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Even though monoclinic CuO has been widely studied for decades, there are still divergent reports regarding its electronic and optical properties, due to its strong correlated interaction among charge, spin, orbital and lattice. For instance, there is a contradiction in the type of band gap: direct or indirect band gap. HSE and LDA+U calculations from Heinemann et al. yield the indirect band gap of 2.74 and 1.39 eV, respectively . Whereas, other articles suggest a direct band gap of about 1.4 eV , , . Moreover, the electronic structure of CuO is still unambiguous, as the calculation electronic structure strongly depends on thecalculated methods or parameters . For instance, different nonlocal Hartree-Fock exchange α in HSE calculation will shift the states and change the band gaps, as shown in Fig. 1.9(a). Similar situations exist in Perdew-Burke-Ernzerhof with exchange correction functional (PBE+U) calculation when tuning the U values (see Fig. 1.9(b)) .
Electrical properties and defect mechanism
Cu2O, Cu4O3 and CuO are intrinsic p-type semiconductors. Among these three oxides, Cu2O has received intensive study, as it possesses larger experimental optical band gap (2.38-2.51 eV)  and higher mobility simultaneously than other p-type semiconductors.
Room temperature mobility of 100 cm2V-1s-1 can be reached in single crystal Cu2O, while thin films deposited at high temperature can produce room temperature mobility of 62 cm2V-1s-1 easily . 5 cm2V-1s-1 has been attained by low temperature (225 oC) atmospheric atomic layer deposition (AALD) . Such high mobility is one of the reasons that Cu2O has drawn much attention in p-type transparent conductive materials , which may originate from its lower effective mass (0.575 mo, mo is the free electron mass) , . However, it is a hard task to get the high mobility in Cu4O3 and CuO.
The origin of p-type conductivity in Cu2O has been studied systematically by calculations and experiments. Theoretical calculations by different methods have confirmed that copper vacancies, including the simple Cu V and the split complex configuration Split Cu V (it can be understood as the insertion of an interstitial copper atom in between two Cu V ), act as shallow hole-producers and account for the intrinsic p-type conductivity , , as shown in Fig. Cu V (0.47 eV) are in agreement with the trap levels measured by deep level transient spectroscopy (DLTS) study at 0.25 and 0.45 eV above the valence band maximum , . Besides, the existence of Split Cu V complex defects has also been proved by Raman . In addition, the role of hydrogen related defects has been theoretically studied . Hydrogen prefers to bind into Cu V under O-poor and O-rich conditions, resulting in a complex H- Cu V defect. This kind of defect has quite low formation energy and acts to kill p-type conductivity .
Raman spectrometry is commonly used in materials science, since vibrational information is specific to the chemical bonds and crystallographic symmetry. Therefore, it provides a fingerprint by which the material can be identified. The Raman scattered light occurs at wavelengths that are shifted from the incident light by the energies of molecular vibrations. The mechanism of Raman scattering is different from that of infrared absorption, and Raman and IR spectra provide complementary information .
Here, micro-Raman spectrometry (Horiba LabRAM HR using a 532 nm laser) was employed to identify the phase structure at room temperature. It is worth noting that extreme care should be paid to the energy density of laser beam to attain the correct spectra of Cu2O and Cu4O3, as these two phases are prone to oxidize into CuO in air at low temperature (< 300 oC). Taking Cu4O3 as an example, the Raman spectrum recorded at low power density of laser beam demonstrates the pure phase Cu4O3, whereas CuO phase is identified using high power density, as shown in Fig. 2.6.
Hall effect measurements
An electrical current along the x (longitudinal) direction in the presence of a perpendicular magnetic field B = (0, 0, B) along z induces an electric field Ey along the transverse (y) direction. The schematic geometry is shown in Fig. 2. 7, where the charge accumulation is due to the Lorentz force. The related transverse voltage is called the Hall voltage (VH), which can be described as nte V IB H (2-1).
where I is the current across the plate length, B is the magnetic field, t is the thickness of the plate, e is the elementary charge, and n is the charge carrier density of the carrier electrons . The Hall effect measurements can determine accurately conductive type (p or n type), carrier density, electrical resistivity, and the mobility of carriers in semiconductors.
