Control of electric field in 4H-SiC UMOSFET: Physical investigation

In this paper, we show how breakdown voltage (V_BR) and the specific on-resistance (R_on) can be improved simply by controlling of the electric field in a power 4H-SiC UMOSFET. The key idea in this work is increasing the uniformity of the electric field profile by inserting a region with a graded doping density (GD region) in the drift region. The doping density of inserted region is decreased gradually from top to bottom, called Graded Doping Region UMOSFET (GDR-UMOSFET). The GD region results in a more uniform electric field profile in comparison with a conventional UMOSFET (C-UMOSFET) and a UMOSFET with an accumulation layer (AL-UMOSFET). This in turn improves breakdown voltage. Using two-dimensional two-carrier simulation, we demonstrate that the GDR-UMOSFET shows higher breakdown voltage and lower specific on-resistance. Our results show the maximum breakdown voltage of 1340 V is obtained for the GDR-UMOSFET with 10 µm drift region length, while at the same drift region length and approximated doping density, the maximum breakdown voltages of the C-UMOSFET and the AL-UMOSFET structures are 534 V and 703 V, respectively.     

Non-Ohmic conduction in polydiacetylene thin films

We have studied the current–voltage (I–V) characteristics of polydiacetylene (PDA) thin films in the temperature region 300–1.7 K. It was found that at electric fields higher than 2 × 10⁴ V/cm, the I–V characteristics are strongly super-linear with negative temperature coefficient of starting voltage. Negative gate voltage increases the source-drain current (the effect is more pronounced at low temperature), whereas the magnetic field up to 7 T does not affect it. The results demonstrate that at low temperature the charge transport is mainly supported due to a charge injection and tunneling from the metallic banks, whereas at higher temperatures the activation energy is related to the band gap mismatch between the different polymer chains or granules.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Low-threshold pure UV electroluminescence from n-ZnO:Al/i-layer/n-GaN heterojunction

Ultraviolet (UV) electroluminescence (EL) of n-ZnO:Al (AZO)/i-layer/n-GaN heterojunctions with different intrinsic layers has been obtained. Rectifying behavior and EL spectra of the heterojunctions are investigated at room temperature. Under positive voltage, a dominant UV emission peak around ～370 nm is observed for both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Nevertheless, the UV emission peak intensity of AZO/i-MgO/n-GaN heterojunction is much stronger than that of AZO/i-ZnO/n-GaN heterojunction at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3 V. The difference of EL spectra and the emission mechanism in these devices are discussed.     

Influence of rapid thermal annealing on electrical and structural properties of double metal structure Au/Ni/n-InP (1 1 1) diodes

The effect of rapid thermal annealing on the electrical and structural properties of Ni/Au Schottky contacts on n-InP have been investigated by current–voltage (I–V), capacitance–voltage (C–V), auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The Au/Ni/n-InP Schottky contacts are rapid thermally annealed in the temperature range of 200–500 °C for a duration of 1 min. The Schottky barrier height of as-deposited Ni/Au Schottky contact has been found to be 0.50 eV (I–V) and 0.86 eV (C–V), respectively. It has been found that the Schottky barrier height decreased with increasing annealing temperature as compared to as-deposited sample. The barrier height values obtained are 0.43 eV (I–V), 0.72 eV (C–V) for the samples annealed at 200 °C, 0.45 eV (I–V) and 0.73 eV (C–V) for those at 400 °C. Further increase in annealing temperature to 500 °C the barrier height slightly increased to 0.46 eV (I–V) and 0.78 eV (C–V) compared to the values obtained for the samples annealed at 200 °C and 400 °C. AES and XRD studies showed the formation of indium phases at the Ni/Au and InP interface and may be the reason for the increase in barrier height. The AFM results showed that there is no significant degradation in the surface morphology (rms roughness of 1.56 nm) of the contact even after annealing at 500 °C.     

