Thermal Annealing Effects on I-V-T Characteristics of Pd/Ge/Ti/Pt/Au-Si-Gaas Contact at High Temperatures

Abstract

Refractory metal (Pd/Ge/Ti/Pt/Au) on semi-insulated GaAs (SI-GaAs) have been prepared and annealed at 380,400 and 425℃. Optical microscope and SEM images show that there is no significant difference in refractory metal surface morphology before and after annealing, compared with the standard AuGeNi electrode of a large number of massive crystals precipitated after annealing, and the specific contact resistivity of Pd/Ge/Ti/Pt/Au-SI-GaAs is the lowest at 400℃ annealing. The current-voltage-temperature (I-V-T) characteristics of the as-deposited and annealed devices have been measured in in the high temperature of 300-400 K with steps of 20 K. The behavior of the forward I-V-T characteristics of inhomogeneous Pd/Ge/Ti/Pt/Au-SI-GaAs contact before and after annealing has been investigated. The experimental results show that the barrier height (Φb) and ideality factor (n) derived from the ideal thermionic emission model were strongly temperature dependent. Such behavior of the parameters is attributed to the inhomogeneous barrier height by assuming a Gaussian distribution at the metal/semiconductor (M/S) interface. We draw Φ_b versus 1/2kT plots to obtain the mean barrier height (`Φ b ) and standard deviation (σ). The values of` Φ_b are 1.373 eV, 1.275 eV, 1.199 eV and 0.130 eV, with  of 0.205 V, 0.184 V, 0.173 V, and 0.193 V for the as-deposited and 380,400 and 425℃ annealed devices, respectively. A modified ln(I₀/T²)-q²σ²/2k²T² versus 1/kT plots give the mean barrier height(Φ_b) and Richardson constant(A^*). The values of `Φ_b are consistent with the previous calculations. A^* of 6.46 A/K²·cm² for 400℃ annealed device is in closer agreement with the known value for SI-GaAs. Thus, it has been seen that the morphology of refractory metal electrode is better than that of AuGeNi electrode, and the Pd/Ge/Ti/Pt/Au-SI-GaAs contact by the thermal annealing at 400℃ is expected to be used as electrodes in SI-GaAs photoconductive switch instead of the standard AuGeNi metal.

Keywords: SI-GaAs, inhomogeneous barrier height, thermal annealing, refractory metal, thermionic emission