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We demonstrated self-aligned amorphous-Indium-Gallium-Zinc-Oxide (a-IGZO) thin-film transistors (TFTs) where the source–drain (S/D) regions were made conductive via chemical reduction of the a-IGZO via metallic calcium (Ca). Due to the higher chemical reactivity of Ca, the process can be operated at lower temperatures. The Ca process has the additional benefit of the reaction byproduct calcium oxide being removable through a water rinse step, thus simplifying the device integration. The Ca-reduced a-IGZO showed a sheet resistance (RSHEET) value of 0.7 kΩ/sq., with molybdenum as the S/D metal. The corresponding a-IGZO TFTs exhibited good electrical properties, such as a field-effect mobility (μFE) of 12.0 cm2/(V s), a subthreshold slope (SS−1) of 0.4 V/decade, and an on/off current ratio (ION/OFF) above 108. Cop. 2015 The Korean Information Display Society.

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. ispartof: Physical Review B vol:93 issue:19 status: published

It is shown in this paper that the finite resistance of the accumulation channel in amorphous In–Ga–Zn–O thin-film transistors (a-IGZO TFTs) is the main cause of the frequency dispersion of the capacitance–voltage curves in these devices. A transmission line model, accounting for the distributed nature of channel resistance, is used to explain this. Multi-frequency analysis techniques for trap density distribution use a lumped series resistance model and attribute dispersion solely to the charging and discharging of trap states. As the resistance–capacitance (RC) time constant values of the IGZO TFTs are in the range of 10–100 µs, a distributed RC network is better suited for the measured frequency range (1 kHz–1 MHz). cop. 2014 The Korean Information Display Society.

In this work we show that the negative bias illumination stress (NBIS) of amorphous Indium Gallium Zinc Oxide (a-IGZO) transistors with an etch stop layer (ESL) deposited by physical vapor deposition (PVD) is substantially better than the NBIS of devices where the ESL layer is deposited by plasma enhanced chemical vapor deposition (PECVD). Both devices show similar transistor characteristics and bias stress in the dark but under NBIS conditions at 425 nm, PVD ESL based transistors show much less threshold voltage shift. The reduction in deep defects due to passivation by PVD layer is responsible for improved performance under NBIS. ispartof: pages:302-304 ispartof: 44th European Solid-State Device Conference - ESSDERC pages:302-304 ispartof: 44th European Solid-State Device Conference - ESSDERC location:Venice Italy date:9 Jan 2014 status: published

We investigate the relationship between charge concentration, exciton concentration, and photocurrent generation in fullerene-containing heterojunction diodes. Impedance measurements on ${\mathrm{C}}_{60}$ diodes reveal a charge buildup at the ${\mathrm{C}}_{60}$/bathocuproine (BCP) interface that can be swept out under reverse bias. In solar cell structures, a similar charge buildup is observed in dark conditions, and increases as a function of incident light intensity. Photoluminescence measurements reveal that the ${\mathrm{C}}_{60}$ exciton concentration is voltage dependent, explained via the process of exciton-polaron annihilation. This process has a negative impact on the generated photocurrent of the solar cells and thereby decreases the fill factor. A combination of electroabsorption, photoluminescence, and impedance measurements reveal a decrease in charge buildup and the associated exciton-polaron annihilation through the use of a BCP/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag cathode.

The first direct measurement is reported of the bulk density of deep states in amorphous IGZO (indium-gallium-zinc oxide) semiconductor by means of deep-level transient spectroscopy (DLTS). The device under test is a Schottky diode of amorphous IGZO semiconductor on a palladium (Pd) Schottky-barrier electrode and with a molybdenum (Mo) Ohmic contact at the top. The DLTS technique allows to independently measure the energy and spatial distribution of subgap states in the IGZO thin film. The subgap trap concentration has a double exponential distribution as a function energy, with a value of ∼1019 cm−3 eV−1 at the conduction band edge and a value of ∼1017 cm−3 eV−1 at an energy of 0.55 eV below the conduction band. Such spectral distribution, however, is not uniform through the semiconductor film. The spatial distribution of subgap states correlates well with the background doping density distribution in the semiconductor, which increases towards the Ohmic Mo contact, suggesting that these two properties share the same physical origin. ispartof: Applied Physics Letters vol:104 issue:8 status: published