Photon-assisted capacitance–voltage study of organic metal–insulator–semiconductor capacitors

The results are reported of a detailed investigation into the photoinduced changes that occur in the capacitance–voltage (C–V) response of an organic metal–insulator–semiconductor (MIS) capacitor based on the organic semiconductor poly(3-hexylthiophene), P3HT. During the forward voltage sweep, the device is driven into deep depletion but stabilizes at a voltage-independent minimum capacitance, C_min, whose value depends on photon energy, light intensity and voltage ramp rate. On reversing the voltage sweep, strong hysteresis is observed owing to a positive shift in the flatband voltage, V_FB, of the device. A theoretical quasi-static model is developed in which it is assumed that electrons photogenerated in the semiconductor depletion region escape geminate recombination following the Onsager model. These electrons then drift to the P3HT/insulator interface where they become deeply trapped thus effecting a positive shift in V_FB. By choosing appropriate values for the only disposable parameter in the model, an excellent fit is obtained to the experimental C_min, from which we extract values for the zero-field quantum yield of photoelectrons in P3HT that are of similar magnitude, 10^?5 to 10^?3, to those previously deduced for π-conjugated polymers from photoconduction measurements. From the observed hysteresis we deduce that the interfacial electron trap density probably exceeds 10¹⁶ m^?2. Evidence is presented suggesting that the ratio of free to trapped electrons at the interface depends on the insulator used for fabricating the device.     

Current–voltage and capacitance–voltage characteristics of Al Schottky contacts to strained Si-on-insulator in the wide temperature range

The electrical characteristics of Al/strained Si-on-insulator (sSOI) Schottky diode have been investigated using current–voltage (I–V) and capacitance–voltage (C–V) measurements in the wide temperature range of 200–400 K in steps of 25 K. It was found that the barrier height (0.57–0.80 eV) calculated from the I–V characteristics increased and the ideality factor (1.97–1.28) decreased with increasing temperature. The barrier heights determined from the C–V measurements were higher than those extracted from the I–V measurements, associated with the formation of an inhomogeneous Schottky barrier at the interface. The series resistance estimated from the forward I–V characteristics using Cheung and Norde methods decreased with increasing temperature, implying its strong temperature dependence. The observed variation in barrier height and ideality factor could be attributed to the inhomogeneities in Schottky barrier, explained by assuming Gaussian distribution of barrier heights. The temperature-dependent I–V characteristics showed a double Gaussian distribution with mean barrier heights of 0.83 and 1.19 eV and standard deviations of 0.10 and 0.16 eV at 200–275 and 300–400 K, respectively. From the modified Richardson plot, the modified Richardson constant were calculated to be 21.8 and 29.4 A cm⁻² K⁻² at 200–275 and 300–400 K, respectively, which were comparable to the theoretical value for p-type sSOI (31.6 A cm⁻² K⁻²).     

Modeling the effect of deep traps on the capacitance–voltage characteristics of p-type Si-doped GaAs Schottky diodes grown on high index GaAs substrates

Numerical simulation, using SILVACO-TCAD, is carried out to explain experimentally observed effects of different types of deep levels on the capacitance–voltage characteristics of p-type Si-doped GaAs Schottky diodes grown on high index GaAs substrates. Two diodes were grown on (311)A and (211)A oriented GaAs substrates using Molecular Beam Epitaxy (MBE). Although, deep levels were observed in both structures, the measured capacitance–voltage characteristics show a negative differential capacitance (NDC) for the (311)A diodes, while the (211)A devices display a usual behaviour. The NDC is related to the nature and spatial distribution of the deep levels, which are characterized by the Deep Level Transient Spectroscopy (DLTS) technique. In the (311)A structure only majority deep levels (hole traps) were observed while both majority and minority deep levels were present in the (211)A diodes. The simulation, which calculates the capacitance–voltage characteristics in the absence and presence of different types of deep levels, agrees well with the experimentally observed behaviour.     

Determination of the interface energy level alignment of a doped organic hetero-junction using capacitance–voltage measurements

A simple method based on capacitance–voltage (C–V) measurements is reported to determine the interface energy level alignment at the junction of 15 mol% Cs₂CO₃ doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) fabricated under high vacuum. The junction properties, such as the depletion layer thickness, built-in potentials and vacuum level shift were calculated with simple Mott–Schottky and Poisson’s equations with the boundary condition of a continuous electric flux density using the information from the C–V data. The interface energy level alignment determined by this method is well matched with the one determined using the in situ ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) experiments performed under ultra-high vacuum. This method can be applied to other semiconductor junctions such as the organic p–n homojunctions and heterojunctions with known energy levels, as long as the metal/semiconductor contact is Ohmic without referring to the photoemission spectroscopies. Moreover, the energy level alignment determined by the C–V measurement gives a more realistic result since the films for the measurements are formed under high vacuum which is a normal device fabrication environment rather than under ultra high vacuum.     

