Schottky Barrier Height Values Research Articles

An Atomically Resolved Schottky Barrier Height Approach for Bridging the Gap between Theory and Experiment at Metal-Semiconductor Heterojunctions.

We propose an atomically resolved approach to capture the spatial variations of the Schottky barrier height (SBH) at metal-semiconductor heterojunctions. This proposed scheme, based on atom-specific partial density of states (PDOS) calculations, further enables calculation of the effective SBH that aligns with conductance measurements. We apply this approach to study the variations of SBH at MoS2@Au heterojunctions, in which MoS2 contains conducting and semiconducting grain boundaries (GBs). Our results reveal that there are significant variations in SBH at atoms in the defected heterojunctions. Of particular interest is the fact that the SBH in some areas with extended defects approaches zero, indicating Ohmic contact. One important implication of this finding is that the effective SBH should be intrinsically dependent on the defect density and character. Remarkably, the obtained effective SBH values demonstrate good agreement with existing experimental measurements. Thus, the present study addresses two long-standing challenges associated with SBH in MoS2-metal heterojunctions: the wide variation in experimentally measured SBH values at MoS2@metal heterojunctions and the large discrepancy between density-functional-theory-predicted and experimentally measured SBH values. Our proposed approach points out a valuable pathway for understanding and manipulating SBHs at metal-semiconductor heterojunctions.

On the temperature dependence of the current conduction mode in non-homogeneous Pt/n-GaN Schottky barrier diode

Forward current-voltage (I–V) measurements of Pt/n-GaN Schottky barrier diode (SBD) are investigated in a wide temperature range (80–400 K). The temperature dependence of the effective Schottky barrier height (SBH) and ideality factor are analyzed on the basis of the thermionic emission (TE) model by considering a double Gaussian distribution (DGD) of SBH due to the presence of barrier height inhomogeneities at the Pt/n-GaN interface. In the high temperature (HT) region (200–400 K), Pt/n-GaN SBD exhibits nearly an ideal behavior in accordance with the TE conduction mode. The obtained values of the homogeneous SBH Φ‾0bn = 1.33 eV and Richardson constant AHT* = 26.86 A cm−2 K−2are in agreement with the theoretical values for n-type GaN. In the low temperature (LT) region (80–160 K), the corresponding values Φ‾0bn = 0.94 eV and ALT* = 14.74 A cm−2 K−2 deduced within the TE mode deviate strongly from the ideal behavior. This deviation is further confirmed by the large ideality factor. In this work, we provide a strong evidence that the tunneling current including thermionic field emission (TFE) largely dominates the conduction mode at low temperature.

First‐Principles Study of Schottky Barrier Heights on Metal/4H‐SiC Polar Interfaces

4H‐SiC based devices have garnered significant interest within the realm of high‐voltage and high‐frequency electronic equipment. Schottky barrier heights (SBHs) play a pivotal role in determining the properties of metal/4H‐SiC contacts and holding significance for electrical performance of 4H‐SiC devices. Herein, the first‐principles method is utilized to investigate the interface properties between various metals (Sc, Mg, Zr, Ag, Al, Ru, Rh, Pd, Ni, Au, Ir, and Pt) and 4H‐SiC. The results reveal an approximate linear relationship between SBH and work function of these metals. Fermi‐level pinning factors are determined to be 0.27 and 0.32 for the Si‐face and C‐face interfaces, respectively. The disparity in pinning factors arises from polarization differences between the two interfaces, which stem from uneven net charge distribution on the Si‐face and C‐face. Meanwhile, the SBH values indicate a strong pinning effect, due to the charge transfer occurring at the metal/4H‐SiC interface. Through further analysis of layer density and differential charge density, the interatomic interaction and charge transfer between metal and SiC atoms are analyzed. This work offers fundamental insights into the structural changes in metal/4H‐SiC interfaces and provides valuable guidance in the potential prediction and optimization of device performance.

