Guided optical modes in randomly textured ZnO thin films imaged by near-field scanning optical microscopy

We report on near-field optical imaging of light transmission through textured zinc oxide (ZnO) thin films grown on glass substrates. Such ZnO layers are commonly used as a transparent conducting oxide for the front contacts in thin-film solar cells. To increase the quantum efficiency of such solar cells, the ZnO surface is randomly textured, resulting in a higher light scattering and promoting light trapping. Here, we study these phenomena microscopically by imaging the local optical mode profiles in such thin films. Our results give clear evidence for the interconversion of propagating and guided evanescent modes. Such information, which cannot be extracted from more conventional far-field optical studies, may prove helpful in further optimizing the efficiencies of thin-film optoelectronic devices.

Figure 1

Schematic view of the collection-mode geometry of NSOM. The incident laser beam illuminates the sample from the substrate side. The transmitted intensity is collected with the fiber tip.

Figure 2

[(a), (c), and (e)] Topography and [(b), (d), and (f)] NSOM transmission images at laser wavelengths of 488, 658, and $780\mathrm{nm}$, respectively, in constant-distance mode. The white lines in the topographic images represent the positions for the cross sections in Fig. 3.

Figure 3

Cross sections of the topography and the NSOM transmission at laser wavelength of (a) $488\mathrm{nm}$, (b) $658\mathrm{nm}$, and (c) $780\mathrm{nm}$ in constant-distance mode. The positions for the cross sections are depicted in Fig. 2 by the white lines.

Figure 4

(a) Topography and (b) NSOM transmission image at $488\mathrm{nm}$ in constant-distance mode. (c) and (d) represent cross sections from (a) and (b), respectively.

Figure 5

(a) Topography and (b) NSOM transmission images at $488\mathrm{nm}$ in constant-distance mode. (c) shows the topographic information for the mixed scanning mode at the base height $z_0$. The corresponding NSOM image is shown in (d). (e) and (f) represent the NSOM images in constant-height mode for the heights $z_0+450\mathrm{nm}$ and $z_0+2450\mathrm{nm}$, respectively. The arrows mark a special structure described in text.

Figure 6

Light rays in a textured thin film to illustrate two different interference effects marked as situations I. and II. described in text.

Figure 7

(Color online) Calculated intensity of the electric field $\left({\mid E\mid }^2\right)$ for a periodic structure of triangular ridges with three different distances between the ridges. In (a), (b), and (c), the peak-to-peak distance is chosen to be 3, 5, and $18\mu \mathrm{m}$, respectively. The intensity scale is normalized to the incident wave.

Figure 8

(Color online) Calculated intensity of the electric field $\left({\mid E\mid }^2\right)$ for a periodic structure of triangular ridges with a distance of $18\mu \mathrm{m}$ between the ridges for three asymmetries. In (a), the field distribution for symmetric ridges is shown. In (b) and (c), the profiles of the ridges are chosen to be asymmetric with the strongest asymmetry in (c). The intensity scale is normalized to the incident wave.