Current–voltage characteristics of organic heterostructure devices with insulating spacer layers

The dark current density in donor/acceptor organic planar heterostructure devices at a given forward voltage bias can either increase or decrease when an insulating spacer layer is added between the donor and acceptor layers. The dominant current flow process in these systems involves the formation and subsequent recombination of interfacial exciplex states. If the exciplex recombination rate limits current flow, an insulating interface layer decreases the dark current. However, if the exciplex formation rate limits the current, an insulating interface layer may increase the dark current. We present a device model to describe this behavior, and we discuss relevant experimental data.     

Theoretical and experimental studies of the current–voltage and capacitance–voltage of HEMT structures and field-effect transistors

The sensitivity of classical n ⁺/n ^– GaAs and AlGaN/GaN structures with a 2D electron gas (HEMT) and field-effect transistors based on these structures to γ-neutron exposure is studied. The levels of their radiation hardness were determined. A method for experimental study of the structures on the basis of a differential analysis of their current–voltage characteristics is developed. This method makes it possible to determine the structure of the layers in which radiation-induced defects accumulate. A procedure taking into account changes in the plate area of the experimentally measured barrier-contact capacitance associated with the emergence of clusters of radiation-induced defects that form dielectric inclusions in the 2D-electron-gas layer is presented for the first time.     

Calculation of the Schottky barrier and current–voltage characteristics of metal–alloy structures based on silicon carbide

A simple but nonlinear model of the defect density at a metal–semiconductor interface, when a Schottky barrier is formed by surface defects states localized at the interface, is developed. It is shown that taking the nonlinear dependence of the Fermi level on the defect density into account leads to a Schottky barrier increase by 15–25%. The calculated barrier heights are used to analyze the current–voltage characteristics of n-M/p-(SiC)_1–x(AlN)_x structures. The results of calculations are compared to experimental data.     

Parasitic capacitance removal of sub-100 nm p-MOSFETs using capacitance–voltage measurements

Capacitance–voltage measurements are performed on sub-100 nm high-k/metal gate p-MOSFETs to extract the intrinsic capacitance per gate length. This is then repeated on simulated devices using finite element modeling to compare to the experimental results. The intrinsic channel capacitance for the simulated devices is isolated from the parasitic capacitance, allowing for the comparison of analytic models of parasitic capacitances to the simulation.     

The influence of a voltage ramp on the measurement of I–V characteristics of a solar cell

For convenience and efficiency the voltage applied to a Si solar cell is often fairly rapidly driven from zero to the open circuit value typically at a constant rate of 1 V per millisecond. During this time the values of the current are determined as a function of the instantaneous voltage thus producing an I–V characteristic. It is shown here that the customary expressions for the current as a function of cell parameters remain still valid provided that the diffusion length in the expression for the dark current is changed from its steady state value L to the effective diffusion length L_i given by

$\text{L}{}_1\text{= L}\text{1+}\text{q}\text{V}\text{?}\text{kT}{}^{\mathrm{\tau }}\text{,}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$

where V is the ramp rate considered constant and τ is the lifetime of minority carriers. This result is true to a very good approximation provided that low level injection prevails.     

Two-peak capacitance–voltage behavior in devices based on electron transport materials

Two peaks are observed in the capacitance–voltage (C–V) characteristics of electron-transport fullerene (C₆₀ or C₇₀)/Bphen based devices. The experimental results suggest that the mobile carriers generated from unintentional doping and trapped carriers are the origins of these two peaks, just the same as those of hole-transport devices. In addition, the polarity of one C–V peak voltage (the voltage corresponding to the peak capacitance) reverses as the Bphen layer thickness increases. The transient photo-voltage (TPV) signals show a polarity reversal of the internal electric field, confirming the related phenomenon.     

