Dependence Of Schottky Barrier Height Research Articles

Electrical characteristics and trap signatures for Schottky barrier diodes on 4H-SiC, GaN-on-GaN, AlGaN/GaN epitaxial substrates

The forward and reverse current transport mechanisms, temperature dependence of Schottky barrier height (SBH) and ideality factor, barrier inhomogeneity analysis, and trap parameters for Schottky barrier diodes (SBDs) fabricated on 4H-SiC, GaN-on-GaN and AlGaN/GaN epitaxial substrates are reported. High SBH is identified for Ni/4H-SiC (1.31 eV) and Ti/4H-SiC (1.18 eV) SBDs with a low leakage current density of <10−8 A cm−2 at −200 V. Thermally stimulated capacitance detects the well-known Z1/2 electron trap at E C—0.65 eV in both 4H-SiC SBDs, while an additional deep-level trap at E C—1.13 eV is found only in Ni/4H-SiC SBDs. The vertical Ni/GaN SBD exhibits a promising SBH of 0.83 eV, and two electron traps at E C—0.18 eV and E C—0.56 eV are identified from deep-level transient Fourier spectroscopy. A peculiar two-diode model behavior is detected at metal/GaN/AlGaN/GaN interface of high-electron mobility transistor (HEMT); the first diode (SBH-1 of 1.15 eV) exists at the standard Metal/GaN Schottky junction, whereas the second diode (SBH-2 of 0.72 eV) forms due to the energy difference between the AlGaN conduction band and the heterojunction Fermi level. The compensational Fe-doping-related buffer traps at E C—0.5 eV and E C—0.6 eV are determined in the AlGaN/GaN HEMT, through the drain current transient spectroscopy experiments.

Analysis of Schottky barrier heights and reduced Fermi-level pinning in monolayer CVD-grown MoS2 field-effect-transistors

Chemical vapor deposition (CVD)-grown monolayer (ML) molybdenum disulfide (MoS2) is a promising material for next-generation integrated electronic systems due to its capability of high-throughput synthesis and compatibility with wafer-scale fabrication. Several studies have described the importance of Schottky barriers in analyzing the transport properties and electrical characteristics of MoS2 field-effect-transistors (FETs) with metal contacts. However, the analysis is typically limited to single devices constructed from exfoliated flakes and should be verified for large-area fabrication methods. In this paper, CVD-grown ML MoS2 was utilized to fabricate large-area (1 cm × 1 cm) FET arrays. Two different types of metal contacts (i.e. Cr/Au and Ti/Au) were used to analyze the temperature-dependent electrical characteristics of ML MoS2 FETs and their corresponding Schottky barrier characteristics. Statistical analysis provides new insight about the properties of metal contacts on CVD-grown MoS2 compared to exfoliated samples. Reduced Schottky barrier heights (SBH) are obtained compared to exfoliated flakes, attributed to a defect-induced enhancement in metallization of CVD-grown samples. Moreover, the dependence of SBH on metal work function indicates a reduction in Fermi level pinning compared to exfoliated flakes, moving towards the Schottky–Mott limit. Optical characterization reveals higher defect concentrations in CVD-grown samples supporting a defect-induced metallization enhancement effect consistent with the electrical SBH experiments.

Effect of Schottky barrier height on quantitative analysis of deep-levels in n-type GaN by deep-level transient spectroscopy

Deep-level transient spectroscopy (DLTS) using Schottky barrier diodes (SBDs) is widely used for quantitative analysis of deep levels. This study focuses on the dependence of Schottky barrier height on apparent time constants and concentrations of electron traps in n-type GaN. DLTS using SBDs with various barrier heights was carried out. Experimental data show that large reverse leakage currents due to low barrier heights resulted in underestimation of time constants and concentrations. Theoretical calculations considering the impact of leakage currents reproduced experimental results well. Based on the calculations, we suggest a minimum required barrier height where accurate time constants and concentrations can be evaluated.

