Photonic crystal thin films were fabricated via the self-assembly of a lattice of silica spheres on silicon (100) substrates. Progressive infilling of the air spaces within the structure with GaAs was achieved using trimethylgallium and arsine under atomic-layer-deposition conditions. Samples with the highest levels of GaAs infill were subsequently inverted using selective etching. Reflectance spectra are interpreted via the Bragg expression and calculated photonic band structure diagrams. For GaAs infilled and inverted samples, the relative positions of the first and second order Bragg reflections are strongly influenced by the wavelength dependent refractive index

Bardosova, M

Pemble, ME

Povey, IM

Renard, J

Thomas, K

Whitehead, DE

Applied Physics Letters

English

University of Salford Institutional Repository

APPLIED PHYSICS LETTERS 89, 104103 2006Photonic crystal thin films of GaAs prepared by atomic layer depositionI. M. Povey, D. Whitehead, K. Thomas, M. E. Pemble,a and M. BardosovaTyndall National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, Republic of IrelandJ. RenardDepartment of Materials Science and Technology, INSA, 20 Avenue des Buttes de Coësmes,35043 Rennes Cedex, FranceReceived 19 December 2005; accepted 12 July 2006; published online 6 September 2006Photonic crystal thin films were fabricated via the self-assembly of a lattice of silica spheres onsilicon 100 substrates. Progressive infilling of the air spaces within the structure with GaAs wasachieved using trimethylgallium and arsine under atomic-layer-deposition conditions. Samples withthe highest levels of GaAs infill were subsequently inverted using selective etching. Reflectancespectra are interpreted via the Bragg expression and calculated photonic band structure diagrams.For GaAs infilled and inverted samples, the relative positions of the first and second order Braggreflections are strongly influenced by the wavelength dependent refractive index. © 2006 AmericanInstitute of Physics. DOI: 10.1063/1.2345359Since the pioneering work of Yablonovitch1 and John2interest in photonic crystals has grown enormously. Photoniccrystals can exhibit a photonic band gap where light possess-ing certain values of wave vector is not allowed to propagatein the material. Opals are natural examples of photonic bandgap materials that may be readily synthesized in the labora-tory. In such materials the nature of the band gap is not onlyinfluenced by the lattice parameters but also the contrast inthe dielectric constant between the opal structure and itsvoids. This contrast can be enhanced by infilling the voidswith a material of higher refractive index and still further byinversion, the selective removal of the host lattice. The ef-fects of infiltrating semiconductor materials into opal havebeen demonstrated, for example, by the work of Astratov etal. with CdS nanoparticles,3 and Blanco et al. and Míguez etal., who have shown that inverted opals made from Si or Gepossess a complete photonic gap, across the whole Brillouinzone.4,5 Materials that possess complete gaps may be particu-larly suited to the fabrication of novel filter or mirror-baseddevices, although the existence of well-defined “pseudogaps”in these materials, which effectively prevent light propaga-tion in certain directions and not others, may also be of hightechnological significance, for example, in terms ofwaveguiding.6Infilling the void spaces within host photonic crystal lat-tices has been demonstrated with a wide range of materialsusing methods such as wet chemical impregnation,7 chemicalvapor deposition8–10 CVD, and, recently, atomic layerdeposition ALD. The ALD technique was demonstrated forthe ZnO,11 TiN,12 and TiO2 and ZnS:Mn Refs. 13 and 14systems and shown to produce uniform and highly conformalinfill from which inverted opals were fabricated with goodphotonic properties. In this present study we extend the ALDmethodology to III/V materials and, in particular, GaAs.GaAs-based photonic crystals are of particular interest interms of both the nature of the photonic gap produced in ahigh refractive index contrast medium and the possibility ofcoupling these effects with both linear and nonlinear opticalprocesses such as second harmonic generation, which wouldaElectronic