Solution-processed thin film transistors incorporating YSZ gate dielectrics processed at 400°C

This work investigates a solution process for yttria stabilized zirconia (YSZ) thin film deposition involving addition of yttria nanoparticles, at 400 °C, in air. Different yttrium doping levels in the YSZ were studied and a wide range of optical, structural, surface, dielectric and electronic transport properties were also investigated. An optimum yttrium doping level of 5 % mol. resulted in the smoothest films (R RMS ~ 0.5 nm), a wide band gap ( ~ 5.96 eV), a dielectric constant in excess of 26, and leakage current of ~ 0.3 nA cm −2 at 2 MV/cm. The solution processed YSZ films were incorporated as gate dielectrics in thin films transistors with solution-processed In 2 O 3 semiconducting channels. Excellent operational characteristics, such as negligible hysteresis, low operational voltages (5 V), electron mobility in excess of 36 cm 2 V −1 s −1 , high on/off current modulation ratio on the order of 10 7 , and low interfacial trap density states (<10 12 cm -2 ) were can used to and of

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A. THIN FILM DEPOSITION AND COMPOSITION
YSZ films with a range of yttrium doping levels were made by depositing different mixtures of zirconium(III) 2,4-pentanedionate and dispersed Y2O3 nanoparticles. The zirconium(IV) 2,4pentanedionate (Alfa Aesar) was dissolved in methanol at a concentration of 38 mg/ml and was stirred at room temperature for about 5 hours to ensure complete dissolution of the precursor. The yttrium oxide nanoparticles of average particle size in the range between 25 and 40 nm (Sigma Aldrich) were dispersed in 0.1 M HCl acid to prepare a 30 mg/ml dispersion that was stirred at room temperature for 12 hours to ensure an agglomeration-free dispersion. Blends of the aforementioned solutions in different ratios (to result in different yttrium to zirconium ratios) were spray coated onto fused silica, glass and ITO-coated glass (sheet resistivity of 10 Ohms/sq.) substrates at 400 o C using a pneumatic airbrush with a nozzle size of 0.3 mm. The airbrush was held at a vertical distance of about 30 cm above the substrate and after a 10 s spray coating period the spray process was interrupted for 60 s to allow the vapors to settle. The cycle was repeated until films of ~100 nm thickness were obtained. This process yielded the best film homogeneity over a 16 cm x 16 cm deposition area. For the TFT device studies that are described later in the manuscript, semiconducting channels of In2O3 were deposited onto the glass/ITO/YSZ stacks at 400 °C. The films were spray coated rom 0.05 M indium chloride (InCl3) solutions in methanol and films of ~ 10 nm thickness were obtained.
Au and Al source and drain contacts for making the metal-insulator-metal devices from glass/ITO/YSZ and glass/ITO/YSZ/In2O3 stacks were thermally evaporated under high vacuum (10 -7 mbar) through a shadow mask. The typical metal contact areas were ~ 0.01 mm 2 and thicknesses ~ 60 nm.

