AVS2011 Session TC+EM+NS-ThA: Transparent / Printable Electronics Part 2

Transparent conducting oxides (TCO) have found application as electrode in emerging markets like that of thin films solar cells or flat panel displays. For this passive functionality the TCO material must combine high transparency preferentially over a wide spectral range and high conductivity. In the last years active transparent devices like photodetectors, transistors or a complete transparent circuitry are envisioned and rely on semiconducting properties of the material. Besides a precise control of the doping level in the active part of devices the creation of space charge regions by rectifying contacts is a prerequisite for active devices. In the emerging field of transparent electronics, only metal-insulator-semiconductor field-effect transistors (MISFETs) were considered so far. In this contribution transparent, high-performance MESFETs, inverters etc. based on ZnO and related ternaries are presented. We discuss design prospects as well as limitations regarding device performance, reliability and stability.

The influence of the contact metal and dielectric passivation layers on the properties of ZnO Schottky diode, used as gate electrode within the MESFETs, as well as sources of non-idealities will be highlighted.

Amorphous oxide semiconductors (AOSs) are expected as an alternative to amorphous/poly-Si for thin-film transistors (TFTs) in next-generation flat-panel displays (FPDs) because AOS TFTs have many advantages such as large field-effect mobilities (>10 cm²(Vs)^-1) and low-temperature process [1]. For more advancing AOS optoelectronic technology, it is important to develop more various devices other than TFTs, and to study some remaining issues such as operation characteristics of AOS devices under light illumination.

In this study, we fabricated good and stable metal-AOS Schottky contacts made of amorphous In-Ga-Zn-O (a-IGZO) and bottom Pt electrodes at temperatures below 200^oC even though it is generally difficult to make high performance oxide Schottky junctions [2]. It was found that the a-IGZO/Pt Schottky contacts have an ideality factor n ~ 1.1 and a Schottky barrier height ф_b ~ 0.9 eV, which were evaluated from their J-V curves using the thermionic emission model. From C-V results, the Schottky junctions operate at the full-depletion condition, whose C corresponds to the geometrical capacitance of the a-IGZO layer, and relative permittivity ε_s of a-IGZO was obtained approximately 13. However, the results of temperature dependences of J-V characteristics were unexplained if we take a simple uniform Schottky barrier model; we found that the barrier potential fluctuations model [3] explained them well, and the mean barrier height ф_{b, m} of 1.2 eV and the net electron affinity χ_s of a-IGZO of 4.2 eV were obtained. On the other hand, the Schottky contacts showed very small open circuit voltages (V_OC’s) < 0.1 V under 100 mWcm^-2 AM1.5 light illumination, which are far smaller than the built-in potential (V_bi ~ 0.4 eV) estimated from the C-V measurements in dark. We also observed that V_OC decays with time after starting the light illumination. We will discuss the mechanism of the small V_OC based on these results.

[1] T. Kamiya et. al. Sci. Technol. Adv. Mater. 11 044305 (2010).

[2] K. Ip et al., J. Cryst. Growth 287, 149 (2006).

[3] J. H. Werner and H. H. Güttler, J. Appl. Phys. 69, 1522 (1991).

Printed electronics is a rapidly growing industry and within this emerging field there are three required material categories critical to fabricating active and passive circuitry – insulators (dielectrics), semiconductors (polymers), and conductors (metals). The increased interest in printable electronics as alternatives to silicon-based technologies is fueled by the promise of large-area, flexible, and ultra-low-cost devices. To enable the growing demands of printing processes this work develops metal-containing inks for the deposition of the coinage metals - copper, silver, and gold. These metals are chip components ranging from interconnects to source and drain contacts in organic field effect transistors. The liquid ink approach is based on fundamental advances in coordination chemistry to fabricate discrete metal complexes that can be heated or irradiated to yield metallic films. Ultimately inkjet printing technologies were employed to deposit these metal inks in specific, predetermined patterns that were directly transformed into active and passive devices. The versatility of this approach holds the possibility of printing any metallic design and pattern on virtually any type of substrate.

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) are transitioning towards commercialization for active-matrix liquid crystal display flat-panel display backplane applications. They also appear to be well-positioned to meet the more demanding challenges associated with active-matrix organic light-emitting device backplanes. Additionally, AOS TFTs offer an attractive approach to printed electronics. The primary focus of this talk will be to discus our novel approach to top-side passivation of bottom-gate indium gallium zinc oxide (IGZO) and zinc tin oxide (ZTO) AOS TFTs. Device performance between passivated and unpassivated AOS TFTs will be compared. Passivation mechanisms will be considered in the context of induced-gap state and device physics electrostatic modeling.

Amorphous oxide semiconductors (AOSs) represented by amorphous In-Ga-Zn-O (a-IGZO) are expected for large-area high-performance flexible electronic devices, because AOSs have large electron mobilities greater than 10 cm²(Vs)^-1 even if fabricated at room temperature (RT). In particular, a-IGZO has good controllablities of carrier concentration, and their thin-film transistors (TFTs) exhibit superior properties including long-term stability.

In this study, we investigated effects of oxidation on operation characteristics of a-IGZO TFTs. Bottom gate, top-contact a-IGZO TFTs were fabricated on SiO₂/c-Si substrates by RF magnetron sputtering. Sputtering conditions were the RF power of 70 W and the total pressure of 0.55Pa. Two oxidation treatments were examined; (i) ozone annealing and (ii) varying a mixing gas ratio of Ar : O₂ from 18 : 2 to 19.8 : 0.2 in standard cc per minute (sccm) during the channel deposition.

For the ozone annealed TFTs, annealing at ≤ 250^oC produced good TFTs, while those annealed at 300^oC caused large hysteresis and S slope. After applying a high V_GS larger than 40 V, the transfer characteristics showed the large V_th of 40V and the small hysteresis. Trap state around Fermi level of the large S state and the large V_th state were ~4x10¹⁷ and ~1x10¹⁷ cm^-3eV^-1, which were estimated by C-V analysis. The large V_th state is very stable in the dark, but the TFT recovers to the initial large S state by light illumination. The photoresponse measurements revealed that deep trap states were formed at 2.3 eV below the conduction band minimum by applying a high V_GS. Thermal desorption spectra showed that weakly-bonded excess oxygens were incorporated in the a-IGZO layer. From these results, we built a subgap DOS model of the trap states of the excess oxygens. We also confirmed similar behaviors in a-IGZO TFTs fabricated in high oxygen pressure conditions.

This study reveals that the control of oxygen stoichiometry is important for obtaining good performance and stability of AOS TFTs.