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High Electron Mobility Thin ‐ Film Transistors Based on Solution ‐ Processed Semiconducting Metal Oxide Heterojunctions and Quasi ‐ Superlattices

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ABSTRACT

High mobility thin‐film transistor technologies that can be implemented using simple and inexpensive fabrication methods are in great demand because of their applicability in a wide range of emerging optoelectronics. Here, a novel concept of thin‐film transistors is reported that exploits the enhanced electron transport properties of low‐dimensional polycrystalline heterojunctions and quasi‐superlattices (QSLs) consisting of alternating layers of In2O3, Ga2O3, and ZnO grown by sequential spin casting of different precursors in air at low temperatures (180–200 °C). Optimized prototype QSL transistors exhibit band‐like transport with electron mobilities approximately a tenfold greater (25–45 cm2 V−1 s−1) than single oxide devices (typically 2–5 cm2 V−1 s−1). Based on temperature‐dependent electron transport and capacitance‐voltage measurements, it is argued that the enhanced performance arises from the presence of quasi 2D electron gas‐like systems formed at the carefully engineered oxide heterointerfaces. The QSL transistor concept proposed here can in principle extend to a range of other oxide material systems and deposition methods (sputtering, atomic layer deposition, spray pyrolysis, roll‐to‐roll, etc.) and can be seen as an extremely promising technology for application in next‐generation large area optoelectronics such as ultrahigh definition optical displays and large‐area microelectronics where high performance is a key requirement.

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Structural analysis of low‐dimensional solution‐processed ZnO and In2O3 layers. a) High‐resolution transmission electron microscope (HRTEM) cross‐section images of the SiO2/ZnO interface. Left: low‐magnification TEM image confirms an ultrathin and continuous ZnO film. Right: higher‐magnification TEM image reveals the presence of polycrystalline ZnO regions with clearly visible lattice fringes. b) AFM phase images of ZnO film with individual grains clearly visible, the calculated rms ZnO surface roughness was ≈0.43 nm. c) GID measurement (X‐ray wavelength λ = 0.0908 nm) shows powder‐like crystallization, i.e., different crystalline orientations, coexisting in the ZnO film. d) HRTEM cross‐section images of the SiO2/In2O3 interfaces reveal the presence of an ultrathin and smooth In2O3 film with highly oriented crystalline domains. e) AFM image indicates the surface roughness of the In2O3 film is ≈0.20 nm. f) GID analysis (X‐ray wavelength λ = 0.0908 nm) shows similar powder‐like crystallization when characterizing the In2O3 layers.
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advs201500058-fig-0001: Structural analysis of low‐dimensional solution‐processed ZnO and In2O3 layers. a) High‐resolution transmission electron microscope (HRTEM) cross‐section images of the SiO2/ZnO interface. Left: low‐magnification TEM image confirms an ultrathin and continuous ZnO film. Right: higher‐magnification TEM image reveals the presence of polycrystalline ZnO regions with clearly visible lattice fringes. b) AFM phase images of ZnO film with individual grains clearly visible, the calculated rms ZnO surface roughness was ≈0.43 nm. c) GID measurement (X‐ray wavelength λ = 0.0908 nm) shows powder‐like crystallization, i.e., different crystalline orientations, coexisting in the ZnO film. d) HRTEM cross‐section images of the SiO2/In2O3 interfaces reveal the presence of an ultrathin and smooth In2O3 film with highly oriented crystalline domains. e) AFM image indicates the surface roughness of the In2O3 film is ≈0.20 nm. f) GID analysis (X‐ray wavelength λ = 0.0908 nm) shows similar powder‐like crystallization when characterizing the In2O3 layers.

Mentions: We have recently demonstrated the ability to grow ultra‐thin layers of ZnO by spin casting a suitable precursor solution.31 Using the same aqueous precursor route, we have grown polycrystalline ZnO layers with thicknesses in the range of 3–10 nm (Figure1a) at 180 °C (see Experimental Section). As‐grown ZnO layers are found to be continuous and conformal with root mean square (rms) surface roughness of ≈0.43 nm as determined by atomic force microscopy (AFM) (Figure 1b). The polycrystalline nature of the ZnO films was also confirmed by grazing incident diffraction (GID) measurements (Figure 1c). The results suggest that ZnO layers exhibit powder‐like diffraction peaks (i.e., no preferred orientation) in agreement with the transmission electron microscope (TEM) data in Figure 1a. It is worth noting that the high‐resolution GID results reported here are the first to reveal the polycrystalline nature of these ultrathin ZnO layers as compared to early work32 where only the (002) peak was detected. Using previously reported methods,33 we have also grown ultrathin layers (5–10 nm) of In2O3 (Figure 1d) by spin casting an aqueous solution of indium nitrate [In(NO3)3] at room temperature followed by thermal annealing at ≈200 °C in air. In2O3 films are found to be continuous and conformal (Figure 1d,e), ultra‐smooth (rms ≈0.2 nm, Figure 1e) and highly polycrystalline (evidence of which are presented in Figure 1d,f) in good agreement with previously reported data.34 Similarly, ultrathin (2–5 nm) films of stoichiometric Ga2O3 were also grown using the same processing steps (Figure S1, Supporting Information).35 Unlike ZnO and In2O3, however, Ga2O3 layers appear to be largely amorphous with no signs of crystallinity as no diffraction peaks could be detected.