Transmission electron microscopy
TEM is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. Then an image is formed from the interaction of the electrons transmitted through the specimen. TEM has revolutionized our understanding of materials by completing the processing-structure-properties links down to atomistic levels. It is now possible to tailor the microstructure of materials to achieve specific sets of properties; the extraordinary abilities of modern TEM to provide almost all the structural, microstructural, and chemical information .
In this thesis, TEM investigation for the microstructure was performed by a state-of -the-art JEOL ARM 200-Cold FEG (point resolution 0.19 nm) fitted with a GIF Quantum ER, as shown in Fig. 2. 8. 200 kV accelerating voltage and 15 μA emission current are used. For this investigation, the cross-section TEM samples of films deposited on silicon substrates were prepared in Sarrebrücken (Germany) using a focused ion beam (FIB)-scanning electron microscope (SEM) dual beam system (FEI Helios 600) using the ‘in situ’ lift-out technique. Final thinning was done with low voltage milling (5 kV or 2 kV) to reduce any possible preparation artefacts.
Table of contents :
Chapter 1 Basic properties of copper oxides and their application in low cost solar cells
1.2 Basic physical properties of copper oxides
1.2.1 Crystal structure
1.2.2 Optical properties and band structures
220.127.116.11 Monoclinic CuO
18.104.22.168 Tetragonal CuO
1.2.3 Electrical properties and defect mechanism
1.2.4 Debating issues
22.214.171.124 n-type Cu2O: fact or fiction?
126.96.36.199 Oxygen vacancy charge in Cu2O: positive or neutral?
1.3 Synthesis methods
1.4 Recent progress of copper oxides based solar cells
1.4.1 Advantages of copper oxides
1.4.2 Poor performance of Cu2O-based solar cells
1.5 Chapter conclusions
Chapter 2 Experimental and calculational details
2.2 Thin film growth
2.3 Thin film characterization
2.3.1 X-ray diffraction
2.3.2 Raman spectrometry
2.3.3 Hall effect measurements
2.3.4 Transmission electron microscopy
2.3.5 Electron energy loss spectroscopy
2.3.6 Photoemission spectroscopy
2.4 Theoretical calculation method
2.5 Chapter conclusions
Chapter 3 Tuning the structure and preferred orientation in reactively sputtered copper oxide thin films
3.2 Structure of copper oxide thin films
3.2.1 Effect of the oxygen flow rate
3.2.2 Effect of the total pressure
3.3 Tuning the preferred orientation in copper oxide thin films
3.3.1 Effect of the substrate nature
3.3.2 Preferred orientation of Cu2O thin films
3.3.3 Preferred orientation of Cu4O3 thin films
3.3.4 Preferred orientation of CuO thin films
3.3.5 Discussion about the preferred orientation in copper oxide thin films
3.4 Evidence of a local homoepitaxial growth mechanism in copper oxide thin films
3.4.1 Study of Cu2O thin films
3.4.2 Can the local homoepitaxial growth be extended to Cu4O3 or CuO?
3.5 Local heteroepitaxial growth of NiO on Cu2O
3.6 Chapter conclusions
Chapter 4 Self-assembled growth of vertically aligned columnar copper oxide nanocomposite thin films on unmatched substrates
4.2 Vertically aligned columnar growth in biphase Cu2O and Cu4O3 thin films
4.2.1 Identifying the phases by X-ray diffraction and Raman spectrometry
4.2.2 Cross-sectional microstructure of biphase Cu2O and Cu4O3 thin films
4.2.3 Top-view microstructure of biphase Cu2O and Cu4O3 thin films
4.2.3 Discussion on the vertically aligned columnar growth mechanism
4.3 Electrical properties of biphase Cu2O + Cu4O3 thin films
4.4 Chapter Conclusions
Chapter 5 Optical properties and electronic structure of copper oxide thin films
5.2 Transmittance enhancement and optical bandgap widening of Cu2O thin films after air annealing
5.2.1 Phase structure evolution
5.2.2 Transmittance enhancement of air annealed Cu2O thin films
5.2.3 Band gap widening of air annealed Cu2O thin films
5.2.4 Photoluminesence analyses on Cu2O thin films
5.3 Electronic structure of binary copper oxide thin films
5.3.1 Band gap
5.3.2 Valence band electronic structure
5.3.3 XPS core level and EELS spectra
5.3.4 Conduction band electronic structure
5.4 Chapter conclusions
Conclusions and outlook