UV-VUV laser induced phenomena in SiO2 glass

UV–VUV laser induced phenomena in SiO2glass Koichi Kajihara a,*,Yoshiaki Ikuta a,Masanori Oto b,Masahiro Hirano a,Linards Skuja a,c,Hideo Hosono a,da Transparent Electro-Active Materials Project,ERATO,Japan Science and Technology Agency,KSP C-1232,3-2-1Sakado Takatsu-ku,Kawasaki213-0012,Japanb Syowa Electric Wire and Cable Co.Ltd,Minami-Hashimoto,Sagamihara229-1133,Japanc Institute of Solid State Physics,University of Latvia,Riga LV1063,Latviad Materials and Structures Laboratory,Tokyo Institute of Technology,4259Nagatsuta,Midori-ku,Yokohama226-8503,JapanAbstractCreation and annihilation of point defects were studied for SiO2glass exposed to ultraviolet(UV)and vacuum UV (VUV)lights to improve transparency and radiation toughness of SiO2glass to UV–VUV laser light.Topologically disordered structure of SiO2glass featured by the distribution of Si A O A Si angle is a critical factor degrading trans-mittance near the fundamental absorption edge.Doping with terminal functional groups enhances the structural relaxation and reduces the number of strained Si A O A Si bonds by breaking up the glass network without creating the color centers.Transmittance and laser toughness of SiO2glass for F2laser is greatly improved influorine-doped SiO2 glass,often referred as‘‘modified silica glass’’.Interstitial hydrogenous species are mobile and reactive at ambient temperature,and play an important role in photochemical reactions induced by exposure to UV–VUV laser light.They terminate the dangling-bond type color centers,while enhancing the formation of the oxygen vacancies.Thesefindings are utilized to develop a deep-UV opticalfiber transmitting ArF laser photons with low radiation damage.Ó2003Elsevier B.V.All rights reserved.1.IntroductionExcimer lasers such as KrF(5.0eV or248nm), ArF(6.4eV or193nm)and F2(7.9eV or157nm) lasers are important coherent light sources in the ultraviolet(UV,<400nm)and vacuum-ultraviolet (VUV,<200nm)wavelength regions.Among the known amorphous optical materials,high-purity synthetic SiO2glass is most widely used for optical components in UV–VUV wavelength pared to crystalline optical materials,amor-phous optical materials are distinctively featured by good shape workability and optical isotropy. However,UV–VUV optical properties of SiO2 glass are often degraded by pre-existing and laser-induced point defects since they induce optical absorption bands andfluctuation of the refractive index[1].In particular,weak optical absorptions near the fundamental absorption edge of SiO2 glass prevented SiO2glass to be used for F2laser optics,which is expected to be a light source in the next-generation photolithography[2].In the present paper,we review our resent studies to develop SiO2glass for UV–VUV lasers.More general reviews on laser-induced point *Corresponding author.Tel.:+81-44-850-9759;fax:+81-44-819-2205.E-mail address:k-kajihara@net.ksp.or.jp(K.Kajihara).0168-583X/$-see front matterÓ2003Elsevier B.V.All rights reserved.doi:10.1016/j.nimb.2003.12.032Nuclear Instruments and Methods in Physics Research B218(2004)323–331/locate/nimbdefects in SiO2glass are available in[1,3].We dis-cuss the effects of topologically disordered struc-ture inherent in SiO2glass on optical absorption near the absorption edge of SiO2glass,and the reduction of the absorption byfluorine doping. We also describe the influence of mobile interstitial hydrogenous species on UV–VUV transparency of SiO2glass through photochemical reactions in-duced by UV–VUV laser irradiations.Finally, utilizing the obtainedfindings,we demonstrate a development of deep-UV(DUV,<300nm)optical fiber which is transparent and tough for ArF laser light.2.Effects of topological disorder on VUV transmit-tance of SiO2glass2.1.Topologically disordered structure of SiO2glassMost forms of the stoichiometric oxides of sil-icon,SiO2,consist of three-dimensionally extend-ing network of corner-linked regular SiO4 tetrahedra[4].Among the crystalline SiO2poly-morphs,optical properties have been most studied for a-quartz.In a-quartz,all the