Schottky Barrier Height Reduction Research Articles

Mechanism of Schottky barrier height modulation by thin dielectric insertion on n-type germanium

Although high channel electron mobility has been reported after some passivation techniques, the performance of n-channel Ge metal-oxide-semiconductor field-effect transistor is still limited by the high Schottky barrier height at the metal/n-Ge contact interface, which comes from the Fermi level pinning effect. Recent experiments demonstrated that the Schottky barrier height can be reduced by inserting a thin dielectric layer between metal and Ge. However, the mechanism has not been well clarified. In this paper, the metal induced gap state model, the dipole layer model, and the fixed charge model are verified by varying contact metals, dielectric thicknesses, as well as the annealing temperatures. The pinning factor is improved slightly by dielectric insertion but its value is independent of the dielectric thickness and is still much lower than the ideal value of the non-pinning case. This pinning effect is consistent with the Fermi level pinning at the metal/TiO2 interface. After thermal process, no interfacial layer forms at the TiO2/Ge interface and the TiO2 crystallizes gradually after annealing but the Schottky barrier height increases. Since the amount of fixed charges in the thin dielectric layer estimated from a metal-insulator-semiconductor structure is about 2 × 1011 cm−2 and is insufficient to produce the observed 0.5 eV Schottky barrier height reduction, it is thus recommended that the main mechanism comes from the change of interface dipoles and the annealing effect is attributed to the short-range ordering of the TiO2 layer. Furthermore, dielectric with low conduction band offset which has good thermal stability should be explored.

Enhanced performance of GaN-based light-emitting diodes with graphene/Ag nanowires hybrid films

Incorporating Ag nanowires with graphene resulted in improved electrical conductivity and enhanced contact properties between graphene and p-GaN. The graphene/AgNWs hybrid films exhibited high transmittance and lower sheet resistance compared to bare graphene. The specific contact resistance between graphene and p-GaN reduced nearly an order of magnitude with the introduction of AgNWs. As a result, light emitting diodes based on the hybrid films showed 44% lower forward voltage and 2-fold higher light output power. The enhanced performance was attributed to the bridging by AgNWs of cracks, grain boundaries in graphene and the reduction of Schottky barrier height at graphene/ p-GaN interface.

Carrier Transport Mechanism at the Interface between Metals and p-Type III–Nitrides Having Different Surface Electronic Structures

We have investigated the carrier transport mechanism for nonalloyed Ni/Au ohmic contacts to p-In0.15Ga0.85N/GaN superlattices (SLs) and Mg delta (δ)-doped GaN using current–voltage–temperature (I–V–T) and specific contact resistance–temperature (Rsc–T) data. The p-SL and Mg δ-doped GaN contacts show a linear I–V behavior, indicating that the Mg δ doping in the GaN matrix as well as p-SL is very effective in improving the electrical properties of p-ohmic contacts. From the Rsc–T and I–V–T measurements, the p-SL and Mg delta-doped GaN contacts show slight temperature-dependence characteristics at the temperatures of ≥293 K. These different behaviors can be described in terms of a reduced energy band-bending, an increased hole carrier density, and a reduced Schottky barrier height.

Local structural phase determination of Ni silicide thin films using EXAFS

AbstractThe local structural characterization of novel contact materials (nickel silicides) on N2+ implanted Si(100) substrates have been performed using Extended X‐ray‐Absorption Fine‐Structure Spectroscopy (EXAFS). The crystal phases of nickel silicides that are stabilized on Si (100) substrates (NiSi or NiSi2) upon varying implantation and post deposition annealing conditions were identified. The questions addressed are whether the final stabilized crystal phases of the nickel silicide layers could be identified and the fraction of the various crystal symmetries could be determined. The identification of the stabilized crystal phases in the nickel silicide formation process is crucial, as it was shown that a Si rich Ni silicide layer at the NiSix/Si interface causes a significant reduction in the electron Schottky barrier height (SBH). EXAFS analyses using experimental and theoretical references were performed on nickel silicide thin films on silicon substrates exposed to different N2+ ion doses prior to nickel deposition and silicidation processes. Using EXAFS analyses, the stabilized phases (e.g. NiSi vs. NiSi2, etc.) and their ratios were determined. EXAFS was proved to be sensitive to the subtle modifications of the crystal structures introduced by these modifications in the processing conditions. EXAFS analyses of the experimental reference structures (NiSi, NiSi2 powder samples) clearly exhibited the robust differences in the Fourier Transformed (FT) EXAFS data in the first shell region. EXAFS fits using theoretical structures corresponding to NiSi and NiSi2 symmetries have successfully identified the distinct crystal phases present in nickel silicide thin films subjected to different doses of N2+ ion implantation. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Schottky Barrier Height reduction using strained silicon-on-insulator and dopant segregation