Synthesis of Template-free Flower-like ZnO Nanorods using a Simple Chemical Bath Technique

Flower-like ZnO nanorods were successfully synthesised using a template-free, low-temperature chemical bath deposition method. The effects of growth time on the structural, morphological, and vibrational properties of flower-like zinc oxide were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated that the crystallite size and density of flower-like ZnO nanorods increased with increasing growth time. Raman spectroscopy confirmed the existence of specific Raman modes that correspond to the wurtzite structure of ZnO. In addition, Raman measurements revealed that increased growth time increases the Raman modes intensity of the flower-like ZnO nanorods. Moreover, the measurements of the Au/ZnO nanorods/FTO Schottky diode showed that all devices functioned as Schottky diodes and rectifying; however, their ideality factor was greater than 1. In addition to the ideality factor, the Schottky barrier height was calculated and slightly increased with growth time. The values of Schottky barrier height obtained were 0.502 eV, 0.523 eV, 0.516 eV and 0.529 eV for samples grown at 1, 2, 3 and 4 h, respectively.

Optical spectroscopic detection of Schottky barrier height at a two-dimensional transition-metal dichalcogenide/metal interface.

Atomically thin two-dimensional transition-metal dichalcogenides (2D-TMDs) have emerged as semiconductors for next-generation nanoelectronics. As 2D-TMD-based devices typically utilize metals as the contacts, it is crucial to understand the properties of the 2D-TMD/metal interface, including the characteristics of the Schottky barriers formed at the semiconductor-metal junction. Conventional methods for investigating the Schottky barrier height (SBH) at these interfaces predominantly rely on contact-based electrical measurements with complex gating structures. In this study, we introduce an all-optical approach for non-contact measurement of the SBH, utilizing high-quality WS2/Au heterostructures as a model system. Our approach employs a below-bandgap pump to excite hot carriers from the gold into WS2 with varying thicknesses. By monitoring the resultant carrier density changes within the WS2 layers with a broadband probe, we traced the dynamics and magnitude of charge transfer across the interface. A systematic sweep of the pump wavelength enables us to determine the SBH values and unveil an inverse relationship between the SBH and the thickness of the WS2 layers. First-principles calculations reveal the correlation between the probability of injection and the density of states near the conduction band minimum of WS2. The versatile optical methodology for probing TMD/metal interfaces can shed light on the intricate charge transfer characteristics within various 2D heterostructures, facilitating the development of more efficient and scalable nano-electronic and optoelectronic technologies.

In-Depth Analysis on Self Alignment Effect of the Fermi-Level Using Graphene on Both n- and p-Type Semiconductors.

Optimizing the contact structure while reducing the contact resistance in advanced transistors has become an extremely challenging problem. Because the existing techniques are limited to controlling only one semiconductor type, either n- or p-type, owing to their work function differences, significant challenges are encountered in the integration of a contact structure and metal suitable for both n- and p-type semiconductors. This is a formidable drawback of the complementary metal-oxide-semiconductor (CMOS) technology. In this paper, we demonstrate the effectiveness of a metal/graphene/semiconductor (MGrS) as a universal source/drain contact structure for both n- and p-type transistors. The MGrS contact structure significantly enhanced the reverse current density (JR) and reduced the Schottky barrier height (SBH) for both semiconductor types. From the analysis of the SBH values and their relationship with the metal work function, which refers to the S-parameter, the van der Waals contact of graphene (Gr) effectively alleviated the Fermi level (FL) pinning for both semiconductor types, reducing the metal-induced gap states (MIGS) at the Gr/semiconductor interface. Furthermore, Gr effectively modulated the work function of the contact metal to yield a position favorable for each semiconductor type. Consequently, a single MGrS contact structure on a Si substrate resulted in excellent Ohmic contacts in both n- and p-type Si, with SBH values reduced to 0.012 and 0.024 eV for n- and p-type Si, respectively. This new approach for integrating the contact structures of semiconductor types will lead to extended capabilities for high-performance device applications and CMOS logical circuitry.

