Poole-Frenkel Conduction Mechanism Research Articles

Poole-Frenkel conduction in CdS single-layered and CdS/SnS2 heterojunction electrode system

In this communication, the prevalence of Poole-Frenkel conduction mechanism in two distinct semiconductor systems, CdS single-layered and CdS/SnS2 heterojunction electrode systems, is reported. X-ray diffraction (XRD) exhibits the formation of CdS quantum dots (QDs). A High resolution transmission electron microscopy (HRTEM) shows a discrete particle distribution of SnS2, tends to assemble into nanosheets. Poole-Frenkel conduction arises due to the trap distribution of CdS dots, modified by SnS2 sheets. Furthermore, the formation of heterojunctions with SnS2 shows promising enhancement in charge transport, characterized by reduced trap density and improved conductivity compared pristine CdS. The findings provide valuable insights into the fundamental charge transport processes in CdS/SnS2 system and offer potential avenues for optimizing the performance of electronic devices.

Effect of tensile and compressive strain on the gate leakage current and inverse piezoelectric effect in AlGaN/GaN HEMT devices

The influence of external strain on the gate leakage current of AlGaN/GaN high-electron-mobility transistors was studied. The magnitude of the leakage current increased by 39% under 0.1% tensile strain but decreased by 23% under −0.1% compressive strain. The leakage current obeyed the Poole–Frenkel conduction mechanism, demonstrating a decrease/increase in the barrier height for electron emission from the trap state into the continuum dislocation state. Furthermore, the magnitude of critical reverse stressing voltage for the onset of degradation induced by the inverse piezoelectric effect became lower/higher under tensile/compressive strain compared with that of the initial state, which was attributed to the modification of the total stress in the film. In addition, using the transient current method, it was established that the detrapping time constant for the traps in the AlGaN barrier increased as a result of the tensile strain, which is ascribed to movement of the dislocation level away from the conduction band.

Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor: Proficient hole transporting material for optoelectronic devices

Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10 compound exhibits three different rectangular columnar (Colr) phases offering an impressive hole mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole transport materials. The charge transport mechanism is governed by the hole polarons hopping through H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout the mesophase range, but it decreases with increasing applied electric field. The current-voltage characteristics of the PQ10 have also been investigated in all three Colr phases and explained via the Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to understand the nature of dielectric permittivity and conductivity as a function of temperature and a correlation is established between the molecular architecture of the Colr phases and aforementioned physical properties. Solar cell simulation has been additionally performed to demonstrate that the PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.

Luminescence from metal-oxide-semiconductor devices with Eu3+-doped CeO2 films on silicon: From broadband to monochromatic emission

We report on the transformation from broadband to monochromatic orange-red emission for the electroluminescence (EL) from the metal-oxide-semiconductor (MOS) devices with europium (Eu3+)-doped CeO2 films annealed at various temperatures as the light-emitting layers. Under the bias voltages exceeding 8 V, the 800 °C-annealed MOS device exhibits dominant Eu3+-related emission at 590 nm with a full width at half maximum of only ∼2 nm. In contrast, the 400 °C-annealed MOS device is activated into broadband emissions associated with the defects in the CeO2 host. Also, the two distinct EL behaviors can co-exist by annealing the CeO2:Eu3+ film at 600 °C. It has been demonstrated that the defect- and Eu3+-dependent EL are related to the Poole-Frenkel (P–F) and trap-assisted tunneling (TAT) electrical conduction mechanisms, respectively. The MOS device with a tunable EL is significant due to its potential applications, e.g., optoelectronic integration.

