GaN-Based Vertical Power Diodes Grown On Si Substrate

Semiconductor power electronic devices are the core components of power conversion,which are widely used in wind power,photovoltaic,rail transit,automobile,household appliances.As a rectifier,power diode is an indispensable component in all kinds of power circuits.Thanks to excellent material properties of GaN(such as high critical breakdown field,high electron saturation velocity,etc.),the device conduction loss of GaN-based power diodes can be greatly reduced under the same breakdown voltage compared to the conventional silicon(Si)-based power diodes.However,bulk GaN substrates with low-defect density are expensive and only available in small sizes,limiting their use for volume production.GaN grown on inexpensive and large-scale Si foreign substrate as a practical enabler is expected to greatly reduce manufacturing costs.In this paper,the epitaxy growth,device fabrication and device performance of GaN-based vertical power diodes(mainly PiN and Schottky Barrier Diode,SBD)grown on Si substrate are studied in detail.The main achievements are as follows:1.Crack-free,low dislocation density and thick GaN grown on Si substrate.A significant amount of compressive strain could build up via a properly designed Al-composition step-graded AlN/AlGaN multilayer buffer.The compressive strain that accumulates via the AlN/AlGaN multilayer buffer can not only compensate for the tensile stress due to the coefficient of thermal expansion mismatch between Si and GaN during cool down,but also induce the inclination and annihilation of threading dislocations at the interfaces.By using this method,we realized the growth of SBD structure on Si substrate with the total thickness of GaN 9.4 ?m(the drift region thickness 5.5 ?m)and the dislocation density?8 × 107 cm-2.Further,PL and CL spectrum measurements showed that there were no obvious deep-level defects related yellow luminance in the drift region GaN layer,indicating a low defect concentration and excellent optical property of GaN grown on Si.High quality wafer makes it possible to obtain high performance device.2.High quality,controllable and repeatable lightly doped n-GaN grown on Si substrate.The carrier concentration of GaN in drift region has strong impacts on device on-resistance and breakdown voltage.In this paper,we investigated the influence of MOCVD growth conditions on carrier concentration of GaN in drift region when it doped with SiH4.It was found that high temperature,high pressure and low growth rate conditions were beneficial to inhibit the incorporation of background impurity C which acts as acceptor in GaN,and high quality GaN could be obtained.The incorporation of Si impurities,however,would increase with the decrease of growth rate.The epitaxy conditions of light doped n--GaN drift region were obtained,and the carrier concentration could be accurately controlled in the range of 0.5?5×1016 cm-3,Which was also repeatable.3.Process development of deep mesa etching for vertical structure PiN and SBD devices on Si substrate.It includes deep mesa etching of GaN in drift region and etching damage removing on sidewall.It was found:(1)the etching selectivity ratio of Ni to GaN is the highest with smoothest mesa sidewall and bottom surface when used Ni as an etching mask material compared to photoresist and SiO2.(2)When using Ni as a mask material,the etching damage on sidewall of GaN mesa can be selectively corroded in alkaline solution.Further analysis shows that there is no obvious linear relationship between reverse leakage current and device size,which proved that alkaline solution can effectively remove etching damage on sidewall and eliminate device surface leakage current.4.Process optimization of Schottky contact between Ni/Au and GaN for SBD devices.It is found that the Schottky barrier height between Ni/Au and n--GaN increased first and then decreased with the increase of annealing temperature from 350 to 650 °C under N2 atmosphere for 5 min,while the ideal factor remains about 1.1.The optimal annealing temperature of Ni/Au Schottky contact with n--GaN is about 550?,with the highest(?1.1 eV,closest to the ideal)Schottky barrier height and lowest device reverse leakage current.5.Based on the above studies,we successfully fabricated vertical structure GaN-based PiN and SBD devices on Si substrate.For PiN power diode,with the drift region thickness of 2.5 ?m,the Ron can decrease to 0.6 m?·cm2.And the breakdown voltage is?370 V without any edge termination.The purpose to study this device is to make a better understanding for the fabrication of junction barrier Schottky power diodes.For SBD power diode,with the drift region thickness of 3 ?m and carrier concentration about 8.5×1015 cm-3,the Ron is about 1.12 m·cm-2,the ideal factor is?1.07,and the Schottky barrier height is?1.02 eV.Without any edge termination structure,the reverse breakdown voltage can reach 395 V,BFOM=139 MW/cm2.The performance of the device is comparable to that of SBD devices on GaN free-standing substrates without edge termination.The device performance can be further improved with proper edge termination.Besides,the mechanisms of reverse leakage current for these two types of device were also studied in detail.