1. INTRODUCTION
1.1. Cuprous oxide
1.1.1 Crystalline structure
Cu2O has a simple cubic structure belonging to 𝑃𝑛3̅𝑚 space group with lattice constant of 4.27 Å and each Cu atom has two neighboring oxygen atoms forming O-Cu-O linear units, as shown in Fig. 1.1. Oxygen atoms are positioned on body centered cubic sublattice and copper atoms are on face centered cubic sublattice [10]. The copper atoms are tetrahedrally coordinated with each neighboring Cu atoms and oxygen atoms are in twofold linear coordination with neighboring oxygen atoms. The bond length of Cu-O is 1.85 Å, separation distance is 3.7 Å for O-O and 3.02 Å for Cu-Cu [11].
Figure 1.1. Crystal structure of Cu2O. a) Cu2O unit cell, b) 4-unit cell representation [12].
3 1.1.2 Band structure and Optical transitions
In Cu2O the valence band maximum and the conduction band minimum lies at the center of the Brillouin zone, as shown in Fig 1.2. The highest valence band is split in to upper Γ7+ band and lower Γ8+ band with energy split of SO= 133.8 meV due to the spin orbit coupling. Because the lowest CB and the split valence bands have the same positive parity, so the transitions from VB (Γ7 + → Γ6+, Γ8 + → Γ6+) to lowest conduction band are dipole forbidden; whereas transition to next CB (Γ7 + → Γ8−, Γ8 + → Γ8− ) are dipole allowed [10].
The band to band direct forbidden transition Γ7 + → Γ6+ has energy of 2.173 eV which is limit of the yellow exciton series and transition from Γ8 + → Γ6+ has energy of 2.304 eV as limit of green exciton series, the direct allowed optical transition Γ7 + → Γ8− has energy of 2.624 eV as limit of blue exciton series, transition from Γ8 + → Γ8− has energy of 2.755 eV as limit of indigo exciton series at 4.2K [5] .
Figure 1.2. The band structure of Cu2O near the center of Brillouin zone and transition of the four excitonic series with spin orbit interaction (λ≠0) and when spin orbit interaction is not
accounted for (λ=0) [11].
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At the top of valence band Γ7 + has light effective hole mass that is nearly half of the electron effective mass. Both light hole and electron effective masses are isotropic and heavy holes and spin orbit hole masses are anisotropic. DFT Calculation and experimentally determined effective electron and hole masses expressed in units free electron mass (m0) along Γ-X (100), Γ-M (110), Γ-R (100) directions are given in Table 1.1 [11].
Table 1.1. Electron and hole effective masses from DFT calculation and experiments [11].
Mass Band Calculated
In p-type oxide semiconductors, oxygen 2p orbitals are strongly localized so that valence band edge has a small dispersion causing the holes to have heavy effective mass that results in low hole mobility. By introducing covalency by hybridization of d orbitals in metal cation and oxygen 2p orbitals the localized band edge is changed to extended structure, consequently the acceptor level is also lowered. The thermally activated holes can migrate within the host lattice due to the extended nature of the valence band edge independent of the doping level. Hence, it is important to find oxide that has a closed shell d orbital (to avoid coloration due d-d transition) that has the same or closer energy level so that it can form bonds oxygen 2p orbitals with considerable covalency that reduce localization and lead to more dispersive VBM. Cu+ and Ag + ions satisfy this electronic structure with closed shell d10s0 configuration [2].
The electronic structure of copper ion ends with 3d104s0 level. In Cu2O, 3d level contributes to the valence band and empty 4s level contribute to the conduction band, as shown
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in Fig 1.3. Cu 3d level is closer to O 2p orbital thus, the valence band maxima is dominated of 3d orbital, resulting in dispersed VBM and light effective hole mass [3]. While this character leads to higher hole mobility, it does not lead to origin of holes in Cu2O. The Cu2O has stable high concentration of holes and the holes originate from the point defects within the crystal [13].
Figure 1.3. Chemical bond between Cu cation with closed shell configuration and oxide ion [2].
Origin of holes in Cu2O has been mainly attributed to two types of Cu vacancy (VCu) and split Cu vacancy (VCusplit) which moves from its initial position towards to Cu vacancy, producing stable defect state by coordinating with 4 neighboring oxygen atoms. VCu produces shallow acceptor like state that can readily release holes about 0.28 eV over VBM with smallest formation energies (around 1 eV) both under Cu rich/ O poor and Cu poor/ O rich conditions, as shown in Fig 1.4. Both Cu vacancies are different regarding to the arrangement of atoms around it, but the formation energies are very close in value [14].
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Figure 1.4. Defect formation energies in Cu2O as a function of the Fermi energy EF under a) Cu rich-oxygen poor, b) Cu poor-oxygen rich conditions. (+), (–), (2–) indicate the charge state [13].
