Synthesis, oxide formation, properties and thin film transistor properties of yttrium and aluminium oxide thin films employing a molecular-based precursor route

Abstract

Combustion synthesis of dielectric yttrium oxide and aluminium oxide thin films is possible by introducing a molecular single-source precursor approach employing a newly designed nitro functionalized malonato complex of yttrium (Y-DEM-NO₂1) as well as defined urea nitrate coordination compounds of yttrium (Y-UN 2) and aluminium (Al-UN 3). All new precursor compounds were extensively characterized by spectroscopic techniques (NMR/IR) as well as by single-crystal structure analysis for both urea nitrate coordination compounds. The thermal decomposition of the precursors 1-3 was studied by means of differential scanning calorimetry (DSC) and thermogravimetry coupled with mass spectrometry and infrared spectroscopy (TG-MS/IR). As a result, a controlled thermal conversion of the precursors into dielectric thin films could be achieved. These oxidic thin films integrated within capacitor devices are exhibiting excellent dielectric behaviour in the temperature range between 250 and 350 °C, with areal capacity values up to 250 nF cm^-2, leakage current densities below 1.0 × 10^-9 A cm^-2 (at 1 MV cm^-1) and breakdown voltages above 2 MV cm^-1. Thereby the increase in performance at higher temperatures can be attributed to the gradual conversion of the intermediate hydroxy species into the respective metal oxide which is confirmed by X-ray photoelectron spectroscopy (XPS). Finally, a solution-processed Y _x O _y based TFT was fabricated employing the precursor Y-DEM-NO₂1. The device exhibits decent TFT characteristics with a saturation mobility (μ _sat) of 2.1 cm² V^-1 s^-1, a threshold voltage (V _th) of 6.9 V and an on/off current ratio (I _on/off) of 7.6 × 10⁵.

Fig. 1. (a) Schematic illustration of the synthesis of bis(diethyl-2-nitromalonato) nitrato yttrium(iii) (Y-DEM-NO₂) 1 and (b) and (c) showing the reaction scheme for the formation of the metal urea compounds of yttrium 2 and aluminium 3.

Fig. 2. ORTEP plot of the molecular structure of Y-UN 2. Vibrational ellipsoids are drawn at the 50% probability. O–Y bond length are in the range 222–226 pm for the urea ligand and 243–249 pm for the nitrate ligand; O–Y–O-bond angles are 90 ± 5° for the urea ligand.

Fig. 3. (a) Thermo gravimetric analysis of 1, 2 and 3 in an oxygen atmosphere and (b–d) differential scanning calorimetry (DSC) for 1, 2 and 3.

Fig. 4. MS intensities of (a) Y-DEM-NO₂1, (b) Y-UN 2 and (c) Al-UN 3 for m/z⁺ peaks corresponding to the TG curves in Fig. 3 respectively.

Fig. 5. Gram–Schmidt intensities (a–c) of 1, 2 and 3 and the corresponding IR signal intensities (d–f) according to the Gram–Schmidt signals of the precursors, respectively.

Fig. 6. (a) and (b) XRD patterns of solution processed Y_xO_y from precursor 1 and 2 annealed at 200 °C, 250 °C, 300 °C and 350 °C. (c) XRD patterns of solution processed Al_xO_y from precursor 3 annealed at 200 °C, 250 °C, 300 °C and 350 °C. (d–f) TEM images of Y_xO_y and Al_xO_y from precursor 1, 2 and 3 prepared at 350 °C.

Fig. 7. (a–c) IR spectra of the thermal transformation of the precursors 1, 2 and 3 into the respective metal oxides at various temperatures.

Fig. 8. (a) O 1s XPS core spectra of samples obtained from Y-DEM-NO₂ precursor 1 annealed for 2 hours each at 200 °C, 250 °C, 300 °C and 350 °C. (b) Atomic concentrations of oxygen (M–O) and hydroxyl as well as carbonate species (M–OH, M–CO₃), related to the total oxygen content and derived from the O 1s XPS spectra of 1 annealed at various temperatures.

Fig. 9. O 1s XPS core spectra of samples obtained from Y-UN precursor 2 annealed for 2 hours at 200, 250, 300 and 350 °C (a) and (b) after 120 s sputtering (with cluster of 300 atoms with 8 keV energy). (c) Atomic concentrations of oxygen (M–O) and hydroxyl as well as carbonate species (M–OH, M–CO₃), related to the total oxygen content and derived from the O 1s XPS spectra taken from the surface as well as (d) in the sub surface layers close to the bulk and annealed at various temperatures.

Fig. 10. (a) O 1s XPS core spectra of samples obtained from Al-UN precursor 3 annealed for 2 hours each at 200 °C, 250 °C, 300 °C and 350 °C. (b) Atomic concentrations of oxygen (M–O) and hydroxyl species (M–OH), related to the total oxygen content and derived from the O 1s XPS spectra of 3 annealed at various temperatures.

Fig. 11. Schematic illustration of the prepared capacitor. The grey layer represents the glass substrate, the purple layer represents a 140 nm ITO film serving as the bottom gate electrode and the blue layer illustrates the Y_xO_y dielectric film. The circular top electrodes are composed of 50 nm gold and on the left-hand side is a 100 nm gold sacrificial contact.

Fig. 12. Capacitance vs. frequency curves of solution processed Y_xO_y and Al_xO_y generated from (a) 1, (b) 2 and (c) 3 annealed at different temperatures. Leakage current density vs. electric field behavior of Y_xO_y and Al_xO_y generated from (d) 1, (e) 2 and (f) 3 annealed at different temperatures.

Fig. 13. Electrical characterization of TFT based on the Y_xO_y-(1)-350 dielectric and a solution processed indium zinc oxide (IZO) semiconductor processed at 350 °C. (a) Transfer characteristics of the device at V_DS = 15 V. (b) Output characteristics of the device measured at 10 V, 15 V and 20 V, respectively.