Research On Hetero-Integration Of PbZr_0.2Ti_0.8O₃ Thin Films With Substrate Lift-off And AlGaN/GaN HEMT

The GaN heteroj unction devices use the stress of AlGaN barrier layer on GaN,and the interface band is bent by the inverse piezoelectric effect to obtain a two-dimensional electron gas(2DEG)layer.Thus the 2DEG density in AlGaN/GaN heterostructures can be modulated via spontaneous polarization effect of ferroelectric materials,which may lead to new characteristics and applications.Therefore,the integration of ferroelectric materials with AlGaN/GaN high electron mobility transistor(HEMT)is a very attractive method for exploring further applications of electronic devices.However,because the traditional integration method is to deposit ferroelectric thin films on AlGaN/GaN heterojunctions directly,it is difficult to obtain high quality and thin ferroelectric thin films due to lattice mismatch,in addition,the high temperature and high oxygen environment during the deposition process will not only cause interdiffusion and interface states are also introduced.These factors will greatly affect the performance of AlGaN/GaN HEMT and the modulation effect to 2DEG.In order to solve the above problems,this theis studies the hetero-integration of Pb(Zr0.2Ti0.8)O3(PZT)and AlGaN/GaN HEMT based on the substrate lift-off technology.The main innovations and research results of this paper are as follows:First,using the substrate lift-off technology,the traditional ferroelectric/GaN integration method is fundamentally improved,and integeated the ferroelectric thin films with substrate lift-off and GaN-based heterojunction for the first time.Compared with the traditional method of deposing PZT on GaN directly,the substrate lift-off technology can not only obtain thinner and higher quality PZT thin films,but also prevent interdiffusion and interface problems during growth.These advantages ensure their excellent ability to regulate 2DEG concentration.More importantly,due to the universality and scalability of the technology,it can be used not only for ferroelectric/GaN but also for heterogeneous integration studies between any other complex oxide films and semiconductor materials or flexible substrates.Second,the single crystal PbZr0.2Ti0.8O3(PZT)thin film was homogeneously epitaxially grown on a SrTiO3(STO)substrate using the PLD system.Because it matches the crystal structure of the substrate,a high-quality single crystal PZT epitaxial layer can be obtained,and XRD And AFM results also prove this.A non-damaged,large-area,high-quality PZT thin film(20 nm)was obtained from the STO substrate by using the substrate lift-off and layer-transfer technology.The comparison of the XRD and AFM data before and after the transfer proves that the PZT thin film remains well during the substrate lift-off.TEM images show that there is a clear interface between the PZT/AlGAN/GaN layer structures.Third,PZT/AlGaN/GaN HEMT was prepared by using a transfer PZT film,and the electrical properties of the PZT film were studied.Experimental results show that the transferred PZT film has good ferroelectricity and insulation with the remanent polarization(Pr)of 15 ?C/cm2.The leakage current density of a 20nm PZT film is only 0.048mA/cm2,when the bias voltage is 1V.By calculation,the carrier mobility(?)of AlGaN/GaN HEMT and PZT/AlGaN/GaN HEMT is 1440 cm2/V·s and cm2/V·s,respectively.The latter is of slightly lower mobility than the former,which indicates that the interface quality of PZT/AlGaN is good,so the mobility is less affected by the interface state and interface trap.Fourth,through the C-V and I-V characteristics measurement of the PZT/AlGaN/GaN HEMTs,the continuous and non-volatile modulation of the 2DEG density at the AlGaN/GaN heterojunction interface was verified by the ferroelectric polarization effect.By poling the PZT/AlGaN/GaN HEMT from 2V to 7V,the device exhibits different threshold voltages(from-0.8V to 2.1 V),indicating that the device's operating type can be changed by applying different poling voltages(depletion type to enhanced type).At the same time,the retention characteristic of the device is excellent,and the variation of the threshold voltage is less than 0.25 V after 105 s.Moreover,according to Hall measurement results,the modulation capacity(T)to 2DEG concentration can reach 350%,which is the highest level reported so far.