F. Cristiano

Visible photoluminescence at room temperature from microcrystalline silicon precipitates in SiO2 formed by ion implantation

We have investigated the photoluminescence of microcrystalline silicon formed in SiO2 layers by ion beam synthesis. 28Si+ ions over the dose range 1 × 1017 to 6 × 1017 cm−2 at energies of 150 keV and 200 keV were implanted into thermal oxide. Samples were annealed in a halogen lamp furnace at temperatures of 900°C, 1100°C and 1300°C for times between 15 and 120 min. The implanted layers were analysed by Rutherford Backscattering Spectroscopy (RBS), Cross-Sectional Transmission Electron Microscope (XTEM) and Photoluminescence (PL) (80 K to 300 K) using an Ar laser of 488 nm wavelength. Room temperature (300 K) visible photoluminescence has been observed from all the samples. XTEM confirms the existence of Si microcrystals (within the SiO2 layers), which typically have a diameter within the range of 2–15 nm. The luminescence peak wavelength was about 600 nm or 800 nm, depending upon processing. Changes in the peak wavelength and intensity from these samples and other samples in which the crystallites were reduced in size by thermal oxidation, show trends which are generally consistent with quantum confinement, however, other mechanisms cannot be ruled out.

Is there an effect of the proximity of a “free-surface” on the formation of End-Of-Range defects?

Abstract We have studied the effect of the proximity of the wafer surface on the formation of End-Of-Range defects. These experiments are aimed at elucidating the behavior, upon annealing, of the Si self-interstitial supersaturation responsible for transient enhanced diffusion of boron in pre-amorphized silicon wafers. By implanting with Ge at constant energy while carefully etching away increasing thicknesses of the amorphous layer the nucleation and growth of End-Of-Range defects have been studied by transmission electron microscopy. Clearly, no influence in the loop population can be shown even when using state-of-the-art “quantitative” electron microscopy. These results are explained by considering that the c a interface is a diffusion barrier for the Si self-interstitial atoms during the nucleation stage, i.e., when the supersaturation is high. Only after the solid phase epitaxial regrowth, i.e., during the coalescence of the loops when the supersaturation is already low, the surface can interact with the loops. However, this interaction is not measurable through the observation of extended defects and this leads to simplifying assumptions for the simulation of Transient Enhanced or Retarded Diffusion in pre-amorphized Si wafers.

INTERSTITIAL TRAPPING EFFICIENCY OF C+ IMPLANTED INTO PREAMORPHISED SILICON : CONTROL OF EOR DEFECTS

Abstract The trapping of Si interstitials by C + implantation has been quantified by following the evolution of EOR defects during thermal annealing. Si (100) n-type wafers have been preamorphised by the implantation of 150 keV Ge + ions to a dose of 2 × 10 15 ions/cm 2 to form a 175 nm thick amorphous layer, followed by C + implantation at 65 keV ( R p = 175 nm to doses ranging from 3 × 10 13 ions/cm 2 to 3 × 10 15 ions/cm 2 . All samples were then annealed at 1000°C for 15 s. These structures have been investigated by RBS, TEM and SIMS. The mean radius of the EOR loops monotonically decreases with increasing dose of C + ions while their density increases. The number of Si self-interstitials stored in the loops also decreases with increasing carbon dose. This effect is associated with the capture of Si self-interstitials by SiC complexes. The effective trapping efficiency of carbon is about 1.18 interstitials per carbon atom, which is consistent with values obtained from studies of B transient enhanced diffusion.

FORMATION OF EXTENDED DEFECTS AND STRAIN RELAXATION IN ION BEAM SYNTHESISED SIGE ALLOYS

Abstract Single crystal SiGe alloy layers have been fabricated by implantation of Ge+ ions at energies ranging from 70 to 400 keV, followed by Si+ post-amorphisation and S olid P hase E pitaxial G rowth (SPEG). It is found that for each Ge+ implantation energy a critical value of the peak germanium concentration exists above which extended defects nucleate in the vicinity of the peak of the germanium depth profile and extend up to the surface. A critical value of the elastic energy stored in the structures ( ∼300 mJ / m 2 ) has been determined above which ion beam synthesised SiGe alloys relax, independently of the implantation energy. This empirical model has been found to successfully account for the results obtained in this work as well as in many other studies reported in the literature. For a regrowth temperature of 700°C, all samples investigated by XRD have been found to be almost fully strained, including samples containing relaxation-induced defects, indicating that, under these conditions, the energy transferred to the defects is very low. Complete strain relaxation is achieved after a second anneal step at 1000°C.

