Field-controlled diffusion – an interesting approach for Schottky-based ZnO devices
Due to its unique materials properties ZnO is a very promising material for optoelectronic components. However, there is still a lack of reliable p-doping in ZnO. Consequently, no high performance pn-diodes and diode based devices like LEDs, laser diodes or photodiodes can be fabricated from this material system. For many applications like photodiodes or FETs Schottky diodes could be used instead. However, the fabrication of high quality Schottky contacts is problematic due to the low absolute energy of the conduction and valence band. Many unconventional fabrication methods have been proposed but none of them provides a reliable, reproducible and convenient method to fabricate Schottky contacts. Field controlled diffusion in ZnO provides an interesting approach for the realization of Schottky-based devices. We have tested this idea by using a ZnO bulk material in which Li doping has been incorporated (e.g., by ion implantation or thermally assisted diffusion process). Depending on the incorporation state (e.g., interstitial or substitutional at a lattice site) the doping ions act as donors or deep acceptors. Electric field-controlled diffusion at an elevated temperature allows a controlled movement of the interstitially incorporated dopants whereas the substitutional incorporated dopants remain localized. This process allows creating depletion regions underneath contact areas. The thickness of the Schottky barrier can be controlled by strength of the electric field. Using this process a larger variety of ZnO devices including Schottky-based diodes, photodetectors, metal-semiconductor field effect transistors (MESFETs) can be produced. This procedure is reversible and can be applied repeatedly. Therefore, the field-controlled diffusion processes provides also a method for revitalization of devices after degradation or (radiation) damage. Besides for Schottky based devices, the field controlled diffusion can be used to control or avoid surface conductivity that has been observed on native ZnO surfaces or to create lateral doping profiles for unique semiconductor structures.
Kiesel, P. ; Schmidt, O. Field-controlled diffusion – an interesting approach for Schottky-based ZnO devices. Invited talk at 14th Semiconducting and Insulating Materials Conference (SIMC-XIV); 2007 May 15-20; Fayetteville; AR; USA.