mail: martyn.pemble@tyndall.ie0003-6951/2006/8910/104103/3/$23.00 89, 10410Downloaded 08 Sep 2006 to 143.239.65.179. Redistribution subject tobe allowed in a material such as GaAs as a result of thedirect band gap and the noncentrosymmetric nature of thebulk unit cell, respectively.15 To date while several two di-mensional photonic band gap structures have been fabricatedusing lithographic methods, reports of fabrication of threedimensional GaAs photonic crystals are limited7 and thereare no reports of growth by CVD or related methods.In this present work we demonstrate that ALD methodsbased upon the use of a conventional metal organic vaporphase epitaxy MOVPE reactor may be used with a highdegree of control to infill opal photonic crystals with GaAs.Furthermore the extent and uniformity of infill is such thatsubsequently, the opal host may be removed, creating a highquality GaAs inverse opal. We demonstrate that the opticalproperties of the resulting material may be understood viathe Bragg equation which generally holds for such materials,provided that accurate refractive index and extinction coeffi-cient values matched to the wavelength of features measuredin the reflectance spectra are employed. In further support ofthe applicability of this simple Bragg model, we show thatthe observed behavior is consistent with that predicted viathe modeling of the photonic band structure using the MITphotonic band MPB package.16Artificial opal thin films were assembled on silicon 100substrates by controlled evaporation, utilizing monodis-persed silica spheres 260 nm in diameter after sinteringprepared by the Stöber synthesis17 Specular reflection mea-surements and electron microscopy confirm that the preparedopals exhibit high levels of uniformity. GaAs growth wasperformed in a horizontal reactor system Thomas Swan Sci-entific Equipment single 2 in. wafer horizontal MOVPE re-actor maintained at 400 °C and 200 torr using trimethylgal-lium TMGa and arsine AsH3 as precursors. Alternatinggas pulses of TMGa 3 s and AsH3 8 s at flow rates of5.310−5 and 4.510−3 mol min−1, respectively, were sepa-rated by a hydrogen purge 2 s to prevent the reactant gasesfrom mixing in the gas phase. Abrupt switching of precursorswas achieved using a pressure-balanced run-and-vent gasmanifold. The flow rate of hydrogen through the reactor was3 −116 dm min throughout precursor pulsing and purging.© 2006 American Institute of Physics3-1 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp104103-2 Povey et al. Appl. Phys. Lett. 89, 104103 2006All samples were sintered under growth conditions priorto infiltration employing a 15 min temperature ramp de-signed to bring the substrate to growth temperature where itwas held for a further 15 min prior to growth. The level ofGaAs infill was tuned using an iterative process, varying thenumber of pulse sequences and calculating the infill usingthe Bragg expression. Inversion of infilled opal structureswas achieved by immersion in 1% HF solution for 20 min.Optical characterization was performed by diffuse reflec-tance measurements, using a Brüker IFS66 spectrometerequipped with a variable angle reflectance stage, and Ramanspectroscopy, using a Spectra Physics 2080/20s argon ionlaser 514.5 nm and a charged coupled device cameramounted on a Dilor XY 800 triple grating spectrometer.Figure 1 shows SEM images recorded after 200 and 800ALD cycles, showing that the infilling of the voids betweenthe silica spheres in the opal structure is initially rough butconformal and virtually complete after 800 cycles. The crys-talline nature of the infilled material is demonstrated by themeasured Raman spectra in Fig. 2 via the presence of the TO263 cm−1 and LO 286 cm−1 phonons for GaAs. The as-signment is supported by the presence of weak but charac-teristic crystalline GaAs features in the x-ray diffractionspectra. Figure 3 shows reflectance data for the bare opalsample and after varying numbers of ALD cycles. This figurereveals that for the bare opal the Bragg peak as measured atan angle of incidence of 12° from normal occurs at ca.592 nm. As the number of ALD cycles is then increased, sothe Bragg peaks shift progressively to higher wavelengthseventually reaching ca. 916 nm after some 800 cycles.The position of the nth order Bragg peak  is related tothe effective dielectric constant of the material eff, theweighted sum of the dielectric constant components, the lat-tice parameter d111, and the angle of incidence  by theexpressionFIG. 