Schottky Nanodiodes Based on Zinc Oxide Thin Films

• Author / Creator

  Shen, Mei

• The unique advantages like low temperature processing with device-level quality and high transparency give ZnO an edge over other semiconductors and extremely attractive for flexible and transparent electronic applications and hybrid circuits integration on sensors. Currently, reliable and reproducible p-type ZnO remains the most daunting obstacle strangling the development of junction devices based on this material system. To enable functional devices applications including UV photodetector, power diode in rectifier circuits like RFID, metal semiconductor field effect transistors (MESFET), source-gated thin film transistor (SGTFT), etc., rectifying Schottky contacts to ZnO have been intensively studied for decades. Many considerable progresses have been achieved in producing of ZnO Schottky contacts on bulk ZnO. However, for many applications, reliable high quality Schottky contacts on ultra-thin polycrystalline ZnO active layer are required. The ultra-thin active layer is beneficial to reduce the turn-on series resistance and promote the flexibility of the devices. Most of the reported metal/ZnO thin film Schottky diodes, particularly for those with an active layer getting down to 100 nm, suffered from large levels of extraneous traps which lead to poor rectifying behavior, low Schottky barrier height and large ideality factor. Numerous fabrication methods and interface pretreatments have been attempted to produce high performance diodes with low defects, high Schottky barrier, low ideality factor and subsequently high rectifying behavior. In the work presented in this dissertation, a novel low temperature plasma-enhanced atomic layer deposition (PEALD) technique with in situ plasma surface pretreatment has demonstrated its feasibility to produce high-performance vertical Schottky nanodiodes with 30 nm thick ZnO active layer grown at near room temperature (50 °C). The best diode achieved in this work demonstrated remarkable room temperature performance with rectifying ratio of ~106, effective barrier height values of 0.76±0.014 eV, ideality factor values of 1.140±0.007, series resistance down to 90 Ω at 1V and breakdown field of at least 1.67 MV/cm. To optimize the fabrication conditions, a series of devices with various ZnO deposition temperatures down to 50 °C and pretreatment and different bottom electrodes were fabricated and characterized by material study and electrical study. The material characterizations showed that the films composition, surface morphology and crystalline structure depended on the ALD recipe and substrate material. The electrical measurements evaluated the electrical performance and indicated the defects level across the Schottky barrier. The effect of barrier inhomogeneity, which widely existed in metal-oxide semiconductor interface, on the temperature dependent behavior of the diode was observed and attributed to the nanocrystalline nature of the materials and interface defects lying in the bandgap. In addition, we demonstrate an interesting time-related improvement of the Schottky nanodiodes performance, which is potentially important for long-term usage. Finally, analytical simulations of the diode’s current-voltage characteristics were carried out taking into account the other mechanisms like image force lowering and thermionic field emission in the carrier transport. The significant roles of other mechanisms besides thermionic emission were revealed for this specific configuration. We believe the advances achieved in this dissertation are important to the development of ZnO based technology, especially in the fields requiring low synthesis temperature.

• Subjects / Keywords

  • Current transport model
  • ZnO thin film
  • Schottky nanodiode

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R33B5WN7S

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.