We investigated the impact of the passivation layer on the stability of indium-gallium-zinc oxide (IGZO) thin film transistors. While the device without any passivation layer showed a huge threshold voltage (Vth) shift under positive gate voltage stress, the suitably passivated device did not exhibit any Vth shift. The charge trapping model, which has been believed to be a plausible mechanism, cannot by itself explain this behavior. Instead, the Vth instability was attributed to the interaction between the exposed IGZO backsurface and oxygen and/or water in the ambient atmosphere during the gate voltage stress.

Origin of threshold voltage instability in indium-gallium-zinc oxide thin film transistors

Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

The basic physics underlying the operation and key performance issues of amorphous-silicon thin-film transistors (TFTs) are discussed. The static transistor characteristics are determined by the localized electronic states that occur in the bandgap of the amorphous silicon. The deep states, mostly consisting of Si dangling bonds, determine the threshold voltage, and the conduction band-tail states determine the field-effect mobility. The finite capture and emission times of the deep localized states lead to a dynamic transistor characteristic that can be described by a time-dependent threshold voltage. The transistors also show longer time threshold voltage shifts due to two other distinct mechanisms: charge trapping in the silicon nitride gate insulator and metastable dangling bond state creation in the amorphous silicon. These two mechanisms show characteristically different bias, temperature, and time dependencies of the threshold voltage shift. Illumination of a TFT causes the generation of electron-hole pairs in the space-charge region leading to a steady-state equal flux of electrons and holes and a reduction in the band-bending. In most applications, the photosensitivity should be minimized. The uniformity of large arrays of transistors for display applications is excellent, with variations in the threshold voltage of 0.5-1.0 V. >

The physics of amorphous-silicon thin-film transistors

We have measured the time and temperature dependence of the two prominent instability mechanisms in amorphous silicon thin‐film transistors, namely, the creation of metastable states in the a‐Si:H and the charge trapping in the silicon nitride gate insulator. The state creation process shows a power law time dependence and is thermally activated. The charge trapping process shows a logarithmic time dependence and has a very small temperature dependence. The results for the state creation process are consistent with a model of Si dangling bond formation in the bulk a‐Si:H due to weak SiSi bond breaking stabilized by diffusive hydrogen motion. The logarithmic time dependence and weak temperature dependence for the charge trapping in the nitride suggest that the charge injection from the a‐Si:H to the nitride is the rate limiting step and not subsequent conduction in the nitride.

Time and temperature dependence of instability mechanisms in amorphous silicon thin-film transistors

We have measured the bias dependence of the threshold voltage shift in a series of amorphous silicon‐silicon nitride thin‐film transistors, where the composition of the nitride is varied. There are two distinct instability mechanisms: a slow increase in the density of metastable fast states and charge trapping in slow states. State creation dominates at low fields and charge trapping dominates at higher fields. The state creation is found to be independent of the nitride composition, whereas the charge trapping depends strongly on the nitride composition. This is taken as good evidence that state creation takes place in the hydrogenated amorphous silicon (a‐Si:H) layer, whereas the charge trapping takes place in the a‐SiN:H. The metastable states are suggested to be Si dangling bonds in the a‐Si:H, and the state creation process similar to the Staebler–Wronski effect. The confirmation of state creation in a thin‐film transistor means that states can be created simply by populating conduction‐band states i...

Bias dependence of instability mechanisms in amorphous silicon thin‐film transistors

When a positive gate voltage is applied to an amorphous-silicon thin-film transistor, electrons become trapped in states close to the silicon-dielectric interface. This is studied by a new technique involving the transient discharge current produced under illumination. It is suggested that the behavior may involve metastable dangling bonds generated within the amorphous silicon as a consequence of the field-effect-induced increase in electron concentration. This constitutes an important new instability mechanism for amorphous-silicon thin-film transistors.

Metastable defects in amorphous-silicon thin-film transistors.

We report simultaneous measurements of the threshold voltage shift under positive bias stress and transient discharge following such stressing, on amorphous silicon - silicon nitride thin film transistors (α-Si : H TFTs). The discharge transients exhibit two components which are both due to emission from deep states within the α-Si : H.

Charge trapping effects in amorphous silicon/silicon nitride thin film transistors

We report on the opto-electronic and structural properties of polysilicon produced by excimer (ArF) laser crystallisation of undoped hydrogenated amorphous silicon (a-Si:H). Micro structure and average grain size are determined by TEM and electron diffraction. A maximum areal grain size of 0.1 μm2 is observed in excimer (ArF) laser crystallised polysilicon. UV reflectivity and FTIR spectroscopy are employed to investigate the degree of crystallinity and atomic bonding configurations. These data are compared with studies of low-temperature (600 °C) furnace crystallised polysilicon.

Polysilicon produced by excimer (ArF) laser crystallisation and low-temperature (600°C) furnace crystallisation of hydrogenated amorphous silicon (a-Si:H)

When a positive gate voltage is applied to an amorphous silicon thin film transistor, electrons become trapped in states close to the silicon/dielectric interface. The density of these states rises continuously during the gate field application period. Various characteristics of the phenomenon are identified and a model is proposed in which metastable silicon dangling bonds are generated as a consequence of field effect induced band bending.

Evidence for metastable defects in amorphous silicon thin film transistors

Numerical modelling is employed to explore the information which can be obtained from studies of the ‘constant photocurrent’ response for disordered semiconductors. Computer simulation is used to determine the experimental behaviour for a range of N(E) distributions of differing functional form. The results constitute input data to which two different interpretive techniques are applied, allowing an assessment of the viability and limitations of such procedures.

A.R. Hepburn

Papers

Metastable defects in amorphous-silicon thin-film transistors.

Charge trapping effects in amorphous silicon/silicon nitride thin film transistors

Polysilicon produced by excimer (ArF) laser crystallisation and low-temperature (600°C) furnace crystallisation of hydrogenated amorphous silicon (a-Si:H)

Evidence for metastable defects in amorphous silicon thin film transistors

Evaluation of the constant photocurrent method for determining the energy distribution of localised states in disordered semiconductors