Electron microscopy is considered to be one of the most efficient and versatile tools in material science for determining the crystallography, microsctructure and composition of a specimen. The resolution limitation of the visible light microscope initially lead to the development of the electron microscope [52]. The best resolution of a visible light microscope is about 300 nm according to the Rayleigh criterion:
δ= 0.61λ
µsinβ (2.5)
whereδis the smallest distance that can be resolved (resolution),λdenotes the wavelength of the radiation, µ represents the refractive index of the viewing medium and β is the semi-angle of collection of the magnifying lens [52]. A resolution of 300 nm corresponds to about 1000 atom diameters, while in some cases a material study requires imaging detail down to an atomic level. An electron microscope provides a higher resolution by taking the advantage of the smaller wavelength of electrons compared to that of visible light. Broglie’s equationλ= 1.22/E1/2 (whereλand E are respectively the wavelength and the energy of an electron) shows that a theoretical resolution of the order of 10−3 nm can be achieved given that the electron has a potential of∼100 keV, which is sufficient for atomic level imaging [52].
The electron microscope focuses an energetic electron beam into a nm sized spot and directs it onto a sample. The interactions between the incident electrons and the atoms in the sample generate a range of secondary signals: backscattered electrons, secondary electrons, Auger electrons, Bremsstrahlung and fluorescence x-rays, cathodoluminescence etc. A scanning electron microscope (SEM) scans over the desired area of a sample, and collects these signals in real time, which is then analyzed and converted to information about the surface topography and the chemical composition of the sample. If the sam- ple is thin enough (<∼ 100 nm), the energetic electrons are able to pass through the material. A transmission electron microscope (TEM) detects the transmitted and the forward-scattered electrons to probe the lattice structure of the specimen. Combining SEM and TEM, a scanning transmission electron microscopy (STEM) technique was es- tablished and is becoming increasingly popular [53]. High resolution TEM (HRTEM) was employed in this thesis for substrate lattice structure characterization and identification of the dopant-atom lattice locations.
2.5. Electron microscopy 27
2.5.1 Transmission electron microscopy
A TEM has a similar illumination system to an optical microscope but uses an electron beam instead of a light beam as shown in the schematic of Figure 2.9. The electron beam is generated by an electron gun and is accelerated to a potential of hundreds of keV. While the 1st condenser lens controls the spot size of the beam on the specimen, the 2nd condenser lens and the condenser aperture determine the beam current (brightness of the image). As the electron beam falls on the specimen, the incident electrons can be divided into two components, one undergoes Bragg scattering and the other is transmitted through the sample. Hence, a TEM has two basic operation modes: diffraction and imaging. In the diffraction mode, a selective area diffraction (SAD) aperture is inserted in the image plane to choose the area of interest of the sample, and the objective lens creates the diffraction pattern in the back focal plane from the scattered electrons. An objective aperture is used in the imaging mode to choose the beam(s) in the diffraction pattern (or the central beam) on the back focal plane, and the objective lens generates the intermediate image in the image plane from the selected beam(s). With the intermediate lens focusing on either the back focal plane or the image plane, a diffraction pattern or a TEM image, respectively, is projected on the fluorescent screen or a CCD (charge-coupled device) camera by the projector lens.
The electron diffraction pattern of a sample carries information about its crystal struc- ture such as the crystallinity and the lattice plane spacing. A discrete scattering pattern means the investigated sample is crystalline and the distances between the diffraction spots represent the atomic plane spacing in reciprocal space. On the other hand, the diffraction pattern of an amorphous or polycrystalline material is a series of concentric rings. The TEM imaging mode is able to switch between bright field and dark field by means of choosing the central beam, or a diffracted beam from the diffraction pattern, respectively, with the objective aperture. HRTEM imaging is achieved by collecting both the central beam and a few diffracted beams, and utilizes their phase contrast [53]. In bright field imaging, the contrast of a TEM image depends on the thickness, composition and lattice damage conditions of the target sample. A higher value of both the sample thickness and the atomic number (Z) of the atoms in the specimen lead to stronger electron beam ab- sorption, which results in the corresponding region appearing darker. Brightness contrast can be created between the low and high Z regions in the case where a sample is com- posed of more than one element and their Z difference is large enough to be distinguished in a TEM image. Thus, the TEM technique is commonly employed to identify the dopant atom location. When the dopant atoms precipitate from the substrate, a contrasted re- gion can be observed because of their Z difference. If both the substrate material and the precipitated dopant are crystalline, a Moir´epattern can be created by the overlap of the substrate and the impurity lattice in the image. TEM can also be used to characterize the lattice disorder of a crystalline sample, while the void defects appear brighter, the regions with high strain are darker in a bright field TEM image.
Figure 2.9: Schematic of a TEM [53].
In this thesis, the TEM images and the electron diffraction patterns were taken with the Phillips CM300 TEM system, at the Center of Advanced Microscopy at the Australian National University. Equipped with a LaB6 filament as the electron source, the system accelerates the electrons with a voltage of 300 kV, reaching a maximum magnification of 750×103 with a resolution of 2 ˚A.
2.5.2 TEM sample preparation
The primary concern for a TEM sample is its thickness; it is essential that the sample is thin enough (<∼100nm) to ensure a sufficiently intense electron beam is transmitted. In the area of material science, there are normally two types of TEM samples: the plan view and the cross section view. While a plan view sample is observed in a direction perpendicular to the original sample surface in a TEM, a cross section sample has the original sample surface parallel to the electron beam direction. There are a range of different sample preparation methods for different materials. In this thesis, cross section TEM samples were prepared and the conventional method was employed, via mechanical milling and ion beam polishing. The cross section TEM sample was selected because it allows an investigation of the dopant depth distribution. At first, the ion implanted sample was
2.6. Raman 29