Decrease In Barrier Height Research Articles

Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

AbstractThis paper reports important developments achieved with CdTe thin‐film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high‐transparency window layer (Cd0.3Zn0.7S) and a device‐activation anneal. These increased the fill factor and open‐circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd0.3Zn0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm2 in order to assess its impact on the device series resistance and fill factor. Finally, micro‐module devices utilising series‐connected 2‐cm2 sub‐cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module‐to‐cell efficiency ratio of 0.9 was demonstrated for a six‐cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on. Copyright © 2015 John Wiley & Sons, Ltd.

Temperature dependent of electrical characteristics of Au/n-GaAs/In Schottky diode with In2S3 interfacial layer obtained by using spray pyrolysis method

A study on Au/n-GaAs Schottky barrier diodes parameters with and without thin In2S3 thin films, fabricated on n-type GaAs grown by Spray Pyrolysis Method (SPM), has been made. The film nanostructure features have been characterized by XRD, SEM and EDAX techniques. The Au/n-GaAs/In and Au/In2S3/n-GaAs/In junctions are investigated the electrical characteristics with Current-Voltage (I–V) measurements at room temperature. The Current-Voltage-Temperature (I–V–T) and Capacity–Voltage–Temperature (C–V–T) characteristics of Au/In2S3/n-GaAs/In structure are determined in the temperature wide range 300–20 K with 20 K steps. Some electrical parameters such as ideality factor n, zero bias barrier height ΦBo and series resistance RS are extracted from I–V–T curves using Thermionic Emission (TE) method and modified Norde functions. It is observed that the barrier height decreases and the ideality factor increases with the decrease temperature. Such behavior is attributed to barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the interface for two regions. Furthermore, from the C–V–T measurement of the structure ΦB(CV), donor concentration (ND), Fermi level (EF) and diffusion potential (VD) have been calculated in the temperature range of 300–20 K at 500 kHz applied frequency.

A different approach to solar cell simulation

Zero barrier height inhomogeneity in the device is generally assumed to conform to Gaussian distribution in the literature. In this study, zero barrier height inhomogeneity has been adopted to obey random distribution. Cell was divided into elementary diodes, which were connected in parallel to each other. The current–voltage characteristics of the cell were obtained by developed device modeling program for various zero barrier height inhomogeneity levels at room temperature in dark and under light conditions. The cell parameters were then calculated by using simulated current–voltage data. It is displayed that increase in zero barrier height inhomogeneity results in Schottky barrier height enhancement and decrease in the diode factor of the cell. Fill factor and Voc values showed a decreasing trend with increasing zero barrier height inhomogeneity. Also, random distribution of zero barrier height effect on interface state density was examined. Interface state density increases with increasing zero barrier height inhomogeneity.

Charge carrier relaxation studies in poly (3, 4-ethylenedioxythiophene) nanofibers

The electrical conductivity and carrier relaxation in poly (3, 4-ethylenedioxythiophene) (PEDOT) nanofibers have been studied over a wide range of frequency and temperature by means of impedance spectroscopy. High resolution transmission electron micrographs confirm the formation of nanofibers with average diameter of 14 nm. The linear increase of imaginary permittivity with decreasing frequency in the log-log plot of e″ versus ω is attributed to the higher contribution of dc conductivity than that of the electrode polarization. The presence of single semicircle in the complex impedance Cole-Cole plot indicates the presence of single charge carrier relaxation mechanism. The perfect matching of the relaxation peak in Z″ and M″ vs. frequency at different temperature confirms the presence of Debye type relaxation. From the temperature dependent behavior of frequency exponent study it is confirmed that the charge transport takes place through correlated barrier hopping mechanism. Decrease of barrier height and increase of density of states with increasing dopant concentration can be corroborated with the conductivity enhancement.

