Effects of hole self-trapping by polarons on transport and negative bias illumination stress in amorphous-IGZO

The effects of hole injection in amorphous-IGZO is analyzed by means of first-principles calculations. The injection of holes in the valence band tail states leads to their capture as a polaron, with high self-trapping energies (from 0.44 to 1.15 eV). Once formed, they mediate the formation of peroxides and remain localized close to the hole injection source due to the presence of a large diffusion energy barrier (of at least 0.6eV). Their diffusion mechanism can be mediated by the presence of hydrogen. The capture of these holes is correlated with the low off-current observed for a-IGZO transistors, as well as, with the difficulty to obtain a p-type conductivity. The results further support the formation of peroxides as being the root cause of Negative bias illumination stress (NBIS). The strong self-trapping substantially reduces the injection of holes from the contact and limits the creation of peroxides from a direct hole injection. In presence of light, the concentration of holes substantially rises and mediates the creation of peroxides, responsible for NBIS.Comment: 8 pages, 8 figures, to be published in Journal of Applied Physic

Ab-initio Investigation of Amorphous Semiconductors

Amorphous oxide semiconductors AOS have been attracting much attention over the last decade for their quality as active layer in thin film transistors (TFT). These materials are of particular interest for the display industry since they are processable on large areas, at low temperature while presenting a large mobility (~10 cm^2/Vs). The amorphous nature guarantees the uniformity of the film over a large area and its low processing temperature allows its deposition on plastic foils. A widely used AOS for TFT applications is amorphous InGaZnO4 (a-IGZO). Out of which numerous proofs of concepts have been built, demonstrating the possibility to create displays and other circuits such as Near Field Communication (NFC) tags. The material is transparent, processable on plastic substrate and compatible with low-cost production techniques. Nevertheless, the stability of a-IGZO based transistors against long gate bias is problematic, most probably due to the presence of defects in the semiconductor. Nonetheless, the nature of these defects, their definition and behavior remain unclear. A better fundamental understanding of the material properties is hence required. In this regard, ab-initio techniques are powerful tools used to study these fundamental properties of matter. However, the disordered nature of the amorphous phase sets some challenges in the investigation process. We highlight in this manuscript that much care is required in the interpretation of the results. On the basis of these techniques, we propose a detailed description of the fundamental properties of a-IGZO, including the role of defects and their implication in the bias stress instabilities observed in a-IGZO TFTs. Although AOS are fascinating materials, they are all naturally n-type doped, forbidding the creation of a complementary circuitry, and hence the development of more complex applications. To understand the root cause of the problem, we investigated the origin of their astonishingly low off-current in TFTs together with the reasons for their absence of p-type performances. From there on, we further explored the possibility to create new amorphous p-type materials able to match the n-type properties of AOS.Abstract iii Contents xiii List of Figures xix List of Tables xxxix 1 Introduction 1.1 Large-Area Electronics . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Thin Film Transistors and Semiconductors . . . . . . . . . . . 3 1.3 Amorphous Materials . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) . . . . . . . 8 1.5 Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.6 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 Modeling Techniques and Methodology Density Functional Theory. . . . . . . . . . . . . . . . . . . . 14 2.1.1 Origin and Formulation . . . . . . . . . . . . . . . . . . 14 2.1.2 Exchange-correlation functional . . . . . . . . . . . . . . 16 2.1.3 Numerical resolution . . . . . . . . . . . . . . . . . . . . 19 2.1.4 Pseudo-Potentials . . . . . . . . . . . . . . . . . . . . . 22 2.1.5 Periodicity . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.1.6 Material Properties . . . . . . . . . . . . . . . . . . . . . 27 2.2 Molecular Dynamics . . . . . . . . . . . . . . . . . . . . . . . . 33 2.3 Creation of Amorphous Models . . . . . . . . . . . . . . . . . . 36 2.3.1 Simulated Annealing . . . . . . . . . . . . . . . . . . . . 39 2.3.2 Genetic Algorithm . . . . . . . . . . . . . . . . . . . . . 40 2.3.3 Basin Hopping . . . . . . . . . . . . . . . . . . . . . . . 45 2.3.4 Direct creation . . . . . . . . . . . . . . . . . . . . . . . 