2. Raman spectrum of an opal sample following 500 ALD cycles ofGaAs growth, showing the TO and LO modes of bulk GaAs at 263 and−1FIG. 1. SEM images of cleaved synthetic opals after a partial infill200 cycles, b near total infill 800 cycles, and c inversion.286 cm , respectively.Downloaded 08 Sep 2006 to 143.239.65.179. Redistribution subject ton = 2d111eff − sin2 1/2, 1eff = fnSiO2 + ikSiO22 + fninfill + ikinfill2 + fnair+ ikair2  n2 − k2SiO2 + n2 − k2infill + n2 − k2air,2where nx and kx are the refractive index and extinction coef-ficient of the component x, respectively.Using Eqs. 1 and 2 and n and k values of 3.57 and0.00, respectively, it is possible to estimate that after800 cycles, the contribution to the composite dielectric con-stant from air is approximately 9%. Thus it may be con-cluded that the voids between the silica spheres are morethan 90% infiltrated with GaAs. It should be noted that dueto geometric considerations it is generally accepted that con-formal growth within a face-centered-cubic opal structurewill create air pockets within the voids that occupy 3.6% ofthe available space thus making the maximum theoreticalinfill 96.4%. The spectrum obtained after some 200 cyclesshows no Bragg peak. This result is a convenient way ofdemonstrating the photonic crystal nature of these samples,because the levels of GaAs infill for this sample are such thatthe weighted average dielectric constant of the material oc-cupying the voids in the opal, i.e., n2−k2GaAs+ n2−k2air, isnow equal to the dielectric constant of the silica. Thus thismaterial possesses no refractive index contrast and as suchthe Bragg reflection is absent. From Eqs. 1 and 2 it maybe estimated that the level of infill at this point is ca. 26%.Peak positions, absolute reflectivity, and calculated levels ofinfill with their corresponding values of n and k are tabulatedin Table I for a range of ALD cycle numbers.Also of note is the appearance of lower wavelength fea-tures in the spectra for sample produced with higher levels ofinfill. These features could be assigned as second order peaksof the Bragg reflection, i.e., when n=2 in Eq. 1. However,the shift from the simple 2 relationship predicted by theBragg equation can only be partially explained by the sig-nificant variation in refractive index and extinction coeffi-cient of GaAs with wavelength, varying from n=4.4 to 3.5and k=2.0 to 0.0 across the region studied400–950 nm.18,19 Furthermore, when infiltration calcula-tions between the first and proposed second order featuresare compared Table I there is considerable disparity, withthe second order features consistently overestimating the in-fill. This leads to the conclusion that the features are notFIG. 3. Color online Diffuse reflectance spectra of opal thin films for a abare opal 0% infill blue line, b 15% infill green line, c 26% infillgray line, d 67% infill black line, and e 91% infill of GaAs red line.Note the progressive redshift of the first order Bragg peaks as the infill levelincreases. At higher levels of infill the partially unassigned secondary fea-tures shift into the accessible spectral window.purely due to a second order process but the convolution of AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp104103-3 Povey et al. Appl. Phys. Lett. 89, 104103 2006more than one band. This argument is supported by thebroadness and hints of structure exhibited by these spectralfeatures.Turning now to a comparison between experimental re-sult and theory, Fig. 4 shows the photonic Brillouin zonepredicted for a GaAs inverted structure calculated using theMIT photonic band package16 using a face-centered-cubiclattice with 260 nm spheres, n=3.62, and k=0.0. The singu-lar refractive index value used in the calculations was chosento match the Bragg reflection peak observed for the invertedstructure. The calculated Brillouin zone can be directly com-pared to reflection data displayed in Fig. 4 from which it isevident that there is good agreement with the position of thefirst order Bragg reflection and the pseudogap