Phase transitions in gold contacts to GaAs

X-ray diffraction was used in situ to study the phase transitions which occurred in 1500 Å Au/GaAs(100) on heating and cooling. The reaction between gold and GaAs took the form Au + Ga → α-(Au-Ga). On heating, α-(Au-Ga) completely dissolved in liquid Au-Ga. On subsequent cooling, β-(Au-Ga) (or Au₇Ga₂) formed. In nitrogen at 1 atm, phase transitions were observed reversibly at 525 ± 25 °C (as a result of the complete dissolution of α-(Au-Ga) on heating) and at 415 ± 5 °C (as a result of the peritectic transformation of β-(Au-Ga) to α-(Au-Ga) and liquid Au-Ga on heating). In a vacuum of 56.7 Pa, similar phase transitions were observed at 425 ± 25 °C and 387 ± 13 °C respectively. The contact morphology remained quite smooth provided that the dissolution of α-(Au-Ga) was avoided.