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Parasitic resistance, particularly source/drain contact resistance becomes one of the most serious problems to extend MOSFET scaling recently. Nickel silicide (NiSi), with advantages of low resistivity and high scalability, has been chosen as the material for source/drain formation. However, its Schottky barrier height (SBH) of 0.65eV for electrons is so high that it would block electrons from tunneling, therefore becomes an obstacle to further reduce the contact resistance, which is necessary to achieve the future scaling.
Among several solutions, high concentration impurity-segregation layers have been introduced at NiSi/Si interfaces to reduce SBH of MS-MOSFETs. Sulfur (S) has been considered to be an efficient material for the segregation-layer to reduce SBH owing to Fermi-level pinning effect. Previous studies have investigated segregation by implanting S before NiSi formation. Because of the high diffusivity of S in Si, S profile becomes broad during silicidation process, which leads to loss of S concentration at the interface. Moreover, S ions spread into the substrate and channel region generate deep impurity levels that induce junction leakage and off leakage, resulting in device performance degradation. In this paper, S implantation after Ni silicidation is proposed to suppress S diffusion because NiSi is expected to be an efficient barrier for S diffusion. In addition, NiSi/Si interface can serve as a potential energetic valley which may trap S during thermal treatment after implantation.
In this work, S was implanted into NiSi/n-Si diodes at the energy of 10keV with dose of 5�1014 and 1015 cm-3 after NiSi is formed. The projection range of S in NiSi is about 6nm, while thickness of NiSi is 16nm. Some devices were annealed at 300C and 450C. The I-V characteristics show that SBH is sufficiently reduced as the annealing temperature becomes higher, and it reaches as low as 3.4meV for 450C annealing. SBH of 3.4meV is much lower than the previously reported value of 70meV for which S was implanted before silicidation. The SIMS analysis result also proves the S profile is much sharper than having S-implantation before Ni silicidation, which supports our hypothesis that S diffusion is suppressed through our process and avoid the loss of S concentration at the interface. Moreover, despite the worry that S-implantation might damage the NiSi/Si interface morphology, cross sectional TEM images show that the interfacial flatness is completely the same as that of non-implanted NiSi/Si, indicating that no degradation occurs by S implantation. In summary, S-implantation after NiSi formation, which provides ultimately low SBH at NiSi/Si interface, is a promising technique to realize ultra-low parasitic resistance source/drain for future LSI beyond 16nm generation.
In the trend of scaling down metal-oxide-semiconductor field effect transistors (MOSFETs), reduction of contact resistance at the silicide/silicon (Si) interface will be essential for higher performance. Nickel silicide (NiSi) is considered as a substi-tute for a present electrode material in MOSFETs, cobalt silicide (CoSi2), because silicidation temperature can be reduced as compared with the case of the conventional CoSi2. Hence, we have focused on the NiSi/Si Schottky interface. An ordinary method to increase the dopant concentration at the interface is ion implantation before silicidation process. The dopant atoms are consequently condensed around the interface by snowplow effect, leading to the effective lowering of the Schottky bar-rier height (SBH) because of the band bending enhancement of the Si layer. However, this band bending technique does not reduce the SBH in further scaled MOSFETs. In this context, we studied another possibility of SBH modulation technique, based on the first-principles calculations. Throughout our calculations, we found that a large atomic-scale dipole between impurity and silicide atoms is generated across the interface. Impurity atoms are expected to be condensed because of a large energy gain at the interfaces, leading to the dramatic reduction of the SBH. Based on these results, we proposed a novel di-pole comforting Schottky (DCS) junction. We have also found that the thickness of the Si layer interfacing with the NiSi layer can be 1nm or less. In the present work, we applied this idea to the actual process through experimental techniques. The calculated results suggest that B implantation after silicidation leads to larger B concentration at the interface than that before silicidation, and thereby larger SBH modulation due to interface dipoles can be produced. Then, the NiSi/Si Schottky diodes were formed by ion implantation after silicidation process for dopants (As, B). We evaluated the interface dipoles contribution to the measured SBH reduction. As a result, the dopant atoms were found to be condensed beyond solubility limits on the interface Si side and we confirmed the generated interface dipoles actually reduces the SBT. Furthermore, we explored the other possibility of another type of impurity atoms applicable to the DCS junction. Among some other impurity atoms (Al, In, Mg), the calculated SBH modulation due to dipoles generated around these impurity atoms were found to be further enhanced in some cases. Based on these understandings, we propose a principle for choosing dopants towards ulti-mate lowering of the contact resistance in ultimately scaled MOSFETs.
