Thermoionic Emission Research Articles

The electrical transport property and gas detection behaviour of polyvinyl alcohol and γ-Fe2O3 nanocomposite film

This paper presents the electrical and gas detection behaviour of polyvinyl alcohol (PVA)/γ-Fe2O3 nanocomposite films. The physical characterization includes X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and UV–Visible spectroscopy. The analysis of both the dc resistivity and ac conductivity using Correlated Barrier Hoping (CBH) conduction process shows that the conductivity enhances with the decreasing of activation energy in case of the PVA/γ-Fe2O3 nanocomposite film which is the indication of interfacial defect states. The current voltage study at various temperatures follows the thermoionic emission model of Schottky Barrier diode. Moreover, the dielectric constant enhances for PVA/γ-Fe2O3 nanocomposite film, which indicates homogeneous distribution of γ-Fe2O3 nanoparticles within PVA matrix and the aggregation of trapped charges found in interface develops an interfacial polarization. As a result, the gas adsorption capacity of the nanocomposite film increases which improves the signal-to-noise ratio, leading to higher gas detection sensitivity.

Electrical characteristics of (AgNPs-PVA)-based Schottky diode and its application as a low-voltage varistor devices

In this report, we studied the influence of silver nanoparticles (AgNPs) on the electrical conductivity of Polyvinyl Alcohol (PVA) as a semiconductor nanocomposites active layer. Here, the Schottky junction is constructed by mechanically pressing a copper (Cu) electrode onto a AgNPs-PVA nanocomposite, which shows rectification behavior at room temperature. The synthesis of silver nanoparticles (AgNPs) was achieved by the physical reduction of silver nitrate using an ultraviolet lamp. The nanocomposite films were created using a casting technique. An ultraviolet spectrophotometer (UV–vis), which displayed maximum absorbance at 430 nm, was used to confirm the synthesis of AgNPs and carry out the optical band gap. The charge carrier transport properties of AgNPs-PVA film were investigated by using impedance spectroscopy and I–V measurements. Then, AC impedance analysis was used to determine grain and grain boundary resistances; current-voltage analysis enabled the barrier height (Φ) to be determined. Moreover, the metal/semiconductor (Cu/AgNPs-PVA) Schottky barrier was confirmed as an equivalent circuit model via the Nyquist plot. Based on thermoionic emission theory, the characteristic I–V induced rectifying Schottky behavior can be understood. Moreover, the AgNPs-PVA nanocomposite exhibited hysteresis behavior under multiple repetitive measurements. For low voltage varistor devices, the nonlinear behavior may be completely utilized.

InxGa1-xN/GaN double heterojunction solar cell optimization for high temperature operation

InxGa1-xN/GaN solar cells are ideal candidates for use in extreme temperature applications. The conversion efficiency potential of double heterostructure solar cells was investigated at high temperatures using physical simulation. For a targeted working temperature, optimized efficiency lies in a compromise between the absorber bandgap energy determined by In composition and the band offsets at the heterointerface directly correlated with the capability for the photogenerated carriers to cross through the barrier by thermoionic emission. An optimized efficiency of 18% is obtained for an In content of 50% at 400K and decreases down to 10% for an In content of 35% at 500K. As the operating temperature goes higher, the indium content needs to be reduced in order to limit the detrimental effect of increasing intrinsic carrier concentration. The consequence is a decreasing efficiency due to the reduced covered range of the solar spectrum. In the same time, the band offsets are no more a limiting parameter, as there are reduced as the In content decreases, and as higher temperature increases the thermionic transport probability. This result shows the interest of InxGa1-xN/GaN double heterostructure design for high temperatures applications.

