different temperatures. There found two broad absorption structures located at around 2.7 and 4.1 eV. These two absorption structures could be related to the charge transfer transitions involving 2p electrons of oxygen ions and 3d electrons of Mn ions, and the intensities of these absorption structures are related to the change of the electronic structures due to the concentration of Mn3+ ions. The intensity variation trend of these absorption peaks with the temperature inversely matches the variation curve of the work functions. That is, the sample with the minimum absorption peak intensity has the maximum work function value and vice versa. This reason is attributed to the work functions increased with increasing oxidized forms. As for constant Mn ions, the low-oxidized cations (Mn3+) increase/decrease correspond to the high-oxidized cations (Mn4+) decrease/increase, thus the work functions decrease/increase.(4) The (Mn1.56Co0.96Ni0.48O4)1-x(LaMnO3)x (MCN-LM, x =0, 0.1, 0.3 and 0.5) composite serial thin films are synthesized and their structural, optical and electrical properties are obtained and compared. The films vary from the pure spinel structure towards the combination of the partial perovskite and partial spinel structures with increasing content of La. It also induces the apparent improvement in the film density and surface roughness. A pronounced difference in the optical functions is found in the visible spectral range. To explain the significant increase of n values above 550 nm wavelength for the composite films, it is believed that the morphological changes or the surface smoothness of these films may be responsible for it. Another reason is that the excitonic interaction tends to be stronger in perovskite oxides due to the stronger ionic bonding in the lattice so that the composite films have a larger refractive indexes compared to pure MCN thin film. The k values of the composite films are greater than that of pure MCN film through most of the visible wavelength ranges and has two broad absorption structures in the visible range. The absorption coefficients for x = 0.5 are peaking at approximately 1.87×105 and 1.43×105 cm-1 for 396 and 561 nm, respectively. The resistance-temperature relationship indicates all of the films are with negative temperature coefficient characters. Especially, the film with x = 0.5 offered the lowest resistance and highest B25/50 constant at 4022 K, suggesting that it is more sensitivity. This study could provide a potential NTC thermistor material with wide spectral range response and higher sensitivity for future generation thermal detectors.

文献类型学位论文
条目标识符http://ir.xjipc.cas.cn/handle/365002/4570
专题材料物理与化学研究室
作者单位中国科学院新疆理化技术研究所
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马超. 锰钴镍氧化物(Mn-Co-Ni-O)薄膜的光学电学性质研究[D]. 北京. 中国科学院大学,2016.
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