tetrahedra are equivalent and two adjacent tetrahedra are bridged by an equal Si A O A Si angle.On the other hand,amorphous SiO2(SiO2glass)is a polymorph of SiO2featured by disordered arrangement of the tetrahedra.Amorphous SiO2includes various structural variants since the topological arrange-ment of the tetrahedra is not unique.However,in contrast to crystalline SiO2,atomic position is not a useful parameter to describe the structure of amorphous SiO2since the translational symmetry is lost.One of the parameters commonly used to characterize SiO2glass is the Si A O A Si angle[5], which has been measured by X-ray diffraction[6], nuclear magnetic resonance[7]and infrared spec-troscopy[8].Here,degree of distribution of the Si A O A Si angle is considered to indicate the degree of disorder in SiO2glass,since there is no distri-bution in the Si A O A Si angle in crystalline SiO2. Several observations have suggested that VUV transparency of SiO2glass depends on the Si A O A Si angle.Decrease in the average Si A O A Si angle accompanied with densification[9]shifts the fundamental absorption edge to lower energy side [10].Further,bandgap energy of SiO2glass is smaller than that of a-quartz[11].Evidently,it is crucial to modify the degree of disorder of SiO2 glass in improving the transparency of SiO2glass near the fundamental absorption edge.Although it is an important subject when developing SiO2glass used with F2laser,systematic studies have not been performed.Transmittance and toughness for F2laser light were examined for SiO2glass samples with differ-ent distribution of the Si A O A Si angle,which was controlled by changing the thermal anneal tem-perature[12].High-purity synthetic SiO2glass (SiOH:2·1018cmÀ3)containing negligible amount of pre-existing defects was thermally an-nealed for120h at900°C,20h at1100°C,1h at 1200°C,or0.1h for1400°C to obtain equili-brated glass structures at anneal temperatures (referred as T f hereafter).Both the optical absorption near the fundamental edge of SiO2 glass(Fig.1)and the optical absorption induced by exposure to F2laser($2.5mJ cmÀ2,8.4·106 pulses)(Fig.1(inset))increased with an increase in T f.Color centers dominating the laser-induced absorption bands were the E0center(a silicon dangling bond,B SiÅ,5.8eV)and the non-bridging oxygen hole center(NBOHC,an oxygen dangling bond,B SiOÅ,4.8and6.8eV).Since comparable324K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331concentrations of the E0centers and NBOHC were formed,photolysis of the Si A O A Si bond is sug-gested to dominate the defect processesB Si A O A Si B!B SiÅþÅO A Si B:ð1ÞNext,defect formation was examined as a function of pulse energy of F2laser.At K10mJ cmÀ2,the concentration of laser-induced defects depended linearly on the pulse energy of F2laser[12,13]. This result indicates that at this pulse energy re-gion the reaction described by Eq.(1)proceeds viaone-photon excitation of the precursors which absorb F2laser photons.Above the threshold pulse energy,however,concentration of laser-in-duced defects increased proportionally with the pulse energy squared(Fig.2)[13].Such quadratic dependence of defect concentration on pulse en-ergy is commonly found for SiO2glasses exposed to KrF or ArF laser pulses.Here defects are cre-ated by the two-photon absorption processes since the one-photon absorption coefficient of SiO2glass is very small for KrF and ArF laser lights.To examine the mechanism of the quadratic depen-dence,the quantum yield of defect formation was evaluated.The quantum yield of F2-laser-induced two-photon defect formation was$3orders of magnitude larger than that expected for the simple two-photon absorption processes[13].This dis-crepancy indicates that two-step absorption via real intermediate state dominates defect creation in SiO2glass exposed to F2laser pulses of J10 mJ cmÀ2.This observation again suggests that electronic states which absorb F2laser photons play an important role in defect creation.Finally,the actual structure of defect precursors is considered.With increasing T f up to1500°C,the density of