| Zn O(3.37 e V)as a direct wide band gap semiconductor material is developed rapidly in recent years.Due to its excellent performance,such as:low defects density,high excitonic binding energy(60 me V),high visible light transmittance,strong ultraviolet absorption and high radiation resistance,Zn O is considered one of the most promising materials preparation of high-performance ultraviolet detector.At present,many researches on Zn O-based UV photodetectors have been reported.However,the detection range of Zn O photodetectors has been unable to meet the needs of practical applications,which undoubtedly requires the band engineering to obtain a larger band gap.Considering that the band gaps of Be O(10.6 e V)and Mg O(7.8 e V)are much larger than those of Zn O,we use Be and Mg co-doped Zn O to form a quaternary alloy substituted by double cation to effectively improve the band gap of Zn O.In this paper,BeMgZnO quaternary alloy thin films were prepared by pulsed laser deposition(PLD).The effects of growth oxygen pressure and substrate on the structure,optical band gap and photoelectric properties of the films were systematically studied.At the same time,the influence of spontaneous polarization field on the photoelectric properties of BeMgZnO photodetectors based on non-polar orientation is explored.The main research contents and conclusions of the thesis are as follows:1.Fixed pulse laser energy(350 m J/Pulse),laser frequency(5 Hz)and substrate temperature(700°C),a series of BeMgZnO quaternary alloy films with different Be and Mg contents were grown on c-plane sapphire substrates by changing deposition oxygen pressure(0 Pa to 4 Pa).And the MSM photoconductive ultraviolet detectors were fabricated by evaporating parallel Au electrodes on the BeMgZnO films surface.The effects of deposition oxygen pressure on the composition,structure,optical properties and photodetector properties of BeMgZnO quaternary alloy films were systematically studied.The results show that BeMgZnO quaternary alloy films deposited on c-plane sapphire substrates by pulsed laser deposition have c-plane orientation and wurtzite structure.With the increase of growth oxygen pressure,the content of Be and Mg in BeMgZnO quaternary alloy films decreases gradually,and the content of Zn increases accordingly.The lattice constant c and band gap width E_gof the films also decrease gradually,while the crystal quality of the films increases continuously.The performance test of MSM photodetectors based on BeMgZnO quaternary alloy films shows that the dark current of BeMgZnO photodetectors decreases,the ratio of light to dark current increases and the response speed increases with the increase of oxygen pressure in thin films deposition.These improvements can be attributed to the improvement of the crystalline quality and the reduction of crystal defects of BeMgZnO quaternary alloy films.2.The non-polar m-plane oriented single-phase BeMgZnO(m-BeMgZnO)quaternary alloy films were successfully grown on m-plane sapphire substrates by pulsed laser deposition at 2 Pa oxygen pressure.Furthermore,two kinds of MSM photodetectors based on m-BeMgZnO were obtained by evaporating the parallel Au electrodes perpendicular to and parallel to the c-axis of the film.The characterization of m-BeMgZnO epitaxial film and the performance test results of photodetectors show that:The in-plane epitaxy of m-BeMgZnO thin films relative to m-plane sapphire substrates is perpendicular to each other's c-axis.The ordering degree of atom arrangement along the c-axis in the film is obviously lower than that along the c-axis in the vertical direction.The crystallization anisotropy can be attributed to the rapid growth of the film along the c-axis leading to the atom not moving enough time to reach the equilibrium position.The spontaneous polarization field along the polar c axis in m-BeMgZnO can assist in enhancing the detection performance of photodetectors.When the parallel electrodes of the detectors are perpendicular to the c axis of the film and the applied electric field is in line with the direction of the internal spontaneous polarization field,the spontaneous polarization field can effectively enhance the separation of photogenerated carriers and enhance the carrier migration speed,thereby improving the response speed and responsivity of photodetectors.Under the promotion of spontaneous polarization field,the rise time of m-BeMgZnO quaternary alloy thin film ultraviolet detector is 2.3 s and the decay time is 0.6 s.And the responsivity of m-BeMgZnO quaternary alloy thin film ultraviolet detector under 300 nm monochrome light is 3.5 A/W.3.BeMgZnO quaternary alloy films with mixed m-plane/c-plane orientation and single c-plane orientation were grown on m-plane and c-plane sapphire substrates by pulsed laser deposition under fixed laser energy(350 m J/Pluse),laser frequency(5 Hz)and substrate temperature(700°C),respectively.Then,MSM BeMgZnO photodetectors were prepared by evaporation method,and I-t,I-V,response test and comparative analysis of the detectors were carried out.The results show that the photocurrent and responsivity of BeMgZnO photodetectors with mixed orientation of c-plane and m-plane are much lower than those of BeMgZnO photodetectors with simple c-plane orientation and m-plane orientation,but their dark current is also smaller and their response speed is faster.The dark current of mixed orientation BeMgZnO photodetectors is 0.001 n A,the photocurrent is 4.8 n A and the ratio of light to dark current is 4800.Its rising time is 0.28 s,and the falling time is only 0.015 s,and the maximum light responsiveness is 0.09 A/W at 300 nm..The excellent photoelectric detection performance of hybrid oriented thin film photodetectors can be attributed to polycrystalline interface defects.The interfacial defects limit the transport of carriers in the dark state,which reduces the dark current.Moreover,the interface defect provides the recombination center,accelerates the recombination of photon-generated carrier,and improves the response speed of mixed orientation BeMgZnO photodetectors. |
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