Study On Schotty And Ohmic Contacts Of ZnO:Al On GaN Thin Films

GaN is one of the most promising materials for high-temperature, high-power, and high-frequency applications as it has a high break down field, a high thermal conductivity and a wide band gap of about3.4eV at room temperature. In recent years, different types of GaN-based electronic devices have been studied, such as Schottky barrier diodes (SBDs), p-n junction photodiodes (PDs), ultraviolet photodetectors, power rectifiers, heterostructural high electron mobility transistors, and high-power metal semiconductor field effect transistors. However, a reliable Schottky or Ohmic contacts on GaN is critical for these devices. Thus, different metals have been deposited on GaN to produce high-performance Schottky contacts.It was found that the responsivity of normal metal-semiconductor-metor photodetectors (MSM) or light emitting diodes (LED) is limited by the presence of the opaque metal contact electrodes in general. Fortunately, it has been shown that the transparent conductive oxide (TCO) films could be used to solve this problem. Recently, GaN-based MSM photodetectors with TCO film electrodes, such as, indium-tin-oxide (ITO), Al-doped zinc oxide (AZO), et al., have also been demonstrated. The AZO films, which show a low electrical resistance and high transmittance in the visible range of the spectrum, have superior physical and optical properties. The minimum electrical resistivity of the AZO films reaches2.5×104Ω cm and the optical transmittance is over90%in the wavelength range from400to800nm. Thus, the AZO films are widely used as a reliable electrode material. The AZO/GaN contacts have an obvious advantage that ZnO films have almost the same lattice parameters as those of GaN films. Therefore, the AZO films could be easily deposited on GaN films. So far, there were few reports about the AZO/GaN Schottky contacts. In this work, the AZO/GaN contacts have been systematically studied, and the main results are summarized as follows:1. The AZO films were deposited on n-GaN films by using an electron beam evaporation system. The I-V measurements showed that the contacts presented a good rectifying behavior. The Schottky barrier heights were calculated out by using the thermionic emission theory. The dependence of the structural and electrical properties of the AZO thin films on different deposition temperatures has been investigated. The diversifications of the AZO/GaN contacts due to the different characters of the AZO films were found. The electric resistivity and optical transmittance of the AZO films are key factors to fabricate the AZO/GaN Schottky contacts.2. The Ag/AZO films were used to form the ohmic contacts to p-GaN. When the Ag/AZO films were heated up to300℃, the Ag films break into into isolated nano-scaled pieces, and the nano Ag will "pinch off" the Fermi-level of p-GaN, which forms an polarization electric field on the front of p-GaN, and produces the ohmic behavior of Ag/AZO to p-GaN.