PECVD, spatial ALD, and PEALD zinc oxide thin film transistors

This thesis describes low-temperature ZnO deposition and thin film transistor (TFT) fabrication for the fastest ZnO circuits reported to date. Using both plasma enhanced atomic layer deposition (PEALD) and, in collaboration with the Eastman Kodak Company, a novel spatial atomic layer deposition (ALD) process, we have fabricated circuits with 4 mum minimum channel length TFTs with propagation delay less than 30 ns/stage. For comparison, we also describe devices fabricated using plasma enhanced chemical vapor deposition that result in much slower circuits.;A key problem in fabricating ZnO devices and circuits is control of the semiconductor trap density and the semiconductor/dielectric interface state density. We believe this strongly limits the performance of ZnO TFTs fabricated by plasma enhanced chemical vapor deposition (PECVD). ZnO TFTs using an Al 2O3 gate dielectric deposited in situ by PECVD showed moderate gate leakage (<105 A/cm2), field-effect mobility of ∼10 cm2/V·s, and threshold voltage of 7.5 V. However, these devices are strongly limited by interface states and reducing the gate leakage results in TFTs with lower mobility. ZnO TFTs fabricated with low-leakage Al2O3 have mobility near 0.05 cm 2/V·s, and five-stage ring oscillators fabricated using these TFTs have a 1.2 kHz oscillation frequency at 60 V, which is substantially slower than simulation results. ZnO TFTs with large gate leakage showed a higher field-effect mobility due to interface state charging through the leaky dielectric.;Although the performance of PECVD ZnO TFTs may be limited by interface or bulk traps, PECVD provides flexibility in depositing low-temperature doped films. We demonstrated boron-doped ZnO thin films, grown at 200°C by PECVD, with resistivity as low as 4 x 10-4 O·cm and with excellent optical transmission (>85% for visible spectrum). The free electron concentration, determined by Hall effect measurement was as high as 1 x 1021/cm3 with mobility of 13.5 cm2/V·s. In this doped ZnO work a low reactivity oxidant, CO2, is used to provide uniform growth over large-area and to simplify the system design. The boron source used was triethylboron (TEB), which is substantially less toxic than commonly used diborane. These results are among the lowest resistivities reported for doped ZnO thin films.;In contrast to PECVD, spatial ALD, and PEALD may result in films with fewer stoichiometric defects and improved semiconductor/dielectric interface. ZnO TFTs fabricated using spatial ALD had a typical field effect mobility of ∼15 cm2/V·s, a threshold voltage of 5 V, subthreshold slope of < 0.3 V/dec, and current on/off ratio >108. Similarly, PEALD ZnO TFTs showed a typical field-effect mobility of ∼16 cm 2/V·s, a threshold voltage of 2.5 V, sub-threshold slope of 80 mV/decade, and current on/off ratio of 1010. Using both spatial ALD and PEALD ZnO TFTs, seven-stage ring oscillators with 4 mum minimum channel length TFTs oscillated at 2.3 MHz, corresponding to a propagation delay of ∼30 ns/stage. These are the fastest ZnO TFT circuits reported to date.