Study On The Growth Of Polarity-Controlled GaN And ZnO Films By MOCVD And The ZnO-GaN Light Emitting Devices

III-nitrides have been attracted extensive attention because of theirs potential application value in light-emitting diodes(LEDs), laser diodes(LDs), microwave power devices, integrated circuit, etc. At present, high-performance LEDs or LDs are usually realized using Ga-polar Ga N template owing to its mature technology and good crystalline quality. However, it has been demonstrated that N-polar LEDs have lower hole injection barriers, batter electron confinement and higher Indium incorporation efficiency as compared to Ga-polar LEDs. Unfortunately, the high background carrier concentration and rough surface morphology of N-polar Ga N films severely hinder the design and development of N-polar Ga N-based devices. In addition, Zn O, as a new-type wide band-gap semiconductor materials, has higher exciton binding energy and optical gain as compared to Ga N. Thus, Zn O should have more advantage than Ga N in the fields of LEDs or LDs. Basing on the above reasons, our researches mainly focus on the growth of polarity-controlled Ga N and Zn O films and the fabrication of Zn O-Ga N heterojunction optioelectronic devices. The detailed research contents are as follows:We investigated the growth of high-quality N-polar Ga N films by metal-organic chemical vapor deposition(MOCVD). The effect of buffer layer thickness, growth temperature and V/III rate on the surface morphology, crystal quality, strain status and optical and electrical properties of N-polar Ga N films have been precisely studied. The fabricated N-polar Ga N films have an atomically smooth surface and its(0002) ?-rocking curve full-wide at half maximum(FWHM) is ~40 arcsec.We achieved p-type doping of N-polar Ga N and studied the effect of growth temperature and Cp2 Mg flow rate on the optical and electrical properties of N-polar Ga N in detail. When the growth temperature and the Cp2 Mg flow rate are 1000 oC and 600 nmol/min, respectively, the hole concentration of the prepared N-polar p-Ga N films is as high as ~3.0×1017 cm-3. We further investigated the effect of Mg doping on the strain statues and chemical etching properties of N-polar Ga N films. Experimental results demonstrated that the etching rate of N-polar Ga N in KOH solution gets slow IV gradually with the increase of Mg dopant concentration. Furthermore, we systematically studied the effects of Mg doping on the thermodynamics and kinetics related factors of chemical wet etching of N-polar Ga N and tentatively proposed etching process model using a simplified ideal atomic configuration.We studied the controllable growth of multi-dimensional O-polar Zn O materials by using MOCVD on Ga N/Al2O3 substrates. The effects of growth temperature, O2 flow rate and growh pressure on the morphology, crystal quality and optical properties of Zn O epitaxy were investigated detailedly. Afterward, Ar+ milling technology was employed to enhance the optical prperties of one-diamional O-polar Zn O nanorods, and the patterned N-polar Ga N templates were used to prepare high-quality O-poalr Zn O nanowall networks. The(0002) ?-rocking curve FWHM of the prepared Zn O nanowall networks is ~208 arcsec.We precisely investigated the effects of interface polarization effect on the electrical and optical properties of n-Zn O/p-Ga N heterojunctions.(1) For Zn-polar n-Zn O/Ga-polar p-Ga N heterojunctions, the interface polarization effect may lead to a narrower depletion region. As a result, the device can emit ultraviolet/green light under forward/reverse bias.(2) For O-polar n-Zn O/N-polar p-Ga N heterojunctions, the interface polarization effect can shift the location of the depletion region from the interface deep into the Zn O side, thereby improving the carriers recombination rate in Zn O layers. The device emits ultraviolet light centered at 385 nm, and the FWHM of electroluminescence(EL) spectra narrows from 48 nm to 24 nm with the increase of forward bias.(3) For O-polar Zn O nanowall networks/N-polar graded p-AlxGa1-xN heterojunctions, It is demonstrated that the interface polarization of O-polar Zn O nanowall networks/N-polar graded p-AlxGa1-xN heterojunction can induce high density two-dimensional hole gas, which can improve hole injection efficiency of the studied diode remarkably.In order to improve the emission efficiency and stability of Zn O-based heterojunction LEDs, Zn O contact heterojunction LEDs were desigined and fabricated:(1) n-Ga N/i-Zn O films/p-Ga N contact heterojunction LEDs were fabricated. Only ~4.57% decay in the light output power was observed from the studied LED after a running time of 10 hours, indicating a good device stability.(2) n-Ga N/i-Zn O nanopencils/p-Ga N contact heterojunction LEDs were fabricated. Ultralow emission onset of ~1.2 ?A was achieved from the LED, and the improved light emission performance can be attributed to the enhanced quantum confinement effect rendered by the top tip of Zn O nanopencils.(3) n-Zn O films/p-h BN/p-Ga N contact heterojunction LEDs were fabricated. A pure ultraviolet emission located at ~378 nm, originating from Zn O, can be observed from the diode under forward bias. More importantly, the FWHM of EL spectra is only 12 nm, suggesting a high-efficient ultraviolet emission obtained.