Opals consist of an ordered array of SiO2 spheres. This leads to a modulation of the refractive index and hence photonic stop bands behaviour over the visible/IR range of the electro-magnetic spectrum. The exact position of the stop bands depends on the size of the silica spheres. However, the refractive index contrast between the SiO2 spheres and air spaces is not great enough to open up a full photonic band gap (PBG), only the pseudogap. To increase the contrast the air spaces are filled with a material of high refractive index such as InP or GaP. To further increase the contrast the SiO2 is removed leaving a III-V framework as the inverse opal structure.



By use of MOCVD we have been able to infill opals with InP and GaP to such a level that has supported the inversion of the composite forming a structure of air holes within a III-V lattice. XRD and Raman confirmed the quality of the III-V infill, while the extent of the infill was studied by SEM and reflectance measurements

López, C

Meseguer, FJ

Nolan, MG

Palacios-Lidón, E

Pemble, ME

Rubio, S

Whitehead, DE

Yates, HM

English

Opals consist of an ordered array of SiO2 spheres. This leads to a modulation of the refractive index and hence photonic stop bands behaviour over the visible/IR range of the electro-magnetic spectrum. The exact position of the stop bands depends on the size of the silica spheres. However, the refractive index contrast between the SiO2 spheres and air spaces is not great enough to open up a full photonic band gap (PBG), only the pseudogap. To increase the contrast the air spaces are filled with a material of high refractive index such as InP or GaP. To further increase the contrast the SiO2 is removed leaving a III-V framework as the inverse opal structure.By use of MOCVD we have been able to infill opals with InP and GaP to such a level that has supported the inversion of the composite forming a structure of air holes within a III-V lattice. XRD and Raman confirmed the quality of the III-V infill, while the extent of the infill was studied by SEM and reflectance measurements