Quaternary, layered, 2D chalcogenide, Mo1−x W x SSe: thickness dependent transport properties

Highly oriented, single crystalline, quaternary alloy chalcogenide crystal, Mo x W1−x S2y Se2(1−y), is synthesized using a high temperature chemical vapor transport technique and its transport properties studied over a wide temperature range. Field effect transistors (FET) with bottom gated configuration are fabricated using Mo0.5W0.5SSe flakes of different thicknesses, from a single layer to bulk. The FET characteristics are thickness tunable, with thin flakes (1–4 layers) exhibiting n-type transport behaviour while ambipolar transfer characteristics are observed for thicker flakes (>90 layers). Ambipolar behavior with the dominance of n-type over p-type transport is noted for devices fabricated with layers between 9 and 90. The devices with flake thickness ∼9 layers exhibit a maximum electron mobility 63 ± 4 cm2 V−1s−1 and an I ON/I OFF ratio >108. A maximum hole mobility 10.3 ± 0.4 cm2 V−1s−1 is observed for the devices with flake thickness ∼94 layers with I ON/I OFF ratio >102–103 observed for the hole conduction. A maximum I ON/I OFF for hole conduction, 104 is obtained for the devices fabricated with flakes of thickness ∼7–19 layers. The electron Schottky barrier height values are determined to be ∼23.3 meV and ∼74 meV for 2 layer and 94 layers flakes respectively, as measured using low temperature measurements. This indicates that an increase in hole current with thickness is likely to be due to lowering of the band gap as a function of thickness. Furthermore, the contact resistance (R ct) is evaluated using transmission line model (TLM) and is found to be 14 kohm.μm. These results suggest that quaternary alloys of Mo0.5W0.5SSe are potential candidates for various electronic/optoelectronic devices where properties and performance can be tuned within a single composition.

Electrical Properties of Ga2O3 Schottky Barrier Diodes with and without Mesa Structure

Devices based on wide bandgap (WBG) semiconductors like silicon carbide (SiC), gallium nitride (GaN), and gallium oxide (Ga2O3) are ideal for high-power electronics in harsh environments. Among the WBG semiconductors, ultrawide bandgap (UWBG) β-phase gallium oxide (Ga2O3), EG ≈ 4.8 eV, is emerging as a replacement for the current commercially available wide bandgap (WBG) power electronics due to its generational improvements in performance and manufacturing cost [1]. Ga2O3 has a high theoretical breakdown electrical field of 8 MV/cm which in turn gives its Baliga’s figure of merit for power devices that is larger than other WBG materials. The availability of high-quality Ga2O3 substrates produced from melt-grown bulk single crystals also facilitates the development of vertical power devices.In the current stage, the vertical Schottky barrier diode (SBD) with Ga2O3 can have an improved current spreading effect when compared to power diode devices from Si, SiC, and GaN. Especially, the Ga2O3 is expected to surpass the trade-off relationship between breakdown voltage (BV) and on resistance (Ron,sp) that other materials have [2]. Nevertheless, the Ga2O3 vertical SBD still cannot achieve the theoretical breakdown electric field. One of the key topics for WBG semiconductors application is a structure for improved electrical management such as field rings, junction termination extension, and field plates to reduce the leakage current in the reverse bias state.Here, we investigate the characteristics of Ga2O3 SBDs with and without mesa structure in extreme environments at high and low temperature. Conventional and mesa Ga2O3 SBDs were fabricated on the halide vapor phase epitaxy (HVPE) grown β-Ga2O3 (001). A Sn-doped substrate with 61016 cm-3 was used for the HVPE growth. In the SBD with mesa structure, the circular mesa with a diameter of 162 μm and a depth of 500 nm was formed around anode electrodes. The Ti/Au metal stack on the back side of the substrate acted as a cathode and the anode electrode deposited Ni/Au/Pt layers. After the fabrication process, current-voltage (I-V) measurements were performed on the SBDs between -5 V and 1 V. From the results, the values of Ron,sp at 0.7 V are 46.4 Ω•cm2 and 46.2 Ω•cm2 in conventional and mesa SBDs, respectively. And additionally, the leakage current at -5 V is reduced by approximately 44.3% in the mesa structure. To obtain the Schottky barrier height (SBH) from I-V and capacitance-voltage (C-V) measurements, the C-V was observed in the range of -5 V to 0.5 V at 100 kHz. The SBH values from I-V and C-V measurements, the mesa SBDs have 1% - 5% larger SBH compared to the conventional structure, and it could be related to lower leakage current. Moreover, the depletion depths of conventional and mesa SBDs were 52.2 nm and 57.8 nm, respectively, from C-V measurements. For considering the electrical characteristics in extreme temperatures, the I-V measurements of both SBDs structure will be measured between 75 K to 525 K. From an initial low temperature measurement series, we determine a Richardson constant of 49 A/cm2 comparable to what has been reported by other groups. Furthermore, we will extend the study by performing deep-level transient spectroscopy (DLTS) to understand the defect information in Ga2O3.

