M. J. Powell

The physics of amorphous-silicon thin-film transistors

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. >

Gap states in silicon nitride

The energy levels of defect states in amorphous silicon nitride have been calculated and the results are used to identify the nature of trap states responsible for charge trapping during transport and the charge storage leading to memory action. We argue that the Si dangling bond is the memory trap in chemical vapor deposited memory devices and is also the center in plasma‐deposited nitride responsible for hopping at low electric fields and for charge‐trapping instabilities in amorphous silicon‐silicon nitride thin‐film transistors.

Time and temperature dependence of instability mechanisms in 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.

Charge trapping instabilities in amorphous silicon‐silicon nitride thin‐film transistors

The most important instability mechanism in amorphous silicon‐silicon nitride thin‐film transistors is charge trapping in the silicon nitride layer, which leads to a threshold voltage shift (ΔVT). We have measured the time, temperature, and gate voltage dependence of ΔVT and conclude that the rate limiting process, in the charge transfer from semiconductor to insulator, is the conduction in the nitride by variable‐range hopping. The threshold shift (under positive bias) is temperature dependent with an activation energy of 0.3 eV. This activation energy is identified with the mean hop energy required to inject charge deep into the silicon nitride at the low applied fields appropriate to transistor operation.

Bias 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...

Resolution of amorphous silicon thin-film transistor instability mechanisms using ambipolar transistors

Bias stress measurements on amorphous silicon‐silicon nitride ambipolar thin‐film transistors give clear evidence for the co‐existence of two distinct instability mechanisms: the metastable creation of states in the a‐Si:H layer and charge trapping in the a‐SiN:H layer. The creation of metastable states in the a‐Si:H is found to dominate at low positive bias, while charge trapping in the nitride dominates at larger positive bias and negative bias.

Relative importance of the Si-Si bond and Si-H bond for the stability of amorphous silicon thin film transistors

We investigate the mechanism for Si dangling bond defect creation in amorphous silicon thin film transistors as a result of bias stress. We show that the rate of defect creation does not depend on the total hydrogen content or the type of hydrogen bonding in the amorphous silicon. However, the rate of defect creation does show a clear correlation with the Urbach energy and the intrinsic stress in the film. These important results support a localized model for defect creation, i.e., where a Si–Si bond breaks and a nearby H atom switches to stabilize the broken bond, as opposed to models involving the long-range diffusion of hydrogen. Our experimental results demonstrate the importance of optimizing the intrinsic stress in the films to obtain maximum stability and mobility. An important implication is that a deposition process where intrinsic stress can be independently controlled, such as an ion-energy controlled deposition should be beneficial, particularly for deposition temperatures below 300 °C.

Amorphous silicon‐silicon nitride thin‐film transistors

A thin‐film field‐effect transistor has been fabricated using glow‐discharge amorphous silicon as the semiconductor and silicon nitride as the insulator. The transistor operates in the electron (n type) accumulation mode and by changing the gate potential from zero to only 3 V a change in the source‐drain conductance of greater than four orders of magnitude is obtained. The results imply upper limits to the density of gap states in amorphous silicon and interface states at the amorphous silicon‐silicon nitride interface of 3×1016 cm−3 eV−1 and 5×1011 cm−2 eV−1, respectively.

The mechanism of photoconductivity in polycrystalline cadmium sulphide layers

The mechanism of photoconductivity in polycrystalline CdS has been studied over the temperature range 100–300 K using Hall‐effect and conductivity measurements in the dark and under white light illumination. Samples were prepared in thin film form by spray pyrolysis and as power‐binder mixtures. Dark conductivities covered the range 10−9–101 Ω−1 cm−1. Dark conductivity is interpreted in terms of a two‐dimensional version of the grain‐boundary barrier model developed by Seto for polycrystalline Si. Except at very low carrier densities, Hall mobilities are found to be thermally activated, and intergrain barrier heights φb are derived for spray pyrolysis layers with doping levels covering the range N = 1014–1018 cm−3. A maximum barrier φbmax ≊0.2 Ev is found at a corresponding doping level, Nmax ≊2×1016 cm−3, which represents the situation where the barrier depletion layers just extend through the whole grain. From this we derive a mean grain diameter of 0.3 μm in good agreement with the result of transmissi...

The electronic properties of plasma‐deposited films of hydrogenated amorphous SiNx (0<x<1.2)

We present the results of a comprehensive series of measurements on glow‐discharge (plasma) ‐deposited silicon nitride films SiNx:H, with x in the range 0<x<1.2. Optical spectroscopy in the visible and infrared regions is used to investigate the nature of the bonding and to assess the role of hydrogen. With increasing x, in the range x<0.7, an increase in the concentration of Si‐H bonds results in an increase in the total hydrogen content; at higher x the rise in the N‐H concentration produces a small increase in the hydrogen content, but even for these samples most of the hydrogen is bonded to silicon. The optical absorption edge due to band‐gap transitions broadens with increasing x due to a change in the nature of the valence band from Si‐Si bonds to N lone‐pair states. Electrical conductivity at high fields and magnetic resonance measurements give information about the defects in the band gap. These results support the Robertson–Powell model in which the principal defect in the band gap of silicon nit...