Velocity renormalization and Dirac cone multiplication in graphene superlattices with various barrier edge geometries

The electronic properties of one-dimensional graphene superlattices strongly depend on the atomic size and orientation of the 1D external periodic potential. Using a tight-binding approach, we show that the armchair and zigzag directions in these superlattices have a different impact on the renormalization of the anisotropic velocity of the charge carriers. For symmetric potential barriers, the velocity perpendicular to the barrier is modified for the armchair direction while remaining unchanged in the zigzag case. For asymmetric barriers, the initial symmetry between the forward and backward momentum with respect to the Dirac cone symmetry is broken for the velocity perpendicular (armchair case) or parallel (zigzag case) to the barriers. At last, Dirac cone multiplication at the charge neutrality point occurs only for the zigzag geometry. In contrast, band gaps appear in the electronic structure of the graphene superlattice with barrier in the armchair direction.Comment: 13 pages, 14 figure

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

Reevaluation of the origin of negative bias stress with and without light exposition in amorphous Indium-Gallium-Zinc-Oxide

Amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) is a promising material for active channels in thin film transistors. In addition to be processable directly on plastic substrate, it offers better electronic and mechanical characteristics than amorphous silicon, which is wildly used for large area application such as in displays. Nevertheless, long time bias stress of a-IGZO based transistor can lead to large shifts of the transfer curve (Ids-Vgs curve). This instability is further enhanced in presence of blue or near UV light and is problematic for the operation of complex circuitry. Its origin is often associated to the presence of oxygen vacancies and the phenomena arising upon illumination are assumed to have the same origin. Based on first-principles simulations and experimental results, we will argue in this work that these are two separated phenomena and propose a new explanation for the negative bias stress instability. Negative bias stress in presence of light will also be discussed. We will argue that this phenomena is incompatible with the nature of isolated oxygen vacancies in a-IGZO.status: accepte

Origin of the apparent delocalization of the conduction band in high mobility amorphous semiconductors

In this paper, we show that the apparent delocalization of the conduction band reported from first-principles simulations for the high-mobility amorphous oxide semiconductor InGaZnO4 (a-IGZO) is an artifact induced by the periodic conditions imposed to the model. Given a sufficiently large unit-cell dimension (over 40Å), the conduction band becomes localized. Such model size is up to four times the size of commonly used models for the study of a-IGZO. This finding challenges the analyses done so far on the nature of the defects and on the interpretation of numerous electrical measurements. In particular, we re-interpret the meaning of the computed effective mass reported so far in literature. Our finding also applies to materials such as SiZnSnO, ZnSnO, InZnSnO, In2O3 or InAlZnO4 whose models have been reported to display a fully delocalized conduction band in the amorphous phase.journal_title: Journal of Physics: Condensed Matter article_type: paper article_title: Origin of the apparent delocalization of the conduction band in a high-mobility amorphous semiconductor copyright_information: © 2017 IOP Publishing Ltd date_received: 2016-12-20 date_accepted: 2017-02-15 date_epub: 2017-05-24status: publishe

Velocity renormalization and Dirac cone multiplication ind graphene superlattices with various barrier-edge geometries

The electronic properties of one-dimensional graphene superlattices strongly depend on the atomic size and orientation of the 1D external periodic potential. Using a tight-binding approach, we show that the armchair and zigzag directions in these superlattices have a different impact on the renormalization of the anisotropic velocity of the charge carriers. For symmetric potential barriers, the velocity perpendicular to the barrier is modified for the armchair direction while remaining unchanged in the zigzag case. For asymmetric barriers, the initial symmetry between the forward and backward momentum with respect to the Dirac cone symmetry is broken for the velocity perpendicular (armchair case) or parallel (zigzag case) to the barriers. At last, Dirac cone multiplication at the charge neutrality point occurs only for the zigzag geometry. In contrast, band gaps appear in the electronic structure of the graphene superlattice with barrier in the armchair direction

Oxygen vacancies effects in a-IGZO: Formation mechanisms, hysteresis, and negative bias stress effects