SiO2glass increases[14]and the average Si A O A Si angle decreases[15].Thus the inset of Fig.1indicates that reduction of the average Si A O A Si angle increases the concentration of de-fect precursors.On the other hand,molecular orbital calculations have suggested that the strain energy of the Si A O A Si bond increases more strongly with decreasing the Si A O A Si angle from the most relaxed angle($140–150°)than with increasing the angle[16,17].Further,Raman spectroscopy indicates that increase in T f enhances 3-and4-membered ring structures of the SiO4 tetrahedral units,which are much strained than the larger ring structures[17].A part of such strained Si A O A Si bonds are most likely to be defect precursors since they release strain energies on dissociation to enhance the stability of resultant defect pairs(Fig.3).2.2.Fluorine doping into SiO2glassObservations shown in Section2.1indicate that the reduction of the strained and unstable Si A O A Si bonds is crucial to enhance the F2laser toughness of SiO2glass.Although low-tempera-ture thermal anneal is a possible approach to de-crease T f,it is not practical since the structural change of SiO2glass is very slow at low tempera-tures.On the other hand,it is possible to accelerate the structural change by incorporating terminal functional groups such as the SiOHand SiClK.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331325groups[18,19],which break up the glass network and reduce the viscosity.This technique allows to prepare SiO2glass with low T f within a reasonable time scale.Indeed,SiO2glass containing the SiOH groups(wet SiO2glass)exhibits good KrF and ArF laser toughness[20,21].We selected the SiF group as a terminal func-tional group to prepare SiO2glass for F2laser because of the following reasons:First,the SiF groups do not exhibit optical absorption withinthe band gap.Second,they are expected to be stable against the exposure to F2laser light since the Si A F bond is stronger than the Si A O bond. Indeed,good VUV transparency[22,23]and radiation toughness to c-rays[22]and ArF laser light[23]have been reported for F-doped SiO2 glasses.Fig.4demonstrates that F-doped SiO2glass exhibits excellent transparency and toughness for F2laser light[24,25].On the other hand,wet SiO2 glass is not transparent for F2laser light since the SiOHgroups have optical absorption at J7.4eV (Fig.4)[26].Another class of technically-impor-tant synthetic SiO2glass is dry(SiOH-free)SiO2 glass,which is widely used for opticalfibers for telecommunication.However,since oxygen vacancies(the Si A Si bonds)generated during elimination of SiOHexhibit optical absorption at 7.6eV,dry SiO2glass is also difficult to use with F2 laser(Fig.4).The transparency and laser toughness are sig-nificantly improved while doping with SiF groups in concentrations up to1mol%,whereas only a slight further improvement is achieved by an additional increase in SiF concentration[27,28]. This observation indicates that the effect of F-doping is not due to the substitutional effects between O and F where the bandgap depends al-most linearly on the SiF content.In contrast,this result suggests that the role of the F-doping is to enhance the structural relaxation where the SiF content of1mol%is sufficient.Fig.5schemati-cally shows how the F-doping reduces the strain of the glass network and enhances the laser toughness.3.Mobile hydrogenous species in SiO2glass3.1.In situ observation of mobile hydrogenous species in SiO2glassSince the corner-shared network of the SiO4 tetrahedra does not allow close packing of the tetrahedra,structure of SiO2glass is relatively sparse.Thus small molecules are frequently found in the lattice interstitial spaces of SiO2glass. Among known interstitials,hydrogenous species, such as the atomic hydrogen(H0)and hydrogen molecule(H2),have attracted attentions since they are mobile and reactive at ambient temperature, and affect various laser-induced defect processes.In situ measurements are useful to study reac-tions involving mobile hydrogenous species since the reactions are relatively rapid.In