Comparison of electrical properties of the thin film fabricated herein using a 0.3 mmol g–1 precursor solution under Ar gas at a flow rate of 1.0 L min–1, versus those of the films formed by other methods. All values were measured at 300 K.
Functional thin films are used in various fields of our life. Many different methods are used to fabricate these films including physical vapor deposition (PVD) and chemical processes. The chemical processes can be used to manufacture thin films in a relatively cheap way, as compared to PVD methods. This chapter summarizes the procedures of the molecular precursor method (MPM), a chemical process, for fabrication of both metal oxide semiconductor Cu2O and metallic Cu thin films by utilizing Cu(II) complexes in coating solutions. The MPM, recently developed and reported by the present authors, represents a facile procedure for thin film fabrication of various metal oxides or phosphates. This method pertinent to the coordination chemistry and materials science including nanoscience and nanotechnology has provided various thin films of high quality. The MPM is based on the design of metal complexes in coating solutions with excellent stability, homogeneity, miscibility, coatability, etc., which are practical advantages. The metal oxides and phosphates are useful as the electron and/or ion conductors, semiconductors, dielectric materials, etc. This chapter will describe the principle and recent achievement, mainly on fabricating the p-type Cu2O and metallic Cu thin films of the MPM.
- molecular precursor method
- thin film
- p-type Cu2O
A sustainable society requires innovative technology where many disciplines interact. Highly functionalized thin films in various devices such as computers, which were developed mainly for the semiconductor industry in the last century, are now widely used in various fields of our daily life. For example, the touch panel in mobile phone uses a transparent conductive thin film and an antireflection thin film on the glass. A product with various thin films makes life more comfortable. Many different methods are used to fabricate such thin films including physical vapor deposition (PVD) such as laser ablation, molecular beam epitaxy, sputtering, and chemical processes . The chemical processes can be used to manufacture thin films in a relatively cheap way compared to PVD methods. The chemical process is a processing technique for preparing thin films, ceramic coatings, and powders. However, it is usually difficult to fabricate high-quality thin films using chemical processes.
Transparent metal oxide thin films of
Recently, we achieved the fabrication of
This chapter summarizes the procedures used in the MPM for the fabrication of both metal oxide semiconductor Cu2O and metallic Cu thin films by utilizing Cu(II) complexes in coating solutions.
2. Molecular precursor method
The decision by the Nobel Prize committee to award the Nobel Prize for chemistry in 1913 to Alfred Werner met with worldwide approval. In a statement, the committee said that Alfred Werner received the prize in recognition of his work on the linkage of atoms in molecules by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry . Today, the metal complexes are used in various applications such as catalysts, luminescence, and medicine. In 1996, one of the authors, M.S., focused on the thin film fabrication of various metal oxides and phosphate compounds using the metal complexes of stable [10–22]. This is the MPM, which is one of the chemical processes used for thin film fabrication. In those days, most of the researchers in the field of thin film formation by chemical processes preferred to use rather unstable metal complexes. It is easy to imagine the capability of polymers to form films because we use polymer films every day. In fact, well-adhered precursor films can be formed on various substrates by coating the solution dispersing the produced oligomers and polymers including metallic species provided by hydrolyzing the unstable metal complexes. These results led us to believe for a long time that only the oligomers and polymers can form precursor films, but the stable metal complexes having discrete molecular weight would not be useful in the formation of such thin films. The MPM was a challenge to this central belief.
The MPM, pertinent to coordination chemistry and materials science including nanoscience and nanotechnology, has been used to fabricate various high-quality thin films with appropriate film thicknesses. As a result, the MPM represents a facile procedure for thin film fabrication of various metal oxides or phosphates, which are useful as electron and/or ion conductors, semiconductors, dielectric materials, etc [24, 25].
