Scanning tunneling microscope investigation of local density of states in Al-doped ZnO thin films

The electrical properties of grain boundaries in technologically relevant oxide thin films are the subject of both applied and fundamental research. Here we present an investigation of the local density of states (LDOS) in sputtered Al-doped ZnO using a scanning tunneling microscope. We observe a pronounced difference in the tunneling conductivity recorded on- and off-grain, with the grain boundary LDOS peaked $~$600 meV below the Fermi level. This provides a direct measurement of the distribution of charge traps that is of relevance in advancing understanding of carrier conduction in this transparent conducting oxide.

Figure 1

Surface topography and optical absorption of the sputtered AZO film. (a) AFM topography map of the as-deposited film, acquired with a sharp tapping mode probe (Nanosensors SSS-NCHR). Grains are approximately 10–50 nm in lateral size. Surface roughness analyzed on 40$\times$40 nm${}^2$ patches is 0.7 $\pm$ 0.2 nm rms. (b) Optical transmission ($T$, dashed black line) and absorption ($A$, solid red line) spectra. Data are measured on a Hitachi U-4001 spectrophotometer with an integrating sphere, and the sample is grown on a 0.5 mm thick sapphire substrate. The film is transparent in an optical window from the bandgap in the UV to the plasma absorption in the IR.

Figure 2

STM constant current topography maps of the same area taken with (a) $-0.9$ V and (b) $-0.1$ V bias applied to the sample relative to the tip. In the inset region of (b), the data have been high-pass filtered to emphasize sharp features; prominent grain boundaries are identified with arrows. Positive height corresponds to the tip retracting from the sample, negative height corresponds to the tip extending toward the sample. Line errors result from electrical and mechanical noise that disrupts the STM feedback loop. The rms surface roughness analyzed on 40$\times$40 nm${}^2$ patches is 0.9 $\pm$ 0.3 nm and 1.3 $\pm$ 0.4 nm for (a) and (b), respectively.

Figure 3

Measured differential tunneling conductivity. (a) $\frac{dI}{dV}$ measured on grains (red) and grain boundaries (black). On-grain curve is the average of 148 scans measured at three different locations on the sample; grain-boundary curve is the average of 132 scans measured at four different locations. The standard deviations of these scans are indicated by the thin dashed lines bounding the averages. All scans were recorded on the same sample with the same tungsten tip, and with the grain and grain-boundary scans interspersed. (b) $\frac{dI}{dV}$ measured on-grain to higher bias in order to fit for the tunneling parameters (see Sec. 3); the teal dashed line shows the fit for $V>$ 0.5.

Figure 4

Grain boundary LDOS and electron trap states. (a) Calculated grain boundary LDOS. Data are normalized to the on-grain ${\rho }_s(E=0)$ and all calculations are performed using constant tip LDOS. The solid black curve shows the Neumann approximation solution for ${\rho }_S(E)$, and the dashed black curves show the scatter in the calculated result given the scatter in the raw $\frac{dI}{dV}$ data (see Fig. 3). The dashed red curve shows a simpler solution calculated from a linear combination of $\frac{dI}{dV}$ and $I(V)$ according to Eq. 5 in Ref. 20. These two solutions give nearly identical results. Also shown (grey dashed) is the normalized conductivity $(\frac{dI}{dV})/T$ that is often used as a proxy for the LDOS. A common feature to all solutions is a clear peak at negative $E-E_F$ that represents electron trap states near the conduction band edge. (b) Energy band schematic showing the distribution of grain boundary electron trap states. The degenerately doped conduction band is represented on the left, with $E_F-E_c$ $=$ 0.52 $\pm$ 0.14 eV. The spectrum of grain boundary LDOS at negative bias is represented by the colormap using data taken from (a). The trap state density is peaked at $E_t~0.1$ eV below the conduction band edge, $E_c$.