Effect of Schottky barrier height lowering on resistance degradation of Fe-doped SrTiO3 thin-film capacitor

It is generally believed that the resistance degradation behavior of bulk and thin-film oxide capacitors arises from the oxygen vacancy migration within the oxide and/or the charge injection at the oxide/electrode interface. The magnitude of the degradation in the resistance has been theoretically studied in the literature by solving the electrochemical transport equations while assuming constant Schottky barrier height. The treatment of constant Schottky barrier height in existing models has led to significant underestimation of the resistance degradation. In this work, I incorporated the dependence of Schottky barrier height on the oxygen vacancy concentration at the interface into the existing model to simulate the degradation process in thin-film oxide capacitors. With the consideration of Schottky barrier height lowering from the interface dipole arising from the accumulation of oxygen vacancies at the cathode interface, I found that the leakage current can be increased by more than one order of magnitude, which is more consistent with experimental observations in comparison to the prediction from existing models.

Origin of Weaker Fermi Level Pinning and Localized Interface States at Metal Silicide Schottky Barriers

The Schottky barriers of transition metal silicides on silicon are characterised by two anomalous features, a face dependence of Schottky barrier heights (SBHs) and a weaker than expected dependenc...

Extending the metal-induced gap state model of Schottky barriers

Fermi level pinning at Schottky barriers strongly limits the minimization of contact resistances in devices and thereby limits the scaling of modern Si electronic devices, so it is useful to understand the full range of behaviors of Schottky barriers. The authors find that some semiconductor interfaces with compound metals like silicides have apparently weaker Fermi level pinning. This occurs as these metals have an underlying covalent skeleton, whose interfaces with semiconductors lead to miscoordinated defect sites that create additional localized interface states that go beyond the standard metal-induced gap states (MIGSs) model of Schottky barriers. This causes a stronger dependence of Schottky barrier height on the metal and on interface orientation. These states are argued to be an additional component needed to extend the MIGS model.

Phase dependence of Schottky barrier heights for Ge–Sb–Te and related phase-change materials

The large difference of dielectric functions between the amorphous and crystalline phases of Ge–Sb–Te based phase-change materials (PCMs) used in memory storage devices also affects their Schottky barrier heights (SBHs) and thus their electrical device properties. Here, the SBHs of each phase of Ge2Sb2Te5, GeTe, GeSe, and SnTe are found by density functional supercell calculations. The Fermi level pinning factor S calculated for the crystalline phases (with a larger dielectric constant) is smaller than their amorphous phases, agreeing well with the empirical relationship linking SBH to a dielectric constant. The relatively large dielectric constant of crystalline PCMs arises from their resonant bonding (metavalent bonding), but their pinning factor is not always as small as empirically expected. The results are useful for optimizing the design of metal contacts for Ge–Sb–Te type phase-change memory devices.

Impacts of ITO interlayer thickness on metal/n-Ge contacts

The dependence of Schottky barrier height (SBH) of metal/ITO/n-Ge contacts on ITO interlayer thickness is experimentally investigated. The SBH of metal/ITO/n-Ge contacts for various metal electrodes of Al, Cu, and Pt is estimated. It is found that the SBH of metal/ITO/n-Ge contacts decreases with increase of ITO thickness from 0 to 4nm, and the dependence of SBH on the metal work function is weak. When the thickness of ITO is larger than 4nm, ohmic metal/ITO/n-Ge contacts are obtained for different metal electrode. The formation of ohmic contact can be ascribed to high carrier concentration of ITO itself and the excellent passivation for Ge surface. The interdiffusion between cap metal and ITO should be responsible for the modulation of SBH of metal/ITO/n-Ge contacts.

Modulation of Fermi level pining position at metal/n-Ge interface by semimetal Ge1−xSnx and Sn interlayers

We have investigated the dependence on the metal work function of Schottky barrier height (SBH) of metal/Ge1−xSnx/n-Ge Schottky diode and have discussed the mechanism of SBH reduction by insertion of a Ge1−xSnx interlayer. The SBH of metal/Ge1−xSnx/n-Ge Schottky diodes for various metal electrodes of Zr, Al, and Au were estimated. Even after the insertion of Ge1−xSnx layer at various metal/n-Ge interface, the dependence of SBH on the metal work function is weak as well as direct metal/n-Ge contact. We also found that the SBH decreases by 0.1–0.2eV by the insertion of a Ge1−xSnx or Sn layer regardless of the metal work function. This result means that the SBH is determined only by the interfacial semimetal Ge1−xSnx layer. Considering that density of states at the EF and momentum of electrons in semimetal Ge1−xSnx and α-Sn are small, those epitaxial interlayers would suppress the metal induced gap state at the metal/Ge interface as well as the disorder induced gap states. Additionally, we demonstrated the reduction of the metal/n-Ge contact resistivity by introduction of a Ge1−xSnx interlayer.

