High Forward Bias Research Articles

Carrier transport mechanisms and photodetector characteristics of Ag/TiOPc/p-Si/Al hybrid heterojunction

Hybrid heterojunction device was fabricated by employing a p-type Si and a thin film of titanyl phthalocyanine (TiOPc). The dark current density–voltage characteristics of the fabricated Ag/TiOPc/p-Si/Al heterojunction were investigated at different temperatures ranging from 294 to 375K to determine the carrier transport mechanisms. At low forward bias, the current was found to follow the thermionic emission mechanism, while at high forward bias, the space charge limited current controlled by exponential trap distribution was found to be the dominated mechanism. The ideality factor and the barrier height were determined. At reverse bias, the conductivity was interpreted in terms of the Schottky effect. The interface state density was determined from the current–voltage characteristics. Also, the photodetector characteristics were studied for the fabricated device under illumination of near-infrared 805nm laser. The responsivity, external quantum efficiency, and detectivity were determined. The photodetector was found to have a rise time of about 23µs.

Investigation of negative dielectric constant in Au/1 % graphene (GP) doped-Ca1.9Pr0.1Co4Ox/n-Si structures at forward biases using impedance spectroscopy analysis

The dielectric properties of Au/(1 % graphene doped-Ca1.9Pr0.1Co4Ox)/n-Si structures were investigated by the impedance spectroscopy method including capacitance–voltage (C–V) and conductance–voltage (G/ω–V) measurements in the frequency range of 10–2 MHz at room temperature. The experimental results show that the real and imaginary parts of dielectric constant (e′, e′′) and electric modulus (M′ and M′′), and ac electrical conductivity (σ ac ) are a strong functions of frequency and voltage, both. Negative dielectric constant behavior was observed at sufficiently high forward bias voltages at low frequencies and it was attributed to the interfacial polarization, interface traps and series resistance. e′ decreases with increasing frequency at sufficiently high biases whereas e′′ increases. Since interfacial polarization and interface states, both, can follow the ac external signal easily at low frequencies, there occurs a contribution to the measured capacitance and conductance. The negative values of e′ correspond to maximum value of e′′. Such contrary behavior in the e′ and e′′ appears as an abnormality when compared to the conventional behavior metal–semiconductor structures with and without interfacial layer. Experimental results confirmed that the dielectric properties of these structures are quite sensitive to the frequency and applied bias voltage, both, especially at low frequencies and high voltages because of density distribution of interface states and interfacial polarization.

Dynamics of electronic transitions and frequency dependence of negative capacitance in semiconductor diodes under high forward bias

We observed qualitatively dissimilar frequency dependence of negative capacitance under high charge injection in two sets of functionally different junction diodes: III-V based light emitting and Si-based non-light emitting diodes. Using an advanced approach based on bias activated differential capacitance, we developed a generalized understanding of negative capacitance phenomenon which can be extended to any diode based device structure. We explained the observations as the mutual competition of fast and slow electronic transition rates which are different in different devices. This study can be useful in understanding the interfacial effects in semiconductor heterostructures and may lead to superior device functionality.

Minimized open-circuit voltage reduction in GaAs/InGaAs quantum well solar cells with bandgap-engineered graded quantum well depths

The use of InGaAs quantum wells with composition graded across the intrinsic region to increase open-circuit voltage in p-i-n GaAs/InGaAs quantum well solar cells is demonstrated and analyzed. By engineering the band-edge energy profile to reduce photo-generated carrier concentration in the quantum wells at high forward bias, simultaneous increases in both open-circuit voltage and short-circuit current density are achieved, compared to those for a structure with the same average In concentration, but constant rather than graded quantum well composition across the intrinsic region. This approach is combined with light trapping to further increase short-circuit current density.

Analysis of current transport properties in nonpolar a-plane ZnO-based Schottky diodes

Using current-voltage (I–V) measurements, we investigated the temperature-dependent transport properties in Ag/nonpolar a-plane ZnO Schottky diodes. The bias-dependent ideality factors were altered by the different temperatures and showed a hump at lower temperatures. The series resistance of the diode depended on the temperatures, which was related to the number of free carriers contributing to the series resistance. For high forward bias, the slope m obtained from the lnI–lnV curves decreased with increasing temperature, assuring the space-charge-limited-current (SCLC) model controlled by an exponential distribution of traps. The reverse-biased current transport was associated with the Schottky effect, with a thermally-assisted tunneling for lower voltages and the Poole-Frenkel effect for higher voltages. The density of localized states (Nt) was obtained by applying the theory of SCLC transport, which yielded a Nt value of 8.32 × 1011 eV−1cm−3.

