Electrical transport characteristics of Au/n-GaN Schottky diodes

The current–voltage measurements were performed in the temperature range (80–300 K) on Au/ n-GaN Schottky barrier type diodes. The Schottky diode shows non-ideal I(V_G) behaviour with ideality factors n equals to 1.18 and 1.81 at 300 K and 80 K, respectively, and are thought to have a metal-interface layer-semiconductor configuration. Under forward bias and for T ≥ 200 K, the electrical current transport was controlled by the thermionic emission (TE) process. However, for T ≤ 200 K, The current was controlled by the thermionic field emission (TFE). The characteristic energy E₀₀ = 3.48 meV was obtained from the I(V_G, T) measurements and agreed very well with the value of E₀₀ = 3.62 meV calculated theoretically. The zero-bias barrier height ϕ_B0 determined from the I(V_G) measurements was 0.84 eV at 300 K and decreases to 0.49 eV at 80 K.     

The electromagnetic properties of the Mn doped Ge/Si quantum dots diodes

We present the electromagnetic properties of Mn doped Ge quantum dots (QDs)/Si electromagnetic diode. The Ge:Mn QDs were grown with a GeH₄/Ar mixed gas under a constant flow at 500 °C by means of a plasma enhanced chemical vapor deposition (PECVD) process. They were then doped with different concentrations of Mn using a magnetron sputtering technique and annealed at 600 °C. The Ge:Mn QD samples show wildly open smooth hysteresis loops. The remnant magnetization M_r and saturation magnetic intensity M_s are functions of the doping concentration of Mn. The electromagnetic diodes fabricated in this way exhibit perfect electromagnetic, current–voltage (I–V) and capacitance–voltage (C–V) properties. The largest voltage and magnetic resistance differences with and without magnetic field are up to 4 V and 169 kΩ, respectively. These electromagnetic properties of the Ge_1?xMn_x QDs/Si diodes can be used to make various electromagnetic devices, including switches and storages devices.     

Preparation and thermomechanical properties of Ag-PVA nanocomposite films

Metal–polymer hybrid nanocomposites have been prepared from an aqueous solution of polyvinyl alcohol (PVA) and silver nitrate (AgNO₃). The silver nanoparticles were generated in PVA matrix by the reduction of silver ions with PVA molecule at 60–70 °C over magnetic stirrer. UV–vis analysis, X-ray diffraction studies, transmission electron microscopy, scanning electron microscopy and current–voltage analysis were used to characterize the nanocomposite films prepared. The X-ray diffraction analysis reveals that silver metal is present in face centered cubic (fcc) crystal structure. Average crystallite size of silver nanocrystal is 19 nm, which increases to 22 nm on annealing the film at 150 °C in air. This result is in good agreement with the result obtained from TEM. The UV–vis spectrum shows a single peak at 433 nm, arising from the surface plasmon absorption of silver nanocolloids. This result clearly indicates that silver nanoparticles are embedded in PVA. An improvement of mechanical properties (storage modulus) was also noticed due to a modification of PVA up to 0.5 wt% of silver content. The current–voltage (I–V) characteristic of nanocomposite films shows increase in current drawn with increasing Ag-content in the films.     

Scanning probe microscopy of single Au ion implants in Si

We have studied 5 MeV Au²⁺ ion implantation with fluences between 7 × 10⁷ and 2 × 10⁸ cm^− 2 in Si by deep level transient spectroscopy (DLTS) and scanning capacitance microscopy (SCM). The DLTS measurements show formation of electrically active defects such as the two negative charge states of the divacancy (V₂(/–) and V₂(–/0)) and the vacancy–oxygen (VO) center. It is observed that the intensity of the V₂(/–) peak is lower compared to that of V₂(–/0) by a factor of 5. This has been attributed to a highly localized distribution of the defects along the ion tracks, which results in trapping of the carriers at V₂(–/0) and incomplete occupancy of V₂(/–). The SCM measurements obtained in a plan view show a random pattern of regions with a reduced SCM signal for the samples implanted with fluence above 2 × 10⁸ cm^− 2. The reduced SCM signal is attributed to extra charges associated with acceptor states, such as V₂(–/0), formed along the ion tracks in the bulk Si. Indeed, the electron emission rate from the V₂(–/0) state is in the range of 10 kHz at room temperature, which is well below the probing frequency of the SCM measurements, resulting in “freezing” of electrons at V₂(–/0).     