B. CHARACTERIZATION
UV-Vis absorption spectroscopy measurements of YSZ films on fused silica were conducted at wavelengths between 180 nm and 1000 nm using an Agilent Cary 5000. The microstructures of the YSZ films on ITO-coated glass was analyzed by Grazing Incidence XRD (GIXRD) experiments (with a grazing incidence angle of 0.6 o ) that were conducted using a Rigaku SmartLab diffractometer with CuKα radiation operating at 45 kV and 200 mA, a Ge(220)x2 monochromator, and a DteX250 1D detector.
The surfaces morphologies of spray coated YSZ onto ITO-coated glass were characterized by contact mode atomic force microscopy (AFM) measurements undertaken in ambient conditions using a Bruker Nanoscope V system and Multimode low noise head using force modulation cantilevers. Also, to further investigate the stacks' interfaces ultrasonic force microscopy (UFM) measurements were carried out under the same conditions using a modified version of Bruker Nanoscope V system. The high resolution UFM measurements were performed using a 4.5 MHz carrier frequency and 2.7 kHz modulation frequency with deflection signal acquired via custombuilt interface. Prior to UFM, the /ITO/YSZ/In2O3/Al stacks underwent Beam-Exit Cross-Sectional Polishing (BEXP) on an angled holder (3° slope) with an overhang of ~300 μm. The beam entry surface was then filed down normal to the beam direction with decreasing size 30 m through a 1 μm diamond impregnated film. The holder was placed within the vacuum chamber and beam-exit cross-sectional polishing was initiated at a vacuum of 2.0 × 10 −5 mbar at a 7 kV accelerating voltage for 10 hours followed by 1 kV for 15 min polishing. Once beam-exit occurred, the voltage was decreased to 1 kV for 20 min to polish the surface. The process resulted in a near-atomically flat cut at approximately 11° through the area of interest with respect to the sample surface through the area of interest. Prior to scanning probe imaging, samples were cleaned in an ultrasonic bath using acetone and methanol for 10 minutes and dried with nitrogen.
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0079195
6 Admittance spectroscopy measurements on Metal-Insulator-Metal stacks (i.e. glass/ITO/YSZ/Al) for different Y2O3 to ZrO2 molar ratio were performed using a Wayne Kerr 6550B precision impedance analyzer at frequencies between 1 kHz and 50 MHz applying a 25 mV AC voltage.
Bottom-gate, top-contact thin film transistors characterization was carried out under vacuum (10 -3 mbar), at room temperature using an Agilent B1500A semiconductor parameter analyzer. Device parameters including electron mobility and interfacial trap density were extracted from the transfer curves in both the linear and saturation regime using the gradual channel approximation.

III. RESULTS AND DISCUSSION
The band gaps of the YSZ films were determined by Tauc plots, as illustrated in Figure S1. The plots indicate a direct band to band transition for the different yttrium doping levels. Figure 1a shows that the fundamental absorption edge of the YSZ films shifts from 5.76 eV to 5.95 eV with an increase of the yttrium content up to a maximum for an yttrium doping level of about 5 mol.% indicating a cubic phase of YSZ. [71] The linear dependence in the semi-logarithmic representation ( Figure S1) reveals an Urbach-type behavior, as previously reported for YSZ. [72] An Urbach-type behavior is attributed to a structural or thermal disorder causing an exponentially decaying density of localized states at the band edge.

Figure 1b
clearly illustrates the increase of the structural disorder with increased yttrium doping levels, as previously reported for YSZ. [73] An increase in structural disorder has been linked to a reduced number of oxygen vacancies in the ZrO2 lattice [72,74] and hence this is expected here for high yttrium doping levels.
The XRD diffraction patterns of YSZ films on ITO-coated glass with different yttrium doping levels is depicted in Figure 2a, whereas those on c-Si are illustrated in Figure S2.
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7
XRD patterns of the different composition YSZ films show the co-existence of both tetragonal and cubic phases as evidenced by the presence of two well-discriminated peaks at about 2θ=30.8° and 2θ=31.5° characteristic of tetragonal (JCPDS 83-113) and cubic (JCPDS 82-1246) ZrO2 respectively. A monotonic increase of the (111) reflection, characteristic of cubic ZrO2 is observed with increasing yttrium doping level up to 5 mol.% [75] Plotting relative intensity ratio of this (111) reflection from the cubic phase to the tetragonal (011) peak (Figure 2b), confirms that the cubic phase of YSZ dominates up to ~5 mol% yttrium doping. As observed in Figure 2b the fraction of cubic ZrO2 is inversely correlated with the average crystal size as has been shown before. [76] Similar results were found for YSZ films that were simultaneously spray coated on c-Si  Notably, pure ZrO2 films (Figure 3d) shows a laminar structure. Such features however are less evident for YSZ films with higher yttrium doping level (Figure 3e and 3f). Also, the quality of the interface between the YSZ and ITO (Figure 3f) deteriorates at high yttrium content and this degrades the device performance, as we show later. Nevertheless, the inset in Figure 3d, 3e and This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. Investigations of the nanomechanical properties of the films using UFM showed significantly higher effective elastic modulus of the YSZ film with yttrium doping level of 5 mol % compared to the pure ZrO2. This confirms stabilization of the stiffer cubic phase of ZrO2 with 5 mol. % yttrium doping as has theoretically been predicted before. [75,77] However, more detailed analysis is outside the scope of the present study.

