Deposition Techniques Thermal CVD Laser-assisted CVD Plasma enhanced CVD Chemical beam epitaxy Vapor phase epitaxy Molecular beam epitaxy (MBE) Laser ablation method

MoS2 film on SiO2 substrate (Few inches wafer scale) 1. Thermal CVD Substrate Source ■ Principle & Process Heat energy is supplied to activate required source and gas-solid phase reaction. ■ Usages & Applications - Thin films (Metal oxide, metal sulfide, silicon, various semiconductors, etc.) ■ Characteristics Advantages High quality product Simple reactor Large-scale synthesis Drawbacks Large quantity carrier gas in need High crystallinity MoS2 film on SiO2 substrate (Few inches wafer scale)

Millimeter-sized boron helix 2. Laser-assisted CVD Millimeter-sized boron helix ■ Principle & Process Chemical reaction of source gas takes place at the spot on the substrate under laser irradiation. ■ Usages & Applications - Thin rod and fibers. (Si, C, B, Al, etc.) ■ Characteristics Advantages Access to control directionality Drawbacks Limitation of mass-production (only suitable for academic research purpose)

Vertically aligned Carbon nanotube 3. Plasma enhanced CVD ■ Principle & Process Discharge electrode excites the reactant gas into a plasma (ionized gas), which induces a chemical reaction and results in the reaction product being deposited on the substrate. ■ Usages & Applications Silicon complex thin film (SiOx, SiNx, amorphous silicon, dopant) Nanowire ■ Characteristics Advantages: Lower temperature(100-400°C) processes compared to conventional CVD (Less damages to sample, good when temperature is restricted) Fast deposition High step coverage (High quality: not as good as thermal CVD) Drawbacks Toxic byproduct gas High cost Chemical contamination Vertically aligned Carbon nanotube Silica film

4. Chemical beam epitaxy ■ Principle & Process Reactants are molecular beam of reactive gases, typically as hydride or metalorganic. Molecular beam (alkyl beam) carries metal atoms. Decomposition happens on the substrate for metal to compose layered structure on the substrate. ■ Usages & Applications Semiconductor films (GaAs, InP, Si1-xGex, etc.) Heterostructures (GaAs/InGaAs films) Carbon doped semiconductors (p-type GaAs) ■ Characteristics Advantages Low growth temperature Uniform film coverage Access to control chemical doping Drawbacks Requires ultra high vacuum (10-10~10-11) >> not good for mass production High carbon contamination Thin films

5. Vapor phase epitaxy ■ Principle & Process Solid metal source is thermally evaporated into vapor, and it deposits on the substrate. ■ Usages & Applications Wide variety of thin films (semiconductors, metals, solar cells, etc.) Metal film is much easier to make. ■ Characteristics Advantages Deposits almost any materials Low contamination Drawbacks Undesired polycrystalline layers Difficult to evaporate materials with low vapor pressure or high melting point. SiC film

Thickness-controlled heterostructure layers 6. Molecular beam epitaxy (MBE) Thickness-controlled heterostructure layers ■ Principle & Process Sublimated gaseous elements (atoms) condense on crystal substrate. Gas molecules (atoms) never collide each other before condensing on the substrate moving in the same direction. (Atomic beam, jet) Substrate is slightly heated. ■ Usages & Applications Thin film, especially semiconductors (GaAs, InP, Si1-xGex, etc.) Heterostructures (GaAs/InGaAs films) ■ Characteristics Advantages Very pure and uniform material Easy control of stoichiometry Drawbacks Very slow deposition rate(1µm/hr) Requires ultra high vacuum (10-10~10-11) >>>Not adequate for mass production (expensive and inefficient)

7. Laser ablation method ■ Principle & Process Al2O3 film ■ Principle & Process High-power laser beam strikes a target, and then vaporized target (ablation plume) deposits onto substrate as thin film (Sputtering) ■ Usages & Applications Thin film of complex compounds with complicated stoichiometry (YBCO, ceramics, etc.) ■ Characteristics Advantages Stoichiometry of a target can be reproduced on substrate (deposits almost any materials) Drawbacks - Requires ultra high vacuum Possibility of incorporating impurities Uneven coverage (Line-of-sight).