Another potential producer of holes is oxygen interstitials (Oi), there are two types of oxygen interstitials at tetrahedral and octahedral sites with almost no distinct difference in their formation energies. Oi have slightly higher formation energy than VCu in both Cu rich/ O poor and Cu poor/ O rich conditions and so the majority of holes are contributed from VCu. Moreover, the potential hole killers are O vacancy which has slightly higher formation energy than VCu but only stable in charge neutral state and Cu interstitials (Cui) have very high formation energy which makes it very unlikely appear. These characteristics make Cu2O native p-type semiconductor and very attractive candidate as p type semiconductor applications [13].
1.1.4 Oxidation of copper oxides
Copper oxide with p-type characteristics exist in three types of phase: Cupric oxide (CuO), Cuprous oxide (Cu2O) and Paramelaconite Cu4O3. In ambient conditions most stable phase is CuO and although the Cu2O phase is unstable and oxidized to CuO. The oxidation process is very slow that Cu2O is considered stable phase at ambient. Cu4O3 phase unstable at ambient atmosphere. Pressure-Temperature phase diagram of copper oxides are theoretically calculated and experimentally determined by different research groups at moderate to high temperatures and is in good agreement with each other [11, 13]. As shown in Figure 1.5, the Cu2O is only stable at low oxygen pressures (10-1–10-4 Torr) and limited temperature region (700-900oC) [15].
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Li et al. studied the oxidation and reduction of copper oxide thin films and offer some insight into equilibrium phase relationship and oxygen in and out diffusion on the phase transformation of copper oxides. They studied the reduction and oxidation of CuO, Cu2O and sputtered Cu4O3 film by annealing in vacuum (P = 210-7 Torr) and oxygen ambient in quartz tube furnace.
Their results indicate that the phase transformation from CuO to Cu2O starts at 350oC and the transformation is completed at 750oC in vacuum. At 350oC for 12 hours of annealing the two phases of the oxides still coexisted, and after annealing temperature was raised to 610oC the CuO was completely transformed to Cu2O phase. The transformation of CuO thin film on Si substrate to Cu2O was completed at 750oC for 30 min in vacuum. Cu2O was then oxidized to CuO phase completely in 15 minutes by annealing in oxygen ambient at 350oC. The rate of oxidation is much faster that reduction in copper oxides. On the other hand, phase transformation from Cu4O3
phase to CuO phase starts at 250oC in oxygen ambient and Cu2O at 340oC in annealing in vacuum both lower temperature than the phase transformation from Cu2O to CuO phase and vice versa. The study suggests the grain boundary, composition and impurity affects the transformation temperature of the CuO to Cu2O [15].
Figure 1.5. Oxygen pressure vs temperature diagram shows stability of copper and its oxide phases [15].
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1.2. Review of recent literature on Cu2O thin film fabrication.
As stated before there are many different techniques to fabricate Cu2O thin films and brief summary of the recent research on the Cu2O film is described in this section.
Thermal oxidation is one of the conventional methods for preparing high-quality oxides because of its simplicity and low cost. The pure Cu2O phases are obtained around 200-250oC and above 300oC CuO phase can be obtained [1]. In 2019, Cheon et al. reported thermal oxidation of epitaxial Cu2O film from single crystal Cu thin film by rapid thermal oxidation method. They successfully fabricated Cu2O film with hole mobility of 41cm2/Vs in temperature range of 800-900oC under argon gas with oxygen partial pressure of 210-3 Torr [5].
Vacuum based methods show the best results in fabricating high quality epitaxial Cu2O thin film with high hole mobility. The highest reported hole mobility of 256cm2/Vs in Cu2O film was fabricated with RF sputtering by Li et al. by introducing low temperature Cu2O buffer layer.
Sohn et al. has reported the Cu2O film grown by RF sputtering can be annealed in vacuum in temperature of 700oC without phase transition into Cu. The hall mobility of the film was increased from 0.02 cm2/Vs to 47.5 cm2/Vs after annealing, demonstrating that Cu2O film can be further annealed in vacuum atmosphere to improve the crystallinity of the film and reduce the strain and disorder within the film for more improved optical and electrical properties [16]. Han et al. also reported similar result in fabricating p-channel thin film transistor with Cu2O thin film grown by sputtering and post deposition annealing in vacuum [6].
Another way to produce high quality epitaxial Cu2O film is trough pulsed laser deposition method (PLD). PLD utilizes high energy laser pulse to grow epitaxial film under high vacuum and high temperature. High quality Cu2O film with Hall mobility in range of 30-107 cm2/Vs were fabricated in temperature range of 500-700oC [1]. Moreover, Farhad et al. have fabricated Cu2O thin film tunable n- and p-type properties by varying the oxygen partial pressure [17].
