2.4 CdO Samples
2.4.1 Surface Preparation in UHV
In order to perform surface sensitive photoemission measurements on CdO, the sam- ple needs to be prepared with a clean and ordered surface in UHV conditions. There are several commonly used approaches to UHV surface preparation including; ion bombardment and annealing (IBA), chemical etching and cleaving. IBA has previ- ously been used to prepare CdO samples [113]. However, an annealing temperature of 420 ◦C was used, resulting in the photoemission spectra still displaying a small
amount of surface contamination.
Several previous studies [66, 114–116] have prepared CdO samples in UHV just by annealing. Figure 2.16 shows the effect of a half hour 600 ◦C UHV anneal on a
-10 -5 0 5 10 Height (nm) -10 0 10 Height (nm) -10 0 10 Distance (Ûm) 0.8 0.6 0.4 0.2 0 1 Height (nm) (c) (d) 0.8 0.6 0.4 0.2 0 1 Distance (Ûm) 0.2 Ûm (a) As grown 0.2 Ûm (b) UHV annealed
Figure 2.17. AFM data illustrating the effect of UHV annealing on the CdO(100)
surface. (a) and (b) show 1µm×1µm AFM height images of an as-grown and UHV
annealed CdO(100) sample, showing the surface roughening and recrystallisation following UHV annealing. Both images are shown with the same indicated colour scale to allow the difference to be observed. (c) and (d) show line profiles taken from the positions indicated across images (a) and (b) to allow the surface roughness to be observed.
ure 2.16 (a)) shows a large contamination peak at ∼ 532 eV which is assigned to
hydroxide and carbonate species, possibly related to the growth, or atmospheric contamination [113, 115]. The Cd 3d5/2 spectra (figure 2.16 (b)) also shows a large
contamination component at ∼ 406 eV which is principally assigned to CdCO3.
These components are fully removed by annealing in UHV. Additionally the C 1s
region (figure 2.16 (c)) shows a significant peak associated with atmospheric contam- ination on the as loaded sample which is fully removed by the annealing. Typical LEED patterns observed from the cleaned CdO(100) surface are shown in figure 2.16 (d-g), exhibiting a sharp (1×1) pattern which indicates a well-ordered surface.
To investigate the effects of UHV annealing on the CdO(100) surface, AFM was taken of both the as-grown surface and following annealing in UHV, shown in figure 2.17. The AFM measurements were performed using a Asylum Research MFP-3D
at the University of Warwick. The as-grown CdO(100) surface exhibits an RMS roughness of ∼ 2 nm, broadly in agreement with similar measurements performed
immediately after the film growth [20]. However, following UHV annealing the sur- face shows much more texture and a RMS roughness of ∼ 5 nm. This increase in
surface roughness may to be related to a recrystallisation of the surface, possibly associated with a loss of oxygen during the annealing process. An oxygen deficient surface is supported by analysis of the XPS shown in figure 2.16 which indicates a surface composition of 56% cadmium to 44% oxygen. This AFM result appears to be contradictory to the LEED measurement which shows the annealed CdO surface is well ordered. This suggests two possibilities, either, the newly formed annealed surface contains grains all of which have the same orientation, or, the outermost layer is disordered and therefore produce only background intensity and the diffrac- tion pattern is generated by ordered underlying layers. It would be of interest to conduct a more through study into the effects of annealing on the surface morphol- ogy to better understand this process. These results show that clean, well-ordered CdO(100) surfaces can be successfully prepared in UHV by controlled annealing at 500 ◦C to 600 ◦C. This approach results in no observable contamination in XPS
and sharp a (1×1) LEED pattern.
This chapter has described the experimental techniques and equipment used to obtain the results presented in this thesis. The key experimental technique used is photoemission which has been described in detail, and the associated surface science technique of LEED has also been described which is used to confirm successful surface preparation where required. Hall effect measurements are described, which are used throughout this thesis to obtain information about the bulk electronic properties of CdO samples. Finally a introduction of the samples studied is given, this included a summary of the growth technique, and a overview of the surface preparation approach used throughout this thesis when a clean CdO(100) surface is required.
CHAPTER
3
Theoretical Background
This thesis is concerned with the electronic properties and structure of CdO, there- fore a number of theoretical approaches are required to understand the CdO band structure. Additionally, CdO exhibits strong downward band bending and signifi- cant electron accumulation at its surface. In order to investigate these phenomena it is therefore necessary to develop a model of the space charge layer at the surface of CdO. In this chapter an outline of the theoretical methods used throughout this thesis are described. Specifically, approximations to semiconductor band structure, density functional theory, space charge regions, and quantised subband states are outlined and discussed.
3.1
Semiconductor Band Structure
The band structure of a semiconductor is a theoretical framework that enables the understanding of the electronic properties of a material. In general this problem is fully expressed by the Schrödinger equation
where E is the electron energy, Ψ(r1,r2,· · · ,rn) is the many body wave function,
and the Hamiltonian (H) is given by [108]
H=X i p2 i 2mi +X j Pj2 2Mj +1 2 X j0,j 0 ZjZj0e2 |Rj−Rj0| −X i,j Zje2 |ri−Rj| + 1 2 X i,i0 0 e2 |ri−ri0| . (3.2)
Wherei andj label the electrons and nuclei respectively, pi and Pj are the electron
and nuclei momentum operators respectively, and ri and Rj are the position of the ith electron and jth nucleus respectively. P0 denotes the sum over non-equivalent
indices only.
However, solving this equation for all the electrons in a material is impractical and the only way forward is to make a series of simplifications. Firstly the electrons are separated into two groups, core and valence. The core electrons are localised around the nuclei and can therefore be combined with the nuclei to form ion cores. Secondly the Born-Oppenheimer [117], or adiabatic approximation is applied, which states that the ion cores appear to be stationary to the electrons as the ion cores are much heavier (Mj mi). Conversely, to the ions, the electrons move so quickly they
only see a time-averaged potential. These approximations allow the Hamiltonian to be written as a sum of three terms
H=Hions(Rj) +He(ri,Rj0) +He−ion(ri, δRj), (3.3)
where Hions(Rj) is the Hamiltonian of the ions moving within their own potential
and the time-averaged electron potential, He(ri,Rj0) is the electronic Hamiltonian
of the electrons moving in the potential established by the frozen ion cores, and
He−ion(ri, δRj) is the Hamiltonian resulting from the ions being displaced from
their equilibrium positions by δRj, commonly referred to as the electron-phonon
and are given by He = X i p2 i 2me + 1 2 X i,i0 0 e2 |ri−ri0| −X i,j Zje2 |ri−Rj0| . (3.4)
Physically these three terms represent the KE of the electrons, the electron-electron and electron-ion interactions. Even solving this simplified electronic Hamiltonian remains impractical due to the large number of electrons present in a real material (>1023 electrons cm−3), so a further approximation is required. This is achieved by
assuming that each electron experiences the same averaged potential V(r), known
as the mean-field approximation. Applying this approximation results in the one- electron Hamiltonian (H1e) which is given by
H1e = p2
2me +
V(r). (3.5)
Substituting this Hamiltonian into equation 3.1 results in the one-electron Schrödinger equation H1eψn(r) = p2 2me + V(r) ! ψn(r) = Enψn(r). (3.6)
The eigenstates of this equation are Bloch electrons, which are independent electrons moving within a periodic potential. This is discussed in the next section.