Oxidation processes of surface hydrogenated silicon nanocrystallites prepared bypulsed laser ablation and their effects on the photoluminescence wavelengthIkurou Umezu, Akira Sugimura, Toshiharu Makino, Mitsuru Inada, and Kimihisa Matsumoto Citation: Journal of Applied Physics 103, 024305 (2008); doi: 10.1063/1.2832392 View online: http://dx.doi.org/10.1063/1.2832392 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/103/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoluminescence of highly porous nanostructured Si-based thin films deposited by pulsed laser ablation J. Appl. Phys. 98, 024310 (2005); 10.1063/1.1985971 Blue photoluminescence of Si nanocrystallites embedded in silicon oxide J. Vac. Sci. Technol. A 23, 978 (2005); 10.1116/1.1871992 Laser annealing of silicon nanocrystal films prepared by pulsed-laser deposition J. Vac. Sci. Technol. B 22, 1731 (2004); 10.1116/1.1767829 Optical properties and luminescence mechanism of oxidized free-standing porous silicon films J. Appl. Phys. 86, 2066 (1999); 10.1063/1.371010 Effects of thermal processes on photoluminescence of silicon nanocrystallites prepared by pulsed laser ablation J. Appl. Phys. 84, 6448 (1998); 10.1063/1.368971

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Oxidation processes of surface hydrogenated silicon nanocrystallitesprepared by pulsed laser ablation and their effects on thephotoluminescence wavelength

Ikurou Umezu,1,a� Akira Sugimura,1 Toshiharu Makino,2 Mitsuru Inada,2 andKimihisa Matsumoto3

1Department of Physics, Konan University, Kobe 658-8501, Japan and Quantum Nano-technologyLaboratory, Konan University, Kobe 658-8501, Japan2High-technology Research Center, Konan University, Kobe 658-8501, Japan3Department of Physics, Konan University, Kobe 658-8501, Japan

�Received 27 April 2007; accepted 15 November 2007; published online 23 January 2008�

Natural oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsedlaser ablation under various hydrogen gas pressures are discussed by measuring the vibrationalfrequency of Si–Hn units on the surface and intensity of Si–O–Si stretching vibration. The surfacesof nanocrystallites are predominantly composed of Si–H bonds and oxidation starts from backbondsof these bonds. The deposited nanocrystal films have a porous secondary structure which dependson the background gas pressure. The oxidation rate observed by infrared absorption measurementsdepended on this porous secondary structure. The oxidation process is discussed by the correlationbetween oxidation rate and porous structure of nanocrystal film. We found that Si–O bond densityincreases with covering the surface of the nanocrystallites during the diffusion of oxygen-relatedmolecules through the void spaces in the porous structure. The surface oxidation of eachnanocrystallite is not homogeneous; after the coverage of easy-to-oxidize sites, oxidation continuesto gradually progress at the post-coverage stage. We point out that the oxidation process at coverageand post-coverage stages result in different photoluminescence �PL� wavelengths. Adsorption of thewater molecule before oxidation also affects the PL wavelength. Defect PL centers which have lightemission around 600 and 400 nm are generated during the coverage and post-coverage oxidationprocesses, respectively. © 2008 American Institute of Physics. �DOI: 10.1063/1.2832392�

I. INTRODUCTION

The study of silicon nanocrystallites is a very active fieldof research because of their interesting physical propertiesand promising applications in advanced electronic and opto-electronic devices.1–3 Optical and electronic properties basedon the quantum size effects have been extensively discussedso far.2,3 Not only the quantum size effects but also the sur-face effects are important to reveal nature of nanocrystallites.However, surface structure of the silicon nanocrystallites isnot well discussed and information on the correlation be-tween surface structure and properties of the silicon nano-crystallites is not enough. Since the surface of the Si nano-crystallites is easily oxidized, the surface oxidation process isessential for the properties of this material. For example,theoretical works suggested that the optical band gap energyslightly decreases with the oxidation of the surface due to thecreation of defects or change in the Si–Si bond length.4,5

Defects are generated during oxidation and some ofthem act as photoluminescence �PL� centers. 6–9,54 Therefore,discussions on the surface oxidation process and defect cre-ation mechanism are very important also from the viewpointof optical properties. Although there are many reports on thecorrelation between PL wavelength and oxidation,4–18 the ef-fects of the oxidation on the PL properties are not clear.Their results were sometimes different between samples and

sometimes lead to controversial discussions. Therefore, con-trol and identification of the surface structure are necessaryto clarify the correlation between the surface oxidation pro-cesses and PL properties.