Temperature dependent transport properties in molybdenum oxide doped α-NPD

The temperature dependent transport properties of molybdenum oxide (MoO₃) doped N,N′-di(1-naphthyl)-N,N′-diphenylbenzidin (α-NPD) were studied over a frequency range of 100 Hz to 1 MHz. The value of trap density and mobility calculated by detailed analysis of current–voltage (I–V) characteristics are 9.43 × 10²⁶ m^?3 and 1.23 × 10^?6 cm² V^?1 s^?1, respectively. The relaxation time for the carriers in the bulk and in the interface region decreases with temperature. The Cole–Cole plot indicates the device can be modeled as the combination of two parallel resistor–capacitor (R–C) circuits with a series resistance of around 70 Ω. The dc conductivity shows two different regions in the studied temperature range with activation energy of E_a ～ 0.107 eV (region I) and E_a ～ 52 meV (region II), respectively. The ac conductivity follows the universal power law and the onset frequency increases with increase of temperature. The temperature dependent conduction mechanism can be explained by correlated hopping barrier (CBH) model.     

Hybrid tandem photovoltaic devices with a transparent conductive interconnecting recombination layer

We demonstrate hybrid tandem photovoltaic devices with a transparent conductive interconnecting recombination layer. The series-connected hybrid tandem photovoltaic devices were developed by combining hydrogenated amorphous silicon (a-Si:H) and polymer-based organic photovoltaics (OPVs). In order to enhance the interfacial connection between the subcells, we employed highly transparent and conductive indium tin oxide (ITO) thin layer. By using the ITO interconnecting layer, the power conversion efficiency of the hybrid tandem solar cell was enhanced from 1.0% (V_OC = 1.041 V, J_SC = 2.97 mA/cm², FF = 32.3%) to 2.6% (V_OC = 1.336 V, J_SC = 4.65 mA/cm², FF = 41.98%) due to the eliminated interfacial series resistance.     

Effect of electron irradiation on characteristics of Mo Schottky barriers on epitaxial silicon

The results of studies on influence of 6 MeV electron irradiation on avalanche breakdown voltage (U_b) and on forward voltage (U_F) at different values of direct current (I_F) for the Mo Schottky diodes on epitaxial silicon of n-type conductivity are presented. It was found out that the avalanche breakdown voltage of the diodes is very sensitive to electron irradiation. A decrease in U_b was observed after electron irradiation with a fluence as low as 1 × 10¹¹ cm^?2. An increase in electron irradiation fluence from 1 × 10¹¹ cm^?2 to 5 × 10¹⁴ cm^?2 resulted in 30% decrease in U_b, however, further increase in electron irradiation fluence from 5 × 10¹⁴ cm^?2 to 3 × 10¹⁶ cm^?2 led to some increase in the avalanche breakdown voltage. Monotonic increases in U_F values at different I_F with the increase in electron irradiation fluence were observed starting from a fluence of 5 × 10¹⁴ cm^?2. Radiation-induced changes in U_b were unstable at room temperature and a significant recovery of U_b occurred after maintaining the irradiated diodes at room temperature for 30 days. Annealing at 120 °C for 20 min resulted in the almost complete recovery of U_b. Radiation-induced changes in U_F values were stable up to 300 °C. Mechanisms of the observed radiation-induced changes in the U_b and U_F values and defects responsible for the changes are discussed.     