situ mea-surements of mobile hydrogenous species in SiO2326K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331glass have been mostly performed by radiolysing the pre-existing SiOHgroup into H0and NBOHC and measuring the following decay kinetics of these species by electron paramagnetic resonance (EPR)[29–33].However,EPR is not suitable to perform measurements in wide temperature range since EPR signals are significantly influenced by temperature and several paramagnetic centers(e.g. NBOHC)are even difficult defect at temperatures as high as ambient temperature due to the line broadening[34].In addition,excitation sources such as X-rays,c-rays and pulsed electron beams photolyze not only the SiO A Hbond but also the Si A O A Si bonds.To selectively photolyze the SiO A Hbond,we used an F2laser since F2laser light directly excites the optical absorption band of the SiOHgroup at J7.4eV to create pairs of H0and NBOHC with a quantum yield as large as$0.1to0.3(Fig.6)[35].Dissociation of the Si A O A Si bonds is negligible here(quantum yield:$10À4).On the other hand,the created NBOHC can be detected down to1014cmÀ3in situ by its1.9eV photolu-minescence(PL)at all temperatures below ambient temperature using a fourth-harmonic of Nd:YAG laser(4.7eV,266nm)as a probe source.During thermal anneal,NBOHCs are annihilated by reacting with mobile H0and H2.Thus diffusion and reactions of H0and H2are sensitively mea-sured by utilizing NBOHC PL as a probe(Fig.6).Wet SiO2samples were irradiated with an F2 laser pulse at10K,and subsequently heated to 330K while measuring the concentration change of NBOHC(Fig.7)[36].The decay of NBOHC was separated into three temperature ranges:(I) below100K,(II)100–160K and(III)above 200K.Such three temperature ranges are also seen in NBOHC decay curves taken with respect to time under a constant temperature(Fig.7(inset)).A simultaneous decay of H0and NBOHC with temperature,which was observed by an EPR measurement,indicates that ranges I and II are due to reaction of NBOHC with H0[36].On the other hand,range III is attributable to reaction of NBOHC with H2judging from the low tempera-ture shift of range III by H2impregnation.K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331327Further,as range III also appeared in the sample which was H2-free before the F2laser irradiation,a part of H0is considered to turn into H2.Activa-tion energy(E a)for diffusion was derived by sim-ulating the decay curves in Fig.7.It was assumed that E a has a distribution reflecting the topologi-cally disordered structure of SiO2glass.The E a value for H0peaked at$0.1and0.2eV,probably attributable to free and shallow-trapped H0, respectively[36].These values are consistent with the reported E a values[31–33,37,38].The E a value for H2peaked at$0.4eV[36].This value also agrees well with those of the precedent studies [30,39–41]which include macroscopic H2diffusion through bulk SiO2glass[39–41].Fig.8shows formation and decay of NBOHC for the samples irradiated with10F2laser pulses at ambient temperature.The‘‘spike’’-like spectrum indicates that a part of NBOHCs rapidly disap-pears as soon as they are formed.Further,decay of NBOHC was much faster in the H2-impreg-nated sample,demonstrating that H2is mobile enough at ambient temperature to rapidly termi-nate NBOHC[35].On the other hand,Fig.8 shows that the concentrations of NBOHC created by F2laser pulses are comparable between the H2-free and H2-impregnated samples.Thus it is evident that H2-impregnated glass leaves less NBOHCs not because of the low quantum yield of NBOHC formation,but because or rapid decay of NBOHC.3.2.Effects of mobile hydrogens on defect formation and annihilationAs shown in Section 3.1,the interstitial H2 effectively reduces the laser-induced dangling bonds such as NBOHC.This phenomenon is attractive to improve the long-term laser tough-ness of SiO2glasses used for UV–VUV optical components.Indeed,it has been concluded that H2-loading generally improves the radiation toughness of SiO2glasses for KrF and ArF lasers [42–44].However,influence of H2on F2laser