A pulsed laser ablation �PLA� in background gas is oneof methods to prepare silicon nanocrystallites.19–21 Usuallyinert gas is used as a background gas to prevent a chemicalreaction between ablated materials and the background gas.Although reactive background gas is often used to preparethin films, reports on the preparation of nanocrystallites inthe reactive background gas are scarce. In this study, hydro-gen is used as a background gas to prepare hydrogenatedsilicon nanocrystallites. We recently reported that the Si–Hbonds mainly exist on the surface area and they are effectivefor termination of the dangling bonds.22,23 This means thatPLA in hydrogen gas is a promising technique to prepare Sinanocrystallites having a well defined surface. However, wereported the correlation between the oxidation and PL wave-length for this surface hydrogenated sample;15,16 the discus-sion on the oxidation process was not enough.

The surface of Si nanocrystallites is naturally oxidizedand the surface structure changes with the oxidation. Theoxidation process for the nonhydrogenated Si nanoparticlesis measured by a few groups by using a transmission electronmicroscope24 �TEM� and by photoelectron spectroscopy.25

Observation of the oxidation process of the surface hydro-genated silicon nanocrystallites is a more important issuea�Electronic mail: umezu@center.konan-u.ac.jp.

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since it has a terminated high quality surface. Not only as thesurface termination, but the surface Si–H bond can be usedas a probe to estimate surface oxidation process. It is wellknown that the stretching vibrational frequency of the Si–Hbond is sensitive to the oxidation of the backbonded atoms.26

Discussion of the surface oxidation process by observing thesurface Si–H bonds was performed for a c-Si wafer based onthe Lucovsky model.27–31 Since Si–Hn stretching vibrationalfrequency can be measured by conventional infrared �IR�absorption measurements, observation of this frequency is asimple and useful characterization method to evaluate theoxidation processes. We apply this method to the surfaceanalysis of the Si nanocrystallites.

The silicon nanocrystallites deposited on the substrate bythe PLA method are an assembly of nanocrystallites, that is ananocrystal film.18,32,33 The deposited surface hydrogenatednanocrystal film has a secondary structure composed ofnanocrystallites as the primary structure.34 The nanocrystalfilm has a porous structure in which the surface area of eachnanocrystallite as well as void spaces between the nanocrys-tallites exist. Not only the individual nanocrystallite, but theporous structure is essential to understand nature of the nano-crystal films. For example, the electric current through theporous structure is important for electroluminescence de-vices based on the nanocrystal film. The oxidation of theporous structure is governed by two diffusion processes ofoxygen; diffusion from the top surface of the nanocrystalfilm through the void spaces and that from the top surface ofeach nanocrystallite toward the center. Therefore, discussionson the oxidation of nanocrystal film as a secondary structureand nanocrystallite itself as a primary structure are impor-tant.

In the present paper, we prepared surface hydrogenatedSi nanocrystal film having different void spaces by changingthe hydrogen backgroung gas pressure. The natural oxidationprocess of the surface hydrogenated silicon nanocrystalliteswas evaluated by Fourier-transform infrared �FTIR� absorp-tion measurements for 250 days. The oxidation due to thediffusion of oxygen through void spaces of the nanocrystalfilm and that of the surface of each nanocrystallite is dis-cussed. The creation of PL centers during the oxidation isalso discussed.