Structural and optical properties of pure ZnO and Al/Cu co-doped ZnO semiconductor thin films and electrical characterization of photodiodes

Abstract Nano-structured semiconductor thin films and p-type Si photodiodes were fabricated with the sol-gel spin coating technique using pure ZnO and co-doped ZnO:Alx:Cuy with x = 1 at.-%, y = 1, 2, 3, 5 at.-%. The structural and optical properties of thin films were examined using an XRD and a UV-spectrophotometer. The thin films have a hexagonal wurtzite crystal structure, and their optical band gap energies decrease with increasing Cu contribution. The electrical properties of photodiodes were assessed via I-V, C-V, (G/ω)-V and phototransient current (I-t, C-t) measurements. The Φ b(I-V), experimental zero-bias barrier height, rectification ratio, ideality factor and Ion/Ioff parameters of the diodes were calculated using thermoionic emission model. In addition, Φ b(C-V), barrier height, Vbi, built-in voltage, Vd, diffusion potential, Nd, donor concentration and Wd, depletion layer width of p-Si/ZnO:Alx:Cuy (x = 1 at.-%, y = 1 at.-%.) photodiodes were obtained using a C-2-V graph plotted at 1 MHz frequency. The photodiodes exhibit rectifying and photosensitive behaviors, and their reverse bias current increases with increasing light intensity. These results indicate that produced diodes can be employed as photodiodes or photosensors in optoelectronic circuits and electronic devices.

Effect of Temperature and Illumination on the Current–Voltage Characteristics of a Al/p-GaSe/In Diode

In this study, Al/p-GaSe/In Schottky diodes (SDs) were fabricated by using a p-type GaSe semiconductor grown by the Brigdman-Stockbarger method. Current–voltage (I–V) measurements of the Al/p-GaSe/In diode were carried out from 100 K to 400 K with steps of 20 K in the +2 V- (−2) V voltage range and in the dark. The characteristic parameters of SDs were calculated from ln(I)-V characteristics according to the thermoionic emission (TE) theory and from modified Norde’s functions depending on the sample temperature. The barrier height (BH) values calculated from ln(I)-V and Norde’s functions were in agreement. The values of BH and ideality factor (IF) calculated from ln(I)-V characteristics for the SD ranged from 0.97 eV and 1.01 at 400 K to 0.45 eV and 2.18 at 100 K, respectively. The value of serial resistance (Rs) calculated from modified Norde’s functions of the SD decreased with increasing temperature. Also, the reverse bias current value of the Al/p-GaSe/In SD under light illumination was higher than the current value in the dark. Thus, the obtained results show that the prepared Al/p-GaSe/In diode might be used as a photodiode in the optoelectronic applications.

Effect of Sb, Tb3+ Doping on Optical and Electrical Performances of SnO2 and Si Based Schottky Diodes

(Sb, Tb3+)-doped SnO2 thin films were deposited on monocrystalline silicon (Si) and on porous silicon (PS) layer from sol-gel spin coating method. The photoluminescence spectrum shows that Tb3+ ions presents higher emission with the PS layer. The fabricated junctions are treated as a metal-semiconductor (MS) Schottky diodes. The current–voltage (I-V) characteristics of SnO2:Sb/p-Si (D1), SnO2:Sb:Tb3+/p-Si (D2) and SnO2:Sb:Tb3+/PS (p) (D3) were measured for these diodes at room temperature. Electronic parameters such as ideality factor, barrier height and series resistance were calculated and compared for the main junctions. Based on the thermoionic emission model, it appears that the contacts presents non-ideal I-V behaviour with a relatively high values of ideality factor (n = 12 for Si based diode, and n = 24 for PS based diode) and a relatively large values of series resistance RS (RS = 2 103 Ω for D3). After the incorporation of Tb3+, the junction characteristics show that the formed diode exhibits high forward current density and a decrease in the series resistance RS (RS = 600 Ω for D2). The non-ideality character of the elaborated MS junctions seems to be principally due to the effects of the interface. The relatively high values of the ideality factor was attributed to the sharing of the applied voltage V by a diffusion potential (VD) across the semiconductor space charge region and a potential (Vi) devoted to the interfacial layer formed between the silicon and the tin oxide. The trapped-limited current was found to dominate the current transport mechanisms through the fabricated junctions. The above results highlight the role of the interfacial layer and interface states in the determination of the electrical performance of Sb-doped SnO2/p-Si, (Sb, Tb3+) co-doped SnO2/p-Si and (Sb, Tb3+) co-doped SnO2/PS(p). They show that the simultaneous presence of Tb3+ ions and porous silicon layer allows an obvious enhancement of both optical and electrical properties of the main junction increasing then the applicability of SnO2/silicon based devices.