Intermolecular interactions in organic semiconductors based on annelated β-oligothiophenes and their effect on the performance of organic field-effect transistors

A series of derivatives based on annelated β-oligothiophenes were synthesized and characterized as active layer in organic field-effect transistors (OFETs). Highest field-effect mobility of 0.52 V^?1 s^?1 for 2,5-dibiphenyl-dithieno[2,3-b:3′,2′-d]thiophene (DBP-DTT), 2.2 cm² V^?1 s^?1 for 2,5-distyryl-dithieno[2,3-b:3′,2′-d]thiophene (DEP-DTT), and 0.16 cm² V^?1 s^?1 for 1,4-di[2-dithieno[2,3-b:3′,2′-d] thiophen-2-yl-vinyl]benzene (DDTT-EP) were obtained, while 2,5-diphenyl-dithieno [2,3-b:3′,2′-d]thiophene (DP-DTT) presents no field-effect behaviors. Their thermal, optical and electrochemical properties, topographical and X-ray diffraction patterns of films, and the single crystal structures were also investigated. With the end-capping groups changing in these materials, the intermolecular interactions could transform from S–S in DP-DTT to S–C in DBP-DTT, to S–π in DEP-DTT, and to the coexisting of S–S and S–π in DDTT-EP. According to the device performances and the results of transfer integral calculations, it was revealed that S–π intermolecular interaction benefits not only improving the mobility but also reducing the threshold voltage (V_T), while S–S intermolecular interaction is not favorable for promoting the mobility.     

Effect of PEDOT:PSS–molecule interface on the charge transport characteristics of the large-area molecular electronic junctions

We have studied the effect of the PEDOT:PSS–molecule contact on the electrical characteristics of molecular junctions consisting of N-alkanedithiol and naphthalenethiol molecules. In this study, we experimentally investigated the properties of PEDOT:PSS-interlayer molecular junctions as they depended on the two kinds of PEDOT:PSS films (the pure PEDOT:PSS film and the dimethyl sulfoxide (DMSO)-modified PEDOT:PSS film) and their thermal annealing treatment. We observed that the electrical properties of these molecular junctions are influenced by the morphology and conductivity of the PEDOT:PSS films and by the thermal treatment. In particular, the resistance of the PEDOT:PSS-interlayer molecular junctions depended on the kind of PEDOT:PSS film and the temperature, within the range of elevated temperatures (higher than room temperature) tested. These experimental results are explained by the change of the interfacial properties of the PEDOT:PSS–molecule contact, which are influenced by the morphology change of the PEDOT:PSS film and the removal of residual DMSO or water from the interface.     

Improvement of oxide thickness determination on MOS structures using capacitance–voltage measurements at high frequencies

It is well known that capacitance–voltage (C–V) measurements provide a simple determination of oxide thickness, but with the scaling down of components the classical method is not appropriated any more. We have observed that for two devices with the same oxide thickness and different surfaces, the classical method is accurate for large area but it is not adapted for the small one. We present a new procedure to make an accurate electrical determination of the oxide thickness on metal-oxide-semiconductor (MOS) structures of low dimensions in U.L.S.I. technology. Our method does not require a measurement in strong accumulation. It is based on C–V measurements at frequencies higher than 1 MHz associated to a non-linear optimisation of the experimental and theoretical band bending versus bias voltage curve (Ψ_S=f(V_g)), in the depletion mode. By this way, a corrective factor is estimated with precision in order to make an accurate determination of the oxide thickness value. We show that the frequency associated to the non-linear optimisation of Ψ_S=f(V_g) is function of the MOS device dimensions and is increased when the surface decreases. The experimental results obtained on low-dimension MOS structures and different oxide thickness are precise and in total agreement with those measured by ellipsometry. By using our new procedure the accuracy of oxide thickness determination is improved.     

Influences of magnetic fields on current–voltage characteristics of gold-DNA-gold structure with variable gaps

Achieving highly sensitive magnetic sensors by means of Metal-DNA-Metal (MDM) structure is a key issue. DNA, being a genetic information carrier in living cells reveals tunable semiconducting response in the presence of external electric and magnetic fields, which is promising for molecular electronics. The influence of magnetic fields up to 1200 mT on the current–voltage (I–V) behavior of Gold-DNA-Gold (GDG) structure having variable gap sizes from 20–50 μm are reported in this work. These structures were fabricated using UV lithography, DC magnetron sputtering and thermal evaporation techniques. DNA strands were extracted from Boesenbergia rotunda plant via standard protocol. The acquired I–V characteristics display the semiconducting diode nature of DNA in GDG structures. The potential barrier for all the structures exhibit an increasing trend with the increase of externally imposed magnetic field irrespective of variable gap sizes. Furthermore, the potential barrier in GDG junction at higher magnetic field strengths (>1000 mT) is found to be considerably enhanced. This enhancement in the junction barrier height at elevated magnetic fields is attributed to the reduction of carrier mobility and augmentation of resistance. The achieved admirable features of magnetic sensitivity suggest the viability of using these GDG sandwiches as a prospective magnetic sensor.     

