Extraction Method And Application Of Density Of States For Oxide TFT

With the development of flat panel display products in the direction of large size,full HD,thinness,and flexibility,oxide thin film transistors?TFTs?have gradually become one of the most potential active drive display technologies,especially in the application of active matrix organic light emitting diode?AMOLED?displays,due to the advantages of high mobility,good uniformity,good transparency,low preparation temperature,and good process compatibility.The interface and bulk density of states?DOS?in the active layer for oxide TFT are the indispensable key parameters for device model and stability analysis.Therefore,it is necessary to deeply understand the device physical properties of oxide TFTs,to design an accurate and reliable extraction method for the interface and bulk density of states.This paper mainly studies extraction methods and applications of interface and bulk DOS for oxide TFTs.Based on the semiconductor device physics,the relationships between the electrical properties and DOS distribution of the oxide TFT device was established.We propose a semi-analytical method to extract the interface DOS and the bulk DOS in indium zinc oxide?IZO?TFTs simultaneously.Then,a flat-band capacitance model considering DOS distribution of TFT devices was studied.Finally,based on the extracted DOS distribution,the DOS distribution of oxide TFT devices under different annealing temperatures,different biasing times,and different illuminations was studied.This paper summarizes the theoretical research progress of energy band structures and trapping effects of oxide TFTs,and analyzes the influences of electrons trapped in deep/tail states on the performance of oxide TFTs.It is shown that the s orbitals of adjacent In atoms in the oxide semiconductor occurs direct overlap,thereby form an efficient electron transport path.And the valence band consists mainly of the 2p orbit of oxygen atoms.The DOS of oxide semiconductor are mainly composed of deep trap states and shallow donor states of oxygen vacancies,and banded states of valence and conduction bands.Besides,The interface DOS may be caused by defects located at the interface,which can exchange charge with the active layers by capturing or emitting electronsA semi-analytical method to extract the interface and bulk DOS in oxide TFTs simultaneously by only using the low-frequency capacitance–voltage characteristics and current–voltage characteristics is proposed.Based on the Poisson equation,the interface and bulk charge density induced by the channel are connected to the gate capacitance,to obtain a semi-analytic expression for extracting the interface and bulk DOS.The results show that the interface DOS is obtained as a superposition of constant deep states and exponential tail states.Moreover,it is shown that the bulk DOS may be represented by the superposition of exponential deep states and exponential tail states.The order of interface DOS at the conduction band is about 10¹22 cm^-2eV^-1.The extracted values of interface and bulk DOS are further verified by comparing the measured transfer and output characteristics of IZO TFTs with the simulation results by a 2D device simulator ATLAS?Silvaco?.Based on the extracted DOS,a physics-based model of flat-band capacitance C_fb for amorphous oxide thin-film transistors has been proposed,considering the influences of free carriers and electrons trapped in deep/tail states.The parameter L_s has been obtained,which represents the screening length in the amorphous oxide semiconductor.Furthermore,the influences of electrons trapped in deep/tail states on the parameter L_s and C_gs-V_gs curve are investigated.The physics-based model of C_fb may be useful for device design,process characterization,or modeling.Finally,the effects of different annealing temperatures,different biasing times,and different illuminations on the distribution of TFT trap states were investigated.The results show that the annealing treatment can reduce the density of states in the oxide film,thereby improving the performance of oxide TFT devices.For the bias stability,it is found that after the bias voltage applied to the IZO TFTs,the density of states increases significantly,and the probability of free electrons trapping by traps increases,which deteriorates the device performance.Under the irradiation of white light,the transfer curve and capacitance curve of the amorphous metal oxide TFT device are obviously shifted to the negative bias.It is found that the DOS distribution has an important influence on the performance of oxide TFT devices.In turn,we can optimize the fabrication process and improve the device performance of TFTs by understanding the relationship between the electrical characteristics and the DOS distribution of oxide TFTs.