Effect of Temperature and Magnetic Dopants on Particle size and Electrical Properties of ZnO Ceramic Varistor

We report here structural and electrical properties of Zn0.95 M0.05O ceramic, M = Zn, Co and Mn. It is found that addition of magnetic doping did not influence the hexagonal wurtzite structure of ZnO. Furthermore, the lattice parameters ratio c/a for hexagonal distortion and the length of the bond parallel u to the c axis were nearly unaffected. The average crystalline diameters, deduced from XRD analysis are 83.75, 72.86 and 70.97 nm for Zn, Mn and Co, which are 15 times lower than those obtained from FESEM micrographs (1570, 1380 and 1150 nm). The breakdown field EB was decreased as the temperature increased, in the following order: Mn> Zn > Co. The nonlinear region was observed for all samples as the temperature increased up to 400 K and completely disappeared with further increase of temperature up to 500 K. The values of nonlinear coefficient, α were between 1.65 and 56 for all samples, in the following order: Mn> Zn > Co. Moreover, the electrical conductivity σ was gradually increased as the temperature increased up to 500 K, in the following order: Co > Zn > Mn. On the other hand, the activation energies were 0.194, 0.155 eV and 0.231 eV for all samples, in the following order Mn, Zn and Co. These results have been discussed in terms of valence states, magnetic moment and thermo-ionic emission which were produced by the doping, and controlling the potential barrier of ZnO.

Dynamics of carrier transport via AlGaN barrier in AlGaN/GaN MIS-HEMTs

Exchange of carriers between the GaN channel and the dielectric/AlGaN interface in AlGaN/GaN metal insulator semiconductor high electron mobility transistors was recently attributed to a serial process of electron transport through the AlGaN barrier and electron trapping/emission at the interface. In this paper, the time constant related to barrier transport is evaluated from the measurements of time onset of threshold voltage drift in stress-recovery experiments. Temperature and forward gate bias dependent studies reveal an activation energy of 0.65 eV for the electron transport at zero bias being consistent with the estimated potential barrier of 0.75 eV at the dielectric/AlGaN interface. Thermo-ionic emission and defect assisted tunneling to near interface states are considered as transport mechanisms.

Conduction Mechanisms in Organic-based Rectifying Diode

The temperature dependent current–voltage characteristics of Ag/ZnPc/p-Si Schottky barrier (SB) diode are investigated in the temperature range of 300–450 K, and in the bias range of ± 1 V. By fitting the experimental data to space-charge limited conduction, bulk-limited Poole–Frenkel emission and thermo-ionic emission theory, it was observed that these models can not be applied to evaluate junction parameters for the investigated SB diode. Preliminary results indicated that the charge transport proceeds by different mechanism for low and high values of the applied voltage under forward and reverse bias conditions. It was found that the charge transport is governed by hopping processes for low values of the forward bias. However, for higher values of the forward bias, the charge transport controlled by the bulk limited procesess. The same voltage dependence was also observed for reverse bias conditions.

Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength

With the help of photoluminescence Spectroscopy (PLS), we have investigated the optoelectronic properties of two different families of InAs quantum dashes (QDashes) grown on misoriented InP(001) substrate with 2[Formula: see text]off miscut angle toward the [110] direction (2[Formula: see text]F type). The lowest full width at half maximum (FWHM) of the PL spectrum measured at 12 K indicates the good self organization of InAs QDashes. The weak ratio of the integrated PL measured in 12–300 K temperature range denotes the good spatial confinement of the photogenerated carriers in InAs QDashes. The fast redshift of the PL peaks energy and the anomalous decrease of the FWHM with the increase of the temperature are attributed to an efficient thermal relaxation process of photogenerated carriers in the vicinal sample. This result is highlighted with the help of theoretical modeling of the PL peak energy as a function of the temperature, using three models (Varshni, “Vina, Logothetidis and Cardona” and Pässler). From experimental and theoretical results, we have evidenced the contribution of longitudinal acoustic-phonons (LA-phonons) in the PL of InAs/InP QDashes, via the deformation potential, especially in high temperatures range. We have attributed this behavior to the strained InAs/InP QDashes and/or to the topography of the vicinal InP(001) substrate which favors the presence of stepped phonons polarized along the steps. These vibrational modes can further interact with the excitons at high temperatures. The measured thermal activation energies of each family of InAs QDashes demonstrate that the InAs wetting layer act as a barrier for the thermoionic emission of photogenerated carriers. This result confirms the good spatial confinement of excitons in this sample.

Mn5Ge3C0.6 /Ge(1 1 1) Schottky contacts tuned by an n-type ultra-shallow doping layer

Mn5Ge3Cx compound is of great interest for spintronics applications. The various parameters of Au/Mn5Ge3C0.6/Ge(1 1 1) and Au/Mn5Ge3C0.6/δ-doped Ge(1 1 1) Schottky diodes were measured in the temperature range of 30–300 K by using current–voltage and capacitance–voltage techniques. The Schottky barrier heights and ideality factors were found to be temperature dependent. These anomalous behaviours were explained by Schottky barrier inhomogeneities and interpreted by means of a Gaussian distribution model of the Schottky barrier heights. Following this approach we show that the Mn5Ge3C0.6/Ge contact is described with a single Gaussian distribution and a conduction mechanism mainly based on the thermoionic emission. On the other hand the Mn5Ge3C0.6/δ-doped Ge contact is depicted with two Gaussian distributions according to the temperature and a thermionic-field emission process. The differences between the two types of contacts are discussed according to the distinctive features of the growth of heavily doped germanium thin films.

Light induced charge transport property analysis of nanostructured ZnS based Schottky diode

In this report we have investigated the light sensing behavior and also discussed the induced charge transport phenomena through the junction made by aluminium and hydrothermally derived zinc-sulfide (ZnS). In this regards the structural, optical and electrical characterization of ZnS was performed well. The optical band gap energy (=3.68 eV) estimated from optical spectra and room temperature conductivity (0.49 × 10−6 S cm−1) measured from current–voltage characteristic explores the inorganic semiconductor behavior of the synthesized material. Depending upon the work function, aluminium (=4.4 eV) was preferred as metal contact to develop a potential barrier within the junction to study the underneath mechanism of charge transport through metal/inorganic-semiconductor interface. Moreover we have fabricated the sandwich structure ITO/ZnS/Al junction to study the effect of incident light on charge transport phenomena and demonstrated the potential applicability of the synthesized material to over crown the research on inorganic nano-semiconductor. The everbound charge transport phenomena within the device was analyzed by thermoionic emission theory. In searching its performance within light sensing electronic device we have deliberated the mobility-lifetime product, diffusion length and density of states (DOS) near Fermi level very aptly.

Conduction mechanism and bandgap engineering in pulsed laser deposited Cd1−xPbxS films

Effect of the doping concentration (Pb2+) on structural, electrical, and optical properties of the Cd1−xPbxS thin films synthesized using pulsed laser deposition was investigated. The main focus of the present study was to inspect the conduction mechanism and tailoring of optical bandgap to enhance their utilization for photo conducing and photovoltaic devices. Study revealed that increased faction of PbS phase leads to formation of ohmic contact. However, electrical conduction is governed by thermo-ionic emission at low fraction of PbS. Optical properties suggest that optical constants and band gap of Cd1−xPbxS films can be tailored by varying “Pb” content.

