Study Of Growth And Properties Of ZnO Films Grown On Si And GaAs Substrates By MOCVD

With the fast developing information technology, communication andnetwork which are depended on optoelectronics and microelectronics becomesthe core of new technology. Short wavelength LEDs and LDs have great usesin light memory, display and laser print, etc. ZnO films attract much moreattention as ZnSe and GaN in optoelectronics research field. ZnO, as apromising wide direct-gap Ⅱ-Ⅵ semiconductor, has remarkable optical andelectrical characteristics,and use in domain semiconductor illumination. It canalso be used in sensors, piezoelectric transducers, transparent conductor,surface acoustic wave devices, and lasers. There are lots of methods have beenused to deposit ZnO films such as pulsed laser deposition (PLD), molecularbeam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD),atomic layer epitaxy (ALE), sputtering, chemical vapor deposition(CVD),e-beam evaporation, sol-gel, spray pyrolysis.In this thesis, we prepare ZnO films on Si and GaAs substrates byMOCVD and firstly found that different treatments to GaAs substrates affectthe orientation of ZnO thin films. Simultaneously, we investigate thoroughlythe influence of the flow rate of DEZn and growth temperature on theproperties of the films, and successfully prepare p-ZnO thin film by Asdiffusion from GaAs substrate. p-Si/n-ZnO and n-GaAs/p-ZnO heterojunctionphotodiodes are fabrication and their characteristics are investigated.It was found that the flow rate of DEZn strongly influenced on thestructural properties and surface morphology of ZnO films. The film grown at3SCCM had only one intense (002) diffraction, and had the biggest grain size.Increasing the flow rate of DEZn, the intense of (002) and other diffractionwere decreased, when it reaches 9SCCM, the crystalline quality of the ZnOfilms was degraded to polycrystal. Considering thermodynamics, when theflow rate of DEZn is above 3SCCM, the growth is under nonequilibriumreaction condition, so it is polycrystal.ZnO films are grown by MOCVD on Si substrates. The effects of thegrowth temperature (Tg) on the properties of the films are analyzed and theoptimized growth conditions are obtained. The results show that the ZnO filmdeposited at 610℃ has the best crystallinity, surface morphology and thebiggest grain size. Increasing the growth temperature, the growth speed ofZnO films was increased. Under low temperature, it depends on the surfacereactive velocity. When the temperature is moderate, it is affected by thequality transport of the source. Under higher temperature, because ZnO filmsdecomposed, the speed decreases with the increasing of the temperature.The effect of different treatments of GaAs substrate to the orientation ofZnO film was firstly analyzed. When the etching time is short, the filmexhibits (002) orientation. XPS measurements indicate that the substrate isAs-rich when the etching time is above 90s, and the orientation of the filmconverts to (100). PL results show that both of the two films have similaroptical properties, the films grown at 640℃ and 630℃ have the best opticalquality.At the same time, the annealing under N2 atmosphere strongly influencedon the structural and optical properties of the ZnO thin films. The optimizedannealing temperature is 700℃. The annealed films have a more intensediffraction and a narrower FWHM owing to the decreasing of lattice mismatchand defects and the increasing of the grain size. The ultraviolet emission inZnO epilayer enhanced obviously after annealed, so annealing can improve thequality of ZnO films.p-ZnO films are firstly prepared by MOCVD through As diffusion fromGaAs substrates. The state of As existed in ZnO films strongly affect theconduction of the film. The unannealed films exhibit n-type conduction owingto most As exist as As-O bond. After annealing, lots of As atoms substitute Olattice bonded with itself of Zn atoms, acted as acceptors, so films presentp-type conduction. We think annealing is the key reason of the conversion ofthe films.n-ZnO/p-Si heterojunction photodiodes are fabricated by MOCVDdeposition of n-ZnO films on p-Si substrates. Ohmic contacts to n-ZnO aremade by applying indium (In) dots with a soldering iron, and to the p-Si byevaporating of Al-Au. Rectifying behaviors are observed in all heterjunctions.The forward turn-on voltage is about 1.5V, the reverse current increaseslinearly with the increase of the reverse bias voltage. The samples deposited at620℃ has the largest reverse current, because the thin SiO2 layer existedbetween n-ZnO and p-Si, confirmed by XPS. The absorption of visiblephotons takes places in the depleted p-Si region, and the photogenerated holesare swept to the charge-neutral p-Si region that is contact with the Inback-electrode, whereas the photogenerated electrons should go to the Au-Alcontacts by way of the depleted n-ZnO layer. As stated above, there is a thinSiO2 layer at the n-ZnO/p-Si interface and hence the photoelectrons may face atransport barrier, while the photogenerated holes in the p-Si experience nosuch barrier. This means that the total photocurrent density (Jph) is now mostlydominated by the photocurrent density due to holes. On the other hand, for UVphoton absorbed in the depleted n-ZnO, Jph is dominated by thephotogenerated density due to electrons.n-GaAs/p-ZnO heterojunction are firstly fabricated by MOCVDdeposition of p-ZnO on n-GaAs substrates in China. Ohmic contacts to thep-ZnO and n-GaAs are made by evaporating Au/Zn and Au/Ge/Ni. All of then-GaAs/p-ZnO heterjunction exhibit typical rectifying behavior. The turn-onvoltage is ～2.5V, the reverse breakdown voltage is ～-4V. Under forwardbias the device produces weak blue EL with a peak emission at 440nm and anintense infrared EL with a peak emission at 1310nm. By comparing the ELspectrum of our LED with the PL spectra of the individual heterostructurelayer it can be concluded that the blue EL emission emerges from the ZnOregion of the device due to the defects or low hole concentration in it, and theinfrared EL emission emerges from the GaAs region.