II. EXPERIMENT

The hydrogenated silicon nanocrystal film was preparedby PLA in H2 gas. After a chamber was evacuated to lessthan 1.0�10−5 Pa, high purity �99.9999%� H2 gas was in-troduced into the vacuum chamber at a flow rate of 10 sccmand maintained at a constant pressure. A fourth harmonic ofthe pulsed Nd:yttrium-aluminum-garnet laser beam was fo-cused onto a single-crystalline Si target. The wavelength,pulse width, repetition rate, and fluence at the single crystalSi target were 266 nm, 10 ns, 10 Hz, and 1.5 J /cm2, respec-tively. Ablated Si species were deposited on a single-crystalline Si wafer or synthetic quartz substrates. The dis-tance between the target and the substrate was 23 mm. Weprepared three kinds of nanocrystal films which have differ-ent secondary structures by changing background gas pres-

sures, 270, 530, and 1100 Pa.34 In addition, nonhydrogenatednanocrystal film was prepared as a reference by introducingHe gas at 530 Pa. After the deposition, nanocrystal filmswere exposed to the atmosphere at room temperature. Thecharacterization of the size, the structure, and the crystallin-ity of the deposited hydrogenated silicon nanocrystalliteswere measured by TEM, scanning electron microscope�SEM�, Raman scattering, and electron spin resonance�ESR�. Chemical bonding configuration was discussed frommeasurement of the FTIR absorption. PL spectra were mea-sured by a 25 cm monochromator with a charge couple de-vice detector. An excitation source was the HeCd laserwhose wavelength is 325 nm. All measurements are per-formed at room temperature. The changes in the FTIR andPL spectra during the exposure to the atmosphere are mea-sured for 250 days.

III. RESULTS

The cross-sectional SEM image of the nanocrystal filmprepared at 270 Pa is shown in Fig. 1�a�. The TEM images ofthe samples prepared at 270 and 1100 Pa are shown in Figs.1�b� and 1�c�. The results of Figs. 1�a� and 1�b� indicate thatthe nanocrystal film is a secondary structure composed ofnanocrystallites. The crystallinity of the nanoparticles wasconfirmed by Raman scattering measurement. Results ofFigs. 1�b� and 1�c� suggest that the porosity of the nanocrys-tal film is higher at a higher background gas pressure. Acorrelation between the background gas pressure and the sec-ondary structure is reported in our previous report.34 Thesecondary structures observed by SEM at hundreds-of-nanometers scale for the nanocrystal film prepared at 270,530, and 1100 Pa were column-, cauliflower-, and fiberlikestructures, respectively. This result indicates that porosity ofthe nanocrystal film increases with the background hydrogengas pressure. On the other hand, the mean diameters of pri-mary structured nanocrystallites observed by TEM did notdepend on the background gas pressure and they are 4–5nm.34–36

The 2100 cm−1 IR absorption peak which relate to thestretching vibrational mode of the Si–Hn bond was observedby FTIR measurements for as-deposited nanocrystal film.The hydrogen content was estimated to be about 20 at. %from the absorption intensity of this vibrational mode byusing a factor determined by Langford et al.37 Good crystal-linity and large hydrogen content suggests that the depositswhich we prepared were hydrogenated silicon nanocrystal-lites and the Si–H bonds mainly exist at the surface area.22,23

The hydrogenated silicon nanocrystallites prepared bythis method were naturally oxidized at room temperature byexposure to the atmosphere. The Si–O bond density wasestimated from the IR absorption intensity of the Si–O–Sistretching mode at around 1080 cm−1. The absorption inten-sity was normalized by the nominal thickness of the nano-crystal film, which was estimated by cross-sectional SEMimages. Since the porosity of the nanocrystal film increaseswith the background gas pressure, the Si–O bond density isunderestimated especially for the nanocrystal film preparedat 1100 Pa. The Si–O bond densities are shown in Fig. 2�a�

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as a function of the exposure time for nanocrystal films pre-pared under 270, 530, and 1100 Pa. The Si–O bond densityrapidly increased at first and then gradually increased. TheIR absorption peak wavenumber at around 2100 cm−1

shifted to higher values as the oxidation progressed as shownin Figs. 3�a� and 3�b� for nanocrystal film prepared at 270and 530 Pa, respectively. This peak shift is due to the changein the electronegativity of the backbonded atoms.26 The insetin Fig. 3�b� shows the IR spectrum measured within 10 minafter exposure to the atmosphere. The peak wavenumber ofthis fresh nanocrystal film is about 2100 cm−1. Although theIR peak absorption wavenumber of the Si–H bond shiftedfrom 2100 to 2250 cm−1, the integrated absorption intensitylittle changed. This indicates that the oxygen atoms attackbackbonds to form Si–O bonds without removing the Si–Hbonds on the surface of the nanocrystallite.

