Ohmic contact behavior of aluminum-doped zinc oxide with carbon-doped p-GaP epilayer for AlGaInP LEDs applications

Aluminum-doped zinc oxide (AZO) thin films used for ohmic contact layers on carbon-doped GaP window layers (p-GaP:C) of AlGaInP light-emitting diodes were fabricated and characterized. AZO thin films with different Zn:Al cycle ratios (15:1, 20:1, and 25:1) were deposited on p-GaP:C window layers through atomic layer deposition. The contact characteristics of the AZO thin films on p-GaP:C were considerably changed from Schottky contact to ohmic contact after rapid thermal annealing (RTA) at 350 °C for 1 min. The most favorable specific contact resistance of AZO/p-GaP:C was evaluated using a circular transmission line model as 6.3 × 10−3 Ω/cm2. Angle-resolved X-ray photoelectron spectroscopy was employed to understand the ohmic contact behavior of AZO/p-GaP:C. After RTA, Zn atoms in the AZO thin films notably diffused into the p-GaP:C layers and Ga atoms diffused out of the p-GaP:C layer. Therefore, the Ga vacancies were occupied by Zn atoms, which increased the doping concentration in the near-surface region of p-GaP:C and reduced the depletion region width of the semiconductor region. Thus, numerous carriers were able to tunnel through the reduced Schottky barrier and those carriers produced the ohmic contact behavior between the AZO and p-GaP:C.