toughness of SiO2glass has been unclear.We examined effects of the interstitial H2on the formation of color centers in SiO2glass exposed to ArF or F2laser light(Fig.9)[45].In H2-free samples,laser irradiation induced a broad UV–VUV optical absorption dominated by the E0 center(5.8eV)and NBOHC(4.8and6.8eV).In H2-impregnated samples,on the other hand, optical absorption at<6.5eV due to the E0center and NBOHC was suppressed,as expected from the results shown in Section3.1.However,impregna-tion of H2enhanced the formation of the oxygen vacancy(the Si A Si bond),and its absorption band centered at7.6eV reduced the transmittance at 7.9eV.We suggest that the Si A Si bonds are created by‘‘photochemical reduction’’of glass net-328K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331work by the interstitial H2,which proceeds simul-taneously with the termination of the dangling bonds.These results conclude that H2loading is not suitable for SiO2glasses used with F2lasers.4.Deep-UV opticalfiberExcellent UV–VUV transparency and shape workability of SiO2glass are suitable properties in preparing opticalfibers transmitting UV–VUV light.Such opticalfibers are attractive to deliver UV–VUV light and to be used for an optical probe in UV–VUV scanning near-field optical micro-scope(SNOM).However,transmittance and radiation toughness at<200nm have not been satisfactory for opticalfibers manufactured so far [46,47].Development of suchfiber seems to bevery difficult because of the following reasons. First,due to the long optical path of opticalfibers, the concentrations of color centers should be sev-eral orders of magnitudes lower than that of the bulk glasses.Second,a part of dangling bonds, which are inevitably created by the deformation during the drawing,often remains in the resultant fibers as the drawing-induced defects[48,49]. Third,as a result of quenching from$2000°C,the fibers generally contains high concentration of the strained Si A O A Si bonds and defect precursors which potentially turn to color centers with expo-sure to UV–VUV light.These problems have been partly overcome by doping SiOHgroups to en-hance the structural relaxation[47].However,ArF laser toughness is not good enough,possibly be-cause a small portion of the SiOHgroups is turned into color centers,which are not significant in bulk SiO2glasses.As discussed in Section2,fluorine doping is also effective in enhancing the structural relaxation. Further,the SiF groups are more stable than the SiOHgroups against exposure to VUV light.Thus we used F-doped SiO2glass for the core part of DUVfiber[50].The clad part also consisted of F-doped SiO2glass with higher SiF content to de-crease the refractive index.As shown in Fig.10,1 m transmittance of the resultantfiber was as high as$60%for ArF laser light.However,it rapidly decreased with ArF laserfluence(Fig.10(inset))because of the formation of the E0center(Fig.10). To suppress the formation of the E0center,the fiber was loaded with H2based on the observa-tions shown in Section3.Transmittance of the H2-loadedfiber degraded only little with exposure to ArF laser up to$105pulses at$50mJ cmÀ2 since the E0center was not accumulated(Fig.10). An additional benefit of thisfiber is that thefiber tip can be easily sharpened by immersion into hydrofluoric acid(Fig.11).This feature is useful for preparation of probe tips for deep-UV SNOM.Fig.11.Scanning electron microscopic photograph of the end of deep-UVfiber sharpened by etching with hydrofluoric acid.K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–3313295.SummaryTopologically disordered structure of SiO2glass is an intrinsic origin of optical absorption near the fundamental absorption edge of SiO2glass,and its reduction byfluorine doping improves transmit-tance and laser toughness of SiO2glass for F2 laser.Doping by the SiOHgroups is also effective to reduce the structural disorder.However,the SiOHgroups absorb F2laser light and can also be precursors of color centers.The interstitial H2 terminates the dangling-bond type defects, whereas it enhances the formation of the oxygen vacancies by photochemical reduction of the SiO2 network.Since effects of the SiOHgroups and interstitial H2are not simple as described above, appropriate control of their content is important to optimize the UV–VUV optical properties of SiO2glass.These knowledge opened a way to de-velop a deep-ultraviolet opticalfiber transmitting ArF laser pulses with maintaining a loss as low as $1.7dB mÀ1.References[1]G.Pacchioni,L.Skuja,D.L.Griscom(Eds.),Defects inSiO2and Related Dielectrics:Science and Technology, NATO Science Series,Kluwer Academic Publishers, Dordrecht,Netherlands,2000.