Atomic force microscope (AFM) was first developed by Binnig and Quate [300] in 1985, as a method of scanning samples with inter-atomic forces between single atoms. The use of an optical technique to detect the position of the tip on the sample surface allows AFM to image non-conductive as well as conductive materials in atomic scale, which is ideal for samples charging up in electron microscopes. The small forces moving the cantilever beam (i.e. 10-18 N) [300] provide high sensitivity in relation to other methods such as scanning tunnelling microscope (STM).
The configuration consists of a flexible cantilever beam connected to a sharp tip, a laser source, a detector system, and an XYZ position control stage on which the sample is placed (Figure 37). The sample moves under the tip and the features on the sample cause movement of the tip and the cantilever attached. The laser beam is
focused onto the back of the cantilever and then it is reflected onto a split optical sensing system (photodiode detector). Hence, the movements of the cantilever cause changes of the laser beam position and a differential voltage is generated as the position of the laser beam changes on the photodiode. Hence, the differential voltage is used to obtain information about the deflections of the cantilever. The data provide surface height information over small (e.g. 500 nm2) or larger areas (e.g. 5 μm2).
Figure 37: Schematic of the AFM main components arrangement.
The information obtained by AFM is strongly affected by the properties of the spring (flexible part of the cantilever) and the tip [301]. The spring should have high resonant frequency in order to reduce the vibrational noise effect, which can be controlled via a high and unvarying ratio of the spring constant k and spring effective mass mo[300].
Detector Laser source Tip Sample AFM stage Cantilever beam
High aspect ratio tips are also suggested to avoid electrostatic cantilever-sample interaction, which may result in a capacitive force [302].
AFM operates in typically two modes, the contact mode and tapping mode. The contact mode consists of the tip contacting the sample‟s surface, with finite forces that deform the tip and sample [301]. The tapping mode on the other hand consists of forces that deflect the tip in a constant distance from the sample‟s surface at 40-50Å [301], hence, it is preferred for scanning „soft‟ materials such as organic materials.
The AFM used in this project was the Bruker Multimode 8, which was connected to NanoScope software that controlled the scanning parameters, such as the frequency of cantilever (scanning speed), image quality, etc. Noise turbulence was a great issue in obtaining good quality images, although the vibration disruptions are partially minimised by the vibration absorbed floor and through an acoustic isolated environment with the use of a foam cover. Using the foam cover did not show a difference in the images, hence it was not used. The parameters providing the best quality images were at frequency 1 Hz and 1024 pixel image using the ScanAsyst mode, since the contact mode was highly affected by the noise vibrations. The ScanAsyst mode is an improved technology of tapping mode developed by Bruker to help the users obtaining better quality images by automatically adjusting the scanning parameters and eliminating the cantilever tuning. This mode uses precise controlled force curves to generate an image, protecting in that way delicate films [303]
. The tip radius used for this mode is 2-12 nm, indicating the resolution of the scan.
From the software, the topographic images showed the surface scanned height and tip friction along the surface, although in this project only the height related images
will be analysed. Those were later on analysed to obtain the root mean square roughness (RMS), the average roughness (Ra), the maximum height, and the image surface area. The RMS and Ra are widely used terms that express the roughness of the surface, with the calculation formulas being the only difference between them. The numerical value of RMS is higher than Ra as it uses the square root of the sum of height difference, and so a large height feature will affect more the RMS value. The Ra uses the average of individual heights. Most of the studies related to the current work used the term RMS to express the level of surface roughness, hence, RMS data are the only data presented in this study in order to compare them with literature. RMS is expressed by Equation 46 [304], where z is the grains height and N is the number of grains. From the relation is shown that RMS is high with larger or sparse grains. However, the relation was not actually used for calculations since the RMS values were automatically estimated by the NanoScope Analysis software.
Equation 46
Chapter 4
Effects of Zr doping on the
microstructure of ZnO films
4.1.
Introduction
This chapter focuses on the microstructure of both Zr-doped and un-doped ZnO films. Microstructure plays an important role in determining the optical and electrical properties of TCOs and it is therefore essential to understand how it is affected by doping, film thickness and also by growth conditions. The microstructure in this work has been investigated using XRD, AFM and TEM to reveal information about the grains preferred orientation and the crystallite size.
Particular focus is given to the preferred orientation of the crystallites and how it is affected by the film thickness. Recently, Singh et al. reported orientation shift in ZnO films deposited by ALD, from c-axis to a-axis as the film thickness increased [226]
. The current work will analyse this effect further by identifying the thickness at which the orientation is shifting to a-axis by using different thickness films and using mathematical models of the surface and strain energies that drive the preferred orientation. This analysis is carried out using un-doped and Zr-doped ZnO films.