ABSTRACT
SigniŒcant progresses on ZnO-based semiconductor materials have been achieved in recent decades. Recently, ZnO-based materials have been regarded as promising widebandgap semiconductors for application in ultraviolet (UV) region optoelectronic devices because of their excellent optical and electrical properties, such as the wide direct bandgap of 3.37 eV, the high exciton binding energy of 60 meV at room temperature, the good chemical properties, and the low cost. Recently, various applications of ZnO-based optoelectronic devices, such as UV photodetectors and light-emitting diodes (LEDs), have been demonstrated. These progresses have been comprehensively reviewed in some review papers [1-5], and the various aspects have been discussed in detail in the relevant chapters of this book. However, there are still some obstacles needed to be overcome. One of the tasks of great interests is to obtain reliable and stable p-type doping and to suppress the native n-type conductivity of the ZnO-based materials. Many efforts have been devoted to solve this problem, and promising successes have been achieved continuously. For example, the vapor-cooling condensation system was developed and established, which demonstrated to be an ideal technique for growing high-quality intrinsic ZnO Œlms [6]. The associated deposition mechanisms were presented [7]. This technique was successfully used to fabricate various devices, such as the ZnO-on-GaN heterojunction LEDs [6] and the UV photodetectors [8], the n-i-p ZnO-based LEDs [9,10], and the single n-ZnO:In/i-ZnO/p-GaN-heterostructured n-i-p nanorod LEDs [11]. The technique has also been used to deposit intrinsic ZnO Œlm as the gate insulator layer for the AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (HEMTs) [12,13].
