掺杂ZnO透明导电薄膜的光电性能与导电机理研究

发布时间：2018-03-01 17:17

本文关键词： ZnO 掺杂 AgNW 迁移率载流子散射　出处：《湖北大学》2016年博士论文　论文类型：学位论文

【摘要】：透明导电薄膜是把光学透明性能与导电性能复合在一体的光电材料,其在平板显示器、太阳能电池、发光二极管、气体传感器、电磁屏蔽视窗、节能玻璃等光电器件领域获得了广泛的应用和日益增大的市场需求。目前应用最广泛的透明导电薄膜是In_2O_3:Sn(ITO)薄膜,但是ITO材料的主要成分In是一种及其昂贵且稀有的元素,限制了 ITO薄膜的实际应用潜力。我们要研究的氧化锌(ZnO)是一种宽禁带Ⅱ-Ⅵ族半导体材料,自然状态下成六方纤锌矿晶体结构。掺杂的ZnO薄膜具有与ITO薄膜相当的高透过率和低电阻率的特点,并且原料丰富、价格低廉、无毒、在氢等离子体气氛中稳定不易被还原,有望成为替代ITO的理想材料。本文首先采用射频磁控溅射法在玻璃衬底上面制备出高质量的A1掺杂ZnO(AZO)透明导电薄膜,用XRD、SEM、XPS和分光光度计等测试手段对沉积AZO薄膜的物相、形貌、成分、光学性能等进行了表征和分析。用Van der Pauw方法对样品的电学特性进行了测量,并分析了 AZO薄膜的导电机制。在制备过程中通过改变衬底温度、氧氩比、功率、气压等工艺条件,研究各个参数对于薄膜光电性能的影响,得出最佳的工艺参数。最终确定用射频磁控溅射法制备AZO薄膜的最佳工艺条件为:掺杂浓度为2 wt%下,氧氩比0:30,衬底温度为常温,工作气压3Pa,功率120W。在此条件下制备的AZO薄膜其透射率达到80%以上,电阻率可达到1 × 10~(-3) Ω·cm数量级。在优化工艺参数的基础上,我们研究了 ZnO薄膜表面界面特性与光电性能的关联,以实现薄膜结构的优化设计。通过对AZO薄膜进行退火处理、改变厚度的处理方法从表面与界面的角度来分析AZO薄膜微结构与光电性能的演变规律,并通过高温高湿的环境稳定性实验来探索薄膜电学性能退化的机理。最终尝试薄膜表面与银纳米线(AgNW)复合的方法,实现降低表面电阻和提高环境稳定性的目的。首先退火结果的比较表明在真空中退火的AZO薄膜比未退火和空气中退火的薄膜显示出更低的电阻率,特别是真空退火后载流子迁移率明显增大,结合Ols电子态分析判断,其增大的迁移率主要源自于真空退火环境中AZO晶界处吸附的O向外界扩散,晶界缺陷浓度降低,对载流子的散射作用减弱。此外,AZO薄膜越厚,结晶程度和择优取向都更好,远离界面处的晶粒逐渐合并长大,形成超大的晶粒,结构也更加致密。当AZO薄膜厚度达到2.5μm时,载流子浓度和迁移率都明显增大,电阻率可以达到8.24×10~(-4)Ω·cm。但是AZO薄膜厚度太厚时,光学透过率会有所下降。然后我们研究了 AZO薄膜的环境稳定特性,在室内环境放置24天后AZO薄膜的方阻增大了 11.1%,而高温高湿环境中薄膜的方阻增大了 95.5%。电学性能测试结果表明薄膜电学性能退化主要原因不是载流子浓度的下降,而是载流子迁移率的降低,结合XPS深度元素分析结果还可以发现O向内部扩散明显,而且含有O_2、H_2O、CO_2等共价键较强的产物。最后我们尝试在AZO薄膜上旋涂一层AgNW并进行真空退火处理,复合薄膜的表面电阻可以从24 Ω/□降低到9.8 Ω/□,环境稳定性也得到大幅改善。结合AZO薄膜的表面特性我们认为,由于AgNW在AZO薄膜表面形成网状结构的导电通道,弱化了薄膜表面的晶界对载流子的散射效果,从而克服了湿热环境通过AZO薄膜晶界带来的负面影响。通过改变掺杂浓度及掺杂元素可以探究导电的机理及主要影响因素。随着A1的掺杂浓度的增加,ZnO薄膜的导电性也发生变化,当Al掺杂质量分数为2%时,薄膜的导电性能最佳,其低的电阻率和高的载流子浓度主要原因是具有相对较高的有效掺杂率。通过在Ar和H_2混合气氛溅射实现H钝化Al掺杂ZnO薄膜可以有效的提高载流子浓度,薄膜的电子浓度达到6.416×10~(20) cm~(-3),电阻率也因此进一步降低到6.49×10~(-4) Ω·cm,此时薄膜的光学带隙也达到了 3.78 eV。H钝化Al掺杂的结果表明H不仅可以影响AZO薄膜的结构,钝化薄膜晶界缺陷,还会扮演电子施主的角色提高薄膜的导电性,故H的掺杂有利于获得高性能AZO透明导电薄膜。相对于ZnO薄膜而言,In、Ga共掺的ZnO薄膜可以明显改善和提高载流子迁移率,其值达到24.01 cm~2/Vs,但是载流子浓度并不高,只有2.74×10~(20)cm~(-3)。XPS测试其元素深度分布可以推测掺杂的ZnO薄膜内部的施主主要来源于替位掺杂缺陷,而不是O空位或间隙Zn缺陷。本文最后论了薄膜中几种主要的载流子散射机制:杂质离子散射、晶界散射和极化光学声子散射机理,确定了每种机理的推理和计算公式。通过ZnO、AZO和IGZO薄膜的变温电学性能测试,特别是不同薄膜的载流子迁移率随温度的变化,探究每种散射机理对薄膜迁移率影响的强弱。首先发现极化光学声子散射限制的迁移率随着温度升高而迅速下降,但是温度升到室温时,迁移率仍然远大于200 cm~2V~(-1)s~(-1),对迁移率的限制不占主导作用。对于ZnO薄膜来说,晶界散射起主要的限制作用,而对于IGZO薄膜来说,由于隧穿电导的作用,杂质离子散射是主要的限制机理。而热电子发射迁移率与电子隧穿迁移率均与载流子浓度和晶界缺陷态密度密切相关,通过图解可以发现,温度、晶粒尺寸、载流子浓度和晶界缺陷态密度四个因素共同影响杂质离子散射、晶界散射和极化光学声子散射这三种电子传输机制之间的竞争与平衡关系。
[Abstract]:The transparent conductive film is the photoelectric material and the conductive performance of the optical properties of the composite in the transparent one, the flat panel display, solar cells, light-emitting diodes, gas sensors, electromagnetic shielding windows, glass and other optoelectronic devices is widely used and the increasing market demand. Currently the most widely used transparent conductive film is In_2O_3:Sn (ITO) film, but the main ingredients of ITO materials and In is an expensive and rare element, limiting the application potential of ITO films. We need to study the Zinc Oxide (ZnO) is a kind of wide band gap II-VI semiconductor material, the six party wurtzite structure into ZnO under natural condition. The film doped with ITO thin film is high transmittance and low resistivity characteristics and abundant raw materials, low price, non-toxic, the hydrogen in the plasma atmosphere is stable and is not easy to be restored, is expected to become The ideal material to replace ITO. Firstly, using RF magnetron sputtering method on glass