Thermionic Emission-diffusion Theory Research Articles

A Simulation Study for Optimizing Grain Size in Poly-Crystalline Silicon Material Using Impedance Spectroscopy

This paper uses impedance spectroscopy as a simulation tool to analyze grain boundaries properties in poly-crystalline materials. The interfacial structure at grain boundaries in poly-crystalline materials plays a decisive role in their electrical behaviors and degrades the performance in integrated circuits, gas sensors and solar photovoltaics. A numerical simulation is carried out to optimize grain size of poly-crystalline material to behave as mono-like material using impedance spectroscopy technique. The grain boundary potential (Vb) obtained from thermionic emission diffusion theory based continuum and localized energy models are utilized for obtaining the resistance (R), capacitance (C) and angular frequency (??) of the bulk of the grain (Rb, Cb and ??b) and that of grain boundaries (Rgb, Cgb and ??gb) using the brick layer model. Two markedly different impedance spectra are obtained, one for the bulk of the grain and another for the grain boundaries. It is found that with an increase in the grain size (Lg), the signatures of grain boundaries spectra diminish and at a particular grain size, disappear completely which indicates that the poly-silicon material behaves comparable to mono-crystalline material. In this simulation work, silicon material is taken as the reference due to its abundant availability and huge market potential for photovoltaic and semiconductor applications but the simulation can be implemented on any poly-crystalline materials. The effect of varying silicon material resistivity (1.0???3.5 ??cm) and temperature (300-420 K) on grain size was also analysed using the impedance spectra. The findings of the paper will be helpful for the silicon ingot growth industries to decide the optimum grain size during the polysilicon materials growth.

Analysis of anomalous transport mechanism across the interface of Ag/p-Si Schottky diode in wide temperature range

We have investigated the detailed temperature dependent mechanism across the interface formed in Ag/p-Si Schottky diode. The Schottky diodes were fabricated by depositing highly pure silver and aluminum metals on the front and back sides of the boron doped p-type semiconductor to form the Schottky and ohmic contacts respectively. The fabricated diode clearly illustrates the rectifying properties between 80 K and 300 K temperature range due to the interface band discontinuity and formation of depleted region. According to thermionic emission diffusion theory the transport mechanisms across the interface reveals abnormal increase in potential barrier and decrease in ideality factor with increase in the temperature due to the potential fluctuations and spatial inhomogeneities at the interface. The inhomogeneities at the interface possess the Gaussian distributions of barrier heights having mean experimental barrier height (ϕ¯b0), 0.91 eV and standard deviation (σ), 0.11 eV respectively. The mean barrier height (ϕ¯b0) of 0.93 eV and Richardson,s coefficient (A∗∗) of 2.7 × 105 Am−2K−2 were obtained from the modified activation energy plot. The Richardson's constant, 2.7 × 105 Am−2K−2 is of the order of known theoretical value of 3.2 × 105 Am−2 K−2 for p-Si. The existence of interface states decreases the capacitance with decrease in the temperature at each applied bias voltage. The higher value obtained for barrier height from CV analysis in comparison to that IV analysis is due the existence of tunneling factor and barrier inhomogeneities. The parameters which characterize the interfacial region were also calculated from CV analysis and found to vary with temperature. The obtained results showed that the fabricated Schottky diode may be a good candidate in the electronic devices.

Unified Physical DC Model of Staggered Amorphous InGaZnO Transistors

In this paper, we propose a unified physical model of InGaZnO [amorphous indium–gallium–zinc-oxide (a-IGZO)] thin-film transistors (TFTs) accounting for both charge injection at the contact and charge transport within the channel. We extract the current-voltage characteristics of the injecting contact from the measurements of a-IGZO TFTs fabricated on plastic foil. We show that the charge injection depends on both the drain and the gate voltages. We model the charge injection in staggered a-IGZO TFTs basing on the thermionic emission–diffusion theory including the charge carrier-dependent electron velocity due to the trap states in the subgap of the a-IGZO semiconductor. Combining the charge injection model with a charge transport model, we accurately and consistently describe the measurements of staggered a-IGZO TFTs with channel-length scaling from $200~\mu m$ to $15~\mu m$ . The proposed unified model is implemented in a circuit simulator and used to design unipolar inverters. The good agreement between simulations and measurements of the inverters further confirms the effectiveness of the proposed approach.

