In recent years, GaN-based materials are widely used in high-power and high-temperature electronic devices due to their excellent thermal conductivity, high thermal stability, high breakdown field and high saturation velocities.
Due to the limited availability of inexpensive homoepitaxial substrate, GaN films are usually grown heteroepitaxially on sapphire, SiC and Si substrates. Among these Si substrate affords many advantages like low cost, good thermal and electrical conductivities, large size availability, and the potential integration of GaN-based devices with well-established Si-based electronics. Despite the large mismatches in the lattice parameters (17%) and thermal expansion co-efficient (52%) between GaN and Si which generates high densities of threading dislocations and defects, Si remains the ideal choice of substrate for GaN epitaxial growth.
In this study, GaN thin films were grown on Si(111) substrate for different Ga flux by plasma-assisted molecular beam epitaxy. Morphological analysis by scanning electron microscope revealed that GaN films exhibit Ga polarity after etching in KOH solution. Current transport processes in GaN films was investigated by current-voltage (I-V) measurements. GaN films prepared at high Ga flux exhibited less leakage current at -2 V. Temperature dependent I-V measurements revealed that the dominant current transport process in GaN films was Poole Frenkel emission in the temperature range 200-260 K. Deep level transient spectroscopy measurements was performed to investigate the deep level defects in GaN films, and the results showed that GaN films have HT1, ET1, and ET2 trap states. The origin of ET1 trap is attributed to the point defects in GaN, and the origin of ET2 trap is related with N interstitial defect. Also, the ET1 signal of GaN films prepared at high Ga flux was small when compared to the films prepared at low Ga flux. Hence, the electrical transport properties of GaN Schottky diodes show a significant increase in the leakage current with the decrease in Ga flux, due to the increase in ET1 trap density. If ET1 trap density increases, relatively more electrons are captured by ET1 trap and were emitted to the dislocations by thermal energy. These many electron emissions contribute to the leakage current rise. Therefore, the ET1 trap density should be reduced in order to ensure low leakage current in GaN Schottky diodes.