A new in-situ technique to reduce threading dislocation density (TDD) within sub- micron growth is demonstrated by using metal-organic vapor-phase epitaxy (MOVPE). We achieved drastic reduction of TDD of AlGaN buffer on SiC substrate by inserting highly-Si- incorporated AlGaN/undoped AlGaN superlattice (SL). TDD of AlGaN was decreased from 2×1010 to 7×107 cm−2 by inserting the SL with the total growth thickness of 0.8νm. Si incorporation in AlGaN SL was estimated to be 1.2×1020 cm−3. This technique is exactly in- situ process without complicated fabrication processes, and the surface is kept flat throughout the total growth. This method is especially useful on SiC wafer in order to prevent cracks with thin growth layer. We confirmed the similar effects for GaN and AlGaN buffer on sapphire substrates.
We demonstrate 230-250 nm efficient ultraviolet (UV) photoluminescence (PL) from AlN(AlGaN)/AlGaN multi-quantum-wells (MQWs) fabricated by metal-organic vapor-phase-epitaxy (MOVPE). Firstly, we show the PL properties of high Al content AlGaN bulk (Al content: 85-95%) emitting from near band-edge. We systematically investigated the PL properties of AlGaN-MQWs consisting of wide bandgap AlGaN (Al content: 53-100%) barrier. We obtained efficient PL emission of 234 and 245 nm from AlN/Al0.18Ga0.82N and Al0.8Ga0.2N/Al0.18Ga0.82N MQWs, respectively, at 77 K. The optimum value of well thickness was approximately 1.5 nm. The emission from the AlGaN MQWs were several tens of times stronger than that of bulk AlGaN. We found that the most efficient PL is obtained at around 240 nm from AlGaN MQWs with Al0.8Ga0.2N barriers. Also, we found that the PL from AlGaN MQW is as efficient as that of InGaN QWs at 77 K.
We demonstrate room temperature intense ultraviolet (UV) emission wavelength ranging 300- 340 nm from InxAlyGa1-x-yN quaternary alloys grown by metal-organic vapor-phase-epitaxy (MOVPE). We found that the UV emission is drastically enhanced by introducing several percent of In into AlGaN. We fabricated single quantum well (SQW) consisting of InxAlyGa1-x-yN quaternary well and barrier, and clearly observed In segregation of sub-micron size from a cathode luminescence (CL) images. The intensity of 320nm-band emission from InAlGaN/InAlGaN QWs were as strong as those of 410nm-band emission from InGaN based QWs, at room temperature. The temperature dependence of photoluminescence (PL) emission for InAlGaN based QWs were much improved in comparison with GaN or AlGaN based QWs. We also grew Mg-doped InxAlyGa1-x-yN quaternary, and obtained hole concentration of 3×1017cm−3 by Hall measurement for high Al content (more than 50%) InxAlyGa1-x-yN quaternary.
We demonstrate 230-250 nm efficient ultraviolet (UV) photoluminescence (PL) from AlN(AlGaN)/AlGaN multi-quantum-wells (MQWs) fabricated by metalorganic vapor-phase-epitaxy (MOVPE). Firstly, we show the PL properties of high Al content AlGaN bulk (Al content: 85-95%) emitting from near band-edge. We systematically investigated the PL properties of AlGaN-MQWs consisting of wide bandgap AlGaN (Al content: 53-100%) barrier. We obtained efficient PL emission of 234 and 245 nm from AlN/Al0.18Ga0.82N and Al0.8Ga0.2N/Al0.18Ga0.82N MQWs, respectively, at 77 K. The optimum value of well thickness was approximately 1.5 nm. The emission from the AlGaN MQWs were several tens of times stronger than that of bulk AlGaN. We found that the most efficient PL is obtained at around 240 nm from AlGaN MQWs with Al0.8Ga0.2N barriers. Also, we found that the PL from AlGaN MQW is as efficient as that of InGaN QWs at 77 K.
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