University of Salford Institutional Repository

GROWTH AND FORMATION OF INVERSE GaP AND InP OPALSH M Yates, D E Whitehead, M G Nolan, M E PembleSchool of Sciences, Cockcroft Building, University of Salford, Salford, M5 4WT, UK.E Palacios-Lidon,S Rubio1, F J Meseguer1, C LopezInstituto de Ciencia de Materiales de Madrid (CSIC), Campus de Cantoblanco, E-28049,Madrid, Spain1also Unidad Asociada CSIC-UPV, Av. Naranjos s/n, V-46022 Valencia, Spain.     Opals consist of an ordered array of SiO2 spheres.  This leads to amodulation of the refractive index and hence photonic stop bandsbehaviour over the visible/IR range of the electro-magnetic spectrum.  Theexact position of the stop bands depends on the size of the silica spheres.However, the refractive index contrast between the SiO2 spheres and airspaces is not great enough to open up a full photonic band gap (PBG), onlythe pseudogap.  To increase the contrast the air spaces are filled with amaterial of high refractive index such as InP or GaP.  To further increasethe contrast the SiO2 is removed leaving a III-V framework as the inverseopal structure.     By use of MOCVD we have been able to infill opals with InP and GaPto such a level that has supported the inversion of the composite forming astructure of air holes within a III-V lattice.  XRD and Raman confirmed thequality of the III-V infill, while the extent of the infill was studied by SEMand reflectance measurements.INTRODUCTION     Over the last decade there has been a great interest in photonic materials, expandingfrom the work of Yablonovitch (1) of mechanically drilled holes in ceramic (microwaveregion), to extensions of Si based microelectronics (growth, lithography, etching etc) forthe ‘log-pile’ structure in initially Si (2) and then GaAs (3).  The alternative approachwas to produce PBG materials by self-organisation, as seen with opal.  Opal consists of aregular fcc structure of silica spheres which provides a periodic modulation of thedielectric constant.  The size of the spheres controls the periodicity and the frequency ofthe visible/IR spectrum in which it operates.  However, the dielectric contrast betweenthe silica spheres and the air gap is low (εSiO2/εair = 2/1) so leads to a pseudo-gap.  Thecontrast can be increased by infilling the air gaps with material of a higher dielectricconstant (SnO2, GaP, InP (4,5,6)).  Theoretical values (7) for a full gap need a dielectriccontrast of 16.   However, this requirement can be halved to a dielectric contrast of 8 ifthe inverted structure of a series of ordered ‘air’ holes is formed (8).  This inversion ofthe composite sample would also improve the contrast yet further.  As yet this has beenachieved with various II-VI compounds (CdSe (9), CdS (10)), group IV (Ge (11), Si(12)) and TiO2 (13) by use of both Silica and polymer based opals.  Until recently no III-V compounds had been successfully infilled to a high enough level to allow inversion.The first success was by Golubev (14) with GaN by impregnating opals with Ga2O3 andheating under NH3.  In this paper we report for the first time the production of invertedIII-V structures of GaP and InP.  Both InP and GaP are included as the method ofproduction is very similar, the only difference being the reactants.  Also thecharacterisation of the composite and inverse opal structures gives similar results.     In earlier work (6,15) we showed that InP and GaP could be placed within opal byCVD and that even low (5%) infill lead to extensive PBG changes.  Further modificationof the MOCVD reactor design has been carried out, such that the reactant gases arechannelled over a constricted space where the opal is placed.  From this it has beenpossible to achieve extremely high loading of the opals, such that an inverted structurehas been obtained.  This is presented as a major technological breakthrough, whichdemonstrates the feasibility of filling opaline systems with conventional III-Vsemiconductors, using equipment, which apart from the reactors themselves, are industrystandard.EXPERIMENTAL     The method used to produce the opals has previously been described, as has themethod of sintering (16).  The sintering is important as it controls the filling fraction andleads to the formation of necks between the opals, which facilitate the removal of theoriginal opal when the inverse structure is formed.  The composite III-V opal structure issoaked in a 1% HF solution for 7 hours to remove the silica.  In the samples chosen theopal sphere sizes were 230nm, 310nm and 358 nm.     The composite structures are grown in a standard atmospheric MOCVD reactor(Electrogas Systems).  The only modification necessary was to the actual reactor.  It hadpreviously been converted to a vertical system (5) in which the opals sat on the top of are-designed carbon susceptor (peppered with 2 mm diameter holes) in an attempt to forcethe reactant gases through the opals rather than over the top surface.  This, although animprovement, still left problems in that not all the opal could be over a hole in thesusceptor and hence over an area where gas could flow.  Also the susceptor was loosefitting, allowing another route for the reactant gases.  To get round these problems asintered quartz platform was fused to the reactor so that all the opal sits on a poroussurface.  However, as this does not allow efficient heating of the samples a carbonsusceptor disk was made with a hole (5 mm diameter) for the sample.     Reactants used were trimethyl gallium (3.0 x 10 –5 mol min-1) or trimethyl indium (1.9x 10-5 mol min-1) (Epichem) with phosphine (8.9 x 10-4 mol min-1) (BOC) in a carrier gasof hydrogen (1 L min-1).  As previously stated (17) a cyclic method is used in which thereactants are sequentially passed through the reactor to allow growth within the opalrather than on the surface.  This cycle is repeated up to 10 times.  Prior to growth thesamples are heated to 200 oC for 1 hr to remove any moisture within the opal pores.During growth the organo-metallic is added at 50 oC and the phosphine at 400 oC at 30minute intervals.     