between thesecond and third bands between the U and L points in theBrillouin zone. The broad spectral feature at ca. 61014 Hz, which is unlikely to be purely a second orderBragg peak, could have a contribution from a full gap be-tween the eighth and ninth bands. This argument is supportedby the presence of a shoulder on the broad secondary featureat ca. 4.51014 Hz that can be attributed to a pseudogapbetween the fourth and fifth bands between the U-L and-L points in the Brillouin zone. However, assignments ofthese higher order bands should be treated with caution asthe dispersions in n and k are not considered at these wave-lengthsIn this letter we have described the production and char-acterization of the first CVD grown GaAs silica opal struc-ture by employing an ALD protocol. It has been demon-strated that ALD is an excellent method for the tunableTABLE I. Positions of the Bragg peaks measurednumbers of ALD cycles. The corresponding valuesestimated from these parameters are also given.ALDcyclesFirst orderBragg peaknm na,b ka,bEstimatedinfill%0 592 0100 616 3.86 0.211 15200 3.77 0.165 26500 840 3.62 0.08 67800 916 3.57 0.00 91aReference 18.bReference 19.FIG. 4. Comparison of the measured reflectance spectrum of an invertedGaAs opal and the corresponding calculated Brillouin zone.Downloaded 08 Sep 2006 to 143.239.65.179. Redistribution subject toinfiltration of GaAs into silica opal thin films, uniformly pen-etrating throughout the structure to give fractional infill of upto ca. 91% of the available space. Optical measurements con-firm the level of infill and nature of the material. In addition,for an inverted GaAs opal, comparison of the optical mea-surements with theoretical calculations demonstrates reason-able agreement with the predicted band structure, althoughunambiguous identification of higher energy features has notyet been achieved.The authors would like to thank M. Schmidt for acquir-ing the Raman spectra and R. H. Tredgold for helpful dis-cussions. This work was supported by Science FoundationIreland.1E. Yablonovitch, Phys. Rev. Lett. 58, 2059 1987.2S. John, Phys. Rev. Lett. 58, 2486 1987.3V. N. Astratov, V. N. Bogomolov, A. A. Kaplyanskii, A. V. Prokofiev, L.A. Samoilovich, S. M. Samoilovich, and Y. A. Vlasov, Nuovo Cimento D17, 1349 1995.4A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S. W. Leonard,C. Lopez, F. Meseguer, H. Miguez, J. P. Mondia, G. A. Ozin, O. Toader,and H. M. van Driel, Nature London 405, 437 2000.5H. Míguez, E. Chomski, F. Garcia-Santamaria, M. Ibisate, S. John, C.Lopez, F. Meseguer, J. P. Mondia, G. A. Ozin, O. Toader, and H. M. vanDriel, Adv. Mater. Weinheim, Ger. 13, 1634 2001.6M. T. Todaro, T. Stomeo, V. Vitale, M. DeVittorio, A. Passaseo, R.Cingolani, F. Romanato, L. Businaro, and E. Di Fabrizio, Microelectron.Eng. 67-68, 670 2003.7Y. C. Lee, T. J. Kuo, C. J. Hsu, Y. W. Su, and C. C. Chen, Langmuir 18,9942 2002.8H. M. Yates, M. E. Pemble, A. Blanco, H. Miguez, C. Lopez, and F.Meseguer, Chem. Vap. Deposition 6, 283 2000.9H. M. Yates, M. E. Pemble, H. Miguez, A. Blanco, C. Lopez, F. Meseguer,and L. Vazquez, J. Cryst. Growth 193, 9 1998.10T. Chunhua, F. Guanghan, Z. Tianming, L. Shuti, and S. Huiqing, PhysicaB 363, 1 2005.11M. Scharrer, X. Wu, A. Yamilov, H. Cao, and R. P. H. Chang, Appl. Phys.Lett. 86, 151113 2005.12A. Rugge, J. S. Becker, R. G. Gordon, and S. H. Tolbert, Nano Lett. 3,1293 2003.13J. S. King, C. W. Neff, C. J. Summers, W. Park, S. Blomquist, E. Forsythe,and D. Morton, Appl. Phys. Lett. 83, 2566 2003.14J. S. King, D. Heineman, E. Graugnard, and C. J. Summers, Appl. Surf.Sci. 244, 511 2005.15A. Yariv, Quantum Electronics, 3rd ed. Wiley, New York, 1989, p. 305.16S. G. Johnson and J. D. Joannopoulos, Opt. Express 8, 173 2001.17W. Stöber, A. Fink, and E. Bohn, J. Colloid Interface Sci. 26, 62 1968.18H. Burkhard, H. W. Dinges, and E. Kuphal, J. Appl. Phys. 53, 655 1982.19eflectance spectroscopy for samples after differentand k for GaAs and the percentage levels of infillsoluteectivity%Secondarypeaknm na,b ka,bEstimatedinfill%512948 488 4.396 0.434 7062 540 4.087 0.286 106by rof nAbreflS. Zollner, J. Appl. Phys. 90, 515 2001. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Photonic crystal thin films of GaAs prepared by atomic layer deposition