The PL spectrum measured in the atmosphere is shownin Fig. 4 for the nanocrystal film prepared at 530 Pa. The PLemission for the nanocrystal film prepared in helium gas ishardly observed in our experimental condition. The neutraldangling bond density of hydrogenated and nonhydrogenated

FIG. 1. �Color online� �a� A cross-sectional SEM image of the sample pre-pared at 270 Pa. TEM images of the sample prepared at �b� 270 and �c� 1100Pa.

FIG. 2. �a� Si–O bond density as a function of exposure time to the atmo-sphere and �b� the ratio of absorption intensity of decomposed SiH�O3�groups to the total absorption intensity at around 2100 cm−1. The circles,squares, and triangles represent nanocrystal films prepared at 270, 530, and1100 Pa, respectively. The broken lines are fitting curves.

FIG. 3. Changes in the IR absorption spectra at around 2100 cm−1 by natu-ral oxidation. Arrows in the figure show the estimated vibrational frequencyof SiH�Si3−nOn� �n=1−3� groups. The inset shows the spectrum measuredwithin 10 min after the exposure to the atmosphere.

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nanocrystal films were measured by ESR. The g value of thesignal was 2.006, which is assigned to that of the Si danglingbond. The dangling bond density of the hydrogenated nano-crystal film was about one order smaller than that of thenonhydrogenated nanocrystal film. These results indicate thatthe PL emission observed for the nanocrystal film preparedin hydrogen gas is due to the decrease in the dangling bonddensity by hydrogenation. The PL spectra changed with theexposure time to the atmosphere. The PL peak wavelengthmeasured in vacuum just after the deposition was about 800nm. The change in the PL wavelength at an early stage wasmeasured within 420 min for the nanocrystal film prepared at1100 Pa. The change in the PL peak wavelength after expo-sure to the atmosphere within 5 min is shown in Fig. 5. The800 nm peak shifted to 700 nm at 1 min after the nanocrystalfilms were exposed to the atmosphere and then graduallydecreased to 660 nm at 5 min. This peak shifted back to alonger wavelength, 730 nm, by reevacuation to about10−1 Pa. Although the peak wavelength did not return to theoriginal value, the shift of the PL wavelength was partiallyreversible for the exposure less than about 5 min. This PLpeak shift was not observed when the nanocrystal film wasexposed in the Ar gas and a similar reversible peak shift wasobserved when exposed in the mixture gas of Ar and H2O.These results indicate that this reversible PL peak shift at aninitial stage of exposure is due to the physical adsorption of

H2O gas in the atmosphere. After the nanocrystal film wasexposed to the atmosphere for 420 min, the PL peak shiftedto 600 nm and this peak did not show any reversible behav-ior by reevacuation to about 10−1 Pa. This means that thephysical adsorption at the initial exposure changed to stron-ger chemical adsorption or chemical bonding after the longerexposure.

This irreversible PL peak shift is shown in Fig. 6 as afunction of Si–O bond density estimated by peak intensity ofthe 1080 cm−1 IR absorption. The PL peak discontinuouslyshifted to a shorter wavelength with oxidation; the three PLbands around 800, 600–700, and 400–500 nm are observed.Hereafter, these PL bands are referred to infrared, red, andblue bands in this paper. The PL peak wavelength mainlydepended on the Si–O bond density while the effect of back-ground gas pressure, that is secondary structure, was small.These experimental results show a strong correlation be-tween the surface oxidation PL wavelength.

