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

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.


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, 1-5 whereas an acceptor traps holes, forming an AX (acceptor (A) deactivated (X)) center. [5][6][7] However, 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.
Investigation of the charge-trapping defects on the atomic scale is an essential prerequisite to overcome the instability issue of the indiumbased amorphous oxide semiconductors. An oxygen-vacancy (V O ) defect has been suggested as a metastable hole-trap center. 34 38 have been suggested as hole-trap centers. An excess O defect model has been previously suggested to describe electron-trap centers based on ozone-treated amorphous InGaZnO 4 . 39 The excess O is characterized as a weakly binding O that results in a peak at~200°C in thermal desorption spectroscopy. Thus, the excess O can be removed using a thermal annealing process. 36,39 Because electron trapping still occurs in the absence of excess O, there should be another cause of electron trapping.
In this paper, we find that undercoordinated indium (In*) acts as an intrinsic electron-trap center in In-based amorphous oxide semiconductors. Conduction electrons are subjected to a strong conductionelectron-ion interaction near the undercoordinated In* and trapped there, forming an In*-M bond. The electron-trapped center is stable in the (2−) charge state; thus, we designate it as a negatively double-charged intrinsic (In*-M) 2 − center in amorphous oxide semiconductors.

MATERIALS AND METHODS
Amorphous InGaZnO 4 is considered as a prototype In-based amorphous oxide semiconductor. For theoretical investigations, the amorphous structures are generated using a melt-and-quench molecular dynamics simulations, 37 and the structural instability of the conduction electrons and the electronic structures are investigated using density-functional theory calculations. 40,41 The projectoraugmented wave pseudopotentials 42,43 and the plane wave basis set with a kinetic energy cutoff of 400 eV are used. The hybrid functional of Heyd-Scuseria-Ernzerhof with a mixing parameter of 0.25 and a screening parameter of 0.2 Å − 1 is used for the exchange-correlation energy of the electrons. 44,45 A rhombohedral 112-atom supercell is adopted, and a 2 × 2 × 2 k-point mesh is used for the Brillouin zone summation. The dimer method is used to find the transition state in the structural changes. 46 In the charged state calculations, for the localized charges, we correct the spurious electrostatic interaction energies between the image charges in supercells using a model charge correction scheme. [47][48][49]

