X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band ͑⌬E v ͒ of SiO 2 / InZnGaO 4 ͑IGZO͒ heterostructures deposited by low temperature plasma enhanced chemical vapor deposition and sputtering at Ͻ50°C, respectively. A value of ⌬E v = 1.43Ϯ 0.15 eV was obtained by using the Ga and Zn 2p3 and In 3d3 and 3d5 energy levels as references. Given the experimental bandgap of 3.2 eV for the IGZO, this would indicate a conduction band offset ⌬E C of 4.27 eV in this system.

The use of Al 2 O 3 as a gate dielectric has been shown to improve the performance and electrical stability of α-InGaZnO 4 (IGZO) thin film transistors. X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the valence band ( E v ) of Al 2 O 3 /IGZO heterostructures deposited by sputtering at <50 • C, respectively. A value of E v = 0.95 ± 0.17 eV was obtained by using the In 2p3/2, Zn 2p3/2 and 3d5/2 energy levels as references. Given the experimental bandgap of 3.2 eV for the IGZO, this would indicate a conduction band offset of 2.35 eV in this system. Due to high saturation mobilities, low threshold voltages, high I ON /I OFF ratios, and transparency in the visible range, amorphous oxide semiconductors have shown exceptional promise as an alternative to amorphous silicon (α-Si) for use in thin film transistors (TFTs). 1-3 In particular α-InGaZnO 4 (IGZO) has garnered a great deal of attention in part due to the ability to be deposited at room temperature, opening up the possibility for device fabrication over a wide range of flexible substrate materials, including plastic and paper. [4][5][6] Transparent oxide based TFTs require high operating voltages in order to achieve high on-off ratios and high mobilities. A gate dielectric with a low trap density at the oxide -dielectric interface can be used to improve device performance, namely decrease leakage currents and threshold voltage, and thus decrease operating voltage. Recent reports have shown that the use of Al 2 O 3 as a gate dielectric improved device characteristics and decreased electrical instabilities due to the decrease in trap states at the interface with IGZO and higher dielectric constant. 7 An all transparent ring oscillator based on IGZO TFTs was able to achieve operating frequencies as high 2.1 MHz with Al 2 O 3 as a gate dielectric 8 due to the lower trap density in Al 2 O 3 as compared to other dielectrics, such as SiN x . 9 In this letter we report an X-Ray Photoelectron Spectroscopy (XPS) study of the valence band offset ( E V ) in an Al 2 O 3 /IGZO heterojunction. The samples were deposited by RF magnetron sputtering for both Al 2 O 3 and IGZO. The valence band offset was determined to be E v = 0.95 ± 0.17 eV.The Al 2 O 3 was deposited by RF sputtering from a single Al 2 O 3 target on both Si and InGaZnO 4 /Si substrates. The IGZO was deposited on both Si and glass substrates (Corning EAGLE 2947) by RF magnetron sputtering using a 3-inch diameter single target of InGaZnO 4 . The temperature at the substrate was ∼40 • C after the α-IGZO deposition, as determined from temperature indicators attached to reference glass substrates during film deposition. The RF power was 140 W with a working pressure of 10 mTorr in a pure Ar ambient for the IGZO.A higher power of 250W at the same pressure was used for Al 2 O 3 . The carrier concentration as determined from Hall measurements was 6.5 × 10 18 cm −3 (n-type). The In/Ga/Zn ratio was measured by X-ray microprobe. The films were amorphous as determined by powder x-ray ...

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