Probing hidden conduction mechanisms in diced silicon solar cells by low frequency noise analysis

Electrical low frequency noise (LFN) measurements are utilized in this work, in order to identify parasitic low voltage transport mechanisms in diced aluminum back surface field (Al-BSF) photovoltaic (PV) cells, which cannot be identified by I–V characterization. A detailed method of LFN analysis is proposed, taking into account the co-existence of various transport mechanisms, that typically occur in parallel in such devices. A precise examination of the 1/f noise level dependency with both current and voltage, combined with static I–V characterizations at various temperatures, enabled the identification of two flicker noise sources appearing at different current regions: one attributed to local linear shunt mechanisms and a second one to a fluctuation in a nonlinear parasitic conductivity localized at the samples’ edges. The latter source exhibits noise signatures resembling either tunneling or Poole-Frenkel transport, while excluding Shockley-Read-Hall recombination processes. This hypothesis was further verified by measurements of thermal activation energies typical of tunnel and Poole-Frenkel conduction mechanisms.

Enhancement of the Electroluminescence from Amorphous Er-Doped Al2O3 Nanolaminate Films by Y2O3 Cladding Layers Using Atomic Layer Deposition.

Amorphous Al2O3-Y2O3:Er nanolaminate films are fabricated on silicon by atomic layer deposition, and ~1530 nm electroluminescence (EL) is obtained from the metal-oxide-semiconductor light-emitting devices based on these nanofilms. The introduction of Y2O3 into Al2O3 reduces the electric field for Er excitation and the EL performance is significantly enhanced, while the electron injection of devices and the radiative recombination of doped Er3+ ions are not impacted. The 0.2 nm Y2O3 cladding layers for Er3+ ions increase the external quantum efficiency from ~3% to 8.7% and the power efficiency is increased by nearly one order of magnitude to 0.12%. The EL is ascribed to the impact excitation of Er3+ ions by hot electrons, which stem from Poole-Frenkel conduction mechanism under sufficient voltage within the Al2O3-Y2O3 matrix.

Interface engineering of p-type quaternary metal oxide semiconductor interlayer-embedded β-Ga2O3 Schottky barrier diode

Here, we propose a p-type copper aluminum oxide (p-CuAlO2) interlayer for the high breakdown and low leakage current of β-Ga2O3 Schottky barrier diodes (SBDs). The XPS and TEM analysis was employed to confirm the surface elemental compositions, chemical states, and microstructural properties of RF sputtered p-CuAlO2 thin films on β-Ga2O3. The electrical and carrier transport properties of Pt/β-Ga2O3 SBD and Pt/p-CuAlO2/β-Ga2O3 heterojunction (HJ) were studied by using current-voltage (I–V) and capacitance-voltage (C–V) measurements. In comparison to the SBD, the HJ displayed superior rectifying behavior, lower reverse leakage current, lower on-resistance, and a larger turn-on voltage. Higher barrier height (BH) was attained for the HJ compared to the SBD, which enabled the p-CuAlO2 interlayer to alter the barrier height. The reverse breakdown voltage of HJ increased by nearly 300 V compared to SBD and reached a maximum of 922 V. The BH values were extracted from the I–V, Hernandez, Cheung's, Mikhelashvili, Norde's, Chot, and surface potential methods found the values are comparable with one another, which indicates their steadiness and validity. The energy distribution profile of the interface state density (NSS) of the HJ decreases compared to the SBD, which indicates that the p-CuAlO2 interlayer facilitated the reduction of NSS of the Pt/β-Ga2O3 SBD interface. The forward log(I)-log(V) plot of the SBD and HJ reveals the ohmic nature at low voltage regions and space-charge limited conduction at higher voltage regions. Results reveal that the reverse leakage current conduction mechanism of the SBD and HJ showed that the Poole-Frenkel conduction mechanism is found to be governed in the lower voltage region, whereas the Schottky conduction mechanism is found to be directed in the higher voltage region, respectively. These outcomes point out that the p-CuAlO2 is a high potential candidate material for forming a heterojunction with β-Ga2O3.