Analysis of temperature dependent forward characteristics of Ni/ β-Ga2O3 Schottky diodes

In this paper, the temperature dependence of the turn-on characteristics of Schottky barrier diodes fabricated on oriented n-type β-Ga2O3 is reported. The barrier height (qΦbn) and ideality factor (n) for Ni- β-Ga2O3 was found to be 1.08 ± 0.05 eV and 1.19 respectively at room temperature. The effective Richardson constant (A**) is determined to be 42.96 A cm−2 K−2, in close agreement with the theoretical value. At low temperatures (85–273 K), the current–voltage characteristics reveal a strong temperature dependence of Schottky barrier heights and ideality factors and a corresponding deviation from the barrier height extracted from capacitance–voltage measurements. A detailed analysis is presented, which suggest that these effects can be attributed to a large barrier inhomogeneity at metal/β-Ga2O3 interfaces, possibly resulting from interfacial defects, which can be modeled using a potential fluctuation model.

Polarisation dependence of Schottky barrier heights at ferroelectric BaTiO3 / RuO2 interfaces: influence of substrate orientation and quality

In situ x-ray photoelectron spectroscopy was employed to examine the change in Schottky barrier height at BaTiO3/RuO2 interfaces upon polarisation reversal for [1 0 0], [1 1 0] and [1 1 1] oriented BaTiO3 single crystals. Compared to previous measurements on BaTiO3/RuO2 interfaces (Chen and Klein 2012 Phys. Rev. B 86 094105), the crystals exhibit a significantly reduced dependence of barrier height on polarisation direction. This is connected to a much higher polarisation of the present cystals, which is comparable to the accepted bulk polarisation of BaTiO3 of and which exhibit the expected dependence on crystal orientation. This indicates a much higher crystal quality in the present experiments, which is also confirmed by a Kolmogorov–Avrami–Ishibashi like polarisation switching dynamics. It is observed that is reduced for the [1 1 0] and [1 1 1] orientation and scales with polarisation as long as crystals from the same batch are used. The fact, that a poor polarisation hysteresis behaviour relates to a high polarisation dependence of Schottky barrier height, indicates that the electrode’s ability to screen ferroelectric polarisation charges depends sensitively on crystal and/or interface quality.

Analysis of inhomogeneous barrier and capacitance parameters for Al/rubrene/n-GaAs (100) Schottky diodes

In this paper, 5,6,11,12-tetraphenylnaphthacene (rubrene) was prepared on n type GaAs (100) substrate by spin coating. The device parameters of Al/rubrene/n-GaAs (100) Schottky diode have been investigated by means of current–voltage (I–V) characteristics in the temperature range 100–300K by steps of 50K and capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics at 1MHz and 300K. It was observed that ideality factors increased and barrier heights decreased with the decreasing temperature. The observed anomaly of temperature dependence of Schottky barrier height and ideality factor are explained by Gaussian distribution of Schottky barrier height in the same temperature ranges. Al/rubrene/n-GaAs Schottky barrier diode has been shown to have a Gaussian distribution with mean barrier height (Φ¯B) of 1.076eV and standard deviation (σs) of 0.119V. Schottky barrier height (ΦB), series resistance (Rs), and the density of interface trap states (Ns s) of the diode were calculated as 1.004eV, 1.18k Ω and 2.145×1011eV−1cm−2 for 1MHz, respectively.

