Preparation And Characterization Of Atomic Layer Deposited Nonpolar ZnO-Based Electroluminescent Devices

ZnO is considered to be a promising semiconductor in optoelectronics because of its wide direct bandgap of 3.37 eV, large exciton binding energy of 60 meV, and high carrier concentration. Recently, ZnO has been widely investigated for using in light emitting devices in the ultraviolet and visible regions owing to its variety of natural defect structures. However, there still remain some obstacles in obtaining high quality and stable p-type ZnO, which induces efficient ZnO p-n homojunction light emitting diodes (LEDs) in a dilemma. As an alternative approach, p-n heterojunction LEDs with n-type ZnO films growing on a variety of p-type substrates have been widely studied. As is known, ZnO is stable in wurtzite structure, which is polarized along c-axis by the spontaneous and piezoelectric polarizations, and thus, the recombination efficiency of electrons and holes is reduced. This is a fatal disadvantage to light emitting devices. The most fundamental approach to avoid polarization effects is to fabricate devices based on semipolar or nonpolar films.In this work, semipolar and nonpolar ZnO films have been prepared by atomic layer deposition (ALD), and several nonpolar ZnO-based electroluminescent devices have been fabricated and characterized. The main conclusions are summed up as follows:1. Atomic layer deposited ZnO films have been prepared on GaN, Si and amorphous substrates. The electrical and structural properties of the ZnO films have been studied. The growth plane of the ZnO films on c-plane GaN substrates changes from (0002) to (10-11). The existence of twin structures and zinc blende grains leads to the formation of 2 kinds of (10-11) grains with different tilting angles, which tilting directions exhibit a six-fold symmetry. However, ZnO films grown on Si and amorphous substrates exhibit a preferred orientation of [10-10], and the films are formed by mosaic crystals.2. Epitaxial m-plane ZnO thin films have been deposited on m-plane sapphire substrates at a low temperature of 200℃ by ALD. The epitaxial relationship between the m-plane ZnO thin films and the sapphire substrates is characterized. Moreover, the residual strain along the ZnO [-12-10] direction is released. Based on the epitaxial m-plane ZnO thin films, metal-insulator-semiconductor light emitting devices are fabricated, and near-infrared random lasing phenomena from the metal-insulator-semiconductor devices are observed and discussed.3. Nonpolar m-plane n-ZnO/p-Si heterojunction LEDs with an AlN intermediate layer are fabricated by ALD. The energy band alignment of the ZnO/AlN/Si heterostructure has been studied by using X-ray photoelectron spectroscopy, and the result corroborates the electron blocking ability of the AlN interlayer. Electroluminescence results at 10 V forward bias reveal that the devices emit a quasi-white light consisted of red, green and blue emission peaks. It is authenticated that the AlN intermediate layer can effectively improve the performance of the n-ZnO/p-Si heterojunction light emitting devices.This work puts emphasis on the applications of atomic layer deposited nonpolar ZnO thin films in electroluminescent devices. And it provides a reference to the research and development of ZnO-based LEDs on p-Si substrates.