[2]V.Liberman,T.M.Bloomstein,M.Rothschild,J.H.C.Sedlacek,R.S.Uttaro,A.K.Bates,C.V.Peski,K.Orvec,J.Vac.Sci.Technol.B17(1999)3273.[3]L.Skuja,H.Hosono,M.Hirano,K.Kajihara,Proc.SPIE5112(2003)2.[4]L.W.Hobbs,X.Yuan,in:G.Pacchioni,L.Skuja,D.L.Griscom(Eds.),Defects in SiO2and Related Dielectrics: Science and Technology,NATO Science Series,Kluwer Academic Publishers,Dordrecht,Netherlands,2000,p.37.[5]A.C.Wright,in:G.Pacchioni,L.Skuja,D.L.Griscom(Eds.),Defects in SiO2and Related Dielectrics:Science and Technology,NATO Science Series,Kluwer Academic Publishers,Dordrecht,Netherlands,2000,p.1.[6]R.L.Mozzi,B.E.Warren,J.Appl.Cryst.2(1969)164.[7]R.Dupree,R.F.Pettifer,Nature308(1984)523.[8]F.L.Galeener,Phys.Rev.B19(1979)4292.[9]R.A.B.Devine,R.Dupree,I.Farnan,J.J.Capponi,Phys.Rev.B35(1987)2560.[10]N.Kitamura,K.Fukumi,K.Kadono,H.Yamashita,K.Suito,Phys.Rev.B50(1994)132.[11]I.T.Godmanis,A.N.Trukhin,K.H€u bner,Phys.StatusSolidi B116(1983)279.[12]H.Hosono,Y.Ikuta,T.Kinoshita,K.Kajihara,M.Hirano,Phys.Rev.Lett.87(2001)175501.[13]K.Kajihara,Y.Ikuta,M.Hirano,H.Hosono,Appl.Phys.Lett.81(2002)3164.[14]R.Br€u ckner,J.Non-Cryst.Solids5(1970)123.[15]A.E.Geissberger,F.L.Galeener,Phys.Rev.B28(1983)3266.[16]N.D.Newton,G.V.Gibbs,Phys.Chem.Miner.6(1980)221.[17]F.L.Galeener,J.Non-Cryst.Solids49(1982)53.[18]G.Hetherington,K.H.Jack,J.C.Kennedy,Phys.Chem.Glasses5(1964)130.[19]A.J.Ikushima,T.Fujiwara,K.Saito,J.Appl.Phys.88(2000)1201.[20]M.Shimbo,K.Sato,Jpn.J.Appl.Phys.34(1995)5640.[21]N.Kamisugi,N.Kuzuu,Y.Ihara,T.Nakamura,Jpn.J.Appl.Phys.36(1997)6785.[22]K.Awazu,H.Kawazoe,K.Muta,J.Appl.Phys.69(1991)4183.[23]M.Kyoto,Y.Ohoga,S.Ishikawa,Y.Ishiguro,J.Mater.Sci.28(1993)2738.[24]H.Hosono,M.Mizuguchi,H.Kawazoe,T.Ogawa,Appl.Phys.Lett.74(1999)2755.[25]M.Mizuguchi,L.Skuja,H.Hosono,T.Ogawa,J.Vac.Sci.Technol.B17(1999)3280.[26]Y.Morimoto,S.Nozawa,H.Hosono,Phys.Rev.B59(1999)4066.[27]H.Hosono,Y.Ikuta,Nucl.Instr.and Meth.B166–167(2000)691.[28]K.Saito,A.J.Ikushima,J.Appl.Phys.91(2002)4886.[29]A.R.Silin,L.N.Skuja,J.Mol.Struct.61(1980)145.[30]D.L.Griscom,J.Non-Cryst.Solids68(1984)301.[31]T.Miyazaki,N.Azuma,K.Fueki,J.Am.Ceram.Soc.67(1984)99.[32]T.E.Tsai,D.L.Griscom,E.J.Friebele,Phys.Rev.B40(1989)6374.[33]I.A.Shkrob,A.D.Trifunac,Phys.Rev.B54(1996)15073.[34]M.Stapelbroek,D.L.Griscom,E.J.Friebele,G.H.SigelJr.,J.Non-Cryst.Solids32(1979)313.[35]K.Kajihara,L.Skuja,M.Hirano,H.Hosono,Appl.Phys.Lett.79(2001)1757.[36]K.Kajihara,L.Skuja,M.Hirano,H.Hosono,Phys.Rev.Lett.89(2002)135507.[37]K.L.Brower,P.M.Lenahan,P.V.Dressendorfer,Appl.Phys.Lett.41(1982)251.[38]D.L.Griscom,J.Appl.Phys.58(1985)2524.[39]R.W.Lee,R.C.Frank,D.E.Swets,J.Chem.Phys.36(1962)1062.[40]R.W.Lee,J.Chem.Phys.38(1963)448.[41]J.E.Shelby,J.Appl.Phys.48(1977)3387.[42]S.Yamagata,Miner.J.15(1991)333.[43]D.R.Sempolinski,T.P.Seward,C.Smith,N.Borrelli,C.Rosplock,J.Non-Cryst.Solids203(1996)69.[44]M.Shimbo,T.Nakajima,N.Tsuji,T.Kakuno,T.Obara,Jpn.J.Appl.Phys.38(1999)L848.[45]Y.Ikuta,K.Kajihara,M.Hirano,S.Kikugawa,H.Hosono,Appl.Phys.Lett.80(2002)3916.330K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331[46]R.K.Brimacombe,R.S.Taylor,K.E.Leopold,J.Appl.Phys.66(1989)4035.[47]K.F.Klein,R.Arndt,G.Hillrichs,M.Ruetting,M.Veidemanis,R.Dreiskemper,J.Clarkin,G.Nelson,Proc.SPIE4253(2001)42.[48]P.Kaiser,J.Opt.Soc.Am.64(1974)475.[49]Y.Hibino,H.Hanafusa,J.Appl.Phys.60(1986)1797.[50]M.Oto,S.Kikugawa,N.Sarukura,M.Hirano,H.Hosono,IEEE Photo.Technol.Lett.13(2001)978. [51]L.Skuja,J.Non-Cryst.Solids179(1994)51.K.Kajihara et al./Nucl.Instr.and Meth.in Phys.Res.B218(2004)323–331331。