Cryogenic processing of metal/GaAs schottky diodes

High quality metal/GaAs Schottky diodes were fabricated with the substrate at low temperature (LT = 77 K) during metal deposition. The barrier height, φ_B, was found to be 0.97 eV for Au metal. The reverse saturation current density, J₀, was 5.6 × 10^-11A/cm². Comparing with the diodes fabricated at room temperature (RT = 300 K), the LT diodes yielded a much higher φ_B and a lower J₀ as confirmed by electroreflectance (ER) measurement. This could be attributed to the amorphous-like reconstruction layer between the metal and GaAs interface. This layer serves as an insulator and gives the LT diode a metal-insulator-semiconductor (MIS)-like structure. In the RT diode, such a layer was destroyed by the reaction of metal with GaAs. The φ_B dependence on the metal work function, φ_m, was investigated by depositing Al, Cu, Au and Pd. A good linear relation was obtained between φ_B and φ_m. This suggests partial Fermi level unpinning in the LT diodes. The transport mechanisms were studied by current-voltage-temperature (I-V-T) techniques. Thermionic emission (TE) theory dominated in the current transport at room temperature for both the RT and LT diodes. At low temperature, the RT diode exhibits an excess current component, while the LT diode yields a tunneling current component. This can be explained by the surface recombination due to the interface states in the RT diode and the very thin insulator-like reconstruction layer in the LT diode.