Electrical characteristics of solution-based IGZO/AlO
Oxide-based electronics have been well established as an alternative to silicon technology; however, typical processing requires complex, high-vacuum equipment, which is a major drawback, particularly when targeting low-cost applications. The possibility to deposit the materials by low-cost techniques such as inkjet printing has drawn tremendous interest in solution processible materials for electronic applications; however, high processing temperatures still required. To overcome this issue, solution combustion synthesis has been recently pursued. Taking advantage of the exothermic nature of the reaction as a source of energy for localized heating, the precursor solutions can be converted into oxides at lower process temperatures. Theoretically, this can be applied to any metal ions to produce the desired oxide, opening unlimited possibilities to materials’ composition and combinations. Solution combustion synthesis has been applied for the production of semiconductor thin films based on ZnO, In2O3, SnO2 and combinations of these oxides, and also for high κ dielectrics (Al2O3). All of which are required for numerous electronic devices and applications such as fully oxide-based thin-film transistors (TFTs). The properties of produced thin films are highly dependent on the precursor solution characteristics; hence, the influence of several processing parameters; organic fuel, solvent and annealing temperature was studied. Although precursor solution degradation/oxide formation mechanisms are not yet fully understood, particularly for thin films, we demonstrate that high-performance devices are obtained with combustion solution-based metal oxide thin films. The results clearly show that solution combustion synthesis is becoming one of the most promising methods for low-temperature flexible electronics.
- solution processing
- combustion synthesis
- environmentally friendly
- metal oxide materials
- thin-film transistors
- semiconductor oxides
- dielectric oxides
1.1. Solution combustion synthesis
Solution combustion synthesis (SCS) is a popular method for the preparation of a wide variety of materials due to its simplicity, broad range of applicability and the possibility of easily obtaining products in the desired composition. This method has been widely used for the development of oxide powder materials; including perovskites, spinels, ferrites; for diverse applications, such as catalysis and solid oxide fuel cells [1–3] and is becoming one of the most convenient methods for the preparation of simple and multicomponent oxides for electronic applications .
SCS is based on a redox system that requires a solution that upon heating to moderate temperatures leads to the development of a strongly exothermic redox reaction, which generally provides the energy for the metal oxide formation. The ignition temperature of the exothermic reaction is significantly lower than the final combustion temperature which results in the material formation; thus allowing the conversion of solution precursors into oxides at lower process temperatures [1–3].
The precursor solution for combustion synthesis is typically constituted by metal nitrates, which are employed simultaneously as metal source and oxidizing agents, and an organic fuel that acts as a reducing agent. However when using metal chlorides, as a metal ion source, a combustion aid is required to provide the oxidizing nitrate ions; in this case, ammonium nitrate is typically used. The most frequently applied fuels are urea, acetylacetone and glycine, amongst others, since these can form stable complexes with metal ions to increase solubility and prevent selective precipitation of the metal ions in solution. This process produces oxide materials of good compositional homogeneity, which is especially important in the synthesis of multicomponent oxides. Historically, water is the most widely used solvent for the combustion synthesis of powder materials [1–3]. Recently, the pursuit of environmental friendly solvents for electronic applications, such as ethanol and water, is growing [5–7]; however, organic solvents, including 2-methoxyethanol and acetonitrile, are currently the most widely used for the production of oxide thin films for electronic applications purposes .
The resulting solution is then heated to evaporate the solvent and when ignition temperature is reached the exothermic reaction takes place. During the combustion reaction, the fuel is oxidized by the nitrate ions. The precursor materials are converted into the metal oxide, and gaseous H2O, CO2 and N2 are the remaining products formed in the combustion reaction. This process can in theory be applied to any desired metal ion.
The solution combustion synthesis of a metal oxide can be represented by the combination of metal nitrate decomposition reaction and fuel oxidation reaction. As an example, respective equations for the combustion synthesis of Al2O3 from aluminium nitrate and using urea as fuel are given as follows [Eq. (1) and (2)] .