Nanoscale thermal imaging of VO2 via Poole–Frenkel conduction

We present a method for nanoscale thermal imaging of insulating thin films using atomic force microscopy (AFM), and we demonstrate its utility on VO2. We sweep the applied voltage V to a conducting AFM tip in contact mode and measure the local current I through the film. By fitting the IV curves to a Poole–Frenkel conduction model at low V, we calculate the local temperature with spatial resolution better than 50 nm using only fundamental constants and known film properties. Our thermometry technique enables local temperature measurement of any insulating film dominated by the Poole–Frenkel conduction mechanism and can be extended to insulators that display other conduction mechanisms.

Zn2SnO4 Thin Film Based Nonvolatile Positive Optoelectronic Memory for Neuromorphic Computing

An optoelectronic synaptic device based on the ITO/Zn2SnO4(ZTO)/ITO structure is fabricated which integrates the electronic memory and optical sensing properties along with synaptic functions. The fabricated device shows over 80% optical transparency for the entire visible region (400–800 nm). Post-oxide annealing treatment is performed in a nitrogen environment at 200 °C. Significant improvements in bipolar resistive switching properties of the device with low SET voltage (+0.93 V) and long DC endurance cycles (∼12000) are observed in the annealed device. The linearity of such memristive synapse is improved for 350 training epochs with a total number of 175000 pulses. The spike time dependent plasticity learning rule for the annealed device is demonstrated through the electric field. The optical sensing capabilities of this device including photonic potentiation (responsivity: 0.52 μA/W), photonic paired pulse facilitation by adjusting time interval between two identical light pulses, learning experience behavior, and multilevel memory feature by the repetition of optical pulse for ∼103 s are demonstrated under the blue light (wavelength “λ” = 405 nm) illumination at 50 mW/cm2. Photonic potentiation and electric depression behavior of the device mimic its nonvolatile synaptic plasticity. The linear fitting of I–V curve illustrates the dominance of Schottky emission and Poole–Frenkel conduction mechanisms at high and low resistance states, respectively. The electric response of the device is explained by the oxygen vacancy based filamentary model. The trapping and detrapping of electrons during the adsorption and desorption processes of atmospheric oxygen molecules on the ITO surface are responsible for the photoconduction phenomenon. To train the Hopfield neural network (HNN) model for image processing of 28 × 28 pixels, the normalized experimental data of long-term potentiation/depression are employed to mimic the learning behavior of the human brain. The convergence of electronic data storage and optical sensor has high potential which provides a path toward the future smart invisible optoelectronics for artificial intelligence.

Influence of the fullerene acceptor derivative on the structure, molecular interaction and optoelectronic properties of poly(3-hexylthiophene) blend films: Elucidating key factors

Blend films made of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) with the weight ratio in the range of 1:0 to 1:2 were prepared using the spin coating method. The influence of PC71BM on the structure, morphology and molecular interactions was assessed using the AFM, XRD, and FTIR spectroscopy measurements. The optical absorbance spectrum of the P3HT:PC71BM blend films inspected that the band gap of P3HT decreased from 2.1 eV to around 1.95 eV with increasing the amount of PC71BM in P3HT. The luminescence spectra of the P3HT:PC71BM blend films indicated that the incorporation of PC71BM with the P3HT:PC71BM weight ratio above 1:1.2 leads to the low emission, implying that 1:1.2 is the optimal blend ratio. The time dependent luminescence intensity measurements showed that the incorporation of PC71BM in P3HT delays the recombination process, thus, allowing an enhancement of the separation of the photo-generated charges. The electrical behavior of the P3HT:PC71BM blend films is described by the space charge limited conduction mechanism, in particular, the Poole-Frenkel conduction mechanism. The inclusion of the Cd0.95Hf0.05S QDs into the hole transport layer PEDOT:PSS leads to an improvement of the solar cell performance. At optimum PC71BM concentration, the highest conversion efficiency of 4.7% with VOC = 0.62 V and JSC = 4.2 mA cm−2 was attained.

Electron Doping Effect in the Resistive Switching Properties of Al/Gd1-xCaxMnO3/Au Memristor Devices.