IV. DISCUSSION

A. Surface structure

The IR peak wavenumber of the hydrogenated siliconnanocrystallites was located at around 2100 cm−1. There arethree possible bonding structures for this vibrational fre-quency; Si–H2 bond in bulk, Si–H2 bond on the surface, andSi–H bond on the surface.28,38 We previously suggested that

FIG. 4. PL emission spectra of the nanocrystal film prepared in hydrogengas at 530 Pa measured at various exposure times to the atmosphere.

FIG. 5. PL peak shift as a function of time within 5 min after exposure tothe atmosphere for the nanocrystal film prepared at 1100 Pa. The PL peakwavelength of the as-deposited nanocrystal film measured without breakingvacuum was 800 nm. This peak shifted to 660 nm at 5 min. The 660 nmpeak shifted back to a longer wavelength, 730 nm, when we evacuated againto about 10−1 Pa.

FIG. 6. PL peak wavelength as a function of Si–O bond density. Thecircles, squares, and triangles represent nanocrystal films prepared at 270,530, and 1100 Pa, respectively.

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the Si–H or Si–H2 bonds predominantly exist on the surfacearea22,23 in our sample since a large amount of the Si–Hbonds exist and the nanoparticles are not amorphous butcrystalline state. This indicates that the observation of2100 cm−1 peak is a good method to discuss surface struc-ture.

The changes in the IR spectra caused by the natural oxi-dation which is shown in Fig. 3 can be explained by theLucovsky model. The Si–H vibrational frequency linearlyincreases with electronegativity of backbonded atoms.26 Thevibrational frequencies of the Si–H groups on the surfacewere estimated in the present paper as follows. The mini-mum and maximum peak wavenumbers observed in Fig. 3are 2100 and 2250 cm−1, respectively. If we assume that thevibrational mode of 2100 cm−1 is assigned to the nonoxi-dized surface Si–H bond, the 2100 and 2250 cm−1 modescorrespond to the Si–H vibrational groups which have mini-mum and maximum numbers of oxygen atoms at the back-bonds; that is SiH�Si3� and SiH�O3� groups on the surface,respectively. The frequencies of intermediate groups,SiH�Si3O� and SiH�SiO2�, are estimated to be about 2150and 2200 cm−1 by interpolation. We can find peaks or shoul-ders in Fig. 3 at around these frequencies and they are shownin the figure by arrows. The existence of four componentssuggests that the predominant structure of the Si–Hn unit isnot Si–H2 but Si–H on the surface. If the original structureis Si–H2, only three components, SiH2, SiH2�SiO�, andSiH2�SiO2�, will be observed. The existence of three compo-nents is not consistent with our experimental results. There-fore, we can conclude that Si nanocrystallites are coveredpredominantly by surface Si–H bonds.

B. Surface oxidation process

The oxidation progresses from the backbonds of theSi–H bonds as shown in Sec. III. The backbond oxidation ofthe Si wafer is reported by many authors.27–30,39–41 Niwanoet al.27 and Zhang et al.29,30 measured the surface oxidationof the hydrogenated surface of the single crystal Si wafer inthe air and in the oxygen ambient, respectively. The formerresult showed a decrease in the Si–H bond density and thelatter showed the backbond oxidation without surface hydro-gen removal. Although our ambient is air, we could hardlyobserve hydrogen removal pointed out by Niwano et al.Since our nanocrystal film is composed of semisphericalnanocrystallite as shown in Fig. 1�b�, a considerable numberof steps and kinks exist on the surface of the nanocrystallite.The oxygen molecules in the air can easily attack the back-bonds at these sites. The difference between bulk single crys-tal and nanocrystal may be a reason of the backbond oxida-tion in the air.

