Research On Interface Control And Electrical Properties Of Lithium Niobate/Semiconductor Integrated Films

Dielectric oxides and semiconductors have been applied widely in various kinds ofelectronic devices, respectively. With electronic systems becoming smaller in volume,faster in speed, much more in functionality, and stronger in property, the integrationbetween dielectric electronic materials with film state and semiconductors has receivedmuch attention. On one hand, multi-functional applications can be expected if theintegration of dielectric oxides and semiconductors would be realized; on the other hand,the modulations of the polarization properties of dielectrics on the transportingproperties of semiconductors would promote the emergence and development ofnew-type devices with high performance.So, lithium niobate （LiNbO₃, LN） was chosen to be integrated with GaN-basedsemiconductor in this thesis. The polarization modulations and coupling effects at theinterface in the LN/GaN integrated films were studied in details. Due to the depletion offerroelectric polarizations on two dimensional electron gas （2DEG）, normally-offLN/AlGaN/GaN enhancement-mode （E-mode） transistors were obtained.Firstly, large quantities of Li vacancies were introduced into the LN films bycontrolling the fabricating conditions of the LN/AlGaN/GaN integrated structures.Based on the process of the electrons trapping from Li vacancies and distrapping,charge-storage LN/AlGaN/GaN nonvolatile memory devices were fabricated and itsoperating mechanisms were explained qualitatively, firstly. By the measurements, theretention time of the device even at300℃could be longer than10years. Theapplications of AlGaN/GaN semiconductors were extended from power electronics intomemory devices by the study.Secondly, single C-axis LN ferroelectric films were deposited on GaN templatesand the LN/GaN integrated films were fabricated. By the measurements of the electricalproperties, obvious ferroelectric polarization modulations on carriers were exhibited inthe integrated structures. On the basis of the above studies, LN films were grown onAlGaN/GaN semiconductors and the modulation effects of the ferroelectric polarizations on2DEG were studied deeply. The experimental results showed that arelative increase （△ns） of the2DEG could be produced by the ferroelectricpolarizations of the LN films and the△nscould reach about7.36×1011/cm2, which wasnot reported in the other similar types of GaN-based heterostructures.Thirdly, without any prepolarization treating, the preferable ferroelectricpolarizations of the LN films were induced by the effect of the polarized surface of theAlGaN/GaN templates. The experimental results showed that the2DEG at theAlGaN/GaN interface could be depleted by the ferroelectric polarizations of the LN film.This would supply a new kind of materials to fabricate E-mode GaN devices.Fourthly, the difficulties of the wet etching of the LN films on AlGaN/GaNtemplates were resolved by using mask layers, and E-mode LN/AlGaN/GaN transistorswere fabricated firstly due to the depletion of the ferroelectric polarizations on the2DEG. The turn-on voltage of the E-mode device could reach+1.0V. When a5nm-ZnO buffer was introduced into the LN/AlGaN interface, the electrical properties ofthe transistor were enhanced greatly, the saturation current increased from97to204mA/mm, and the transconductance increased from27to46mS/mm. The researchsupplied a new route to develop GaN logic circuits with less power consumption.In addition, the interface controlling means using ZnO nano layers were spread intothe fabrications of the Si-based heterostructures and the LN/ZnO buffer/Si integratedstructures were fabricated. Obvious photoswitching characteristics were observed firstlyin the fabricated integrated films and the mechanisms were proposed by theenergy-band diagram of the integrated films.