Predominance of Alternate Diffusion Mechanisms for the Interstitial-Substitutional Impurity Gold in Silicon

摘要

Gold is the model interstitial-substitutional (i-s) impurity in Si with nearly all atoms residing on substitutional sites, Aus , but diffusion occurs through the small fraction of fast-moving interstitial Au atoms, Au_i , because direct Aus atom migration via the usual vacancy and interstitialcy mechanisms are negligible when compared to that due to the rapid Aui migration and changeover to or from Au_s . The Au_s - Au_i changeover involves Si native point defects, selfinterstitials ( I ) and/or vacancies (V ), via the Kick-Out mechanism (KO),1 and/or the Frank- Turnbull mechanism (FT).2 In KO a Au_i produces a Aus and generates an I or vise versa via Aui ⇔Aus + I . In FT a Aui produces a Aus and consumes a V or vise versa via i s Au +V ⇔ Au . Thus, Au diffusion involves non-equilibrium I and/or V . In dislocation-free Si, these nonequilibrium point defects are relaxed via diffusion to or from the Si surfaces, resulting in the changeover being controlled by proximity to the wafer surfaces, which controls the point defect concentrations. Modeling and experiments have shown that Au indiffusion is dominated by KO~1,3,4 associated with I supersaturation, relieved by I outdiffusion to the two wafer surfaces. This yields the characteristic symmetrical U-shaped Aus profile, although the Au source is only on one Si surface. The competing FT would also show the wafer surface proximity effect, but it will instead produce two erfc-function type Aus profiles, one at each surface. Earlier modeling of gettering of Au by an Al layer assumed also that the KO dominates the Au outdiffusion process,5,6 with an associated I undersaturation to be relieved by I indiffusion from both wafer surfaces. We have previously performed experiments of gettering Au from Si using Al.7 The experimental and modeling results are in qualitative agreement. However, the same qualitative feature would also hold for the FT mechanism. In this work, fittings of our Al gettering results of Au showed that the FT mechanism dominates Au outdiffusion, while the KO dominates Au indiffusion. In the present study we use the experimental results obtained by our group previously, reported elsewhere in detail~7 Briefly, Au was diffused at 950°C for 16 h into 530μ thick FZ Si wafer from one side. Next, a control anneal without an Al layer was done at 1000°C for 8 h and Al gettering at 1000°C for 30 min, 2 h and 8 h was done with a 1μ thick Al layer on only one surface. The Au concentration profiles were measured by spreading resistance profiling (SRP). Figure 1 shows that of the 30-min case, which will be used for our analysis.