Spectroscopic analysis of a novel Nd3+-activated barium borate glass for broadband laser amplification

Spectroscopic parameters of a novel Nd³⁺-activated barium borate (BBONd) glass have been analyzed for broadband laser amplification. The Judd–Ofelt (JO) intensity parameters were determined through a systematic analysis of the absorption spectrum of Nd³⁺ ions in the BBONd glass. High values of the JO intensity parameters reveal a great centro-symmetrical loss of the Nd³⁺ sites and high covalency degree of the ligand field. The very high Ω₆ intensity parameter value makes evident both a great structural distortion of the Nd³⁺ sites and a strong electron–phonon coupling between Nd³⁺ and free OH⁻ ions, which is consistent with the phonon energy maximum (3442.1 cm⁻¹) recorded by Raman spectroscopy. This strong electron–phonon coupling favors high effective bandwidth and gain bandwidth values of the laser emission (⁴F_3/2 → ⁴I_11/2) of Nd³⁺ ions. The electric-dipole oscillator strengths of all the Nd³⁺ absorption transitions, and in particular that of the hypersensitive transition (⁴I_9/2 → ⁴G_5/2), are enhanced by this great structural distortion of the host. Broadband laser amplification of the ⁴F_3/2 → ⁴I_11/2 emission (1062 nm) of Nd³⁺ ions in the BBONd glass pumped at 805 nm (⁴I_9/2 → ⁴F_5/2 + ²H_9/2) is evaluated through the main fluorescent parameters in competition with non-radiative processes. In general, the BBONd glass exhibits spectroscopic parameters comparable with those reported in the literature for broadband laser amplification into the IR region.     

Effects of illumination on electrical parameters of Ag/n-CdO/p-Si diode

The current–voltage (I–V) characteristics of the Ag/n-CdO/p-Si diode were investigated under various white light (visible light) illuminations. The electrical parameters such as ideality factor (n), zero-bias-barrier height (Φ_b) and series resistance (R_S) of Ag/n-CdO/p-Si MIS diodes were determined by using the forward bias current–voltage measurements. The Ag/n-CdO/p-Si diode exhibits a non-ideal behavior due to the interfacial layer, the interface states and the series resistance. The ideality factor is increased, while the barrier height is decreased with decreasing illumination intensities. The values of R_S obtained from Cheung and Norde methods are decreased with increasing illumination intensity. The distribution profile of the interface states (N_SS) as a function of energy distribution (E_SS − E_V) was extracted from the forward I–V measurements under various illumination intensities. The interface state densities were observed to be strongly illumination dependent and are decreased with increasing illumination intensities.     

Space charge limited conduction in bulk Se100 − xSbx chalcogenide glasses

The present paper reports the measurements on space charge limited conduction in bulk glassy Se_100 − xSb_x (x = 2, 4, 6, 8). I–V characteristics have been measured at various fixed temperatures. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields (E ∼10⁴ V/cm), the current becomes superohmic.At high fields (10⁴ V/cm), current could be fitted to the theory of space charge limited conduction in case of uniform distribution of localized states in the mobility gap of these materials. Using the theory of SCLC for the uniform distribution of the traps, the density of localized states near Fermi level is also calculated. Density of localized states initially increases with Sb concentration till 6 at.% of Sb and then decreases with further increase of Sb. This decrease is explained in terms of average coordination number.     

