Preparation And Properties Of Nonpolar/Semipolar ZnO-based Thin Films And Structure

ZnO, owing to its wide direct-bandgap, is considered as a promising material for application in photoelectronic devices, such as light-emitting diodes (LEDs), laser diodes (LDs) and detectors. However, being wurtzite structure, ZnO-based heterostructures usually grow along the c axis and typically have a large internal electric field due to lattice polarization effect. As a result, the internal electric field seperates the wave functions of electrons and holes in real space and the so-called quantum-confined Stark Effect (QCSE) red-shifts the ultraviolet emission. This leads to reduced optical gain and thus limits the device efficiency. To overcome the detrimental effects of the internal electric fields, the growth of ZnO along nonpolar/semipolar direction has been proposed.In this paper, we prepared transition metals (Mn, Co) and Na/Ga codoped ZnO thin films by pulsed laser deposition (PLD) on various substrates, including R-plane sapphire, glass and quartz. The influences of growth conditions and doping effect on preferred orientation (PO) and properties of the ZnO films have been investigated in detail and several films and structures with various PO have been controllably fabricated. The main work included:1. Mn-Na codoped ZnO thin films were prepared on R-plane sapphire by PLD. The results indicate that substrate temperature, growth pressure and doping have significant influences on crystalline quality and properties of the as-prepared ZnO thin films. The 5 at.%Mn and 1 at.%Na codoped film which grown at 600℃under the pressure of 0.02 Pa had an acceptable crystalline quality and the best electrical properties with a-axis PO.2. Moreover, the growth of undoped pure ZnO films proceeded by using the Zn(Mn, Na) film as a buffer layer, which retained a nonpolar characteristic. Subsequently, two heteroj unctions named n-Zn(Mn, Na)O/n-ZnO/i-MgZnO/p-NiO with the same buffer layer and n-ZnO/i-MgZnO/p-NiO have been fabricated. I-V characteristic results show that threshold voltage and reverse leakage current of the device with buffer layer reduced.3. Co-Ga codoped semipolar ZnO thin films were prepared on glass substrates by PLD. The Zn(Co, Ga)O thin film grown at 500℃with the pressure of 40 Pa had semipolar PO and the best crystalline quality. Moreover, undoped ZnO films with inherited PO were prepared using the semipolar Zn(Co, Ga)O films as a buffer layer.4. A series of different elements doped ZnO thin films were grown on quartz substrates by PLD. It is found that certain conditions, such as high substrate temperature, high oxygen pressure, high laser energy, small target distance and a relatively large Mn content, favor the growth of film along nonpolar/semipolar direction.