The surface oxidation depends on the background gaspressure during PLA as shown in Fig. 2�a�. Since the poros-ity of the nanocrystal film increases with background gaspressure, this result indicates that the surface oxidation pro-cess depends on the porosity of the nanocrystal film. A sim-plified schematic view of our sample is shown in Fig. 7. Thenanocrystal film has a porous structure in which the surfacearea of each nanocrystallite as well as void spaces between

the nanocrystallites exist. The void spaces, that is porosity,increase and the size of primary nanocrystallite does notchange with background gas pressure.34 Therefore, the effectof background gas pressure on the oxidation is mainly notdue to the primary nanocrystallites but due to the porosity ofthe nanocrystal film. Here we assume that the oxidation rateis mainly determined by the nonoxidized surface area ofnanocrystallites in the nanocrystal film. The surface areacovered by oxide, �, is determined by the equation,

d�/dt = k�s − �� , �1�

where k is the proportional constant and s is the total effec-tive surface area. The value of s depends on the numberdensity of the nanocrystallites in the nanocrystal film whichcorrelates to the void spaces. By solving this differentialequation, � is given as

� = s�1 − exp�− kt�� . �2�

The values of s and k can be obtained by fitting this equationto the experimental results shown in Fig. 2. The fitted curvesare shown in broken lines in Fig. 2�a� and the fitting param-eters are shown in Table I. The values for s are expressed asdimensionless numbers. Fair agreement between the fittedlines and experimental data indicates that the area of nonoxi-dized surfaces of nanocrystallites in the nanocrystal filmmainly determines the curvature of the Fig. 2�a�.

The oxygen atoms for the oxidation are provided byoxygen-related molecules such as O2 and H2O in the atmo-sphere. The oxygen-related molecules on the surface of thenanocrystal film reach the surface of each nanocrystalitethrough the void spaces. The value of k depends on the ratefor the oxygen-related molecules to diffuse from the surfaceof the nanocrystal film to the surface of each nanocrystallitethrough void spaces and on the rate for the oxygen-relatedmolecules to react with the surface of each nanocrystallite.The oxidation starts from the surface of each nanocrystalliteand the Si–O bond density on the surface saturates when allof the surface area of the nanocrystallites in the nanocrystalfilm are oxidized. As shown in Figs. 1�b� and 1�c�, porosity

FIG. 7. A simplified schematic view of structure of deposits.

TABLE I. The best fitted values of s and k. These values are obtained byusing Eq. �2� and fitted curves are shown in Fig. 2�a�.

Hydrogen gas pressure �Pa� s k �1 /s�

270 16 4.6�10−8

530 38 1.2�10−7

1100 22 2.0�10−6

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of the nanocrystal film increases with the hydrogen back-ground gas pressure. The increase in the value k with thebackground gas pressure suggests that an increase in the dif-fusion rate of the oxygen-related molecules through the voidspaces determines the process, while the value s cannot bediscussed in the present paper because we could not evaluatethe total surface area. Furthermore, the Si–O bond densityobtained in our experiment is a nominal value as mentionedearlier. Since the porosity of the nanocrystal film prepared at1100 Pa is much larger than that of the other two nanocrystalfilms, the nominal thickness and bond density of this nano-crystal film are over- and underestimated, respectively. How-ever we cannot qualitatively discuss porosity dependence ofthe s value and Si–O bond density, the correlation betweenthe porosity and k value indicates that the surface of eachnanocrystallite is easily oxidized and the oxidation rate islimited mainly by the diffusion of oxygen-related moleculesthrough the void spaces.

Although the number of Si–O bonds in the nanocrystalfilm is discussed, the number of oxygen atoms bonded to oneSi atom is not discussed earlier. The number of Si–O bondsbackbonded to Si–H bond can be estimated by observing the2100 cm−1 IR peak. The ratio of oxygen rich SiH�O3�groups, RO3

to the total number of SiH�Si3−nOn� �n=0−3�groups was estimated by the integrated intensity of the de-composed SiH�O3� absorption component divided by the to-tal integrated intensity at around 2100 cm−1. The obtainedvalues are plotted as a function of the exposure time and theyare shown in Fig. 2�b�. This figure indicates that the numberof backbonded Si–O3 increases with increasing exposuretime. Since the shape of Fig. 2�b� resembles Fig. 2�a�, weassume that Si–O bond density increases with the increasingnumber of the backbonded oxygen atoms. The broken linesin Fig. 2�b� are fitting curves calculated using Eq. �2� withthe same values of k given in Table I for the different gaspressure condition. Fair agreement between the experimentaldata and the fitting curves indicates that the number ofSiH�O3� unit increases with the Si–O bond density.

