Ab-initio study on optical and electrical properties of oxide semiconductors

Abstract

Oxide semiconductor (OS) is a promising candidate for the application to large-area and flexible opto-electrical devices, because OS which is deposited at low temperature exhibits the outstanding electrical properties showing the high electron mobility (~10 cm2/Vs) in comparison with the widely used semiconductors such as amorphous Si (a-Si). In addition, since OS is transparent in the most visible and near-infrared regions due to their large band gap, it is often called transparent oxide semiconductor and can be expected to offer a solution for fabrication of the transparent electronic device on flexible substrates. In spite of the advantages of OS, previous experimental studies reported various obstacles to be overcome for realization of the device utilizing OSs. For example, the OS based thin film transistor (TFT) suffers from device instabilities under illumination and bias stress. In addition, the underlying mechanism for the charge transport in OS still remains unclear, which hinders improvement of the OS based device performance. In this research, we investigate the optical and electrical properties of OSs using ab-initio calculations on the basis of density functional theory (DFT) to suggest the strategy to improve the performance of OSs in terms of electron transport and reliability. First, in order to investigate the electron transport in OSs, we introduce the model Hamiltonian for the conduction band within tight-binding approximation and carry out the calculation of the electron mobility considering several types of scattering processes. It is found that the interaction between metal s and oxygen p orbitals under tetrahedral and octahedral local atomic structure gives the quasi-linear dispersion of the conduction band which plays an important role in determining the electron mobility in OSs. In addition, we reveal that the electron mobility in a multi-component OS like InGaZnO4 is dominated by the cation-disorder scattering process. Next, we investigate the influence of hydrogen impurity in OS. Previously, various models to explain the threshold voltage shifts under illumination and bias stress were suggested based on oxygen vacancy defect, but the clear mechanisms for that are still controversial. In this study, the DFT calculation results turn out that hydrogen in OSs can have +1/-1 charge state depending on Fermi level. This bistability of hydrogen enables to cause the threshold voltage shift by alternating its charge state under illumination and bias stress. In a second part, the optical properties are mainly dealt with. For the more realistic modelling, we study the ab-initio calculation methodology based on GW approximation for obtaining the accurate band gap of OS. The DFT calculation usually results in 30~40% underestimation of the band gap in typical insulator and GW method improves the accuracy of the calculated band gap by correcting exchange-correlation energy of the conventional DFT calculation. However, previous GW calculation results for OSs still yield ~10% lower band gap than experimental value. Thus, we investigate the theoretical reason for such wrong description of GW method for OSs and suggest new method to further improve the predictive power of GW method, which shows the mean absolute relative error (MARE) of ~3%. Finally, we study the visible light absorption of amorphous In-Ga-Zn-O since the device degradation critically occurs when exposed to visible and UV light. It is found that the relative downshift of the conduction band position comparing to that of crystalline phase occurs as well as long tail states near the valence band edge appear in amorphous phase. This is one of the main reasons for amorphous OSs to absorb the visible light without any defects.