The influence of high energy electron irradiation on the Schottky barrier height and the Richardson constant of Ni/4H-SiC Schottky diodes

The influence of high energy electron (HEE) irradiation from a Sr-90 radio-nuclide on n-type Ni/4H–SiC samples of doping density 7.1×10¹⁵ cm⁻³ has been investigated over the temperature range 40–300 K. Current–voltage (I–V), capacitance–voltage (C–V) and deep level transient spectroscopy (DLTS) were used to characterize the devices before and after irradiation at a fluence of 6×10¹⁴ electrons-cm⁻². For both devices, the I–V characteristics were well described by thermionic emission (TE) in the temperature range 120–300 K, but deviated from TE theory at temperature below 120 K. The current flowing through the interface at a bias of 2.0 V from pure thermionic emission to thermionic field emission within the depletion region with the free carrier concentrations of the devices decreased from 7.8×10¹⁵ to 6.8×10¹⁵ cm⁻³ after HEE irradiation. The modified Richardson constants were determined from the Gaussian distribution of the barrier height across the contact and found to be 133 and 163 A cm⁻² K⁻² for as-deposited and irradiated diodes, respectively. Three new defects with energies 0.22, 0.40 and 0.71 eV appeared after HEE irradiation. Richardson constants were significantly less than the theoretical value which was ascribed to a small active device area.     

Potentiality of trap charge effects and SiON induced interface defects in a-Si3N4/SiON based MIS structure for resistive NVM device

Potential application of amorphous silicon nitride (a-Si₃N₄)/silicon oxy-nitride (SiON) film has been demonstrated as resistive non-volatile memory (NVM) device by studying the Al/Si₃N₄/SiON/p-Si metal–insulator–semiconductor (MIS) structure. The existence of several deep trap states was revealed by the photoluminescence characterizations. The bipolar resistive switching operation of this device was investigated by current–voltage measurements whereas the trap charge effect was studied in detail by hysteresis behavior of frequency dependent capacitance–voltage characteristics. A memory window of 4.6 V was found with the interface trap density being 6.4 × 10¹¹ cm⁻² eV⁻¹. Excellent charge retention characteristics have been observed for the said MIS structure enabling it to be used as a reliable non-volatile resistive memory device.     

Role of Schottky barrier height at source/drain contact for electrical improvement in high carrier concentration amorphous InGaZnO thin film transistors

We report the fabrication of bottom-gate thin film transistors (TFTs) at various carrier concentrations of an amorphous InGaZnO (a-IGZO) active layer from ~10¹⁶ to ~10¹⁹ cm⁻³, which exceeds the limit of the concentration range for a conventional active layer in a TFT. Using the Schottky TFTs configuration yielded high TFT performance with saturation mobility (μ_sat), threshold voltage (V_TH), and on off current ratio (I_ON/I_OFF) of 16.1 cm²/V s, −1.22 V, and 1.3×10⁸, respectively, at the highest carrier concentration active layer of 10¹⁹ cm⁻³. Other carrier concentrations (<10¹⁹ cm⁻³) of IGZO resulted in a decrease of its work function and increase in activation energy, which changes the source/drain (S/D) contact with the active layer behavior from Schottky to quasi Ohmic, resulting in achieving conventional TFT. Hence, we successfully manipulate the barrier height between the active layer and the S/D contact by changing the carrier concentration of the active layer. Since the performance of this Schottky type TFT yielded favorable results, it is feasible to explore other high carrier concentration ternary and quaternary materials as active layers.     

