Study On Structure And Process Of High-voltage GaN-on-Silicon Power Devices

Gallium nitride（GaN）is a typical representative of third-generation semiconductor materials.Due to its outstanding advantages,such as wide band gap,high electron drift speed and high breakdown field strength,GaN-based high electron mobility transistors（HEMT）have become one of the key options for high frequency,high-efficiency,and high-voltage applications.However,there are still many problems to be solved in GaN-based HEMT.On the one hand,its breakdown voltage has not reached the theoretical value of GaN materials,and device still suffers from ohmic contact and gate Schottky contact instability.On the other hand,considering safety and cost,it is necessary to realize normally-off devices.This paper studies the structure and process of high-voltage GaN-on-Silicon power devices.Firstly,the preparation process of the mixed contact drain electrode was studied,and the influence of different contact drain electrode structures on the breakdown voltage of the device was compared.Then,from the perspective of testing the real breakdown voltage of the device on-chip,the electrode protection layer technology was proposed.Secondly,the Ni Ox/Si Nx and Al₂O₃/Si Nx composite gate dielectric process and the low-temperature Au-free ohmic structure was applied in the gate-first process to improve the dynamic performance of the gate.Finally,high-voltage and large gate width cascode and recessed GaN-on-Silicon power devices were fabricated.The main research contents are as follows:（1）Two methods for the growth of source-drain electrodes with electron beam/magnetron sputtering hybrid preparation and electron beam oblique angle/vertical hybrid incidence were proposed.Then,two hybrid contact drain electrode structures（Schottky/Ohmic/Schottky drain and Schottky/Ohmic drain）were realized by one-step lithography without additional process steps.Benefit from the exist of the Schottky contact in the hybrid contact,an additional"shallow depletion region"is introduced on the drain side,which helps to optimize the electric field distribution.As a result,compared with the traditional method of electron beam evaporation of metal,both electrode preparation methods improved the electrode morphology after high temperature annealing,the breakdown voltage of the device was increased by 27.3%and 28.5%,and the figure of merit（FOM）was as high as 239 and 195 MW/cm²,respectively,（2）A method of introducing an electrode protection layer to obtain the real breakdown voltage of the device through on-chip testing was proposed.The introduction of the electrode protective layer can fill the air gap between the electrodes and prevent the device from premature failure due to air breakdown between the electrodes.When the thickness of the electrode protective layer does not completely fill the air gap between the electrodes,the device is prone to air breakdown at the lead-out electrode.Under these circumstances,the breakdown voltage of the device increases with the increase of the thickness of the electrode protective layer.When the thickness of the electrode protection layer completely fills the air gap between the electrodes,the thickness of the electrode protection layer will no longer affect the breakdown voltage of the device.At this time,selecting the electrode protection layer material with a higher critical breakdown field strength can further improve the breakdown voltage of the device until the device breakdown no longer occurs in the lead-out electrode area.Finally,a Si O₂ electrode protective layer with a critical breakdown field strength of 10.3 MV/cm and a thickness of 1.5μm was employed,and the average breakdown voltage of the device was increased from 482 V of the sample without electrode protective layer to 1116 V.（3）To avoid plasma-enhanced chemical vapor deposition（PECVD）equipment from causing active plasma source damage to the（Al）GaN surface,Ni Ox and Al₂O₃ metal oxide layers were proposed as its barrier layers,and Ni Ox/Si Nx and Al₂O₃/Si Nx composite gate dielectric structure improves the interface quality of the gate dielectric/（Al）GaN.Both Ni Ox and Al₂O₃ metal oxide layers were formed by thermal oxidation of their elemental metals.Compared with the device with single Si Nx layer,the breakdown voltage of the device prepared by the composite gate dielectric technology is increased by 36%and 52%,respectively.The current collapse under the off-state stress of 200 V is reduced by 90%and58%,respectively.Besides,the interface state density of the device is reduced by 81.8%and 46.6%,respectively.（4）Aiming at the problem of gate performance degradation caused by high temperature annealing in gate-first process,a gate-first technology based on low-temperature gold-free ohmic structure is proposed.The device employed high-quality in-situ Si Nx layer as both the passivation layer and gate dielectric layer.The dielectric electric field strength and FOM of the low-temperature Au-free device reached 13.5 MV/cm and424 MW/cm²,respectively.They increased by 20.5%and 61.8%compared with the high-temperature Au-based ohmic device.Through time-dependent dielectric breakdown（TBBD）testing of both devices,the gate failure rate was set at 63.2%and expected to operate for 20 years.It is found that the gate operating voltage of the low-temperature Au-free ohmic device is still as high as 11.2 V,which is 21.7%higher than that of the high-temperature Au-based ohmic device.（5）The cascode and recessed normally-off devices were prepared.According to the ohmic contact process,gate width and thickness of the lead-out electrode of the device,its influence on the electrode contact resistance and on-resistance of the device was explored.The contact resistance of Ti/Al/Ni/Cu W metal system under 900℃-N₂-30s annealing is only 0.43Ω·mm.In addition,it is found that with the increase of gate width and thickness of gate-source-drain lead-out electrode,the on-resistance of the device first decreases and then tends to be saturated.At a result,the cascaded device with source field plates has a threshold voltage of+2.7 V,a saturation current of 40 A,an on-resistance of 116.5 mΩ,a breakdown voltage of 1600 V,and an off-state leakage current of only 1.82μA;the threshold voltage of the recessed-gate device is+2 V,the saturation current is 10 A,the breakdown voltage is 960 V,and its off-state leakage current is only 3μA.