Schottky Barrier Height (SBH) lowering at the source/drain contacts appears as an inescapable requirement for CMOS downscaling in the sub-20nm regime that legitimates metallic source/drain technology as a relevant solution. For that sake, we have developed a test structure which enables the extraction of the effective SBH by the combination of experimental data to a transport model taking into account thermionic and tunnel emission. In this study, we demonstrate the compatibility and the cumulative effects of two SBH lowering techniques: dopant segregation integration and wafer-based biaxial tensile strain.

Selenium Segregation for Effective Schottky Barrier Height Reduction in NiGe/n–Ge Contacts

In this letter, we report the demonstration of an effective electron Schottky barrier height (ΦBn) reduction technology for NiGe/n-type Germanium (n-Ge) contacts using ion implantation of selenium (Se) followed by its segregation at NiGe/n-Ge interface. Se was found to segregate at NiGe/n-Ge interface after germanide formation. Nickel monogermanide was formed using a 350°C 30-s anneal. Se segregation gives ΦBn as low as ~0.13 eV.

RF dynamic and noise performance of Metallic Source/Drain SOI n-MOSFETs

This paper presents a detailed study of the RF and noise performance of n-type Schottky barrier (SB) MOSFETs with a particular focus on the influence of the Schottky barrier height (SBH) on the main dynamic and noise figures of merit. With this aim, a 2D Monte Carlo simulator including tunnelling transport across Schottky interfaces has been developed, with special care to consider quantum transmission coefficients and the influence of image charge effects at the Schottky junctions. Particular attention is paid to the microscopic transport features, including carrier mean free paths or number of scattering events along the channel for investigating the optimization of the device topology and the strategic concepts related to the noise performance of this new architecture. A more effective control of the gate electrode over drain current for low SBH (discussed in terms of internal physical quantities) is translated into an enhanced transconductance gm, cut-off frequency fT, and non-quasistatic dynamic parameters. The drain and gate intrinsic noise sources show a noteworthy degradation with the SBH reduction due to the increased current, influence of hot carriers and reduced number of phonon scatterings. However, the results evidence that this effect is counterbalanced by the extremely improved dynamic performance in terms of gm and fT. Therefore, the deterioration of the intrinsic noise performance of the SB-MOSFET has no significant impact on high-frequency noise FoMs as NFmin, Rn and Gass for low SBH and large gate overdrive conditions. The role of the SBH on Γopt, optimum noise reactance and susceptance has been also analyzed.

Impacts of image force on the Schottky barrier height at metal-carbon nanotube contacts

By using first-principles calculations and classical image force model, we studied the image force induced Schottky barrier (SB) height reduction in carbon nanotube (CNT)-metal contact. A low dielectric constant leads to a stronger band bending and therefore strongly reduces the SB height. This reduction depends on carrier concentration and CNT diameter. For CNT with carrier concentration of 1020 cm−3, the reduction in SB height caused by image force is up to 0.7 eV when CNT diameter is less than 20 Å. Our results demonstrate that image force induced SB height reduction is important for the design of CNT-based nanoelectronic devices.

Hydrogen Detection with Noble Metal-TiO2 Schottky Diodes

Fast detection of hydrogen at a wide concentration range is desired for many applications. We report detecting hydrogen using Au-TiO2-Ti and Ag-TiO2-Ti diodes. While hydrogen-Au and hydrogen-Ag interactions are very different, at a constant biasing voltage, the measured current in both diodes is highly sensitive to the partial pressure of hydrogen contamination in the surrounding atmosphere. Work function variations were investigated by connecting the I-V specifications to the energy barrier height established at the Au-TiO2 and Ag-TiO2 junctions. Electronic features of the devices were described based on the assumption of two different hydrogen-noble metal interactions: Hydrogen reduces Ag work function by reducing the adsorbed oxygen species from the silver surface, while Au work function is reduced by the same mechanism as well as the direct adsorption of hydrogen species to the gold surface. Both of these mechanisms result in hydrogen detection by Schottky barrier height reduction and current increase.

