The Investigation Of InGaMgO/InGaZnO Heterojunctions And Related Devices

In recent decades,researchers have found that the electron confinement structure,which could spatially separate the electrons and the ionized donors,was realized at the interface of heterojunction.According to such structure,the scattering effect of ionized donors can be effectively suppressed,resulting in the formation of two-dimensional electron gas?2DEG?,and a significantly enhancement of carrier mobility.In 2004,Prof.Ohtomo demonstrated that 2DEG could be formed at the interface of the SrTiO₃?STO?/LaAlO₃?LAO?heterojunction,which inspired the interesting of study the oxide heterojunction worldwide.Although the carrier mobility of 2DEG at the interface of STO/LAO heterojunction is extremely high at low temperature?4 K?,it is less than 10cm²/Vs at room temperature.Thus,the application of STO/LAO heterojunction is limited.Amorphous InGaZnO thin film is one of the most important oxides owing to its outstanding properties,such as high carrier mobility?¹0 cm²/Vs?,low cost,and good reliability.Moreover,the thin-film transistors?TFTs?based on the InGaZnO have been applied to flat panel displays.From the perspective of materials,InGaZnO belongs to the serious of RAMO₄ compounds,in which R represents In or Sc,A represents Al or Ga,M represents Ca,Cd,Mg or Zn.It was reported that the band gaps of all the RAMO₄compounds could be modulated by their compositions.Accordingly,RAMO₄ compounds have the potential to construct oxide heterojunction with good properties.According to the theoretical calculation,InGaMgO,in which Zn is completely replaced by Mg,shows the most suitable band structure to construct heterojunction with InGaZnO.Therefore,this work focused on the study of InGaMgO/InGaZnO heterojunction,expecting such an architecture could improve the carrier mobility of InGaZnO,and promote the development of oxide electronics.In our work,InGaZnO thin films were deposited by the radio-frequency?RF?sputtering.Afterwards,the transmittance spectra of InGaZnO thin films were measured by the UV-VIS spectrophotometer.It was found that InGaZnO exhibited good transmittance in the visible range?over 80%?,and its band gap was determined to be 3.0eV.Besides,the photo-sensitivities of InGaZnO thin films were characterized by fabricating metal-semiconductor-metal?MSM?photodetectors.In addition,the TFTs based on InGaZnO thin films are also fabricated and measured in this work.The field-effect mobility of InGaZnO TFT was 10 cm²/Vs,which was consistence with the previously reported one.The InGaMgO thin films were deposited by the RF sputtering.Furthermore,the crystal structure,band gap,carrier mobility,and photo-sensitivity of InGaMgO was determined by the X-Ray Diffraction?XRD?,UV-VIS spectrophotometer,Hall effect measurement,and I-V measurement,respectively.Afterwards,the TFTs based on InGaMgO thin films with different post deposition annealing?PDA?treatments were fabricated,and the opto-electrical properties of these devices were tested.Based on the investigation of InGaZnO and InGaMgO,the InGaMgO/InGaZnO heterojunction was deposited,and its band alignment was characterized via the X-Ray photoelectron spectroscopy?XPS?measurement.A disruption in the conduction band was observed at the interface of the heterojunction,which constructed a potential well and a potential barrier.Considering the n-type natures of these oxides,electrons could be accumulated in the potential well and depleted in the barrier layer.Hence,such an architecture could be used to confine electrons,and thus favors the appearance of 2DEG.According to the band alignment,the TFTs based on InGaMgO/InGaZnO heterojunctions were fabricated and tested.It was found that the mobility of such device was 30 cm²/Vs,which was three times larger than the mobility of InGaZnO TFT.This work investigated the effect of dielectric constant on the electrical properties of InGaMgO/InGaZnO TFTs through changing the dielectric layer from SiO₂ to ZrO₂.As a result,the mobility of InGaMgO/InGaZnO TFT was increased from 30 cm²/Vs to 45.3cm²/Vs.On the other hand,this work also studied the effect of InGaMgO thickness on the opto-electrical properties of InGaMgO/InGaZnO TFT,because InGaMgO acted as a photo absorb layer in this architecture.It was found that 200 nm was the most suitable thickness for InGaMgO thin films.If the InGaMgO was too thin,it could not completely absorb the illuminated light.Otherwise,the photo generated electrons could not be effectively transported to the interface of InGaMgO/InGaZnO heterojunction.