45 2.3.5 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 53 2.4 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3 Structure and properties of amorphous and crystalline InGaZnO 4 3.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 3.2 Structural characteristics . . . . . . . . . . . . . . . . . . . . . 66 3.3 From crystalline to amorphous IGZO . . . . . . . . . . . . . . . 69 3.4 Conduction band . . . . . . . . . . . . . . . . . . . . . . . . . . 72 3.5 Valence band and tail states . . . . . . . . . . . . . . . . . . . . 74 3.6 Impact of mechanical strain . . . . . . . . . . . . . . . . . . . . 77 3.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4 Oxygen vacancies in a-IGZO. 4.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 4.2 Influence of the Model Size . . . . . . . . . . . . . . . . . . . . 85 4.3 Effect of Oxygen Vacancies . . . . . . . . . . . . . . . . . . . . 88 4.3.1 Position of the Vacancies Kohn-Sham Levels . . . . . . 88 4.3.2 Position of the Vacancies Charges Transition Level . . . 90 4.3.3 Occurrence of meta-stable vacancies . . . . . . . . . . . 95CONTENTS xv 4.4 Discussion and correlation with experiments . . . . . . . . . . . 98 4.4.1 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . 98 4.4.2 Negative Bias Stress . . . . . . . . . . . . . . . . . . . . 99 4.4.3 Negative Bias Illumination Stress . . . . . . . . . . . . . 100 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102 5 Holes, polarons and NBIS in a-IGZO. 5.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.2 Effect of hole injection . . . . . . . . . . . . . . . . . . . . . . . 110 5.3 Origin of NBIS . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 5.4 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . .120 6 The Delocalization Problem 6.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 6.2 Localization of the Conduction Band States . . . . . . . . . . . 124 6.3 Modeling of the Localization Process . . . . . . . . . . . . . . . 125 6.4 Impact on the Conduction Mechanism . . . . . . . . . . . . . . 130 6.5 Possible Effects on the Study of Defects . . . . . . . . . . . . . 131 6.6 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . .132 7 A simplified model for defects in a-IGZO 7.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 7.2 Defects in Amorphous a-IGZO . . . . . . . . . . . . . . . . . . 140 7.2.1 Metal-Metal Defects . . . . . . . . . . . . . . . . . . . . 143 7.2.2 Peroxide Defects . . . . . . . . . . . . . . . . . . . . . . 146 7.2.3 Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . 150 7.2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 152 7.3 Doping Mechanism in a-IGZO . . . . . . . . . . . . . . . . . . . 152 7.4 Instabilities Mechanism in A-IGZO . . . . . . . . . . . . . . . . 156 7.4.1 Negative Bias Stress . . . . . . . . . . . . . . . . . . . . 156 7.4.2 Positive Bias Stress . . . . . . . . . . . . . . . . . . . . 158 7.4.3 Negative Bias Illumination Stress . . . . . . . . . . . . . 159 7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 8 A method to quantify the delocalization of the electronic states in amorphous semiconductors and its application to assessing charge carrier mobility of p-type amorphous oxide semiconductors. 8.1 Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 8.2 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 8.2.1 Convergence . . . . . . . . . . . . . . . . . . . . . . . . 170 8.2.2 ISWO sensitivity to inter-atomic interactions . . . . . . 171 8.2.3 ISWO sensitivity to the electronic interactions . . . . . 173 8.2.4 Validation on crystals and molecules . . . . . . . . . . . 174 8.3 Application to amorphous structures . . . . . . . . . . . . . . . 177 8.4 Analysis of the properties of P-type oxide semiconductors candidates 179 8.5 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 9 On the possibility of amorphous P-type semiconductors based on Gold, Silver and Copper. 185 9.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 9.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 9.3 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . .192 10 Conclusion and Outlook 10.1 Creation of Relevant Amorphous Model . . . . . . . . . . . . . 194 10.2 Modeling Challenges . . . . . . . . . . . . . . . . . . . . . . . . 195 10.3 Interpretation Challenges . . . . . . . . . . . . . . . . . . . . . 195 10.4 Properties of a-IGZO . . . . . . . . . . . . . . . . . . . . . . . . 197 10.5 Amorphous p-types . . . . . . . . . . . . . . . . . . . . . . . . . 199 10.6 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 10.6.1 Modeling opportunities . . . . . . . . . . . . . . . . . . 200 10.6.2 Future Prospects on the Fundamental of a-IGZO . . . . 203 10.6.3 On the Possibility of Amorphous P-types . . . . . . . . 204 Bibliography 207 Curriculum vitae 233 List of publications 235status: publishe

