Zero-bias Schottky Barrier Height Research Articles

Impact of Surface States in Graphene/p-Si Schottky Diodes.

Graphene-silicon Schottky diodes are intriguing devices that straddle the border between classical models and two-dimensional ones. Many papers have been published in recent years studying their operation based on the classical model developed for metal-silicon Schottky diodes. However, the results obtained for diode parameters vary widely in some cases showing very large deviations with respect to the expected range. This indicates that our understanding of their operation remains incomplete. When modeling these devices, certain aspects strictly connected with the quantum mechanical features of both graphene and the interface with silicon play a crucial role and must be considered. In particular, the dependence of the graphene Fermi level on carrier density, the relation of the latter with the density of surface states in silicon and the coupling between in-plane and out-of-plane dynamics in graphene are key aspects for the interpretation of their behavior. Within the thermionic regime, we estimate the zero-bias Schottky barrier height and the density of silicon surface states in graphene/type-p silicon diodes by adapting a kown model and extracting ideality index values close to unity. The ohmic regime, beyond the flat band potential, is modeled with an empirical law, and the current density appears to be roughly proportional to the electric field at the silicon interface; moreover, the graphene-to-silicon electron tunneling efficiency drops significantly in the transition from the thermionic to ohmic regime. We attribute these facts to (donor) silicon surface states, which tend to be empty in the ohmic regime.

Low-temperature electrical properties and barrier inhomogeneities in ITO/β-Ga2O3 Schottky diode

This study reports the electronic properties of the indium tin oxide (ITO)/β-Ga2O3 Schottky diode through the temperature (50–320 K)-dependent current density–voltage, capacitance–voltage, and admittance spectroscopy experiments. The room-temperature turn-on voltage of the ITO/β-Ga2O3 diode was observed to be 0.83 V, and it exhibited a slow increase with decreasing temperature. The ITO/β-Ga2O3 diode's ideality factor (n) varied from 3.92 to 1.05, and the zero-bias Schottky barrier height (φbo) varied from 0.31 to 1.28 eV. The temperature-dependent n and φbo indicate the spatial inhomogeneities of the potential barrier at the Ga2O3-ITO interface. We exploit the Gaussian distribution model to explain the Schottky barrier inhomogeneities, which could be associated with the defects observed by the admittance spectroscopy method.

Graphene/4H-SiC Schottky photodetector operating in the visible spectrum range

In this work, we present the first experimental results on a Schottky photodetector based on Silicon Carbide (SiC) and Graphene (Gr) designed to operate in the visible spectral range. While SiC has been extensively investigated for various applications in the ultraviolet domain, there are only a few works in the visible range, where SiC exhibits negligible optical absorption. To overcome such intrinsic limit of SiC, we exploit the properties of a single layer of Gr to enhance, significantly, the photodetection performance of the device operating, in our experiments, at the wavelength of λ=633 nm. From the current-voltage (I-V) characteristics, a series resistance of 3.7 kΩ, an ideality factor of 8.4, and the zero-bias Schottky barrier height of 0.755 eV have been calculated. Finally, the internal responsivity, as function of the reverse bias applied to the device, has been measured demonstrating a maximum value exceeding 1 mA/W at -5V.

Light-induced modification of the Schottky barrier height in graphene/Si based near-infrared photodiodes

The impact of light on the Schottky barrier height (SBH) in p-type graphene/n-type Si (p-Gr/n-Si) based near-infrared photodiodes is investigated. Hall effect and optoelectronic transport measurements carried out under illumination of 905 nm wavelength light showed that zero-bias SBH in such photodiodes can be effectively tuned in a range between 0.7 and 0.9 eV consistent with the variation in their open-circuit voltage. Shockley-Read-Hall model, which considers the charge recombination through mid-gap and interface states at the p-Gr/n-Si heterojunction, is used to explain the experimentally observed nonlinear dependence of SBH on the incident light. Light induced tunability of SBH at the graphene/semiconductor heterojunction is of great importance especially for the development of new generation optically driven devices in which graphene acts as a functioning element.

