NiO基p型透明导电氧化物薄膜及其二极管的研究

【作者】 杨铭；

【导师】 张群；

【作者基本信息】 复旦大学 ， 物理电子学， 2011， 博士

【摘要】 p型透明导电氧化物(TCO)薄膜是近年来半导体材料领域的研究热点之一。在已经报道的p型TCO薄膜材料中,还没有任何一种材料的性能可以提升至满足实用化的水平。p型氧化物薄膜在材料的导电机理、合成工艺、器件制备等方面都有待进一步研究。基于此,本论文开展了利用新型脉冲等离子体沉积(PPD)技术制备p型TCO薄膜的研究工作,主要研究了Cu、K掺杂的p型NiO透明导电氧化物薄膜,以及由它们和n型TCO薄膜所构成的透明氧化物薄膜二极管。NiO是一种典型的宽禁带p型半导体材料,对其进行适当的掺杂可以提高其在室温下的电导率；PPD是一种基于脉冲电子束烧蚀靶材形成薄膜的技术。开展利用PPD技术制备掺杂氧化镍p型薄膜的研究工作,对于拓展透明导电氧化物薄膜材料的新体系、以及开发p型氧化物薄膜制备工艺,具有重要的意义与价值。主要研究内容与结论如下：基于Cu外层电子结构3d10的能级与O2p6相近、能够在金属-氧化学键中引入共价键成分来诱导形成扩展价带结构、从而降低氧离子电负性对空穴载流子强局域化作用的考虑,首先开展了关于掺铜氧化镍p型透明导电薄膜的研究工作。采用PPD方法制备了p型Nil-xCuxO(x=0～0.9)薄膜,Cu掺杂含量的提高有利于改善薄膜的电学性能,但是不利于光学性能；NiO导电的机理主要源于Ni3+存在,薄膜中的间隙氧能产生Ni3+,较高基底温度与较低氧气压条件下沉积薄膜都会减少间隙氧的含量,进而降低电导率；经工艺优化后制备的p型Ni0.9Cu0.10薄膜,室温下电导率达到5.17 S cm-1,可见光区(400～700 nm)的平均透射率为60%。开展了关于掺钾氧化镍p型透明导电薄膜的研究工作。利用PPD制备了p型Nil_xKxO(x=0～0.4)薄膜,所制备的薄膜同样没有杂质相的生成,且随沉积温度的提高晶格结构更趋于完善；K置换Ni提供更多的空穴数量,提高了薄膜的电导率；基底温度提高会改善薄膜在可见光区域的透明性,但是同时会减少薄膜中Ni3+的含量,不利于薄膜的电学性能；提高沉积薄膜时的氧气压可以同时改善薄膜的电学性能与光学性能。经工艺优化后制备的p型Ni0.75K0.250薄膜,室温下电导率达到4.25 S cm-1,可见光区的平均透射率接近60%。采用基于密度泛函理论的第一性原理方法模拟计算了Cu、K掺杂的NiO超晶胞结构。计算结果表明,两种元素的掺杂都使NiO能带的费米能级进入价带顶,呈现p型半导体特征,并且没有在禁带中引入深能级态。