有机半导体薄膜生长原位实时测量方法的研究

针对原位实时监测有机半导体薄膜生长情况的需求, 提出了差分反射光谱法与场效应晶体管法结合的光电联合测量方法, 设计研制了测量系统. 以并五苯有机分子为例, 通过自制底栅底接触式场效应管微结构, 实验测试了热蒸发法生长导电膜层过程中光电信号的演变与相互关联. 光谱信号显示, 并五苯以薄膜态结构进行生长, 光谱随生长进程变化显著. 实验数据与四相结构模型仿真结果的良好吻合, 表明因薄膜增厚引起干涉条件的改变是光谱变化的主因, 由此推算出薄膜生长速率为0.23 nm/min. 当薄膜等效厚度达到28 nm时, 场效应管的导电性显著增强, 标志着并五苯有效传输层的形成. 此后, 薄膜厚度持续增加, 但测试电流增长缓慢, 说明该结构进入电学特性饱和区. 光电联合法不仅有助于研究有机半导体薄膜的光谱信息、电学特性和薄膜结构之间的相互对应关系, 也为发展原位监测有机半导体薄膜制备过程, 探索最佳工艺提供了新的研究手段.

Study on in-situ real-time measurement for thin film growth of organic semiconductors

We propose an approach for in-situ real-time measuring the optical and electric properties of a thin film in parallel during the process of growth. The method is developed based on two techniques: differential reflectance spectroscopy (DRS) and field effect transistor (FET) structure based electrical characteristics testing method. In order to demonstrate the performance of the method, FETs with a bottom-gate structure are manufactured and the pentacene organic thin film is deposited by vacuum thermal evaporation as a transport layer on the top of the transistor, i.e. the insulator substrate of SiO₂. The optical and electrical properties of the organic thin film are in-situ investigated during its growth. As obtained from the optical spectra, the DRS signal moves up and down along the wavelength. Its fluctuation amplitude increases quickly and is very sensitive to the variation of the thickness of the top most film since the shutter of the molecular evaporation source is open. A good agreement between the experimental data and the computational results with a four-layer structure model of Si/SiO₂/pentacene/air suggests that the DRS signal here is mainly due to the interference that exists in the multilayer interfaces. In addition, there are two characteristic peaks at 629 nm (1.97 eV) and 673 nm (1.84 eV) appearing occurs clearly in the DRS spectra at the initial stage of the growth. It means that the pentacene layer forms a thin film phase structure. Furthermore, the growth rate is evaluated to be 0.23 nm/min. When the effective thickness of the pentacene layer reaches 28 nm, calculated from the growth rate and the measured time, the conductivity of the organic FET becomes noticeable. It implies that an electrical conducting layer is already formed. After that, the thickness of the conducting layer continuously increases, while the current between the drain and the source increases slowly and turns to be saturated. After a 15-hour film growth, the sample has a threshold voltage of -20 V and the charge carrier mobility is 3.1×10^-3 cm²/(V· s). These data confirm that the sample is an FET although its electronic properties are not good enough. These results show that the proposed approach is a useful measurement tool to build the relationships among the data of the optical spectrum, the electrical property, and the structure of the thin films. Hence, it is valuable for both the explanation of the growth mechanism of the thin film in research and the optimization of its preparation process in industry.