Size-dependent capacitance study on InGaN-based micro-light-emitting diodes

We report a detailed study on size-dependent capacitance, especially the negative capacitance (NC), in InGaN-based micro-pixelated light-emitting diodes (μLEDs). Similar to conventional broad-area LEDs, μLEDs show NC under large forward bias. In the conventional depletion and diffusion capacitance regimes, a good linear relationship of capacitance with device size is observed. However, the NC under high forward bias shows slight deviation from above-mentioned linear relationship with device size. This behaviour can be understood if the effects of current density and junction temperature on NC are considered. The measured temperature dependence and frequency dispersion of the capacitance underpin this point of view. The NCs of two reference broad-area LEDs were also measured and compared with that of μLED clusters with the same total size. A stronger NC effect is observed in the μLED clusters, which is attributed to the increased number of sidewall defects during fabrication process.

Current–voltage characteristics and inhomogeneous barrier height analysis of Au/poly(o-toluidine)/p-Si/Al heterojunction diode

Thin films of poly(o-toluidine) (POT) with amorphous structure were prepared onto the surface of p-Si single crystal by spin coating technique. The electrical conduction mechanisms and related parameters of the Au/POT/p-Si/Al heterojunction diode were investigated using current–voltage (I–V) characteristics in temperature range 298–378 K. The device showed rectifying behavior. At relatively low forward applied voltages; the current through the junction has been analyzed on the bases of the standard thermionic emission theory. The ideality factor decreases with increasing temperature, whereas the barrier height increases. This behavior could be elucidated in terms of inhomogeneity model of barrier heights. In addition, the values of series resistance, ideality factor and effective barrier height at zero-bias are determined at different temperatures by using Cheung’s functions. At relatively high forward bias voltages, analysis of the double logarithmic I–V characteristics indicates that transport through the device is controlled by a space-charge-limited current process characterized by single trap of distribution in which the related parameters are estimated. Data analysis in reverse direction showed that the current can be described in terms of Poole–Frenkel mechanism at low applied voltage and in terms of Schottky mechanism at higher applied voltage.

Temperature and light dependent diode current in high-efficiency solution-processed small-molecule solar cells

This paper reports on the detail analysis of the DC electrical and photoelectrical properties of the high-efficient (η=8.01% under standard 100mW/cm2 AM1.5 illumination) small molecule bulk heterojunction (SM BHJ) solar cells p-DTS(FBTTh2)2/PC70BM. In this SM BHJ solar cell, the dark diode current is determined by the multistep tunnel-recombination via interface states at low forward bias (V<0.65V) and the interface state assisted thermionic emission at high forward bias (V>0.65V). The effect of illumination on the diode current was also quantitatively investigated. It was observed a reduced Shockley–Read–Hall recombination via interface states at large forward bias (from the maximum power point to the open-circuit conditions). The expression of the load I–V characteristic of the illuminated high-efficient SM BHJ solar cells was derived in the presence of the light dependent series and shunt resistance.

Forward and reverse electron transport properties across a CdS/Si multi-interface nanoheterojunction

The electron transport behavior across the interface plays an important role in determining the performance of optoelectronic devices based on heterojunctions. Here through growing CdS thin film on silicon nanoporous pillar array, an untraditional, nonplanar, and multi-interface CdS/Si nanoheterojunction is prepared. The current density versus voltage curve is measured and an obvious rectification effect is observed. Based on the fitting results and model analyses on the forward and reverse conduction characteristics, the electron transport mechanism under low forward bias, high forward bias, and reverse bias are attributed to the Ohmic regime, space-charge-limited current regime, and modified Poole—Frenkel regime respectively. The forward and reverse electrical behaviors are found to be highly related to the distribution of interfacial trap states and the existence of localized electric field respectively. These results might be helpful for optimizing the preparing procedures to realize high-performance silicon-based CdS optoelectronic devices.

Forward Current Transport Mechanisms of Ni/Au—InAlN/AlN/GaN Schottky Diodes

We fabricate two Ni/Au-In0.17Al0.83N/AlN/GaN Schottky diodes on substrates of sapphire and Si, respectively, and investigate their forward-bias current transport mechanisms by temperature-dependent current-voltage measurements. In the temperature range of 300–485 K, the Schottky barrier heights (SBHs) calculated by using the conventional thermionic-emission (TE) model are strongly positively dependent on temperature, which is in contrast to the negative-temperature-dependent characteristic of traditional semiconductor Schottky diodes. By fitting the forward-bias I–V characteristics using different current transport models, we find that the tunneling current model can describe generally the I–V behaviors in the entire measured range of temperature. Under the high forward bias, the traditional TE mechanism also gives a good fit to the measured I–V data, and the actual barrier heights calculated according to the fitting TE curve are 1.434 and 1.413 eV at 300K for InAlN/AlN/GaN Schottky diodes on Si and the sapphire substrate, respectively, and the barrier height shows a slightly negative temperature coefficient. In addition, a formula is given to estimate SBHs of Ni/Au—InAlN/AlN/GaN Schottky diodes taking the Fermi-level pinning effect into account.

