ALD uses self-limiting chemical reactions to grow thin films at a rate of approximately 0.1 nm/cycle. For this work, ~90 nm (1000 cycles) of amorphous alumina was deposited on ~530 nm of thermally grown SiO

2 on a silicon wafer substrate.

● ALD alumina grown using TMA/water (T = 150°C)● Sapphire (Sumitomo) nanoparticles in isopropanol dropcast

on alumina surface● Samples annealed at 900-1050°C● Samples analyzed using scanning electron microscopy (SEM)

and electron backscatter diffraction (EBSD)

α-Al2O

3 thin films are used for scratch-resistant

and optical coatings and in electronics. However, fabrication of α-Al

2O

3 substrates from the molten

state requires temperatures above 2000°C. Here, we examine the transformation of amorphous, atomic layer deposited alumina into polycrystalline sapphire films at low temperatures via seeded lateral solid phase epitaxy (SLSPE).

Introduction

Experimental

Materials Science and Engineering and Center for Advanced Materials and Nanotechnology, Lehigh University

Seeded Crystallization of ALD Alumina Thin Films

Crystal growth rate varies linearly with crystallization temperature. Growth rates were observed to plateau after long heating times, possibly due to solid state reactions between the amorphous alumina and SiO

2.

Effect of annealing ambient

Annealing in air resulted in the highest growth area (12 μm2), possibly due to water vapor in air.

Annealed for 12 hr at 1050°C. Scale bar: 2 μm.

Textural propertiesCorrelating crystal orientation and growth

The sapphire crystal structure is represented by the hexagonal unit cell. EBSD data were used to correlate the crystallographic orientation of the crystalline regions with their shape.

An image of the HCP unit cell orientation as determined by EBSD data (B) is used to determine the projection of the [0001] direction onto the plane in the SEM image (A) and EBSD map (C). The plot and image below show that the short axis of the crystal corresponds to this [0001] direction.

Conclusions● Crystal growth rates exhibit Arrhenius behavior● Air annealing environment yields largest crystals● Crystal aspect ratio is closest to unity for c-axis

oriented crystals (θ≈0) ● Largest crystals show crystal aspect ratio closest

to unity● Crystal short axis correlates with [0001]

direction; The [0001] direction is the slowest growth direction

● Activation energy for crystallization may be lowered by SLSPE

William Mushock, Animesh Kundu, Michael Kracum, Ling Ju, Roderick Marstell, Bo Bao

Acknowledgements

Growth rate vs temperature

Crystal short axis correlates with [0001] direction

References

Crystal growth area vs aspect ratio

Atomic layer deposition (ALD) is a growth technique used to produce thin, conformal films of precise thicknesses.1,2 As-deposited ALD alumina films are amorphous and many studies have focused on their crystallization through post deposition annealing.3 However, the impact of seeding on the crystallization behavior of ALD alumina has not been reported. We propose that by depositing nanoparticle sapphire seeds on the amorphous film before annealing, the nucleation barrier for crystal growth can be lowered.

1. George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 110, 111-131 (2010).2. Miikkulainen, V., Leskela, M., Ritala, M. & Puurunen, R. L. Crystallinity of inorganic films grown by atomic layer

deposition: Overview and general trends. J. Appl. Phys. 113, 021301 (2013).3. Afanas’ev, V. V., Stesmans, A., Mrstik, B. J. & Zhao, C. Impact of annealing-induced compaction on electronic

properties of atomic-layer-deposited Al2O

3. Appl. Phys. Lett. 81, 1678-1680 (2002).

4. http://images.1233.tw/unit-cell-z/5. White, C.W., et al. Ion implantation and annealing of crystalline oxides and ceramic materials. Nuclear

Instruments and Methods in Physics Research. B32, 11-22 (1988).

SEM images of crystals grown at different temperatures. Top row scale bar: 2.5 μm. Bottom row scale bar: 2 μm.

Crystallized at 1050°C for 12 hours. No SiO2

intermediate layer. Scale bar: 25 μm.

Hannah Maret, Derek Weisberg, Helen M. Chan, Nicholas C. Strandwitz

Results and Discussion

The largest growth area corresponds to crystals with aspect ratio closest to unity and θ≈0.

θ = 7.08°, Aspect ratio = 1.04

θ = 89.0°, Aspect ratio = 2.60

Annealed for 12 hr at 1050°C. Scale bar: 1 μm.

Crystallized for 12 hr at 1050°C. Scale bar: 1 μm.

Crystal growth rates were measured using SEM micrographs. In the micrograph shown in pane A, white spots represent sapphire nanoparticles and the lighter gray areas show crystallized regions. Pane B is an EBSD map, which identifies the crystalline phase and orientation of individual crystal domains surrounding the seed crystals. The crystals are indexed to α-Al

2O

3 as shown in the

phase confidence map in pane C.

Annealed at 950°C for 72 hours.

[4]

Crystal aspect ratio increases with misalignment angle of c-axis (θ)

Annealed at 1050°C for 12 hours. All scale bars: 1 μm.

● Sapphire seeds nucleate single crystalline domains (T > 900°C)

● Crystallized regions index to α-Al2O

3● Seed orientation is random

● Circular crystals exhibit the fastest axis growth rate● The oblong long axis growth rate is comparable to

the circular radial growth rate● At constant temperature, axis growth rate

decreases with time

Growth rate according to crystal axis

Activation energy for crystallization The Arrhenius equation was used to plot growth data where k is the crystal axis growth rate . Best fit lines were generated and E

a interpreted from the

slope.

The average Ea for the

SLSPE α-Al2O

3 transformation reaction was E

a=2.82 eV (compare

to literature values for the γ→α transformation where E

a≈3.6 eV5)