Nitrogen grain-boundary passivation of In-doped ZnO transparent conducting oxide

We have investigated the properties and conduction limitations of spray pyrolysis grown, low-cost transparent conducting oxide ZnO thin films doped with indium. We analyze the optical, electrical, and crystallographic properties as functions of In content with a specific focus on postgrowth heat treatment of these thin films at $320{}^{\circ }\mathrm{C}$ in an inert, nitrogen atmosphere, which improves the films electrical properties considerably. The effect was found to be dominated by nitrogen-induced grain-boundary passivation, identified by a combined study using $in$ situ resistance measurement upon annealing, x-ray photoelectron spectroscopy, photoluminescence, and x-ray diffraction studies. We also highlight the chemical mechanism of morphologic and crystallographic changes found in films with high indium content. By optimizing growth conditions according to these findings, ZnO:In with a resistivity as low as $2\times {10}^{-3}\mathrm{\Omega }\mathrm{cm}$, high optical quality $\left(T\approx 90\%\right)$, and sheet resistance of $32\mathrm{\Omega }/\square$ has been obtained without any need for postgrowth treatments.

Figure 1

Example of the measured resistivity of undoped ZnO and ZnO:In thin films (0.6% and 5%) as a function of temperature during the nitrogen anneal cycle. The observed hysteresis illustrates the significant reduction in resistivity. The right panel shows the resistivity before and after the anneal cycle for all films and the ratio of improvement.

Figure 2

Carrier concentration and Hall mobility of as-grown (black) and ${\mathrm{N}}_2$ annealed (blue) ZnO:In. The decrease in resistivity during the anneal cycle is predominantly caused by a change in carrier mobility. Only for nominally undoped samples and In content below 0.4% was an increase in carrier concentration during annealing noted.

Figure 3

XPS spectra of a ZnO:In sample after a nitrogen anneal cycle (blue) and $in$ situ Ar ion etching (black), removing the samples top layer. The weak N $1s$ signal with two structures at 399.8 and 401.0 eV is significantly reduced after short Ar-ion etching removing approximately 1 nm of material. Additional oxygen components (OH, $\mathrm{C}-\mathrm{O},\mathrm{C}=\mathrm{O})$ and residual carbon are also reduced, while the Zn $2p$ and O $1s$ (Zn-O) signal increases. Raw data (symbols) and fitted lines are shown. Zn signals have been scaled by a factor of 0.2, the weak nitrogen by a factor of 50, and in addition constant offsets have been employed for better visual comparison of the components and spectra.

Figure 4

Room-temperature photoluminescence measurement of ZnO:In films before and after nitrogen annealing. The spectra have been area normalized for easier comparison. Most prominently the green and blue defect luminescence intensity and energetic position is altered during the anneal process for doping levels below 2%. The spectra of 0.2, 0.6, 1.25, and 5% samples prior (solid lines) and after ${\mathrm{N}}_2$ anneal (dashed lines) are shown exemplary on the right with characteristic features of as-grown films at 3.3, 2.74, and 2.54 eV indicated for better comparison. The spectral ranges for various defect emission bands (orange, green, blue, and violet) are indicated. False color plots are generated by extrapolating 12 discrete spectra with 0.2%–10% In content (see experimental details).

Figure 5

(a) XRD patterns of ZnO:In as a function of the In/Zn ratio. Films up to 1% In/Zn ratio are highly textured with the ZnO $c$ axis perpendicular to the surface. The gray-scale plot is generated by extrapolating 12 discrete diffraction patterns of samples with 0.2%–10% In content. (b) Close-up of the region around $2\mathrm{\Theta }$ values of ${35}^{\circ }$ for low, medium, and highly doped films showing the change in texture and small shift to lower angles. (c) shows calculated texture coefficients for the (002) and (101) reflex and $a$- and $c$-lattice parameters derived from a Rietveld refinement of the XRD data. Dashed lines indicate the expected values for wurtzite ZnO (JCPDS pattern No. 36-1451).

Figure 6

(a)–(d) Scanning electron microscopy images of ZnO:In after ${\mathrm{N}}_2$ annealing with increasing In/Zn ratio $(1\times 1\mu \mathrm{m}$ area). The films are made of compact layers of stacked ZnO grains. Individual grains show a hexagonal structure. For low In/Zn ratio they are vertically stacked, while for higher concentration grains are randomly tilted. (e) There is also an overall reduction of grain size, consistent with the coherent domain size reduction as derived from Rietveld refinements of the measured XRD patterns and using the Scherrer equation for the (002) reflex.

Figure 7

Selected optical properties of the ZnO:In films. All films show a high transparency in the visible range with clearly defined thin-film interference fringes. The latter are used to calculate the film thickness $d$. The onset of optical absorption $\left(E_{abs}\right)$, haze $\left(H\right)$, and average sample transmittance and reflectance $\left(T_{av},R_{av}\right)$ is also shown.