Figure 1 shows the Co3O4 thin films, which were first fabricated using the molecular precursor solutions. To date, more than 40 kinds of metal oxides or phosphates have been easily fabricated. Figure 2 shows the general protocol for fabricating the titanium dioxide thin films. First, a water-resistant coating solution was prepared by the reaction of a neutral [Ti(H2O)(edta)] complex with dipropylamine in ethanol, where edta represents ethylenediamine-
Cuprous oxide, Cu2O, with a cubic structure is a potential candidate for
Figure 4 shows the Arrhenius plot of the Cu2O thin film on the Na-free glass substrate over the temperature range 160–300 K. Hall effect measurements of the thin film indicated that the single phase Cu2O thin film is a typical
The resistivity of the Cu2O thin film fabricated using the MPM was lower than that of both films obtained by the oxidation of a copper film and by the dc reactive magnetron sputtering process [26–28]. It was observed that the carrier concentration tends to be high and the carrier mobility is low for the Cu2O thin film fabricated by the MPM, compared to the thin films formed by previously reported processes (Table 1).
|Method||Annealing condition||Thickness||Carrier concentration||Mobility||Resistivity|
|Temperature||Atmosphere||Time||nm||(cm–3)||(cm2 V–1 s–1)||Ω cm|
|Molecular precursor method||450||Ar gas flow||10 min||50||4.8||76||4.8|
|Oxidation method ||400||10% O2 in Ar gas||5 min||230||6.1||370||6.1|
|Magnetron sputtering ||200||O2 gas pressure of 1.1 × 10−3 torr in vacuum||–||130||0.36||1.8||0.36|
|Pulsed laser deposition ||500||In vacuum||6 h||100||32||100||32|
The method described is the first example of fabrication and characterization of
3. Kinetic study of Cu2O thin film fabrication
In order to clarify the precise mechanism of Cu2O formation from the Cu(II) complex, a kinetic study was performed using XRD . In the study, it was clarified that the thermal reaction of the precursor film, which consists of a dibutylammonium salt of a [Cu(edta)]2− complex ion, first produced metallic Cu species in Ar gas containing <10 ppm of air as an impurity. The Cu phase appeared gradually, and the amount of the phase could be determined from the area of the (111) peak of Cu. The activation energy (1.5 × 102 kJ mol–1) of the reduction reaction from the Cu(II) complex to metallic Cu species was obtained by an Arrhenius plot over the temperature range 230–250°C. Above this temperature range, the Cu2O phase was formed by the oxidation of the Cu phase under Ar gas flow. The amount of the Cu2O phase could be determined from the area of the (111) peak. The activation energy (1.4 × 102 kJ mol–1) of Cu2O formation from the Cu phase was obtained by the Arrhenius plot over the temperature range 400–450°C. In order to examine the stability of the formed Cu2O phase, the oxidation reaction rate from Cu2O to the CuO phase in an identical atmosphere was also measured over the temperature range 450–475°C. The activation energy of the oxidation reaction from Cu2O to the CuO phase was determined to be 1.0 × 102 kJ mol–1. It was observed that the quality of the
The XPS spectra of the films are shown in Figure 5. The peak positions of the Cu 2p3/2 level in spectra (a)–(d) are 933.5, 932.7, 932.5, and 933.2 eV in Figure 5A, respectively. The peaks observed in (a) and (d) can be assigned to the Cu2+ ion, and the broad peak at 944 eV observed in (d) is typical of CuO. In contrast, the peaks observed in (b) and (c) can be assigned to metallic Cu and/or Cu+ ions. These XPS results are consistent with the XRD results.
In Figure 5B and C, the XPS spectra of the C1s and N1s peaks, respectively, are shown. No impurities such as nitrogen or carbon atoms can be found in the XPS spectra of the resultant Cu2O thin films, although the metallic Cu0 thin film includes a certain amount of nitrogen and carbon atoms. It was thus clarified that single phase Cu2O formation was completed by the removal of organic residues in the Cu0 thin film. The co-presence of nitrogen and carbon atoms was thus shown to have an important role in preventing the oxidation of the produced Cu2O phase. The presence of nitrogen and carbon atoms may also help in organizing the stepwise reactions.