Unidirectional solidification of a Zn-rich Zn–2.17 wt%Cu hypo-peritectic alloy

Unidirectional solidification of a Zn-rich Zn–2.17 wt%Cu hypo-peritectic alloy has been carried out to investigate the microstructure evolution over the growth velocity range 0.02–4.82 mm/s at a temperature gradient of 15 K/mm by means of the Bridgman technique. Regular and plate-like two-phase cellular structures were observed in samples grown at growth velocities V above 0.48 and 2.64 mm/s, respectively. The dominant microstructure in samples grown below 0.22 mm/s was dendrites of primary ε in a matrix of secondary η. Intercellular spacing Λ decreased with increasing growth velocity V such that ΛV^1/2 is a constant of 316±55 μm^3/2/s^1/2. Secondary dendrite arm spacing λ₂ of primary ε decreased with increasing V such that λ₂V^1/3 is a constant of 14.9±0.9 μm^4/3/s^1/3. The observed transition from regular cells to plate-like cells of η is discussed on the basis of competitive growth and crystallographic effect.     

Analysis of the device characteristics of AlGaN/GaN HEMTs over a wide temperature range

In this study, we investigate the behavior of the current–voltage (I–V) characteristics of AlGaN/GaN HEMT in the temperature range of 223–398 K. Temperature dependent device characteristics and the current transport mechanism are reported. It is observed that the Schottky barrier height Φ increases and the ideality factor n decreases with temperature. There is a linear relationship between the barrier height and the ideality factor, which is attributed to barrier height inhomogeneities of AlGaN/GaN HEMT. The estimated values of the series resistances (R_s) are in the range of 144.2 Ω at 223 K to 74.3 Ω at 398 K. The Φ, n, R_s, G_m and Schottky leakage current values are seen to be strongly temperature dependent.     

Effect of solidification parameters on the microstructure of Sn-3.7Ag-0.9Zn solder

In this work, Sn–Ag–Zn alloy of eutectic composition (Sn-3.7wt.%Ag-0.9wt.%Zn) was directionally solidified upward at a constant temperature gradient (G = 4.33 K/mm) in a wide range of growth rates (V = 3.38–220.12 μm/s) and a constant growth rate (V = 11.52 μm/s) with different temperature gradients (G = 4.33–12.41 K/mm) using a Bridgman type directional solidification furnace. The microstructure was observed to be a rod Ag₃Sn structure in the matrix of β–Sn from the directionally solidified Sn-3.7wt.%Ag-0.9wt.%Zn samples. The values of eutectic spacing (λ) were measured from transverse section of samples. The dependency of eutectic spacing on the growth rate (V) and temperature gradient (G) were determined with linear regression analysis. The dependency of λ on the values of V and G were found to be λ = 10.42V ^? 0.53 and λ = 0.27G ^? 0.48, respectively. The values of bulk growth were also determined to be λ²V = 86.39 μm³/s by using the measured values of λ and V. The results obtained in present work were compared with the previous similar experimental results obtained for binary and ternary alloys.     

Immobilization of cholesterol esterase in mesoporous silica materials and its hydrolytic activity toward diethyl phthalate

Cholesterol esterase (CE, cholesteryl ester hydrolase, EC 3.1.1.13) from porcine pancreas (molecular weight 400–500 kDa) exhibits hydrolytic activity toward various toxic organic phthalate esters. CE was confined in the nanospace (diameter 3–30 nm) of five types of mesoporous silica (MPS) that differ in structural properties such as pore diameter, pore volume, and particle morphology. These structural properties were characterized by transmission electron microscopy, small-angle X-ray diffraction, N₂ adsorption–desorption experiments, solid-state ¹³C nuclear magnetic resonance (NMR), and solid-state ²⁹Si NMR. Catalytic activities of immobilized and free CE were evaluated by the hydrolysis of diethyl phthalate in phosphate buffer solutions containing an organic cosolvent. Optimal activity recovery was achieved when CE was immobilized in n-decane-functionalized MPS, which had a large pore size (22.5 nm). The immobilization also protected against effects of temperature within the range 30 °C–60 °C; CE immobilized in n-decyl-functionalized MPS exhibited better thermal stability than in non-functionalized MPS or free CE. Moreover, it retained approximately 60% of its catalytic activity even after six catalytic cycles.     