Graphene-GaN Schottky diodes

The electrical characteristics of graphene Schottky contacts formed on undoped GaN semiconductors were investigated. Excellent rectifying behavior with a rectification ratio of ∼107 at ±2 V and a low reverse leakage current of 1.0 × 10−8 A/cm2 at −5 V were observed. The Schottky barrier heights, as determined by the thermionic emission model, Richardson plots, and barrier inhomogeneity model, were 0.90, 0.72, and 1.24 ± 0.13 eV, respectively. Despite the predicted low barrier height of ∼0.4 eV at the graphene-GaN interface, the formation of excellent rectifying characteristics with much larger barrier heights is attributed to the presence of a large number of surface states (1.2 × 1013 states/cm2/eV) and the internal spontaneous polarization field of GaN, resulted in a significant upward surface band bending or a bare surface barrier height as high as of 2.9 eV. Using the S parameter of 0.48 (measured from the work function dependence of Schottky barrier height) and the mean barrier height of 1.24 eV, the work function of graphene in the Au/graphene/GaN stack could be approximately estimated to be as low as 3.5 eV. The obtained results indicate that graphene is a promising candidate for use as a Schottky rectifier in GaN semiconductors with n-type conductivity.

Effect of metal intermixing on the Schottky barriers of Mo(100)/GaAs(100) interfaces

The electronic and structural properties of Mo(100)/GaAs(100) interfaces and Mo diffusion into GaAs are explored using first principle calculations. Our results show that the interface undergoes substantial atomic rearrangement with respect to the bulk structures and the bilayer of the GaAs adjacent to the interface becomes conducting. We study the n-type Schottky barrier height's dependence on Mo interdiffusion in the GaAs, with values ranging from ∼0.9 eV to ∼1.39 eV. This range is caused by the diffusants acting as additional n–type doping at the surface and their interaction with the metal-induced gap states.

Temperature-dependent electrical transport properties of (Au/Ni)/n-GaN Schottky barrier diodes

The temperature-dependent electrical properties of (Au/Ni)/n-GaN Schottky barrier diodes (SBDs)have been investigated in the wide temperature range of 40–400K. The analysis of the main electrical characteristics such as zero-bias barrier height (ΦB0), ideality factor (n) and series resistance (Rs) were found strongly temperature dependent. Such behavior is attributed to barrier inhomogeneities by assuming a Gaussian distribution (GD) of barrier heights (BHs) at the interface. It is evident that the diode parameters such as zero-bias barrier height increases and the ideality factor decreases with increasing temperature. The values of series resistance that are obtained from Cheung׳s method are decreasing with increasing temperature. The temperature dependence of Schottky barrier height (SBD) and ideality factor (n) are explained by invoking three sets of Gaussian distribution of (SBH) in the temperature ranges of 280–400K, 120–260K and 40–100K, respectively. (Au/Ni)/n-GaN Schottky barrier diode have been shown a Gaussian distribution giving mean BHs (Φ¯B0) of 1.167, 0.652 and 0.356eV and standard deviation σs of 0.178, 0.087 and 0.133V for the three temperature regions. A modified ln(I0/T2)−q2σ2/2k2T2 vs. 1/kT plot have given Φ¯B0 and A⁎ as 1.173eV and 34.750A/cm2K2, 0.671eV and 26.293A/cm2K2, 0.354eV and 10.199A/cm2K2, respectively.

Micro-structural and temperature dependent electrical characterization of Ni/GaN Schottky barrier diodes

Micro-structural investigation of Ni/GaN Schottky barrier diodes has been carried out using high-resolution transmission electron microscopy and electron diffraction spectrum in order to emphasize the role of Ni/GaN interface in controlling the Schottky diode behavior. Variable temperature Hall effect measurement of GaN samples along with the current–voltage (I–V) characteristics of Ni/n-GaN Schottky barrier diodes have been measured in 100–380 K temperature range. Results are analyzed in terms of thermionic emission theory by incorporating the concept of barrier inhomogeneity at the metal/semiconductor interface. The observed anomaly of temperature dependence of Schottky barrier height and ideality factor are explained by invoking two sets of Gaussian distribution of SBH in the temperature ranges of 100–180 K and 220–380 K, respectively. The value of A** (effective Richardson constant) as determined from the modified Richardson plot is 29.2 A/(cm2 K2), which shows an excellent agreement with the theoretical value (26.4 A/(cm2 K2)) in the temperature range of 220–380 K.