The overall combustion reaction can thus be written as Eq. (3)
Note that these are theoretical reaction equations that do not consider possible secondary reactions such as nitrates decomposition, urea hydrolysis, thermal decomposition and also fuel-oxidizer adducts; however, the overall reaction allows the calculation of a stoichiometric condition that can be used as a reference [3, 6].
There are many essential variables in solution combustion synthesis, including metal ion, metal precursor type and concentration, content of organic fuel, combustion aid and oxidizer/fuel proportion; which is determinant for the thermodynamics of the oxide formation .
A concept for determining the stoichiometric proportion of oxidizer and fuel for SCS was adopted from the Jain method , which is based on propellant chemistry and allows the calculation of the reducing/oxidizing valences (OV/RV) of a redox mixture. In this method, oxygen and nitrogen are considered oxidizers with the respective valence of −2 and 0. On the other hand, carbon, hydrogen and metal ions are considered as reducing elements with their final valences of +4 and +1 and the corresponding metal valence, respectively. Consequently, urea has a reducing valence of +6 (RV = 4 + 4 × 1 + 2 × 0 − 2) whilst aluminium nitrate has an oxidizing valence of −15 (OV = 3 + 2 × 0 − 3 × 6), resulting in a 15/6 (or 5/2) reducing/oxidizing valences for an optimal stoichiometric redox mixture of urea-aluminium nitrate.
The relation between the redox stoichiometry and the molar ratio of reagents can be determined by the reducing/oxidizing valences (RV/OV) of the reagents and is given by Eq. (4)
The oxygen consumption or production is controlled by the fuel/oxidizer ratio (
A fuel-lean condition is obtained when
1.2. Solution-based oxide electronics
The evolution from rigid silicon-based electronics to flexible electronics requires the use of new materials with novel functionalities that allow non-conventional, low-cost and environmental friendly processing technologies [9, 10]. Metal oxide-based electronics have been well established as an alternative to silicon technology, demonstrating exceptional electronic performance as active semiconductor components, which can be tuned for the applications, where high transparency/electrical conductivity is demanded [11, 12]. The major investment of several high-profile companies: SHARP, SAMSUNG, LG, BOE has led to the commercialization of oxide-based display backplanes in a very short period of time, and the global market is expected to increase [9, 10]. The current typical processing techniques require complex high-vacuum equipment which is a major drawback, particularly when targeting low-cost applications.
Recently, the demands for low-cost flexible electronics has led to a remarkable development of solution-based production methods and solution-processed inorganic metal oxide semiconductor materials for high-performance thin-film transistors (TFTs), and such devices have demonstrated competitive results when compared with materials obtained by physical techniques [4, 13].
Several solution-based chemical synthetic routes have been exploited for the preparation of these oxides because of their simplicity, versatility, and scale-up capability; however, the application of solution combustion synthesis to the production of thin films for TFT applications was first reported in 2011 . Since then, significant research efforts have been put on the development of semiconductor materials such as indium oxide and indium, zinc tin and gallium-based multicomponent oxides [13–16] and more recently of dielectric materials [5, 6, 17]. However, most of the research as focussed on the use of toxic solvents (2-methoxyethanol) and scarce materials (indium), which can be a major drawback in the upscaling of this technology. Consequently, the challenge remains to unveil the combination of solution combustion synthesis processing parameters that allow an environmentally friendly production of high-quality insulator and semiconductor thin films at low temperature and their combination in fully combustion solution-processed TFTs.
