Surface studies of crystalline and amorphous Zn–In–Sn–O transparent conducting oxides

a r t i c l e i n f o Keywords: Work function Indium tin oxide Zinc indium tin oxide Transparent conducting oxide Ultraviolet photoelectron spectroscopy X-ray photoelectron spectroscopy Thin film X-ray and ultraviolet photoelectron spectroscopy (UPS) studies were made of in situ RF magnetron-sputtered crystalline (c) and amorphous (a) Zn–In–Sn–O (ZITO) thin films, ex situ pulsed laser deposited c-and a-ZITO thin films, and bulk ZITO ceramics. Cosubstitution of Zn and Sn for In results in an increase of the In core level binding energy at a given Fermi level compared to that measured in undoped and Sn-doped In 2 O 3 (ITO). In plots of work function vs. Fermi level, in situ c-ZITO and a-ZITO films have low ionization potentials (7.0– 7.7 eV) that are similar to undoped In 2 O 3. In contrast, dry-air-annealed in situ films, ex situ films, and bulk ceramics have higher ionization potentials (7.7–8.1 eV) that are more similar to ITO and match well with previous work on air-exposed surfaces. Kelvin Probe measurements were made of select a-ZITO films exposed to air and ultraviolet/ozone-treated so as to measure work functions under conditions commonly employed for device fabrication. Results (4.8–5.3 eV) were in good agreement with the UPS work functions of oxygen-exposed materials and with literature values. Lastly, a parallelogram plot of work function vs. Fermi level shows that a wider range of work functions is achievable in ZITO materials as compared to other transparent conducting oxides (Sb-doped SnO 2 , Al-doped ZnO, Sn-doped In 2 O 3), making ZITO more versatile for applications. Transparent conducting oxides (TCOs) are employed as transparent electrodes in flat-panel displays, electrochromic windows, light-emitting diodes (LEDs), photovoltaics (PVs) and flexible electronics [1,2]. Electronic surface potentials (Fermi level, work function) of TCOs play an especially significant role in organic-based devices such as organic LEDs and PVs because they determine the barrier height for carrier injection/extraction [3–6]. Indium tin oxide (ITO) is used in most organic-based devices, largely owing to commercial availability. However, there are drawbacks to ITO, including: 1) chemical instability (ITO is easily etched by poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), known as PEDOT:PSS [7]), 2) relatively low-temperature crystallization of amorphous ITO (crystallization of amorphous films can occur below 200 °C [8,9]), 3) the inhomogeneity of surface properties of crystalline (c-)ITO films resulting in " hot " and " dead " spots on the surface of c-ITO electrodes [5,10], and 4) the volatility …