substrate were prepared by doping A1 high quality ZnO (AZO) transparent conductive thin film, XRD, SEM, XPS and photometer for deposition of AZO thin film phase, morphology, composition and optical properties of the characterization and analysis. The electrical characteristics of samples using Van der Pauw method were measured and analyzed the conduction mechanism of AZO thin films. By changing the substrate temperature in the preparation process, oxygen argon ratio, power, pressure and other process conditions, effects of various parameters on the electrical properties of thin films, the optimum parameters the final to determine the optimum conditions for preparation of AZO films by RF magnetron sputtering as the doping concentration is 2 wt%, ratio of oxygen and argon at 0:30, the substrate temperature is room temperature, working pressure 3Pa, 120W. power under the condition of AZO thin films prepared by the transmittance reached 80% Above, the resistivity can reach 1 * 10~ (-3). The level of CM. Based on the optimization of process parameters, we studied the association of surface and interface properties of ZnO thin film and photoelectric properties, in order to achieve the optimal design of thin film structure. Based on the AZO thin films were annealed, the thickness change processing method from surface and interface the perspective of the evolution and photoelectric properties of AZO thin film micro structure, and through the stability test of high temperature and high humidity environment to explore the mechanism of the degradation of electrical properties of the films. The final attempt to film and silver nanowires (AgNW) composite method, can reduce the surface resistance and improve the stability of the environment. The first annealing that the AZO films annealed in vacuum than annealed without annealing in air and show lower resistivity, especially after annealing in vacuum carrier mobility increased significantly with Ols State analysis, the increasing mobility mainly from the vacuum annealing environment in AZO grain boundaries. The adsorption of O to the outside diffusion, grain boundary defect concentration decreased, reduced scattering effect on the carrier. In addition, the AZO film is thicker, degree of crystallinity and orientation are better, far away from the interface with grain gradually grew up, the formation of large the grain, the structure is more compact. When the thickness of AZO film reached 2.5 m, carrier concentration and mobility are obviously increased, the resistivity can reach 8.24 * 10~ (-4) cm. AZO. But the film thickness is too thick, the optical transmittance will decline. Then we study the environmental stability characteristics of AZO thin films in the indoor environment, after 24 days of AZO thin film resistance increased by 11.1%, while the resistance film of high temperature and high humidity environment. The test results show that the electrical properties of 95.5%. thin film electrical property degradation is not the main reason for the carrier The decrease of the concentration, but lower carrier mobility, combined with the results of XPS depth elements can also be found in O to the internal diffusion is obvious, but also contains O_2, H_2O, CO_2 and other strong covalent products. Finally we try on the AZO film coated with a layer of AgNW and vacuum annealing treatment, surface resistance of composite films can from 24 to 9.8 ohms / lower - Omega / -, environmental stability has been greatly improved. Combined with the surface properties of AZO films we think, because the AgNW forming conductive channel network structure on the surface of AZO film, the weakening of the scattering effect on the carrier of grain boundary film surface, so as to overcome the negative effects of hygrothermal environment through the AZO grain boundary film bring. By changing the doping concentration and doping elements can be conductive to investigate the mechanism and the main influence factors. With the increase of A1 doping concentration, the conductivity of ZnO films is also changing, when Al The doping concentration is 2%, the best performance of the conductive film, low resistivity and high carrier concentration is the main reason for having relatively high effective doping ratio. Through the realization of H passivation of Al doped ZnO films on Ar and H_2 mixed atmosphere sputtering can effectively improve the carrier concentration, the electron concentration reached to 6.416 * 10~ (thin film 20) cm~ (-3), so the resistivity is further reduced to 6.49 * 10~ (-4). Cm, the optical band gap of the films reached 3.78 eV.H passivation doped Al results show that H not only can influence the structure of AZO thin film, passivation film boundary defects, the conductivity will play the role of improving film electron donor therefore, H doping is beneficial to obtain high performance AZO transparent conductive films. Compared with the ZnO film, In, ZnO Co doped Ga thin films can be obviously improved and enhanced carrier mobility, its value reached 24.01 cm~ 2/Vs, but the current carrying Concentration is not high, only 2.74 * 10~ (20) cm~ (-3).XPS test ZnO film internal donor the depth profile of elements can be doped mainly from substitutional doping defects, rather than the O vacancy or interstitial defects. Finally, Zn on several main carrier scattering mechanism in the film: impurity ions scattering and grain boundary scattering, polar optical phonon scattering mechanism, reasoning and calculation formula for each mechanism is determined. Through ZnO, the electrical properties of temperature test of AZO and IGZO films, especially films of different carrier mobility versus temperature on each scattering mechanism of the film mobility influence. The first migration polar optical phonon scattering limit rate decreased rapidly with the increasing of temperature, but the temperature to room temperature, the migration rate is still far greater than 200 cm~2V~ (-1) s~ (-1), the mobility restriction does not play a dominant role for the ZnO. Film, play the main role in the grain boundary scattering limit, and for IGZO thin films, due to the tunneling conductance effect, scattering of impurity ions is limited. The main mechanism of the thermal electron emission and electron tunneling mobility mobility and carrier concentration and grain boundary defect density is closely related, through diagrams can be found in temperature grain size, carrier concentration, grain boundary and defect density of four common factors that affect the scattering of impurity ions, the relationship between competition and balance between grain boundary scattering and polarization optical phonon scattering of the three electron transfer mechanism.

【学位授予单位】：湖北大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TB383.2

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