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Susceptance and conductance dispersions as well as Nyquist plots in the frequency range between 1 kHz and 15 MHz of representative YSZ dielectrics in metal-insulator-metal devices are illustrated in Figure S3. The plots reveal stable systems whose equivalent circuit consists of a capacitor with high shunt and low series resistances.
The geometric dielectric constant at 1 kHz (calculated from Bode plots that are shown in Figure 4) as a function of the yttrium doping level is shown in Figure 5a showing a peak in the dielectric constant for YSZ with 5 mol. % yttrium doping. This peak corresponds to the highest fraction of the cubic YSZ phase.

Figure 5b
shows the band gap versus the static dielectric constant of solution-processed gate dielectric films along with a large number of dielectric films deposited using vacuum-based techniques, [8,[78][79][80][81][82][83][84] demonstrating the attractive positioning in terms of band gap and dielectric constant of our YSZ.
Also, as shown in Figure 5c, a minimum leakage current density for the highest dielectric constant is ~0.3 nA/cm 2 at an electric field of 2 MV/cm further confirms the deposition of YSZ films of superior high dielectric strength and dielectric constant compared to that of other dielectrics deposited from solutions [8,25,79,81,[84][85][86][87][88][89][90] . The same applies to the dielectric constant values 9 of those films deposited by vacuum based techniques on various substrates and substrate temperatures. [27,57,[91][92][93][94][95][96] The Poole-Frenkel plots for YSZ with different yttrium doping levels are shown in Figure   6 indicating the Poole-Frenkel effect as the dominant conduction mechanism at room temperature, an expected finding for such relatively thick and low leakage current insulator. [97] Next, the performance of the YSZ as a gate dielectric in bottom-gate top-contact thin films transistors was explored. The In2O3 semiconducting channel layer was previously characterized independently and found to be the cubic phase with lattice constant a of 10.0949 Å and band gap of 3.58 eV [80] (Figure S4).
A set of transfer characteristics (operating at saturation) are illustrated in Figure 7a. As illustrated in Figure 7b and Figure 7c, a high electron mobility of ~36 cm 2 /Vs and high on/off current modulation ratio, in excess of 10 7 , were measured for the TFTs incorporating YSZ with 5 mol.% yttrium doping. An interfacial trap density of ~10 12 cm -2 was measured for the TFT at the optimal 5 mol.% yttrium level, again indicative of a high-quality interface between the YSZ and In2O3. The variation of the conduction threshold (Figure 7d) is consistent with both the variation in the dielectric constant, k and Dit. The variation of the onset-of-conduction or conduction threshold is predominantly determined by the voltage drop across the YSZ gate-dielectric (Vins) and the voltage drop across the YSZ-semiconductor interface (Vint). The latter is a function of the interface state density (Dit) whereas the former a function of the dielectric constant, k. As seen in Fig. 7d, Dit (and hence Vint) reaches its minimum and k its highest value at an yttrium doping level of 5 mol.% as seen in Figure 5a, giving rise to a low conduction threshold.
In summary, we fabricated YSZ thin films with a range of yttrium concentrations, by spraying pyrolysis of blends of metalorganic precursor solutions of zirconium (IV) 2,4pentanedionate) with Y2O3 nanoparticles dispersions in air at 400°C. An optimum composition This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0079195
corresponding to 5 mol. % of yttrium in YSZ gives a wide band gap (5.96 eV), high dielectric constant (26), low leakage current (0.3 nA cm -2 at 2 MV/cm), and smooth surface (Rrms 0.5 nm), over a large substrate area of ~256 cm 2 . Furthermore, In2O3-based TFTs were fabricated by spraying an In2O3 semiconducting channel onto the optimum YSZ dielectric film. The TFTs showed excellent characteristics, such as high electron mobility (36 cm 2 V −1 s −1 ), high on/off current modulation ratio (10 7 ) low interfacial trap density states (<10 12 cm -2 ), enhancement mode operation and negligible hysteresis. Finally, the high quality YSZ films made using the simple, low temperature and low-cost process could have potential applications in thermal barrier coatings, fuel cells, and other electrochemical devices.

Supporting Information
Supporting Information is available from the AIP Publishing or from the author. The data that support the findings of this study are available from the corresponding author upon reasonable request.