Povey, I. M.

Whitehead, Debra

Thomas, K.

Pemble, Martyn

Bardosova, M.

Renard, J.

E-space: Manchester Metropolitan University's Research Repository

I. M. Povey

D. Whitehead

K. Thomas

M. E. Pemble

M. Bardosova

J. Renard

Crossref

Photonic crystal thin films were fabricated via the self-assembly of a lattice of silica spheres on silicon (100) substrates. Progressive infilling of the air spaces within the structure with GaAs was achieved using trimethylgallium and arsine under atomic-layer-deposition conditions. Samples with the highest levels of GaAs infill were subsequently inverted using selective etching. Reflectance spectra are interpreted via the Bragg expression and calculated photonic band structure diagrams. For GaAs infilled and inverted samples, the relative positions of the first and second order Bragg reflections are strongly influenced by the wavelength dependent refractive index. (c) 2006 American Institute of Physics. (DOI:10.1063/1.2345359

Povey, Ian M.

Whitehead, D. E.

Thomas, Kevin K.

Pemble, Martyn E.

Bardosova, Maria

Irish Universities

Cork Open Research Archive

Title Photonic crystal thin films of GaAs prepared by atomic layer depositionAuthor(s) Povey, Ian M.; Whitehead, D. E.; Thomas, Kevin K.; Pemble, MartynE.; Bardosova, Maria; Renard, J.Publication date 2006Original citation Povey, I. M., Whitehead, D., Thomas, K., Pemble, M. E., Bardosova, M.and Renard, J. (2006) 'Photonic crystal thin films of GaAs prepared byatomic layer deposition', Applied Physics Letters, 89(10), pp. 104103.doi: 10.1063/1.2345359Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.2345359http://dx.doi.org/10.1063/1.2345359Access to the full text of the published version may require asubscription.Rights © 2006 American Institute of Physics.This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Povey, I. M., Whitehead, D., Thomas, K., Pemble, M.E., Bardosova, M. and Renard, J. (2006) 'Photonic crystal thin filmsof GaAs prepared by atomic layer deposition', Applied PhysicsLetters, 89(10), pp. 104103 and may be found athttp://aip.scitation.org/doi/abs/10.1063/1.2345359Item downloadedfromhttp://hdl.handle.net/10468/4385Downloaded on 2018-08-23T18:52:40ZPhotonic crystal thin films of GaAs prepared by atomic layer depositionI. M. Povey, D. Whitehead, K. Thomas, M. E. Pemble, and M. BardosovaJ. RenardCitation: Appl. Phys. Lett. 89, 104103 (2006); doi: 10.1063/1.2345359View online: http://dx.doi.org/10.1063/1.2345359View Table of Contents: http://aip.scitation.org/toc/apl/89/10Published by the American Institute of PhysicsPhotonic crystal thin films of GaAs prepared by atomic layer depositionI. M. Povey, D. Whitehead, K. Thomas, M. E. Pemble,a and M. BardosovaTyndall National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, Republic of IrelandJ. RenardDepartment of Materials Science and Technology, INSA, 20 Avenue des Buttes de Coësmes,35043 Rennes Cedex, FranceReceived 19 December 2005; accepted 12 July 2006; published online 6 September 2006Photonic crystal thin films were fabricated via the self-assembly of a lattice of silica spheres onsilicon 100 substrates. Progressive infilling of the air spaces within the structure with GaAs wasachieved using trimethylgallium and arsine under atomic-layer-deposition conditions. Samples withthe highest levels of GaAs infill were subsequently inverted using selective etching. Reflectancespectra are interpreted via the Bragg expression and calculated photonic band structure diagrams.For GaAs infilled and inverted samples, the relative positions of the first and second order Braggreflections are strongly influenced by the wavelength dependent refractive index. © 2006 AmericanInstitute of Physics. DOI: 10.1063/1.2345359Since the pioneering work of Yablonovitch1 and John2interest in photonic crystals has grown enormously. Photoniccrystals can exhibit a photonic band gap where light possess-ing certain values of wave vector is not allowed to propagatein the material. Opals are natural examples of photonic bandgap materials that may be readily synthesized in the labora-tory. In such materials the nature of the band gap is not onlyinfluenced by the lattice parameters but also the contrast inthe dielectric constant between the opal structure and itsvoids. This contrast can be enhanced by infilling the voidswith a material of higher refractive index and still further byinversion, the selective removal of