Fabrication And Characterization Of Li Or/and N Doped ZnO-based Thin Film Transistors

Zinc Oxide based thin-film-transistors (TFTs) have get highly attention at home and abroad because of its wide application prospect in the flat-panel displays and transparent electronics field. However, there tend to be a high carrier concentration in the intrinsic ZnO films prepared by conventional technologies due to the existence of Zinc interstitial (Zni) and Oxygen vacancy (Vo) in ZnO, leading to larger off-current and smaller on-off ratio of the current in the intrinsic ZnO-TFTs. While the reported performance of the doped ZnO-TFTs needs to be improved further. So there is a key role for regulating the carrier concentration in the active layer to play in the improvement in the performance of ZnO based TFT.As the suitable candidates for substituting Zn and O (Lizn and No), respectively, Li and N could be used to control the carrier concentration by intentional doping into ZnO that is expected to improve the performance of ZnO-TFTs. Based on these considerations above, Li or/and N doped ZnO TFTs were prepared in this thesis. And the electrical properties of the ZnO-based TFTs were improved dramatically by optimizing the preparation of active layers. The main contents are as follows:(1) Li doped ZnO (ZnO:Li) films were deposited on SiO2/p-type Si substrate by radio frequency magnetron sputtering (RFMS). And The fabrication and electrical characterization of ZnO:Li-TFTs were studied in this work. Here we studied the effects of annealing temperature, thickness of active layer and ratio of channel width to length (W/L) on the performance of the device. The results show that the device has the best performance with a 30 nm-thick active layer and annealed at 800 ℃, exhibiting a field effect mobility of 7.6 cm2/V s and an on/off current ratio of 2.3×107. Compared with the reported ZnO-TFTs doped by the elements such as Al, Mg or Ga, ZnO:Li-TFT shows an improvement in her performance.(2) N doped ZnO (ZnO:N) films were deposited on SiO2/p-type Si substrate by RFMS. And the fabrication and electrical characterization of ZnO:N-TFTs were studied in this work. We studied the effects of several main factors, such as annealing temperature, thickness of active layer, W/L and the storage time, on the performance of the device. The results show that the device has the best performance with a 30 nm-thick active layer and annealed at 800℃, exhibiting a field effect mobility of 22.1 cm2/V s, an on/off current ratio of 1.1×108, and an on current of 9.4×10-3 A. Compared with the reported ZnO-TFTs doped by the elements such as Al, Mg or Ga, ZnO:N-TFT shows a dramatical improvement in the device performance. And compared with the reported ZnO-TFTs without doping, her on/off current ratio has a dramatical improvement. The study found that the smaller the channel length L, the greater the improvement in the performance of device. And the device shows a good stability that she still keep a high mobility μSAT= 19.3 cm2/V s after 120 days in the air.(3) In this work, we fabricated the bottom-gate ZnO:(Li,N)-TFT with her active layers deposited by RFMS, and discussed the influence of annealing temperature, thickness of active layer, W/L and storage time on her performance. The results show that the device has the best performance with 30 nm-thick active layers annealed at 800 ℃, while the mobility is as high as 33.6 cm2/V s, the on/off current ratio is as high as 1.1×108, and an on current is 8.2×10-3 A. Compared with the reported ZnO-TFTs without doping and doped by Al, Mg, Ga and so on, ZnO:(Li,N)-TFT has a dramatical improvement in her performance. The study found that the smaller the channel length L, the greater the improvement in her performance. And the device shows a good stability that she still keep a high mobility μSAT=31.2 cm2/V s after 180 days in the air.(4) ZnO:(Li,N)-TFTs were fabricated by RFMS technology without thermal annealing treatment in the study. We studied the effects of thickness of active layer, UV-Ozone, ratio of Ar to O2 gas flow, AL2O3 buffer layer and storage time on the performance pf the device here. The results show that the device has the best performance with a mobility of 13.5 cm2/V s, an on/off current ratio of 8.1×107, and a threshold voltage of 7.6 V as the thickness of active layer is 20 nm. With placed in the air, the mobility of the device increases to 15.7 cm2/V s after 60 days and then reduces to 10.9 cm2/V s after 120 days that shows that the device has a good stability. The study shows that the performance of ZnO:(Li,N)-TFT could not be improved by increasing the flow of oxygen and Al2O3 buffer layer; however, the performance could be improved by the UV-Ozone treatment on her active layer, exhibiting a mobility of 15.3 cm2/W s, an on/off current ratio of 1.2×108, and a threshold voltage of 9.9 V.