Characterization of effective conductive layer of nano organic thin film using reflectance spectroscopy

We propose an optical approach for analyzing the formation of the conductive layer during organic thin film growth. The relationship between the properties of multi-layer film, such as thickness and optical coefficients, and the corresponding differential reflectance spectrum (DRS) is derived as math formula based on the effective medium approximation. With the deduced formula, the thickness of the deposited film, for example, electron transport layer in this paper, can be estimated according to the measured DRS data. But, in fact, the fitting error always exists. It is, on the other hand, a useful evidence to indicate the actual situation of the thin film. A concept of the normalized fitting error (NFE) is offered here to equivalently assess the fitting results of all DRS data in the growth process. The curve of NFE versus time is proposed to analyze the growth revolution of the thin film and reveal the inner physical mechanism. In order to demonstrate the performance of the proposed method, an organic field effect transistor (OFET) with a bottom-gate structure is fabricated and pentacene organic thin film is deposited by vacuum thermal evaporation, as an electron transport layer, on the top of the transistor, i. e., an insulator substrate of Si/SiO2. The reflected optical spectrum and the current between the drain and the source of the OFET device are investigated in real time in the growth process. It has been reported that pentacene has three kinds of crystal structures and their optical properties differ from each other. The actual phase of the pentacene film in our experiment is discussed at first. The fitting results show that the pentacene layer exists mainly in thin film phase here. Then, the thickness of SiO2 layer is determined to be 296 nm, which is close to the design value of 300 nm. With those parameters, a four-layer model is used to calculate the thickness of the organic film. The thickness data indicate that the film appears to be linearly growing and the growth rate is 0.2 nm/min. Next, the NFE is plot as a function of time. In this plot, the curve of the NFE increases quickly at the beginning of the growth and reaches to a positive peak at 70 min. After that, the NFE decreases and then keeps constant for a while. When the measured current-time curve is added into this plot, one finds that the increase of the current happens at the same time with the peak of the NFE. It implies that the NFE is related to the structure change of the organic film and thus linked indirectly to the electronic property. The peak of the NFE, to a certain extent, reveals the completeness of the organic conductive layer. As a result, the presented optical approach is valuable for analyzing the electronic status of the organic thin film, especially if the electronic test cannot be performed.