Analysis of Oxidation Behavior in Nanocrystal-FeSi 2 / Si Composites by Rutherford Backscattering Spectrometry and Computation of Diffusion Flux

We have investigated oxidation behaviors of nanocrystal (NC) -FeSi2/Si composites by Rutherford Backscattering Spectrometry (RBS) and computation of diffusion fluxes. The oxidation is controlled by diffusion of the O flux from the surface and the Si flux toward the Matano-Boltzmann (M-B) interface. The time dependence of the location of the M-B interface was determined. We proposed a model for oxidation of the composite that is different from oxidation of Si controlled by O diffusion through SiO2 at the surface.


Introduction
Enhancement of photoluminescence from -FeSi2/Si heterojunctions based on a confinement effect of electrons and holes in -FeSi2 nanocrystals of less than 10 nm in size have been reported [1].Radiative recombination of electron-hole pairs excited is controlled by densities of electrons and holes in -FeSi2 potential wells with a band offset of the conduction band (Ec) and that of the valence band (Ev).From photocurrent measurements using the -FeSi2/Si heterojunction, it was revealed that both band offsets were large enough to confine both electrons and holes in the -FeSi2 well, however, the Ev became too small to confine the holes [2].This knowledge teaches that almost the holes may be thermally activated and they escaped to outside of the well, when the temperature increased.The Ev for holes becomes near zero at room temperature.This situation supports the fact that room temperature light emission is very small or limited.From above consideration, we have looked for an appropriate heterostructure with large band offsets for both bands in which the electrons and holes can be effectively confined.One of candidates is a -FeSi2/SiO2 heterojunction, which may be easy to synthesize from -FeSi2/Si heterojunctions by sample oxidation.Nakamura et al. have reported synthesis of nanocrystal -FeSi2 (NC-) embedded in SiO2 by using oxidation of samples in air, and they succeeded in observation of evident photoluminescence at 250 K [3].Moreover, Morita et al. [4] have reported time dependent growth of SiO2 in oxidation of -FeSi2/Si, and they found no structural and quantitative changes of NC- by using infrared absorption (IRA) measurements, and pointed that selective oxidation took place in the sample.These reports indicate that the oxidation process is effective to synthesize -FeSi2/SiO2 heterojunction with enough large band offsets for electrons and holes even above room temperature from the -FeSi2/Si heterojunction.In this study, toward development of higher control of sample oxidation, oxidation behavior in Si layers composited with NC--FeSi2 was investigated by Rutherford backscattering spectrometry (RBS) and computation of diffusion flux using elements depth profiles that was deduced from RBS.

Experiments
Precursor samples including NC- (the average size less than 10nm) embedded in Si were synthesized by an ion beam synthesis (IBS) process.Oxidation of the sample was carried out at 900 o C in an electrical furnace in circulating dry air.Formation of NC- was confirmed by far-IRA measurements.2.0 MeV-4 He + Rutherford Backscattering Spectrometry (RBS) were measured at the backscattering angle of 165 o .For the energy calibration of each channel, high purity Fe plates, Si wafers, Al foils, Au films were employed.Depth profiles of elements compositions were deduced from random RBS spectra by SIMNRA ® [5].A diffusion fluxes Ji of element i in depth after oxidation duration t were computed by the following equation composited in MultiDiFlux distributed by Dayananda of Purdue University [6].
where x0 is the location of the Matano-Boltzmann plane and c(x) is a concentration of each element at depth x.

Results and Discussions
Figure 1 shows random RBS spectra of the samples oxidized for each times.It was observed that Fe spectrum near the Fe edge at KFeE0 shifted to the lower energy, however, the width of spectrum indicated small change.Growth of silicon oxide near the sample surface was confirmed from increase of both the yield and the width of oxygen spectrum near KOE0.Using SIMNRA analysis of these random RBS spectra, we obtained details of depth profiles of Fe, Si and O for each samples oxidized for 2-8 h as shown in Figs. 2 (a)-(d).Growth of SiO2 near the surface was observed with increasing the oxidation time.Such an apparent shift of Fe distribution toward the deeper location was caused by growth of SiO2 with 2.6 times of volume expansion at the sample surface.As an interesting fact, it was found that the composition at the interface between Si and O atoms was fixed at the composition ratio of Si/O=1 during the oxidation.This fact may be a new finding for a conventional oxidation process of Si The reason has not been solved, however, presence of Fe implants may affect the oxidation process of Si.Quantitative discussion requires details of element diffusion behaviors.The diffusion fluxes of Si and O and their time dependence were computed.Figures 3 (a This fact suggests that Fe atoms in the iron silicide does not contribute to the oxidation process.At the interface between the oxide and the silicide at the front edge of Fe profile, the oxide layer with oxygen deficiency such as SiO2-x was formed.This behavior will be discussed later by using results of Figs. 4 and 5.In comparison with oxidation of Si which is limited by diffusion of O atom in SiO2, this oxidation behavior obtained from this analysis may be quite unique.As shown in Fig. 4 (right), the difference in their location between each interfaces is defined as M.The M for 2 h oxidation was about 30 nm in thickness.The M layer with M in thickness can be considered to be the portion in which Si and oxygen fluxes meet together.Si atoms are supplied into this M layer more than oxygen atoms, therefore, this layer becomes an oxygen deficient oxide such as SiO2-x.The time dependence of M is given by M = dM-B(Si)-dM-B(O)=79.61t 0.33 -54.75t 0.43 .Figure 5 shows the observed (blue dots) and estimated (red curve) M values as a function of the oxidation time.The M value becomes the maximum at 2-3 h for oxidation, then decreases.After oxidation for 45-46 h, at least the M is estimated to be zero in thickness.