Non-contact approach to extract Schottky barrier characteristics for gas sensitive nanostructured interfaces

Formation of high precision nanorod based surface morphologies can be engineered with nanotechnology to mimic the function of sensory hairs. Gas sensitive I-V characteristics unique to these surfaces require initialisation under voltage bias and temperature elevation to exploit their band structures and therefore achieve optimal gas response. Prior to detection, devising effective approaches to optimise nanorod based sensors can provide access and efficient control to downsize and significantly miniaturise current gas sensors. In this work, we devised a novel non-contact analysis approach via ultraviolet photoelectron spectroscopy (UPS) to extract specific band structure parameters specifically to locate particular bias for optimal sensor initialisation. The results from five cases all indicate how fabricated nanostructured Schottky barrier heights can be evaluated from UPS data and how their I-V data are correlated and also validate our band structure postulations. The obtained Schottky barrier height values from simulated band structures provided an optimal biasing condition of 0.8 V for Au-ZnO nanorod hydrogen gas sensory operation at 150 °C, demonstrating experimental validity with gas-electrical measurements. Further, the gas response data show exponential dependence with gas concentration which is highly relevant for programmable trigger response controlled sensing function based applications.

Schottky barrier heights and electronic transport in Ga2O3 Schottky diodes

The Schottky contact, formed at the interface between a metal and a semiconductor, is instrumental in defining the electrical properties of Schottky barrier diodes (SBDs). The characteristics of the Schottky contact are contingent on the properties of interacting metal and semiconductor properties. Herein, we studied the carrier-transport mechanisms and electrical characteristics at room and elevated temperatures. These SBDs employ pre-treated Ga2O3 thin films and either Ni or Au Schottky contacts. The SBDs pre-treated (pre-T) via annealing at 900 °C under an N2 atmosphere for the Ni contact showed highest on/off ratio at room temperature. They also demonstrated ideality factors and Schottky barrier heights (SBHs) that remained relatively stable between 298 K and 523 K. To ascertain the SBH, ideality factors (n) derived from the thermionic emission (TE) and thermionic field emission (TFE) models were used, and results were subsequently compared. Moreover, SBDs employing Ni as the anode material exhibited lower SBHs than those employing Au. The pre-T Ni SBD was best described by the TFE model, wherein the SBH and ideality factor varied by 0.14 eV and 0.13, respectively, between 298 K and 523 K. Conversely, for pre-T Au, untreated Ni, and untreated Au SBDs, neither TE and TFE provided a satisfactory fit due to the ideality factor is greater than 2 at room temperature and the variation of SBH and n with temperature. These suggests that the transport mechanism should be described by other physical mechanisms. Without pre-treatment, both the Ni and Au SBDs exhibited more significant variation in the SBH and n with temperature. SBHs values were determined using measurement of current, capacitance and x-ray photoelectron spectroscopy, and were found to depend on the interface quality, indicating inhomogeneous SBH. Our results suggest that the use of annealing pre-treatments and anode metals with low work functions holds considerable potential for reducing Schottky barrier heights in Schottky diodes, thereby enhancing their electrical performance.