Characterization of extended defects in SiGe alloys formed by high dose Ge+ implantation into Si

The synthesis of SiGe/Si heterostructures by Ge+ ion implantation is reported. 400 keV Ge+ ions were implanted at doses ranging from 3 × 1016 to 10 × 1016 ions/cm2 into (001) Si wafers, followed by Si+ amorphisation and low temperature Solid Phase Epitaxial Regrowth (SPER). TEM investigations show that strained alloys can be fabricated if the elastic strain energy (Eel) of the SiGe layer does not exceed a critical value (E′el) of about 300 mJ/m2, which is independent of the implantation energy. Our analysis also suggests that “hairpin” dislocations are formed as strain relieving defects in relaxed structures. A “strain relaxation” model is proposed to explain their formation.

MeV ion implantation induced damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3-μm-thick relaxed Si1−xGex alloys of high crystalline quality with 2 MeV Si+ ions has been studied as a function of Ge content (x=0.04, 0.13, 0.24, or 0.36) and Si dose in the dose range 1010–2×1015 cm−2. The accumulation of damage with increasing dose has been investigated by Rutherford backscattering spectrometry, optical reflectivity depth profiling, and transmission electron microscopy. An enhanced level of damage, and a strong decrease in the critical dose for the formation of a buried amorphous layer in Si1−xGex is observed with increasing x. Electron paramagnetic resonance studies show that the dominant defects produced by the implantation are Si and Ge dangling bonds in amorphouslike zones of structure similar to a-Si1−xGex films of the same x, and that the effect of increasing the ion dose is primarily to increase the volume fraction of material present in this form until a continuous amorphous layer is formed. A comparative study of the op...

Ge+ ion implantation : a competing technology ?

Difficulties with the incorporation of SiGe epitaxial growth technologies into established silicon whole wafer processes are delaying the commercialisation of SiGe technology. In this paper the possibility of using an alternative, well proven technology, namely ion implantation, is considered. Experiments reported in the literature suggest that both strained and relaxed device worthy SiGe layers, with graded interfaces, can be synthesised by Ge + ion implantation followed by a two step thermal anneal to achieve solid phase epitaxial growth and annihilation of electrically active defects. It is concluded that for specific device architectures both heterojunction bipolar and MOS devices could be commercially realised whilst other applications, such as buried SiGe waveguides and devices incorporating compositionally graded layers, may also be viable. The technology warrants further investigation.

Defects in -implanted Si studied by slow positron implantation spectroscopy

Slow positron implantation spectroscopy has been applied to the investigation of point defects formed during the synthesis of buried SiGe alloy layers in Si by high-dose implantation and post-amorphization with . It is seen that omitting the post-amorphization stage prior to annealing leaves a damaged layer, containing open volume defects, extending beyond the -implanted overlayer. Provided that the Ge dose is low enough to allow planar crystal regrowth, post-amorphization appears to inhibit defect formation. Samples implanted with higher doses contain as-yet unidentified defects.

Dopant activation in Sb-implanted relaxed Si1−xGex alloy layers grown on compositionally graded buffers

Supersaturated solid solutions of substitutional, electrically active Sb have been obtained by ion implantation of relaxed epitaxial Si1−xGex alloy layers grown on compositionally graded buffers. Substitutional and nonsubstitutional Sb fractions in relaxed Si0.85Ge0.15, Si0.65Ge0.35 and Si0.50Ge0.50 alloy layers implanted to a dose of 5×1015 Sb cm−2 and annealed isothermally at temperatures ranging from 400 to 850°C have been studied by Rutherford backscattering/channeling, transmission electron microscopy and Hall-effect and sheet resistivity measurements. A supersaturated solution of Sb corresponding to a peak carrier concentration of 4×1020 cm−3 and an electrically active fraction of 40% of the implanted dose is observed by Hall measurements for the case of Si0.85Ge0.15 and Si0.65Ge0.35 alloys annealed at 550°C.

Structural studies of ion beam synthesised SiGe/Si heterostructures for HBT applications

Abstract Si/Si 1− x Ge x /Si heterostructures which are potentially suitable for HBT fabrication have been synthesised by implanting Ge + ions into a Si(100) substrate upon which a Si overlayer was deposited, followed by Si + -induced post-amorphization and low temperature Solid Phase Epitaxial Growth. RBS and TEM investigations have shown that the Ge induced end-of-range (EOR) defects are annihilated during regrowth, as well as extended defects observed in some C + co-implanted samples, resulting in good crystalline structures.