We report on the resistive switching (RS) properties of Al/Gd1–xCaxMnO3 (GCMO)/Au thin-film memristors. The devices were studied over the whole calcium substitution range x as a function of electrical field and temperature. The RS properties were found to be highly dependent on the Ca substitution. The optimal concentration was determined to be near x = 0.9, which is higher than the values reported for other similar manganite-based devices. We utilize an equivalent circuit model which accounts for the obtained results and allows us to determine that the electrical conduction properties of the devices are dominated by the Poole–Frenkel conduction mechanism for all compositions. The model also shows that lower trap energy values are associated with better RS properties. Our results indicate that the main RS properties of Al/GCMO/Au devices are comparable to those of other similar manganite-based materials, but there are marked differences in the switching behavior, which encourage further exploration of mixed-valence perovskite manganites for RS applications.

Existence of bipolar and unipolar resistive switching in CaZrO3 thin film device

Resistive Random Memory (RRAM) effect has been investigated in Pt/CaZrO3/Pt/Si thin-film device, which is demonstrated bipolar resistive switching (BRS) and unipolar resistive switching (URS) phenomenon. BRS has a wide voltage window to SET and RESET the device during the repetition of the cycle. In the URS measurement, the device was SET at a lower voltage window and RESET to a higher voltage window. The resistance ratio (RHRS/RLRS) is ≈102 in BRS and 103 in URS, respectively. It revealed a good retention time ≈103 s and ≈104 s in BRS and URS, respectively, to retain the data. During URS measurement, the device was SET and RESET at a compliance current of 5 μA and 1 mA, respectively. Poole-Frenkel (P-F) and Schottky emission (S-E) conduction mechanisms provide unrealistic dielectric constant. SCLC conduction mechanism has been dominated in the phenomenon of resistive switching. Formation and rupturing of thin conducting filament depend upon the migration of oxygen vacancies and oxygen ions. This device illustrates BRS and URS phenomenon and low compliance current, which may be suitable for nonvolatile memories.

Resistive Switching Characteristics of Nonvolatile Memory with HSQ Film Using Microwave Irradiation.

In this study, we fabricated a resistive random access memory (ReRAM) of metal-insulator-metal structures using a hydrogen silsesquioxane (HSQ) film that was deposited by a low-cost solution process as a resistance switching (RS) layer. For post-deposition annealing (PDA) to improve the switching performance of HSQ-based ReRAMs, we applied high energy-efficient microwave irradiation (MWI). For comparison, ReRAMs with an as-deposited HSQ layer or a conventional thermally annealed (CTA) HSQ layer were also prepared. The RS characteristics, molecular structure modification of the HSQ layer, and reliability of the MWI-treated ReRAM were evaluated and compared with the as-deposited or CTA-treated devices. Typical bipolar RS (BRS) behavior was observed in all the fabricated HSQ-based ReRAM devices. In the low-voltage region of the high-resistance state (HRS) as well as the low-resistance state, current flows through the HSQ layer by an ohmic conduction mechanism. However, as the applied voltage increases in HRS, the current slope increases nonlinearly and follows the Poole-Frenkel conduction mechanism. The RS characteristics of the HSQ layer depend on the molecular structure, and when the PDA changes from a cage-like structure to a cross-linked network, memory characteristics are improved. In particular, the MWI-treated HSQ ReRAM has the largest memory window at the smallest operating power and demonstrated a stable endurance during the DC cycling test over 500 times and reliable retention at room (25 °C) and high (85 °C) temperatures for 10⁴ seconds.