Modeling for ammonia gas concentration detection of GaN-based sensors

Excellent properties of gallium nitride (GaN) make it an ideal material for realizing gas sensors, especially for ammonia (NH[Formula: see text] detection. Although many researchers have pursued to describe the characteristics of GaN-based NH3 gas sensors by different approaches, few models have been reported. In this paper, with the consideration of the exponential distribution of interfacial states, a model for ammonia concentration detection of GaN gas sensors has been presented. The Poisson equation is applied to model the effect of defect states on the potential. By taking advantage of the current-voltage characteristics, the value of Schottky barrier height can be obtained. The concentration of the adsorbed NH3 gas is derived by exploiting the surface potential. It indicates that densities of acceptor interfacial trap states are in the order of 10[Formula: see text][Formula: see text]cm[Formula: see text]eV[Formula: see text]. The current increases with the NH3 concentration at the same applied voltage. In addition, detailed investigations of physical mechanisms and the analysis of the sensitivity have been depicted. It shows that the sensitivity followed an approximately exponential dependence on NH3 density. Results compared well with experimental data that verify the proposed model and simulation method.

Inhomogeneous heterojunction performance of Zr/diamond Schottky diode with Gaussian distribution of barrier heights for high sensitivity temperature sensor

Herein, we demonstrated the fabrication and operation of vertical diamond Schottky barrier diode (SBD) as a high-performance temperature sensor. The current transport mechanism and temperature sensor properties of Zr/p-diamond SBD were comprehensively investigated with operating temperatures up to 448 K. At room temperature, Zr/p-diamond SBD exhibits outstanding performances with a high rectification ratio of 2.28 × 1010, a low specific on-resistance of 0.5 mΩ•cm2, a large Schottky barrier height (SBH) of 1.43 eV, a low ideality factor of 1.77, and an extremely low saturation current of 4.42 × 10−22 A. The enhanced-performance with a higher forward current and a lower reverse leakage current is observed to be across a wide temperature range from 323 to 448 K. Additionally, a significant improvement of rectifying current curve with a highest rectification ratio over 1012 is obtained at 423 K. The SBH increases whereas the ideality factor decreases with temperature increasing, indicating barrier inhomogeneities at Zr/p-diamond interface. The temperature-dependent electrical transport characteristics on spatially inhomogeneous Schottky contacts can be successfully described by potential fluctuations model. The extracted original value of Richardson constant (A*) is 0.0153 A/cm2•K2, which is much lesser than theoretical value of 96 A/cm2•K2. The discrepancy of A* value from theoretical value has been well explained by thermionic emission model with Gaussian distribution of barrier inhomogeneity. The values of mean SBH and A* extracted from modified Richardson plot depending on SBH inhomogeneity are 1.76 eV and 105.51 A/cm2•K2, respectively. With a Gaussian distribution take into account, the A* value is close to the theoretical value, and confirmed the existence of SBH inhomogeneity at Zr/p-diamond interface. Furthermore, for Zr/p-diamond SBD operating as a temperature sensor, the voltage drop across the diode decreases linearly with increasing temperature at a specific forward current level, contributing to a highest sensor sensitivity of 6.8 mV/K. The reported results in this work are among the best to date in diamond SBD temperature sensor. Although not optimized for temperature sensor, the high and thermally stable rectifying barriers make Zr/p-diamond SBD a strong candidates for high-performance diamond rectifying diode and temperature sensor.

Natural band alignment of BAlN and BGaN alloys

The natural band alignment of BAlN and BGaN alloys was investigated using the atomic solid-state energy scale approach. The band edge positions relative to the vacuum level were determined for BAlN and BGaN alloys, and the band offset values for each heterostructure were estimated. The results suggest that the natural band alignment of BAlN and BGaN alloys behaves according to the common anion rule. Further, the Schottky barrier height (SBH) was calculated based on the results of band alignment for BAlN and BGaN alloys. The predicted SBH values are expected to be an important guideline for boron nitride and its related alloy device design.

The Variation of Schottky Barrier Height Induced by the Phase Separation of InAlAs Layers on InP HEMT Devices

We investigated the effect of phase separation on the Schottky barrier height (SBH) of InAlAs layers grown by metal–organic chemical vapor deposition. The phase separation into the In-rich InAlAs column and Al-rich InAlAs column of In0.52Al0.48As layers was observed when we grew them at a relatively low temperature of below 600 °C. From the photoluminescence spectrum investigation, we found that the band-gap energy decreased from 1.48 eV for a homogeneous In0.52Al0.48As sample to 1.19 eV for a phase-separated InxAl1−xAs sample due to the band-gap lowering effect by In-rich InxAl1−xAs (x > 0.7) region. From the current density–voltage analysis of the InAlAs Schottky diode, it was confirmed that the phase-separated InAlAs layers showed a lower SBH value of about 240 meV than for the normal InAlAs layers. The reduction in SBH arising from the phase separation of InAlAs layers resulted in the larger leakage current in InAlAs Schottky diodes.