Effect of series resistance on the performance of silicon Schottky diode in the presence of tin oxide layer

The current–voltage (I–V) characteristics of Al/SnO₂/p-Si (MIS) Schottky diodes prepared by means of spray deposition method have been measured at 80, 295 and 350 K. In order to interpret the experimentally observed non-ideal Al/SnO₂/p-Si Schottky diode parameters such as, the series resistance R_s, barrier height Φ_B and ideality factor n, a novel calculation method has been reported by taking into account the applied voltage drop across interfacial oxide layer V_i and ideality factor n in the current transport mechanism. The values obtained for V_i were subtracted from the applied voltage values V and then the values of R_s were recalculated. The parameters obtained by accounting for the voltage drop V_i have been compared with those obtained without considering the above voltage drop. It is shown that the values of R_s estimated from Cheung’s method were strongly temperature-dependent and decreased with increasing temperature. It is shown that the voltage drop across the interfacial layer will increase the ideality factor and the voltage dependence of the I–V characteristics. The interface state density N_ss of the diodes has an exponential growth with bias towards the top of the valance band for each temperature; for example, from 2.37 × 10¹³ eV⁻¹ cm⁻² in 0.70−E_v eV to 7.47 × 10¹³ eV⁻¹ cm⁻² in 0.62−E_v eV for 295 K. The mean N_ss estimated from the I–V measurements decreased with increasing the temperature from 8.29 × 10¹³ to 2.20 × 10¹³ eV⁻¹ cm⁻².     

An organic electroluminescent device made from a gadolinium complex

A gadolinium ternary complex, tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) (phenanthroline) gadolinium [Gd(PMIP)₃(Phen)] was synthesized and used as a light emitting material in the organic electroluminescent (EL) devices. The triple layer device with a structure of indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (20 nm)/Gd(PMIP)₃(Phen) (80 nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (bathocuproine or BCP) (20 nm)/Mg: Ag(200 nm)/Ag(100 nm) exhibited green emission peaking at 535 nm. A maximum luminance of 230 cd/m² at 17 V and a peak power efficiency of 0.02 lm/w at 9 V were obtained.     

The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 °C to 100 °C. The current-dependent electroluminescence (EL) spectra, current–voltage (I–V) curves and luminescence intensity–current (L–I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of ?3.7 mV/K and the emission peak wavelength increased with a slope of +0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 °C and from 22.4 to 35.6 °C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 °C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of ?0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs.     

Effects of trench oxide and field plates on the breakdown voltage of SOI LDMOSFET

To improve the breakdown voltage, we propose a SOI-based LDMOSFET with a trench structure in the drift region. Due to the trench oxide and underneath boron implanted layer, the surface electric field in the drift region effectively reduced. These effects resulted in the increment of breakdown voltage for the trenched LDMOS more than 100 V compared with the conventional device. However, the specific on-resistance, which has a trade-off relationship, is slightly increased. In addition to the trench oxide on the device performance, we also investigated the influence of n− drift to n+ drain junction spacing on the off-state breakdown voltage. The measured breakdown voltages were varied more than 50 V with different n− to n+ design spaces and achieved a maximum value at L_DA = 2.0 μm. Moreover, the influence of field plate on the breakdown voltage of trench LDMOSFET was investigated. It is found that the optimum drain field plate over the field oxide is 8 μm.     

Memory characteristics and tunneling mechanism of Pt nano-crystals embedded in HfAlOx films for nonvolatile flash memory devices

A non-volatile flash memory device based on metal oxide semiconductor (MOS) capacitor structure has been fabricated using platinum nano-crystals(Pt–NCs) as storage units embedded in HfAlO_x high-k tunneling layers. Its memory characteristics and tunneling mechanism are characterized by capacitance–voltage(C–V) and flat-band voltage-time(ΔV_FB-T) measurements. A 6.5 V flat-band voltage (memory window) corresponding to the stored charge density of 2.29 × 10¹³ cm⁻² and about 88% stored electron reserved after apply ±8 V program or erase voltage for 10⁵ s at high frequency of 1 MHz was demonstrated. Investigation of leakage current–voltage(J–V) indicated that defects-enhanced Pool-Frenkel tunneling plays an important role in the tunneling mechanism for the storage charges. Hence, the Pt–NCs and HfAlO_x based MOS structure has a promising application in non-volatile flash memory devices.     

