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F doped ZnO

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Like indium tin oxide, ZnO thin film is a transparent conducting oxide. Some advantages over ITO include:

  • low material cost,
  • non-toxicity
  • high chemical stability under hydrogen plasma

Applications

  • LEDs
  • solar cells
  • liquid crystal displays

Doping agents for ZnO

  • Al-doped (AZO)
  • Ga-doped ZnO (GZO)
  • F-doped AnO (FZO)  atomic radius (0.136 nm) is similar to that of oxygen (0.132 nm). Covalent bond lengths are also similar to oxygen.
ZnF2_1
Left p group periodic table of elements. Wang (2014) substituted F- for doping anions Ga and Al in ZnO films. Atomic radii of uncharged and a representative anion are shown. Fig 5 from Wang Figure 7 (Wang 2014) shows that these films are a good long pass filter regardless of sputtering temperature. The transmisttance of the 300oC prepared material is slighted blue shifted to that prepared at room temperature.

Hall mobility is a measurement of how fast an electron moves through a semiconductor. It may be influenced by electron scepters and donors as well as impurities and defects. X-ray diffraction, not shown here, suggest that sputtering at higher temperature decreases the defects.

Much discussion was given to the blue shifting, or the Moss-Burstein Effect.  When the wavelength is equal to 300 nm, the visible light absorbed by the thin films is due to a quantum phenomenon called band edge absorption. A Burstein-Moss shift of the absorption edge to the shorter wavelength region is due to the increase in carrier concentration.  The resistance of these FZO films was stable over the course of 25 days.

Methods, in brief

  • 1.5 wt% ZnF2 (99.995%) mixed with ZnO powder (99.999%), dried, ground
  • calcined at 600°C for 1 h, ground, mixed with polyvinyl alcohol (PVA) as binder
  • mixed powder was uniaxially pressed into pellets of 5-mm thickness and 54-mm diameter using steel die.
  • FZO pellet was sintered at 1,060°C for 3 h, consider Dr Fritsch DCS sintering press
  • Deposition was on Corning 1737 glass substrates using an RF sputtering system. MSE customers may also consider
  • The chamber pressure was pumped to 5 × 10−6
  • The substrate temperature was changed from room temperature (RT) to 300°C.
  • The deposition process, the deposition pressure was controlled at 5 × 10−3 Torr.  Pressure in the chamber was controlled by introducing argon.

Reference

Deposition of F-doped ZnO transparent thin films using ZnF2-doped ZnO target under different sputtering substrate temperatures   Fang-Hsing Wang, Cheng-Fu Yang, and Yen-Hsien Lee Nanoscale Res Lett. 2014; 9(1): 97. Free Paper

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