Magnetron-sputtered high performance Y-doped ZnO thin film transistors fabricated at room temperature

In this report, sputtered-grown undoped ZnO and Y-doped ZnO (ZnO:Y) thin film transistors (TFTs) are presented. Both undoped ZnO and ZnO:Y thin films exhibited highly preferred c-axis oriented (002) diffraction peaks. The ZnO:Y thin film crystallinity was improved with an increase of (002) peak intensity and grain size. The electrical properties of ZnO:Y TFTs were significantly enhanced relative to undoped ZnO TFTs. ZnO:Y TFTs exhibited excellent performance with high mobility of 38.79 cm² V⁻¹ s⁻¹, small subthreshold swing of 0.15 V/decade, and high I_on/I_off current ratio of the order of 8.17 × 10⁷. The O_1s X-ray photoelectron spectra (XPS) showed oxygen vacancy-related defects present in the ZnO:Y TFTs, which contributed to enhancing the mobility of the TFTs.     

AlGaN/GaN Schottky barrier diodes on silicon substrates with various Fe doping concentrations in the buffer layers

This study demonstrated AlGaN/GaN Schottky barrier diodes (SBDs) for use in high-frequency, high-power, and high-temperature electronics applications. Four structures with various Fe doping concentrations in the buffer layers were investigated to suppress the leakage current and improve the breakdown voltage. The fabricated SBD with an Fe-doped AlGaN buffer layer of 8 × 10¹⁷ cm^− 3 realized the highest on-resistance (R_ON) and turn-on voltage (V_ON) because of the memory effect of Fe diffusion. The optimal device was the SBD with an Fe-doped buffer layer of 7 × 10¹⁷ cm^− 3, which exhibited a R_ON of 31.6 mΩ-cm², a V_ON of 1.2 V, a breakdown voltage of 803 V, and a buffer breakdown voltage of 758 V. Additionally, the low-frequency noise decreased when the Fe doping concentration in the buffer layer was increased. This was because the electron density in the channel exhibited the same trend as that of the Fe doping concentration in the buffer layer.     

High electron mobility triazine for lower driving voltage and higher efficiency organic light emitting devices

The triazine compound 4,4′-bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1′-biphenyl (BTB) was developed for use as an electron transport material in organic light emitting devices (OLEDs). The material demonstrates an electron mobility of ∼7.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at a field of 8.00 × 10⁵ V cm⁻¹, which is 10-fold greater than that of the widely used material tris(8-hydroxyquinoline) aluminum (AlQ₃). OLEDs with a BTB electron transport layer showed a ∼1.7–2.5 V lower driving voltage and a significantly increased efficiency, compared to those with AlQ₃. These results suggest that BTB has a strong potential for use as an OLED electron transport layer material.     

In-situ MBE Si as passivating interlayer on GaAs for HfO2 MOSCAP’s: effect of GaAs surface reconstruction

We report a study of MOS capacitors having a dielectric of HfO₂ and an interlayer of Si deposited in-situ, by MBE on GaAs surfaces prepared with various surface-reconstructions. Interface state densities of about 1 × 10¹² eV⁻¹cm⁻² have been obtained. Capacitors on the Ga-rich surface, measured with peripheral illumination, show signs of a possible inversion layer.     

Electrical properties and barrier modification of GaAs MIS Schottky device based on MEH-PPV organic interfacial layer

We report fabrication and electrical characterization of GaAs based metal-interfacial layer-semiconductor (MIS) device with poly[2-methoxy-5-(2^/-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), as an interfacial layer. MEH-PPV raises the barrier height in Al/MEH-PPV/p-GaAs MIS device as high as to 0.87 eV. A Capacitance-Voltage (C–V) characteristic exhibits a low hysteresis voltage with an interface states density of 1.69×10¹¹ cm⁻² eV⁻¹. Moreover, a high transition frequency (f_c) of about 50 kHz was observed in the accumulation mode. The photovoltaic response of Al/MEH-PPV/p-GaAs device was measured under the air masses (AM) 1.0 and 1.5. The open circuit voltage (V_OC), short circuit current (I_SC), fill factor and the efficiency of the Al/MEH-PPV/p-GaAs device were found to be 1.10 V, 0.52 mA, 0.65, and 5.92%, respectively, under AM 1.0 condition.     