the host lattice. The ef-fects of infiltrating semiconductor materials into opal havebeen demonstrated, for example, by the work of Astratov etal. with CdS nanoparticles,3 and Blanco et al. and Míguez etal., who have shown that inverted opals made from Si or Gepossess a complete photonic gap, across the whole Brillouinzone.4,5 Materials that possess complete gaps may be particu-larly suited to the fabrication of novel filter or mirror-baseddevices, although the existence of well-defined “pseudogaps”in these materials, which effectively prevent light propaga-tion in certain directions and not others, may also be of hightechnological significance, for example, in terms ofwaveguiding.6Infilling the void spaces within host photonic crystal lat-tices has been demonstrated with a wide range of materialsusing methods such as wet chemical impregnation,7 chemicalvapor deposition8–10 CVD, and, recently, atomic layerdeposition ALD. The ALD technique was demonstrated forthe ZnO,11 TiN,12 and TiO2 and ZnS:Mn Refs. 13 and 14systems and shown to produce uniform and highly conformalinfill from which inverted opals were fabricated with goodphotonic properties. In this present study we extend the ALDmethodology to III/V materials and, in particular, GaAs.GaAs-based photonic crystals are of particular interest interms of both the nature of the photonic gap produced in ahigh refractive index contrast medium and the possibility ofcoupling these effects with both linear and nonlinear opticalprocesses such as second harmonic generation, which wouldbe allowed in a material such as GaAs as a result of thedirect band gap and the noncentrosymmetric nature of thebulk unit cell, respectively.15 To date while several two di-mensional photonic band gap structures have been fabricatedusing lithographic methods, reports of fabrication of threedimensional GaAs photonic crystals are limited7 and thereare no reports of growth by CVD or related methods.In this present work we demonstrate that ALD methodsbased upon the use of a conventional metal organic vaporphase epitaxy MOVPE reactor may be used with a highdegree of control to infill opal photonic crystals with GaAs.Furthermore the extent and uniformity of infill is such thatsubsequently, the opal host may be removed, creating a highquality GaAs inverse opal. We demonstrate that the opticalproperties of the resulting material may be understood viathe Bragg equation which generally holds for such materials,provided that accurate refractive index and extinction coeffi-cient values matched to the wavelength of features measuredin the reflectance spectra are employed. In further support ofthe applicability of this simple Bragg model, we show thatthe observed behavior is consistent with that predicted viathe modeling of the photonic band structure using the MITphotonic band MPB package.16Artificial opal thin films were assembled on silicon 100substrates by controlled evaporation, utilizing monodis-persed silica spheres 260 nm in diameter after sinteringprepared by the Stöber synthesis17 Specular reflection mea-surements and electron microscopy confirm that the preparedopals exhibit high levels of uniformity. GaAs growth wasperformed in a horizontal reactor system Thomas Swan Sci-entific Equipment single 2 in. wafer horizontal MOVPE re-actor maintained at 400 °C and 200 torr using trimethylgal-lium TMGa and arsine AsH3 as precursors. Alternatinggas pulses of TMGa 3 s and AsH3 8 s at flow rates of5.310−5 and 4.510−3 mol min−1, respectively, were sepa-rated by a hydrogen purge 2 s to prevent the reactant gasesfrom mixing in the gas phase. Abrupt switching of precursorswas achieved using a pressure-balanced run-and-vent gasmanifold. The flow rate of hydrogen through the reactor was16 dm3 min−1 throughout precursor pulsing and purging.aElectronic mail: martyn.pemble@tyndall.ieAPPLIED PHYSICS LETTERS 89, 104103 20060003-6951/2006/8910/104103/3/$23.00 © 2006 American Institute of Physics89, 104103-1All samples were sintered under growth conditions priorto infiltration employing a 15 min temperature ramp de-signed to bring the substrate to growth temperature where itwas held for a further 15 min prior to growth. The level ofGaAs infill was tuned using an iterative process, varying thenumber of pulse sequences and calculating the infill usingthe Bragg expression. Inversion of infilled opal structureswas achieved by immersion in 1% HF solution for 