In the sol-gel method for Cu2O thin film formation, the CuO film is annealed at 900°C for 5 h in nitrogen for the partial removal of oxygen atoms from the initial oxide thin film. In contrast, the MPM eliminates the organic components in order to form the Cu2O thin film at the abovementioned lower temperature. It is interesting that the difference between these two methods is in the kind of atoms that must be removed. Furthermore, it is important that the formation route of the
It was first shown that the expected Cu2O formation using the MPM occurred via an unexpected intermediate Cu0 phase formed by the thermal decomposition of the molecular precursor involving a Cu(II) complex salt. The XRD measurement of the crystallized thin films was useful in determining the activation energies of the redox reactions from the Cu(II) complex to Cu0, from Cu0 to Cu2O, and from Cu2O to CuO (Figure 6). The redox reactions of the metals and organic ligands occurred stepwise with annealing of the thin films under moderate conditions. Consequently, the ligand in the molecular precursor plays an important role in fabricating excellent
4. Fabrication of copper thin films
From the kinetic study of the Cu2O thin film formation, it was elucidated that the Cu0 species formed as an intermediate was oxidized to the resultant Cu2O thin film during the heat treatment, and the oxidizing agent is the oxygen present in the commercially available Ar gas as an impurity (<2 ppm) . However, the intermediate Cu thin film obtained through the reaction using the precursor film is not electrically conductive. Therefore, in order to fabricate transparent metal copper thin films, we examined novel precursor solutions [35, 36]. A novel precursor solution containing a Cu2+ complex of EDTA and a Cu2+ complex of propylamine derived from formic acid, and the amine was prepared by mixing the two precursor solutions. The concentration of total copper in the ethanolic precursor solution was adjusted to 0.35 mmol g–1. The spin-coating method was used for precursor film formation on a Na-free glass substrate. The spin-coated precursor films were preheated in a drying oven at 70°C for 10 min and then, heat-treated at 350°C for 15 min under an Ar gas flow of 1.5 L min–1 to fabricate thin films in a tubular furnace with a quartz glass tube. The resultant thin film is hereby denoted as
The single peak at 2θ = 43.7° for
The scheme indicates that four Cu complexes are required to construct one FCC copper unit cell. During the heat treatment of the precursor complexes in Ar gas flow containing <2 ppm of oxygen as impurity, neighboring complexes react with each other. The valency of copper was reduced from +2 to 0 by the thermal decomposition of the complexes of EDTA and butylamine ligands in Ar gas. In the process, Cu2O involving Cu and the neutral carbon atom is produced in the
The polycrystalline Cu lattices were gradually structured by reducing the valency of the Cu2+ ion with carbon atoms, and the Cu grains were simultaneously grown by annealing. This reaction mechanism involving the reduction reaction caused by carbon atoms may be comparable to the modern and indirect steel-making system using corks. The tensile strength of the
Figure 10 presents the transmittance and reflectance spectra of the thin films. The transmittance spectra of
Recently, we attempted to embed copper in narrow trenches (0.2–1.0 μm wide and 5.0 μm deep) by using the MPM. A new precursor solution was prepared by dispersing the Cu nanopowder (20–40 nm) into the abovementioned Cu precursor solution. Si substrates with the trenches were immersed in this precursor solution under ultrasonic vibration for 1 min and then slowly withdrawn from the solution. The dip coating and heat treatment steps were repeated twice. The cross-sectional FE-SEM images of the treated substrate indicate that the embedded copper fills the trenches without voids.
Thermal reactions of metal complex films useful for ceramic thin film production such as Cu2O did not attract much attention for a long time, with the exception of the CVD procedure. However, we indicated that the MPM provides facile and unique routes to obtain
The importance of the metal complex in the MPM was presented by using the unprecedented thin film fabrication of
It is important that most of the originally included atoms in the MPM system are not involved in the resultant thin films if the amounts and treatment are appropriate. Therefore, the role of the ligand of the metal complex resembles that of auxiliary lines to solve geometrical problems in mathematics. In the sol-gel process, the similarity of the gel composition and the final oxides is desirable and is supposed to be an advantage of the method, though the rearrangement of the polymerized amorphous species to the crystalline requires much energy. From this point of view, the concept of the MPM is quite different from that of the conventional sol-gel method and has many potential applications.
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