Evolution of the electrical and structural properties during the growth of Al doped ZnO films by remote plasma-enhanced metalorganic chemical vapor deposition

Abstract

Al-doped zinc oxide (AZO) films were deposited by means of remote plasma-enhanced metalorganic chemical vapor deposition from oxygen/diethylzinc/trimethylaluminum mixtures. The electrical, structural (crystallinity and morphology), and chemical properties of the deposited films were investigated using Hall, four point probe, x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), electron recoil detection (ERD), Rutherford backscattering (RBS), and time of flight secondary ion mass spectrometry (TOF-SIMS), respectively. We found that the working pressure plays an important role in controlling the sheet resistance Rs and roughness development during film growth. At 1.5 mbar the AZO films are highly conductive (Rs<6 Ω∕□ for a film thickness above 1200 nm) and very rough (>4% of the film thickness), however, they are characterized by a large sheet resistance gradient with increasing film thickness. By decreasing the pressure from 1.5 to 0.38 mbar, the gradient is significantly reduced and the films become smoother, but the sheet resistance increases (Rs≈100 Ω∕□ for a film thickness of 1000 nm). The sheet resistance gradient and the surface roughness development correlate with the grain size evolution, as determined from the AFM and SEM analyses, indicating the transition from pyramid-like at 1.5 mbar to pillar-like growth mode at 0.38 mbar. The change in plasma chemistry/growth precursors caused by the variation in pressure leads to different concentration and activation efficiency of Al dopant in the zinc oxide films. On the basis of the experimental evidence, a valid route for further improving the conductivity of the AZO film is found, i.e., increasing the grain size at the initial stage of film growth.

Controlling the resistivity gradient in aluminum-doped zinc oxide grown by plasma-enhanced chemical vapor deposition

“…23 Gas phase-based techniques, as MO-CVD, have also shown potential to grow high quality aluminumdoped ZnO (ZnO:Al) layers 24,25 at deposition rates as high as $14 nm/s 26 on a large surface area (>10 cm 2 ). 25,26 In the plasma CVD techniques, the overall heat load of the process is lowered, as the substrate temperature is reduced down to the range of 100-200 C. For example, in the past years, we have shown that the expanding thermal plasma (ETP) leads to good quality ZnO:Al layers deposited up to 1 nm/s with a resistivity of 8 Â 10 À4 X cm for 1100 nm film thickness, at a substrate temperature of 200 C, as reported by Volintiru et al 27 One of the drawbacks of the CVD processes, [27][28][29] unlike the sputtering approach, is the development of a gradient in resistivity as function of the film thickness, usually present over a large thickness range. In the case of ZnO:Al sputtering, the gradient is typically limited and confined within a small thickness range, i.e., below 200 nm.…”

“…The exact correction procedure is reported in more detail by Volintiru et al 27 At high DEZ conditions the resistivity gradient is significantly reduced, and present up to $300 nm film thickness (Figure 1(b)). Therefore, the procedure to correct for resistivity gradient is not applicable for high DEZ at the thickness above 300 nm, so resistivity of top layer at 1100 nm remains 4.0 Â 10 À4 X cm.…”

“…24,27,32 The technique is based on generating a plasma jet, which expands into a vacuum chamber, dissociates precursors injected downstream and generates reactive species, responsible for film deposition. The plasma source is a cascaded arc unit, where a DC discharge is generated between three cathodes and an anode plate at sub-atmospheric pressure (typically 0.3-0.4 Â 10 5 Pa) in argon.…”

“…The effective (measured) resistivity of 8.3 Â 10 À4 X cm at 1100 nm achieved by Volintiru et al, 27 resulted in a resistivity value of 2.5 Â 10 À4 X cm for the top layer. Similarly, effective resistivity of 3.0 Â 10 À4 X cm measured at 1100 nm for low DEZ conditions in this work would be equal to 1.1 Â 10 À4 X cm for the top ZnO:Al layer.The exact correction procedure is reported in more detail by Volintiru et al 27 At high DEZ conditions the resistivity gradient is significantly reduced, and present up to $300 nm film thickness (Figure 1(b)). Therefore, the procedure to correct for resistivity gradient is not applicable for high DEZ at the thickness above 300 nm, so resistivity of top layer at 1100 nm remains 4.0 Â 10 À4 X cm.…”

“…Earlier, an attempt has been performed by Volintiru et al 27 to study the impact of the process parameters (e.g., the process pressure in the deposition chamber) in the ETP with the aim of reducing the gradient in resistivity, described above. The gradient in resistivity was effectively reduced by reducing the process pressure, however, the overall resistivity increased up to $10 À2 X cm in the whole thickness range, because the growth mode shifted from pyramidto pillar-like, leading to a limited grain size extending through the whole film bulk.…”