Properties of Inverse Opal Photonic Crystals Grown By Atomic Layer Deposition J.S. King 1, C. W. Neff 1, W. Park 2, D. Morton 3, E. Forsythe 3, S. Blomquist 3, and C. J. Summers 1 (1) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA (2) Department of Electrical and Computer Engineering, University of Colorado, Boulder, CO (3) U.S. Army Research Laboratory, Adelphi, MD
Outline I. Rationale II. III. Introduction to Photonic Crystals and Atomic Layer Deposition Fabrication Methodology IV. Characterization V. Results VI. Conclusion VII. Future Work VIII.Acknowledgements
I. Rationale Immediate Goal Investigate Atomic Layer Deposition (ALD) as an effective infiltration route for fabricating inverse opals for use in the visible Why? Good results with Chemical Bath Deposition, MOCVD ( Blanco, Norris, Romanov, etc.): porosity or incomplete filling is often observed. We propose ALD as an effective alternate method. Flexible deposition technique for oxides, semiconductors and metals Easily scalable to mass production (already used in IC fab) Ultimate Goal High dielectric material based luminescent inverse opals with microcavity defects
II. Introduction: Photonic Crystals and Inverted Opals PCs offer unprecedented control of light as is well known from the work of Yablonovitch, John, and many others. Highly efficient optic and electro-optic devices; i.e. LEDs, Lasers, Waveguides, luminescent materials Most work has been on 2D structures. 3D luminescent PC s, including microcavity-based structures offer potential for wavelength control and low threshold emission Inverted Opal Structure (FCC) Predicted by Busch and John As demonstrated in silicon for the IR regime by Blanco, et.al., one of few known 3D structures that should exhibit a full PBG in the visible Only structure currently experimentally practical
II. Introduction: Photonic Crystals and Inverted Opals Requirements 3D periodic structure w/ high refractive index contrast (> 2.8) n>3.5 for 10% PBG GaP, TiO 2, SnS 2, Fe 2 O 3, all can be grown using ALD Low optical absorption Lattice constant ~ 140-350 nm High crystalline quality, conformal coatings High filling fractions- Typical filling fractions ~ 10 50 % of available pore volume reported for infiltration schemes
III. Fabrication Methodology Self-assembled thin film provides periodicity Sedimentation of monodisperse colloidal silica in a confinement cell followed by sintering to form synthetic opal (modification of method of Gates and Xia) After sintering, the opal is then infiltrated with high index material (ALD) 2% HF etch used to remove silica spheres, leaving air holes Infiltrated material must be compatible with etching scheme Self Assembly ALD Etch Sintered Opal Infiltrated Opal Inverted Opal
Atomic Layer Deposition Thin film grown by sequential deposition of reactants results in monolayer-by-monolayer growth Advantage: surface-controlled growth instead of source-controlled
Atomic Layer Deposition Available Materials: ZnS, SiO 2, TiO 2, ZnSe, Fe 2 O 3, SnS 2, GaP, W, Ta,... Multi-layered materials possible, allowing tailoring of refractive index and luminescent behavior ZnS:Mn used for initial demonstration: well studied ALD material, but insufficient index (n~2.5) for full PBG: Pseudo-gap behavior Luminescent Center: Mn 2+ doping using MnCl 2 every 100 th pulse ZnCl 2 N 2 Purge H 2 S N 2 Purge ZnCl 2 N 2 Purge H 2 S