In this work, we focus on the environmentally friendly solution combustion synthesis of oxide-based materials for electronic applications, including insulator and semiconductor thin films by studying the influence of synthetic and processing parameters, such as fuel type; solvent and annealing temperature, on their electrical properties. Fully combustion solution-processed indium-free AlO
2. Experimental details
2.1. Precursor solution preparation and characterization
The preparation of all precursor solutions was performed in a similar manner. Typically, the metal salts were dissolved in 25 mL of solvent, either 2-methoxyethanol (2-ME, C3H8O2, Fluka, 99%), ethanol (C2H6O, Merck, 99.5%) or deionized water, to yield solutions with the desired metal ion concentration. Then, the fuel (urea; CO(NH2)2, Sigma, 98%) and ,if required, the combustion aid (ammonium nitrate; NH4NO3, Roth, 98%) were added to the prepared solutions which were maintained under constant stirring until complete dissolution. The urea to metal nitrate molar proportion was determined by the Jain method, as described in Section 1.1, to guarantee the redox stoichiometry of the reaction for each material; namely 1.6:1 for Zn2+ metal ions and 2.5:1 for In3+ and Al3+ metal ions. In the case of metal chloride precursors, a combustion aid is required and the molar proportion of metal ion:NH4NO3:urea used was 1:1:1 for tin chloride solutions.
Semiconductor ZTO precursor solutions were prepared by mixing zinc oxide and tin oxide precursor solutions in a 2:1 proportion. Zinc oxide precursor solution was prepared as described earlier in 2-methoxyethanol or ethanol to yield solutions with 0.05 M concentration. Tin oxide precursor solutions were prepared by mixing tin chloride (SnCl2⋅2H2O, Sigma, 98%), urea and ammonium nitrate to yield solutions with 0.05 M concentration. Individual solutions were magnetically stirred for 12 h before ZTO solution preparation.
Dielectric aluminium oxide precursor solutions were prepared by dissolving aluminium nitrate nonahydrate (Al(NO3)3⋅9H2O, Fluka, 98%) and urea in 2-methoxyethanol, ethanol or water, to yield solutions with 0.1 M concentration. Prior to their use, all solutions where magnetically stirred for at least 15 min and filtrated through a 0.45 µm hydrophilic filter. All reagents were used without further purification.
Thermal characterization of precursor solutions was performed by thermogravimetry and differential scanning calorimetry (TG-DSC). TG-DSC analyses were performed on precursors dried for 12 h at 80°C under air atmosphere up to 550°C with a 10°C/min heating rate in an aluminium crucible using a Simultaneous Thermal Analyzer, Netzsch (TGA-DSC—STA 449 F3 Jupiter).
2.2. Thin-film deposition and characterization
Prior to deposition all substrates (silicon wafer and soda-lime glass) were cleaned in an ultrasonic bath at 60°C in acetone for 15 min, then in 2-isopropanol for 15 min. and dried under N2; followed by a 30 min. UV/Ozone surface activation step using a PSD-UV Novascan system. Thin films were deposited by spin coating the precursor solutions at 2000 rpm for 35 s (Laurell Technologies) followed by an immediate hotplate annealing in ambient conditions; this procedure was repeated to obtain the desired thickness.
The films’ structure was assessed by glancing angle X-ray diffraction (GAXRD) performed by an X’Pert PRO PANalytical powder diffractometer using with Cu Kα line radiation (λ = 1.540598 Å) with angle of incidence of the X-ray beam fixed at 0.9°. The surface morphology was investigated by atomic force microscopy (AFM, Asylum MFP3D) and scanning electron microscopy (SEM-FIB, Zeiss Auriga Crossbeam microscope). Cross section of produced films and devices was performed by focussed ion beam (FIB). In FIB milling experiments, Ga+ ions were accelerated to 30 kV at 5 pA and the etching depth was around 250 nm. Spectroscopic ellipsometry measurements of thin films deposited on silicon substrates were made over an energy range of 1.5–6.0 eV with an incident angle of 70° using a Jobin Yvon Uvisel system to determine film thickness.
