Thermionic Emission Model Research Articles

Material characterization of thermally evaporated ZnSn2Te4 thin films

Polycrystalline and stoichiometric ZnSn2Te4 (ZST) thin films were deposited on glass substrates by sequential evaporation of elemental powder sources. The deposited films were annealed in nitrogen atmosphere at annealing temperature ranging 100–300 °C. Under post-annealing treatments, the composition, structural, surface morphological, optical and electrical characteristics of the films were investigated. Annealing treatments lead to maintain the structural characteristics with the possible change in atomic concentration of the constituent elements in limit of detection and crystallinity of the films increased with increasing annealing temperature. Grainy surface morphology was observed in as-grown and annealed films and densely packed appearance of the surface of the samples indicates uniform deposition of the film over the entire substrate surface. Under the aim of visible light harvesting in the applications of thin film photovoltaics, normal-incidence transmittance measurements were performed and the direct band gap values were found in the range of 1.8–2.1 eV. Temperature dependent conductivity characteristics of the films were investigated under dark condition and the observed conductivity profiles were found in Arrhenius behavior with temperature dominated by the thermionic emission model.

Dynamic variation of carrier transport properties of recessed Au-free ohmic contacts to InAlN/AlN/GaN on Si-wafer

The temperature dependence of specific contact resistivity of recessed ohmic contacts was investigated to examine the carrier conduction mechanism in Ti/Al/W ohmic contacts on InAlN/AlN/GaN on Si wafer. The lowest specific contact resistivity of 2.36 × 10−5 Ω·cm2 was obtained from a sample with a half-etched InAlN layer. The barrier heights and donor concentrations of recessed contacts with various recess depths were calculated using a thermionic field emission model that showed good fitting with the experimental data. It was found that the rate of tunneling in carrier conduction decreased with increasing recess depth.

Thermally enhanced photoelectric emission from GaAs photocathode

Thermally enhanced photoelectric emission (TEPE) is a method for solar energy conversion, and it combines the quantum approach and the thermal approach. Different from photon-enhanced thermionic emission (PETE), TEPE is based on the photoelectric emission model rather than the thermionic emission model. In this paper, numerical calculations for the conversion is proposed under detailed balance condition. By using a double dipole layer surface model, the theoretical conversion efficiency of TEPE conversion from a GaAs photodiode is calculated. The emission current of TEPE increases with the temperature until all the excited electrons are emitted. An experimental setup with a NEA GaAs vacuum photodiode was applied. Assuming a phosphorus-doped diamond film with a work function of 0.9 eV as the anode, the energy conversion efficiency can be estimated, and it increases from 4.92% to 5.11% as the temperature rises from 20 °C to 80 °C.

Electrical properties of Au–Cu/ZnO/p-Si diode fabricated by atomic layer deposition

The electrical properties of the Au–Cu/ZnO/p-Si diode were investigated with the temperature dependent current–voltage measurements in a wide temperature range from 220 to 360 K with 20 K steps and also frequency dependent capacitance–voltage and conductance–voltage measurements using admittance spectroscopy by changing frequency from 1 to 1000 kHz. The ZnO thin film layer was deposited by atomic layer deposition technique (ALD) to obtain homogenous interface layer and this layer surface was characterized with AFM analyses. Assuming thermionic emission (TE) model, diode parameters such as barrier height and ideality factor were determined for the Au–Cu/ZnO/p-Si diodes and the strong temperature dependence of these values were modeled by TE model with modified by Gaussian distribution of the barrier height. Therefore, the observed non-ideal current behavior was discussed on the basis of barrier inhomogeneity and the presence of native oxide interfacial layer in the diode. The mean barrier height was found as 1.38 eV with 0.18 standard deviation, and the Richardson constant modified by this model was obtained as 28 A/cm2k2 close to the theoretical value. Additionally, the distribution profile of the interface states and series resistance value were evaluated by the capacitance and conductance characteristics. It could be seen from the results that both of them strongly depends on the device frequency. As a result of these works, fabricated Au–Cu/ZnO/p-Si diodes may be used for next technological application in wide range temperature in industry.

