Electrical characterization of Al-doped GaN thin films deposited by electrodeposition

Abstract

The objective of this work was to prepare and study the electrical properties of gallium nitride and Al-doped gallium nitride thin films grown by electrodeposition on different substrates for use in optoelectronic devices. High-quality gallium nitride and Al-doped gallium nitride thin films were successfully prepared by electrodeposition on indium tin oxide (ITO) coated glass and silicon (Si) substrates. The structure of the thin films were verified using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Schottky diodes were manufactured by depositing gold and gold/nickel as Schottky contacts and aluminium contacts as ohmic contacts in a resistive thermal evaporation system. The electrical properties of GaN and Al-doped GaN thin films have been studied using current-voltage (IV) and capacitance-voltage (CV) measurements. Defects in the films have been investigated by deep level transient spectroscopy. Good rectification was obtained with the Schottky diodes on the GaN and Al-GaN thin films deposited on the Si substrate. The GaN and Al-doped GaN thin films were irradiated using α-particles from an americium 241 metal foil with an energy of 5.4 MeV and a fluence rate of 7.1 × 106 cm-2.s-1. Electrical properties of the Au/GaN/Al and Au/Al-doped GaN/Al Schottky diodes was studied using current-voltage (IV) and capacitance and voltage (CV) measurements at the room temperature (300 K) as well as in the (300 – 50 K) temperature range. As the temperature decreased, the barrier height from IV measurement decreased from 0.68 eV to 0.13 eV, and the ideality factor increased from 2 to 7.3. Temperature-dependent current voltage (IVT) measurements of Au/Ni/GaN/Al have showed that the barrier heights and ideality factor increase with increasing temperature, while temperature-dependent capacitance voltage CVT measurements showed a decrease in barrier heights with increasing temperature. This discrepancy has been attributed to the effect of barrier height inhomogeneity. In this thesis, deep level transient spectroscopy (DLTS) was used to investigate the electrically active defects in gallium nitride and Al-doped GaN before and after irradiation using -particles. ii For the first project, the defects at 0.49 eV to 0.47 eV and 0.29 eV were observed in Au/Ni/GaN/Al Schottky diodes grown using different current densities. In the as-grown material, only one defect state at 0.32 eV below the conduction band was observed in Au/GaN/Al. However, after irradiation with high-energy α-particles, four defect states at 0.10 eV, 0.20 eV, 0.42 eV and 0.51 eV were observed. The defect state at 0.10 eV had a capture cross section that was found to be five orders of magnitude larger compared to a similar study, and is therefore believed to be a different defect, not previously observed, while the other defects are the most common defect species observed in n-GaN. In Al-doped GaN defects were observed at 0.44 eV, 0.34 eV and 0.37 eV. The activation energies of the defects observed after irradiation were 0.61 eV for the undoped GaN, and 0.42 eV and 0.56 eV for the doped films with 0.5 at. % Al and 1 at. % Al, respectively. The radiation-induced defects found in this study were similar to defects previously found in GaN were prepared with different techniques.