Sung Man Lee

Fabrication of amorphous-carbon-nitride field emitters

To improve silicon field emitters, an amorphous-carbon-nitride (a-CN) coating was applied by helical resonator plasma-enhanced chemical vapor deposition. By this process, a-CN was very uniformly coated on silicon tips without any damage. Microstructural and electrical investigation of the silicon and a-CN coated field emitters were performed. a-CN coating lowered turn-on voltage and increased emission current. Negative electron affinity of carbon nitride is suggested for enhancing emission current.

Interfacial reaction and formation mechanism of epitaxial CoSi2 by rapid thermal annealing in Co/Ti/Si(100) system

A ternary compound of Co3Ti2Si is suggested as a reaction barrier for the formation of epitaxial CoSi2 in the Co/Ti/Si system when adopting the rapid thermal annealing process. It controls Co diffusion to the Si substrate, followed by formation of epitaxial CoSi2. After the epitaxial CoSi2 was formed, the interfacial morphology of the upper layer/ CoSi2 interface was very different according to silicidation temperature, that is, the interface was planar at 800 °C, but rough at 900 °C. This was attributed to the reaction between the upper layer consisting of Co–Ti–Si and the CoSi2 layer at 900 °C, which resulted in Ti-rich precipitates at the surface. The Ti-rich precipitates acted as a diffusion sink of dopant, thus, the leakage current density for the silicidation temperature of 900 °C was much higher than that for the temperature of 800 °C. These results suggest that the silicidation temperature is one of the most critical factors in determining the leakage current of the p+n junction diode.

Structural, Optical, and Field Emission Properties of Hydrogenated Amorphous Carbon Films Grown by Helical Resonator Plasma Enhanced Chemical Vapor Deposition.

Hydrogenated amorphous carbon films have been prepared by helical resonator plasma enhanced chemical vapor deposition using CH4 and H2 mixtures. Films with various physical properties were obtained from different deposition conditions. The structural and optical properties of hydrogenated amorphous carbon (a-C:H) films were more sensitive to the substrate bias than the substrate temperature. This reflects that the energetic ion bombardment modified the films more effectively than the thermal energy. The a-C:H films deposited with no bias applied show characteristics of polymeric films with a large content of C–H bond while the a-C:H films deposited as a function of the substrate temperature at a bias of 40 W show characteristics ranging from diamond-like carbon (DLC) to graphitic nature with a significantly reduced C–H bond. From elastic recoil detection analysis, the hydrogen content in the films also significantly reduced with an increase of substrate temperature at a bias of 40 W. The field emission from bare Si emitters and a-C:H coated Si emitters has been examined in an ultrahigh vacuum chamber. The field emission characteristic of the a-C:H coated Si emitters is better than that of the bare Si emitters. For the a-C:H coated Si emitters, the emission current of the a-C:H coated (at 150°C/40 W) Si emitters is higher than the that of the a-C:H coated (at 260°C/40 W) Si emitters. This difference in field emission characteristic is attributed to the structural and optical properties as well as hydrogen content.

Control of Co flux through ternary compound for the formation of epitaxial CoSi2 using Co/Ti/Si system

A ternary compound of Co3Ti2Si is suggested as reaction barrier for the formation of epitaxial CoSi2 in the Co/Ti/Si system. It has a role to control Co diffusion to the Si substrate, followed by formation of CoSi2. After Co3Ti2Si was formed, CoO and Ti oxide were formed at surface, depending on Ti thickness. In the case of Ti oxide being at surface, the outdiffusion of Ti in ternary compound was accelerated. Then, the decomposition of Co3Ti2Si occurred by reaction with Ti oxide, resulting in uniform epitaxial CoSi2. However, in the case of CoO being at surface, the Ti outdiffusion was suppressed, followed by thermally decomposition of Co3Ti2Si. This caused nonuniform Co supply to form nonuniform CoSi2.

Improved thermal stability of Ni silicide on Si (100) through reactive deposition of Ni

The effect of reactive deposition of Ni on the thermal stability of Ni silicide has been investigated in this study. In the case of room-temperature-deposited Ni, the agglomeration of Ni silicide, which induced the thermal instability during subsequent annealing, started to appear at 600 °C and the sheet resistance was increased abruptly after high-temperature anneals. However, when the Ni was deposited on the heated Si substrate (reactive deposition of Ni), the sheet resistance of Ni silicide film exhibited a constant value of about 7.91 Ω/□ at the whole reaction temperature, especially at 900 °C.