As shown in Fig. 2�b�, the value of RO3does not reach

unity for the nanocrystal film prepared at 1100 Pa, eventhough the Si–O bond density was saturated after 20 days.One of the reasons why the value of RO3

does not reach unityis the inhomogeneous oxidation rate. Since the observednanocrystallite is hemispherical, the Si nanocrystallite iscomposed of �111�-and �100�-like surfaces and many kinksand edges. Exothermic energies for the oxidation of thesesurfaces and these sites are estimated from calculation offormation energy using a program package for semiempiricalmolecular orbital calculations �MOPAC�. The exothermic en-ergy to generate one Si–O–Si unit for flexible structure isestimated from following chemical reaction:

Si2H6 + H2O = Si2OH6 + H2. �3�

The exothermic energy for this reaction is 20 kcal /mol. Theexothermic energies to form Si–O–Si units are estimatedfrom this value and the number of Si–O–Si units; it isshown in Fig. 8 by a solid line as a function of the number ofSi–O–Si units. The structure of model calculation are hy-drogenated Si �111� and dimerized �100� surface on Si53H56

and Si62H54 clusters and they are shown in inset of Fig. 8.The H2O molecules were placed on the surface and the for-mation energy was calculated by creating Si–O–Si units andby H2 molecules from H2O molecules. The Si–O–Si unitsare placed at the backbonds around the center of the �111�surface or the dimer of �100� surface. The crosses in thefigure show the results when the Si–O–Si units are placed atthe edge site of the nanocrystallite, as shown in the inset ofFig. 8. The exothermic energies to form Si–O–Si units onthe �111� and �100� surface are shown in Fig. 8 as solidcircles and triangles. The exothermic energies to form theSi–O–Si unit on the �111� and �100� is smaller than that atedge sites. The main difference between the �111� or �100�surface and the edge sites is structural flexibility. The Siatoms on the �111� and dimerized �100� surfaces have threerigid Si–Si bonds at the back of Si–H bond. This means thatstress and strain are induced to form the Si–O–Si unit atbackbonds. On the other hand, such structural restriction issmaller at the edge site. The results of calculated exothermicenergy suggest that easy-to-oxidize and hard-to-oxidize sitesexist on the surface of the hemispherical nanocrystallite,which have inhomogeneous surface structures. The stress in-duced during the growth plays a critical role in the growthprocedure itself.42,43 This effect may enhance the inhomoge-neous oxidation. Some of the hard-to-oxidized sites are notcompletely oxidized and do not reach stoichiometric compo-sition within the observed time scale. The oxidation rateshown in Fig. 2 is mainly explained by the diffusion of anoxygen-related molecule through the void spaces. Thismeans that the easy-to-oxidize sites are covered by Si–Obonds during the diffusion of the oxygen-related moleculesthrough the void spaces of the nanocrystal film.

We propose a model of oxidation from these results asfollows. The oxygen-related molecules, which exist at thesurface of the nanocrystal film, at first diffuse through thevoid spaces to reach the surface of each nanocrystallite. Theoxidation then starts from the backbonds of the Si–H bondby breaking Si–Si bonds at easy-to-oxidize sites of eachnanocrystallite. The surface coverage is not homogeneousand not complete due to the inhomogeneous nature of thesurface of Si nanocrystallites. The surface of each nanocrys-

FIG. 8. The exothermic energy as a function of number of Si–O–Si units.The solid circle, triangle, and cross correspond to the �111� surface, �100�surface, and edge. The solid line indicates estimated exothermic energy togenerate Si–O–Si bonds in the molecule.