Conclusion
We have investigated oxidation behaviors of NC-/Si composites by RBS and computation of diffusion flux.The oxidation is controlled by diffusion of O atoms from the surface and that of Si atoms toward the Matano-Boltzmann (M-B) interface.The time dependence of location of the M-B interface was determined.This oxidation situation is far from oxidation of Si controlled by O diffusion through SiO2 at the surface.We have proposed the oxidation model for the composite and known that the composite cannot be fully oxidized even after oxidation for 8 h.Further long time oxidation reaching 45-46 h may be required to obtain fully oxidized composites.

Fig. 1 .
Fig. 1.Random RBS spectra of the NC-/Si composite oxidized for each times.After the oxidation, the oxygen (O) spectrum at the O-edge KOE0 appeared on the base of Si spectrum.

Fig. 2
Fig.2Depth profiles of Fe, Si and O atoms deduced from their random RBS spectra by using the SIMNRA code[5].The times for sample oxidation in dry air at 900 o C were 2-8 hours.The Fe depth profile indicated small change with increasing the oxidation time.This fact suggests that Fe atoms in the iron silicide does not contribute to the oxidation process.At the interface between the oxide and the silicide at the front edge of Fe profile, the oxide layer with oxygen deficiency such as SiO2-x was formed.This behavior will be discussed later by using results of Figs.4 and 5.

Fig. 3
Fig. 3 Changes of diffusion fluxes of Si and O atoms when the oxidation time increased.The plus sign means diffusion direction from the surface to the deep portion, and the minus sign means a contrary diffusion direction.The Matano-Boltzmann interfaces indicated by read and blue lines are indicated for Si and O fluxes, respectively.

011105- 3 Fig. 4 (
Fig. 4 (left) Matano-Boltzmann (M-B) interfaces for Si and O atomic fluxes computed from diffusion fluxes shown in Figs.3 (a)-(d).In the right figure, the difference in location between their M-B interfaces (M) is ~27 nm which is associated with difference in their diffusion fluxes.

Figure 4 (
left) shows changes of the diffusion flux as a function of the oxidation time.It was found that the Si flux JSi decreased rapidly after oxidation for 2 h, then approached the oxygen flux Jo.Figure 4 (right) shows the location (or depth) of the Matano-Boltzmann interfaces from the surface of growing oxide.The Si interface was deeper than the oxygen interface, and their locations did not become same when the oxidation time increased.The location of the M-B interface dM-B (nm) for each fluxes was fitted as a function of the oxidation time.We obtained dM-B(Si)=79.61t 0.33 and dM-B(O)=54.75t 0.43 (t is the oxidation time in hour) for Si and O fluxes, respectively.The exponent of O diffusion flux has larger than that of the Si diffusion flux, and the preexponential factor of O diffusion flux has smaller than that of the Si diffusion flux.This time dependence is close to that observed by IRA [3].This result can be understood from a big difference in amount of diffusion fluxes between Si and O atoms as shown in Figs.3 (a)-(d).

Fig. 5
Fig.5Thickness of the M layer as a function of the oxidation time.The blue dots are observed and the red curve is calculated from the equation M (nm)= 79.61t 0.33 -54.75t 0.43 , where t is the oxidation time in hour.

Fig. 6
Fig. 6 Schematics of difference in oxidation behaviors on Si substrates and on silicides.The two yellow circles indicate points where the reaction Si+2O=SiO2 proceeds.The right figure shows a detail structure around the silicide and the oxide layers, and Si and oxygen fluxes that made the M layer (the maximum thickness~27 nm) with oxygen deficiency as indicated in Fig. 5.This behavior obtained in Fig.5may be attributed to pronounced decrease in the Si flux JSi, when growth of oxide from the sample surface proceeds.Figure6shows schematics of difference in oxidation behaviors on Si substrates and on the silicide obtained in this study.In the left figure, you can see the Si flux from the Si substrate and goes through the silicide and onto the silicide surface, also the oxygen flux comes from air into the surface oxide.Oxidation of Si is dominated by the oxygen diffusion in the oxide and reaction rate.In contrast, in silicide/Si composites the Si flux coming through the silicide layer and onto the surface dominates the growth of Si oxide layer on the silicide, because the oxygen flux is blocked by the silicide layer and cannot go through the silicide.Figure6(right) shows a detail of layer structures deduced from the M-B interfaces and diffusion fluxes.There is a difference in the M-B interface of Si and oxygen fluxes.The Si atoms are supplied into this intermediate layer denoted by the M more than the oxygen atoms, so that the M layer becomes an oxide layer with oxygen deficiency.However, from analysis in Fig.5, we estimate to synthesize non oxygen deficient oxide after sample oxidation for 45-46 h.

Figure 6 (
right) shows a detail of layer structures deduced from the M-B interfaces and diffusion fluxes.There is a difference in the M-B interface of Si and oxygen fluxes.The Si atoms are supplied into this intermediate layer denoted by the M more than the oxygen atoms, so that the M layer becomes an oxide layer with oxygen deficiency.However, from analysis in Fig.5, we estimate to synthesize non oxygen deficient oxide after sample oxidation for 45-46 h.