Transcript ATOMIC LAYER DEPOSITION @ GEORGIA TECH E. Graugnard, J. S. King, D.

ATOMIC LAYER DEPOSITION
@ GEORGIA TECH
E. Graugnard, J. S. King, D. Heineman, and C. J. Summers
School of Materials Science and Engineering,
Georgia Institute of Technology, Atlanta, GA, USA
Outline
• Introduction to Photonic Crystals
• Opals
• Inverse Opal
– Requirements for Photonic Band Gaps: high
filling fraction, smooth, conformal, high
refractive index
• Infiltration using ALD
– Meets above requirements
• Results: ZnS:Mn, TiO2, Multi-layers
• Summary
Photonic Crystals
z
1D
2D
3D
y
x
Periodic in
one direction
Periodic in
two directions
Periodic in
three directions
(Joannopoulos)
• Photonic Crystal – periodic modulation of dielectric constant
• Exhibits a “Photonic Band Gap” (PBG) where propagation of a range of
photon energies is forbidden.
• For visible wavelengths, periodicity on order of 150 – 500 nm.
• Introduction of “dielectric defects” yield modes within the PBG.
• Luminescent 2D & 3D PC structures offer the potential for controlling
wavelength, efficiency, time response and threshold properties (phosphors,
displays, solid state lighting, etc.).
Real Photonic Crystals:
Applications for thin films
1-D
2-D
3-D
3D Photonic Crystals:
Opals & Inverse Opals
• For 3D PC’s: “top-down” approaches are difficult.
– “Bottom-up” approach: self-assembly
• Most common 3D photonic crystal is the opal.
– Close-packed silica spheres in air
• Opal is used as a template to create an inverse opal.
– Close-packed air spheres in a dielectric material
ALD
3D-PC
Opal
26% air
Inverse Opal
74% air for high
dielectric contrast
SiO2 Opal Films
• Opal films are polycrystalline, 10 m thick, FCC films
with the (111) planes oriented parallel to the surface.
• For visible spectrum, lattice constant ~ 140 – 500 nm.
1 µm
300 nm
Challenge: growth of uniform films within a dense,
highly porous, high surface-area, FCC matrix
Opal Infiltration: Growth Issues
Geometrical Constraints
• Narrowest pathway (bottleneck)
into opal is through (111) planes.
• Consideration of geometry
predicts pore closure at 7.75% of
sphere diameter.
• Monte Carlo simulations show
this is ~ 86% infiltration of voids.
• Tetrahedral void size is
0.46×rsphere ~ 32–115 nm.
• Octahedral void size is
0.82×rsphere ~ 57–205 nm.
Opal Films: Growth Issues
Increased Surface Area
• Surface area of opal film is much larger than an
equivalent planar area:
Aopal
Afilm
0.74 l  w  t 4πr
2.22t



3
4 3πr
lw
r
2
• For a 10 m thick opal film with 200 nm diameter spheres:
Aopal/Afilm = 222
Aopal = 0.089 m2
Opal Infiltration:
Requirements
• Uniform Infiltration
– Material must be distributed uniformly throughout the opal
• Controlled Filling Fraction
– Must be able to precisely control the void space filling
• Conformal and Smooth Surfaces
– Creates lower porosity infiltrations
– Creates air pockets at the center of the opal voids,
enhancing the PBG
• High Refractive Index, Transparent, & Luminescent
Materials
– For a full PBG, the refractive index contrast (with air) must
be > 2.8
• ALD is the only technique to meet all of these
requirements
Inverse Opal Fabrication
Methods
• Good results with Chemical Bath Deposition,
• Solution precipitation
• Chemical vapor deposition CVD, and MOCVD
- (Blanco, Norris, Romanov, etc. ZnS – 50%, CdS ~ 96%)
• Low pressure chemical vapor deposition (LPCVD)
• Nanoparticle co-sedimentation
• Liquid metal infiltration
• However: porosity or incomplete filling is often observed
- Exception has been LP-MOCVD of Si
• Atomic Layer Deposition
Inverse Opal:
Fabrication
• Self-assembled silica opal template
– 10 μm thick FCC polycrystalline film, (111) oriented.
