The microstructure of un-doped ZnO thin films was investigated as a function of growth temperature. AFM was carried out on ZnO films grown at 150°C, 200°C, 250°C and 275°C with 500 cycles of ALD. Figure 38 shows detailed area (500 nm2) scans of these samples (excluding 150°C as it is very similar to the 200°C result). The height topography across selected cross sections is shown as dotted lines in the figures.
Figure 38: AFM images of un-doped films grown at a) 200°C, b) 250°C, and c) 275°C. The dotted line section shows the height changes of the surface map, which
The root mean square (RMS) values obtained using the NanoScope Analysis software, do not show much variation, ranging from 2.25 nm (250°C) to 2.34 nm (275°C), with the 200°C film having RMS=2.31 nm. The small variation indicates negligible changes in surface roughness as temperature increased, although the grains appear different in shape. From the three films, the grains appear as needle shaped at the low temperature with features of 50-60 nm length. It is assumed that the length of the needle shaped grains is the c-axis and their width is the a-axis. The grains in films grown at 275°C appear rounder in shape compared to the ones at 200°C, while the grains at 250°C appear both elongated and round. The round grains could be elongated grains oriented perpendicular to the substrate. It is difficult to accurately assess the length of the grains along the c-axis using AFM as the orientation of individual grain is unknown. It is, however, possible to estimate the width of each grain along the a-axis by measuring the shortest dimension of the AFM features.
The a-axis dimensions at low temperature are approximately the same as the width of the round feature at high temperature after measuring the average dimensions of the grains using AxioVision Rel. 4.8 software (i.e. 24.7 nm width at 200°C and 23.2 nm at 275°C). The grains‟ height on the other hand presented by the plotted peaks shown on the AFM images, was shown to increase at 275°C indicating possible transition of the grains orientation. As a result, the surface topography indicates that as the temperature increases the grains become more perpendicular to the substrate (i.e. grains lie perpendicular to the image angle and so appeared rounder, but are still needle shaped).
In order to support the AFM data and specify the grains orientation, XRD measurements were carried out on the same samples. The measurements are displayed in Figure 39, consisting of XRD diffraction patterns of 99-62 nm films grown from 150-275°C (i.e. thickness reduced with temperature increase as the number of cycles was kept fixed but the growth rate was decreased). The graph also shows the reference patterns of hexagonal wurtzite ZnO by space group P63mc [305] and the diffraction pattern from uncoated Si wafer substrate.
All the films are polycrystalline, with the highest intensity peak typically indicating a preferred orientation in that plane. The current films grown at low temperatures show dual orientation (two peaks are dominant) with a rise of (0002) and ( ̅ ) peaks corresponding to the c-axis and a-axis orientation respectively. At high growth temperature the (0002) orientation is the most favourable as the ( ̅ ) peak gradually suppresses and disappears. The ̅ peak is very small and broad in all films, although in the reference pattern it has the strongest signal. The ( ̅ ) peak does not appear, but the ̅ diffraction appears as a very small peak up to 225°C. From the reference pattern the ( ̅ ) peak has the lowest intensity of all diffractions, hence, it is very difficult to be seen, especially when its related peak
̅ is very low. The suppression of the planes in the z-direction such as ̅ and ̅ suggests that the grains are closed packed as grown horizontally or perpendicularly to the substrate, hence, there is no much space for grains to grow in other directions. Note that the existence of both (0002) and ( ̅ ) peaks do not correspond to planes of the same unit cell as the cells oriented in the c-axis do not allow diffractions of a-axis planes. Therefore, the a-axis diffractions refer to grains oriented in the a-axis and the c-axis to grains oriented in the c-axis direction.
XRD results are therefore consistent with AFM, with the 200°C film showing elongated grains corresponding to a-axis oriented grains, and with the 275°C film showing smaller round features corresponding to c-axis oriented grains (i.e. vertical to the substrate). Additionally, as shown in AFM the elongated grains were reduced in number at 250°C, which agrees with the intensity reduction of the ( ̅ ) peak, while the rounder features increased accordingly to the (0002) XRD peak intensity. As temperature reach its maximum of 275°C, no elongated grains appeared in AFM which corresponds to the disappearance of the ( ̅ ) peak in XRD. The data agreement between the two techniques at this stage suggests that the grains at low temperature grow parallel to the substrate, while as temperature increased the grains gradually grow in direction perpendicular to the substrate.