20 min.Optical characterization was performed by diffuse reflec-tance measurements, using a Brüker IFS66 spectrometerequipped with a variable angle reflectance stage, and Ramanspectroscopy, using a Spectra Physics 2080/20s argon ionlaser 514.5 nm and a charged coupled device cameramounted on a Dilor XY 800 triple grating spectrometer.Figure 1 shows SEM images recorded after 200 and 800ALD cycles, showing that the infilling of the voids betweenthe silica spheres in the opal structure is initially rough butconformal and virtually complete after 800 cycles. The crys-talline nature of the infilled material is demonstrated by themeasured Raman spectra in Fig. 2 via the presence of the TO263 cm−1 and LO 286 cm−1 phonons for GaAs. The as-signment is supported by the presence of weak but charac-teristic crystalline GaAs features in the x-ray diffractionspectra. Figure 3 shows reflectance data for the bare opalsample and after varying numbers of ALD cycles. This figurereveals that for the bare opal the Bragg peak as measured atan angle of incidence of 12° from normal occurs at ca.592 nm. As the number of ALD cycles is then increased, sothe Bragg peaks shift progressively to higher wavelengthseventually reaching ca. 916 nm after some 800 cycles.The position of the nth order Bragg peak  is related tothe effective dielectric constant of the material eff, theweighted sum of the dielectric constant components, the lat-tice parameter d111, and the angle of incidence  by theexpressionn = 2d111eff − sin2 1/2, 1eff = fnSiO2 + ikSiO22 + fninfill + ikinfill2 + fnair+ ikair2  n2 − k2SiO2 + n2− k2infill + n2 − k2air,2where nx and kx are the refractive index and extinction coef-ficient of the component x, respectively.Using Eqs. 1 and 2 and n and k values of 3.57 and0.00, respectively, it is possible to estimate that after800 cycles, the contribution to the composite dielectric con-stant from air is approximately 9%. Thus it may be con-cluded that the voids between the silica spheres are morethan 90% infiltrated with GaAs. It should be noted that dueto geometric considerations it is generally accepted that con-formal growth within a face-centered-cubic opal structurewill create air pockets within the voids that occupy 3.6% ofthe available space thus making the maximum theoreticalinfill 96.4%. The spectrum obtained after some 200 cyclesshows no Bragg peak. This result is a convenient way ofdemonstrating the photonic crystal nature of these samples,because the levels of GaAs infill for this sample are such thatthe weighted average dielectric constant of the material oc-cupying the voids in the opal, i.e., n2−k2GaAs+ n2−k2air, isnow equal to the dielectric constant of the silica. Thus thismaterial possesses no refractive index contrast and as suchthe Bragg reflection is absent. From Eqs. 1 and 2 it maybe estimated that the level of infill at this point is ca. 26%.Peak positions, absolute reflectivity, and calculated levels ofinfill with their corresponding values of n and k are tabulatedin Table I for a range of ALD cycle numbers.Also of note is the appearance of lower wavelength fea-tures in the spectra for sample produced with higher levels ofinfill. These features could be assigned as second order peaksof the Bragg reflection, i.e., when n=2 in Eq. 1. However,the shift from the simple 2 relationship predicted by theBragg equation can only be partially explained by the sig-nificant variation in refractive index and extinction coeffi-cient of GaAs with wavelength, varying from n=4.4 to 3.5and k=2.0 to 0.0 across the region studied400–950 nm.18,19 Furthermore, when infiltration calcula-tions between the first and proposed second order featuresare compared Table I there is considerable disparity, withthe second order features consistently overestimating the in-fill. This leads to the conclusion that the features are notpurely due to a second order process but the convolution ofFIG. 