Novel flexible inkjet-printed Metal-Insulator-Semiconductor organic diode employing silver electrodes

Abstract The fabrication of diodes by inkjet printing has been hampered by difficulties in printing low work function metals required to establish Schottky barriers with organic semiconductors. This limitation has been a major roadblock in the use of inkjet printing technology for the development of organic rectifying diodes. This paper presents the fabrication and characterization of a novel inkjet-printed metal- organic insulator- organic semiconductor diode on flexible plastic substrates. The structure consists of a polymeric insulator/semiconductor interface sandwiched between two silver electrodes. It is proposed that the rectification properties are due to a voltage controlled leakage current across the insulator/semiconductor interface. The current across the insulator is caused by the formation of a semiconductor brush-like morphology into the underneath porous insulator layer. The carrier injection into the insulator follows a thermionic emission model. Temperature dependent measurements reveal an interfacial barrier height between 0.97 eV and 0.36 eV depending on the morphology and type of insulator layer used. Metal- organic insulator- organic semiconductor based diodes show rectification ratios up to 150 at |10V| and a current density up to approximately 1 μAcm−2. The simple fabrication process of the diodes also makes it advantageous for scaling up to roll-to-roll production.

Improving the optical and electrical properties of NiO/n-Si photodiode by Li dopant

In this study, Li-doped NiO solution was produced using a sol–gel spin coating technique. The manufactured solution was deposited onto the n-Si substrate to obtain a homogeneous thin film that could be used for photodiode production. Next, we measured the morphological, electrical, and optical parameters of the manufactured photodiode. First, the morphological properties of the thin film were studied by atomic force microscopy (AFM). Using AFM analysis software, the roughness and grain size of the thin film were estimated as 8.2–10 and 227–239 nm respectively. The optical transparency and the band gap of the Li-doped NiO thin film were studied. The optical measurements of the thin film were taken using a Shimadzu UV–Vis–NIR 3600 spectrophotometer. The transmittance of the thin film was 83.6%, and the band gap was 3.57 eV. The current–voltage (I–V) characteristic of the photodiode was measured in the dark and under various intensities of illumination. We found that the current of the photodiode changed depending on the intensity of illumination. As the intensity of illumination increased, the current also increased, from 6.3 × 10−7 to 3.36 × 10−4 A. These data indicate that the photodiode is sensitive to illumination intensity and, therefore, could be used as an optical sensor. The barrier height and ideality factor of the photodiode were also determined using the thermionic emission model. The barrier height and ideality factor of the Al/n-Si/LiNiO/Au photodiode were 0.81 eV and 3.7 respectively. Also, the capacitance–voltage (C–V), the interface density (Dit), and the serial resistance of the photodiode were found to change with changing frequency. Taken together, these data show that the Li doping ratio improved the optical and electrical properties of NiO. Based on these findings, we propose that Li-doped NiO could be incorporated into optoelectronic devices, such as photodiodes and photosensors.

Effect of Annealing Temperature on Structural and Electrical Properties of Al/ZrO2/p-Si MIS Schottky Diodes

The structural and electrical properties of Al/ZrO2/p-Si Schottky barrier diodes (SBDs) have been investigated at different annealing temperatures (300, 400, 500 and 600 °C). X-ray diffraction (XRD) analysis shows that the film annealed at 600 °C exhibits better crystalline nature with monoclinic phase. X-ray photoelectron spectroscopy (XPS) analysis reveals that the oxidation state of ZrO2 film is Zr4+. The scanning electron microscopy (SEM) image shows that the film annealed at 600 °C exhibits sub-micro-sized and square-shaped grains. The thermionic emission (TE) model determines the diode parameters such as barrier height (ΦB), ideality factor (n), series resistance (Rs) and saturation current density (Js) from J–V characteristics and Cheung’s method. The ideality factor of the Al/ZrO2/p-Si diodes decreases (3.772–3.442) with increasing annealing temperature (300–600 °C).

Modeling the time-dependent characteristics of perovskite solar cells

We proposed two different time-dependent modeling approaches for variation of device characteristics of perovskite solar cells under stress conditions. The first approach follows Sah-Noyce-Shockley (SNS) model based on Shockley–Read–Hall recombination/generation across the depletion width of pn junction and the second approach is based on thermionic emission model for Schottky diodes. The connecting point of these approaches to time variation is the time-dependent defect generation in depletion width (W) of the junction. We have fitted the two models with experimental data reported in the literature to perovskite solar cell and found out that each model has a superior explanation for degradation of device metrics e.g. current density and efficiency by time under stress conditions. Nevertheless, the Sah-Noyce-Shockley model is more reliable than thermionic emission at least for solar cells.