Characterisation was carried out with X-ray diffraction (Siemens D5000), micro-Raman 514.5 nm Ar line (Renishaw 1000) and scanning electron microscopy.  To studythe photonic band gap behaviour of the samples Bragg reflectance measurements wereused.  Incident plane parellel white light (Tungsten (NIR) and Xe arc (visible)) at 15o tonormal to the (111) orientation of the composite and inverse structures was used.RESULTS AND DISCUSSION     Visually, the infilled opals are highly opalescent and deeply coloured.  The actualcolour seen depends on the angle it is viewed.  The uniformity of the colour across thesample relates to the uniformity of the infill and quality of the opal.     The infilled GaP and InP was polycrystalline, as seen from the XRD (figure 1) withstrong diffraction peaks for the (111), (220) and (311) orientations.Figure 1 - X-ray diffraction of InP and GaP composite opals.  The peaks at 38o and 44oon the GaP-opal sample come from the XRD sample holder.    Also the micro-Raman (figure 2) shows strong signals for the LO and TO phonons.However, it can be seen that the position of the GaP LO and TO phonon bands (390 cm-1,357 cm-1) are at lower Raman shifts than that of bulk literature values (402, 366 cm-1)(18).Figure 2 – Raman for the III-V composites.020004000600080000 200 400 600InP-opal compositeGaP waferGaP-opal compositeRaman shift, cm-1Intensity0408012016020 30 40 50 60InP-opalGaP-opal(311)(220)(111)2ϕcountsThis reduction in Raman shift could be due to strain within a GaP sample (19).  Acomparison with a GaP wafer, that should be approximately strain free, is shown to havethe highest Raman shift values (411 cm-1, 373 cm-1).  A similar result was seen for theInP with the LO and TO phonons of InP (291 cm-1, 328 cm-1), which again are lowerthan the literature values of 304 and 346 cm-1.  The softening of the phonons can be alsodue to the nanocrystalline quality of the semiconductors loaded.  The Raman k-vectorselection rules are relaxed in the case of amorphous and nanocrystalline semiconductors.Therefore, Raman peaks are broader and the frequency becomes softer as accurs in ourcase.  Alternatively (or as well) it could be related to increased III-V porosity with theRaman shift to lower values (20).     SEM (Figure 3) depth profiles of a cross-section through the opal host (φ = 358nm)showed that it was uniformly deposited within the opal and penetrated heavily to at least200 µm (figure 3b).(a)       (b)  (c)       (d)Figure 3 - SEM images (a) near surface and (b) depth of 200 µm of the interior of theGaP infilled opal sample, (c) inverted GaP opal (air holes 358 nm), (d) inverted InP opal(air holes 230 nm).It can be seen that the GaP forms a thick skin round the opal spheres consisting of lots ofsmall crystallites. The same type of behaviour can be seen with InP.  It is possible thatthe infill takes place by the growth of many small independent crystallites that graduallyfill up the space round the opal spheres. It was also possible to invert these structures, asseen in Figure 3c (GaP) and 3d (InP).  The hexagonal structure of air holes and GaP/InPwalls can be clearly be seen.  The order within the samples stretches over 10’s ofmicrons.      At a given wavelength InP and GaP have similar, but not identical, refractive indices.For example at a wavelength of 700 nm the refractive indices of InP and GaP are 3.48and 3.23 respectively.  The difference is relatively small, but it is still large enough tohave an influence on the PBG effects of the infilled and inverted opals.  The opticalproperties of the III-V opal composites and inverse structures gave broadly the sameresult (differences in overall intensity due to opal quality, shifts in position of the bandsdue to bare opal size and amount of infill).  From the position of the Bragg stop band it ispossible to estimate the extent of III-V infill within the opal.  The higher the extent ofinfill the greater the wavelength shift from the bare opal.    The reflectance data is complex in the cases of InP and GaP and will be reported inmore detail in another publication.  In general the 1st order Bragg stop band is taken to bethe most intense, well defined peak round a/λ=0.65 (see Figure 4).  However, if thisapproach is taken, then the data (in Figure 4) implies that there is little to no shift in stopband position following infill.  In fact the infill calculation predicts a level of at the most7% pore volume.  This is obviously not correct, as there is enough GaP (or InP) withinthe pores to support inversion.  As the Bragg peak is very sensitive to the infilling itshould be wider than that for the bare opal, but it is actually narrower.  Finally, this peaklies to the lower wavelength side of two of the infilled samples shown, which wouldimply a reduction in the average refractive index of the composite opal, which is notpossible.Figure 4 – Bragg diffraction showing the shift in wavelength (and hence infill for GaP)depending on the number of growth cycles. Sphere size of the bare opal used is 310 nm.The Bragg stop band has been taken to be the higher wavelength, low intensity peak.Using this, the infill can be calculated by Bragg’s Law.  However, these values shouldonly be taken as an approximate guide for comparison.  Possible reasons are that Bragg’sLaw is only strictly accurate for weak dielectric contrast where(ε1-ε2)/(ε1+ε2)<1and that the choice of dielectric constant value in the calculation could be inaccurate dueto unintentional doping (21) of the GaP or InP.00.250.500.751.000.3 0.5 0.7 0.9 1.15 cycles5+5 cycles10 cyclesa/λReflectance (a.u.)     Samples were prepared in two different ways. In the first one, the sample was infilledin five growth cycles and then characterised. After this, a new infiltration (five morecycles) was carried out. The second one consisted of performing ten growth cycles,without the extraction of sample from the reactor. Reflectance measurements are shownin Figure 4 for the GaP-opal composite.  For clarity the bare opal has not been shown,which consists, within this wavelength range, of a single peak at 670 nm (a/λ=0.654).    