Adsorbate-induced enhancement of the spectral response in graphene/silicon-based Schottky barrier photodetectors

The impact of atmospheric adsorbates on the spectral response and response speed of p-type graphene/n-type Silicon (p-Gr/n-Si) based Schottky barrier photodetectors are investigated. Wavelength resolved photocurrent and transient photocurrent spectroscopy measurements conducted under high-vacuum conditions revealed that the atmospheric adsorbates such as O2 and H2O stuck on graphene electrode lead to hole doping in graphene and therefore shift its Fermi level towards higher energy states below its Dirac point. Such a shift in graphene’s Fermi level due to adsorbates increases the zero-bias Schottky barrier height of the p-Gr/n-Si heterojunction from 0.71 to 0.78 eV. Adsorbate induced increment in the barrier height promotes the separation of photo-generated charge carriers at the depletion region and leads to an improvement in the maximum spectral response (e.g., from 0.39 to 0.46 AW−1) and response speed of the p-Gr/n-Si photodetector in the near-infrared region. The experimentally obtained results are expected to give an insight into the adsorbate related variations in the rectification and photo-response characters of the heterojunctions of graphene and other 2D materials with different semiconductors.

Barrier Height Inhomogeneity in Mixed-Dimensional Graphene/Single CdSe Nanobelt Schottky Junctions

Mixed-dimensional graphene/single CdSe nanobelt (NB) Schottky junctions are fabricated, and the influence of Schottky barrier inhomogeneity on the electrical transport mechanism is investigated. The ideality factor increases and the zero-bias Schottky barrier height (SBH) decreases monotonically, as temperature decreases from 300 to 80 K. The temperature-dependent electrical transport characteristics can be explained by the SBH inhomogeneity. We use a spatial potential fluctuation model to analyze the conduction mechanism, where the SBH with a Gaussian distribution is assumed. The standard deviations of SBH distribution are up to 13.06% and 14.09% of the mean value of zero-bias SBH in 80–140 and 140–300 K, respectively, implying strong SBH inhomogeneity in typical graphene/CdSe NB junctions.

Impact of carbon impurities on the initial leakage current of AlGaN/GaN high electron mobility transistors

We have systematically studied the origin of high gate-leakage currents in AlGaN/GaN high electron mobility transistors (HEMTs). Devices that initially had a low gate-leakage current (good devices) are compared with ones that had a high gate-leakage current (bad devices). The apparent zero-bias Schottky barrier height of bad devices (0.4 < ϕB0 < 0.62 eV) was found to be lower than that of the good devices (ϕB0 = 0.79 eV). From transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analysis, we found that this difference is due to the presence of carbon impurities in the nickel layer in the gate region.

Numerical simulations of the electrical transport characteristics of a Pt/n-GaN Schottky diode

In this paper, using a numerical simulator, we investigated the current–voltage characteristics of a Pt/n-GaN thin Schottky diode on the basis of the thermionic emission (TE) theory in the 300 to 500 K temperature range. During the simulations, the effect of different defect states within the n-GaN bulk with different densities and spatial locations is considered. The results show that the diode ideality factor and the threshold voltage decrease with increasing temperature, while at the same time, the zero-bias Schottky barrier height (Φb0) extracted from the forward current density–voltage (J–V) characteristics increases. The observed behaviors of the ideality factor and zero-bias barrier height are analyzed on the basis of spatial barrier height inhomogeneities at the Pt/GaN interface by assuming a Gaussian distribution (GD). The plot of apparent barrier height (Φb,App) as a function of q/2kT gives a straight line, where the mean zero-bias barrier height () and the standard deviation (σ0) are 1.48 eV and 0.047 V, respectively. The plot of the modified activation energy against q/kT gives an almost the same value of and an effective Richardson constant A* of 28.22 A cm−2 K−2, which is very close to the theoretical value for n-type GaN/Pt contacts. As expected, the presence of defect states with different trap energy levels has a noticeable impact on the device electrical characteristics.