Time-dependent mechanical-electrical coupled behavior in single crystal ZnO nanorods.

Nanoscale time-dependent mechanical-electrical coupled behavior of single crystal ZnO nanorods was systematically explored, which is essential for accessing the long-term reliability of the ZnO nanorod-based flexible devices. A series of compression creep tests combined with in-situ electrical measurement was performed on vertically-grown single crystal ZnO nanorods. Continuous measurement of the current (I)-voltage (V) curves before, during, after the creep tests revealed that I is non-negligibly increased as a result of the time-dependent deformation. Analysis of the I-V curves based on the thermionic emission-diffusion theory allowed extraction of nanorod resistance, which was shown to decrease as time-dependent deformation. Finally, based on the observations in this study, a simple analytical model for predicting the reduction in nanorod resistance as a function of creep strain that is induced from diffusional mechanisms is proposed, and this model was demonstrated to be in an excellent agreement with the experimental results.

Asymmetric behavior in flexible piezoelectric strain sensors made of single ZnO nanowires.

Piezoelectric strain sensors were fabricated using single ZnO nanowires on flexible polystyrene substrates and the asymmetric behavior in this kind of piezoelectric strain sensor made of single nanowire was firstly introduced. The I~V curves of the sensors under a serious of tensile and compressive strains were measured and agreed with the thermionic emission-diffusion theory. The Schottky barrier heights were obtained. The change in Schottky barrier heights of the sensors under compressive strain is much slower than that under tensile strain and the asymmetric elastic response of the sensors is caused by the bending of the ZnO nanowires under compressive strains when the applied strain is larger than a critical strain εcr. And the conclusion that the critical strain εcr only relates to the geometry parameters of the nanowire is drawn. The reduction of the axial compressive strain in compression was calculated. This asymmetric behavior in strain sensors is firstly introduced and is important for the design and fabrication of this type of strain sensors.

New study of the abnormal behavior of the low temperature dependence of the current in inhomogeneous Schottky diode

AbstractIn this study, we show clearly why unexpected observations have been reported in the current–voltage curves of Schottky diodes, containing barrier inhomogeneities generated by using the analytical results based on a Gaussian distribution model of barrier heights. The Chand's calculations have shown that the current (saturation current) at low temperatures may exceed the current (saturation current) at high temperatures when the effective barrier height is calculated from an appropriate integral with integration limits −∞ and +∞. In this new study, we show that the method followed by Chand to remove these anomalies is not accurate enough. We prove that the origin of these anomalies stems from the nature of a proper function f(φ) that moves to the negative barrier heights and takes large value of the integral at low temperatures than at high temperatures when it has large standard deviation (σ) and the discrepancies are not due to the integration limits as Chand concluded. In order to obtain results consistent with the thermionic emission–diffusion theory, the standard deviation must have lower values. Copyright © 2014 John Wiley & Sons, Ltd.

Self-powered ultraviolet photodetectors based on selectively grown ZnO nanowire arrays with thermal tuning performance

A self-powered Schottky-type ultraviolet photodetector with Al-Pt interdigitated electrodes has been fabricated based on selectively grown ZnO nanowire arrays. At zero bias, the fabricated photodetector exhibited high sensitivity and excellent selectivity to UV light illumination with a fast response time of 81 ms. By tuning the Schottky barrier height through the thermally induced variation of the interface chemisorbed oxygen, an ultrahigh sensitivity of 3.1 × 10(4) was achieved at 340 K without an external power source, which was 82% higher than that obtained at room temperature. According to the thermionic emission-diffusion theory and the solar cell theory, the changes in the photocurrent of the photodetector at zero bias with various system temperatures were calculated, which agreed well with the experimental data. This work demonstrates a promising approach to modulating the performance of a self-powered photodetector by heating and provides theoretical support for studying the thermal effect on the future photoelectric device.