Electron-leakage-related low-temperature light emission efficiency behavior in GaN-based blue light-emitting diodes

The typical light emission efficiency behaviors of InGaN/GaN multi-quantum well (MQW) blue light-emitting diodes (LEDs) grown on c-plane sapphire substrates are characterized by pulsed current operation mode in the temperature range 40 to 300 K. At temperatures lower than 80 K, the emission efficiency of the LEDs decreases approximately as an inverse square root relationship with drive current. We use an electron leakage model to explain such efficiency droop behavior; that is, the excess electron leakage into the p-side of the LEDs under high forward bias will significantly reduce the injection possibility of holes into the active layer, which in turn leads to a rapid reduction in the radiative recombination efficiency in the MQWs. Combining the electron leakage model and the quasi-neutrality principle in the p-type region, we can readily derive the inverse square root dependent function between the light emission efficiency and the drive current. It appears that the excess electron leakage into the p-type side of the LEDs is primarily responsible for the low-temperature efficiency droop behavior.

Visible and near-infrared electroluminescence from TiO2/p+-Si heterostructured device

We report on visible and near-infrared (NIR) electroluminescence (EL) from the device based on the TiO2/p+-Si heterostructure, in which the TiO2 film is composed of anatase and rutile phases. As the device is applied with sufficiently high forward bias with the positive voltage connecting to p+-Si, the visible EL peaking at ∼600 nm along with the NIR EL centered at ∼810 nm occur simultaneously. It is proposed that the oxygen vacancies in the anatase TiO2 and Ti3+ defect states in the rutile TiO2 are the responsible centers for the visible and NIR EL, respectively.

High performance vertical tunneling diodes using graphene/hexagonal boron nitride/graphene hetero-structure

A tunneling rectifier prepared from vertically stacked two-dimensional (2D) materials composed of chemically doped graphene electrodes and hexagonal boron nitride (h-BN) tunneling barrier was demonstrated. The asymmetric chemical doping to graphene with linear dispersion property induces rectifying behavior effectively, by facilitating Fowler-Nordheim tunneling at high forward biases. It results in excellent diode performances of a hetero-structured graphene/h-BN/graphene tunneling diode, with an asymmetric factor exceeding 1000, a nonlinearity of ∼40, and a peak sensitivity of ∼12 V−1, which are superior to contending metal-insulator-metal diodes, showing great potential for future flexible and transparent electronic devices.

Temperature‐dependent ac current‐voltage‐capacitance characteristics of GaN‐based light‐emitting diodes under high forward bias

AbstractTemperature‐dependent ac current‐voltage‐capacitance characteristics between 90 K and 440 K were investigated for InGaN/GaN multi‐quantum‐wells blue light‐emitting diodes (LEDs) under high forward bias. Under large forward current, deep saturation of junction voltage Vj was observed over a wide temperature range. It was found that Vj corresponds to the separation energy of electron and hole quasi‐Fermi levels and its temperature dependence could be well explained by the flat band model. After the pinning of quasi‐Fermi levels, the forward current is dominated by the drift current in the p ‐GaN access layer. Considering Poole‐Frenkel effect, a quantitative model was developed to reproduce the cur‐rent‐voltage characteristics of LEDs under high forward bias. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electrical Properties and Carrier Transport Mechanism of Au/&lt;i&gt;n&lt;/i&gt;-GaN Schottky Contact Modified Using a Copper Pthalocyanine (CuPc) Interlayer

We investigated the electrical characteristics of Au/n-GaN Schottky rectifier incorporating a copper pthalocyanine (CuPc) interlayer using current­voltage (I­V), capacitance­voltage (C­V) and conductance­voltage (G­V) measurements. The barrier height of the Au/CuPc/n-GaN Schottky contact was higher than that of the Au/n-GaN Schottky diode, indicating that the CuPc interlayer influenced the space charge region of the n-GaN layer. The series resistance of Au/CuPc/n-GaN Schottky contact extracted from the C­V and G­V methods was dependent on the frequency. In addition, the series resistance obtained from C­V and G­V characteristics was comparable to that from Cheung’s method at sufficiently high frequencies and in strong accumulation regions. The forward logI­logV plot of Au/CuPc/n-GaN Schottky contact exhibited four distinct regions having different slopes, indicating different conduction mechanisms in each region. In particular, at higher forward bias, the trap-filled space-charge-limited current was the dominant conduction mechanism of Au/CuPc/n-GaN Schottky contact, implying that the most of traps were occupied by injected carriers at high injection level. [doi:10.2320/matertrans.M2013449]