Hybrid graphene/silicon Schottky photodiode with intrinsic gating effect

We propose a hybrid device consisting of a graphene/silicon (Gr/Si) Schottky diode in parallel with a Gr/SiO2/Si capacitor for high-performance photodetection. The device, fabricated by transfer of commercial graphene on low-doped n-type Si substrate, achieves a photoresponse as high as and a normalized detectivity higher than in the visible range. It exhibits a photocurrent exceeding the forward current because photo-generated minority carriers, accumulated at Si/SiO2 interface of the Gr/SiO2/Si capacitor, diffuse to the Gr/Si junction. We show that the same mechanism, when due to thermally generated carriers, although usually neglected or disregarded, causes the increased leakage often measured in Gr/Si heterojunctions. We perform extensive I–V and C-V characterization at different temperatures and we measure a zero-bias Schottky barrier height of 0.52 eV at room temperature, as well as an effective Richardson constant A** = and an ideality factor , explained by a thin (<1 nm) oxide layer at the Gr/Si interface.

Temperature dependence of forward and reverse bias current–voltage characteristics in Al–TiW–PtSi/n-Si Schottky barrier diodes with the amorphous diffusion barrier

The forward and reverse bias current–voltage (I–V) characteristics of Al–TiW–PtSi/n-Si SBDs which are fabricated by a magnetron sputtering method, were measured in the temperature and applied bias voltage ranges of 80–360K and (−25)–(5)V, respectively. The diode matrix contains 14 Al–TiW–PtSi/n-Si diodes with the rectifier contact areas between 1×10−6cm2 and 14×10−6cm2. It was found that the ideality factor n decreases, while the zero-bias Schottky barrier height (ΦB0) increases with increasing temperature. The conventional Richardson plot was also found to be nonlinear especially at low temperatures. Therefore, ΦB0 vs. q/kT plot was drawn to obtain an evidence of a Gaussian Distribution (GD) of barrier heights (BHs), and mean BH and zero-bias standard deviation values were obtained from this plot as Φ¯B=0.856eV and σ0=95mV, respectively. Thus, a modified ln(I0/T2)-q2σ02/2k2T2 vs. q/kT plot gives Φ¯B and the Richardson constant (A∗) as 0.820eV and 169Acm−2K−2, respectively. On the basis of obtained results, it can be concluded that the hexagonal emptiness in crystal lattice of Si(111) strongly influence the electric parameters of Al–TiW–PtSi/n-Si structures and its I–V characteristics can be successfully explained on the basis of a thermionic emission (TE) mechanism with Gaussian distribution (GD) of the barrier heights (BHs).The crossing of the experimental semi-logarithmic reverse bias lnI–V plots appears as an abnormality considering the conventional behavior of ideal SBDs. This behavior could be expected for semiconductors in the temperature region where there is no carrier freezing-out which is non-negligible at low temperatures.

Temperature-dependent electrical properties of (Pt/Au)/Ga-polarity GaN/Si(1 1 1) Schottky diode

We report on temperature dependent electrical properties of (Pt/Au)/Ga-polarity GaN/Si(1 1 1) Schottky diode using current–voltage characteristics in the temperature range of 200–375 K. The basic diode parameters, such as zero-bias Schottky barrier height (ϕB0) and ideality factor (‘n’) were extracted from I–V using standard thermionic emission (TE) theory. The estimated ϕB0 and ‘n’ are found to be 0.50 eV and 3.2 for 200 K, 0.78 eV and 1.4 for 375 K. The calculated series resistance (Rs) by Cheung’s method shows unusual behavior with increasing temperature. The Rs value exhibits by diode at 200 K is about 534 Ω, which then decreases to 132 Ω for 300 K and thereafter increases to 154 Ω at 375 K, respectively. It was seen that ϕB0, ‘n’ and Rs are strongly temperature dependent. There is also a linear correlation between ϕB0 and ‘n’ due to the inhomogeneities of the barrier heights (BHs). The estimated Richardson’s constant (A∗) from intercepts at the ordinate of linear fit to the conventional activation energy plot is about ∼5.57 × 10−3 A cm−2 K−2,which is much lower than the known value of n-type GaN (26.4 A cm−2 K−2). Such behavior is also attributed to barrier inhomogeneities at the contact interface. In order to prove the same, results have been interpreted based on the assumption of Gaussian distribution (GD) over the BHs. We calculated the mean barrier height (Φ¯) is 1.04 eV and standard deviation (σs) to be 145 mV. In addition to this, obtained A∗ from the modified Richardson plot ln(Js/T2)-(e2σ02/2k2T2) versus 1/T is 27.62 A cm−2 K−2, which is in good agreement with the theoretical value of GaN.