Double Gaussian barrier distribution of permalloy (Ni0.8Fe0.2) Schottky contacts to n-type GaN

The temperature-dependent current-voltage (I-V) characteristics of permalloy (Ni0.8Fe0.2) Schottky contacts to n-type GaN have been investigated. Magnetization measurements revealed the ferromagnetic behavior of Ni0.8Fe0.2 film on n-type GaN. The Schottky barrier parameters, such as the barrier height and ideality factor, determined by thermionic emission depended on the measurement temperature, suggesting the presence of lateral inhomogeneity in the Schottky barrier. The experimental data modified by the thermionic emission model along with a Gaussian distribution of the barrier heights indicated the presence of a double Gaussian barrier distribution in the Ni0.8Fe0.2/n-type GaN Schottky contact. The mean barrier heights and standard deviations for each Gaussian distribution were 0.84 & 1.32 eV and 0.10 & 0.17 eV over temperature range of 125–200 K and 225–400 K, respectively. The noise spectral density of the current fluctuations measured as a function of frequency (f) at room temperature followed a 1/fγ dependence with a γ value close to unity, irrespective of the applied forward bias. The 1/f-type noise was attributed to the barrier inhomogeneity existing at the Ni0.8Fe0.2/n-type GaN Schottky interface as revealed from the temperature-dependent I–V characteristics.

Electronic Transport Mechanism for Schottky Diodes Formed by Au/HVPE a-Plane GaN Templates Grown via In Situ GaN Nanodot Formation.

We investigate the electrical characteristics of Schottky contacts for an Au/hydride vapor phase epitaxy (HVPE) a-plane GaN template grown via in situ GaN nanodot formation. Although the Schottky diodes present excellent rectifying characteristics, their Schottky barrier height and ideality factor are highly dependent upon temperature variation. The relationship between the barrier height, ideality factor, and conventional Richardson plot reveals that the Schottky diodes exhibit an inhomogeneous barrier height, attributed to the interface states between the metal and a-plane GaN film and to point defects within the a-plane GaN layers grown via in situ nanodot formation. Also, we confirm that the current transport mechanism of HVPE a-plane GaN Schottky diodes grown via in situ nanodot formation prefers a thermionic field emission model rather than a thermionic emission (TE) one, implying that Poole–Frenkel emission dominates the conduction mechanism over the entire range of measured temperatures. The deep-level transient spectroscopy (DLTS) results prove the presence of noninteracting point-defect-assisted tunneling, which plays an important role in the transport mechanism. These electrical characteristics indicate that this method possesses a great throughput advantage for various applications, compared with Schottky contact to a-plane GaN grown using other methods. We expect that HVPE a-plane GaN Schottky diodes supported by in situ nanodot formation will open further opportunities for the development of nonpolar GaN-based high-performance devices.

Performance analysis of Schottky diodes based on Bi doped p-ZnO thin films

In the present paper, Bismuth (Bi) doped ZnO thin film have been deposited using sol gel spin coating technique to produce p-type ZnO films on n-Si substrate. The deposited Bi doped ZnO thin films were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), hot point probe and hall measurement. The XRD and AFM characterization results shows uniform growth of the film over the entire substrate having polycrystalline nature of the film and Hall measurement and hot point probe method shows p-type nature of the deposited Bi doped ZnO film. Further, Palladium (Pd) and Nickel (Ni) metal contacts have been deposited over separate Bi doped ZnO thin films using vacuum coating method to fabricate Schottky diodes. The current–voltage characteristics have been analyzed by using conventional thermionic emission model, Cheung's method and Norde's technique to calculate the barrier height, ideality factor and series resistance of the fabricated Schottky diodes. From the calculations, it is observed that Cheung's method gives more realistic results for estimating diode parameters.

Influence of $$\hbox {Si}_{3}\hbox {N}_{4}$$ Si 3 N 4 layer on the electrical properties of Au/n-4H SiC diodes

