Study Of ZnO&MgZnO Based Heterostructure Ultraviolet Light Emitting Devices

Wide band-gap semiconductor ZnO is regarded as an ideal material for ultravioletoptoelectronic devices, due to its high exciton binding energy of60meV, low growthtemperature and low cost, etc. Deep ultraviolet light emitting and laser diodes are quitevaluable in many fields such as high density memory and military. As a result, the study onZnO based light emitting device has attracted more and more attentions. Especially,wavelength-tunable deep ultraviolet emission can be realized from MgZnO ternary alloys,since it offers certain advantages including continuously tunable band gaps from3.37to7.8eV and tailored physicochemical properties, which facilitate band gap engineering. However,the difficulty in p-type ZnO hinders the development of ZnO homojunctions, most studieshave focused on the heterojunction light emitting device.In this thesis, we develop the heterojunction light emitting diodes (LED) and laser diodes(LD) based on ZnO and MgZnO alloy materials, the characteristics of electroluminescenceand electrically pumped laser are studied in detail. The primary contents are decrisbed asfollows:High quality (Mg)ZnO film is the most important component in the fabrication of lightemitting devices. As a result, we firstly study the effect of substrate temperature, oxygenpressure and target component on the compositions, defect states, optical, and electricalproperties of MgZnO alloy films. We suggest an optimized condition for the growth of highquality, high Mg-content, single-phase wurtzite MgZnO films.As to ZnO based light emitting devices, p-n heterojunction is firstly taken intoconsideration. Due to the identical crystal structure and similar physical propertities betweenGaN and ZnO, a series of p-GaN/n-ZnO and p-GaN/n-MgZnO diodes are fabricated.Combined with the study of photoluminescence, I-V characteristic and band alignment, thephysical mechanism of electroluminescence is rationally analyzed. It is proposaled that theradiative recombination at around400nm in p-GaN/n-ZnO arised from the interfacetransition at the junction. While the electroluminescence at around390nm inp-GaN/n-MgZnO is found independent of Mg composition, and is attributed to the transitionfrom the shallow donor defects to Mg-related acceptor level in p-GaN itself.It is very hard to realize a pure ZnO exciton emission in such p-n heterojunctions. So themetal-insulator-semiconductor (MIS) heterojunction is designed towards ZnO exciton LD. Siwafer is used as the substrate for the compatibility with silicon-based electronics. Electricallypumped exciton random laser is realized in this ZnO film based MIS heterostructure.Compared with the same MIS diode on lattice-matched GaN substrate, the MIS LD on Si wafer exhibits a similar excellent performance. It is ascribed to the island-like morphology ofpolycrystalline ZnO film induced from the large lattice mismatch. Such a high disorder inheterojunction interface enhances the optical scattering, which favors a low threshold randomlasing. Based on the band gap engineering, MgZnO films are devoloped to substitute ZnO asthe active layer in MIS heterojunctions. Wavelength-tunable electroluminescence andelectrically pumped random lasing are realized in MgZnO-based MIS LDs. Furthermore, theMIS heterojunction is successfully transferred to a Zn foil substrate. A flexible ZnO LD isachieved, and the MIS device also exhibits a function of memory and storage.The bandoffset in the heterojunctions reduces the quantum efficiency of light emittingdevices, while nano-sized diode can be an effective route since it offers higher carrierinjection rate. Therefore, nanoscale device is developed by using ZnO/MgO core/shellnanowires in the MIS structure. The morphology of ZnO nanowires can be well controlled bythe precursor concentration and seed layer. Electrical transport characteristics and randomlaser behavior were discussed in detail. Compared with the identical planar device, thecore/shell nanowire LD exhibits a lower threshold and higher emission efficiency. Theimproved device performance and the simple fabrication technique suggest that the ZnO/MgOcore/shell nanowires can serve as good building blocks for high-efficiency UV light-emittingdevices.