불소계열 플라즈마 처리를 통한 수직형 UV LED용 ITO/Al 기반 반사전극의 전기적/광학적 특성 최적화

We optimize electrical and optical properties of thermal and SF6 plasma treated indium tin oxide (ITO)/Al based reflector for high-power ultraviolet (UV) light-emitting diodes (LEDs). After thermal and <TEX>$SF_6$</TEX> plasma treatments of ITO/Al reflector, the specific contact resistance decreased from <TEX>$1.04{\times}10^{-3}\;{\Omega}{\cdot}cm^2$</TEX> to <TEX>$9.21{\times}10^{-4}\;{\Omega}{\cdot}cm^2$</TEX>, while the reflectance increased from 58% to 70% at the 365 nm wavelength. The low resistance and high reflectance of ITO/Al reflector are attributed to the reduced Schottky barrier height (SBH) between the ITO and AlGaN by large electronegativity of fluorine species and reduced interface roughness between the ITO and Al, respectively.

Contact Resistance Reduction to FinFET Source/Drain Using Novel Dielectric Dipole Schottky Barrier Height Modulation Method

Recent experiments have demonstrated the ability to alleviate Fermi-level pinning, resulting in reduced Schottky barrier heights (SBHs) and reduced contact resistivity by inserting thin layers of dielectric at the contact interface. In this letter, FinFETs with dielectric SBH tuning layers are investigated and shown to have reduced contact resistance over the control wafer. The reduced contact resistivity results in an ≈25% increase in drive current as well as a reduction of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/D</sub> by 100 Ω · μm. Contact chain measurement shows a 10-Ω · μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> reduction in specific contact resistivity over the control wafer associated with a 100-meV reduction in SBH. Routes to further improvements in device performance are discussed, including key material considerations for dielectric tuning layers.

Schottky barrier height reduction for metal/n-GaSb contact by inserting TiO2 interfacial layer with low tunneling resistance

Fermi level pinning near GaSb valence band edge leads to high Schottky barrier height for metal/n-type GaSb contacts. However, this effect can be alleviated by depinning of the Fermi level with the introduction of thin interfacial dielectric. In this paper, the use of TiO2 allows depinning of the Fermi level without introducing excessive tunneling resistance due to the low conduction band offset, estimated by synchrotron radiation photoemission spectroscopy. It is shown the insertion of TiO2 results in reduction in Schottky barrier height and greater than four orders of magnitude increase in current density for metal contacts on n-type GaSb.

Ohmic contact behaviour of Co/C/4H-SiC structures

The electrical contact properties of Co/4H-SiC structures are investigated. A carbon interfacial layer between a Co film and SiC is used to improve the Ohmic contact properties significantly. The C film is deposited prior to Co film deposition on SiC using DC sputtering. The high quality Ohmic contact and specific contact resistivity of 2.30 × 10−6 Ω · cm2 are obtained for Co/C/SiC structures after two-step annealing at 500 °C for 10 min and 1050 °C for 3 min. The physical properties of the contacts are examined by using XRD. The results indicate that the Co-based metal contacts have better structural stability of silicide phases formed after the high temperature annealing and carbon-enriched layer is produced below the contact, playing a key role in forming an Ohmic contact through the reduction of effective Schottky barrier height for the transport of electrons. The thermal stability of Au/Co/C/SiC Ohmic contacts is investigated. The contacts remain Ohmic on doped n-type (2.8 × 1018 cm−3) 4H-SiC after thermal aging treatment at 500 °C for 20 h.

Mg0.55Zn0.45O solar-blind ultraviolet detector with high photoresponse performance and large internal gain

A Schottky type metal-semiconductor-metal solar-blind ultraviolet detector was fabricated on high quality wurtzite Mg0.55Zn0.45O epitaxial film. Photoresponse spectra show a responsivity peak of 22 mA/W under 130 V bias. A sharp cutoff was recognized at a wavelength of 270 nm, and a temporal response measurement indicates a fast decay time of less than 500 ns. A large internal gain was observed and interpreted by a reduced Schottky barrier height model, which fits well with the experimental data.