• Infiltration of opal with high index materials
– ZnS:Mn n~2.5 @ 425 nm (directional PBG)
– TiO2 (rutile) navg~ 3.08 @ 425 nm (omni-directional PBG)
Self
Assembly
ALD
Sintered Opal
Etch
Infiltrated Opal
Inverted Opal
Opal Infiltration:
Atomic Layer Deposition of ZnS:Mn
• Atomic layer deposition (ALD) is a CVD
variation that utilizes sequential reactant pulses.
ALD Growth of ZnS
Zn2+
S2-
H+
(a) Chemi + physisorption
Cl-
Nitrogen
(b) Physisorbed layer removal
(c) Formation of ZnS
(a) ZnCl2 Pulse
(b) N2 Purge
(d) Removal of H2S and HCl
~0.78 Å/cycle growth rate
Growth temperature: 500 C
Nitrogen
(c) H2S Pulse
(d) N2 Purge
• Halide precursors are solids: high deposition temperature.
• Mn2+ luminescent centers added by MnCl2 doping pulse.
Opal Infiltration:
Atomic Layer Deposition of ZnS:Mn
• ZnS:Mn Infiltrations
– Initial conditions:
• ZnCl2/H2S - 660ms/660ms
• N2 purge - 550ms
– Optimum conditions:
• ZnCl2/H2S – 2s/2s
• N2 purge - 2s
– 10s MnCl2 pulse every 100th
cycle
– Performed at US Army
Research Laboratory (ARL)
using a Microchemistry F-120
ALD of ZnS:Mn:
Scanning Electron Microscopy
(111)
Silica Spheres
220 nm infiltrated opal
ZnS:Mn
460 nm infiltrated opal
Growth Conditions: 500ºC, ZnCl2 – 660 ms, H2S – 660 ms
Ti
Opal Infiltration:
O
H
Cl
Atomic Layer Deposition of TiO2
TiCl
n(OH )(s)  TiCl4 ( g )  (O)n TiCl4 n (s)  nHCl
( g4)
Ti
H
O
Cl
(O)n TiCl4n (s)  (4  n) H2O( g )  (O)n Ti(OH )4n (s)  (4  n) HCl( g )
(b)
(a)
N2 purge
TiCl4
(b)
(a)
N2 purge
Ti H2O
H
O
Cl
(c)
(d)
N2 purge
TiCl4
H 2O
(c)
•
•
•
(d)
(a)
(e)
N2 purge
N2 purge
Liquid precursors:
high vapor pressure at low T.
TiCl4 is highly reactive with the oxide film.
Result: Wide deposition temperature window: RT to 600 C
(c)
H 2O
Opal Infiltration:
Atomic Layer Deposition of TiO2
• TiO2 Infiltrations
– Initial conditions:
• TiCl4/H2O - 1s/1s
• N2 purge - 1s
– Optimum conditions:
• TiCl4/H2O - 4s/4s
• N2 purge - 10s
– Performed at Georgia Tech
using a custom built hotwall, flow-style reactor
Schematic of Georgia Tech
TiO2 ALD System
Needle
Valve
TiCl4
Solenoid
Valve
Pressure
Gauge
Tube Furnace
MFC
N2
Sample
Entry
MFC
Sample
Mass Flow
Controllers
H2 O
Exhaust
To rough pump
and gas scrubber
Pulse lengths and cycles computer controlled
• Determine processes for self-limiting growth on planar substrates and for opal infiltrations
– Determine growth rate vs. temperature relationship
– Optimize pulse and purge lengths
– Determine growth rates for varying conditions
• Characterize: crystal structure, film morphology, chemical composition, & optical constants
Planar Thin Film Growth:
Growth
(nm/cycle)
cycle(nm/cycle)
per Rate
Growth
Growth Rate vs. Substrate Temperature
• 3 distinct regions of
growth that
correspond with
development of
crystal structure
0.200
amorphous
anatase
rutile
0.150
0.100
0.050
0.000
0
100
200
300
400
500
600
700
800
o
Substrate Temperature ( C)
1000 cycles
2000 cycles
4000 cycles
0.5s H2O pulse, 1s TiCl4 pulse, 4s purge, 1000 cycles
– 100 - 200oC amorphous
• Higher growth rate
– 200 - 500oC anatase
– 500 - 700oC rutile
• Decreased density of
reactive surface
species (-OH groups)
at higher temperatures
ALD of TiO2
Surface Roughness: planar TiO2 films
•
•
•
Large ALD temperature window allows optimization of surface morphology.
