Fabrication And Property Study Of Oxide Thin Film Transistor

Thin film transistor(TFT) is one of core devices of active matrix organic light-emitting diode(OLED). As OLED is a current-driven device, the traditional amorphous silicon TFT cannot meet the need of OLED due to its low mobility, while the polycrystalline silicon TFT has shortcomings such as high preparation cost, existing boundary and poor uniformity which combine to restrict the competitiveness of polycrystalline silicon TFT in the market. With high mobility, high transparency in the visible light range, simple preparation process and low cost, the oxide TFT which has been developed in recent years has become the most promising TFT technology. This paper studied the preparation process, electrical properties and stability of oxide TFT. Main research contents and innovations are as follows:1. Selection of insulating layer materials and growing method- Al2O3 and atomic layer deposition(ALD). The insulating layer is an important part of the TFT device, and the insulation material and preparation process of selection are the key for the device quality. This paper compared the characteristics of TFTs with the low dielectric constant Si O2(ε=3.9) and high dielectric constant HfO2(ε=25) as insulating layer. The study showed that the mitigation of devices with high dielectric constant as insulating layer was improved obviously; the mitigation of devices with magnetron sputtered silicon oxide as insulating layer was 2 cm2/Vs, while that for devices with hafnium oxide and silicon oxide as the double insulation layer was 8.6 cm2/Vs. It was confirmed through comparison that the high dielectric constant insulating layer can improve the device mobility and reduce the driving voltage. However, it has a poor property of leakage, which has a serious impact on the performance of the device. This is because the high dielectric constant materials are always in a form of polycrystalline and a leakage channel forms in the grain boundaries, resulting in leakage. The ideal insulation material should be amorphous and has a relatively high dielectric constant. Among all metal oxides, only Al2O3 meet the condition, so Al2O3 should be the first choice for insulating material.The second step is the selection of preparation method. First of all, this paper studied the performances of device with magnetron sputtered alumina as the insulating layer and found that the stability of the device is still unsatisfactory. The maximum interface trapping density of device with thermal silicon oxide as the insulating layer was 4.2×1011 cm-2, while that for the device with Al2O3 as the insulating layer was 3.4×1012 cm-2. The paper believed the large interface trapping density of Al2O3 is caused by the preparation method. Although the method of magnetron sputtering is widely used, particle energy in the sputtering process is very strong, which will damage the interface and in turn affect the device stability. Hence, this paper adopted the technology of atomic layer deposition(ALD) which is more advanced, so as to give full play of Al2O3. Compared with the device with sputtered Al2O3 as gate insulator, the one with ALD-Al2O3 as gate insulator demonstrated better performances as the mobility increased from 4.4 cm2/Vs to 5.2 cm2/Vs, the threshold voltage decreased from 9.9 V to 4.3 V, and the interface states reduced from 3.4×1012 cm-2 to 9.1×1011 cm-2. As a result, in this paper all the devices in the following researches were prepared on the basis of ALD-Al2O3.2. Study on the stability of ALD-Al2O3-based TFT: This paper studied the influence of growth conditions of ALD and the thickness of gate insulator on the electrical properties and stability of IGZO-TFT. We obtained devices with higher mobility and stability through optimizing the thickness and growth temperature of ALD-Al2O3.However, as the dielectric constant of Al2O3 was still not high enough, the improvement of mobility was restricted. We used ALD-Al2O3 as the buffer layer to modify the high dielectric constant Zr O2(ε=25), and as a result, the leakage of Zr O2 was reduced by two orders of magnitude and the device performance was improved significantly, i.e.: the mobility increased from 9.8 cm2/Vs to 14 cm2/Vs, the on/off increased from 4×105 to 3.5×106, the maximum density of surface states at the channel-insulator interface decreased from 4.3×1012 cm-2 to 2.5×1012 cm-2, the stability of the device was improved significantly, and the threshold voltage shift was reduced from 1.6 V to 0.79 V after 3,600s’ stress duration. Using ALD-Al2O3 to modify the high dielectric constant insulating layer has not only maintained Al2O3’s advantages of low leakage and good interface, but also improved the dielectric constant.3. Extraction of the density of states and the relationship between the density of states and the stability: The instability of TFT is not only associated with the defects at active layer / insulator interface(such as positive bias stress), but also related to the active layer in semiconductor with internal energy level. Especially in the light or under the bias of stress under thermal excitation, TFT shows the threshold voltage drift. The study on subgap density of states(DOS) of semiconductor active layer is of great significance for the study of TFT stability. This paper adopted the Temperature Stress method to activate electrons by raising the temperature and study the gate voltage changing speed, so as to obtain the density of states and then explain its impact on the device stability.The IZO-TFT was prepared in 150 nm ALD-Al2O3. The threshold voltage of the device was-0.46 V, the mobility was 17.9 cm2/Vs, the on/off was 108, and the subthreshold swing was 0.13 V/dec. After Temperature Stress experiment and calculation, we obtain the maximum activation energy of 0.96 e V with a change rate of 1.39 e V(V)-1. The density of state was 1017-1015 e V-1 cm-3 which was lower than the recently reported 1018-1016 e V-1 cm-3, indicating that the stability of the device is good.We also studied the effects of active layer thickness on the density of state and stability. Research results showed that the device with higher stability displayed lower density of states. We also studied the effect of sputtering power density on the density of states and the stability, which also proved the association between the stability and the density of states.4. Application of double active layer: the active layer is an important part of TFT devices. IZO and IGZO are the two mostly widely-used active materials at present. IZO has the advantage of high carrier concentration while IGZO has high stability. When we used IZO and IGZO double layer composite structure as the active layer, the device not only showed high stability as IGZO-TFT, but also exhibited high mobility as IZO-TFT. TFT with IZO/IGZO double active layer demonstrated an ideal threshold voltage of 0.8 V, a high mobility of 14.4 cm2/Vs and a subthreshold swing as low as 0.13 V/dec. The density of states was lower than that of TFTs with IGZO or IZO as the active layer, which showed good stability.