RESULTS AND DISCUSSION
The charge density of the lowest conduction band in amorphous InGaZnO 4 is shown in Figure 1a. The conduction electrons are delocalized as expected because, in amorphous InGaZnO 4 , the lowest conduction band states are mainly characterized by the In-5s-like atomic orbital states, and their effective overlap through the In atomic sites results in a low electron effective mass, which is the reason for the high electron mobility in amorphous InGaZnO 4 . Interestingly, the s-like conduction electrons in amorphous InGaZnO 4 are not found to be homogeneous, but they are highly concentrated in the depicted the local atomic structure as shown in Figure 1a.
The place where the conduction electrons are highly concentrated is found to be near the undercoordinated In* atom. In crystalline Inoxides, such as In 2 O 3 and crystalline InGaZnO 4 , the In atoms have sixfold coordination with nearby O atoms. In amorphous InGaZnO 4 , the coordination number of some In atoms, such as the In depicted in Figure 1a, is depleted to fivefold coordination, and the mean value of the In coordination number has been measured to be~5.5 (see the running coordination numbers and shaded region in Figure 1b). 50 In Figure 1c, we plot the integrated charges in the Wigner-Seitz volume around the In atoms with a radius of 1.677 Å, as a function of the In coordination number. The In coordination number is determined by counting the number of O atoms that have a valence charge density minimum along the In-O lines higher than 0.2 ea/Å 3 . This criterion approximately corresponds with the number of O atoms within 2.6 Å of the central In atom. There is a tendency that the integrated charge increases as the In coordination number decreases. The In* atom indicated by the red circle in Figure 1c is fivefold coordinated and has the highest local-integrated charge among the In atoms in the system, indicating structural instability, which will be discussed below. The local deficiency of O atoms around the In atom can accommodate the conduction electrons most likely via electrostatic attraction, which is important in ionically bonded materials. The variation in the integrated charges with the same In coordination number observed in Figure 1c can be attributed to strained In-O bonds and a variety of local-field effects in the amorphous structure. Conduction electron crowding can occur near an In atom in amorphous InGaZnO 4 , and the undercoordinated In atoms are more likely to be the In* atoms, which can accommodate more conduction electrons. Conduction electron crowding does not occur at all of the undercoordinated In atoms, but at least one (In*) of the undercoordinated In atoms in the system experiences electron crowding.
The conduction electron crowding near the undercoordinated In* implies strong conduction-electron-ion interaction. We placed two electrons (2e − ) in a 112-atom supercell (1.553 × 10 21 cm − 3 ) and investigated the changes in the atomic structure. The charge neutrality is satisfied by assuming a uniform background (2+) charge. In the presence of conduction electrons, the original In* configuration (Figure 2a) is no longer stable, but a new In*-M bond (in this case, an In*-Ga bond) configuration (Figure 2c) is generated. Because the conduction electrons are more concentrated near the undercoordinated In*, the atomic structure near the In* atom is affected by them. We denote the original atomic configuration as the normal state (NS) and the In*-M bond configuration as the electron-trapped state (In*-M). The transition state (TS) between them is shown in Figure 2b.
We would like to describe the changes in the atomic structure between NS and In*-M. An O atom that has a tetrahedral bonding configuration with one In*, one Ga and two Zn (see Figure 2a (Figure 2c) is an electron trap as well, but it is intrinsic in amorphous InGaZnO 4 . It can be interpreted as a small polaron that is more strongly localized after forming the In*-M bond. Figure 3 shows the calculated local electronic density-of-states near the In* and Ga atoms, as the NS is transformed into the In*-M structure in the (2-) charge state. In the NS+2e − , there is a defect state inside the conduction band (indicated by In* at the top of Figure 3) that originates from the undercoordinated In* atom. The charge density shown in Figure 1a includes this defect state. As the NS+2e − is transformed into (In*-M) 2 − , the defect level decreases; at TS 2 − , the defect level crosses the Fermi level near the conduction band minimum, and then, the defect state emerges inside the band gap, which is occupied by two electrons. In (In*-M) 2 − , we find a well-isolated state inside the band gap. The charge density of the (In*-M) 2 − deep state is shown in the inset of The electron trap and detrap mechanisms in amorphous oxide semiconductors can, therefore, be expressed by the reaction NS þ 2e À 2ðIn Ã À MÞ 2À : The calculated potential energy surfaces in the structural transition between the NS and In*-M configurations are shown in Figure 4a    Undercoordinated indium as an intrinsic electron-trap center H-H Nahm and Y-S Kim same number of positive