Microstructure Properties And New Effects Of Polarized Wide Bandgap Semiconductors

Throughout the research area of material science and even physics, today, GaN and ZnO could be taken as the hottest topics. While still undergoing a lot of applied studies and developments, the fundamental researches which could lead us to a deep understanding can not be ignored, microstructure characterization is one of them. Both in the family of wide bandgap semiconductors, GaN and ZnO are more related because of polarity. In this paper, we study the correlationship between microstructure and physical properties of polarized GaN and ZnO materials. The main innovative results are:1. Local piezoelectric polarization enhancement effect induced by edge dislocation in GaN layerA comparative study of the spatial distributions of microstructure and electrical properties has demonstrated that compressive strain existed surrounding edge dislocations in GaN layer, which will generate a local piezoelectric polarization filed with the opposite direction to that in AlGaN layer, and thus help to localize two-dimensional electron gas (2DEG). This type of compressive strain is a hydrostatic strain component, which will always associate with edge dislocation unless there is no n type background carrier in GaN. The resulted local piezoelectric polarization enhancement effect could be one of the reasons that GaN electronic devices operate well despite high dislocation densities.2. Effect of lateral growth on the microstructure and optical properties in GaNThrough a systematically investigation of metal-organic chemical vapor deposition (MOCVD) growth mechanisms, a lateral growth model of high temperature GaN epitaxial layer has been proposed. It is found that under high lateral growth rate, dislocation is easy to bend into subgrains away from c axis, which deteriorates the luminience efficiency of GaN. While straightly propogated dislocation and large grain size can increase the light extraction efficiency of the film, which could be one of the reasons that GaN photoelectronic devices operate well despite high dislocation densities.3. Effect of polarity on GaN wet etching processesThrough a thoroughly observation of GaN wet chemical etching, it has been demonstrated that polarity results in different etch pit shapes of screw and edge dislocations in GaN epilayers. For the pure screw dislocation it is easy to be etched along the surface steps that dislocation terminates. Consequently a small Ga-polar plane is formed to prevent further vertical etching, which results in an etch pit of inverted truncated hexagonal pyramid. However, for the pure edge dislocation it is easy to be etched along dislocation line, inducing an etch pit of inverted pyramid. This mechanism can be used in industry to detect defects of high densities in GaN epitaxial layer; moreover, wet etching can also be used to fabricate GaN multilayer quantum dots. These two applications have obtained two Chinese national invention patents.4. Switching effects of piezoelectric potential on the electronic transport of ZnO nanowire devicesWe have investigated the effects of piezoelectric potential in a ZnO nanowire on the transport characteristics of the nanowire based field effect transistor through numerical calculations and experimental observations. Under different straining conditions, including stretching, compressing, twisting, and their combinations, a piezoelectric potential is created throughout the nanowire to modulate/alternate the transport property of the metal-ZnO nanowire contacts, resulting in a switch between symmetric and asymmetric contacts at the two ends, or even turning an Ohmic contact type into a diode. Correctly manipulation the magnitude and distribution of piezoelectric potential in nanowires, a group of electronic devices can be fabricated.5. Fatigue free effect of ZnO nanowire.The dynamic mechanical properties of ceramic ZnO nanowire (NW) have been investigated under resonance cyclic loading condition using in-situ transmission electron microscopy (TEM). After mechanical deformation at the resonance frequency at a vibration angle of 5.2o for 1010 cycles, no failure or any defect generations have been found. We believe that the dislocation-free nature of NW and the large surface-to-volume ratio contribute to the NW's ability to undergo deformation without fatigue or fracture, proofing its durability and toughness for nanogenerators and nanopiezotronics.