On the other hand, solution-based methods are attractive for large scale production of film in electronic applications. Solution based methods are wide in range like chemical bath
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deposition, electrochemical deposition, hydrothermal synthesis, sol-gel, spray pyrolysis etc. Here some of the notable results using solution-based fabrication of Cu2O thin films are described briefly.
One of the solution-based methods is spray pyrolysis. Spray pyrolysis methods utilizes a simple downward spraying of precursor solution containing Cu ions onto a heated substrate. The method is easy and quick and well suited for large scale fabrications. Nitta et al. reported Cu2O film with varying growth orientation from [111] to [110] using copper sulfate and NH3 and NaOH as reaction solution to obtain film with band gap in range of 2.05 - 2.20 eV at 70oC. the growth orientation was influenced by the ratio of NH3 and NaOH, with higher NH3 concentration leading to growth preference in [110] direction [18]. Also, Rivera et al. reported obtaining Cu2O film by spray pyrolysis at temperature range of 330-340oC. At below this temperature range, the film had Cu phase and above the 340oC the film had CuO phase. Thus the processing window for obtaining Cu2O is very narrow at ambient atmosphere [19].
Chemical bath deposition is a simple coating method by dipping a substrate into a bath of a precursor solution. But the method is not very suitable for growing film on large substrates and requires relatively large amount of precursor solution and the solution is not reused after the deposition process. But this problem has been addressed by using modified technique to reduce the amount of precursor solution used. One of the examples is reported by Altindemir et al. They fabricated Cu2O film with high hole mobility in range of 23-174 cm2/Vs using four different copper salts as precursor at 60oC using successive ionic layer adsorption method, a modified chemical bath deposition method [9].
Another of well-known solution-based method is electrochemical deposition technique.
This method deposits metal oxide film from simple metal ion complex solution. One advantage of this method is possibility of varying the growth preference of Cu2O thin film by modifying the solution pH and applied voltage during the deposition process. In 2015, Wu et al. reported fabrication of Cu2O film using electrochemical deposition method on ITO substrate with varying pH of solution to control the growth orientation of the Cu2O film from [100] to [110] to [111]
with pH value of 9.0, 10.5 and 12.3 respectively. They showed that the growth orientation of the
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film can be effectively controlled by the pH of the solution in this method [7]. However, since the electrochemical deposition method requires the use of conductive substrate the accurate determination electrical characterization of the resulting film is complicated [20].
Next, Sol-gel method is one of wet chemical techniques widely used in fabrication of metal oxide materials. The method typically utilizes metal alkoxide precursor molecules to produce solution “sol” and gradually forms the “gel” diphasic system containing both liquid and solid phase. The process runs through several stages starting with colloidal solution preparation by hydrolysis condensation and then after gelation followed by drying processes. This method can be used for producing a wide range of micro and nanostructures like particles, tubes, hollow spheres, cages etc. Also, precursors can be deposited onto various substrates to produce thin films using dip coating or spin coating method [21]. Some of the results of sol gel derived Cu2O thin films are described below. under inert atmosphere and 500oC for 1 hour under reducing conditions [8].
Nagai et al. have demonstrated the fabrication of Cu2O film by different approach, using molecular precursor Cu2+ complex with EDTA and spin coating on glass substrate, followed by a post treatment at 450oC in Ar gas, which resulted in a transparent Cu2O film with optical band gap of 2.3eV and hole mobility of 4.8 cm2/Vs [23].
Kim et al. successfully fabricated p-channel thin film transistor with sol gel processed Cu2O in two step annealing under nitrogen and oxygen atmosphere for the first time. The as-annealed film under N2 had mainly Cu phase indicating need for additional annealing under O2. The oxygen partial pressure (PO2) were 0.04, 0.2, 0.9 Torr, and Cu2O film was obtained at PO2 of 0.04 and 0.2 Torr and at 0.9 Torr, CuO phase has appeared. The resulting TFT have field effect mobility of 0.16 cm2/Vs and Cu2O film had hall mobility of 18.9 cm2/Vs [24].
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Jang et al. fabricated TFT with sol gel processed Cu2O and CuO thin film with filed effect mobility of 2.010-3 cm2/Vs and 1.010-2 cm2/Vs respectively. The Cu2O film was obtained at 200oC for 4 hours in air. Annealing time played an important role in obtaining Cu2O, when annealed for 1 hour, film was in metallic Cu phase and when annealed for 12 hours, the film consisted of Cu2O and Cu phases [25].