High Electron Mobility Thin ‐ Film Transistors Based on Solution ‐ Processed Semiconducting Metal Oxide Heterojunctions and Quasi ‐ Superlattices
Structural analysis of low‐dimensional solution‐processed ZnO and In2O3 layers. a) High‐resolution transmission electron microscope (HRTEM) cross‐section images of the SiO2/ZnO interface. Left: low‐magnification TEM image confirms an ultrathin and continuous ZnO film. Right: higher‐magnification TEM image reveals the presence of polycrystalline ZnO regions with clearly visible lattice fringes. b) AFM phase images of ZnO film with individual grains clearly visible, the calculated rms ZnO surface roughness was ≈0.43 nm. c) GID measurement (X‐ray wavelength λ = 0.0908 nm) shows powder‐like crystallization, i.e., different crystalline orientations, coexisting in the ZnO film. d) HRTEM cross‐section images of the SiO2/In2O3 interfaces reveal the presence of an ultrathin and smooth In2O3 film with highly oriented crystalline domains. e) AFM image indicates the surface roughness of the In2O3 film is ≈0.20 nm. f) GID analysis (X‐ray wavelength λ = 0.0908 nm) shows similar powder‐like crystallization when characterizing the In2O3 layers.
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advs201500058-fig-0001: Structural analysis of low‐dimensional solution‐processed ZnO and In2O3 layers. a) High‐resolution transmission electron microscope (HRTEM) cross‐section images of the SiO2/ZnO interface. Left: low‐magnification TEM image confirms an ultrathin and continuous ZnO film. Right: higher‐magnification TEM image reveals the presence of polycrystalline ZnO regions with clearly visible lattice fringes. b) AFM phase images of ZnO film with individual grains clearly visible, the calculated rms ZnO surface roughness was ≈0.43 nm. c) GID measurement (X‐ray wavelength λ = 0.0908 nm) shows powder‐like crystallization, i.e., different crystalline orientations, coexisting in the ZnO film. d) HRTEM cross‐section images of the SiO2/In2O3 interfaces reveal the presence of an ultrathin and smooth In2O3 film with highly oriented crystalline domains. e) AFM image indicates the surface roughness of the In2O3 film is ≈0.20 nm. f) GID analysis (X‐ray wavelength λ = 0.0908 nm) shows similar powder‐like crystallization when characterizing the In2O3 layers.
Mentions: We have recently demonstrated the ability to grow ultra‐thin layers of ZnO by spin casting a suitable precursor solution.31 Using the same aqueous precursor route, we have grown polycrystalline ZnO layers with thicknesses in the range of 3–10 nm (Figure1a) at 180 °C (see Experimental Section). As‐grown ZnO layers are found to be continuous and conformal with root mean square (rms) surface roughness of ≈0.43 nm as determined by atomic force microscopy (AFM) (Figure 1b). The polycrystalline nature of the ZnO films was also confirmed by grazing incident diffraction (GID) measurements (Figure 1c). The results suggest that ZnO layers exhibit powder‐like diffraction peaks (i.e., no preferred orientation) in agreement with the transmission electron microscope (TEM) data in Figure 1a. It is worth noting that the high‐resolution GID results reported here are the first to reveal the polycrystalline nature of these ultrathin ZnO layers as compared to early work32 where only the (002) peak was detected. Using previously reported methods,33 we have also grown ultrathin layers (5–10 nm) of In2O3 (Figure 1d) by spin casting an aqueous solution of indium nitrate [In(NO3)3] at room temperature followed by thermal annealing at ≈200 °C in air. In2O3 films are found to be continuous and conformal (Figure 1d,e), ultra‐smooth (rms ≈0.2 nm, Figure 1e) and highly polycrystalline (evidence of which are presented in Figure 1d,f) in good agreement with previously reported data.34 Similarly, ultrathin (2–5 nm) films of stoichiometric Ga2O3 were also grown using the same processing steps (Figure S1, Supporting Information).35 Unlike ZnO and In2O3, however, Ga2O3 layers appear to be largely amorphous with no signs of crystallinity as no diffraction peaks could be detected.

View Article: PubMed Central - PubMed

ABSTRACT

High mobility thin‐film transistor technologies that can be implemented using simple and inexpensive fabrication methods are in great demand because of their applicability in a wide range of emerging optoelectronics. Here, a novel concept of thin‐film transistors is reported that exploits the enhanced electron transport properties of low‐dimensional polycrystalline heterojunctions and quasi‐superlattices (QSLs) consisting of alternating layers of In2O3, Ga2O3, and ZnO grown by sequential spin casting of different precursors in air at low temperatures (180–200 °C). Optimized prototype QSL transistors exhibit band‐like transport with electron mobilities approximately a tenfold greater (25–45 cm2 V−1 s−1) than single oxide devices (typically 2–5 cm2 V−1 s−1). Based on temperature‐dependent electron transport and capacitance‐voltage measurements, it is argued that the enhanced performance arises from the presence of quasi 2D electron gas‐like systems formed at the carefully engineered oxide heterointerfaces. The QSL transistor concept proposed here can in principle extend to a range of other oxide material systems and deposition methods (sputtering, atomic layer deposition, spray pyrolysis, roll‐to‐roll, etc.) and can be seen as an extremely promising technology for application in next‐generation large area optoelectronics such as ultrahigh definition optical displays and large‐area microelectronics where high performance is a key requirement.

No MeSH data available.


Related in: MedlinePlus