Cu掺杂NiO的价带顶主要由Ni 3d、Cu 3d和O2p态贡献,导带底则主要来源于Ni 4s态；K掺杂NiO的价带顶主要由Ni 3d、K 2p和O 2p态贡献。K掺杂引入的杂质能级对费米能级附近态密度的贡献没有Cu掺杂大。掺杂Cu离子引入的3d能级与NiO价带顶附近的能级发生了杂化,形成的杂化轨道减弱了O 2p能级对空穴载流子的局域性,预示着在NiO中掺Cu能实现更好的p型掺杂效果,有利于提高p型NiO的电导率。开展了关于p-Ni0.9Cu0.1O/n-IWO透明氧化物异质结的研究。掺钨氧化铟(IWO)是一种具有低电阻率、高迁移率和同时具备可见-近红外区(700～2500nm)高透射率的新型TCO材料,在太阳能电池透明电极、近红外光传感器等领域具有应用价值。通过调整溅射过程中的氧分压,可以控制IWO薄膜的电阻率在半导体的范围内,从而获得了与p型Ni0.9Cu0.1O薄膜电学性能相匹配的透明氧化物半导体n-IWO薄膜,可见光区平均透射率达到85%,光学禁带宽度3.65 eV。在此基础上,采用反应直流磁控溅射法、PPD和热蒸发法,设计并制备了IWO/n-IWO/p-Ni0.9Cu0.1O/Al四层结构的薄膜二极管。二极管在电流-电压(Ⅰ-Ⅴ)测试中体现了良好的整流特性,整流比接近90。所制备的薄膜电极与半导体层薄膜间的接触均为欧姆接触,没有产生肖特基势垒。分析了pn异质结的能带结构,理论计算得到的内建电势差与二极管的阈值电压大小基本符合。本部分内容对透明氧化物薄膜二极管的制备做出了初步探索,为室温生长全透明非晶氧化物薄膜二极管奠定了实验基础。非晶透明导电氧化物由于具有结构均一、高载流子迁移率、工艺温度低、易于大面积成膜等优点而特别适用于柔性显示技术。为了进一步拓展TCO薄膜在柔性电子学领域的应用空间,需要制备由该类材料构成的有源光电功能器件。这里利用反应直流磁控溅射法和PPD技术,并结合掩膜,在室温下生长基于α-IWO/n-IZO/p-Ni0.9Cu0.1O/α-IWO四层结构的全透明非晶氧化物薄膜二极管。通过逐层摸索寻找各层薄膜的最优化工艺。制备的α-IWO薄膜电极的最低电阻率达到5.75×10-4Ωcm,可见光与近红外透射率均大于80%；n型半导体层采用铟锌氧化物(IZO),IZO薄膜具有随沉积过程中氧分压变化的可控电阻率；整个薄膜二极管的各层薄膜均为非晶结构,且表面均匀平整。I-V测试结果显示,载流子浓度与电导率相接近的p型Ni0.9Cu0.1O和n型IZO薄膜的组合获得了较好的整流曲线,整流比达到40；整个二极管在可见光范围内的平均透射率接近50%,属于半透明状态,在柔性透明电子器件领域具有良好的应用前景。更多还原

