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Oxide semiconductor TFT

Amorphous Oxide Semiconductor

Since the discovery of the amorphous-InGaZnO (a-IGZO) by Hideo Hosono in 2004, amorphous oxide semiconductors (AOS) have been attracting great attention.

Ionic characteristics of oxides with two or more different metal atoms (with different electronegativity) is strong enough that valence band is formed by the oxygen 2p orbital and conduction band is formed by the ns or np orbital of metal cation.

Oxides with large enough cations (n>4) have ns orbitals that overlaps each other. Hence, oxides (i.e. IGZO) will have no directionality in their conduction band and have high mobility even in the amorphous phase.

Relatively high mobility, transparency and low deposition temperature has been the merits of amorphous oxide semiconductors. Due to these merits, AOS materials are being used in display and memory devices.

Gate Dielectric Effect on the NBITS (IGZO TFT)

Hole current was directly observed in oxide semiconductors under illumination. TFTs subjected to stress conditions in the dark and under light illumination showed a negligible difference in Vth shift at wavelengths longer than the threshold wavelength while a substantial difference was observed at wavelengths shorter than it.

Hole trapping was found to be one of the reasons for the NBITS instability in oxide semiconductor

Related paper:

Y. J. Chung, et al., Electrochemical and Solid-State Letters, 6, 14, G35-G37 (2011)

J. H. Kim, et al., Applied Physics Letters, 23, 98, 232102 (2011)

J. H. Kim, et al., Physica Status Solidi Rapid Research Letters, 5-6, 5, 178-180 (2011)

Optical Modeling of Light Induced Phenomena in IGZO TFT

An optical model based on the transfer matrix method was used to predict the behavior of amorphous IGZO TFT and MOSCAP devices under illumination condition. The simulation, considering the interference of light in the multi-layer structure, results well agree with experimental hole current and NBIS instability data. Such method can be applied to any general system, with optical information of the films, results can be applied to minimize NBIS instability in an arbitrary AOS TFT for the display devices.

ITO/IGZO bilayer stacks in OTFT

The optimized device portrayed a high saturation mobility of ~80 cm2/V s with off current values lower than 10-11A with 5nm ITO at the gate dielectric / semiconductor interface. The negative bias illumination conditions showed that a significant amount of ITO layer also acted as a hole filter layer, and hole current and threshold voltage shift values measured under photo-generated charge carriers were annihilated before reaching the gate insulator.

Related paper:

Y. J. Chung, et al., Applied Physics Letters, 105, 013508 (2014)

Defect Analysis of AOS (ZTO)

The defects in AOS systems have been a major research topic in the field because defects, especially oxygen vacancies, have a crucial influence on the conductivity and device reliability. On that point, MOCVD could be a viable method to fabricate the channel layer with better electrical properties and lower defect concentrations compared to other deposition methods. For these reasons, we investigated on physical, chemical, and electrical properties of MOCVD ZnxSnyOz (ZTO) thin films with various Zn/Sn atomic compositions.

It is reported that free carriers are originated from the oxygen vacancies and carrier trapping occurs at the oxygen vacancies or at orbital states related to the oxygen vacancies. By varying composition, we observed that the relative area of the oxygen vacancy-related peak was changed and found out the relationship between the cation ratio and oxygen vacancy concentration.

To examine the defect states and their influence on the reliability, NBIS analysis was performed. Depending on composition of ZTO, quite different electrical performances were shown. Applying NBIS stress for 1000 s, the transfer curves of the Zn-rich ZTO TFT showed a hump, but the transfer curves of the Sn-rich ZTO TFT exhibited a parallel shift to the negative bias direction. We determined these phenomena were attributed to the difference in the oxygen vacancy energy states generated in the ZTO band gap by light illumination.

Recently, device simulation tool is widely used to understand the experimental results or consider various possible factors. Using ATLAS simulations on the transfer curves of the ZTO TFTs, we found that the Zn- and Sn-related oxygen vacancies generated deep donor like trap states at 0.3 eV and shallow states at 0.1 eV from the conduction band (or mobility) edge, which induced the hump as well as shift and only parallel shift in the transfer curves of the Zn-rich and Sn-rich films, respectively

Related Paper :

Un Ki Kim et al., J. Mater. Chem. C, 1, 6695–6702 (2013)

P-type Tin Monooxide (SnO)

Since p-type oxide semiconductor materials with high enough reliability are not reported yet, it is difficult to realize the complementary circuit made from only oxide semiconductor materials. Thus, the research on the p-type oxide materials has been recognized to be very important and valuable.

