Germanium Photodetectors for Silicon Microphotonics by Direct Epitaxy on Silicon
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Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
high as 3500 cm 2fV-s was also measured. 11 In this paper, we present a study on the optimization of the cyclic thermal annealing process and demonstrate the effect of improved materials quality on the performance of Ge/Si photodetectors. EXPERIMENT A hot wall UHV/CVD was used for Ge heteroepitaxy on Si. The base pressure of the UHV/CVD chamber is 3x10 9 Torr. The substrates were p-type Si (100) wafers with resistivity in the range of 0.5-2 Q-cm. Before heteroepitaxy, Si wafers were cleaned in Piranha solution (H 2SO 4 : H2 0 2 = 3 : 1) for 10 minutes. Next, native oxide was removed by dipping the wafers in aqueous HF solution (HF : H20 = 1 : 5) for 15 seconds. Heteroepitaxy of Ge on Si was initiated at 350'C with a flow of 10 sccm of GeH4 (15% in Ar). The total pressure during Ge heteroepitaxy was 15 mTorr. After 30 nm of Ge was deposited on Si, the furnace temperature was raised to 600'C and 1 gim of Ge was deposited on Si. Finally, the wafers were cyclically annealed between a high annealing temperature (TH) and a low annealing temperature (TL). TD densities were examined using plan-view transmission electron microscopy (TEM), and etch-pit density counting (EPD). For EPD, the etchant was a mixture of CH 3COOH (67ml), HNO 3 (20ml), HF (10ml) and 12 (30mg). An optical microscope was used to image the etch pits. For TEM experiments, JEOL 2000FX and JEOL 2010 transmission-electron-microscopes were used. We calibrated the magnification using a diffraction grating replica. Metal-semiconductor-metal (MSM) planar interdigitated photodetectors were fabricated on as-grown and cyclic thermal annealed Ge epilayers grown on Si. Detailed information regarding the fabrication of MSM photodetectors has been reported earlier. 7 The photo-response of MSM photodetectors were measured as a function of bias voltage using standard lock-in technique. RESULTS Table I shows a summary of the annealing parameters and measurements of TD densities in Ge deposited on Si. Plan-view TEM was used to estimate the TD densities. The error bars are the 95% confidence intervals of the average TD densities. A comparison of the TD densities of samples B, C, and D shows that increasing the number of annealing cycles is more effective in reducing TD densities than increasing the annealing time at TH=900°C. Thermal stress induced dislocation glide and annihilation has been proposed as the mechanism responsible for TD density reduction in cyclic thermal annealing processes. 12' 13 According to this mechanism, a process optimization scheme can be proposed based on the optimization of dislocation glide velocity. By increasing the dislocation glide velocity the distance a threading dislocation travels Table I. Annealing parameters and characterization results of samples A, B, C, and D. All samples were grown by two-step UHV/CVD. TH and TL are the high and low temperatures of the cyclic thermal annealing process. Threading-dislocation densities were measured by planview TEM. Sample ED TH(o
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