Preparation And Characterization Of Nonpolar ZnO Heterojunctions By Atomic Layer Deposition

ZnO with a high exciton binding energy of60meV is a promising material for photonic applications, such as ultraviolet light emitting diodes (LED) and lasers. Herein lies, however, an obstacle of homojunction diodes of ZnO, namely p-type doping, which still remains a problem in obtaining high quality and stable p-type ZnO. Although some p-n homojunctions of ZnO LED have been reported, their electroluminescence intensities are usually very weak. As an alternative approach, heterostructured LEDs have been fabricated with n-type ZnO on the top of a variety of p-type substrates. As is known, ZnO is polarized along its c-axis of wurtzite structures caused by the spontaneous and piezoelectric polarizations, and thus, electrons and holes are separated in crystal. This causes negative effects on the device performance and decreases the internal quantum efficiency. Therefore, there has been considerable interest to grow high-quality nonpolar or semipolar ZnO films to avoid polarization effects. In this work, some nonpolar and semipolar devices have been fabricated by atomic layer deposition (ALD) and molecular beam epitaxy and the characteristics of these devices have been systematic studied. The main results are summarized as follows:1. Nonpolar m-plane ZnO films have been deposited on p-type Si substrates by ALD. Moreover, with a10nm Al2O3bufferlayer, the crystalline quality, optical and electrical properties of the heteroj unctions have been improved.2. ZnO films have been deposited on the p-type GaN (0002) substrate by ALD and the growth direction of the ZnO films were [70-74]. With the XRD and HRTEM results, a new structure of the ZnO film was presented. Moreover, the heterostructured LED can be lighted on under an ultralow driven current density of15mA/cm2. With a10nm Al2O3bufferlayer, the growth direction of the ZnO films on GaN (0002) changed from [70-74] into [10-10]. The nonpolar heteroj unction showed different emission behaviors under forward and reverse biases. In addition, with an InGaN bufferlayer, the crystalline quality of the ZnO films had been dramatically improved and the white light might be directly obtained with this structure in theory.3. Nonpolar and semipolar GaN and ZnO films were deposited on r-plane and m-plane sapphire substrates by molecular beam epitaxy and ALD, respectively. Furthermore, a-plane ZnO/GaN heterostructured LEDs were fabricated.In this work, nonpolar and semipolar ZnO heterstructured LEDs have been prepared. It provides a way to fabricate nonpolar ZnO heterojunctions by ALD for optoelectronic devices.