As discussed before, one of the approaches for the active semiconducting layer fabrication is to use a precursor solution. For this method, the metal-oxide semiconductor is syn-thetized using a metal precursor in solvent. After its deposition on the template, the film is pre-annealed in order to remove the solvent and post-annealed to form the active semi-conducting layer. During the annealing step, the main processes for the achievement of an active semiconductor are the precursor decomposition followed by a hydrolysis process where the metal is bonded to hydroxyl groups. The final step is the dehydroxylation and alloying of the film; the hydroxyl groups are removed, the metal is bounded to oxygen, and neighboring metal-oxide molecules become interconnected [KYK14]. For amorphous metal-oxide systems as GIZO and IZO, which present more than a single metal type in their chemical compositions, the same steps occur [KYK14]. However, the initial solution is a combination of all precursors. For GIZO systems, for example, the precursor contents
Sensor Technology Department Semiconductor 3.2
Figure 3.6: Schematic diagram of the thermogravimetric analyses of nitrate-, acetate- and chloride-based precursors. Adapted from [KYK14].
indium, gallium and zinc precursors and its electrical characteristics depend on the partial concentration of each one of them [KDS+09, JHM+10, FKD+12].
The choice of which precursor is used for the synthesis also plays an important role in the integration process itself as it influences the required annealing temperature, as well as the chemical compatibility of the solution with the substrate or with previously deposited layers. Precursors for metal-oxide systems are commonly nitrate-, acetate- or chloride-based. The temperature necessary for an adequate synthesis of the semiconduc-tor can be evaluated by a thermogravimetric analysis. Figure 3.6 depicts a schematic diagram showing the requirements for each precursor group. Based on the synthesis tem-perature and on the flexible substrates limitations, acetate- and nitrate-based precursors were evaluated.
Sol-gel processes of metal-oxide compounds are commonly defined as the formation of an oxide network originated by a polycondensation of molecular precursors [HW90].
For the achievement of ZnO, commonly a zinc salt is required, and depending on its characteristics (precursor type), the reaction temperature is influenced as prior discussed.
First experiments were performed employing zinc acetate (Zn(Ac)2) as zinc salt. Generally the Zn(Ac)2 is dissolved in an alcoholic or another organic solvent and subsequently an alkaline solution is added to it [OKY97, YI02, HFKI04]. The routine used in this study
3 Integration Paderborn University
is based on the work of Hosono et al. [HFKI04] in which a non-basic solution method was described for the preparation of ZnO precipitates. The general reaction is described as:
5 (Zn(Ac)2·2 H2O) −−→ Zn5(OH)8(Ac)2·2 H2O + 8 AcH −−→ 5 ZnO + 10 AcH + 5 H2O.
(3.1) For the reaction accomplishment, the following chemicals were mixed. First, 5.5 g of Zn(Ac)2 was dissolved in 25 ml of deionized water7. Then, the solution was heated to 60◦C under vigorous stirring and, subsequently, 1.9 g of 2-methoxyethanol (2-ME) was dropwisely added to the mixture. Afterwards, water was added until the total volume of the precursor reached 50 ml. After 1 h under vigorous stirring, the aqueous solution was deposited onto the sample by spin-coating technique. It was deposited on the wafer during the low-spin phase at 800 rpm followed by a high-spin step of 3000 rpm for 30 s at room temperature. Although the high-spin phase influences the thickness of the final layer, variations from 2000 rpm to 4000 rpm have not significantly changed the electrical performance of the integrated devices. Immediately after the solution deposition, the sample was soft-baked at 115◦C on a hot-plate for 5 min and then baked in a convection oven at 200◦C for 1 h. Due to the temperature difference between the solution and the substrate during the deposition, the formation of a reliable film was not fully achieved.
The deposited films possessed a high density of cracks, particles and agglomerations, and the material did not cover the whole sample. Additionally, the relative low temperature employed was not sufficient to achieve a complete synthesis reaction of the zinc acetate pre-cursor, as already expected taking into account the thermogravimetric analysis [KYK14]
and the TFTs characteristics described in Section 4.1. Moreover, in order to increase the interparticle connection quality, the solution was deposited onto a nanoparticulated film or mixed with a dispersion containing ZnO nanoparticles [LJK+07, LJJ+08]. The electrical characteristics of the transistors produced in this way were evaluated in Section 4.2.
Aiming at a low synthesis temperature, zinc nitrate-based precursors were used as they possess advantages regarding their thermal compatibility to flexible substrates. The precursor synthesis is based on the work of Meyers et al. [MAH+08], where an ammine-hydroxo zinc ink for ZnO TFTs is reported. This method uses simple cation hydration chemistries and highly reactive aqueous precursors to obtain a high quality ZnO with
7 In this study, deionized water using reverse osmosis with minimum resistivity of 18 MΩ was used in all steps and from now on the term water refer to deionized water. If for a particular reason another water type (e.g. distilled or non-purified) is used, the term water is clarified.