As seen in Figure 4 the Bragg peak for 10 cycles (a/λ=0.51, 52% pore volume) is athigher wavelength than that for 5+5 cycles (a/λ=0.55, 35% pore volume) which is againgreater than that for 5 cycles (a/λ=0.56, 30% pore volume).  From this it is obviouslymore efficient making the infiltration in one run of 10 cycles (peaks are shifted to higherwavelengths) than 2 runs of 5 cycles.  Both these methods, as expected, infill the opal toa higher level than just 5 growth cycles.  The reduced efficiency is probably due to a thinfilm of GaP grown on the top of the opal. This gradual closing of the opal pores reducesthe amount of infiltration inside the opal.  Also, the removal of the sample from thereactor leads to the influx of air and moisture, which in turn will require that the sampleis re-baked before infilling.  This may have a detrimental effect on the sample.  Only 10cycle growth samples support the inversion for GaP, while for InP 4 to 6 cycles(depending on the size of the opal spheres infiltrated) was sufficient.  The faster growthof the InP, under the growth conditions used, may relate the difference in the Ga-C andIn-C bonds in the organo-metallic reactant.  The lower breaking strength (22) of the In-Cbond making it more thermodynamically favourable that the trimethyl indium reactsfaster, hence speeding the reaction.     Figure 5a shows the behaviour on the Bragg stop-band on changing between bare toGaP-opal composite to GaP inverse structure.  As expected the bandwidth increases asthe dielectric contrast increases from bare opal (silica/air 2.1 to composite 5 to inverse10).  An approximate doubling of the dielectric contrast leads to atleast a doubling of theexperimental gap to mid-gap ratios ∆λ/λ (6%, 22%, 46%).(a)                    (b)Figure 5 – Bragg reflectance of (a) GaP inverse opal, GaP-opal composite and bare opal(sphere size 358 nm) (b) InP inverse opal, InP-opal composite and bare opal (sphere size230 nm).00.050.100.150.3 0.4 0.5 0.6 0.7 0.8bare opalcompositeinversea/λReflectance (a.u.)00.20.40.60.3 0.4 0.5 0.6 0.7 0.8compositeinversebarea/λReflectance (a.u.)This is particularly noticeable with the inverse structure as this is a much strongerscatterer due to the fill factor being 0.26 not 0.74.  The position of the stop-band willbasically depend on the average dielectric constant as the underlying structure is notchanged on infilling or inversion.   The average dielectric constant increases from that ofthe bare opal 1.8 to the inverse structure 2.4 (65% pore volume) (totally infilled 3.1) tothe composite 3.2 (34% pore volume) (totally infilled 3.9).  As this increases the stop-band energy decreases as seen in figure 5a.    This behaviour is all replicated for the equivalent InP set of structures, as seen inFigure 5b.  As the opal used is much smaller (230 nm spheres rather than 358 nm) thewavelength range in which the stop bands are observed is at a lower wavelength andhence the dielectric constant used is higher.  Again as the dielectric contrast increases(silica/air 2.1 to composite 6 to inverse 12.5) there is a broadening of the experimentalgap to mid-gap ratios ∆λ/λ (4%, 35%, 43%).  However this is greater than expected socould be due to the presence of a range of bands.  The stop band energy is seen toincrease from that of the composite (96% pore volume), through the inverted structure(98% pore volume), to the bare opal as the average dielectric constant decreases (4.8, 3,1.8).    By use of a a/λ scale on the x-axis it makes it possible to compare directly the InP andGaP reflectance results, removing the differences caused by the wavelength position oropal sphere size.  For both the composite and the inverse structures the shift for the GaPis less than that for the InP, which is in agreement with a lower dielectric constantaverage for the GaP-composite and GaP inverse structures, due to less GaP deposition.The band width of the composite is wider for InP than GaP confirming the higherdielectric constant contrast for the InP-composite.  For the inverse structures the bandwidths are similar with greater error in their measurement.CONCLUSIONS     Very high levels of good quality GaP and InP have been grown within the opalstructures as seem by X-ray diffraction and Raman.  The behaviour of the (111) Braggpeak fitted well with the expected behaviour relating to the dielectric contrast/averageand the fill factor.  It was found that a 10 cycle growth was necessary for inversion of theGaP structure while only 4 - 6 cycles were needed for the InP structure.  These relating tothe faster growth rate of InP over GaP.  Also of interest was that a single experiment of10 cycles was more efficient than 2 experiments of 5 cycles. Increased numbers ofgrowth cycles, as expected, increased the amount of infill, but with ever decreasingamounts at each cycle due to the gradual reduction in the size of the pores within theopal.  It has successfully been shown, for the first time that inverted opaline structures ofGaP and InP can be grown.ACKNOWLEGDEMENTSThis work is supported by the EC PHOBOS project 27731 and the EPSRC in the UK.                                                 REFERENCES1 E. Yablonovitch, T.J. Gmitter, K.M. Leung, Phys.Rev.Lett.,67, 2295 (1991)2 E. Ozbay, E. Michel, G. Tuttle, R. Biswas, M. Sigalas, K.-M. Ho Appl.Phys.Lett., 64,2059 (1994)3 N. Yamamoto, S. Noda, A. Chutinan, Jpn.J.Appl.Phys., 37, L1052 (1998)4 H.M. Yates, M.E. Pemble, A. Blanco, H. Miguez, C. Lopez, F.J. Meseguer, ChemicalVapor Deposition, 6, 283 (2000)5 D.E. Whitehead, M.E. Pemble, H.M. Yates, A. Blanco, C. Lopez, H. Miguez, F.J.Meseguer, J. de Physique IV,12, 63, (2002)6 H. Miguez, A. Blanco F.J. Meseguer, C. Lopez, H.M. Yates, M.E. Pemble, V. Fornes,A. Mifsud, Phys.Rev.B, 59, 1553, (1999)7 H. Sozuer, J.Haus, R.Inguva Phys.Rev.B, 45, 13962 (1992)8 K. Busch, S. John, Phys.Rev.E, 58, 3896 (1998)9 Y.A. Vlasov, N. Yao, D.J. 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Growth and formation of inverse GaP and InP opals

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