Reactions of Pd on (100) and (110) GaAs surfaces

The reactions of Pd on atomically clean or air-exposed (100) and (110) GaAs surfaces at temperatures between 20 to 500°C in different ambients were investigated by transmission electron microscopy. Interfacial reactions quite different from previous x-ray results were observed and two new Pd-Ga-As ternary phases were identified for the first time. At lower temperatures (T≲250°C) the formation of a ternary phase PdGa ∼0.3As∼0.2, which has a hexagonal structure very similar to that of Pd2Ge or Pd2Si with a 0=b0=0.672 nm and c0=0.340 nm, was observed. This ternary phase is epitaxially oriented with (12̄0) ternary∥(100)GaAs and [001]ternary∥ [011]GaAs on (100) GaAs substrates, and with (11̄0) ternary∥(110)GaAs and [001] ternary∥[11̄0]GaAs on (110) GaAs substrates. At temperatures between 350 and 500°C only one phase, PdGa, was observed to form in a high vacuum environment, whereas in a forming gas ambient, either a mixture of PdAs2 and another ternary phase PdGa ∼0.6As∼0.4 (at 350°C) or a mixture of PdAs2 and PdGa (at 500°C) was observed. The ternary phase PdGa∼0.6As∼0.4 is also hexagonal in structure with a0=b0=0.947 nm and c0=0.374 nm. The PdGa phase formed at high temperature is epitaxially oriented on (100) substrates with (110) PdGa∥(100)GaAs and [1̄11] PdGa∥[011]GaAs, but is randomly oriented on (110) substrates. All these observations indicate that the Pd-GaAs reactions at T≳350°C are very sensitive to the ambient conditions but not as sensitive to the GaAs surface cleanliness or substrate orientation. Correlation of these structural observations to ultraviolet and x-ray photoelectron spectroscopy data obtained from the same reacted interfaces are also discussed.