Sensor Technology Department Semiconductor 3.2
crystallization at low temperatures. Moreover, it avoids the use of metal-organic com-pounds, which require a high activation and diffusion energy, and employs an all inor-ganic hydroxo-condensation [BPS06, MAH+08]. As a result of the weak acidity of the ZnO2+ ions, the energy required for the dehydration of the Zn(OH)2 and for the oxide crystallization is much lower than for In(OH)3 and Sn(OH)4 [GGKL67, Sat05]. For these reasons, the fabrication of pure ZnO through this method is promising. Conjointly, the use of ammonia adds the advantage of its extreme volatility and labile bonding affording a low-temperature, rapid and low volume-loss decomposition process in comparison to other nitrogen-based ligands.
The following routine has the goal to provide a ZnO film with low impurity concentra-tion as well as to maintain a low synthesis temperature. Due to the polarizaconcentra-tion of the NO3– charge cloud by acidic metal cations, the purity of the initial soluble Zn(NO3)2 is important for the achievement of a low-temperature process. For this reason, 99.998 % pure Zn(NO3)2 purchased from Alfa Aesar Co. was used. The zinc salt was dissolved in water to a total concentration of 0.5 M Zn. Under vigorous stirring 10 ml of 2.5 M NaOH was slowly added to 15 ml of the solution during the course of 5 min. The reaction can be expressed as:
Zn(NO3)2(aq) + 2 NaOH(aq) −−→ Zn(OH)2(s) + 2 NaNO3(aq). (3.2)
The hydroxide slurry was then centrifuged and the supernatant removed. 20 ml of water was added and stirred for 3 to 5 min, this step was followed by another centrifugation and supernatant removal. The process was repeated 5 times in order to reduce Na+and NO3–
contaminations caused by incomplete reaction. After a final centrifugation, the precipitate was dissolved in 50 ml of 25 % aqueous ammonia to form the precursor as:
Zn(OH)2(s) + x NH3(aq) −−→ Zn(OH)2(NH3)x(aq). (3.3)
Although ZnO could be dissolved directly in ammonia, a complete dissolution of large-grain ZnO powder is difficult to be achieved because of the kinetic obstacles involving the metal oxides [MAH+08]. Additionally, the simple dissolution of zinc salt in ammonia also degrades the low temperature synthesis as the precursor contains non-basic counterions [MAH+08]. In order to avoid this effect and to improve the dissolubility, fresh Zn(OH)2
precipitates should be dissolved in ammonia. This process is already used for more than 90
3 Integration Paderborn University
years for the synthesis and purification of bulk ZnO [DJ27]. Finally, after the deposition of the precursor on the sample, the following reaction is expected:
Zn(OH)2(NH3)x(aq) −−→ ZnO(s) + x NH3(g) + H2O(g). (3.4)
If the initial compounds are free of contaminations and the solution is properly puri-fied during the rinse and centrifugation steps, the dehydration and oxide crystallization occur at temperatures below 100◦C [BAL94, MAH+08] enabling the process to flexible substrates.
In order to decrease the chemical complexity and to avoid multiple steps during the centrifugation and supernatan removal, Theissmann et al. have proposed an alternative synthesis process [TBS+11]. For this reaction, commercially available zinc oxide hydrate (ZnO · x H2O) was directly dissolved in ammonia. The solubility of the compound in am-monia was reported as sufficient due to the crystal water, and the final precursor solution is similar to the one previously described [TBS+11]. On the other hand, this reduced synthesis process requires a higher annealing temperature or annealing in a specific atmo-sphere to achieve transistors with adequate electrical characteristics [TBS+11, BSTS12].
To reduce contamination and to maintain the crystallization of the zinc oxide at lower temperatures for the process to be suitable to flexible substrate, this synthesis method was not applied but rather the procedure proposed by Meyers et al.
For the deposition of the zinc nitrate precursor, spin-coating technique was used. The stock precursor was filtered by 0.45 µm PTFE syringe filter and deposited on a steady wafer prior to the low-spin phase of the template at 800 rpm for 7 s and a high-spin step of 3000 rpm for 30 s. Immediately after the precursor deposition, the wafer was soft-baked at 115◦C on a hot-plate for 5 min. In order to achieve a thicker layer, the process was repeated up to 5 times. The sample was then baked in a convection oven at 150◦C for 1 h to ensure homogeneity and a complete crystallization of the layer.
When SiO2 was used as gate dielectric, the precursor has shown a good wetting to the substrate and a uniform film could be observed. By increasing the number of the precur-sor depositions, the layer uniformity is reduced, nonetheless the film is still homogenous.
One of the drawbacks of the employment of precursors for the achievement of active semi-conducting layers is the strict and necessary use of specific chemicals in its composition.
As the precursor used contains ammonia and oxidation elements, the deposition of the
Sensor Technology Department Semiconductor 3.2
ammine-hydroxo zinc solution on the high-k nanocomposite (explained in Section 3.3.2) damages this dielectric. Furthermore, material agglomerations and formation of clusters on the film were also observed. This effect prevails the full use of this precursor on flexi-ble substrates when the high-k dielectric is used without a protection layer, for instance.
Additionally, as the precursor reacts with aluminum (metal used for the drain and source electrodes), the employment of different transistors setups is limited or the transistor per-formance is reduced due to an unstable contact between the metal electrodes and the semiconducting layer. The electrical performance of the transistors integrated using the nitrate-based precursor is discussed in Section 4.1.