The Preparation And Properties Research Of Polycrystalline GaN Film

First,the poly-GaN films are prepared by MOCVD method and the preparation parameters were obtained.The influence of the preparation parameters and the annealing process were studied systemically,which is important to the structure and component,the electrical and luminescence properties.The poly-GaN films are prepared on the silicon and sapphire substrate by MOCVD method.The nitrogen and gallium precursors are ammonia and TMGa respectively. During the experiments,the flow ratio(Ⅲ/Ⅴ)was 1:250,1:500 and 1:1000;the chamber pressure was 50Torr,100Torr,200Torr,300Torr and 400Torr;the temperature was 900℃,950℃,1000℃and the growing time was 1 hour.Some films were annealed at 900℃for 1 or 2 hours.All X-ray diffractometer patterns revealed our films were poly-GaN,which could be shown according to X-ray photoelectron spectroscopy.The best deposition condition is 1:250(the flow ratio)at 300Torr pressure due to the greater intensity,smaller FWHM, better crystallization.It is recognized that the sapphire is more appropriate as a substrate than the silicon due to the greaer intensity of diffraction peak and the smaller FWHM. The annealing process has a irnportant role in the selective oritentation.It is shown that the conductive type of our films is n-type and the carrier concentration is 10¹⁸according to the results of the Hall measurements.The multi-barrier caused by the defects would involve in the transport of electron,the number of free electrons would decrease and the mobility would reduce.During the different reaction pressures,when the airflow density is too low,a mass of defects and traps of the films would reduce the number of free electrons;and the high airflow density would lead to an incomplete reaction between the reactants,the quality of the fdms would be poor, which would involve in the number and transport of free electrons,so the mobility reduced.It is approved that larger testing voltage could active more free electrons and increase the drift velocity,which could help more electrons to get across the barrier resulted from the grain boundary(GB),and the mobility would be higher.The annealing did not have effects on the electrical properties.Although the process increased the grain size,the different crystal lattice and grain size would involve in the transition of free electron,and the mobility would be lower.So it is necessary to take a profound research to the influence of the annealing process to the electrical properties of poly-GaN films.There are three Illumination peaks occurred at 374nm,400nm,450nm respectively. The UV emission(374nm)originated from band edge emission for that the peak energy is close to the optical -gap energy of single crystal GaN.It is suggested that the peak at 400nm originated from the transition from the electrons on conduction band to the acceptor carbon while the blue emission(450nm)originated from the DX center to the acceptor carbon in our films.During different growth conditions,first,the defect density of the films atⅢ/Ⅴ=1: 250 is lower,and luminescence intensity is the greatest,the film has intrinsic luminescence;the influence of the defects due to the chamber pressure is important to the luminescence properties of the films,the relative peaks are different,too.The luminescence intensity of the films deposited on sapphires is higher than that deposited on silicon.The films deposited on sapphires had a wild peak between 400nm and 450nm, which indicates the lattice mismatch between GaN and sapphire.The influence of the lattice mismatch could be eliminated by depositing the buffer layer earlier.The annealing process has a negative role in the films.It increases the defect density and reduces the luminescence properties.It is important to have a total identification to the samples before the annealing,and the time of this process should be noticed,too. According to the photoluminescence of different temperature,when the temperature is below 200K,the band-gap would reduce if the temperature raised,the energy among the impurity level would decrease responsibly,and the redshifi would appear;when the temperature climbed to the room temperature,the thermal action would increase the exciting energy,which could make electron transit to the higher level,and the blueshift would appear,it is because that the Auger recombination due to the defects would reduce the luminous efficiency,and the photoluminescence intensity decreased suddenly.