Undercoordinated indium as an intrinsic electron-trap center in amorphous InGaZnO4

Ho-Hyun Nahm,Yong-Sung Kim

doi:10.1038/am.2014.103

Abstract

Undercoordinated indium (In*) is found to be an intrinsic defect that acts as a strong electron trap in amorphous InGaZnO4. Conduction electrons couple with the under-coordinated In* via Coulomb attraction, which is the driving force for the formation of an In*–M (M=In, Ga, or Zn) bond. The new structure is stable in the electron-trapped (2–) charge state, and we designate it as an intrinsic (In*–M)2− center in amorphous InGaZnO4. The (In*–M)2− centers are preferentially formed in heavily n-doped samples, resulting in a doping limit. They are also formed by electrical/optical stresses, which generate excited electrons, resulting in a metastable change in their electrical properties. Korean scientists have identified a key clue that can enhance the long-term stability of transparent semiconductors in ‘wearable’ computers. Recently, a glass-like oxide containing the metals indium, gallium and zinc (InGaZnO4) has found wide use in flexible electronic devices because its speedy transistor characteristics can drive high-resolution optical displays. However, the capabilities of this see-through semiconductor often degrade over time due to a phenomenon called charge trapping. Yong-Sung Kim from the Korea Research Institute of Standards and Science and co-workers simulated the amorphous structure of InGaZnO4 by performing first-principles quantum computations and discovered a previously unnoticed trapping site — ‘under-coordinated’ indium atoms that snare extra electrons through strong electron–ion interactions. Processing conditions that specifically supress populations of under-coordinated indium should be an essential part of future manufacturing efforts, suggest the authors. Undercoordinated indium (In*) is found to be an intrinsic defect that acts as a strong electron trap in amorphous InGaZnO4. Conduction electrons couple with the under-coordinated In* via Coulomb attraction, which is the driving force for the formation of an In*–M (M=In, Ga, or Zn) bond. The new structure is stable in the electron-trapped (2–) charge state, and we designate it as an intrinsic (In*–M)2− center in amorphous InGaZnO4. The (In*–M)2− centers are preferentially formed in heavily n-doped samples, resulting in a doping limit. They are also formed by electrical/optical stresses, which generate excited electrons, resulting in a metastable change in their electrical properties.

Highlights

The identification of charge-trapping defects on the atomic scale has been achieved in crystalline semiconductors
We find that undercoordinated indium (In*) acts as an intrinsic electron-trap center in In-based amorphous oxide semiconductors
MATERIALS AND METHODS Amorphous InGaZnO4 is considered as a prototype In-based amorphous oxide semiconductor

Summary

Introduction

The identification of charge-trapping defects on the atomic scale has been achieved in crystalline semiconductors. A donor can capture carrier electrons with large lattice relaxations, forming a DX (donor (D) deactivated (X)) center,[1,2,3,4,5] whereas an acceptor traps holes, forming an AX (acceptor (A) deactivated (X)) center.[5,6,7] in amorphous semiconductors, even though many charge-trapping phenomena that can modify electronic device characteristics[8] and be applied to nonvolatile memory devices[9] have been observed, the atomic and electronic structures of the charge-trapping defects lack clear understanding. Indium (In)-based amorphous oxide semiconductors are considered as a promising material for next-generation thin-film electronics and optoelectronics because they have high electron mobility, transparency, flexibility and uniformity.[29,30,31,32,33] the success of these applications has been limited by the lack of stability in their electrical properties owing to charge trapping

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Results

Conclusion