The Investigation Of The P-ZnO And P-NiZnO Films Grown By MOCVD And The Light Emitting Device

ZnO is stable hexagonal system under atmospheric pressure, belongs to the wurtzitestructure, and it is a semiconducting material with a direct wide band gap. Its bandwidth（3.37eVat room temperature） is similar to GaN，but the exciton binding of60meV is muchhigher than20meV of GaN. It has the very big development potential in ultra-violet opticaldevices application. However, due to very low intrinsic donor defects formation energy，ZnOnon-doped thin iflm material is n-type, so it is much diiffcult to realize p-type doping, theproblem still has so far not been effectively resolved and stall the development offabrication of ZnO photoelectric devices. Around this key problem, this paper carried out aseries of research work, sandwich doping method research innovatively proposed isbeneifcial to fabricate p-ZnO thin iflm material, based on this,by MOCVD growth method,we manufactured p-ZnO/n-GaN heterojunction light-emitting devices, and achieved thenear-band-edge emission in ZnO; according to the NiO normally for the characteristics ofp-type conductive maetrial,we also make a systematic study NixZnO_1-xO alloy material tofabricate p-type material.In this paper, ifrst chapter ifrstly give the fundamental properties of ZnO. Then，wereview the development track of the research on ZnO of scientiifc community in recent yearsand some signiifcant results that are about light-emitting device research and fabrication，andare paid highly attention.In addition, the latest progress of related device preparation, forexample, in2005Tsukazaki etc. of Tohoku university institute for materials research,employing N2as the source and using MBE method, successfully achieved better repetitive p-type ZnO thin iflm materials, and ifrst manufactured ZnO homojunction blue-UV LEDs onScAlMg04substrate by adjusting the growth temperature, which obtained the much betterexperimental results; in2011，Jianlin Liu，coming from University of California，ifrstly grew alayer n-ZnO on sapphire substrate adopting MBE method, then epitaxy Sb:p-ZnO NWs onn-ZnO thin iflm by vapor deposition, based on this,obtained zinc oxide-based homojunctionLED etc. achievement. Following we give a brief introduction to several common methodsand technological means fabricating ZnO thin iflms，as metal-organic chemical vapordeposition（MOCVD），molecular beam epitaxy（MBE）, magnetron sputtering and puled laserdeposition（PLD）.The second chapter mainly introduces the MOCVD equipment which is employed indepositing ZnO thin iflms by our team. We make two innovations. Firstly, Diethylzinc （DEZn）and oxygen were used as the precursors of oxygen and zinc for the convenience of dopingand controlling the concentration. DEZn and O2will react at room temperature before theyengage with each other，and the product particles will fall on the substrate which willinfluence the iflm quality.We introduce the precursors into the reactor through separateinjectors to avoid the pre-reaction to improve the quality. Secondly, we add the plasmasystem which contributes to ionizing the p-type doping gas（N2、NO、N02、MH3etc.） in thechamber. We have other innovations such as the high-speed rotating substrate, airbrush withuniform jackets and double lfow plan. This chapter also introduces some measurementscharacterizing ZnO iflm quality, such as x-ray diffraction（XRD）、 X-ray photoelectronspectroscopy（XPS）、 Field emission scanning electron microscopy （SEM）、photoluminescence（PL）> Hall-effect measurements （Hall） etc.The third chapter was major in the growth of ZnO iflm on the substrates of sapphire andGaAs by MOCVD. In the study, we found that the growth temperature obviously inlfuenceevery properties of the iflm. With the temperature increasing, The crystallization of ZnO thiniflm performance improvement, the grain size increases and the surface of the iflm becomesmooth. Meanwhile, with the temperature increasing，the iflm’s Optical quality improvesvery much. But we can’t heat the chamber more than650。C. So all of our ZnO iflms we fabricate are made of many nanorods.and they can’t be called good crystal thin iflm。The ZnO iflm grown on a GaAs substrate by MOCVD，turns into p-type conductiveafter annealing due to the diffusion effect of the As,, and its optical and electrical propertiesare studied. Some literature with ifrst principles calculation results show that the doping of Asin the ZnO can form Aszn-2Vzn complex defects acceptor, leading a p-type conduction.Some research group previously reported the incorporaiton of As could form the acceptorlevels, but their positions were not conifrmed. We annealed the GaAs substrate by heating.The As atoms can diffuse into the ZnO epitaxial layer, converting the epitaxial layer intop-type conduction. The P-ZnO variable temperature PL spectra show this material is n typeconduction before annealing. Donor related radiative recombination luminescencepeaks dominate in the low temperature PL spectra. When annealed, it is converted into n typeand acceptor related radiative recombination luminescence peaks dominate in thecorresponding low temperature PL spectrum. Our experiment