Microstructure evolution study of Pd-Ge ohmic contact formation on GaAs

The microstructure evolution and the corresponding solid-state reactions th at take place during the formation of the Pd-Ge ohmic contacts on GaAs were studied using constant-heating-rate differential calorimetry (DSC) and cro ss-sectional transmission electron microscopy (XTEM). DSC analysis at diffe rent scan rates was performed on Pd(20 nm)/Ge(150 nm)/Pd(50 nm) thin film s tacks that were lifted off the substrate and four solid-state reactions wer e identified. Specimens heated at temperatures that coincide with the DSC p eaks were quenched in a He atmosphere and the resulting microstructure was characterized by XTEM. Variable constant-heating-rate DSC experiments allow ed us to determine the activation energy associated with each solid-state r eaction by the Kissinger plot method. The results were as follows: for Pd:G e interdiffuson, the activation energy Q=1.03 eV, for hexagonal Pd2Ge forma tion Q=1.12 eV, for orthorhombic PdGe formation Q=1.33 eV and for Ge crysta llization Q=1.8 eV. Based on these correlations, the mechanisms that contri bute to the formation of an optimal ohmic contact microstructure were ident ified. (C) 1999 American Institute of Physics. [S0021-8979(99)07114-5].