2.3. Electronic device fabrication and characterization
Metal-insulator-semiconductor (MIS) capacitors were produced by AlO
Thin-film transistors (TFTs) were produced in a staggered bottom-gate, top-contact structure by depositing AlO
The core of oxide-based electronics for displays applications are thin-film transistors (TFTs). Figure 1 shows a schematic representation of a TFT structure and respective transfer plot. TFTs are three terminal field-effect devices, whose working principle relies on the modulation of the current flowing in a semiconductor placed between two electrodes (source and drain). A dielectric layer is inserted between the semiconductor and a transversal electrode (gate), being the current modulation achieved by the capacitive injection of carriers close to the dielectric/semiconductor interface, known as field effect .
The semiconductor and gate insulator are equally important material components in field-effect transistors; consequently, the development of both materials by solution techniques is essential .
3.1. Amorphous semiconductor oxides
Amorphous oxide semiconductors (AOSs) have drawn significant attention in the field of flat panel displays during the last decade, due to their high carrier mobility when compared to amorphous silicon (a-Si). Indium-gallium-zinc oxide (IGZO) is the most explored semiconductor due to its superior field effect mobility and enhanced electrical performance [11, 12]. However, alternative semiconductor materials that rely on abundant and non-toxic elements are required due to environmental demands. Zinc-tin-oxide (ZTO) is a promising indium and gallium-free alternative and impressive results have already been obtained in TFTs applications [19, 20].
Several solution-processed ZTO-based TFTs have been reported; however, processing temperature above 400°C and a toxic organic solvent, such as acetonitrile, 2-ethoxyethanol or 2-methoxyethanol are usually required . The use of non-toxic solvents has been pursued and ethanol  and water  based ZTO TFTs have already been reported.
The overall ZnO formation reaction can thus be written as follows Eq. (7).
Using the Jain method, the reducing valence of urea is +6 and the oxidizing valence of zinc nitrate is –10. In order to assure the redox stoichiometry 5/3 (or 1.6), moles of urea are required per mole of zinc nitrate.
The tin oxide formation can be represented by the following equation:
In the combustion synthesis of tin oxide, a combustion aid, ammonium nitrate, was added to the precursor solution as commonly performed for chlorine-based metal salts. According to Jain’s method, the oxidizer valence of ammonium nitrate (NH4NO3) is −2 and to achieve
ZTO precursor solution was prepared after mixing individual of zinc and tin oxide precursor solutions of 0.05 M concentration in a 2:1 proportion, respectively. Thermal analysis of ZTO precursor solutions was performed to investigate the influence of solvent on their decomposition behaviour. TG-DSC measurements were performed on precursors dried for 12 h at 80°C. Figure 2 shows the DSC results for ZTO precursors up to 350°C as above this temperature no further events were observed.
The combustion reaction of the organic fuel with the metal nitrates typically leads to an intense exothermic peak with corresponding abrupt mass loss. For ZTO precursors, these were observed at 125°C and 108°C for 2-ME and ethanol-based solutions, respectively. A smaller peak is observed at 275°C which can be related to secondary reactions [3, 4] that can occur during thermal decomposition of the reagents.
The precursor solution degradation mechanism is expected to be different in bulk and thin-film form which explains the need for higher thin-film processing temperature than the one determined by thermal analysis. Devices annealed at temperatures below 350°C did not show effective gate modulation as such, the processing temperature was fixed at this temperature although the minimum temperature required for full degradation of the precursors is 275°C.
Structural characterization of the films (Figure 3a) indicates that amorphous films are obtained up to 350°C for both precursor solution solvents, as no diffraction peaks were observed. Morphological surface analysis show that smooth and uniform films are obtained regardless of solvent; however, 2-ME-based ZTO films demonstrate a lower surface roughness, 0.2 nm, when compared to ethanol-based films; 5 nm.
Combustion solution-based ZTO TFTs were produced by spin-coating precursor solutions onto commercial Si/SiO2 substrates. Electrical characterization of TFTs was performed by measuring the transfer and output characteristics of the devices in ambient conditions in the dark. Figure 4 shows a cross section SEM image of ZTO TFTs produced on Si/SiO2, with Al source-drain electrodes, obtained after FIB milling and electrical characterization of 2-ME and ethanol-based ZTO TFTs.