Phase Separation Induced Schottky Barrier Height Change in InAlAs/InP Heterostructure-based HEMT Devices

The effect of phase separation phenomenon in InAlAs layers grown by metal-organic chemical vapor deposition on the Schottky barrier height (SBH) is investigated. The phase separation into In-rich and Al-rich InxAlyAs columns of InAlAs (x = 0.52, y = 0.48) layers was observed when we grow them at a relatively low temperature below 600 °C. From the photoluminescence spectrum investigation, we found that the band-gap energy decreased from 1.48 eV for a homogeneous In0.52Al0.48As sample to 1.19 eV for a phase-separated InxAl1-xAs sample due to the band-gap lowering effect by In-rich InxAl1-xAs (x > 0.7) region. From the current density-voltage analysis of the InAlAs Schottky diode, it was also confirmed that the phase-separated InAlAs layers showed a lower SBH value of about 250 meV than it for the normal InAlAs layers. The reduction of SBH arising from the phase separation of InAlAs layers resulted in the larger leakage current in InAlAs Schottky diodes.

Current-voltage-measurement temperature characteristics depending on the barrier-forming contact metal thickness in Au/Cu/n-Si/Au–Sb/Ni rectifying contacts

We experimentally have introduced the current–voltage-temperature (I–V-T) characteristics of the Au/n-type Si (D1), Au/Cu/n-Si (D2), Au/Cu(4 nm)/n-Si (D3) and Au/Cu(2 nm)/n-Si (D4) Schottky-barrier diodes (SBDs) in the temperature range of 40–320 K with the step of 10 K. The copper Schottky contact (SC) dots for the D2, D3 and D4 SBDs have the thickness of 100 nm, 4 nm and 2 nm, respectively. The top Au contact has been used as a shield to prevent oxidation of the copper SC metal. The Schottky barrier height (SBH) Φb0 values of 0.61 eV, 0.66 eV and 0.69 eV, and the ideality factor values of 1.04, 1.04 and 1.02 for the D2, D3 and D4 SBDs have been obtained at 300 K, respectively. The SBH value has increased with decreasing Cu thin film thickness. An average series resistance Rs value of 10.6 Ω has been obtained for the Au/Cu/n-Si diodes at 300 K. The obtained Rs and the ideality factor values are almost independent of the SC metal thickness. In the SBH vs temperature plots, the SBH value in the D3 and D4 diodes with 2 nm and 4 nm metal thickness almost remained unchanged from 320 K to 120 K, and in the D2 SBD from 320 K to 160 K. Therefore, it can be said from these results that the quality of the SBDs has increased with decrease in Cu Schottky film thickness.

Effects of Carbon Incorporation on Electrical Characteristics and Thermal Stability of Ti/TiO2/n-Ge MIS Contact

The effects of carbon incorporation on the thermal stability of the interfacial TiO₂ layer and the electrical characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were investigated. The improved thermal stability and contact characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were characterized in terms of Schottky barrier height (SBH) and specific contact resistivity (&#x03C1;_<i>c</i>) using the Schottky diode and circular transmission line model (CTLM). The values of SBH and &#x03C1;_<i>c</i> increased after the rapid thermal annealing (RTA) above 550 &#x00B0;C. The current density&#x2013;bias voltage (<i>J&#x2013;V</i>) curves of the Schottky diode showed a change of contact characteristics from Ohmic-like behavior to rectifying. This thermal instability was mainly caused by the decomposition of the interfacial TiO₂ layer after high-temperature annealing. The structural degradation was confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses. When carbon ions were incorporated into the interfacial TiO₂ layer, the SBH and &#x03C1;_<i>c</i> values showed relatively stable characteristics as the RTA temperature increased up to 600 &#x00B0;C. The EELS mapping showed that the diffusion of oxygen from the interfacial TiO₂ layer was effectively suppressed thanks to the incorporation of carbon. Thus, the carbon incorporation can improve the thermal stability of the interfacial TiO₂ layer and the metal&#x2013;insulator&#x2013;semiconductor contact characteristics for Ge-based device applications.