Comparison of the electronic structure of amorphous versus crystalline indium gallium zinc oxide semiconductor : structure, tail states and strain effects

We study the evolution of the structural and electronic properties of crystalline indium gallium zinc oxide (IGZO) upon amorphization by first-principles calculation. The bottom of the conduction band (BCB) is found to be constituted of a pseudo-band of molecular orbitals that resonate at the same energy on different atomic sites. They display a bonding character between the s orbitals of the metal sites and an anti-bonding character arising from the interaction between the oxygen and metal s orbitals. The energy level of the BCB shifts upon breaking of the crystal symmetry during the amorphization process, which may be attributed to the reduction of the coordination of the cationic centers. The top of the valence band (TVB) is constructed from anti-bonding oxygen p orbitals. In the amorphous state, they have random orientation, in contrast to the crystalline state. This results in the appearance of localized tail states in the forbidden gap above the TVB. Zinc is found to play a predominant role in the generation of these tail states, while gallium hinders their formation. Last, we study the dependence of the fundamental gap and effective mass of IGZO on mechanical strain. The variation of the gap under strain arises from the enhancement of the anti-bonding interaction in the BCB due to the modification of the length of the oxygen–metal bonds and/or to a variation of the cation coordination. This effect is less pronounced for the amorphous material compared to the crystalline material, making amorphous IGZO a semiconductor of choice for flexible electronics. Finally, the effective mass is found to increase upon strain, in contrast to regular materials. This counterintuitive variation is due to the reduction of the electrostatic shielding of the cationic centers by oxygen, leading to an increase of the overlaps between the metal orbitals at the origin of the delocalization of the BCB. For the range of strain typically met in flexible electronics, the induced variation in the effective mass is found to be negligible(less than 1%).status: publishe

Defects in Amorphous Semiconductors: The Case of Amorphous Indium Gallium Zinc Oxide

© 2018 American Physical Society. Based on a rational classification of defects in amorphous materials, we propose a simplified model to describe intrinsic defects and hydrogen impurities in amorphous indium gallium zinc oxide (a-IGZO). The proposed approach consists of organizing defects into two categories: point defects, generating structural anomalies such as metal - metal or oxygen - oxygen bonds, and defects emerging from changes in the material stoichiometry, such as vacancies and interstitial atoms. Based on first-principles simulations, it is argued that the defects originating from the second group always act as perfect donors or perfect acceptors. This classification simplifies and rationalizes the nature of defects in amorphous phases. In a-IGZO, the most important point defects are metal - metal bonds (or small metal clusters) and peroxides (O-O single bonds). Electrons are captured by metal - metal bonds and released by the formation of peroxides. The presence of hydrogen can lead to two additional types of defects: metal-hydrogen defects, acting as acceptors, and oxygen-hydrogen defects, acting as donors. The impact of these defects is linked to different instabilities observed in a-IGZO. Specifically, the diffusion of hydrogen and oxygen is connected to positive- and negative-bias stresses, while negative-bias illumination stress originates from the formation of peroxides.status: publishe

