Preparation Of Mg_xZn_1-xO By Sol-Gel Method And Study On Ultraviolet Photodetectors

The wide bandgap semiconductors,represented by zinc oxide,silicon carbide and gallium nitride,have merits of wide bandgap,high critical breakdown electric field,fast carrier migration speed,high thermal conductivity and stable physical and chemical properties.The application of wide bandgap semiconductors in power electronic devices,radio frequency devices,photoelectric devices and other devices possesses great superiority,being compared with traditional silicon or gallium arsenide devices.These devices have broad application prospects in new energy power,aerospace,scientific research,medical treatment,military field and so on.In this dissertation,the process of preparing MgxZn1-xO films by sol-gel method was studied.The MgxZn1-xO films and metal-semiconductor-metal(MSM)ultraviolet(UV)photodetectors(PDs)were then prepared and characterized.Finally,Silvaco TCAD was used for 4H-SiC separate absorption and multiplication(SAM)avalanche photodetectors(APD)structural optimization simulation.The details of the works and achievements in this dissertation are as follows:1.The process of preparing MgxZn1-xO films by sol-gel method was optimized.The standard process of sol-gel method could not be directly applied in our works,because the preparation steps of sol-gel methods and the properties of grown materials are dependent on the equipment condition of respective laboratories.Therefore,based on the standard process,the preparation steps of the annealing,spin coating,soft-baking,static time,and plasma treatment were optimized and the MgxZn1-xO films with outstanding properties were prepared.2.MgxZn1-xO films with Mg content of 0.05,0.10,0.15 and 0.20 were prepared and characterized.SEM technique was introduced to characterize the surface morphology of the films,and a surface of particle stacking without crack was observed.XRD results showed that the film maintains the wurtzite structure of ZnO,which was further proved by the Raman shifts.The analysis of the transmission spectra revealed the values of the MgxZn1-xO films bandgap,which were 3.38,3.44,3.50 and 3.55 eV with Mg content of 0.05,0.10,0.15 and 0.20,respectively.3.MgxZn1-xO MSM UV PDs were fabricated and tested.According to the spectral response,the cutoff wavelength and maximum photocurrent wavelength showed blueshift with the Mg content increasing,indicating the enlargement of bandgap.The responsivities were between 0.24 A/W to 0.31 A/W,corresponding to quantum efficiencies of 86.8%to 109.0%.The quantum efficiencies were more than 1,which could be caused by the photoconductive gain.Moreover,the difference of the quantum efficiencies of the devices with different Mg content was small,showing a stable response performance.The dark currents of the devices were kept in the order of 10 pA,corresponding to a current density of 5 nA/cm2,which was the lowest dark current of sol-gel MgxZn1-xO MSM UV PDs reported till now.And,the dark current decreased gently with the increasing of Mg content,which could be caused by the enhancement of Schottky barrier height.The photocurrent increased continuously with increasing bias from 0 to 20 V,indicating the existence of photoconductive gain.The noise equivalent power(NEP)were about 1.9×10-15 W,and the normalized detectivity(D*)were about 1.5×1011 cm·Hz1/2/W.4.By means of simulation,the effects of mesa's parameters,such as positive bevel,negative bevel,angle and etching depth on the electric field distribution and breakdown voltage of 4H-SiC SAM APD were studied.The simulation results showed that the positive bevel structure was helpful to reduce the surface electric field intensity,suppress the surface breakdown,and maintain the bulk breakdown of 4H-SiC SAM APD.The negative bevel would increase the surface electric field intensity,thus promoting the surface breakdown and reducing the breakdown voltage.For the simulation of angle,the breakdown voltage of positive bevel structure was almost independent of the angle,which further proved that the breakdown of positive bevel structure is bulk breakdown.Therefore,positive bevel structure instead of negative bevel structure should be employed for the APD fabrication.In brief,we have accomplished the preparation of high quality MgxZn1-xO films and the fabrication of MgxZn1-xO detector with high performance.In addition,we have modeled the APD with different mesa structures and concluded that the mesa with positive bevel should be employed to avoid the premature breakdown.