Fabrication And Photoelectric Properties Of N-ZnO/p-GaN Heterojunction Light Emitting Device

Zinc oxide （ZnO）, a II-VI oxide semiconductor with a wide direct band gap （3.37eV） and alarge exciton binding energy （60meV） at room temperature, has been considered as a potentialcandidate material for short-wavelength optoelectronic applications, especially for blue toultraviolet （UV） light emitting diode （LED） and UV detector devices. However, althoughprogress has been made in p-type doping of ZnO, ZnO-based homojunction LEDs has sufferedfrom lack of reproducible and high-quality p-type materials. As an alternative approach tohomojunction, n-ZnO/p-GaN heterojunction has been proposed as attractive candidates fordevice applications, for ZnO and GaN have similar lattice structure （wurtzite） and relativelysmall lattice mismatch （1.8%）. In this study, ZnO thin films and nanorods were grown by pulsedlaser deposition （PLD） and hydrothermal synthesis on p-GaN to form n-ZnO/p-GaNheterojunction LEDs, respectively. The optoelectronic properties of different devices have beenstudied, and an appropriate band model of PN junction has been constructed for illuminating themechanism of the carrier recombination characteristics and luminescence properties of ZnO/GaNbased LED.The major findings of this thesis are as follows:1. ZnO films have been deposited on Al₂O₃（0001） substrate under the different experimentalcondition by PLD technique. With the X-ray diffraction （XRD）, atomic force microscopy （AFM）and photoluminescence （PL）, the infuences of the substrates temperature, oxygen pressure andannealing temperature on the photoluminescence properties of ZnO thin flms were characterized.The results show that, the ZnO thin flms which were deposited in10Pa oxygen atmospheresand at450℃substrate temperature have the best surface morphology and crystalline quality.And the crystalline quality of ZnO film first increases and then decreases with the annealingtemperature increasing, reaching the best at800℃. It was shown that the substrates temperature,oxygen pressure and annealing temperature have significant effect on the photoluminescenceproperties of ZnO thin films. Furthermore, the highly oriented ZnO nanorods are prepared on theAl₂O₃（0001） substrate by hydrothermal method, and the effects of seed layer on the structureand photoluminescence of ZnO nanorods are investigated.2. ZnO flms were grown on the p-GaN substrate by PLD and the n-ZnO/p-GaNheterojunction LEDs were fabricated. The electroluminescence （EL） properties of then-ZnO/p-GaN heterojunction diodes have been studied and all the EL spectra display onebroadened emission band centered at about430nm. Compared with the PL spectra, it can be easily identified that the EL emission of n-ZnO/p-GaN LED originates from the deep levelrecombination in the p-GaN layer. The mechanisms of radiative recombination in n-ZnO/p-GaNheterojunction LEDs have been understood by examining the energy band structure. Althoughthe barrier heights for electrons and holes are almost the same, the electron injection from n-ZnOcan prevail over the hole injection from p-GaN, by reason that the electron concentration andmobility in the n-ZnO films are higher than the hole concentration and mobility in the p-GaNlayer.3. To block the injection from ZnO and suppress the emission from GaN, n-ZnO/p-GaNheterojunction light emitting diodes with different interfacial layers were fabricated by PLD.Wide band gap materials such as MgO, AlN, ZnS, Ga₂O₃and i-ZnO,have been introduced inton-ZnO film/p-GaN LEDs to fabricate the PIN light emitting diodes. Due to the largeconduction-band offsets （ΔEC） for MgO/ZnO （ΔEC=3.55eV） and AlN/ZnO （ΔEC=3.29eV）interfaces, the interfacial layers can effectively block electrons injection from ZnO and improvedEL performances can be observed from n-ZnO/MgO/p-GaN and n-ZnO/AlN/p-GaN LEDs.Moreover, although the ΔECfor the Ga₂O₃/ZnO （1.85eV） and ZnS/ZnO （0.45eV） interface aremuch smaller than that of the MgO/ZnO, AlN/ZnO interfaces, a much smaller barrier heightsΔEV（0.20eV and0.14eV） for hole can make hole injection from GaN to ZnO more easily. As aresult, signifcant improvement of ultraviolet EL was observed in n-ZnO/ZnS/p-GaN andn-ZnO/Ga₂O₃/p-GaN heterojunction LED. Furthermore, an n-ZnO:Ga/p-GaN heterojunctionLED with i-ZnO interfacial layer was fabricated. The i-ZnO layer with a low electronconcentration in the n-ZnO:Ga/i-ZnO/p-GaN structure leads to the injection of electrons fromn-ZnO:Ga and holes from p-GaN into the i-ZnO layer to produce ultraviolet emission.4. Heterojunction LEDs based on arrays of ZnO nanorods were fabricated on p-GaN films byhydrothermal method. Without any phosphors, white-light EL from ZnO nanorods/p-GaNheterojunction LEDs operated at reverse breakdown bias was observed. The tunneling effect inthe interface can be charged for the possible mechanisms. With the ZnO seed layer, the ELspectra of ZnO nanorods/i-ZnO/p-GaN exhibit a broad emission peak from ultraviolet to visibleand the EL mechanisms infuenced by the interfacial layer are discussed.