Low Cost Zinc Oxide Films for High Performance Rigid and Flexible Optoelectronic Devices

• Author / Creator

  Benlamri, Mourad

• From the fabrication perspective, zinc oxide (ZnO) offers the possibility of thin film growth through the use of low-cost solution techniques. The solution-processed ZnO films, usually of polycrystalline formation, are suitable for use in field-effect transistors and in such applications as electronic displays, transparent conductive electrodes, light-emitting diodes, radio frequency identification tags, and power inverters. Chemical bath deposition, spray pyrolysis, and sol-gel are some of the most commonly used solution techniques to synthesize ZnO thin films. Another solution technique, rarely utilized for optoelectronic devices fabrication, is the electrodeposition technique. My works utilized the sol-gel and electrodeposition techniques to grow high-quality ZnO thin films that provide the required electrical properties to produce highly performant optoelectronic devices in general and high-power devices in particular.
  I present a fundamental study of the growth, and hence the resulting electrical properties, of ZnO films formed by the sol-gel process on two different high-quality substrates: silicon nitride (Si3N4) and silicon oxide (SiO2). A high effective electron mobility of 33 cm2V-1s-1 was achieved in the undoped ZnO thin films grown on Si3N4. The introduction of Si3N4 as growth substrate thus resulted in a mobility improvement by a factor of 2.5 with respect to the commonly used SiO2. The ZnO films grown on Si3N4, prepared by low-pressure chemical vapor deposition, revealed bigger grain sizes and better crystalline quality in comparison to the films grown on thermal SiO2. These results show that the nucleation and growth mechanisms of sol-gel films are substrate dependent. Interface trap densities measured in high-κ hafnium oxide (HfO2)/ZnO MOSCAPs were about ten times lower in those fabricated on Si3N4 substrates.
  Although the sol-gel ZnO films have achieved a relatively high electron mobility, they still suffer from high, uncontrollable doping densities in the general range of 1017 1020 cm 3. I present low-doped and highly textured ZnO films, synthesized by an optimized low-cost and low-temperature electrodeposition technique on copper substrates, which possess an equilibrium free electron concentration of 2.8 × 1014 cm 3 and a minimum electron mobility as high as 80 cm2V 1s 1. These electrical parameters tend toward those obtained in hydrothermally-grown single crystals of bulk ZnO. The resulting Schottky diodes made, for the first time to our best knowledge, by electrodeposited ZnO on copper films, demonstrate rectification ratios of 106, low on-state resistance, and ideality factors as low as 2. These results show a route forward for the growth of solution-processed ZnO of high electronic quality.
  A flexible Kapton substrate was employed to grow a high-quality ZnO thin film optimized for optoelectronic performance by the electrodeposition technique, which allows process temperatures of less than 100 °C. Schottky diodes made of the electrodeposited ZnO film and underlying copper contact on flexible Kapton substrate exhibited high rectification ratios (106) and low ideality factors (2 to 3) that surpass the current state-of-the-art for reported solution-processed ZnO films. The diodes withstood reverse voltages in excess of -3 V with no breakdown. By performing bending tests, we demonstrated that the diodes have potential capabilities to retain device operationality under mechanical deformations.
  Because of the capabilities that the electrodeposition process provides in terms of moderate doping and high film thicknesses, its suitability for fabrication of power devices was explored by investigating its electrical breakdown strength. To our knowledge, this is the first investigation of breakdown field strength in electrodeposited ZnO material. Test vehicles in the form of Schottky diodes were made through a simple fabrication process with no passivation, no edge effect reduction and no dry etch. The devices exhibited high electrical performance with an ideality factor of 1.10, a carrier concentration of 2.5  1015 cm 3 and a critical electric field at breakdown of 800 kV cm 1, which attest to the suitable quality of the ZnO film. The obtained value for the critical electric field surpasses the best reported value by one order of magnitude. Drastic changes in carrier concentration were observed for different growth conditions.
  Planar microwave resonators employing ZnO films directly electrodeposited on copper strip lines were fabricated to study the photosensing capabilities and charge carrier lifetimes in these films as well as the effects of humidity on them. The film characterization parameters were extracted by analysing the S-parameters of the resonator device in the microwave frequency region.

• Subjects / Keywords

  • solution-process
  • Electrodeposition
  • ZnO
  • low-cost
  • power
  • Schottky
  • zinc oxide

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-ya11-8774

• License

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