Near-white emitting QD-LED based on hydrophilic CdS nanocrystals

In this work we report fabrication of a nanocrystal (NC)-based hybrid organic–inorganic LED with structure of ITO/PEDOT:PSS/PVK/CdS-NCs/(Al or Mg:Ag). The hydrophilic CdS NCs were synthesized using a novel aqueous thermochemical method at 80 °C and sizes (around 2 nm) were controlled by thioglycolic acid (TGA) as the capping agent. The favorite feature of these NCs is their relatively high emission intensity and broad, near-white emission. The hydrophilic CdS NCs were successfully spin coated using Triton X-100 as the wetting agent. The fabricated LEDs demonstrated a turn on voltage about 7 V for Al metallic contact. The electroluminescence was a broad spectrum at 540 and 170 nm width, which was about 50 nm red shifted compared to photoluminescence spectra. The CIE color coordinates of the LED at (0.33, 0.43) demonstrated a near white light LED with an emission on green–yellow boundary of white. Annealing of the device up to 190 °C had a positive effect on the performance, possibly due to better contacts between layers. Replacing Al contacts with Mg:Ag reduced the turn-on voltage to 6 V and changed CIE color coordinate to (0.32, 0.41). The EL peak was also shifted to 525 nm, with a brightness of 15 Cd/m² at working voltage of 15 V. The current efficiency and external quantum efficiency of device were 0.08 Cd/A and 0.03% at current densities higher than 10 mA/cm².     

Photovoltaic properties of black silicon microstructured by femtosecond pulsed laser

Prototype devices based on black silicon have been fabricated by microstructuring 250 μm thick multicrystalline n doped silicon wafers using femtosecond pulsed laser in ambient gas of SF₆ to measure its photovoltaic properties. The enhanced optical absorption of black silicon extends across the visible region and all the black silicons prepared in this work exhibit enhanced optical absorption close to 90% from 300 nm to 800 nm. The highest open-circuit voltage (V_oc) and short-circuit current (I_sc) under the illumination of He–Ne continuous laser at 632.8 nm were measured to be 53.3 mV and 0.11 mA, respectively at a maximum power conversion efficiency of 1.44%. Upon excitation with He–Ne continuous laser at 632.8 nm, external quantum efficiency (EQE) of black silicon as high as 112.9% has also been observed. Development of black silicon for photovoltaic purposes could open up a new perspective in achieving high efficient silicon-based solar cell by means of the enhanced optical absorption in the visible region. The current–voltage characteristic and photo responsivity of these prototype devices fabricated with microstructured silicon were also investigated.     

Fabrication and characteristics of photodiode based on ZnO nanowire arrays and spray-coated regioregular P3HT layers

A photodiode based on well-aligned ZnO nanowire arrays (ZNAs) and spray-coated regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) layers hybrid hetero junctions was fabricated, and its electrical characteristics in dark and under illumination with a solar simulator were investigated. Current–voltage (I–V) data of ITO/ZNAs/P3HT/Ag device in dark and under illumination showed typical diode characteristics. A rectification ratio (RR) of 22.7 at 1.7 V and a low turn-on voltage of 0.4 V in dark were obtained. Also, the photodiode with high photo-response in the order of 0.31 A/W at -2 V using 80 mW/cm² illumination power was observed. Upon increasing illumination power from 40 to 100 mW/cm², the RR value for the photodiode continuously was improved with a highest value of 12.5.     

Ferroelectric polarization and resistive switching characteristics of ion beam assisted sputter deposited BaTiO3 thin films

In this work, 150 nm thick polycrystalline BaTiO₃ (BTO) films were deposited on Pt/TiO₂/SiO₂/Si substrate by ion beam assisted sputter deposition technique. The bias voltage dependent resistive switching (RS) and ferroelectric polarization characteristics of Au/BTO/Pt devices are investigated. The devices display the stable bipolar RS characteristics without an initial electroforming process. Fittings to current–voltage (I–V) curves suggest that low and high resistance states are governed, respectively, by filamentary model and trap controlled space charge limited conduction mechanism, where the oxygen vacancies act as traps. Presence of oxygen vacancies is evidenced from the photoluminescence spectrum. The devices also display P–V loops with remnant polarization (P_r) of 5.7 μC/cm² and a coercive electric field (E_c) of 173.0 kV/cm. The coupling between the ferroelectric polarization and RS effect in BTO films is demonstrated.     

I–V characteristics and magnetic field profile studies in high Tc BSCCO based Helmholtz coil

A double pancake wound high T_c Helmholtz coil has been fabricated using commercial grade BSCCO tape. The current voltage (I–V) characteristics of this HTS coil has been carried out using standard four probe technique. From the I–V characteristics, the critical current based on self field dependent of HTS coil and quality index “n” value have been calculated. Magnetic field profiles have been studied along the axis of this coil for various amplitudes of applied current ranging from 10 A to 50 A at 77 K. The measured field profiles have a very good agreement with that of theoretical values predicted in the literature.     

