Investigation Of GaN Quasi-vertical Schottky Barrier Diodes

Compared with traditional materials,Gallium Nitride(GaN)has shown significant advantages including high breakdown electric field,high electron mobility and saturation velocity,strong reliability in high temperature and irradiation.The GaN devices exhibit great potential in power and radio frequency applications.However,during the research of conventional lateral devices,several problem such as the current collapse and self-heating effect have limited the further development GaN devices.Under this situation,the vertical-structure GaN devices provides the new approach to break the obstacles,because it is estimated to achieve a higher current density,stronger reliability and more uniform thermal distribution.What'more,the quasi-vertical devices fabricated on foreign substrates would be easier to be put into industrial application due to the low fabrication cost.Among the electronic devices,the Schottky barrier diode,as a basic two-port component,has the advantages of low turn-on voltage and high switching frequency compared with other devices.it plays an important role in rectification,detection and limiter.Based on the above background,a series of key issues about quasi-vertical GaN SBDs including fabrication process develpment,structure optimization,current transport mechanism and reliability were investigated in this paper.This work is mainly supported by the National Key Science&Technology Special Project.The main research achievement are shown as follows:1.At first,the key fabrication process was developed in this paper including GaN deep groove etch,ohmic contact optimization on n⁺-GaN and so on;in the epilayer structure aspect,the patterned sapphire substrate was firstly used to fabricate the quasi-vertical devices.the density of dislocation for epitaxial layer grown on the patterned sapphire substrate was reduced one order of magnitude compared to epitaxial layer grown on the flat sapphire substrate.This would benefits to suppress the leakage current;in the device structure aspect,ohmic metal was deposited cross the mesa to increase the forward current;in the layout design aspect,the electrical performance of circle-type and finger-type SBDs were compared,and the current density distribution was analyzed assisted by TCAD simulation.We found that the finger-type devices has smaller on-resistance compared to circular-type devices due to the larger Perimeter/area ratio.In addition,the anode size and mesa size was also optimized in this paper,and the optimizing layout structure was achieved,which set up the experimental foundation for the following high-performance device fabrication.2.Based on the relationship between the Schottky barrier height and metal work function,the low work function metal(tungsten)was used as the Schottky contact.The extremely low turn-on voltage(V_on)of 0.39 V was achieved in our fabricated SBDs.Compared with conventional SBDs by Nickel Schottky contact,the Von of W-SBD is decreased about 0.21V,while the leakage current increased only about one order of magnitude,indicating the better electrical performance for the W-SBDs.In order to give explanation about this,the leakage mechanism was analyzed by comparing the leakage current of the two types of SBDs.It is proved that the leakage current is dominated by thermionic emission when low reverse bias was applied.With the increase of reverse bias,the Variable Range Hopping mechanism related to dislocation changed to dominate,resulting to the small difference between two types of devices.On the other hand,in order to explain the temperature-dependent variation of Schottky barrier height and ideality factor,the barrier distribution inhomogeneities was considered in this paper.The W-SBD showed a slightly larger inhomogeneities compared with Ni-SBD.What'more,after inserting this model,the modified Richardson constant become consistent with theoretical value.The analysis in this paper provided the direction for device optimization.3.Based on the leakage current mechanism mentioned above,the effect of oxygen Plasma surface treatment on quasi-vertical SBDs has been investigated.With the increase of oxygen Plasma treat power,the suppression of leakage current became more effective,the stability of leakage current under high temperature became better.However,this could also increase the V_on.Based on this,the oxygen Plasma termination was adopted in quasi-vertical GaN SBD.The thickness of drift layer is 1.3?m,the turn-on voltage,breakdown voltage and on-resistance for our fabricated SBDs is 0.71 V,190 V and 0.2 m?×cm2,respectively.The related values reached the highest level of the corresponding devices in the world.The average electric field was calculated to be 1.48 MV/cm,which is the highest value in GaN SBDs.The mechanism behind this was verified by the X-ray photoelectron spectroscopy and Kelvin probe force microscopy and assisted by the TCAD simulation.The surface potential is increased after the oxygen Plasma treatment,which results in the suppression of the leakage current.4.In the end,the reliability of quasi-vertical SBDs under electrical stress and gamma irradiation were studied in this paper.It is proved that the quasi-vertical GaN SBDs shows current collapse free under the pulse and stress measurement.On the on-state stress reliability assessment,the degradation trend was related to the stress voltage and stress time:If the stress voltage is below 4 V,the degradation shows more serious with the increase of stress voltage and stress time;If the stress voltage is beyond 4.5 V,the degradation was relieved.The self-heating and minority-carrier injection was supposed to be account for the two different changes.In the aspect of gamma irradiation reliability,the device still work effectively after a 3 Mrad gamma irradiation.What's more,the on/off ratio and leakage current were improved after gamma irradiation.The reason behind this was explain by the high resolution X-ray diffractometer,cathodoluminescence spectroscopy and temperature dependent current-voltage measurement.It is proved that the performance improvement was related to the defect density reduce and weak barrier inhomogeneities.