Current–voltage temperature characteristics of Au/n-Ge (1 0 0) Schottky diodes

The variation in electrical characteristics of Au/n-Ge (100) Schottky contacts have been systematically investigated as a function of temperature using current–voltage (I−V) measurements in the temperature range 140–300K. The I–V characteristics of the diodes indicate very strong temperature dependence. While the ideality factor n decreases, the zero-bias Schottky barrier height (SBH) (ΦB) increases with the increasing temperature. The I–V characteristics are analyzed using the thermionic emission (TE) model and the assumption of a Gaussian distribution of the barrier heights due to barrier inhomogeneities at the metal–semiconductor interface. The zero-bias barrier height ΦB vs. 1/2kT plot has been used to show the evidence of a Gaussian distribution of barrier heights and values of ΦB=0.615eV and standard deviation σs0=0.0858eV for the mean barrier height and zero-bias standard deviation have been obtained from this plot, respectively. The Richardson constant and the mean barrier height from the modified Richardson plot were obtained as 1.37Acm−2K−2 and 0.639eV, respectively. This Richardson constant is much smaller than the reported of 50Acm−2K−2. This may be due to greater inhomogeneities at the interface.

Temperature-dependence of barrier height of swift heavy ion irradiated Au/n-Si Schottky structure

The electrical characteristics of swift heavy ion (SHI) irradiated Au/n-Si (100) structure has been investigated in a wide temperature range (50–300K). The forward bias current–voltage (I–V) measurements have been used to extract the diode parameters as a function of temperature. The Zero-bias Schottky barrier height decreases with decreasing temperature. However, the flat-band barrier height is almost independent of the temperature. These results are interpreted using the models of Fermi level pinning. The behavior of Schottky diode parameters is explained by taking into account the role of the irradiation induced defects at Au/n-Si (100) interface.

On the intersecting behaviour of experimental forward bias current–voltage (I–V) characteristics of Al/SiO2/p-Si (MIS) Schottky diodes at low temperatures

In this study, we have investigated the intersection behaviour of forward and reverse bias current–voltage (I–V) characteristics of Al/SiO2/p-Si Schottky diodes in the temperature range of 79–325 K. The crossing of the experimental semi-logarithmic ln(I)–V curves appears as an abnormality when seen with respect to the conventional behaviour of ideal Schottky diodes. Experimental results show that this crossing of ln(I)–V curves is an inherent property of even Schottky diodes. The ideality factor n was found to decrease, while the zero-bias Schottky barrier height (SBH) ΦB0 increases with increasing temperature. The conventional Richardson plot is found to be nonlinear in the temperature range measured. However, the ln(I0/T2) versus 1000/nT plot gives a straight line corresponding to activation energy 0.233 eV. It is shown that the values of series resistance Rs estimated from Cheung's method were strongly temperature dependent and abnormally increased with increasing temperature. In addition, the temperature dependence of energy distribution of interface states density Nss profiles was obtained from the forward bias I–V measurements by taking into account the bias dependence of the effective barrier height Φe and ideality factor n. All these behaviours indicate that the thermionic emission (TE) cannot be the main current transport mechanism, especially at low temperatures.

Evaluation of Schottky barrier height of TiN∕p-type Si(100)

Zero-bias Schottky barrier height (ϕBp) of reactive-sputtered TiN(∼40nm) on p-type Si(100) was determined at room temperature and the effect of heat treatment was investigated by forward current–voltage (I–V) measurements. The TiN remained amorphous following annealing. The zero-bias barrier heights for the as-deposited and annealed specimens were in the range of 0.53–0.64V. The lower ϕBp values of 0.53–0.54V are related to as-deposited and 673K annealed specimens, whereas the 0.62–0.64V values refer to samples annealed at 773–783K. Forward I–V measurements of as-deposited (300K) TiN∕Si diodes were performed in the temperature range of 220–285K. The barrier height was determined by the activation energy method, resulting in ϕBp=0.58V.