The amorphous oxide semiconductor Indium-Gallium-Zinc-Oxide (a-IGZO) has gained a large technological relevance as a semiconductor for thin-film transistors in active-matrix displays. Yet, major questions remain unanswered regarding the atomic origin of threshold voltage control, doping level, hysteresis, negative bias stress (NBS), and negative bias illumination stress (NBIS). We undertake a systematic study of the effects of oxygen vacancies on the properties of a-IGZO by relating experimental observations to microscopic insights gained from first-principle simulations. It is found that the amorphous nature of the semiconductor allows unusually large atomic relaxations. In some cases, oxygen vacancies are found to behave as perfect shallow donors without the formation of structural defects. Once structural defects are formed, their transition states can vary upon charge and discharge cycles. We associate this phenomenon to a possible presence of hysteresis in the transfer curve of the devices. Under NBS, the creation of oxygen vacancies becomes energetically very stable, hence thermodynamically very likely. This generation process is correlated with the occurrence of the negative bias stress instabilities observed in a-IGZO transistors. While oxygen vacancies can therefore be related to NBS and hysteresis, it appears unlikely from our results that they are direct causes of NBIS, contrary to common belief.status: publishe

Interplay between hopping and band transport in high-mobility disordered semiconductors at large carrier concentrations: The case of the amorphous oxide InGaZnO

We suggest an analytic theory based on the effective medium approximation (EMA) which is able to describe charge-carrier transport in a disordered semiconductor with a significant degree of degeneration realized at high carrier concentrations, especially relevant in some thin-film transistors (TFTs), when the Fermi level is very close to the conduction-band edge. The EMA model is based on special averaging of the Fermi-Dirac carrier distributions using a suitably normalized cumulative density-of-state distribution that includes both delocalized states and the localized states. The principal advantage of the present model is its ability to describe universally effective drift and Hall mobility in heterogeneous materials as a function of disorder, temperature, and carrier concentration within the same theoretical formalism. It also bridges a gap between hopping and bandlike transport in an energetically heterogeneous system. The key assumption of the model is that the charge carriers move through delocalized states and that, in addition to the tail of the localized states, the disorder can give rise to spatial energy variation of the transport-band edge being described by a Gaussian distribution. It can explain a puzzling observation of activated and carrier-concentration-dependent Hall mobility in a disordered system featuring an ideal Hall effect. The present model has been successfully applied to describe experimental results on the charge transport measured in an amorphous oxide semiconductor, In-Ga-Zn-O (a-IGZO). In particular, the model reproduces well both the conventional Meyer-Neldel (MN) compensation behavior for the charge-carrier mobility and inverse-MN effect for the conductivity observed in the same a-IGZO TFT. The model was further supported by ab initio calculations revealing that the amorphization of IGZO gives rise to variation of the conduction-band edge rather than to the creation of localized states. The obtained changes agree with the one we used to describe the charge transport. We found that the band-edge variation dominates the charge transport in high-quality a-IGZO TFTs in the above-threshold voltage region, whereas the localized states need not to be invoked to account for the experimental results in this material.status: publishe

Flexible metal-oxide thin film transistor circuits for RFID and health patches

We discuss in this paper the present state and future perspectives of thin-film oxide transistors for flexible electronics. The application case that we focus on is a flexible health patch containing an analog sensor interface as well as digital electronics to transmit the acquired data wirelessly to a base station. We examine the electronic performance of amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) during mechanical bending. We discuss several ways to further boost the electronic transistor performance of n-type amorphous oxide semiconductors, by modifying the semiconductor or by improving the transistor architecture. We show analog and digital circuits constructed with several architectures, all based on n-type-only amorphous oxide technology. From circuit point of view, the discovery of a p-type amorphous semiconductor matching known n-type amorphous semiconductors would be of great importance. The present best-suited p-type is SnO, but it is poly-crystalline in nature and shows some ambipolarity due to the presence of n-type SnO2. In search of a better p-type semiconductor, preferably amorphous, we present recent insights into the band structure of potential amorphous oxide p-type semiconductors.status: publishe