Diode Parameters of Mesa Structural n-Type Nanocrystalline FeSi<sub>2</sub>/p-Type Si Heterojunctions Prepared by Lift-Off Photolithography

Mesa structural n-type nanocrystalline-FeSi2/p-type Si heterojunctions were successfully fabricated by a lift-off technique combined with a photolithography process. Their current-voltage characteristics were measured at low temperatures range from 300 K down to 60 K. We estimated their diode parameters such as ideality factor, barrier height and series resistance based on the thermionic emission theory and Cheung’s method. From the estimation by the thermionic emission theory, the obtained results show an increase of ideality factor and a decrease of barrier height at low temperatures. The estimation by Cheung’s method shows that the values of ideality factor and barrier height are in agreement with those obtained from the thermionic emission theory. The obtained series resistances from dV/d (lnJ)-J and H(J)-J plots, which are approximately equal to each others, are increased at low temperatures.

Electron-phonon interaction in three-barrier nanosystems as active elements of quantum cascade detectors

The theory of electron tunneling through an open nanostructure as an active element of a quantum cascade detector is developed, which takes into account the interaction of electrons with confined and interface phonons. Using the method of finite-temperature Green’s functions and the electron-phonon Hamiltonian in the representation of second quantization over all system variables, the temperature shifts and electron-level widths are calculated and the contributions of different electron-phonon-interaction mechanisms to renormalization of the spectral parameters are analyzed depending on the geometrical configuration of the nanosystem. Due to weak electron-phonon coupling in a GaAs/Al0.34Ga0.66As-based resonant tunneling nanostructure, the temperature shift and rf field absorption peak width are not very sensitive to the electron-phonon interaction and result from a decrease in potential barrier heights caused by a difference in the temperature dependences of the well and barrier band gaps.

Electrically conductive polyaniline as hole-injection layer for MEH-PPV:BT based polymer light emitting diodes

Polyaniline (PANI) was synthesized by oxidative polymerization of aniline at different temperatures (5, 10, 15, 20 and 25 °C). The influence of polymerization temperature on sheet resistance of PANI was investigated, and the one prepared at 15 °C which showed lowest resistivity was chosen for further analysis. PANI was subsequently used as hole-injection layer (HIL) in polymer light emitting diodes (PLEDs) with structure of poly(ethylene terephthalate) (PET)/indium tin oxide (ITO)/PANI/MEH-PPV:BT/aluminum (Al). The PLEDs with emission layer made from a blend of poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and an electron transporting material, benzothiadiazole (BT), were fabricated at room conditions without using glove boxes. Our results showed an improvement in performance of our PANI-based fabricated PLEDs (PET/ITO/PANI/MEH-PPV:BT/Al) compared to the conventional devices that use poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PET/ITO/PEDOT:PSS/MEH-PPV:BT/Al) as their HIL. The hole injection barrier height (φ) of the fabricated PLEDs were then estimated using the Fowler–Nordheim (FN) field-emission tunneling theory and revealed that the barrier height decreases by increasing the BT concentration in the MEH-PPV:BT blend layer.

Influence of illumination intensity on the electrical characteristics and photoresponsivity of the Ag/ZnO Schottky diodes

In this article, the ZnO thin films were grown by RF-magnetron sputtering on ITO glass substrates. The Schottky diodes with the configuration of Ag/ZnO/ITO have been fabricated and it has been observed that the diodes exhibit a good rectification. The structural and optical properties of the ZnO films were investigated by X-ray diffractometry and spectrophotometry. The current–voltage (I–V) characteristics of the Ag/ZnO diode were measured under various illuminations. We use the forward bias current–voltage measurements to determine the electrical parameters such as ideality factor, barrier height and series resistance of the diode. The Ag/ZnO Schottky diode exhibits a non-ideal behavior due to the interfacial layer, the interface states and the series resistance. It is found that the barrier height and ideality factor values are strong functions of illumination intensity. The results show that the ideality factor and the barrier height decrease with increasing illumination intensity. The values of Rs obtained from Cheung and Norde methods are decreased with increasing illumination intensity. Photoresponse characteristics of the diode have been analyzed and it is clear that the diode shows a fast response. It is evaluated that the prepared diodes can be used as optoelectronic devices.

