Takeru Saito

Surface chemical states and oxidation resistivity of ‘ecologically friendly’ semiconductor (β-FeSi2) thin films

Abstract Surface chemical states and oxidation resistivity of the ‘ecologically friendly’ semiconductor β-FeSi 2 have been investigated. Previously, we studied β-FeSi 2 thin films prepared by the ion-beam sputter deposition method (IBSD) on an Si(100) substrate. Through these studies, it was observed that the oxidation of the formed FeSi 2 does not proceed so much even in the air atmosphere compared with elemental Si or Fe or other compound semiconductors. In the present study, the obtained films were analyzed by synchrotron–radiation X-ray photoelectron spectroscopy (SR-XPS) in order to obtain chemical states and depth profiles of films non-destructively. From the XPS spectra, the results indicated FeSi 2 films have strong oxidation resistivity. The XPS spectra reveal that oxidation resistivity is caused by a very thin SiO 2 layer covering the formed β-FeSi 2 . The results of cross sectional observation by transmission electron microscope and surface morphological analysis by scanning electron microscope are consistent with the SR-XPS results.

Fabrication and stability of binary clusters by reactive molecular ion irradiation

Abstract We have observed a strongly enhanced cluster formation, such as silicon and carbon emitted from the target surface, by molecular ion irradiation. It is expected that binary and multielement clusters are easily fabricated through the reaction between the irradiated ion and target atom using reactive molecular ions. In the present study, boron–carbon (B–C) and silicon–carbon (Si–C) binary clusters have been fabricated by reactive molecular ion irradiation (C 6 F 5 + ; 4 keV, 1 μA/cm 2 ) of the boron or silicon surface. On the basis of the results, Si n − m C m binary clusters, which contain more than two carbon atoms ( m ⩾2), were scarcely observed. On the other hand, the relative intensities of the B n − m C m clusters ( m ⩾2) are higher than those of Si n − m C m ( m ⩾2). It can be considered that the B n − m C m ( m ⩾2) clusters are more stable than Si n − m C m ( m ⩾2) since the framework of the boron clusters easily alternate with one another. These results show that the structure of the cluster strongly affects the stability.

Silicon cluster formation by molecular ion irradiation: Relationship between irradiated ion species and cluster yield

Abstract When an ion beam is irradiated to a solid surface at an extremely high flux (∼mA/cm2), it is known that a remarkable formation of clusters is observed during the sputtering process. We have observed strongly enhanced cluster formation at quite low fluxes (∼μA/cm2) by irradiation of molecular ion. In the present study, various molecular ions are irradiated to a silicon surface. The relationship between irradiated ion species [ SF n + (n=1,5), Xe + , Ar + ] and the obtained silicon cluster yield is investigated. Observed mass spectra show that relative yield of the Sin cluster Y(Sin) for SF 5 + (m / e=127) irradiation is substantially higher than that for SF+ (m/e=51) irradiation. In the case of monatomic Xe+ (m/e=132) and Ar+ (m/e=40) irradiation, which have fairly the same mass compared with SF5+ and SF+, respectively, the cluster yield for Xe+ is also higher than that for Ar+ irradiation. However, the difference of the cluster yield for molecular ions is larger than that for monatomic ions [Y( Si n ) ( SF 5 + )⪢Y( Si n ) ( SF + ),Y( Si n ) ( Xe + )>Y( Si n ) ( Ar + )] . These behaviors imply that the size of the incident ions greatly affects the cluster yield compared with the mass of the ions.

STRONGLY ENHANCED SECONDARY ION EMISSION BY MOLECULAR ION IRRADIATION

Strongly enhanced secondary ion emission is observed by irradiation of molecular ion, SF5 +. It is well known that the sputter yield increases significantly when the solid surface is irradiated by ions at an extremely high flux, such as in the order of mA/cm2. In the present study, 4 keV SF5 + ions irradiate a beryllium surface at relatively low flux, 1 μA/cm2. The Xe+ and Ar+ monatomic ions also irradiate at the same condition for comparison. The emitted beryllium and beryllium cluster ions are observed. The ordering of the secondary ion intensities (I) of monatomic Be+ is I(SF5 +) > I(Xe+) > I(Ar+). For the SF5 + irradiation, formation of the cluster ions (Be3 +, Be6 +) is also observed. The irradiating molecular ions strongly affect the surface atom emission, since a high-density collision cascade can be formed in the irradiated local area by atoms dissociated from molecular ions. The enhanced cluster formation is also observed by the molecular ion irradiation.