The analysis of the electrical characteristics reveals that ZTO TFTs show an
3.2. Dielectric oxides
Insulator materials suitable for TFT gate dielectric applications must fulfil some criteria, namely i) high bang gap, with favourable conduction band offset to avoid high gate leakage; ii) high dielectric constant (high-κ) the added capacitance can compensate interface traps, thus improving the transistor performance and allow low operating voltage; iii) good interface properties, which can be achieved using amorphous dielectrics [11, 22, 23].
Solution-based high-κ dielectrics, such as Al2O3, HfO2, ZrO2, have demonstrated high performance and suitability for the application in metal oxide semiconductor-based TFTs [5, 6, 17, 24–28]. Amongst these, aluminium oxide is one of the most developed materials from solution synthesis since several aluminium precursors salts are readily availability at low cost. The influence of processing parameters on the solution combustion synthesis of aluminium oxide was studied.
The decomposition reactions for aluminium, urea and the overall aluminium oxide formation reaction are already represented by Eq. (1), (2) and (3), respectively. According to Jain’s method and to ensure redox stoichiometry, 2.5 mol of urea per mole of aluminium nitrate were used to prepare aluminium oxide precursor solutions with 0.1 M concentration. The influence of several processing parameters such as fuel, solvent and annealing temperature on the properties of dielectric AlO
Thermal analysis of AlO
Thermal behaviour of AlO
Thin films of AlO
Generally, structural and morphological properties of AlO
The effect of processing parameters on the electrical properties of these films was studied by assessing the electrical performance of capacitors and thin-film transistors comprising the solution-based AlO
The electrical characteristics of AlO
IGZO TFTs comprising combustion solution-based AlO
TFT performance is assessed trough the analysis of the turn-on voltage (
|2-ME||350||−0.36||1 × 105||17.3|
|250||−0.30||8 × 104||9.6|
|200||−0.25||2 × 104||13.5|
|Water||350||0||5 × 105||9.2|
|250||−0.10||5 × 105||6.9|
|200||0||7 × 103||12.9|
Despite the non-ideal capacitance-voltage characteristics obtained for low-temperature AlO
3.3. Fully solution-based oxide TFTs
The realization of printed electronics requires all solution-based devices; consequently, semiconductor and dielectric materials must be combined in TFTs. Fully combustion-solution-based TFTs comprising the developed dielectric and semiconductor materials have been successfully fabricated. Figure 9 shows transfer characteristics of these devices [5, 6].
The combination of solution-processed dielectric and semiconductors yielded TFTs with a good overall performance, demonstrating very low clockwise hysteresis, close to zero
|Ref||Solvent||Gate dielectric||T (°C)||Mobility (cm2 V−1 s−1)|
The use of indium-free semiconductor materials, such as ZTO and GZTO, although environmentally relevant somehow lead to a compromise of solution-processed TFTs performance both in saturation mobility and processing temperature.
Fully solution-based TFTs were obtained at maximum processing temperature of 200°C when combining the developed 2-methoxyethanol-based AlO
The application of solution combustion synthesis to prepare oxide materials for electronic devices was successfully achieved for dielectric and semiconductor thin films using varied processing parameters. We have clearly demonstrated that dielectric AlO
This work was partly funded by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT—Portuguese Foundation for Science and Technology under the projects POCI-01-0145-FEDER-007688, Reference UID/CTM/50025 and EXCL/CTM-NAN/0201/2012; European Community FP7 2007-2013 project i-FLEXIS Grant Agreement n. 611070. D. Salgueiro acknowledges FCT-MEC for doctoral grant SFRH/BD/110427/2015. Authors would like to acknowledge J. V. Pinto for XRD, D. Nunes for SEM-FIB and T. Sequeira for AFM measurements.