Investigation on the mechanism of charge generation in organic heterojunctions: Analysis of I–V and C–V characteristics

The charge generation mechanism of organic heterojunction (OHJ) consisted of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and different hole transporting materials (HTMs) are studied systematically by current-voltage (I–V) and capacitance-voltage measurements. The analysis of I–V characteristics of the devices based on OHJs at forward and reverse voltages by comparing the thickness of HTM layers finds that a forward and reverse symmetrical I–V curve is observed at thin HTM layers and the forward current becomes larger than the reverse current with the increase of HTM thickness, fully illustrating the effectiveness of OHJ charge generation. Moreover, the I–V characteristics at different temperatures indicate that the efficient charge generation is originated from electron tunneling rather than diffusion. And the C–V and capacitance-frequency (C–F)characteristics further illustrate the highly efficient charge generation ability of OHJs so that the charge density is as high as 4.5 × 10¹⁷ cm⁻³, guaranteeing the high conductivity of OHJs, which is very beneficial to developing highly efficient OLEDs using OHJs as charge injector and generator.     

Investigation of current–voltage characteristics of vertically stacked all-NbCN Josephson junctions

Current–voltage characteristics of vertically stacked all-NbCN Josephson junctions has been investigated with a purpose to use them as an element of integrated circuits. It has been shown that increases of microwave power in the junction definition process using electron cyclotron resonance (ECR) etching causes reduction of the junction quality parameter. From results of a measurement of current–voltage characteristics for an array composed of five-fold vertically stacked NbCN/MgO/NbCN junctions, it has been found that a very high uniformity in critical currents can be achieved.     

Influence of PVK ： NPB hole transporting layer on the characteristics of organic light-emitting devices

A doping system consisting of NPB and PVK is employed as a composite hole transporting layer （CHTL）. By adjusting the component ratio of the doping system, a series of devices with different concentration proportion of PVK ： NPB are constracted. The result shows that doping concentration of NPB enhances the competence of hole transporting ability, and modifies the recombination region of charge as well as affects the surface morphology of doped film. Optimum device with a maximum brightness of 7852 cd/m^2 and a power efficiency of 1.75 lm/W has been obtained by choosing a concentration proportion of PVK ： NPB at 1：3.     

Methods for measuring the current–voltage characteristics of photodiodes in a multirow infrared photodetector

Methods for measuring the current-voltage characteristics (I–V curves) of photodiodes in a 6 × 576 mercury-cadmium-tellurium (MCT) multirow photodetector designed for operation in the longwave part of the infrared (IR) spectral range are analyzed. The I–V curve is plotted using the resultes of measurements of output signals of a large-scale readout integrated circuit (ROIC) hybridized with a row of IR photodiodes. The method of independent current measurement at each point of the I–V curve is compared to the method of additive current measurements. A method of determining optimum working points of photodiodes by plotting and analyzing the dependence of the differential resistance of photodiode on the bias voltage is proposed. Distributions of photodiode currents for a sample of a 6 × 576-element focal plane array (FPA) based on MCT photodiodes with a p-type conductivity substrate having the cutoff wavelength of λ_0.5 = 10.5 μm are considered.     

Temperature dependent of the current–voltage (I–V) characteristics of TaSi2/n-Si structure

Tantalum silicide (TaSi₂) thin films were deposited on n-type silicon single crystal substrates using a dual electron-gun system and with Ta and Si targets. The electrical transport properties of the TaSi₂/n-Si structures were investigated by temperature-dependent current–voltage (I–V) measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by thermionic-emission and space-charge-limited current mechanisms at low and high voltage respectively. On the other hand, the reverse current is limited by the carrier generation process.     

Effect of transverse electric field on the longitudinal current–voltage characteristic of graphene superlattice

The current density induced along the axis of graphene superlattice in the presence of ac and dc electric fields has been calculated. The dc electric field vector is assumed to have both transverse and longitudinal components with respect to the superlattice axis. The constant component of the current density is shown to oscillate with a change in the ac field amplitude. The longitudinal current–voltage characteristic of graphene superlattice contains a portion with negative differential conductivity. The maximum of the longitudinal current–voltage characteristic shifts to larger values of the longitudinal component of dc field with an increase in the transverse component of electric field.