Enhanced emission of 2.7 μm from Er3+/Nd3+-codoped LiYF4 single crystals

An Er³⁺/Nd³⁺ co-doped LiYF₄ single crystal of ～Φ 12 mm × 95 mm size with high quality was grown by a Bridgman method. The luminescent properties of the crystals with different Er³⁺ and Nd³⁺ concentrations were studied. Compared with the Er³⁺ single-doped LiYF₄ crystal extremely enhanced emission at 2.7 μm from the Er³⁺/Nd³⁺ co-doped LiYF₄ was observed upon excitation of an 800 nm laser diode. Meanwhile, the green up-conversion emission and near infrared emission at 1.5 μm from Er³⁺ in the co-doped crystals were effectively restricted. The luminescent mechanisms for the Er³⁺/Nd³⁺ co-doped crystals were analyzed and the possible energy transfer processes were proposed. The energy transfer efficiencies for (Er³⁺:⁴I_13/2, Nd³⁺:⁴I_9/2) → (Er³⁺:⁴I_15/2, Nd³⁺:⁴I_15/2) and (Nd³⁺:⁴F_3/2, Er³⁺:⁴I_15/2) → (Nd³⁺:⁴I_9/2, Er³⁺:⁴I_9/2) were calculated. It was found that Er³⁺/Nd³⁺ co-doped single crystal may be a potential host for 2.7 μm lasers.     

Energy transfer and energy level decay processes of Er3+ in water-free tellurite glass

This report details the fundamental spectroscopic properties of a new class of water-free tellurite glasses studied for future applications in mid-infrared light generation. The fundamental excited state decay processes relating to the ⁴I_11/2 → ⁴I_13/2 transition in singly Er³⁺-doped Tellurium Zinc Lanthanum glass have been investigated using time-resolved fluorescence spectroscopy. The excited state dynamics was analyzed for Er₂O₃ concentrations between 0.5 mol% and 4 mol%. Selective laser excitation of the ⁴I_11/2 energy level at 972 nm and selective laser excitation of the ⁴I_13/2 energy level at 1485 nm has established that in a similar way to other Er³⁺-doped glasses, a strong energy-transfer upconversion by way of a dipole–dipole interaction between two excited erbium ions in the ⁴I_13/2 level populates the ⁴I_11/2 upper laser level of the 3 μm transition. The ⁴I_13/2 and ⁴I_11/2 energy levels emitted luminescence with peaks located at 1532 nm and 2734 nm respectively with luminescence efficiencies of 100% and 8% for the higher (4 mol.%) concentration sample. Results from numerical simulations showed that a population inversion is reached at a threshold pumping intensity of ∼57 kW cm⁻² for a CW laser pump at 976 nm for [Er₂O₃] = 2 mol.%.     

2–3 μm emission and fluorescent decaying behavior in Ho3+-doped tellurium germanate glass

In this work, we report the 2.05 μm emission and ∼3 μm broadband spectra of Ho₂O₃-doped 33GeO₂–30TeO₂–27PbO–10CaO (in mol%) glass under 640 nm laser excitation. Clear emission spectra due to the ⁵I₇–⁵I₈ transition and the ⁵I₆–⁵I₇ transition in Ho³⁺ are observed. The 2.05 μm emission intensity and the full width at half maximum (FWHM) of the ∼3 μm broadband depend on the Ho concentration. The peak stimulated emission cross-section of Ho³⁺ is 6.57 × 10⁻²¹ cm² at 2.05 μm, as calculated by the McCumber theory. The emission spectra are recorded and the maximum emission intensity at 2.05 μm is obtained at a doping level of 0.5 mol% Ho₂O₃ in the glass. A broad and flat emission band from 2700 nm to 3050 nm is observed in 2 mol% Ho₂O₃-doped tellurium germanate glass. The lifetime of the ⁵I₇ state decreases with the increase in Ho³⁺ concentration due to non-radiative relaxation processes. An energy transfer coefficient of 271.88 mol⁻¹ s⁻¹ is obtained.     