Below 150 C, ultra-smooth amorphous film results ( 2 Å RMS roughness).
400 C, 2 hr. heat treatment forms anatase, Roughness increase of only 2 Å!
• Refractive index increases from 2.5 to 2.85 (@425 nm).
500° C Deposition
100° C Deposition
Low T ALD + Heat Treatment = Smooth, conformal, high index!
ALD of TiO2
Surface Roughness: AFM Images
• Formation of polycrystalline structure results in surface roughening of
the film, which increases with increased deposition temperature.
• Surface roughness prevents direct high temperature ALD in opals
100oC
2 Å RMS roughness
300oC
21 Å RMS roughness
600oC
96 Å RMS roughness
AFM images acquired with a Park Instruments Inc. CP Autoprobe
and processed with WSxM 3.0 from Nanotec Electronica S.L.
ALD of TiO2
(111)
300 nm
433 nm opal with TiO2
crystallites deposited at 600ºC.
Polycrystalline TiO2 grown at
high temperatures produces very
rough surface coatings.
224 nm opal with TiO2
deposited at 500ºC.
The opal structure is lost at the
outer surface for complete TiO2
infiltrations at high temperatures.
ALD of TiO2 at 100ºC
(111)
Cross-sections
300 nm
433 nm opal infiltrated
with 20 nm of TiO2
433 nm opal infiltrated
with TiO2
433 nm TiO2 inverse opal
• TiO2 infiltration at 100ºC produces very smooth and
conformal surface coatings with rms roughness ~2Å.
• Heat treatment (400C, 2 hrs.) of infiltrated opal converts it to
anatase TiO2, increasing the refractive index from 2.35 to 2.65,
with only a 2Å increase in the rms surface roughness.
XRD of Infiltrated Opals
350
Silicon
(400)
Fired 400 deg. 2 hrs
(101)
300
Intensity (cps)
(200)
250
(103)
(004)
(112)
200
(204/213)
(215)
(105)
(301)
(211)
150
100
50
As-infiltrated TiO2
0
10
20
30
40
50
60
70
80
2
• XRD data for 100C 433 nm infiltrated TiO2 opal (lower
curve), and same sample after 400C 2 hour heat treatment
(upper curve).
Incomplete Opal Penetration
(111)
220 nm ZnS:Mn inverse opal
200 nm TiO2 inverse opal
• For small opal sphere sizes, uniform infiltration
becomes difficult creating air cavities when the
opal is inverted.
Optimized TiO2 Infiltration
• Pulse and purge times were increased to optimize
infiltration in opals with small sphere sizes.
2 µm
433 nm TiO2 inverse opal
Anatase TiO2 Inverse Opal
U
FCC Brillouin zone
433 nm inverse opal, ion milled (111) surface
Anatase TiO2 Inverse Opal
433 nm inverse opal fracture surface
TEM of TiO2 Shells
• (a) TEM image of TiO2
shell structures after
annealing. The inset shows
an electron diffraction
pattern confirming the
polycrystalline structure.
• (b) HR-TEM image
showing lattice fringes that
match the (101) planes of
anatase TiO2.
Inverse Opal Reflectivity:
Theoretical Comparison
Normalized Reflectivity
300
• Agreement: full index attained!
Flat band peaks
400
Wavelength (nm)
• TiO2 infiltration of 330 nm opal.