Electrical and photovoltaic properties of SnSe/Si heterojunction

Thin film of SnSe is deposited on n-Si single crystal to fabricate a p-SnSe/n-Si heterojunction photovoltaic cell. Electrical and photoelectrical properties have been studied by the current density–voltage (J–V) and capacitance–voltage (C–V) measurements at different temperatures. The fabricated cell exhibited rectifying characteristics with a rectification ratio of 131 at ±1 V. At low voltages (V<0.55 V), the dark forward current density is controlled by the multi-step tunneling mechanism. While at a relatively high voltage (V>0.55 V), a space charge-limited-conduction mechanism is observed with trap concentration of 2.3×10²¹ cm⁻³. The C–V measurements showed that the junction is of abrupt nature with built-in voltage of 0.62 V which decreases with temperature by a gradient of 2.83×10⁻³ V/K. The cell also exhibited strong photovoltaic characteristics with an open-circuit voltage of 425 mV, a short-circuit current density of 17.23 mA cm⁻² and a power conversion efficiency of 6.44%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50 mW cm⁻².     

Electrical and optical properties of Cu2ZnSnS4 grown by a thermal co-evaporation method and its diode application

Cu₂ZnSnS₄ (CZTS) is low cost and constitutes non-toxic materials abundant in the earth crust. Environment friendly solar cell absorber layers were fabricated by a thermal co-evaporation technique. Elemental composition of the film was stated by energy dispersive spectroscopy (EDS). Some optical and electrical properties such as absorption of light, absorption coefficient, optical band gap charge carrier density, sheet resistance and mobility were extracted. Optical band gap was found to be as 1.44 eV, besides, charge carrier density, resistivity and mobility were found as 2.14×10¹⁹ cm⁻³, 8.41×10⁻⁴ Ω cm and 3.45×10² cm² V⁻¹ s⁻¹, respectively. In this study Ag/CZTS/n-Si Schottky diode was fabricated and basic diode parameters including barrier height, ideality factor, and series resistance were concluded using current–voltage and capacitance–voltage measurements. Barrier height and ideality factor values were found from the measurements as 0.81 eV and 4.76, respectively, for Ag/CZTS/n-Si contact.     

Charge trapping in ultrathin Gd2O3 high-k dielectric

Charge trapping in ultrathin high-k Gd₂O₃ dielectric leading to appearance of hysteresis in C-V curves is studied by capacitance-voltage and current-voltage techniques. It was shown that the large leakage current at a negative gate voltage causes the generation of the positive charge in the dielectric layer, resulting in the respective shift of the C-V curve. The capture cross-section of the hole traps is around 2 × 10⁻²⁰ cm². The distribution of the interface states was measured by conductance technique showing the concentration up to 7.5 × 10¹² eV⁻¹ cm⁻² near the valence band edge.     

IGZO thin film transistors with Al2O3 gate insulators fabricated at different temperatures

Thin film transistors (TFTs) with bottom gate and staggered electrodes using atomic layer deposited Al₂O₃ as gate insulator and radio frequency sputtered In–Ga–Zn Oxide (IGZO) as channel layer are fabricated in this work. The performances of IGZO TFTs with different deposition temperature of Al₂O₃ are investigated and compared. The experiment results show that the Al₂O₃ deposition temperature play an important role in the field effect mobility, I_on/I_off ratio, sub-threshold swing and bias stability of the devices. The TFT with a 250 °C Al₂O₃ gate insulator shows the best performance; specifically, field effect mobility of 6.3 cm²/Vs, threshold voltage of 5.1 V, I_on/I_off ratio of 4×10⁷, and sub-threshold swing of 0.56 V/dec. The 250 °C Al₂O₃ insulator based device also shows a substantially smaller threshold voltage shift of 1.5 V after a 10 V gate voltage is stressed for 1 h, while the value for the 200, 300 and 350 °C Al₂O₃ insulator based devices are 2.3, 2.6, and 1.64 V, respectively.     