Effects of LiF/Al back electrode on the amorphous/crystalline silicon heterojunction solar cells

To improve the quantum efficiency (QE) and hence the efficiency of the amorphous/crystalline silicon heterojunction solar cell, we have employed a LiF dielectric layer on the rear side. The high dipole moment of the LiF reduces the aluminum electrode's work–function and then lowers the energy barrier at back contact. This lower energy barrier height helps to enhance both the operating voltage and the QE at longer wavelength region, in turn improves the open-circuit voltage (V_oc), short-circuit current density (J_sc), and then overall cell efficiency. With optimized LiF layer thickness of 20 nm, 1 cm² heterojunction with intrinsic thin layer (HIT) solar cells were produced with industry-compatible process, yielding V_oc of 690 mV, J_sc of 33.62 mA/cm², and cell efficiencies of 17.13%. Therefore LiF/Al electrode on rear side is proposed as an alternate back electrode for high efficiency HIT solar cells.     

Stoichiometric and non-stoichiometric films in the Si–O–N system: mechanical,electrical, and dielectric properties

A novel low-temperature (600–850 °C), chemical vapor deposition method, involving a simple reaction between disiloxane (H₃Si–O–SiH₃) and ammonia (NH₃), is described to deposit stoichiometric, Si₂N₂O, and non-stoichiometric, SiO_xN_y, silicon oxynitride films (5–500 nm) on Si substrates. Note, the gaseous reactants are free from carbon and other undesirable contaminants. The deposition of Si₂N₂O on Si (with (1 0 0) orientation and a native oxide layer of 1 nm) was conducted at a pressure of 2 Torr and at extremely high rates of 20–30 nm min⁻¹ with complete hydrogen elimination. The deposition rate of SiO_xN_y on highly-doped Si (with (1 1 1) orientation but without native oxide) at 10⁻⁶ Torr was ∼1.5 nm min⁻¹, and achieved via the reaction of disiloxane with N atoms, generated by an RF source in an MBE chamber. The phase, composition and structure of the oxynitride films were characterized by a variety of analytical techniques. The hardness of Si₂N₂O, and the capacitance–voltage (C–V) as a function of frequency and leakage current density–voltage (J_L–V) characteristics were determined on MOS (Al/Si₂N₂O/SiO/p-Si) structures. The hardness, frequency-dispersionless dielectric permittivity (K), and J_L at 6 V for a 20 nm Si₂N₂O film were determined to be 18 GPa, 6 and 0.05–0.1 nA cm⁻², respectively.     

Synthesis and crystal structures of a new (2,3-(CH3)2C6H3NH3)H2XO4 (X=P,As)

The aim of encapsulation of 2,3-dimethylanilinium cation in (H₂XO₄)_n polymeric anion chains is to build acentric frameworks that are efficient for non-linear optical (NLO) applications. The synthesis and structures of two new inorganic–organic NLO crystals with general formula (2,3-(CH₃)₂C₆H₃NH₃)H₂XO₄ (X=P, As) are reported. The magnitude of their second harmonic generations (SHG) responses was found to be between the KDP and urea. They crystallize with monoclinic unit-cells and are isotopic. We have determined the structure of phosphoric salt. The following unit-cell parameters were found: a=8.866(3) Å, b=5.909(6) Å, c=10.644(5) Å, β=112.44(1)°, V=515.5(5) Å³ and D_X=1.412 g cm⁻³. The space group is P2₁ with Z=2. The structure was refined with R=0.041 (R_w=0.057) for 1652 reflections with I≥3σ(I). It exhibits infinite (H₂PO₄)_nⁿ⁻ chains. The organic groups (2,3-(CH₃)₂C₆H₃NH₃)⁺ are anchored between adjacent polyanions through multiple hydrogen bonds. Chemical preparation, crystal structure, calorimetric and spectroscopic investigation are described.