Interdiffusion Behavior of Si/Si 1-x Ge x Layers in Inert and Oxidizing Ambients

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Mat. Res. Soc. Symp. Proc. Vol. 532 ©1998 Materials Research Society

EXPERIMENTAL The SiGe/Si test structure was grown by Molecular Beam Epitaxy (MBE) at a temperature of 520 °C. As shown in Figure 1, the structure consists of a lightly p-doped (100) Si substrate with an undoped 100 un Si buffer, followed by an undoped 50 nm Si0 .85Ge 0 .15 layer and an undoped 50 nm Si cap. The Ge concentration was verified by Rutherford Backscattering Spectroscopy (RBS) and the layer thicknesses were verified by cross-sectional Transmission Electron Microscopy (XTEM). 50nm

Si Cap

5Onm

lwes

10Onm

S•Buffer

Figure 1. Schematic of sample structure. The test wafer was first cut into I x 1 cm pieces which were cleaned using a regimen of deionized water, H2S0 4 :H20 2 (1:2) and H20:HF (10:1) and then dried with N 2 . Samples were furnace annealed at 900 °C for 330 min and 1000 °C for 43 min using N 2 flowing at 1.5 /mrin as the inert ambient and pure 02 flowing at 1.5 I/min as the oxidizing ambient. Samples were also rapid thermal processed (RTP) at 1100 °C for 60 to 240 s and at 1200 TC for 60 to 180 s using a Heatpulse 2101 with Ar flowing at 1.5 /main as the inert ambient and 02 flowing at 1.5 1/min as the oxidizing ambient. Since Ar and N 2 have similar thermal conductivities the thermal profiles of samples processed in these gases are expected to be similar. The Ge depth versus concentration profiles for as-grown and annealed samples were determined by Secondary Ion Mass Spectroscopy (SIMS) using a Perkin Elmer PHI 6600 quadrapole analyzer with a 6 kV oxygen beam. The profile depth scales were determined from Tencor Alpha-Step 500 surface profiler measurements of the SIMS sputtered craters. RESULTS AND DISCUSSION The diffused Ge profiles were analyzed using the Florida Object Oriented Process Simulator (FLOOPS). The measured Ge profile for the as-grown structure was used as the initial condition for the FLOOPS diffusion simulations. The software was adapted to the analysis of Ge interdiffusion by introducing Ge as a dopant in Si in the regions where there was SiGe alloy. The interdiffusivity is extracted from the FLOOPS calculated profile with the best fit to the SIMS measured profile for the annealed sample. In the present work, this diffusivity is taken to be a function of temperature only, ignoring possible concentration and stress dependencies. The oxidation of Si introduces silicon self-interstitials into the lattice [2]. The pair model is applied to those samples annealed in an oxidizing ambient. This model accounts for the

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possibility of both interstitial and vacancy mechanisms being operative by using an effective diffusivity given by[1]: D = D * fI ý1 +- (I 1-f I) C ~vv( 1

where D* is diffusivity under inert conditions, f, is the fraction of diffusion which occurs via interstitials, C, and Cv are the interstitial and vacancy concentrations respectively and Cj* and Cv" are the equilibrium interstitial and vacancy concentrations respectively. Keeping D* at the value extracted from the inert diffusion expe

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