Interfacial Charge-Transfer Loss in Dye-Sensitized Solar Cells

Interfacial charge-transfer loss factors (CTLFs) of dye-sensitized solar cells (DSCs) were studied using a thermionic emission–diffusion theory and electrochemical impedance spectroscopy (EIS). It is found that the subsidiary CTLFs can be quantified including a native charge-transfer loss at the TiO2/electrolyte interface in DSC devices. The obtained CTLFs as a probability parameter α were identified as almost matching to dominant resistance-capacitance (RC) time constants obtained from the EIS and its numerical calculation. The quantified CTLFs based on a detailed physical model of quantum-mechanical tunneling and reflection of charge carriers can explain a degradation of open-circuit voltage in DSC devices.

Unified tunnelling-diffusion theory for Schottky and very thin MOS structures

We derive general formulae for calculating the transport of free charge carriers in a MOS structure with a thin insulating layer. In particular, we obtain relationships for boundary concentrations of free charge carriers on the insulator–semiconductor interface and for the current densities flowing through the MOS structure. Our direct tunnelling-diffusion approach makes the well known thermionic emission–diffusion theory for the Schottky interface applicable also to metal–insulator–semiconductor barriers with a very thin insulator layer. We demonstrate how direct tunnelling through the insulating layer and drift–diffusion of free charge carriers in the semiconductor affect the I–V and C–V curves and the boundary concentrations needed to numerically solve the continuity equations.

Band discontinuity measurements of the wafer bonded InGaAs∕Si heterojunction

p -type InGaAs∕Si heterojunctions were fabricated through a wafer fusion bonding process. The relative band alignment between the two materials at the heterointerface was determined using current-voltage (I-V) measurements and applying thermionic emission-diffusion theory. The valence and conduction band discontinuities for the InGaAs∕Si interface were determined to be 0.48 and −0.1eV, respectively, indicating a type-II band alignment.

Extended thermionic emission-diffusion theory of charge transport through a Schottky diode

An extended thermionic emission-diffusion theory of charge carrier transport through a Schottky diode is derived considering the effect of the Richardson constant for electrons passing from the metal to the semiconductor. As it is evident from the obtained expressions for the current density, the Richardson contact strongly affects the reverse mode of the I- V characteristics. If the ratio of Richardson constants for the metal and semiconductor is larger or smaller than unity, then the reverse current density will be lower or higher, respectively, than the reverse current density obtained, without considering the above effect.

Barrier-height non-uniformities of PtSi/Si(111) Schottky diodes

The forward current-voltage characteristics of PtSi Schottky contacts on epitaxial n-type Si (111) is studied in the temperature range from 100 to 300 K. A current contribution in excess to that predicted by thermionic emission diffusion theory is caused by a few thousand patches of reduced Schottky barrier height in a device area of 3.14 mm2. The typical lateral extent of these patches is 70–250 nm. It correlates with the size of surface bumps observed. The number of patches is reduced upon increasing the silicidation temperature up toTsiI=550°C. Possible non-uniformities of the typical crystallite grain size of 20 nm are not resolved due to effective pinch-off.

The Influence of Drift-Diffusion Processes onI–V Characteristics of Si Schottky Diodes

A more accurate approximation for the diffusion velocity in the space-charge layer of Schottky diodes is obtained. The influence of the dependence of the diffusion velocity on the applied voltage to the slope of I–V characteristics, calculated by the combined thermionic emission-diffusion theory of current in Schottky diodes, is taken into account. It is shown that this influence is of the same magnitude as the image force effect on I–V characteristics of Schottky diodes on Si substrates with donor doping around 1015 cm−3. [Russian Text Ignored.]