2. Raman spectrum of an opal sample following 500 ALD cycles ofGaAs growth, showing the TO and LO modes of bulk GaAs at 263 and286 cm−1, respectively.FIG. 3. Color online Diffuse reflectance spectra of opal thin films for a abare opal 0% infill blue line, b 15% infill green line, c 26% infillgray line, d 67% infill black line, and e 91% infill of GaAs red line.Note the progressive redshift of the first order Bragg peaks as the infill levelincreases. At higher levels of infill the partially unassigned secondary fea-tures shift into the accessible spectral window.FIG. 1. SEM images of cleaved synthetic opals after a partial infill200 cycles, b near total infill 800 cycles, and c inversion.104103-2 Povey et al. Appl. Phys. Lett. 89, 104103 2006more than one band. This argument is supported by thebroadness and hints of structure exhibited by these spectralfeatures.Turning now to a comparison between experimental re-sult and theory, Fig. 4 shows the photonic Brillouin zonepredicted for a GaAs inverted structure calculated using theMIT photonic band package16 using a face-centered-cubiclattice with 260 nm spheres, n=3.62, and k=0.0. The singu-lar refractive index value used in the calculations was chosento match the Bragg reflection peak observed for the invertedstructure. The calculated Brillouin zone can be directly com-pared to reflection data displayed in Fig. 4 from which it isevident that there is good agreement with the position of thefirst order Bragg reflection and the pseudogap between thesecond and third bands between the U and L points in theBrillouin zone. The broad spectral feature at ca. 61014 Hz, which is unlikely to be purely a second orderBragg peak, could have a contribution from a full gap be-tween the eighth and ninth bands. This argument is supportedby the presence of a shoulder on the broad secondary featureat ca. 4.51014 Hz that can be attributed to a pseudogapbetween the fourth and fifth bands between the U-L and-L points in the Brillouin zone. However, assignments ofthese higher order bands should be treated with caution asthe dispersions in n and k are not considered at these wave-lengthsIn this letter we have described the production and char-acterization of the first CVD grown GaAs silica opal struc-ture by employing an ALD protocol. It has been demon-strated that ALD is an excellent method for the tunableinfiltration of GaAs into silica opal thin films, uniformly pen-etrating throughout the structure to give fractional infill of upto ca. 91% of the available space. Optical measurements con-firm the level of infill and nature of the material. In addition,for an inverted GaAs opal, comparison of the optical mea-surements with theoretical calculations demonstrates reason-able agreement with the predicted band structure, althoughunambiguous identification of higher energy features has notyet been achieved.The authors would like to thank M. Schmidt for acquir-ing the Raman spectra and R. H. Tredgold for helpful dis-cussions. This work was supported by Science FoundationIreland.1E. Yablonovitch, Phys. Rev. Lett. 58, 2059 1987.2S. John, Phys. Rev. Lett. 58, 2486 1987.3V. N. Astratov, V. N. Bogomolov, A. A. Kaplyanskii, A. V. Prokofiev, L.A. Samoilovich, S. M. Samoilovich, and Y. A. Vlasov, Nuovo Cimento D17, 1349 1995.4A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S. W. Leonard,C. Lopez, F. Meseguer, H. Miguez, J. P. Mondia, G. A. Ozin, O. Toader,and H. M. van Driel, Nature London 405, 437 2000.5H. Míguez, E. Chomski, F. Garcia-Santamaria, M. Ibisate, S. John, C.Lopez, F. Meseguer, J. P. Mondia, G. A. Ozin, O. Toader, and H. M. vanDriel, Adv. Mater. Weinheim, Ger. 13, 1634 2001.6M. T. Todaro, T. Stomeo, V. Vitale, M. DeVittorio, A. Passaseo, R.Cingolani, F. Romanato, L. Businaro, and E. Di Fabrizio, Microelectron.Eng. 67-68, 670 2003.7Y. C. Lee, T. J. Kuo, C. J. Hsu, Y. W. Su, and C. C. Chen, Langmuir 18,9942 2002.8H. M. Yates, M. E. Pemble, A. Blanco, H. Miguez, C. Lopez, and F.Meseguer, Chem. 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Positions of the Bragg peaks measured by reflectance spectroscopy for samples after differentnumbers of ALD cycles. The corresponding values of n and k for GaAs and the percentage levels of infillestimated from these parameters are also given.ALDcyclesFirst orderBragg peaknm na,b ka,bEstimatedinfill%Absolutereflectivity%Secondarypeaknm na,b ka,bEstimatedinfill%0 592 0 51100 616 3.86 0.211 15 29200 3.77 0.165 26500 840 3.62 0.08 67 48 488 4.396 0.434 70800 916 3.57 0.00 91 62 540 4.087 0.286 106aReference 18.bReference 19.FIG. 4. Comparison of the measured reflectance spectrum of an invertedGaAs opal and the corresponding calculated Brillouin zone.104103-3 Povey et al. Appl. Phys. Lett. 89, 104103 2006

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Photonic crystal thin films of GaAs prepared by atomic layer deposition

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