Temperature‐Dependent Electrical Transport Properties of Single‐Walled Carbon Nanotube Thin Films Prepared by Electrohydrodynamic Atomization Technique

Herein, the temperature‐dependent electrical transport properties of single‐walled carbon nanotube (SWCNT) thin films prepared by the electrohydrodynamic atomization technique, are reported. The physico–chemical properties of SWCNT thin films are characterized using X‐ray diffraction, Raman, transmission electron, and field‐emission scanning electron microscopy techniques. The electrical transport measurements are carried out from 285 to 20 K. A semiconducting behavior is observed when the temperature goes down to 20 K. The prevalent transport mechanism is analyzed with a variable‐range hopping model, which is well aided by Raman analysis. Furthermore, the thermal dependence of the electrical conductivity of the SWCNT thin film is investigated using the Arrhenius model, which reveals the conductivity of the SWCNT thin film at low‐temperature shows in the order of mega‐Ohm resistance with a small activation energy of 1.81 J mol−1, and an exponential decrement in conductivity is observed while decreasing the temperature to 20 K, which further confirms the semiconducting behavior of the SWCNT thin film. The Poole–Frenkel conduction mechanism shows best fit for the temperatures 50, 75, and 100 K, and the results provide potential insights into the science and development of SWCNT research.

On high-field conduction and I–V measurements in quaternary Se–Te–In–Pb nano-chalcogenide thin films

In this paper, multi-component Se79-xTe15In6Pbx (x = 0, 1, 2, 4, 6, 8 ,and 10 at.wt.%) nano-chalcogenide alloys were prepared by using melt quenching technique and their electrical properties and sub-micron structural features have been reported and discussed in detail. Sharp characteristic peaks in x-ray diffraction (XRD) spectra signify nano-range of each alloy under investigation and validated by field emission scanning electron microscopy (FESEM). The thin films of investigated nano-chalcogenide alloys were deposited on glass substrates by thermal evaporation technique. In-depth analysis of DC conductivity for studied nano-chalcogenide alloys was carried out in the temperature range 303–333 K and in the voltage range 0–150 V, to realize the conduction mechanism. I–V characteristics illustrate ohmic behavior in the voltage range 0–50 V while at high voltage non-ohmic behavior is detected. Moreover, pre-exponential factor (σo) along with activation energy (ΔE) deduced from the variation of DC conductivity with temperature, which reveals that thermally assisted tunneling of charge carriers among the localized states occurs in band tails responsible for conduction in the analyzed systems. The detailed analysis indicates that SCLC theory is not appropriate to describe the conduction process, as ln(I/V) against V plots is almost straight, although slope of these plots does not reduce linearly with temperature. The linear relation among ln(I) against V1/2 demonstrates that the conduction mechanism is either Poole–Frenkel type or Schottky emission. The obtained outcomes demonstrate that Poole–Frenkel conduction mechanism is most appropriate to describe the conduction. The deduced values of activation energy are less than 0.98 eV, which once again validated the presence of Poole–Frenkel type conduction in the examined composition. The deduced results are further corroborated by photoluminescence studies.

Electroluminescence from light-emitting devices based on erbium-doped ZnO/n-Si heterostructures: Enhancement effect of fluorine co-doping.

We report on erbium (Er) related electroluminescence (EL) in the visible and near infrared (NIR) regions from the light-emitting device (LED) based on the Er-doped ZnO (ZnO:Er)/n-Si isotype heterostructure formed by sputtering ZnO:Er film on n-Si/n+-Si epitaxial wafer. Herein, the ZnO:Er film exhibits n-type in electrical conduction. The aforementioned LED is electroluminescent only under sufficiently high forward bias with the negative voltage connecting to n+-Si substrate. Such forward bias enables the electrons from n-Si to enter into the ultra-thin SiOx (x ≤ 2) layer inherently existing between the ZnO:Er film and n-Si via Poole-Frenkel conduction mechanism and, subsequently, to drift into the ZnO:Er film thus becoming hot electrons, which impact-excite the Er3+ ions to emit characteristic visible and NIR light. Furthermore, the Er-related EL from the aforementioned LED can be significantly enhanced through adopting the strategy of co-doping F- ions into the ZnO host, which brings about twofold primary effects. Firstly, due to the atomic size compensation between F- and Er3+ ions, the ZnO crystal grains become larger to accommodate much more optically active Er3+ ions. Secondly, the partial substitution of F- ions for O2- ions around the Er3+ ion reduces the symmetry of pseudo-octahedral crystal field of Er3+ ion, thus increasing the probabilities of intra-4f transitions of Er3+ ions. We believe that this work sheds light on developing efficient silicon-based LEDs using the Er-doped oxide semiconductors.