Figure 39: XRD patterns of ZnO films (500 ALD cycles), with reference patters displayed for comparison to Si substrate pattern (see chapter 3) and ZnO pattern of
space group P63mc [305]. 20 24 28 32 36 40 44 48 52 56 60 Si Substrate 1012 Int en sity (CPS) 2(degree) 1010 0002 1011 1120 ZnO P63MC 150C 200C 225C 250C 275C
Figure 40: XRD and AFM data comparison of similar thickness films (55-62 nm) grown at 200°C and 275°C respectively.
The growth temperature appears to affect the degree of orientation shown by the increased XRD intensity. However, it has to be examined whether this is an effect of having different film thickness. A direct comparison of XRD and AFM data is shown in Figure 40 for films grown at different temperatures both with similar thickness (i.e. 55-62 nm). From the AFM data the grains were found to be narrower and longer for the 200°C film compared to the 275°C film, with their length to width ratio of 1.9 and 1.3 respectively (i.e. 39.9/21.3 nm at 200°C and 31.0/23.2 nm at 275°C). The image shows a-axis oriented grains at 200°C, consistent with XRD data showing a small ( ̅ ) peak. The round features showing in the 275°C film indicate c-axis oriented grains, consistent with the high XRD intensity of the (0002) peak, while the appearance of the ̅ peak suggests the existence of a few a-axis
oriented grains. The higher (0002) peak intensity for the 275°C film indicates the formation of larger crystallites at high growth temperatures. Hence, regardless the film thickness, the films grow at both a-axis and c-axis directions at low temperatures, while at high temperatures the grains preferred growing in the c-axis direction. Further analysis is necessary to specify whether the orientation shift in preferential growth (dual or c-axis) depends on the thickness, as the peaks intensity changes between the 55 nm (Figure 40) and 95 nm (Figure 39) films grown at the same temperature.
The reason for the grain size increase at high temperature is found within the lattice changes during the growth. In particular, the grain growth occurs by the movement of the grain boundaries and the diffusion of atoms from the one side of the boundary to the other [306]. The boundaries movement is driven by the tendency to reduce the grain boundary energy as the total energy increases, which is done by reducing the boundary area when the grains are larger. Therefore, at high growth temperatures where the total energy is raised, the grain boundaries energetically prefer to be reduced in size by the formation of larger grains. In addition, the atoms mobility increases with increased temperature expressed by Boltzmann transport model (i.e. kinetic energy increases due to the gained thermal energy). Therefore, at high growth temperatures the atoms are expected to move into larger grains, forming very large grains and much smaller ones. This is consistent with the AFM images, as there are many large features and a few small ones in the film grown at 275°C.
In comparison to previously reported studies on ZnO films grown by ALD, the temperature effect is consistent with a selected-area-diffraction (SAD) study of ZnO films grown by ALD using DEZ, showing a higher degree of orientation and larger
grains at high temperature [307]. The low temperature film in [307] also showed dual orientation similar to the data in the current study.
Figure 41: Temperature dependence of lattice strain dependence for ZnO films.
From the XRD data it is also useful to analyse the lattice strain and observe any effects of altering the deposition temperature. The lattice strain (Figure 41) was obtained by comparing the measured d‒spacing to literature values of
̅ =2.815 Å and =2.604 Å [17]. The results show a linear increase in
tensile strain in the c-axis as temperature increases, corresponding to spacing decrease. The strain in the a-axis (i.e. ( ̅ ) peak) shows an overall reduction with temperature, although there is some scattering due to the low signal from the plane.
150 200 250 300 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 (10-10) (0002)
Strain
(%)
Temperature (C)
This suggests that as temperature increases the c-axis oriented grains are under more tensile strain and the grains in the a-axis preferred direction are under less tension.