IV. Characterization Methods Scanning Electron Microscopy (SEM) Film quality, conformal nature, sphere sizing Optical: Reflectivity (8 degree) lattice constant-sintered opals, filling fraction-infiltrated opals Photoluminescence (PL) Band edge and Mn 2+ excitation Crystallinity Modulation by photonic crystal Angular dependence
V. Results SEM ZnS:Mn Infiltrated Opal 460 nm 460 nm Sacrificial Sample Infiltrated Opal Trench Cut Via FIB
SEM ZnS:Mn Infiltrated/Inverse Opal d= 240 nm d= 220 nm
Calculation of Filling Fraction Periodicity yields Bragg diffraction Derivative of Bragg law allows calculation of (111) peak position Opals grown oriented with (111) planes parallel to substrate Resulting peak shift after infiltration allows calculation of filling fraction, f filling % for ideal FCC = f/0.26 x 100% avg λ = 2d0.816 ε avg Sin ε = ε 0.74 + (0.26 f ) ε + f Silica ε = avg ε silica ε. ( 0.74) ) ZnS 1 2 air θ 0.26 fε ZnS d= silica sphere diameter ε ZnS = ZnS bulk dielectric constant ε avg = effective dielectric constant θ = angle of incidence
Reflectivity 300 nm ZnS:Mn Opal ZnS Background subtracted Reflectivity (arb.units) 30 25 20 15 10 5 0 301 nm SiO 2 OP/ZnS:Mn/ f= 90% Stand. Inf. Cond. Infiltrated 826 nm Bragg peak 400 500 600 700 800 900 1000 Reflectivity (arb. units) 30 25 20 15 10 5 0 301 nm SiO 2 OPI/ZnS:Mn/ f= 94% Stand. Inf. Cond. Etched 708 nm Bragg peak 400 500 600 700 800 900 1000 Wavelength (nm) Wavelength (nm)
Reflectivity - Summary (111) Bragg Peak Wavelength (nm) 1000 900 800 700 600 500 400 300 200 100 0 Infiltrated Sintered Opal 0 100 200 300 Opal Sphere Radius (nm) From the trendline, the average fill fraction is 90% Increased deviation as sphere size decreases Uncertainty in measurements of sphere size and wavelength d ~ 5-10 nm λ ~ 5 nm
PL Results ZnS:Mn Infiltrated Opal Intensity (cps) 3.50E+06 3.00E+06 2.50E+06 2.00E+06 1.50E+06 1.00E+06 5.00E+05 PL standard collection angle = 22.5º Band Edge Excitation (330 nm) 225 nm SiO 2 OP/ZnS:Mn 330 exc. f= 62.5% Stand. Inf. Cond. Cl - Mn ++ Intensity (cps) 7.00E+06 6.00E+06 5.00E+06 4.00E+06 3.00E+06 2.00E+06 1.00E+06 Mn 2+ Direct Excitation (390 nm) 225 nm SiO 2 OP/ZnS:Mn 390 exc. f= 62.5% Stand. Inf. 0.00E+00 400 500 600 700 Wavelength 0.00E+00 450 550 650 750 Wavelength (nm)
Angle Dependent PL Results ZnS:Mn Inverse Opal 2.00E+07 1.80E+07 1.60E+07 225 nm SiO 2 OPI/ZnS:Mn 390exc f= 76.4% Stand. Inf. Cond. d=225 nm 10º PL Intensity (cps) 1.40E+07 1.20E+07 1.00E+07 8.00E+06 6.00E+06 4.00E+06 2.00E+06 508 nm Refl. Peak 18 7.5º 0º 22.5º Infiltrated Detection Angle, θ 390 nm excitation 0.00E+00 400 450 500 550 600 650 700 Wavelength (nm)
VI. Conclusions I. Successful ALD ZnS:Mn infiltration of opals Filling fractions close to 100% achieved, calculated using Reflectance SEM confirms high quality, conformal coatings II. Etched silica spheres to form inverted opal Resulting inverted opal ~ 3 5 microns thick, robust III. Luminescent Properties PL confirms high crystalline quality PL shows interesting changes, angle dependence
VII. Future Work I. Further optimization of ALD process for smaller opals to maximize depth of penetration of high index material II. Thorough investigation of angular dependence of reflectivity and PL III. Use ALD to fabricate inverted opals in high-n materials systems, i.e. TiO 2, Fe 2 O 3, SnS 2 GaP, etc. IV. Work on incorporation of dopant spheres in opal for microcavity fabrication
VIII. Acknowledgements Work supported by: U.S. Army Research Office MURI Contract# DAAA19-01-1-0603 Sandia National Laboratories Contract # 18499