Ru/Ti Schottky Contacts on N-type In-P (100): Temperature Dependence of Current-Voltage (I-V) Characteristics

The current-voltage characteristics (I-V) of Ru/Ti/n-InP (100) schottky diodes have been measured in the temperature range of 200-400oC. These are analyzed using thermionic emission theory (TE) by incorporating the concept of barrier inhomogeneity at the metal/semiconductor interface through a Gaussian distribution factor. The forward and reverse I-V characteristics are analyzed to estimate the Schottky barrier parameters. It has revealed that decrease of barrier height and increase of ideality factor as the temperature increases. The barrier height (Фb) is found to be 0.82eV, 0.81eV, 0.80eV and 0.79eV for as-deposited sample, annealed at 200oC, 300oC and 400oC respectively. The ideality factor in the same temperature range are found to be 1.19, 1.26, 1.59, 1.64 and this is good agreement with the Cheung's function dV/dln(I) vs. I. The series resistance values are evaluated in the range of 16MΩ – 62 MΩ with increasing temperature. The nonlinearity in the Richardson plot and Schottky barrier heights may be attributed to the formation of phosphide phases at the Ru/Ti/n-InP interface.

Reversible influence of ultrasound on γ-irradiated Mo/n-Si Schottky barrier structure

The influence of ultrasonic loading on current–voltage characteristics has been investigated in Mo/n-n+-Si structures irradiated by 60Co γ-rays. The longitudinal ultrasonic waves were of 9.6MHz in frequency and had the intensity approaching 1.3W/cm2. The observed reversible acoustically induced increase in forward and reverse currents was as large as 60%. The ultrasound has been found to affect thermionic emission mainly due to Schottky barrier height decrease. The observed effects are related to acoustically induced ionization of the defects located at the metal–semiconductor interface. It has also been found that in the result of γ-irradiation, the ultrasonic wave-defect interaction is modified. Ultrasonic loading, however, has been found to have no effect either on direct or phonon-assisted tunneling.

Electrical characterization of Ni/Al0.09Ga0.91N Schottky barrier diodes as a function of temperature

The current–voltage characteristics (I–V) of Ni/Al0.09Ga0.91N Schottky barrier diodes (SBDs) prepared with a photolithography and lift-off techniques were investigated in the wide temperature range of 100–310K. The I–V characteristics of the devices were analyzed on the basis of the thermionic emission (TE) theory. An abnormal decrease in the experimental effective barrier height and an increase in ideality factor with a decrease in the temperature were observed. The temperature dependence of the effective Schottky barrier height (SBH) was explained with the presence of the laterally barrier height inhomogeneity at the metal-semiconductor interface. In addition, the modified Richardson plots were used to determine experimental Richardson constants in the three temperature regions.

Growth and temperature dependent characterization of pulsed laser deposited Ag/n-ZnO/p-Si/Al heterojunction

The Ag/n-ZnO/p-Si(100)/Al heterojunction diodes were fabricated by pulsed laser deposition of zinc oxide (ZnO) thin films on p-type silicon. The X-ray diffraction analysis shows the formation of ZnO thin film with hexagonal structure having strong (002) plane as preferred orientation. The energy band gap of ZnO films simultaneously deposited on quartz substrate was calculated from the measured UV–Visible transmittance spectra. High purity vacuum evaporated silver and aluminum thin films were used to make contacts to the n-ZnO and p-silicon, respectively. The current–voltage and capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterostructures were measured over the temperature range of 80–300 K. The Schottky barrier height and ideality factor were determined by fitting of the measured current–voltage data into thermionic emission diffusion equation. It is observed that the barrier height decreases and the ideality factor increases with decrease of temperature and the activation energy plot exhibit non-linear behavior. This decrease in barrier height and increase in ideality factor at low temperature are attributed to the occurrence Gaussian distribution of barrier heights. The capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterojunction diode were also studied over the wide temperature range. Capacitance–voltage data are used to estimate the barrier height and impurity concentration in n-type ZnO.

