The Lateral diffusion length of Zn in Si-doped GaAs under a SiO
2 or SiN
x diffusion mask was evaluated using (1) the TLM resistor pattern method, (2) the p-n capacitance method, (3) the EBIC method, and (4) the cleaved stain-etching method. The SiO
2 was deposited at 250°C using SiH
4 and N
2O gases, and SiN
x was deposited by plasma-chemical-vapor-deposition at 300°C using SiH
4 and N
2 gases. Employing the open-tube method, Zn was diffused into GaAs through windows opened in the dielectric films at 600_??_650°C for 2.5_??_22.5 minutes in a nitrogen gas ambient. ZnAs
2 was used as a diffusion source. The anomalous fast lateral diffusion of Zn was observed only for the SiO
2-deposited GaAs. The ratio of the lateral to vertical diffusion length was about 4 for 0.3μm-thick SiO
2 and was strongly dependent on SiO
2 film thickness. For SiN
x on the other hand, the ratio was about 0.9 regardless of film thickness. Similar results were obtained by each evaluation method. The anomalous diffusion mechanism for SiO
2 is not clear, however, the results of our systematic experiment suggests that the defects at GaAs surface, which were produced during SiO
2 deposition, correlate with the anomalous diffusion.
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