Low temperature characteristics in amorphous indium-gallium-zinc-oxide thin-film transistors down to 10 K (original) (raw)
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Physical Review B, 2016
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.
Journal of Applied Physics, 2011
For nondegenerate bulk semiconductors, we have used the virial theorem to derive an expression for the temperature T j of the transition from the regime of "free" motion of electrons in the c-band (or holes in the t-band) to their hopping motion between donors (or acceptors). Distribution of impurities over the crystal was assumed to be of the Poisson type, while distribution of their energy levels was assumed to be of the Gaussian type. Our conception of the virial theorem implementation is that the transition from the band-like conduction to hopping conduction occurs when the average kinetic energy of an electron in the c-band (hole in the t-band) is equal to the half of the absolute value of the average energy of the Coulomb interaction of an electron (hole) with the nearest neighbor ionized donor (acceptor). Calculations of T j according to our model agree with experimental data for crystals of Ge, Si, diamond, etc. up to the concentrations of a hydrogen-like impurity, at which the phase insulator-metal transition (Mott transition) occurs. Under the temperature T h % T j /3, when the nearest neighbor hopping conduction via impurity atoms dominates, we obtained expressions for the electrostatic field screening length K h in the Debye-Hückel approximation, taking into account a nonzero width of the impurity energy band. It is shown that the measurements of quasistatic capacitance of the semiconductor in a metal-insulator-semiconductor structure in the regime of the flat bands at the temperature T h allow to determine the concentration of doping impurity or its compensation ratio by knowing K h .
Charge transport in amorphous InGaZnO thin-film transistors
Physical Review B, 2012
We investigate the mechanism of charge transport in indium gallium zinc oxide (a-IGZO), an amorphous metal-oxide semiconductor. We measured the field-effect mobility and the Seebeck coefficient (S = V / T ) of a-IGZO in thin-film transistors as a function of charge-carrier density for different temperatures. Using these transistors, we further employed a scanning Kelvin probe-based technique to determine the density of states of a-IGZO that is used as the basis for the modeling. After comparing two commonly used models, the band transport percolation model and a mobility edge model, we find that both cannot describe the full properties of the charge transport in the a-IGZO semiconductor. We, therefore, propose a model that extends the mobility edge model to allow for variable range hopping below the mobility edge. The extended mobility edge model gives a superior description of the experimental results. We show that the charge transport is dominated by variable range hopping below, rather than by bandlike transport above the mobility edge.
Temperature and electric-field dependence of hopping transport in low-dimensional devices
Physical review. B, Condensed matter, 1994
A theoretical calculation has been performed for the variable-range-hopping (VRH) conduction mechanism in the presence of temperature and electric field for quasi-two-dimensional (QTD) and quasi-onedimensional (QOD) systems. In the present calculation, it is assumed that the localized states are randomly distributed both in energy and space coordinates. The states both below and above the Fermi level are included in the calculation of the hopping range and conductivity. The present approach diS'ers significantly from the percolation method and others in the calculation of the mobility and the conductivity. The expressions for the hopping range, the mobility, and the conductivity are obtained for the constant and the energy-dependent density of states. The expression of the conductivity for the constant density of states can be reduced to that of Mott in certain approximations. The em'ect of electronelectron interaction in the calculation of the conductivity and hopping range has been included through the density of states. After some approximations, the present expression of the conductivity can be reduced to that of Efros and Shklovskii. The logarithm of the conductivity follows the (1-P~)~' and +I-P' electric-field dependence for QTD and QOD systems, respectively, in the presence of the electron-electron interaction and a weak electric field. Here P is directly proportional to an electric field. The present calculations are applied to explain the recent conductivity experiments on PrBa&Cu307 y (PBCO) films. A possible crossover from Mott-type VRH to Efros-and-Shklovskii-type VRH has been observed in PBCO.
Physical Review B
Unraveling the dominant charge transport mechanism in high-mobility amorphous oxide semiconductors is still a matter of controversy. In the present study we extended the random band-edge model suggested before for the charge transport and Hall-effect mobility in such disordered materials [Fishchuk et al., Phys. Rev. B 93, 195204 (2016)], and also describe the field-effect-modulated thermoelectricity in amorphous In-Ga-Zn-O (a-IGZO) films under the same premises. The model is based on the concept of charge transport through the extended states and assumes that the transport is limited by the spatial variation of the position of the band edge due to the disorder potential, rather than by localized states. The theoretical model is formulated using the effective medium approximation framework and describes well basic features of the Seebeck coefficient in disordered materials as a function of energy disorder, carrier concentration, and temperature. Carrier concentration dependencies of power factor and thermoelectric figure of merit have been also considered for such systems. Besides, our calculations reveal a remarkable turnover effect from a negative to a positive temperature dependence of Seebeck coefficient upon increasing carrier concentration. The suggested unified model provides a good quantitative description of available experimental data on the Seebeck coefficient and the charge mobilities measured in the same a-IGZO transistor as a function of the gate voltage and temperature by considering the same charge transport mechanisms. This promotes a deeper understanding and a more credible and accurate description of the transport process in a-IGZO films.
Temperature dependent electronic conduction in semiconductors
Physics Reports, 1980
Activation energies for electronic conduction in crystalline 2.5. High field effects 114 solids 61 2.6. Conclusion 120 1.1. Introduction 61 3. Electronic conduction in molecular crystals 1.2. Location of the Fermi level in semiconductors 71 3.1. Introduction 1.3. Temperature induced changes in activation energies 82 3.2. Electronic structure of molecular crystals 1.4. Some complicating features in activation energy 3.3. Narrow-band conduction analysis 90 3.4. Polaron conduction 1.5. Deep-level impurities in semiconductors 96 3.5. Molecular theories 2. Electronic conduction in disordered semiconductors 101 3.6. Highly-conducting materials 2.1. Introduction 101 3.7. Trapping 2.2. The electronic structure of disordered semiconductors 102 3.8. Summary and conclusions 2.3. Extended state conduction 105 References 2.4. Conduction in localised states 107 * Now at Royal Signals and Radar Establishment, Malvern.
Temperature dependent electron transport in amorphous oxide semiconductor thin film transistors
2011 International Electron Devices Meeting, 2011
A temperature-dependent mobility model in amorphous oxide semiconductor (AOS) thin film transistors (TFTs) extracted from measurements of source-drain terminal currents at different gate voltages and temperatures is presented. At low gate voltages, trap-limited conduction prevails for a broad range of temperatures, whereas variable range hopping becomes dominant at lower temperatures. At high gate voltages and for all temperatures, percolation conduction comes into the picture. In all cases, the temperature-dependent mobility model obeys a universal power law as a function of gate voltage.
2006
When a strong, though non-Arrhenius temperature dependence of electrical resistivity is observed, one usually concludes that the underlying mechanism is variable-range hopping. Unexpectedly, such observations are also made for many semiconductor systems at elevated temperatures, where a variable-range hopping mechanism seems unlikely. A satisfactory explanation for this observation is still lacking up to now. The authors demonstrate that a non-Arrhenius resistivity behavior may also arise in a band transport picture by thermal activation of charge carriers from a reservoir into the transport-carrying band states, provided the energy distribution of reservoir states is sufficiently broadened or the density of band states exhibits tails.