First principle study on modulating of Schottky barrier at metal/4H-SiC interface by graphene intercalation

In the production of SiC electronic devices, one of the main challenges is the fabrication of good Ohmic contacts due to the difficulty in finding the metals with low Schottky barriers of wide band gap SiC. Therefore, reducing the Schottky barrier height (SBH) at the metal/SiC interface is of great importance. In this paper, the effects of graphene intercalation on the SBH in different metals (Ag, Ti, Cu, Pd, Ni, Pt)/4H-SiC interfaces are studied by combining the average electrostatic potential and local density of states calculation methods based on first-principles plane wave pseudopotential density functional theory. The calculation results show that single-layer graphene intercalation can reduce the SBH of metal/4H-SiC contact. When the two layers of graphene are inserted, the SBH are further reduced. Especially, the contact between Ni and Ti exhibits negative SBH values, inferring that good Ohmic contacts are formed. When layers of graphene continue to increase, the SBH no longer changes obviously. By analyzing the differential charge density and the local density of states of the interface, the mechanism of SBH reduction may be that the dangling bonds on the SiC surface are saturated by the graphene C atoms and the influence of the metal-induced energy gap state at the interface is reduced, thereby reducing the interface state density. In addition, graphene and the corresponding new phases at the interface have low work functions. Moreover, an interfacial electric dipole layer may be formed at the SiC/graphene interface which also contributes to barrier reduction.

Characteristics of Al/Ge Schottky and ohmic contacts at low temperatures

Schottky barrier contact has been fabricated by thermal deposition of Al on (100) p-Ge (impurity concentratioñ1010/cm3 at 80K) that shows n-type conductivity above 180K due to thermally generated carriers. Both p and n-type Ge exhibits ideal Schottky behavior with low reverse current and near unity ideality factors obtained from the linear form of temperature dependent current-voltage (I–V) characteristics. The diode current at various temperatures change its direction at non-zero applied bias that reflects a shift in position of charge neutrality level (CNL) from the Fermi level of Ge. With the rise in temperature, Schottky barrier height (SBH) steadily increases for p-Ge that can be understood on the basis of observed variation in CNL. Values of SBH determined from the zero bias Richardson plot agrees well with that estimated from the Schottky-Mott rule for strongly pinned interface. Activation energies are determined from the Richardson plot at various forward voltages and found to decrease with applied bias for n-Ge but reduces to zero for p-Ge that shows work function similar to Al. Annealing of Al/Ge induces regrowth of p-type Al doped Ge layer that exhibits gradual reduction of Al concentration towards p-Ge crystal. Al doped Ge (p+)/Ge (p) junction thus fabricated shows linear I–V characteristics in the extrinsic region (below 180K). In the intrinsic region (above 180K), rectification is observed in the I–V curve due to temperature dependent change in conductivity of both Al doped Ge layer and Ge crystal.

Vertical GaN Schottky barrier diodes with area-selectively deposited p-NiO guard ring termination structure

In this study, the effect of p-NiO guard ring length on the properties of vertical GaN Schottky barrier diode was investigated extensively. The forward biased current-voltage characteristics of all diodes with different guard ring length follow the thermionic emission theory, showing nearly the same ideality factor and Schottky barrier height value. While the series resistance increases by nearly two times with the increasing guard ring lengths because the increasing depletion region at the p-NiO/n-GaN junction compresses the conduction path. Under reverse bias, the breakdown voltage of the SBD without any termination is relatively low due to the electric field crowding. The p-NiO guard ring is effective to enhance the breakdown voltage although the leakage current is comparable. However, the breakdown voltage decreases with the increasing guard ring length, which may be ascribed to the tunneling leakage caused by the imperfect p-NiO/n-GaN interface.