Analysis of the electrical properties of Cr/n-BaSi2 Schottky junction and n-BaSi2/p-Si heterojunction diodes for solar cell applications

Current status and future prospects towards BaSi2 pn junction solar cells are presented. As a preliminary step toward the formation of BaSi2 homojunction diodes, diodes with a Cr/n-BaSi2 Schottky junction and an n-BaSi2/p-Si hetero-junction have been fabricated to investigate the electrical properties of the n-BaSi2. Clear rectifying properties were observed in the current density versus voltage characteristics in both diodes. From the capacitance-voltage measurements, the build-in potential, VD, was 0.53 V in the Cr/n-BaSi2 Schottky junction diode, and the Schottky barrier height was 0.73 eV calculated from the thermoionic emission theory; the VD was about 1.5 V in the n-BaSi2/p-Si hetero-junction diode, which was consistent with the difference in the Fermi level between the n-BaSi2 and the p-Si.

Current–voltage and photovoltaic characteristics of n-Ge10Se80In10/p-Si heterojunction

In this work, the analysis of electrical properties Au/n-Ge10Se80In10/p-Si/Al heterojunction is studied. The dark forward current–voltage characteristics showed a thermoionic emission mechanism at low voltages (V≤0.4V) followed by a SCLC mechanism at high voltages (V≤0.5V). The junction parameters like series resistance, rectification ratio, ideality factor, effective barrier height, and total trap concentration were determined. The capacitance–voltage (C–V) characteristics of n-Ge10Se80In10/p-Si devices were also investigated. The barrier height value obtained from the C–V measurements was found to be 0.56eV. Solar cell parameters were also evaluated under illumination of 6mW/cm2 and the power conversion efficiency was estimated as 1.5%.

The dependence of the ideality factor on the charge density of a conjugated polymer (MEH-PPV) doped with some laser dyes

Blends made of poly[2-methyl-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) conducting polymer and cresyl violet 670 (CV) and coumarin 460 (C460) have been utilized for preparation of organic Schottky diodes. The I–V characteristics of these diodes are then determined and interpreted in terms of thermoionic emission (TIE) conduction process. Upon blending MEH-PPV with CV and C460 dyes, the conduction is seen to completely be through TIE conduction. The turn on voltage for MEH-PPV diode is found to be 2.0V and decreases with CV blending in contrast to the blend of MEH-PPV-C460. The Schottky parameters are determined where the potential barrier is found to be around 0.75V which is consistent with reported values. The charge densities of the respective blends are calculated and found to reduce by in MEH-PPV-C460 blends while it has been enhanced in the CV samples by an order of magnitude compared that of MEH-PPV. The correlation between the ideality factor with the dielectric constant and the charge density is inspected. It is found that the ratio between the charge density of the polymeric blend and the charge density of the Al-electrode is crucial for obtaining low ideality factor.

Effect of temperature on the electrical properties of Zn0.95M0.05O (M = Zn, Fe, Ni)