Effect of gate insulator thickness on device performance of InGaZnO thin-film transistors

Top-contact thin-film transistors (TFTs) are fabricated in this work using atomic layer deposition (ALD) Al₂O₃ as the gate insulator and radio frequency sputtering InGaZnO (IGZO) as the channel layer so as to investigate the effect of Al₂O₃ thickness on the performance of IGZO-TFTs. The results show that TFT with 100-nm-thick Al₂O₃ (100 nm-Al₂O₃-TFT) exhibits the best electrical performance; specifically, field-effect mobility of 5 cm²/Vs, threshold voltage of 0.95 V, I_on/I_off ratio of 1.1×10⁷ and sub-threshold swing of 0.3 V/dec. The 100 nm-Al₂O₃-TFT also shows a substantially smaller threshold voltage shift of 1.1 V after a 10 V gate voltage is applied for 1 h, while the values for TFTs with an Al₂O₃ thickness of 220 and 280 nm are 1.84 and 2 V, respectively. The best performance of 100 nm-Al₂O₃-TFT can be attributed to the larger capacitance and the smaller amount of total trap centers possessed by a thinner insulator compared to the thicker ones.     

Photo-patternable polyimide gate insulator with fluorine groups for improving performance of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiopene (C10-BTBT) thin-film transistors

Surface properties of gate insulators strongly affect the device performance of organic thin-film transistors (OTFTs). To improve the performance of OTFTs, we have developed photo-sensitive polyimide gate insulator with fluorine groups. The polyimide gate insulator film could be easily patterned by selective UV exposure without any photoinitiator. The polyimide gate insulator film, fabricated at 130 °C, has a dielectric constant of 2.8 at 10 kHz, and leakage current density of <1.6 × 10^?10 A/cm² while biased from 0 to 90 V. To investigate the potential of the polyimide with fluorine groups as a gate insulator, we fabricated C₁₀-BTBT TFTs. The field-effect mobility and the on/off current ratio of the TFTs were measured to be 0.76 ± 0.09 cm²/V s and >10⁶, respectively.     

Effect of interface traps for ultra-thin high-k gate dielectric based MIS devices on the capacitance-voltage characteristics

The impact of states at the Al₂O₃/Si interface on the capacitance-voltage C-V characteristics of a metal/insulator/semiconductor heterostructure (MIS) capacitor was studied by a numerical simulation, by solving Schrodinger-Poisson equations and taking the electron emission rate from the interface state into account. Efficient computation and accurate physics based capacitance model of MOS devices with advanced ultra-thin equivalent oxide thickness (EOT) (down to 2.5 nm clearly considered here) were introduced for the near future integrated circuit IC technology nodes. Due to the importance of the interface state density for a low dimension and very low oxide thickness, a high frequency C-V model has been developed to interpret the effect of interface state density traps which communicate with the Al₂O₃/Si and their influence on the C-V characteristics. We found that these states are manifested by jumping capacity in the inversion zone, for a density of interface, higher than 1 × 10¹¹ cm^− 2 eV^− 1 during a p-doping of 1 × 10¹⁸ cm^− 3. This behavior has been investigated with various doping, temperature, frequency and energy levels on the C-V curves, and compared with the MIS structure that contains a standard SiO₂ insulator.     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

Growth and characterization of indium oxide diodes prepared by reactive magnetron sputtering

The electrical properties of Cr/Pt/Au and Ni/Au ohmic contacts with unintentionally doped In₂O₃ (U-In₂O₃) film and zinc-doped In₂O₃ (In₂O₃:Zn) prepared by reactive magnetron sputtering deposition are described. The lowest specific contact resistance of Cr/Pt/Au and Ni/Au is 2.94 × 10⁻⁶ and 1.49 × 10⁻² Ω-cm², respectively, as determined by the transmission line model (TLM) after heat treatment at 300 °C by thermal annealing for 10 min in nitrogen ambient. The indium oxide diodes have an ideality factor of 1.1 and a soft breakdown voltage of 5 V. The reverse leakage current prior to breakdown is around 10⁻⁵ A.     