【Abstract】 The p-type transparent conducting oxide (TCO) thin films have been widely researched in recent years. However, until now, the properties of reported p-type TCO films can’t meet the commercial applications yet. The condution mechanism, synthetization method of the materials and device fabrication need to be further studied. In this paper, we study Cu-doped and K-doped NiO p-type transparent conducting films by a newly pulsed plasma deposition (PPD) method, and further the diodes composed of the p-type NiO-based thin films. NiO is a kind of typical wide band gap p-type semiconductor. The conductivity of NiO could be increased by doping with appropriate elements. PPD is a kind of deposition technique based on the ablation of the target. The study on p-type doped NiO films by PPD method is a meaningful work for exploring newly p-type TCO materials systems and preparation methods. Main experimental results and conclusions are as follows:The energy level of Cu 3d10 is comparable to that of O 2p6 and the formation of covalent bonding is possible, which could lead to large dispersion in the valence band and reduction in localization of positive holes. The study on Cu-doped NiO transparent p-type conducting films was first performed. The Ni1-xCuxO(x=0～0.9) thin films were prepared by PPD method. The conductivity will be improved with high content of dopant while the transparency will be deteriorated. The conduction mechanism of NiO was mainly ascribed to the existence of Ni3+. Ni3+ could be formed by the excess oxygen in films. The conductivity would be decreased due to the decreased amount of Ni3+, which is mainly caused by low depositon temperature and working pressure. The Ni0.9Cu0.1O film exhibits the highest conductivity of 5.17 S cm-1, with an average transmittance of 60% in the visible region (400～700 nm).K-doped NiO transparent p-type conducting films was studied. The Nii-xKxO (x =0～0.4) films were prepared by PPD method. There was no impurity peaks detected in films. With the deposition temperature increased, the diffraction peaks would become stronger, which indicate the better crystalline structure. The conductivty of the films were promoted due to the K doping. The transparency was improved with the increase of deposition temperature while the conductivity was deteriorated due to the decrease amount of Ni3+. Both the electrical and optical properties of the films would be improved if deopsited in a high oxygen atmosphere. The Ni0.75K0.25O film deposited at room temperature exhibits the highest conductivity of 4.25 S cm-1, with an average transmittance of～60% in the visible region.First principles based density functional theory have been performaned for Cu-doped and K-doped NiO supercells, respectively. The calculation results showed that both the two doped elements make the fermi level of NiO into the valence band maxium (VBM), which was a typical characteristic of the p-type semiconductors. Besides, the doped elements didn’t introduce the deep level in the gap. The VBM of Cu-doped NiO was mainly composed of Ni 3d, Cu 3d and O 2p states, respectively, while the conduction band minimun (CBM) consisted mainly of Ni 4s states; The VBM of K-doped NiO was mainly composed of Ni 3d, K 2p and O 2p states, respectively. The impurity level introduced by the K doping contributed less to the states of density near the fermi level in contrast to Cu doping. The 3d level introduced by Cu doping were hybridized with the energy levels from VBM. The formed hybridized orbits would weaken the localization of O 2p level. The results indicate that Cu doping have a better effect on the increase of p-type conductivity.Transparent oxide hetero-junction of P-Ni0.9Cu0.1O/n-IWO was investigated. Tungsten doped indium oxide (IWO) is a newly kind of TCO material with low resistivity, high mobility and high transparency in both visible and near infrared region (700～2500nm). IWO films have potential applications in solar cells and near infrared sensors. The resistivity of IWO film could be controlled to match the p-Ni0.9Cu0.1O layer by adjusting the oxygen partial pressure during the sputtering process, with an average transmittance of 85% in visible region and energy band gap of 3.65 eV. The diode with the structure of IWO/n-IWO/p-Ni0.9Cu0.1O/Al was fabricated by dc magnetron sputtering, PPD method and thermal evaporation. Current-voltage (Ⅰ-Ⅴ) curve of the diode exhibits nonlinear and rectifying characteristics. The ratio of forward current to the reverse current is about 90. The ohmic nature of the contacts between electrode layer and semiconductive layer was confirmed. The equilibrium energy band diagram of the pn heterojunction was given. The value of the built-in voltage calculated theoretically is approximately identical with the threshold voltage. The results give the reference for the next step of fabrication of amorphous transparent all oxide film diode at room temperature.Amorphous tranparent conducting oxides are highly favorable for applications in flexible displays because they have many inherent advantages such as robust properties with regard to lattice mismatch and low temperature deposition on large substrates. The optoelectronic devices composed of such materials should be fabricated to enlarge the applications in the field of flexible electronics. In this paper, amorphous transparent all oxide film diode with structure ofα-IWO/n-IZO/p-Ni0.9Cu0.1O/α-IWO was fabricated at room temperature by dc magnetron sputtering and PPD technique. Each layer was studied respectively to obtain the optimal deposition condition. IWO film as electrode layer showed a resistivity of 5.75×10-4Ω. cm, with an average transmittance over 80% in both visible and near infrared region. Indium zinc oxide (IZO) was used as n-type semiconductive layer. The resistivity of IZO films could be adjusted by the oxygen partial pressure during deposition. Each layer of the diode showed amorphous structure and smooth surface. Diode composed of p-Ni0.9Cu0.1O and n-IZO layers with matched properties obtained the better nonlinear and rectifyingⅠ-Ⅴcharacteristics, with a ratio of 40 of forward current to the reverse current and an average transmittance of～50% in visible region. The results indicate a promosing application in flexible transparent electronic devices.更多还原