Atomic layer deposition Chengcheng Li 2013/6/27

What is ALD ALD (Atomic Layer Deposition) Deposition method by which precursor gases or vapors are alternately pulsed on to the substrate surface. Precursor gases introduced on to the substrate surface will chemisorb or surface reaction takes place at the surface Surface reactions on ALD are complementarity and self-limiting

ALD Example Cycle for Al 2 O 3 Deposition Cambridge NanoTech Inc., Cambridge, MA USA

ALD vs CVD Chemical vapor deposition(CVD) One or more gases or vapors react to form a solid product: Solid product can be: film particle nanowire nanotube Reaction started by: heat mix two vapors plasma

Atomic layer deposition(ALD) Sequential, self-limiting surface reactions make alternating layers: Benefits of ALD: Atomic level of control over film composition Uniform thickness over large areas and inside narrow holes Very smooth surfaces (for amorphous films) High density and few defects or pinholes Low deposition temperatures (for very reactive precursors)

Criteria for Both CVD & ALD Precursors Sufficient volatility (> 0.1 Torr at T < 200 ℃ ) No thermal decomposition during vaporization Precursors and byproducts don’t etch films

Criteria for ALD Precursors Reactivity with substrate Reactivity with surface of growing film Thermal decomposition allowed or even needed Criteria for CVD Precursors Self-limited reactivity with substrate Self-limited reactivity with the surface made by reaction of the film with the other precursor Thermal decomposition not allowed

Usefulness of Precursors for CVD & ALD Some precursors work only in CVD, but not ALD: Ni(CO) 4, W(CO) 6, many alkoxides Some precursors work in both CVD and ALD: many beta-diketonates and amidinates Most ALD precursors also work in CVD Some CVD precursors also work in ALD

To realize self-terminating, reactions must be irreversible and saturating Self-terminatingReversible saturating Ir/reversible saturating Irreversible nonsaturating

Effect of temperature

Effect of cycle numbers Two dimension growth Random deposition Island growth

N-dope: N 2p W-dope: increase visible activity reduce recombination W,N-doped TiO 2 Narrow Eg Increase recombination (trap for charge carriers)

The Journal of Physical Chemistry C 113(20): eV

1 ALD for TiNx Journal of Photochemistry and Photobiology A: Chemistry 177 (2006) 68–75

Precursors: TiCl 4, NH 3, H 2 O Deposition temperature: 500 ℃ Substrate: ITO N-TiO2 deposited by TiN/TiO2 ALD process TiN will be partially oxided during TiO2 ALD Process, leading to TiO 2-x N x TiCl4+NH3TiCl4+H2O

Process

2

substrate: n+ Si precursor: TiCl 4 and ammonia water Temperature: 400 ℃ Thickness ： 55nm after 1000 cycles (GPC 0.55A/cycle ) NH 3 ： H 2 O in vapor 420 ： 1 Film N concentration: 1.60 atom %

XPS 1.12% 0.48% About 70% of total N

IPCE

2.25eV

Next Focus + doped TiO2

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