Among many candidates, tin monoxide (SnO) has received a lot of attention as a p-type active layer because pseudo-closed Sn 5s and O 2p orbitals having close energy levels for effective interaction.

They could form more isotropic and delocalized hybridized orbitals that would compose a VBM potentially giving more effective hole transport.

A wide range of process conditions for p-type SnO phase does exist.

We have investigated the characteristics of p-type SnO thin films by co-sputtering technique with pure SnO and Sn targets. By controlling the co-sputtering conditions, p-type conductivity was observed in Hall-effect measurements. Deposited films exhibit a hole carrier concentration in the range of 3.1 1017 – 2.6 1023 cm-3; electrical conductivity between 0.266 – 9.91 103 S cm-1(see figure below); and Hall mobility around 10.3 cm2/V s at 50 % of Sn/SnO power ratio (The reported value is the average of 4–7 measurements performed on samples of every deposition condition). Among the various conditions of co-sputtering, the Sn-rich condition prevents oxidization from SnO to SnO2 and enhances the hole mobility due to valence band maximum (VBM) delocalization.

Electronic Structure of Crystalline and Amorphous ZnSnO3

Amorphous ZnSnOX is promising oxide semiconductor of structural and electronic properties similar to those of IGZO. In particular, ZnSnO3 (ZTO) is highly promising due to its high mobility and easy fabrication. However, ZTO is less stable compared to separated phases (Zn2SnO4+SnO2), so detailed studies on its electronic structures and accompanied properties have been limited. Especially, comparative studies on the detailed theoretical aspects of electronic structures of the crystalline and amorphous ZTO are still lacking. We theoretically investigated electronic structures of crystalline and amorphous ZTO based on ab-initio calculations using Vienna ab-initio simulation package (VASP) code. The results were compared with electronic structure of ZTO films deposited by ALD technique and characterized by O K-edge X-ray absorption spectroscopy (XAS). ab-initio calculation shows that that the orbital character of the lowest CB was mostly Sn 5s for both phases. However, the CB state was more localized for amorphous structure compared with the crystalline case, although the degree of localization was not significant (left figure). The difference in magnitude of localization of CB is also captured as the difference in F-peak of XAS spectra between annealed (nano-crystalline) and as-deposited (amorphous) ZTO films (right figure). The higher F-peak intensity can be interpreted as a signature of strong Sn 5s localization in the as-deposited sample, since O K-edge XAS reflects the unoccupied PDOS of the metal ions that hybridize with the O 2p orbital states through the O 1s<->2p dipole transition.

Related Paper:

J. Lee, et al., Appl. Phys. Lett. 102, 242111 (2013)

Property of Interface between ZnSnO3 and gate dielectric (SiO2)

Compared to individual amorphous ZnSnO3 (a-ZTO) and amorphous SiO2 (a-SiO2), information on characteristic atomic structures arising at the interface between them and associated electronic structures are quiet limited. For example, it has been well known that atomic structure of bulk a-ZTO can be characterized by diverse coordination numbers (CN=4~6) and polyhedral connectivities(corner, edge and face-sharing) of M-O(M=Zn, Sn) polyhedra(fig. a) while that of a-SiO2 shows strict corner-shared Si-O tetrahedra(fig. b). However, it is important to understand the interface properties in order to control and enhance the TFT performance such as device stability. We simulated the eight interface structures(a-ZTO/a-SiO2) using ab-initio molecular dynamics simulation using VASP code (fig. c) and investigated atomic structure using radial distribution function and electronic structure using PDOS. Atomic structure at the interface features reduction of CN of M and O atoms consisting of M-O polyhedra. It results in O-terminated M-O bonding (fig. f) and M-O-M bonding (fig. h) structures where interfacial gap states consisting of O 2p states appeared (fig. g and i). In contrast, CN of Si was nearly unchanged indicating its strong tendency to form corner-sharing SiO4 tetrahedra as seen from its bulk structure, which we attribute to the reason of reduction of CN of M and O atoms in M-O polyhedra. The result shows how the differences in the coordination states and the connectivity of oxygen polyhedra between a-ZTO and a-SiO2 play a role in bringing characteristic atomic and electronic structures at the interface between them.

Related Paper:

J. Park, et al., Phys. Status Solidi B(accepted)