Study on the charging current of surface traps in AlGaN/GaN high electron mobility transistors with a slot gate structure

The charging current of surface traps in AlGaN/GaN high electron mobility transistors with a slot gate structure was investigated. A slot was formed in the middle of the gate metal via the lift-off process. Even though the slot gate only partially controlled the channel, the transient drain current gradually decreased toward a saturated off-state value with increasing measurement time after application of an off-state gate voltage. This indicated that the slot gate was still capable of turning off the entire channel. Analysis of the experimental results indicated that electrons were injected from the gate and trapped in the slot region, resulting in the depletion of the slot channel. An equivalent charging current by the surface traps on AlGaN could be inferred via the charge conservation principle. Temperature-dependent measurements of the charging current showed that it could be well fitted by the Poole-Frenkel conduction mechanism, with an extracted trap energy level of 0.129 eV.

Effect of laser illumination on the electrical conductivity of single-crystal diamond needles

Herein, we present the results of electrical conductivity studies of single-crystal diamond needles. Pyramid-shaped needles of about hundreds of micrometers in length and about tens of micrometers in thickness were produced by oxidation of polycrystalline diamond films fabricated by chemical vapor deposition. The electrical conductivity of the needles was characterized via current measurements using a field ion microscope in the dark and under laser illumination. Considering the effects of laser intensity and wavelength on the measured current, we determine that the observed conduction regime corresponds to competing Ohmic and Poole-Frenkel conduction mechanisms. The measured values are in good agreement with the values reported previously for bulk and thin film diamond. The model that is proposed explains the electric conduction properties as well as light absorption and subsequent sample heating, with the presence of structural defects and impurities in diamond.

Temperature dependent lateral and vertical conduction mechanisms in AlGaN/GaN HEMT on thinned silicon substrate

This paper presents the influence of Si substrate thinning by grinding and dicing on the mechanical and electrical properties of AlGaN/GaN HEMT grown on 6-inch Si substrate. By experimentally removing 96% of the Si substrate, an increase in the lateral breakdown voltage by 13% is achieved for a wide temperature range. A decreasing vertical breakdown voltage with substrate thinning shows the necessity of substrate replacement to maintain the vertical isolation. Poole-Frenkel and space charge limited conduction mechanisms are identified for the vertical leakage in forward and reverse bias down to thicknesses of 200 μm, respectively. The increase in chip bow and the resulting increase in tensile strain in the epitaxial layers resulting from the substrate removal are correlated to a 40% decrease in 2DEG charge carrier concentration along with a 27% increase in threshold voltage. These findings are explained by strain-induced gate barrier and polarization charge variations.

Energy Transfer Under Electrical Excitation and Enhanced Electroluminescence in the Nanolaminate Yb,Er Co‐Doped Al2O3 Films

The energy transfer from Yb3+ to Er3+ ions under electrical excitation which enhances the Er‐related 1531 nm electroluminescence is evidenced, within the nanolaminate Yb,Er co‐doped Al2O3 films fabricated by atomic layer deposition. The light‐emitting devices based on these nanolaminates present an external quantum efficiency up to 24% and the power efficiency of 2.8×10−3, ≈60% higher than the solely Er‐doped Al2O3 device. The decay dynamics also support, with the radiative lifetime of the Yb3+ emission reduced while that of Er3+ emission rises. The Er3+ and Yb3+ ions are impact‐excited by hot electrons, which stem from Poole–Frenkel conduction mechanism in Al2O3 matrix. This work certifies the potential of sensitizing Er3+ ions by the adjacent Yb3+ ions under electrical excitation, and applications of the rare earth‐doped nanolaminates in silicon‐based optoelectronics.