Intrinsic delay of permeable base transistor

Permeable base transistors (PBTs) fabricated by vacuum deposition or solution process have the advantages of easy fabrication and low power operation and are a promising device structure for flexible electronics. Intrinsic delay of PBT, which characterizes the speed of the transistor, is investigated by solving the three-dimensional Poisson equation and drift-diffusion equation self-consistently using finite element method. Decreasing the emitter thickness lowers the intrinsic delay by improving on-current, and a thinner base is also preferred for low intrinsic delay because of fewer carriers in the base region at off-state. The intrinsic delay exponentially decreases as the emitter contact Schottky barrier height decreases, and it linearly depends on the carrier mobility. With an optimized emitter contact barrier height and device geometry, a sub-nano-second intrinsic delay can be achieved with a carrier mobility of ∼10 cm2/V/s obtainable in solution processed indium gallium zinc oxide, which indicates the potential of solution processed PBTs for GHz operations.

Single Gaussian distribution of barrier height in Al/PS–ZnPc/p-Si type Schottky barrier diode in temperature range of 120–320 K

Possible current-transport mechanism in aluminum/polystrene–zincphthalocyanine/ptype silicon Schotky barrier diode (Al/PS–ZnPc/p-Si; SBD), for the forward bias current–voltage (I–V) characteristics were carried out in the temperature range of 120–320 K. The high value of ideality factor (n), especially at low temperatures, was attributed to the existence of PS layer, barrier in-homogeneities and particular density distribution of surface states between metal and semiconductor. An abnormal decrease in the zero-bias barrier height (BH) and increase in n with decreasing temperature which leads to non-linearity in the Richardson plot, have been observed. Linear relationship between BH and n was also observed. BH was plotted as a function of q/2kT to obtain evidence of Gaussian distribution (GD) of the BHs. The mean BH and its standard deviation (σ) were obtained as 1.03 eV and 0.117 V from the slope and intercept of this plot, respectively. Thus, the modified ln(Io/T2) − q2σo2/2k2T2 versus q/kT plot gives mean BH and the modified Richardson constant \({\text{A}}_{\bmod }^{*}\) as 1.043 eV and 29.824 A cm−2 K−2, respectively. This value of the Richardson constant is very close to the theoretical value of 32 A cm−2 K−2 for p-type Si. Therefore, non-ideal behavior of forward-bias I–V characteristics in Al/PS–ZnPc/p-Si might be successfully explained in terms of the thermionic emission mechanism with single GD of BHs.

Electrical characterization of Ni/n-ZnO/p-Si/Al heterostructure fabricated by pulsed laser deposition technique

The ZnO thin films are grown on the p-Si for the heterojunction fabrication by pulsed laser deposition method. X-ray diffraction study showed that the texture of the film is hexagonal with a strong (002) plane as preferred direction. High purity vacuum evaporated nickel and aluminum metals were used to make contacts to the n-ZnO and p-Si, respectively. The current–voltage characteristics of Ni/n-ZnO/p-Si(100)/Al hetero structure measured over the temperature range 80–300K have been studied on the basis of thermionic emission diffusion mechanism. The equivalent Schottky barrier height and diode ideality factor are determined by fitting of measured current–voltage data in to thermionic diffusion equation. It is observed that the barrier height decreases and the ideality factor increases with decrease of temperature and the activation energy plot exhibit non-linear behavior. These characteristics are attributed to the Gaussian distribution of barrier heights. The capacitance–voltage characteristics of Ni/n-ZnO/p-Si(100)/Al heterojunction diode are also studied over wide temperature range. From the measured capacitance–voltage data the built in voltage and impurity concentration in n-type ZnO is estimated.

Double Gaussian distribution of barrier height for FeCrNiC alloy Schottky contacts on p-Si substrates