Polarization dependence of Schottky barrier heights at interfaces of ferroelectrics determined by photoelectron spectroscopy

Ferroelectric polarization in thin films is stabilized by screening charges in the metal electrodes. Imperfect screening of the polarization charge strongly modifies the film's capacitance and should lead to a variation of the Schottky barrier height at the interface with polarization direction. An experimental approach based on photoelectron spectroscopy is introduced which allows us to quantitatively determine Schottky barrier heights at ferroelectric/metal interfaces in dependence on polarization. The procedure is exemplified for BaTiO3 single crystals with RuO2 and Pt electrodes, revealing a variation of Schottky barrier height of 1.1 and 0.65 eV in dependence on polarization for RuO2 and Pt electrodes, respectively. Inhomogeneous barrier switching is observed for Pt electrodes, which may be related to defect formation during metal deposition.

Composition dependence of Schottky barrier heights and bandgap energies of GaNxAs1−x synthesized by ion implantation and pulsed-laser melting

We present a systematic investigation on the band structure of the GaNxAs1−x alloys synthesized using nitrogen ion implantation followed by pulsed-laser melting and rapid thermal annealing. The evolution of the nitrogen-concentration depth profile is consistent with liquid-phase diffusion, solute trapping at the rapidly moving solidification front, and surface evaporation. The reduction of the Schottky barrier height of the Γ-like threshold at nitrogen composition up to x=0.016 is studied by ballistic electron emission microscopy (BEEM) and determined quantitatively using the second voltage derivative BEEM spectra to be −191±63 meV per x=0.01, which is close to the corresponding slope for samples grown by low-temperature molecular beam epitaxy. This slope is also consistent with the bandgap narrowing measured on the same samples by photomodulated reflectance and is consistent with the band anticrossing model for the splitting of the conduction band in the GaNxAs1−x alloys. Lithographically patterned GaNxAs1−x dots are imaged by BEEM. Analysis of BEEM spectra of the locally confined dots indicates an alloying-induced decrease in the Schottky barrier height of four times the thermal energy at room temperature.

Schottky barrier heights of metal contacts to n-type gallium nitride with low-temperature-grown cap layer

The Schottky barrier heights of metal contacts, including WSi0.8, Cr, Ti, Pt, and Ni, on n-type gallium nitride (GaN) with a GaN cap layer grown at low-temperature (LTG) were studied. Higher barriers can be formed by introducing LTG GaN on top of the conventional structures. The higher Schottky barrier observed in samples with the LTG GaN cap layer may be due to the facts that the high-resistivity LTG GaN layer may passivate the surface defects (pits) formed from threading dislocations or it may cause the Fermi-level pinning effect at the metal/semiconductor interface, revealing a weak dependence of Schottky barrier height on the metal work function. The measured barrier heights of the LTG GaN-capped samples were 1.02–1.13eV.

Dependence of local electronic structure in p-type GaN on crystal polarity and presence of inversion domain boundaries

Scanning probe techniques including scanning capacitance microscopy, scanning capacitance spectroscopy, scanning Kelvin probe force microscopy, and atomic force microscopy have been used to assess structure and local electronic properties of Ga-face and N-face p-type GaN and of inversion domain boundaries in p-type GaN. Epitaxial layers of p-type GaN were grown by molecular-beam epitaxy, and by adjustment of the Ga:N flux ratio samples containing both Ga-face and N-face material were obtained. Under identical growth conditions, net incorporation of electrically active Mg acceptors was found to be more efficient for material with Ga-face polarity. Only a very small dependence of surface potential on polarity was observed, in contrast to results reported for n-type GaN, in which a substantial dependence of Schottky barrier height on polarity has been found. In addition, elevated net concentrations of ionized Mg acceptors were observed in Ga-face regions in the immediate vicinity of some, but not all, inversion domain boundaries, consistent with theoretical suggestions that incorporation of high concentrations of Mg within an inversion domain boundary can lead to increased concentrations of Mg acceptors near the inversion domain boundary.