Spectroscopic observation of the field induced carrier doping effect in ultrathin layers of organic semiconductors

为了给我们理解超薄有机半导体层中的场致载流子掺杂(FICD)现象提供依据，我们首先采用了一种理论方法。在该方法中，考虑了金属/绝缘体/半导体/金属四层模型。几种物理近似的引入简化了模型，使我们能够根据费米分布和高斯定律来解析地求解描述模型的联立微分方程组。通过将参数设置为采用Langmuir-Blodgett(LB)技术制备绝缘层和半导体层的情况，考察了所获得的溶液的行为。这种方法显示了下面两点：i)尽管FICD现象变为非线性，但即使当DS和半导体层中的态密度都非常小时，累积层的厚度也保持比半导体层(DS)的厚度小得多；ii)在另一种CaS…中FICD现象与无机器件基本相同。作为一种实验方法，我们计划将改进的红外反射吸收光谱(IR-RAS)技术应用于由LB膜制成的金属/绝缘体/半导体/金属四层器件。制备了两种改进型IR-RAS技术。低频类型在每次傅立叶变换(FT)扫描完成后开启和关闭施加到设备的U.C.偏置电压，以便光谱仪记录有偏置电压和不有偏置电压的RAS信号之间的差异。高频类型对器件施加交流(1-100 kHz)偏置电压，并在红外探测器-信号前置放大器和模数转换器之间插入相敏探测器，使光谱仪也记录有偏置电压和不加偏置电压时RAS信号的差异。低频类型已应用于四层器件，其中烷基TCNQ的LB膜(作为半导体层)，脂肪酸的LB膜(作为绝缘层)，BO-脂肪酸混合物的LB膜(作为金属层之一)，探测到了蒸发的金膜(作为另一金属层)和偏压引起的RAS信号的变化。然而，不幸的是，观察到的变化并不是完全可逆的，因此我们得出结论，一些电化学效应发生了。目前，我们正在尝试通过优化样品结构、偏置电压的大小和频率等方法来消除电化学效应，观察纯FICD效应。

In order to provide a basis for understanding the field-induced carrier doping (FICD) phenomenon in ultrathin organic semiconductor layers, firstly, we have adopted a theoretical approach. In this approach, a metal/insulator/semiconductor/metal four-layer model has been considered. Introduction of several physical approximations has simplified the model and allowed us to solve analytically the simultaneous derivative equation describing the model according to the Fermi distribution and the Gauss law. The behavior of the obtained solution has been examined by setting the parameters to the cases in which the insulator and semiconductor layers are made by Langmuir-Blodgett (LB) technique. This approach has shown the next two points : i) Though the FICD phenomenon becomes nonlinear, the thickness of the accumulation layer is kept much smaller than the thickness of the semiconductor layer (ds) even when both of ds and the density of states in that layer are very small ; ii) In the other cas … More es the FICD phenomenon is essentially the same as that in the inorganic devices.As an experimental approach, we have planned to apply modified infrared reflection-absorption spectroscopy (IR-RAS) techniques to metal/insulator/semiconductor/metal four-layer devices made of LB films. Two types of the modified IR-RAS technique have been prepared. The low-frequency type switches on and off the U.C. bias voltage applied to the device after each Fourier-transform (FT) scan completes so that the spectrometer records the difference between the RAS signals with and without the bias voltage. The high-frequency type applies A.C. (1-100 kHz) bias voltage to the device and inserts a phase-sensitive detector between the IR detector-signal preamplifier and the analogue-to-digital converter so that the spectrometer records the difference between the RAS signals with and without the bias voltage, too.The low-frequency type has been applied to four-layer devices with LB films of alkylTCNQ (as the semiconductor layer), LB films of fatty acid (as the insulator layer), LB films of the BO-fatty acid mixture (as one of the metal layer), and evaporated gold film (as the other metal layer) and changes in the RAS signal induced by the bias voltage have been detected. Unfortunately, however, the observed changes are not fully reversible and then we have concluded that some electrochemical effect occurs. At present, we are trying to eliminate the electrochemical effect and to observe the pure FICD effect by optimizing the sample structure, the magnitude and the frequency of the bias voltage, etc. Less