• ~88% filling fraction
• 2.65 Refractive Index
Band Diagram
500
600
700
2-3 PPBG
800
Fabry-Perot
fringes
900
1000
1100

L
Sintered Opal
Band Diagram
300
500
400
600
Wavelength (nm)
Wavelength (nm)
Normalized Reflectivity
400
Flat band
peaks
700
800
2-3 PPBG
900
1000
1100
Normalized Intensity
Band Diagram
10-11
8-9
5-6
PPBG's
500
600
700
2-3 PPBG
800
900
L
Infiltrated Opal

1000
L
Inverse Opal

Precise Digital Opal Infiltration
Void filling fraction of opal
as function of ALD Cycles
calculated from reflectivity
ALD Cycles
Coating Thickness (% radius)
Void Space Filling (%)
FCC (111) Pore Closure ~86%
TiO2 Coating Thickness as
function of ALD cycles
Slope: 0.039% /cycle
Growth Rate: 0.0512 nm/cycle
ALD Cycles
• Optical verification of maximum filling fraction.
• ALD allows for ultra-fine control of opal infiltration.
Two-Layer Inverse Opal
TiO2
ZnS:Mn
20 nm ZnS:Mn/20 nmTiO2/ Inverse Opal
Three-Layer Inverse Opal
• SEM of TiO2/ZnS:Mn/TiO2
inverse opal
330 nm sphere size
Luminescent multi-layered inverse opals
fabricated using ALD
Photoluminescence:
ZnS:Mn/TiO2 Composite
(a) 2-layer TiO2/ZnS:Mn/air
(14 nm/20 nm) inverse opal
(b-f) 3-layer TiO2/ZnS:Mn/TiO2
inverse opal after backfilling
with TiO2 by
(b) 1 nm
(c) 2 nm
(d) 3 nm
(e) 4 nm
(f) 5 nm
Relative Intensity
• 433 nm opal
• 337 nm N2 laser excitation
• Detection normal to surface
Cl
-
Mn
2+
108%
(a)
(b)
(c)
(d)
400
(e)
(f)
500
600
700
Wavelength (nm)
800
Using ALD of TiO2 to create
novel 2D structures.
X. D. Wang, E. Graugnard, J. S. King,
C. J. Summers, and Z. L. Wang
TiO2 Coated ZnO Arrays
Aligned ZnO nano-rods in a
hexagonal matrix on a
sapphire substrate.
Aligned ZnO nano-rods coated
with 100 nm of TiO2 at 100°C.
TiO2 Coated ZnO Arrays
Aligned ZnO nano-rods coated with
100 nm of TiO2 at 100°C.
Aligned ZnO nano-rods coated with
50 nm of TiO2 at 100°C.
TEM image of a TiO2 coated
ZnO nano-rod.
TiO2 Bowl Arrays
• TiO2 bowl arrays can be
used for particle sorting.
TiO2 Bowl Arrays
• TiO2 bowl arrays can be
used for particle sorting.
Summary
• ALD is an ideal deposition method for PC fabrication.
• Fabricated high quality inverse opal photonic crystals in the visible spectrum
using ALD.
• TiO2 ALD conditions optimized for complete, uniform infiltrations with
smooth and conformal coatings.
– Growth/Anneal protocol developed to form anatase inverse opals
• Precise control enables novel photonic crystal structures:
– Inverse opals with void space air pockets (enhanced PBG)
– Achieved maximum infiltration of 86%
– Perfect match between reflectivity and calculated band structure
– Multi-layered luminescent inverse opals
• Modification of photoluminescence by precise infiltration
– Increased Mn2+ peak intensity by 108%
• Pathway for photonic crystal band gap engineering.
• Novel structures created with ALD
– TiO2/ZnO aligned nano-rod arrays
– TiO2 nano-bowl arrays
Acknowledgments
•
•
•
•
Curtis Neff
Davy Gaillot
Tsuyoshi Yamashita
US Army Research Lab: S. Blomquist, E.
Forsythe, D. Morton
• Dr. Won Park, U. Colorado
• Dr. Mike Ciftan, US Army Research Office:
MURI “Intelligent Luminescence for
Communication, Display and Identification”