We report here the structural and electrical properties of Zn0.95M0.05O ceramic varistors, M = Zn, Ni and Fe. The samples were tested for phase purity and structural morphology by using X-Ray diffraction XRD and scanning electron microscope SEM techniques. The current-voltage characteristics J-E were obtained by dc electrical measurements in the temperature range of 300–500 K. Addition of doping did not influence the hexagonal wurtzite structure of ZnO ceramics. Furthermore, the lattice parameters ratio c/a for hexagonal distortion and the length of the bond parallel to the c axis, u were nearly unaffected. The average grain size was decreased from 1.57 μm for ZnO to 1.19 μm for Ni sample and to 1.22 μm for Fe sample. The breakdown field EB was decreased as the temperature increased, in the following order: Fe > Zn > Ni. The nonlinear region was clearly observed for all samples as the temperature increased up to 400 K and completely disappeared with further increase of temperature up to 500 K. The values of nonlinear coefficient, a were between 1.16 and 42 for all samples, in the following order: Fe > Zn > Ni. Moreover, the electrical conductivity s was gradually increased as the temperature increased up to 500 K, in the following order: Ni > Zn > Fe. On the other hand, the activation energies were 0.194 eV, 0.136 and 0.223 eV for all samples, in the following order: Fe, Zn and Ni. These results have been discussed in terms of valence states, magnetic moment and thermo-ionic emission, which were produced by the doping, and controlling the potential barrier of ZnO.

Anisotropic properties and conduction mechanism of TlInSe2 chain semiconductor

TlInSe2 chain crystals were prepared using the modification of the Bridgman technique. The grown crystals were identified by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). We investigate the anisotropy of transport properties for the first time for TlInSe2 crystals. Temperature dependence of the dc electrical conductivity, Hall coefficient, Hall mobility, and charge carrier concentration were investigated in the temperature range 184–455 K. The conduction mechanism of TlInSe2 crystals was studied, and measurements revealed that the dc behavior of the grown crystals can be described by Mott’s variable range hopping (VRH) model in the low temperature range, while it is due to thermoionic emission of charge carriers over the chain boundaries above 369 K. The Mott temperature, the density of states at the Fermi level, and the average hopping distance are estimated in the two crystallographic directions. The temperature dependence of the ac conductivity and the frequency exponent, s, is reasonably well interpreted in terms of the correlated barrier-hopping CBH model.

Synthesis, characterization and dielectric properties of novel axial coumarin-substituted titanium(IV) phthalocyanines

The oxotitanium(IV) phthalocyanine (2) and axial coumarin-substituted derivatives (5 and 6) have been synthesized, and characterized by IR, UV–Vis, fluorescence, 1H NMR, MALDI-TOF mass spectrometry and elemental analysis. It was observed that the reaction of novel oxotitanium(IV) phthalocyanine with orthodihyroxycoumarin compounds gives its derivatives, substituted by coumarin at the axial position. The electronic and interface state density properties of the Ag/Pc/p-Si diodes have been investigated by current–voltage (I–V) and conductance–frequency–voltage (G–ω–V) methods. It was found that, the conduction mechanism of the diodes is controlled by the thermoionic emission mechanism. From the analysis of frequency, ω, dependence of electrical conductivity, σac, it was found that the σac obeys the power law given by σac=σ0ωs. The interface state density of the diodes were determined from (G/ω)–ω plots. It was found that the kind of substitution affects both the interface and ac electrical properties of the phthalocyanine complexes considerably. The obtained results show that the Ag/6/p-Si heterojunction is a good candidate for the electronic device applications.

Schottky barrier diode parameters of Ag/MgPc/p-Si structure

An Ag /Pc/p- Si Schottky barrier (SB) diode was fabricated. The current-voltage (I-V), capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were carried out to determine the characteristic parameters such as barrier height, ideality factor and series resistance of the SB diode. The non-linear behavior of ln (I) vs. ln (V) and ln (I/V) vs. V1/2 plots indicated that the thermoionic emission theory can be applied to evaluate junction parameters for the investigated SB diode rather than space-charge limited conduction (SCLC) mechanism and bulk-limited Poole–Frenkel emission. The bulk doping concentration NB and fixed oxide charges Nf was determined from the measured high frequency C-V curve and was found to be 9.5 × 1014 cm-3 and 2.3 × 1013 cm-2, respectively. The values of barrier height obtained from Norde's function were compared with those from the forward bias current-voltage characteristic, and it was seen that there was a good agreement between barrier heights from both methods.