| The ZnO TFT (Thin Film Transistor) has demonstrated improved electron mobility over entrenched, a-Si, technologies and is receiving considerable attention as an alternative TFT technology due to high transparency in the visible region and potential for flexible electronics on conformal substrates. This work studied the material properties of ZnO thin films deposited by pulsed laser deposition, provided a qualitative discussion of TFT operation, applied the buried-channel approach to enhance channel mobility, and demonstrated a complementary logic inverter circuit based on a n-channel ZnO TFT and a p-channel ZnTe TFT.The material study in this work focused on optimal deposition conditions needed to realize low-resistivity ZnO thin films. Various techniques were used to illustrate the effect of decreasing polycrystalline ZnO grain size on carrier mobility and background carrier concentration as a function of thickness. Building on these results, discussion of TFT operating principles concentrated on explaining its dependence on carrier concentration and active-channel thickness. The resulting performance of surface-channel ZnO TFTs demonstrated saturation mobility of muSAT = 1.8cm2/Vs, current on/off ratio of ION/IOFF > 109, and off current of IOFF=10pA.A buried-channel approach to suppress the effect of electron trapping on carrier transport by isolating the ZnO active channel was presented. Quantum confined structures, such as quantum wells, enhance speed of electronic devices due to carrier confinement. Quantum wells based on the ZnO/MgxZn 1-xO (x&le0.3) material system have been grown on c-plane sapphire substrates. Luminescent properties characterized by low-temperature photoluminescence revealed quantum confinement with a systematic blueshift as a function of decreasing ZnO well width in the range from 3nm to 10nm. An enhancement in saturation mobility of 3.9cm2/V-s was achieved by utilizing the ZnO/MgZnO heterojunctions to fabricate a buried-channel TFT.Finally, incorporation of a ZnTe TFT, exhibiting relatively high hole mobility (&sim5cm2/Vs), and ZnO TFTs in a complementary logic inverter circuit demonstrated reasonable transfer characteristics. Inverter behavior is demonstrated, with a high output voltage of VOH >14V and low output voltage of VOL<0.2V are observed for a supply voltage of 15V. The small signal gain for the transfer characteristic is found to be -dVout/dVin>5 at Vin=6V. |
