Study Of The Surface Work Function Adjustment Of AZO Film By Plasma Immersion Ion Implantation

Transparent conductive oxide films (TCO’s) have high transmittance in visible light region、high reflectivity in infrared region, and low resistance. Thus, TCO have been widely applied in solar battery, display panel, shielding and antistatic film, gas-sensitive sensor, infrared stealth materials, OLED, and etc. Among various TCO’s, ITO is the most applicable with its low resistivity, high visible light transmittance, and relatively high work function. However, in reducing or plasma atmosphere, it will be unstable and cause In precipitation. When temperature is higher than 700K, ITO deteriorates; mainly results from In diffusion out of substrate. For example, when ITO serves as anodes of OLED, In spreads into organic layer, which affects photoelectric property of OLED devices. Besides, Though as rare metal only 0.1 ppm in earth crust, In will be exhausted and its price will increase higher and higher, LCD, Solar battery, and especially OLED demand a large amount of In for ITO production. Therefore, developing new TCO materials instead of ITO has attracted a lot of attention in recent years.In all the substitute materials of ITO, ZnO and doped with Al, Ga, In, Zr, B and lanthanon are well developed. Among of them, AZO is best candidate for its excellent photoelectric property, rich resource, environment protection and low cost. Nevertheless, surface work function (SWF) of AZO, merely 4.5 eV, highly mismatches the organic materails’ HOMO energy level, and thus a big hole injection barrier will be existed at the interface of anode and organic layer. That barrier resists hole injection and has negative influence on operation voltage, efficiency and life time of OLED device. To raise SWF of AZO to the same level as organic layer’s HOMO energy level is the key for AZO application in OLED anode. Although there is a few literature about AZO work function adjustment now, the surface treating methods for ITO SWF adjustment can apply for AZO too for their similar properties. As has been recognized, oxygen plasma treatment is among most frequently used and effective way in raising work function; however, hole injection barrier remains high. Up until now, mechanism of work function variation is of high controversy, and how long can work function stay in a higher level after oxygen plasma treatment is still uncertain.In order to solve these problems, this thesis introduce plasma immersion ion implantation (PⅢ) method to adjust SWF for AZO. Kelvin probe, XPS, XRD, AFM, four probe tester, and ultraviolet-visible spectrophotometer respectively indicate SWF, surface chemical composition, crystal structure, exterior morphology and photoelectric property of treated AZO film. SWF adjustment of AZO and the mechanism have been studied, the fruition is listed below:(1) It is the first systematical research in PⅢ adjusting SWF of AZO film, including how gas type, bias voltage, treating time, and pulse width change AZO surface property. Both oxygen PⅢ and oxygen ICP treated sample are compared to reference sample in the five aspects with foregoing analyzing methods. The results show that as oxygen PHI improves SWF substantially, it does no obvious damage to crystal structure, exterior morphology, and photoelectric property of AZO film.(2) After optimizing experiment parameter, the thesis gives the best condition for PⅢ treatment for AZO. Oxygen was used as the working gas, in the condition of the following: base vacuum 1.0×10-3 Pa, gas flux 50 sccm, RF power 88W,-500V biased potential supply with pulse width 10 μs at 1 kHz, treating time 30 minutes, the SWF of AZO can be up to 1.1 eV, enough for matching HOMO energy level of organic materials. On the other hand, SWF of sample treated by oxygen ICP only increases 0.6 eV. Besides, all samples treated by Oxygen PⅢ have higher SWF than those treated by oxygen ICP. So we can see that, Oxygen PⅢ is more efficient improving surface work function than oxygen ICP.(3) According to experiments and analysis, we find that eliminating surface pollution and reducing oxygen vacancies basically result in higher work function of AZO, and reducing the surface oxygen vacancies probably play a major role. Both oxygen ICP and Oxygen PⅢ can eliminate pollution on AZO surface and increase oxygen content. More oxygen in surface reduced oxygen vacancies and carrier concentration, and thus Fermi level gets down. Then, work function is up. Compared with oxygen ICP, oxygen PⅢ treated AZO deeper. In spite of film passively attaching oxygen atoms, PⅢ actively injects atoms into several or even up to less than 20 atomic layers’depth. More oxygen atoms reduce oxygen vacancies and deeper location in crystal structure restricts diffusion. Hence, oxygen PⅢ treatment is more effective and lasting than oxygen ICP.(4) According to study the variety mechanism of work function after treatment, we find that aging effect of treated AZO film depends on interaction between secondary pollution, oxygen diffusion, light irradiation, surface electrostatic field and other factors. After oxidized treatment, AZO film is more easily to adsorb impurities in the air. Oxygen diffusion will reduce surface oxygen content, which engenders more oxygen vacancies. Additionally, light irradiation and electrostatic field speed up this process and thus surface treated by oxygen PⅢ deteriorates as time goes by. In terms of the variety mechanism of work function, more surface pollution and oxygen vacancies lead to lower work function, which also counts for reasonable explanation for treatment timeliness.