Diffusion effects in p-type PtSi Schottky diodes under reverse bias

At low temperature, the dominant current transport process of a p-type PtSi Schottky diode on a moderately doped Si substrate is thermionic emission over the barrier. In this paper, current has been measured as a function of temperature at a fixed reverse bias for several Schottky diodes. It is found that diffusion effects due to the limitation upon the diffusion rate through the space-charge region can become significant under some conditions. At moderate field, these conditions are calculated to be at a temperature of 150 K or greater, depending on the doping density of the devices. I-V-T measurements were carried out on several Schottky devices fabricated in slightly different ways, and the diffusion effect was observable in some of these devices. A numerical routine is utilized to fit the experimental data to a combined thermionic emission-diffusion theory in these regions. It is found that the experimental results fit this theory well. In addition, the acceptor density at which diffusion becomes the dominant current at 80 K under moderate field is calculated to be around 5×1011 cm−3.

Transient analysis of a triangular-barrier bulk unipolar diode

The transient analysis of a triangular-barrier bulk unipolar diode (TB) is based on the conduction currents associated with both the majority and minority carriers together with a displacement current associated with the space change regions. During the turn-on and turn-off transients the conduction current, which is determined by thermionic emission-diffusion theory, is shown to have negligible effect compared with the displacement current. The transients and steadystate currents of a typical GaAs TB are compared to those from a device simulator which includes thermionic emission over the barrier, and are shown to be in excellent agreement. 10% time constants are determined by an analytic expression for both turn-on and turn-off and are shown to be equivalent and dependent on the applied DC bias and independent of the position of the p+plane in the undoped region.

Parameter sensitivity analysis for computer modelling of metal-semiconductor junctions

The aim of this paper is to give the sensitivity of the I- V characteristics of metal-semiconductor diodes to various parameters used in computer modelling. Based on the Crowell-Sze combined thermionic emission-diffusion theory, the primary sensitivities to the barrier height, effective recombination velocity and effective diffusion velocity are given. The secondary sensitivities to the diffusion coefficient, doping density etc. are also discussed. An approximation for the field dependence of the optical phonon scattering factor is given.

Thermionic-emission diffusion model of current conduction in polycrystalline silicon

A comprehensive model of current conduction in polycrystalline silicon based on thermionic-emission diffusion theory is developed. On the basis of this model, a more general expression for the effective majority-carrier mobility is derived which reduces to thermionic-emission (TE) theory-based expressions under certain simplifying assumptions and also helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results.

Conduction in Polycrystalline Silicon: Generalized Thermionic Emission-Diffusion Theory and Extended State Mobility Model

ABSTRACTWe present a general theory of conduction in polysilicon. The theoretical framework reconciles two apparently divergent approaches for modeling conduction processes in polysilicon and provides a physical basis to correctly interpret and to point out the deficiencies of previously reported thermionic and thermionic field emission theory. This model is based on an extended state mobility in the disordered grain boundary and the thermionic emission-diffusion theory for conduction of current. The attractive features of our theory are (a) it can explain the experimental data without the use of an artificial factor, f, (b) the conduction process is characterized explicitly by the inherent material properties of the grain and the grain boundary. Our model is particularly suited for describing the electrical properties of laser restructured polysilicon, where because of large grain size the diffusion process is expected to be dominant.

Minority carrier effects upon the small signal and steady-state properties of Schottky diodes

Schottky diode theory is re-evaluated by applying the combined thermionic emission-diffusion theory to both the majority and minority carrier flows across the metal-semiconductor contact. Under both steady-state d.c. and small signal a.c. conditions, numerical solutions to the semiconductor transport equations subject to boundary conditions determined from the combined theory are used to investigate the effects of minority carriers upon the properties of uniformly doped Schottky diodes. High injection effects and contact limitations are shown to influence the minority carrier injection ratio and the total stored minority carrier charge. It is further shown that the small signal impedance of a large class of Schottky diodes becomes inductive under moderate forward bias.