Yu et al. reported solution processed TFT with Cu2O film using one step vacuum annealing. The sol gel deposited films were post treated at 400 - 700oC for 2 hours under base pressure of 210-6 Torr. At 400oC the film consisted of CuO phase and as the temperature increased the CuO phase was converted to Cu2O and at 600 and 700oC, the film was fully converted to Cu2O. With increasing temperature (400-600oC) the field effect mobility was increased from 0.05 to 0.29 cm2/Vs, however at 700oC the mobility dropped to 10-2 cm2/Vs [26].
Similar results of applying vacuum annealing to improve electrical performance of Cu2O film were also reported by Han et al. after deposition of Cu2O thin film by reactive sputtering. The deposited Cu2O did not go through phase conversion after vacuum annealing at temperature up to 700oC for 10 min under base pressure of ~ 7.110-4 Torr. In contrast to results of Yu et al, after annealing at 700oC the electrical properties of the film did not deteriorate but improved due to reduction of disorder in the film at high temperature annealing, revealed by analyzing the Urbach energy of the film [6].
Sohn et al. fabricated Cu2O thin film transistor film by vacuum annealing (210-6 Torr) CuO film deposited by RF magnetron sputtering on SiO2/Si substrate at 350 and 500oC for 7 min. At 350oC the film consisted of both CuO and Cu2O phase and at 500oC the film only consisted of Cu2O. Cu2O film had hall mobility of 47.5 cm2/Vs and field effect mobility of 0.07 cm2/Vs [16].
In 2015, temperature dependent spectral studies were performed by Yu et al. They were able to obtain pure Cu2O thin film through sol gel method and rapid thermal annealing under N2
atmosphere in temperature range of 800-900oC for 15 min [27].
12 1.3. Research purpose/motivation
When fabricating thin films by sol gel method, characteristics of the film can be regulated by many factors involved in the process. The typical process of growing oxide semiconductor thin film involves precursor solution preparation, coating, and annealing. After the deposition, for formation of uniform and improving the structural quality of films high temperature post annealing step. As described previously, in oxygen rich atmosphere the annealing temperature over 300oC results in CuO phase change, while in oxygen poor atmosphere like in vacuum, under nitrogen and argon gas or reducing atmosphere then moderate to higher temperatures can be used to fabricate Cu2O. Cu2O is sensitive to phase conversion depending on the annealing environment, and difficult phase to obtain as opposed to CuO. In reviewing of recent research works on sol-gel growth of pure Cu2O, processing window for obtaining pure Cu2O can be very narrow and could require precise control of the temperature and oxygen pressure. Therefore, unlike complicated route adopted by the other research groups, such as chemically modified Cu-O structures before annealing or precise control of post-annealing temperature and duration at a relatively high vacuum above 10-4 Torr, we attempt to grow Cu2O films by post-annealing preformed CuO or non-crystallized Cu-O structures under an optimum oxygen partial pressure over the temperature range from 600 oC to 900oC. The high temperature adopted here is expected to obtain a high crystalline quality Cu2O. In other words, the purpose of this work is to introduce a simple approach to fabricate Cu2O thin film using solution method, specifically sol-gel technique with one step annealing in vacuum atmosphere in broad range of temperature.
We aimed to study the following in this work:
- Effects of the post annealing temperature, atmosphere, and duration on obtaining pure Cu2O phase.
- Determine the optimal conditions to obtain Cu2O thin film.
- Effects of the post annealing condition in the structural properties of the film, investigated by spectroscopic methods
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2. EXPERIMENTAL SECTION
2.1. Cu2O thin film growth
In this work, the precursor solution was prepared using copper nitrate trihydrate salt (Cu (NO3)2 · 3H2O) as a copper source, monoethanolamine (MEA) as a stabilizer and isopropanol (IPA) or water as a solvent. The precursor solution was prepared with 0.6 moles of copper ion:
MEA with a ratio of 1:0.5. After the addition of the solvent, the solution was stirred at 55oC for 1 hour with magnetic stirring at 450 rpm. Then, the solution was filtered through Whatman 52 filter paper and aged at room temperature for up to 7 days.
Aged precursor solution was deposited on sapphire (0001) substrate (cleaned in acetone, methanol, and water consecutively in ultrasonic bath for 10 min and dried in ambient atmosphere) by spin coating precursor solution for up to 5 layers. Preheating steps in range of 200-750oC were employed between each successive layer to evaporate the remaining organic residues. Finally, after the deposition, the samples were subjected to post annealing step at various temperatures in range of 400-900oC in vacuum atmosphere (with or without air-flow) in quartz tube furnace with heating ramp of 10oC/min.
Figure 2.1. Schematic of Cu2O deposition process.
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Detailed growth parameters of the samples are summarized in Table 2.1. and Table 2.2.
Table 2.1. Growth parameter of Cu2O thin film samples annealed in vacuum
Table 2.1. Growth parameter of Cu2O thin film samples annealed in vacuum