Method to quantify the delocalization of electronic states in amorphous semiconductors and its application to assessing charge carrier mobility of p-type amorphous oxide semiconductors

Amorphous semiconductors are usually characterized by a low charge carrier mobility, essentially related to their lack of long-range order. The development of such material with higher charge carrier mobility is hence challenging. Part of the issue comes from the difficulty encountered by first-principles simulations to evaluate concepts such as the electron effective mass for disordered systems since the absence of periodicity induced by the disorder precludes the use of common concepts derived from condensed matter physics. In this paper, we propose a methodology based on first-principles simulations that partially solves this problem, by quantifying the degree of delocalization of a wave function and of the connectivity between the atomic sites within this electronic state. We validate the robustness of the proposed formalism on crystalline and molecular systems and extend the insights gained to disordered/amorphous InGaZnO4 and Si. We also explore the properties of p-type oxide semiconductor candidates recently reported to have a low effective mass in their crystalline phases [G. Hautier et al., Nat. Commun. 4, 2292 (2013)]. Although in their amorphous phase none of the candidates present a valence band with delocalization properties matching those found in the conduction band of amorphous InGaZnO4, three of the seven analyzed materials show some potential. The most promising candidate, K2Sn2O3, is expected to possess in its amorphous phase a slightly higher hole mobility than the electron mobility in amorphous silicon.status: publishe

Defects in amorphous semiconductors : the case of amorphous indium gallium zinc oxide

© 2018 American Physical Society. Based on a rational classification of defects in amorphous materials, we propose a simplified model to describe intrinsic defects and hydrogen impurities in amorphous indium gallium zinc oxide (a-IGZO). The proposed approach consists of organizing defects into two categories: point defects, generating structural anomalies such as metal - metal or oxygen - oxygen bonds, and defects emerging from changes in the material stoichiometry, such as vacancies and interstitial atoms. Based on first-principles simulations, it is argued that the defects originating from the second group always act as perfect donors or perfect acceptors. This classification simplifies and rationalizes the nature of defects in amorphous phases. In a-IGZO, the most important point defects are metal - metal bonds (or small metal clusters) and peroxides (O-O single bonds). Electrons are captured by metal - metal bonds and released by the formation of peroxides. The presence of hydrogen can lead to two additional types of defects: metal-hydrogen defects, acting as acceptors, and oxygen-hydrogen defects, acting as donors. The impact of these defects is linked to different instabilities observed in a-IGZO. Specifically, the diffusion of hydrogen and oxygen is connected to positive- and negative-bias stresses, while negative-bias illumination stress originates from the formation of peroxides.status: publishe

Mechanical and electronic properties of thin-film transistors on plastic, and their integration in flexible electronic applications

The increasing interest in flexible electronics and flexible displays raises questions regarding the inherent mechanical properties of the electronic materials used. Here, the mechanical behavior of thin-film transistors used in active-matrix displays is considered. The change of electrical performance of thin-film semiconductor materials under mechanical stress is studied, including amorphous oxide semiconductors. This study comprises an experimental part, in which transistor structures are characterized under different mechanical loads, as well as a theoretical part, in which the changes in energy band structures in the presence of stress and strain are investigated. The performance of amorphous oxide semiconductors are compared to reported results on organic semiconductors and covalent semiconductors, i.e., amorphous silicon and polysilicon. In order to compare the semiconductor materials, it is required to include the influence of the other transistor layers on the strain profile. The bending limits are investigated, and shown to be due to failures in the gate dielectric and/or the contacts. Design rules are proposed to minimize strain in transistor stacks and in transistor arrays. Finally, an overview of the present and future applications of flexible thin-film transistors is given, and the suitability of the different material classes for those applications is assessed