Observation of Defects that Reduce Schottky Barrier Height in 4H-SiC Schottky Contacts Using Electrochemical Deposition of ZnO

We performed the electrochemical deposition of ZnO on 4H-SiC epilayers and characterized Schottky diodes fabricated on the same epilayers in order to find the positions of defects that cause the low Schottky barrier height. We found that the positions where ZnO was deposited corresponded to the positions of contacts with lower Schottky barrier heights than the other contacts. After the removal of the ZnO layer, the surfaces of the ZnO-deposited positions were observed by atomic force microscopy. Photoluminescence mapping was also performed to observe the distribution of stacking faults. Then, the epilayer was etched using molten salt and the resulting etch pits were observed. Finally, we discussed the types of defects that reduce the Schottky barrier height.

Observation of Defects that Reduce Schottky Barrier Height in 4H-SiC Schottky Contacts Using Electrochemical Deposition of ZnO

We performed the electrochemical deposition of ZnO on 4H-SiC epilayers and characterized Schottky diodes fabricated on the same epilayers in order to find the positions of defects that cause the low Schottky barrier height. We found that the positions where ZnO was deposited corresponded to the positions of contacts with lower Schottky barrier heights than the other contacts. After the removal of the ZnO layer, the surfaces of the ZnO-deposited positions were observed by atomic force microscopy. Photoluminescence mapping was also performed to observe the distribution of stacking faults. Then, the epilayer was etched using molten salt and the resulting etch pits were observed. Finally, we discussed the types of defects that reduce the Schottky barrier height.

Increase in current density for metal contacts to n-germanium by inserting TiO2 interfacial layer to reduce Schottky barrier height

Metal contacts to n-type Ge have poor performance due to the Fermi level pinning near the Ge valence band at metal/Ge interfaces. The electron barrier height can be reduced by inserting ultrathin dielectrics at the metal-semiconductor interface. However, this technique introduces tunneling resistance from the large conduction band offset (CBO) between the insulator and Ge. In this work, the CBO between TiO2 and Ge is estimated to range from −0.06 to −0.26 eV so tunneling resistance can be reduced. By inserting 7.1 nm TiO2 between Al and n-Ge, current densities increased by about 900× at 0.1 V and 1200× at −0.1 V compared to contacts without TiO2.

Sharp Reduction of Contact Resistivities by Effective Schottky Barrier Lowering With Silicides as Diffusion Sources

An extremely low contact resistivity of 6-7 × 10-9 Ω·cm2 between Ni0.9Pt0.1Si and heavily doped Si is achieved through Schottky barrier engineering by dopant segregation. In this scheme, the implantation of B or As is performed into silicide followed by a low-temperature drive-in anneal. Reduction of effective Schottky barrier height is manifested in the elimination of nonlinearities in IV characteristics.

The effect of pulsed laser annealing on the nickel silicide formation

The pulsed laser annealing (PLA) is used to assist nickel silicide transformation for Schottky barrier height reduction and tensile strain enhancement and the effect of different laser power are investigated. In this report, a two-step annealing process which combine the conventional rapid thermal annealing with pulsed laser annealing is proposed to achieve a smooth silicon-rich NiSi x interfacial layer on (1 0 0) silicon. With optimized laser energy, a 0.2 eV Schottky barrier height (SBH) modulation is observed from Schottky diode electrical characterization. Furthermore, PLA provides sufficient effective temperature during silicidation which also lead to increased tensile stress of silicide film than the two-step RTA silicide is also investigated. The SBH modulation and tensile stress enhancement benefits of PLA silicidation are considered as an alternative to the conventional rapid thermal annealing for ultra-scaled devices performance enhancement.

Nanowire-metal heterostructures for high performance MOSFETs

In this work we report on the formation, of copper-germanide/germanium nanowire (NW) heterostructures with atomically sharp interfaces. The copper-germanide (Cu3Ge) formation process is enabled by a chemical reaction between metallic Cu pads and vapor-liquid-solid (VLS) grown Ge-NWs. The atomic scale aligned formation of the Cu3Ge segments is controlled by in situ SEM monitoring at 310 °C thereby enabling length control of the intrinsic Ge-NW down to a few nm. The single crystal Cu3Ge/Ge/Cu3Ge heterostructures were used to fabricate Ω-gated Ge-NW field effect transistors with Schottky Cu3Ge source/drain contacts. Temperature dependent I/V measurements revealed the metallic properties of the Cu3Ge contacts with a maximum current carrier density of 5 × 107 A/cm2. Prior to the gate deposition the intrinsic Ge-NW was modified with a focussed Ga+ ion beam. According to the thermoionic emission theory we determined an effective Schottky barrier height reduction from 218 meV to about 115 meV due to Ga+ implantation.