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tallite is considered to be covered mainly by SiH�O3� groupssince the data of Fig. 2�b� can be fitted by the same k valueobtained by Fig. 2�a�. After the effective surface area ofeasy-to-oxidize sites is covered by SiH�O3� groups, oxida-tion of hard-to-oxidize sites or oxidation from the surface tothe center of each nanocrystallite continue to graduallyprogress. We refer these oxidation stages to surface coverageand post-coverage stages. The oxidation process of the nano-crystal film is composed of these two stages. The correlationbetween the oxidation process and PL wavelength is dis-cussed in the next subsection.

C. Defect PL centers formed during oxidation

The PL peak wavelength changes with the oxidation ofthe surface. Figure 5 shows that the PL wavelength of theas-deposited nanocrystal film is 800 nm. Allan et al. pointedout a self-trapped exciton state at the dimer on the surface bytheoretical calculation.44 The PL from the self-trapped exci-ton is experimentally observed by Kobitski et al. at around800 nm.45 Since the 800 nm PL peak is sensitive to surfaceconditions such as the adsorption of H2O, the self-trappedexciton is one of the possible candidates for the origin of thisband.46

When the nanocrystal film is left in the atmosphere morethan 420 min, the reversible PL peak shift was not observed.This means that the origin of PL has changed from the ad-sorped H2O molecule to some oxidation-induced defect cen-ters. The PL band discontinuously changes from the red toblue band by the oxidation as shown in Fig. 6. This resultindicates that the change in the PL peak energy is due to thechange in the species of the defect center, which contributesto the PL emission during the oxidation. Figure 6 indicatesthat the Si–O bond densities which show the red and bluePL bands are 0–20 and 20–40 in arbitrary units. By compar-ing Figs. 2 and 6, we can mention that the former and latterbond densities correspond to the surface coverage and post-coverage stages, respectively. This means that defects formedat coverage and post-coverage stages are different and theyare origins of the red and blue PL centers, respectively. Thesmaller exothermic energy during oxidation of the �111� and�100� surfaces suggests that creation of defect is expecteddue to large stress and strain. The red band is considered tobe originated from the Si=O bond6,54 or nonbridging oxygenhole center47–49 on the surface. The blue band is frequentlyreported in the silicon oxide.50–53 Possible candidates for theorigin of the emission at around 400 nm are twofold coordi-nated silicon lone pair centers50,51 �O2=Si:� and defects asso-ciated with the OH group.52,53 Unfortunately, we could notobserve IR absorption peaks originated by these oxygen re-lated defect centers. However, we cannot identify the defectcenter from our experimental results; we are able to concludethat defect formation process and formed defect centerschange with the progress of oxidation.

V. CONCLUSIONS

We prepared hydrogenated silicon nanocrystallites byPLA in hydrogen background gas. The Si–Hn bonds wereobserved by FTIR measurement and they predominantly lo-

cate on the surface area. The analysis of a 2100 cm−1 ab-sorption peak suggests that the predominant Si–Hn group isSi–H bond on the surface of nanocrystallites. The porosityof the secondary structure of the nanocrystal film, that is voidspaces, increases with increasing background gas pressure.

The oxidation of the nanocrystal film depended on thevoid spaces in the porous structure. The oxygen-related mol-ecules on the surface of the nanocrystal film diffuse to thesurfaces of each nanocrystallite through the void spaces andthe oxidation starts from the backbonds of the surface Si–Hbonds with keeping the Si–H bond density unchanged. Theoxidation of nanocrystallites is not uniform and easy-to-oxidize and hard-to-oxidize sites exist due to inhomogeneoussurface structure. During the diffusion of the oxygen-relatedmolecules through the void spaces of the nanocrystal film,the easy-to-oxidize sites are covered by Si–O bonds. Thedefects formed during the coverage and post-coverage stagesgenerate red and blue defect PL centers, respectively. Thesurface oxidation process is governed by void space in thesecondary structured nanocrystal film and surface of primarystructured nanocrystallites.

ACKNOWLEDGMENTS

This work was partially supported by a Grant-in-Aid forScientific Research from the Japan Society for the Promotionof Science, Nippon Sheet Glass Foundation for MaterialsScience and Engineering, and the Hirao Taro Foundation ofthe Konan University Association for Academic Research.

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