The effect of various concentrations of PVDF-HFP polymer gel electrolyte for dye-sensitized solar cell

A PVDF-HFP gel electrolytes based DSSCs were fabricated successfully, where gel electrolytes with 2.5 wt.%, 5 wt.%, 10 wt.% and 15 wt.% PVDF-HFP are included, respectively. Linear sweep voltammetry (LSV), photocurrent–voltage measurements and electrochemical impedance spectra (EIS) were measured. As the results shown, the apparent diffusion coefficient (D_app) of I⁻ and I₃⁻ decreased as PVDF-HFP increased. D_app of I⁻ and I₃⁻ are decreased from 1.87 × 10^− 6 to 0.67 × 10^− 6 cm²/s and 3.28 × 10^− 6 to 0.88 × 10^− 6 cm²/s, respectively. For the solar cell measurements, the short circuit current density (J_sc) were affected by the ion motilities, which was decreased from 11.58 mA/cm² to 8.17 mA/cm², and the energy converting efficiency (η %) was decreased from 5.17% to 2.79%. For electrochemical impedance spectra (EIS) measurements, the ionic diffusion impedance for the redox-couple (I⁻/I₃⁻) in the gel electrolyte was also increased with the concentration of PVDF-HFP from 0.61 Ω to 8.17 Ω. In the Bode Plots, the electron lifetime (τ_e) of the 2.5 wt.% and 5 wt.% PVDF-HFP electrolytes was increased from 40.52 ms to 48.48 ms and 41.29 ms, respectively. However, τ_e was decreased in the concentrations of 10 wt.% and 15 wt.% PVDF-HFP, due to the ion motilities that were decreased by excessing PVDF-HFP polymer. For gel electrolyte, the cell of 2.5 wt.% PVDF-HFP exhibited a better J_SC of 10.89 mA/cm², a higher energy conversion efficiency (η) of 4.75%, a higher fill factor (FF) of 61.26%, and a smaller R of 1.06 Ω than the 15 wt.% PVDF-HFP based cell.     

Evaporated iron disulfide thin films with sulfurated annealing treatments

FeS₂ thin films were grown on a glass substrate using a physical vapor deposition technique at room temperature. Subsequently, the thin films were annealed in two different atmospheres: vacuum and vacuum-sulfur. In the vacuum-sulfur atmosphere a graphite box was used as sulfur container and the films were sulfurated successfully at 200–350 ºC. It was found that annealing in a vacuum-sulfur atmosphere was indispensable in order to obtain polycrystalline FeS₂ thin films. The polycrystalline nature and pure phase were determined by XRD and Raman techniques and the electrical properties by the Hall effect. Using the sulfurating technique, the n-type semiconductor was prepared at 200–350 °C and a p-type at 500 °C. The carrier concentrations were between 1.19×10²⁰ and 2.1×10²⁰ cm⁻³. The mobility was 9.96–5.25 cm² V⁻¹ s⁻¹ and the resistivity was 6.31×10⁻² to 1.089×10⁻² Ω cm. The results obtained from EDS showed that the films prepared in the vacuum-sulfur atmosphere were close to stoichiometric and that the indirect band gap varied between 1.03 and 0.945 eV.     

A study of charge transport in a novel electroluminescent poly(phenylene vinylene-co-fluorenylene vinylene) based π-conjugated polymer

The charge transport properties in a novel electroluminescent poly{[2-(4′,5′-bis(3″-methylbutoxy)-2′-p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207–300 K) and electric fields (1.5 × 10⁵–6.1 × 10⁵ V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 × 10⁻⁵ cm²/V s at 2.5 × 10⁵ V/cm. This increased to 8.4 × 10⁻⁵ cm²/V s at 6.1 × 10⁵ V/cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler’s Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor μ_∞ = 1.2 × 10⁻³ cm²/V s, energetic disorder parameter σ = 82 meV and positional disorder parameter Σ = 1.73. The average inter-site separation (a = 7 Å) and the charge localization length (L = 3.6 Å) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.