In this study, the effect of $$\hbox {Si}_{3}\hbox {N}_{4}$$ insulator layer on the electrical characteristics of Au/n-4H SiC diode was investigated. The current–voltage ( $${I{-}V}$$ ), capacitance–voltage ( $${C{-}V}$$ ) and conductance–voltage ( $$G/w{-}V$$ ) measurements were carried out at room temperature condition. Under thermionic emission model, electrical parameters as zero-bias barrier height ( $$\Phi _{\mathrm{Bo}}$$ ), ideality factor (n), interface states ( $$D_{\mathrm{it}}$$ ), and series ( $$R_{\mathrm{s}}$$ ) and shunt ( $$R_{\mathrm{sh}}$$ ) resistances were estimated from forward bias $${I{-}V}$$ analyses. The values of n and $$\Phi _{\mathrm{Bo}}$$ were about 1.305 and 0.796 eV for metal–semiconductor (MS) rectifying diode, and 3.142 and 0.713 eV for metal–insulator–semiconductor (MIS) diode with the insertion of $$\hbox {Si}_{3}\hbox {N}_{4}$$ layer, respectively. Since the values of n were greater than the unity, the fabricated diodes showed non-ideal $${I{-}V}$$ behaviour. The energy distribution profile of $$D_{\mathrm{it}}$$ of the diodes was calculated by taking into account of the bias dependence of the effective barrier height ( $$\Phi _{\mathrm{e}}$$ ) and $$R_{\mathrm{s}}$$ . The obtained $$D_{\mathrm{it}}$$ values with $$R_{\mathrm{s}}$$ are almost one order of magnitude lower than those without $$R_{\mathrm{s}}$$ for two diodes. According to Cheung’s model, $$R_{\mathrm{s}}$$ were calculated and these values were found in increasing behaviour with the contribution of $$\hbox {Si}_{3}\hbox {N}_{4}$$ insulator layer. In addition, the $$J_{\mathrm{R}}{-}V$$ plot behaviours with linear dependence between ln( $$J_{\mathrm{R}}$$ ) vs. $$V^{0.5}$$ indicated that the dominant conduction mechanism in the reverse bias region was Schottky effect for both MS and MIS diodes. In the room temperature $${C{-}V}$$ measurements, different from the results of MIS diode, the values of C for MS diode was observed in decreasing behaviour from ideality with crossing the certain forward bias voltage point ( $$\sim $$ $$2.5\ \hbox {V}$$ ). The decrease in the negative capacitance corresponds to the increase of G / w.

Investigation of temperature dependent electrical characteristics on Au/Ni/β-Ga2O3 Schottky diodes

Au/Ni/β-Ga2O3 Schottky barrier diodes were fabricated on a 15 μm drift layer mechanically exfoliated from (100)-oriented β-Ga2O3 bulk. The temperature dependent current density-voltage (J-V) characteristics from 300 K to 550 K were investigated and the barrier height and ideality factor were determined. Compared with the thermionic emission (TE) model, the forward J-V behavior follows thermionic field emission (TFE) model and the reverse J-V characteristics can be explained by Poole-Frenkel model.

Electrical transport and capacitance characteristics of metal-insulator-metal structures using hexagonal and cubic boron nitride films as dielectrics

Metal-insulator-metal capacitor structures using thick hexagonal and cubic boron nitride (hBN and cBN) films as dielectrics are produced by plasma jet-enhanced chemical vapor deposition, and their electrical transport and capacitance characteristics are studied in a temperature range of 298 to 473 K. The resistivity of the cBN film is of the order of 107 Ω cm at 298 K, which is lower than that of the hBN film by two orders of magnitude, while it becomes the same order as the hBN film above ∼423 K. The dominant current transport mechanism at high fields (≥1 × 104 V cm−1) is described by the Frenkel-Poole emission and thermionic emission models for the hBN and cBN films, respectively. The capacitance of the hBN film remains stable for a change in alternating-current frequency and temperature, while that of the cBN film has variations of at most 18%. The dissipation factor as a measure of energy loss is satisfactorily low (≤5%) for both films. The origin of leakage current and capacitance variation is attributed to a high defect density in the film and a transition interlayer between the substrate and the film, respectively. This suggests that cBN films with higher crystallinity, stoichiometry, and phase purity are potentially applicable for dielectrics like hBN films.

Electromechanical Characterization of Single GaN Nanobelt Probed with Conductive Atomic Force Microscope

The electromechanical characterization of the field effect transistor based on a single GaN nanobelt was performed under different loading forces by using a conductive atomic force microscope (C-AFM), and the effective Schottky barrier height (SBH) and ideality factor are simulated by the thermionic emission model. From 2-D current image, the high value of the current always appears on the nanobelt edge with the increase of the loading force less than 15 nN. The localized (I–V) characteristic reveals a typical rectifying property, and the current significantly increases with the loading force at the range of 10–190 nN. The ideality factor is simulated as 9.8 within the scope of GaN nano-Schottky diode unity (6.5–18), therefore the thermionic emission current is dominant in the electrical transport of the GaN-tip Schottky junction. The SBH is changed through the piezoelectric effect induced by the loading force, and it is attributed to the enhanced current. Furthermore, a single GaN nanobelt has a high mechanical-induced current ratio that could be made use of in a nanoelectromechanical switch.

Influence of the Interaction Between Graphite and Polar Surfaces of ZnO on the Formation of Schottky Contact

We show that the interaction between graphite and polar surfaces of ZnO affects electrical properties of graphite/ZnO Schottky junctions. A strong interaction of the Zn-face with the graphite contact causes interface imperfections and results in the formation of laterally inhomogeneous Schottky contacts. On the contrary, high quality Schottky junctions form on the O-face, where the interaction is significantly weaker. Charge transport through the O-face ZnO/graphite junctions is well described by the thermionic emission model in both forward and reverse directions. We further demonstrate that the parameters of the graphite/ZnO Schottky diodes can be significantly improved when a thin layer of ZnO2 forms at the interface between graphite and ZnO after hydrogen peroxide surface treatment.