The electrical properties of Schottky contact with a quadripartite alloy FeCrNiC on p-Si have been investigated in the temperature range of 80–320K, for the first time. An abnormal decrease in the apparent barrier height (φap.) and an increase in the apparent ideality factor (nap.) with a decrease in the temperature were elucidated by the current–voltage (I–V) characteristic of the FeCrNiC/p-Si structure. The conventional Richardson plot exhibits non-linear behaviour at temperature below 180K with the linear portion to be used for the calculation of activation energy and Richardson constant (A*) as 0.352eV and 8.3×10−3AK−2cm−2, respectively. The observed anomalies were explained on the basis of the thermionic emission (TE) theory by incorporating the concept of inhomogeneous multiple barriers at Metal–Semiconductor (MS) interface. It has been seen that the apparent barrier height φap. exhibits double Gaussian distribution (DGD) feature with the mean BH (ϕ¯b0) of 0.695 and 0.646eV, accompanied by their standard deviations (σ0) of 0.082 and 0.070eV in 320–180K and 180–80K regions, respectively. These values of the ϕ¯b0 have been confirmed with the modified Richardson plot [ln(J0/T2)−(q2σ02/2k2T2) vs. 1/T] as 0.690eV and 0.633eV at the demarcated temperature regions, respectively. Richardson constant A* has also been calculated from the modified Richardson plots as 33.43AK−2cm−2 and 28.47AK−2cm−2 that belong to two distinct temperature ranges. Their average value exactly matched the theoretical value of 31.6AK−2cm−2 for the holes in p-type Si. Our results confirm the predictions of the multiple GD approach of nanoscale spatial BH inhomogeneities at the MS interface.

Annealing effect on Schottky barrier inhomogeneity of graphene/n-type Si Schottky diodes

The current–voltage characteristics of graphene/n-type Si (n-Si) Schottky diodes with and without annealing were measured in the temperature range of −120 to 30 °C and analyzed on the basis of thermionic emission theory. It is found that the barrier height decreases and the ideality factor increases with the decrease measurement temperatures. Such behavior is attributed to Schottky barrier inhomogeneities. It is shown that both the barrier height and the ideality factor can be tuned by changing the annealing temperature. Through the analysis, it can be suspected that a SiOx layer at the graphene/n-Si interfaces influences the electronic conduction through the device and stoichiometry of SiOx is affected by annealing treatment. In addition, both Schottky barrier inhomogeneity and the T0 effect are affected by annealing treatment, implying that stoichiometry of SiOx has a noticeable effect on the inhomogeneous barriers of graphene/n-Si Schottky diodes.

Schottky barrier inhomogeneity for graphene/Si-nanowire arrays/n-type Si Schottky diodes

The current–voltage characteristics of graphene/Si-nanowire (SiNW) arrays/n-type Si Schottky diodes with and without H2O2 treatment were measured in the temperature range of −150 ∼ 150 °C. The forward-bias current-voltage characteristics were analyzed on the basis of thermionic emission theory. It is found that the barrier height decreases and the ideality factor increases with the decreased temperatures. Such behavior is attributed to barrier inhomogeneities. It is shown that both Schottky barrier inhomogeneity and the T0 effect are affected by H2O2 treatment, implying that charge traps in the SiNWs have a noticeable effect on Schottky barrier inhomogeneity for graphene/SiNWs/n-type Si diodes.

Highly efficient inverted organic solar cells using amino acid modified indium tin oxide as cathode

In this paper, we report that highly efficient inverted organic solar cells were achieved by modifying the surface of indium tin oxide (ITO) using an amino acid, Serine (Ser). With the modification of the ITO surface, device efficiency was significantly enhanced from 0.63% to 4.17%, accompanied with an open circuit voltage (Voc) that was enhanced from 0.30 V to 0.55 V. Ultraviolet and X-ray photoelectron spectroscopy studies indicate that the work function reduction induced by the amino acid modification resulting in the decreased barrier height at the ITO/organic interface played a crucial role in the enhanced performances.

Barrier inhomogeneities at vertically stacked graphene-based heterostructures

The integration of graphene and other atomically flat, two-dimensional materials has attracted much interest and been materialized very recently. An in-depth understanding of transport mechanisms in such heterostructures is essential. In this study, vertically stacked graphene-based heterostructure transistors were manufactured to elucidate the mechanism of electron injection at the interface. The temperature dependence of the electrical characteristics was investigated from 300 to 90 K. In a careful analysis of current-voltage characteristics, an unusual decrease in the effective Schottky barrier height and increase in the ideality factor were observed with decreasing temperature. A model of thermionic emission with a Gaussian distribution of barriers was able to precisely interpret the conduction mechanism. Furthermore, mapping of the effective Schottky barrier height is unmasked as a function of temperature and gate voltage. The results offer significant insight for the development of future layer-integration technology based on graphene-based heterostructures.