Electric field induced metal–insulator transition in VO2 thin film based on FTO/VO2/FTO structure

A VO₂ thin film has been prepared using a DC magnetron sputtering method and annealing on an F-doped SnO₂ (FTO) conductive glass substrate. The FTO/VO₂/FTO structure was fabricated using photolithography and a chemical etching process. The temperature dependence of the I–V hysteresis loop for the FTO/VO₂/FTO structure has been analyzed. The threshold voltage decreases with increasing temperature, with a value of 9.2 V at 20 °C. The maximum transmission modulation value of the FTO/VO₂/FTO structure is 31.4% under various temperatures and voltages. Optical modulation can be realized in the structure by applying an electric field.     

Structural,optical and electrical properties of europium picrate tetraethylene glycol complex as emissive material for OLED

A new europium complex [Eu(Pic)₂(H₂O)(EO4)](Pic)·0.75H₂O was synthesized and used as the emission material for the single layer device structure of ITO/EO4–Eu–Pic/Al, using a spin-coating technique. Study on the optical properties of the [Eu(Pic)₂(H₂O)(EO4)](Pic)·0.75H₂O complex where EO4=tetraethylene glycol and Pic=picrate anion, had to be undertaken before being applicable to the study of an organic light emitting diode (OLED). The electrical property of an OLED using current–voltage (I–V) measurement was also studied. In complex, the Eu(III) ion was coordinated with the EO4 ligand as a pentadentate mode, one water molecule, and with two Pic anions as bidentate and monodentate modes, forming a nine-coordination number. The photoluminescence (PL) spectra of the crystalline complex in the solid state and its thin film showed a hypersensitive peak at 613.5–614.9 nm that assigned to the ⁵D₀→⁷F₂ transition. A narrow band emission from the thin film EO4–Eu–Pic was obtained. The typical semiconductor I–V curve of device ITO/EO4–Eu–Pic/Al showed the threshold and turn on voltages at 1.08 and 4.6 V, respectively. The energy transfer process from the ligand to the Eu(III) ion was discussed by investigating the excitation and PL characteristics. Effect of the picrate anion on the device performance was also studied.     

Improved electrical parameters of vacuum annealed Ni/4H-SiC (0 0 0 1) Schottky barrier diode

The reported work has been focused on the improvement of electrical parameters of Schottky diode using vacuum annealing at mild temperature in Ar gas ambient. Nickel Schottky barrier diodes were fabricated on 50 μm epitaxial layer of n-type 4H-SiC (0 0 0 1) substrate. The values of leakage current, Schottky barrier height (?_B), ideality factor (η) and density of interface states (N_SS) were obtained from experimentally measured current–voltage (I–V) and capacitance–voltage (C–V) characteristics before and after vacuum annealing treatment. The data revealed that ?_B, η and reverse leakage current for the as-processed diodes are 1.25 eV, 1.6 and 1.2 nA (at ?100 V), respectively, while for vacuum annealed diodes these parameters are 1.36 eV, 1.3 and 900 pA (at same reverse voltage). Improved characteristics have been resulted under the influence of vacuum annealing because of lesser number of minority carrier generation due to incessant reduction of number of available discrete energy levels in the bandgap of 4H-SiC substrate and lesser number of interface states density at Ni/4H-SiC (0 0 0 1) interface.     

Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres

Fe-doped ZnO porous microspheres composed of nanosheets were prepared by a simple hydrothermal method combined with post-annealing, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller N₂ adsorption–desorption measurements and photoluminescence (PL) spectra. In this paper we report Fe doping induced modifications in the structural, photoluminescence and gas sensing behavior of ZnO porous microspheres. Our results show that the crystallite size decreases and specific surface area increases with the increase of Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (V_O and Zn_i) to acceptor (V_Zn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppb-level n-butanol at 300 °C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area.     

Fabrication and electrical characterization of Schottky diode based on 2-amino-4, 5-imidazoledicarbonitrile (AIDCN)

The junction characteristics of the organic compound 2-amino-4, 5-imidazoledicarbonitrile (AIDCN) on p-type silicon substrate are studied in detail. AIDCN is deposited on silicon substrate using thermal evaporator. Current–voltage (I–V) characteristic of the device is measured at room temperature. The Au/AIDCN/p-Si device shows non-linear I–V characteristic with rectification ratio of 7.2×10³ at 5 V. The electronic device parameters such as barrier height, ideality factor, and series resistance are calculated using I–V data and observed to be 0.74 eV, 3.00, and 3.73×10⁴ Ω respectively.