Barrier Inhomogeneities in n-ZnO/p-Si Heterojunctions Fabricated with ZnO Nanorods

The p-Si/n-ZnO nanoheterostructures were prepared by the sol–gel and hydrothermal method. The structural characterization carried out by the x-ray diffraction and field emission scanning electron microscope confirms the growth of ZnO nanorods on p-type silicon substrate. The current–voltage (I–V) characteristics of the junction show a rectifying diode-like behavior. The analysis indicates the presence of a high value of series resistance (3.2 KΩ) at the interface. The barrier height extracted from the temperature variation study using the thermionic emission model was of the order of 167.7 meV and the Richardson constant value was of the order of 1.1096E−6 A cm−2 K−2 which is much lower than the standard value, i.e., 32 A cm−2 K−2 for ZnO. Such a discrepancy has been resolved and explained using the Gaussian distribution of barrier heights. The refined value of the barrier height and Richardson constant as calculated from the modified Richardson plot was 1.24 eV and 35.16 A cm−2 K−2, respectively. These are in agreement with the standard reported values.

Dislocation-Related Electron Transport in Au Schottky Junctions on AlGaN/GaN

We investigated the electrical properties of Au/AlGaN/GaN Schottky junctions as a function of temperature by analyzing the current–voltage (I–V) measurements. The barrier height increased with increasing temperature, but the ideality factor decreased. Increases in temperature are associated with barrier inhomogeneity. The modified Richardson plots for Al0.25Ga0.75N yielded a higher Richardson constant, 77.3 A cm−2K−2, than theoretically predicted (30.0 A cm−2 K−2). This indicates that the thermionic emission (TE) model with barrier inhomogeneity is not suitable for explaining the transport characteristics of the junction. We fitted the experimental I–V data to predictions based on various transport mechanisms, such as TE, generation-recombination, and tunneling currents. The dominant transport mechanism at all temperatures was found to be caused by the tunneling current. The dislocation model of the tunneling current yielded a dislocation density of 2.96 × 106 cm−2.

Electrical characterization of the temperature dependence in CdTe/CdS heterojunctions deposited in-situ by pulsed laser deposition

The I-V and C-V characteristics of CdTe/CdS heterojunctions deposited in-situ by Pulsed Laser Deposition (PLD) were evaluated. In-situ deposition enables the study of the CdTe/CdS interface by avoiding potential impurities at the surface and interface as a consequence of exposure to air. The I-V and C-V characteristics of the resulting junctions were obtained at different temperatures, ranging from room temperature to 150 °C, where the saturation current (from 10−8 to 10−4 A/cm2), ideality factor (between 1 and 2), series resistance (from 102 to 105 Ω), built-in potential (0.66–0.7 V), rectification factor (∼106), and carrier concentration (∼1016 cm−3) were obtained. The current–voltage temperature dependence study indicates that thermionic emission is the main transport mechanism at the CdTe/CdS interface. This study also demonstrated that the built-in potential (Vbi) calculated using a thermionic emission model is more accurate than that calculated using C-V extrapolation since C-V plots showed a Vbi shift as a function of frequency. Although CdTe/CdS is widely used for photovoltaic applications, the parameters evaluated in this work indicate that CdTe/CdS heterojunctions could be used as rectifying diodes and junction field effect transistors (JFETs). JFETs require a low PN diode saturation current, as demonstrated for the CdTe/CdS junction studied here.

Gate-bias and temperature dependence of charge transport in dinaphtho[2,3-b:2′,3′-d]thiophene thin-film transistors with MoO3/Au electrodes

We investigated the gate-bias and temperature dependence of the voltage–current (V–I) characteristics of dinaphtho[2,3-b:2′,3′-d]thiophene with MoO3/Au electrodes. The insertion of the MoO3 layer significantly improved the device performance. The temperature dependent V–I characteristics were evaluated and could be well fitted by the Schottky thermionic emission model with barrier height under forward- and reverse-biased regimes in the ranges of 33–57 and 49–73 meV, respectively. However, at a gate voltage of 0 V, at which a small activation energy was obtained, we needed to consider another conduction mechanism at the grain boundary. From the obtained results, we concluded that two possible conduction mechanisms governed the charge injection at the metal electrode–organic semiconductor interface: the Schottky thermionic emission model and the conduction model in the organic thin-film layer and grain boundary.