uniform background charges. They correspond to 1.553, 2.330, 3.106, 3.883, 4.659 and 6.212 × 10 21 cm − 3 , respectively. The calculated α energy barrier as a function of n is shown in Figure 4b, which is reduced with increasing n. When the carrier density is 4.7 × 10 21 cm − 3 , the α barrier is found to be zero. The structural recovery from (In*-M) 2 − to the NS+2e − state can take place when the deep (In*-M) 2 − electronic state inside the band gap releases the two electrons. The (In*-M) 2 − level can be increased from (In*-M) 2 − to TS 2 − in Figure 3 by thermal excitation, and when it crosses the Fermi level, the two trapped electrons are released. The recovery energy barrier (β) through the thermal process is calculated to be 0.74 eV, as shown in Figure 4a, which increases as the conduction electron density increases (the level of the Fermi sea is higher). The structural recovery can also occur via optical or electrical excitation of the (In*-M) 2 − electrons into the empty conduction bands. For the (In*-M) 2 − → NS+2e − detrapping process, the required photon energy depends on the Fermi level, and when it is at the conduction band minimum, the minimum required photon energy is estimated to be 2.1 eV.
The electron-trapping (In*-M) 2 − centers are likely to form in heavily n-doped amorphous InGaZnO 4 . (In*-M) 2 − acts as a donorcompensating center that reduces the electron carrier concentration. Experimentally, the carrier concentration in n-type amorphous InGaZnO 4 has not surpassed 10 20 cm − 3 (the doping limit) by controlling oxygen partial pressure or hydrogen incorporation. 51 The doping limit has been measured to be much lower than the dopant concentration, 51 implying the presence of deep electrontrapping centers in amorphous InGaZnO 4 . 32,52 The formation of (In*-M) 2 − can also occur by optical or electrical excitation of electrons as the n-type doping in amorphous InGaZnO 4 . Electrical stress, positive gate bias stress (PBS) or current stress (CS), in which the (n-type) thin-film transistors are turned on, can be applied, and the threshold voltage has been known to be positively shifted owing to its metastability. PBS and CS generate a high concentration of carrier electrons in the amorphous InGaZnO 4 channel, and via the forward reaction in Equation (1), electron trapping (In*-M) 2 − centers can be formed. A negatively charged deep level has been hypothesized to be created in experiments, accompanied by a positive shift of the threshold voltage. 20 The experimentally measured thermal activation energy for electron trapping (E a,trap ) is in the range of 0.22-0.95 eV [15][16][17][18][19]22 under PBS and 0.08-0.14 eV 20 under CS. The α energy barrier in the (In*-M) 2 − formation corresponds to these values, which vary depending on the carrier concentration (Figure 4b). For no10 21 cm − 3 , a larger supercell is needed, which is not currently accessible, but it can be extrapolated to the n = 0 limit (α = 5.2 eV and β = 0 eV in the neutral state as shown in Figure 4a). In the range of n 4 10 20 cm − 3 , which is typical under PBS and CS conditions, the estimated α energy barriers are 0.0-1.4 eV in good agreement with the experiments (0.08-0.95 eV). [15][16][17][18][19][20]22 The thermal activation energy for electron detrapping (E a, detrap ) (after stopping the PBS or CS) has also been measured. This value can be interpreted as the β energy barrier in the (In*-M) 2 − → NS+2e − transition. Without external stresses, the carrier density is typically no10 20 cm − 3 (below the doping limit) in the presence of both normal shallow donors and electron-trapping (In*-M) 2 − centers, and the estimated β energy barriers are 0.0-0.7 eV in the n range. The measured values are E a,detrap = 0.23 and 0.97 eV. 22,19 The issue that a uniform background charge with PAW formalism gives rise to an additional total energy term has been recently addressed. 53 This term is not included in this study, and the energies obtained are only qualitative at best. For the (2 − ) charge state, the error is typically o0.2 eV according to reference 53 , which is smaller than the energy differences obtained in this study. Therefore, we do not need to make any qualitative changes to our conclusions. The α and β barrier estimations shown in Figure 4b could be quantitatively affected by the additional total energy term, but their trends would be unaffected.

CONCLUSIONS
In conclusion, an intrinsic electron-trapping center in amorphous InGaZnO 4 is identified. The conduction electrons are attracted to undercoordinated In* and subjected to a strong electron-ion interaction. The driving force to form In*-M bonds is induced by trapped electrons. The negatively double-charged (electron-trapped) intrinsic (In*-M) 2 − centers in amorphous InGaZnO 4 have an important role in pinning the Fermi level in heavily n-doped samples, and metastable positive-shifts of the threshold voltage in thin-film transistors under PBS or CS, which generate excited electrons. To suppress the PBS and CS instabilities and enhance the n-doping limit, a reduction in the number of undercoordinated In* in amorphous InGaZnO 4 is essential.