Effect of Ti-capping thickness on the formation of an oxide-interlayer-mediated-epitaxial CoSi2 film by ex situ annealing

A modified oxide mediated epitaxy process using a single deposition and ex situ annealing by Ti capping has been suggested in this study. It has been shown that in the case of pure Co on SiOx-covered Si, the reaction between Co and Si did not occur up to 800 °C during ex situ annealing. However, Co silicidation occurred in the case of Ti-capped Co on SiOx-covered Si. The crystalline nature of CoSi2 formed in this case strongly depends on the Ti capping thickness. When a thin Ti capping layer of thickness less than 5 nm was used, Ti oxidation occurred nonuniformly, and the morphology of the surface Ti oxide layer was very rough. This caused an exposure of Co to the oxygen in the ambient, resulting in the formation of polycrystalline CoSi2 due to the suppressed Co diffusion towards the Si substrate. In the case of Ti capping thickness being more than 10 nm, however, a uniform Ti oxide surface layer, which blocks the incoming oxygen retarding Co diffusion, was formed, and it led to uniform Co diffusion into ...

PREDICTION OF SOLID-STATE AMORPHIZING REACTION USING EFFECTIVE DRIVING FORCE

It is proposed that the nucleation and growth of the amorphous phase through the solid‐state amorphizing reaction in thin‐film diffusion couples can be predicted by using the concept of effective driving force. The effective driving force consists of two factors: (i) the thermodynamic driving force given by maximum free‐energy difference between the physical mixture of binary elements and the amorphous phase (ΔGmax), and (ii) the kinetic factor given by a ratio of the effective radius of the interstitial site in the host matrix to the atomic radius of the diffusing species (Rm/d). From the comparison of reported experimental results, it is shown that the criterion of effective driving force holds well for predicting the nucleation of the amorphous phase in metal/silicon systems as well as that of metal/metal systems. In addition, the concept of effective driving force holds well for predicting the growth tendency of the amorphous phase in metal/silicon systems.

The Field Emission Characteristics of a-C:H Thin Films Prepared by Helical Resonator Plasma Enhanced Chemical Vapor Deposition

The field emission characteristics of hydrogenated amorphous carbon (a-C:H) films prepared by helical resonator-plasma enhanced chemical vapor deposition (HR-PECVD) are examined. a-C:H films are deposited with CH4/H2 and CH4/Ar gases under different substrate RF bias conditions. The properties of a-C:H films are investigated by Raman spectroscopy, Fourier transform IR (FT-IR), UV spectroscopy and elastic recoil detection (ERD). Field emission characteristics of a-C:H coated on Si whiskers which are grown by the vapor-liquid-solid (VLS) method are tested under ultrahigh vacuum. Highly efficient field emission characteristics are achieved in the specimen deposited at a substrate RF bias higher rather than in the ground deposition condition regardless of the nature of the reactant gas. As the substrate RF bias is changed from ground to a higher RF substrate bias, the deposited a-C:H films have lower hydrogen contents and higher sp2-bonds. Therefore, the field emission characteristics of a-C:H thin films are affected by the hydrogen contents of the films rather than by the sp3/sp2 ratio.

Relationship between optical properties and microstructure of CeO2–SiO2 composite thin films

CeO2–SiO2 composite thin films were prepared by e-beam evaporation and ion beam assisted deposition using an End-Hall ion source. The refractive index of composite thin films exhibited a maximum value at 20%–35%SiO2 fraction, indicating the high packing density. Optical analysis revealed that the transmittance and reflectance spectra of composite films were consistent with the results of the refractive index. The results from x-ray diffractometry, atomic force microscopy and scanning electron microscopy measurements showed that composite thin films containing 20%–35%SiO2 concentration had a dense and smooth amorphous surface, compared to the roughened granular structure of the pure SiO2 and CeO2 thin films. The composite thin films with 20%–35%SiO2 concentration exhibited a higher resistance to water absorption than the CeO2 thin films in spite of the highest refractive index.

Enhancement of electron emission from silicon tips by nitrogen doped amorphous carbon coating

Silicon field emitters have been coated with nitrogenated amorphous carbon (a-C:N) in order to enhance the electron emission current. The coating was produced using a helical resonator plasma enhanced chemical vapor deposition system. The a-C:N film is amorphous and hydrogenated with about 30 at % hydrogen. Nitrogen is also included in the amorphous network and reduces the work function by doping. a-C:N coating enhances significantly the emission current of silicon tips.