We discuss the heavily-doped Zn diffusion properties that result from two-step rapid thermal annealing processes, and characterize the use of Au/Zn-based metal alloys as ohmic contacts, in a double floating guard ring avalanche photodiode (APD) structure with recess etching. Electrochemical capacitance voltage measurements were carried out on the APD, to determine the concentration of holes that result from Zn diffusion, transfer length method measurements were used to determine the specific surface contact resistance, and bandwidth measurements were used to characterize the APD. The hole concentration and specific contact resistance of the APD structure were found to be 1 x 10(19) cm(-3) and 2.8 x 10(-6) Omega cm(-2), respectively. In addition, the gain-bandwidth product of the APD was found to be over 80 GHz. The results confirmed that our APD operated satisfactorily with good reliability. (c) 2006 Elsevier B.V. All rights reserved.