results further demonstratedAszn-2Vzn complex defects acceptor determined by the primary principle exists and isreasonable. We use two different methods, respectively calculates the As related acceptorlevel positions in ZnO materials band. Both results, close to the calculation result, are locatedabove the top of valence band about140meV, which means that is a kind of shallowacceptor.Chapter Four introduces a GaAs sandwiching As doping method that our study groupinnovates. This solves the As doping problem of ZnO thin iflms grown on other non GaAsmaterial substrates by MOCVD. Meanwhile, the p-ZnO:As/n-GaN light emitting devicesprepared by this p type ZnO thin iflm materials, were observed the lasing phenomenon, andwe conifrmed that the emission luminescence comes from p ZnO side of the p-ZnO/n-GaNheterostructure devices.In the study of ZnO-based materials and devices, the preparation of p-type ZnO is themain issue that scientists all over the world strive to solve. We will discuss the ZnO iflmsgrown on a GaAs substrate in Chapter3. In this way, we got the p-type ZnO，because the Asfrom the substrate diffuse into the ZnO iflms. In order to achieve As doped in ZnO iflms grown on other substrate, and avoid to use the matalorganic source of As which are highlytoxic as well, we developed a new method to achieve As doped with the use of GaAsinterlayer. In the process, we will presputter a GaAs interlayer on the substrate withsubsequent epitaxy of ZnO iflm by MOCVD，and then make the As atoms diffuse into theZnO iflm through annealing at a high temperature to achieve the inversion of conductivityfrom n-type to p-type. In this chapter we will discuss the way to preparation the GaAsinterlayer in details. And also, we have studied the structural, electrical, opticalcharacterization and the X-ray photoelectron spectroscopy of ZnO iflm grown by MOCVDon sapphire substrate with two different thickness of4nm and8nm GaAs interlayer.After got the p-type ZnO iflm，we produced p-ZnO/n-GaN heterojunction luminescentdevice with it. Because of the good lattice matching and the similar band gap between ZnOand GaN，before the preparation of this device, some researchers have developedn-ZnO/p-GaN, n-ZnO/MgO/p-GaN luminescent devices with p-GaN. In particular, then-ZnO/MgO/p-GaN luminescent devices have achieved ZnO stimulated emission of electricalinjection. But the carrier concentration in p-type GaN is still low today, general at the1017order of magnitudes. Moreover, the low mobility and high resistivity were also found inp-type GaN. Due to the characterizations of p-type GaN described above. The carriers indevices can easily transfuse into p-type GaN from n-type ZnO and the recombination ofelectrons and holes in p-type GaN is the main origin of light. The n-type GaN is more mature,the carrier concentration and mobility are higher,so carriers can inject into the p-ZnO fromthe n-GaN. We have produced the p-ZnO/n-GaN heterojunction luminescent device andachieved ZnO stimulated emission of electrical injection with laser wavelength located onabout380nm，threshold current of47mA and threshold current density of2.6A/cm2whichare better than many reports about ZnO stimulated emission before.Under the present process conditions, neither p-type ZnO iflm grown on the GaAssubstrate or interlayer nor the lasing p-type ZnO iflm achieved from devices, which both areC-axis-oriented iflms arranged by columnar nanocrystal, can be deifned as the ideal crystaliflm. Although p-type ZnO iflm is already achieved, it is still too early to say we have solved the problems during the doping of the p-type ZnO iflm. That is to say，there is no real p-typeZnO single crystal iflm.The research results of NixZnO_1-xO alloy are introduced in Chapter5.Since NiO areusually the features of the p-type conductive material, we prepare NixZnO_1-xO alloycreatively for the purpose of p-type ZnO materials. We use light-assisted metal organic vapordeposition（MOCVD） technique to grow NiZnO with different Ni and oxygen concentration,and the crystal structure, optical and electrical properties of the samples are investigated. Theresults show that, NiZnO iflms which have wurtzite structure are successfully prepared on thec-sapphire substrate by using PA-MOCVD method. NiZnO keeps wurtzite structure as withZnO, and NiO phase separation is not observed until the Ni concentration is0.18.The c-axislattice constant of NiZnO iflm decreases gradually with the increase of Ni content. The testresults of optical properties show that the basic absorption edge of NiZnO alloy movestowards the low end with the increase of Ni content. Especially the charge carrierconcentration and the resistivity and electrical characteristics are analyzed, experiments andPL spectra show that，different Ni content also affects the electrical conductivity type ofNiZnO. When the Ni content X value exceeds a certain range, NixZnO_1-xO alloy changesinto p-type conductive. The intrinsic donor defects in the NiZnO iflm can be compensatedeffectively by increasing the Ni and O content. High hole concentration p-type NiZnO iflmscan be obtained by controlling Ni and O elements content in the iflm sample.