AlGaN/GaN Schottky Diode Fabricated by Au Free Process

An AlGaN/GaN Schottky diode is fabricated using an Au free process. Both ohmic and Schottky contact are studied. The ohmic contact results show low contact resistance and good surface morphology. The specific resistance (ρc) and contact resistance (Rc) of the ohmic contact are ~ 3.5×10-5 Ω×cm2 and 2.1 Ω·mm, respectively. Several current transport mechanisms are employed to analyze the measured I- V curves of the diode. The data have demonstrated that at a lower forward bias the combination of generation recombination and tunneling current mechanisms are dominant, whereas the thermionic emission mechanism becomes more significant at a higher forward bias. The effective Schottky barrier height is estimated to be 0.99 eV. The breakdown voltage of AlGaN/GaN Schottky diode is ~ 125 V.

Transport and luminescence phenomena in electroformed silicon nitride-based light emitting diode

Hydrogenated amorphous silicon nitride-based heterojunction pin diode was electroformed under sufficiently high forward bias stress leading to its instant nanocrystallization at room temperature. In order to investigate the origin of the accompanying bright visible light emission, current-voltage and electroluminescence (EL) characteristics of the electroformed diode were scanned over temperature. Electrical transport mechanism was analysed in the low, medium and high electric field regimes. Temperature and field dependence of thermal activation energy were discussed. EL characteristics were studied using the spectral energy distribution and the injection current dependence of its intensity. Thermal quenching data of EL and photoluminescence intensities were compared. Finally, the relationship between the electrical transport and luminescence phenomena was attempted to be interpreted within the frame of a self-consistent model.

A Study of Light-Sensitive Ideality Factor and Voltage-Dependent Carrier Collection of CdTe Solar Cells in Forward Bias

Two features that distinguish CdS/CdTe solar cells from traditional c-silicon cells have been widely confirmed by experiment: 1) The ideality factor of the diode current is sensitive to photon flux input, and 2) the photocurrent is voltage dependent. Here, numerical simulation of one-sided p-type CdTe junctions has been performed in order to interpret the experimental characteristics under forward bias. In these calculations, we assumed a distribution of trapping states within the CdTe bandgap. Detailed discussions have been divided into two parts: light-dependent forward current and voltage-dependent carrier collection. The former has demonstrated that recombination loss increases with increased optical generation. As a result, the diode current, in a certain forward bias region, depends on illumination. With this mechanism, the ideality factor of simulated J-V curves shows a variation consistent with experimental curves. Carrier collection loss under voltage bias is the combined result of carrier diffusion, drift, and recombination during transport. Simulations that are based on our assumption have demonstrated that collection loss increases at higher forward bias and depends on wavelength, carrier mobility, and carrier lifetime.

Improved open-circuit voltage of silicon nanowires solar cells by surface passivation

The surface recombination at the surface of silicon nanowires (SiNWs) deteriorates the performance of SiNWs solar cells and thus the reduction of the SiNWs surface recombination becomes a crucial issue. In this paper, we observe an improved SiNW surface passivation by hydrogenated amorphous silicon (a-Si:H). The results show that a thicker i-layer results in a higher open-circuit voltage Voc. That can be ascribed to the better passivation by thicker intrinsic a-Si:H. The dark current–voltage data reveal that the reverse leakage current and the diode ideality factor at high forward bias decrease monotonically with increasing the thickness of i-layer. Moreover, for the first time, we observe that the lower Voc is associated with the capacitance–voltage (C–V) curve shifting toward higher positive voltage. We propose that the shift of the curve is related to the capacitance affected by the surface states. Finally we prove that the improvement in the NW solar cell performance, especially the Voc, can be attributed to the reduction of the surface states on SiNWs.

2D layered insulator hexagonal boron nitride enabled surface passivation in dye sensitized solar cells

A two-dimensional layered insulator, hexagonal boron nitride (h-BN), is demonstrated as a new class of surface passivation materials in dye-sensitized solar cells (DSSCs) to reduce interfacial carrier recombination. We observe ~57% enhancement in the photo-conversion efficiency of the DSSC utilizing h-BN coated semiconductor TiO2 as compared with the device without surface passivation. The h-BN coated TiO2 is characterized by Raman spectroscopy to confirm the presence of highly crystalline, mixed monolayer/few-layer h-BN nanoflakes on the surface of TiO2. The passivation helps to minimize electron-hole recombination at the TiO2/dye/electrolyte interfaces. The DSSC with h-BN passivation exhibits significantly lower dark saturation current in the low forward bias region and higher saturation in the high forward bias region, respectively, suggesting that the interface quality is largely improved without impeding carrier transport at the material interface. The experimental results reveal that the emerging 2D layered insulator could be used for effective surface passivation in solar cell applications attributed to desirable material features such as high crystallinity and self-terminated/dangling-bond-free atomic planes as compared with high-k thin-film dielectrics.