Ferroelectric Polarization Induced Transport Regulation Of Two-dimensional Electron Gas In AlGaN/GaN Heterojunction

The most important characteristics of GaN-based crystals are spontaneous polarization and piezoelectric polarization.AlGaN/GaN heterojunction devices can make the interface band bent due to the inverse piezoelectric effect causing by the stress action of AlGaN barrier layer on GaN as a result of which producing Two Dimensional Electron Gas(2DEG).The concentration of 2DEG is directly related to the crystal structure of the material,that is,to the thickness of AlGaN barrier layer.In the conventional AlGaN/GaN heterojunction,the2DEG concentration at the interface is controlled by changing the thickness of AlGaN barrier layer and the piezoelectric effect generated by gate voltage.The change of the thickness of the barrier layer changes the stress at the GaN interface,thus regulating the 2DEG concentration.But this kind of interface stress control ability is limited,and can not apply continuous adjustable stress range.The problem of gate control is that the gate area determines the controlled 2DEG range.In the process of device preparation,the reduction of gate size will lead to the increase of gate leakage,the influence of parasitic resistance and capacitance.And the concentration change caused by gate control 2DEG will be restored to the initial state with gate voltage withdrawal.Polarization effect of ferroelectric material is flexible,adjustable,reversible and non-volatile.Ferroelectric oxide film(Pb Zr_0.2Ti_0.8O₃,PZT)can be introduced on AlGaN/GaN heterostructure materials utilizing heterogeneous integration technology,and non-volatile regulation can be imported by ferroelectric polarization on the basis of gate-controlled concentration of 2DEG,meanwhile,ferroelectric domain structure is flexible for building AlGaN/GaN HEMT devices of different structures.At present,the research on ferroelectric controlled interface 2DEG concentration is still limited to theoretical simulation and macroscopic transport measurement.There is no direct evidence for the presence of a 2DEG variation with ferroelectric domain in a microscopic local system.In this paper,the Scanning Probe Microscope(SPM)was applied to regulate the concentration of 2DEG in AlGaN/GaN heterojunction.Atomic Force Microscopy(AFM),as a high-resolution,flexible and simple characterization method,is used to microregulate and characterize the 2DEG concentration at the interface of AlGaN/GaN heterojunction.In this paper,Conductive Atomic Force Microscope(C-AFM)and Piezoresponse Force Microscope(PFM)are used to situ manipulation the concentration on the interface so as to realize the local variation of the carrier concentration by injecting charge on the samples which have the ferroelectric layer PZT/AlGaN/GaN heterostructure,and it put forward new ideas for the preparation of small size devices.The work includes the following parts:First,C-AFM was used to complete the microscopic electrical measurement of AlGaN/GaN materials,including the characterization of current distribution and the regulation of 2DEG polarization.C-AFM was used to carry out microscopic electrical measurements of AlGaN/GaN materials with different barrier layer thicknesses,and the measurement voltage range of AlGaN/GaN materials with different barrier layer thicknesses was determined.C-AFM was used to control the 2DEG polarization on the AlGaN/GaN material with the barrier layer thickness of 22 nm.The results show that the higher the absolute value of negative voltage is,the smaller the current in the channel is,and the concentration of 2DEG in the channel decreases until it is depleted.In the same region,changing the position of the probe and the aspect ratio of scanning for many times can realize the polarization of the nano channel,the current of the nano channel is reduced,and the concentration of 2DEG in the channel is reduced.However,only the AlGaN/GaN two-layer materials,the influence of piezoelectric polarization is gradually disappears with the voltage withdrawal.Without ferroelectric layer,the regulation of 2DEG is easy to be lost and cannot be maintained.Secondly,PFM and C-AFM were used to complete the regulation of ferroelectric polarization on the microstructure characteristics of AlGaN/GaN heterojunction,including2DEG polarization regulation and microstructure characterization analysis.The PZT/AlGaN/GaN heterojunction was prepared by introducing ferroelectric layer PZT on the surface of AlGaN/GaN samples by heterogeneous integration method.PFM was used to regulate the ferroelectric polarization of AlGaN/GaN heterojunction.The electrical characteristics of AlGaN/GaN heterojunction before and after polarization were characterized and compared by C-AFM.The microstructure transport characteristics and2DEG concentration of AlGaN/GaN heterojunction were controlled by ferroelectric polarization successfully.These results indicate that ferroelectric regulation of AlGaN/GaN heterojunction is feasible,non-volatile and recoverable.Thirdly,PFM and C-AFM were used to study the ferroelectric regulated AlGaN/GaN HEMT devices and their microscopic transport characteristics,including 2DEG polarization regulation and microscopic characterization analysis.EBL lithography was used to conduct field isolation and metal electrode evaporation on the surface of AlGaN/GaN materials.PZT/AlGaN/GaN HEMT devices were fabricated by introducing ferroelectric layer PZT by heterogeneous integration method.PFM was used to regulate ferroelectric polarization of AlGaN/GaN HEMT devices,and C-AFM was used to characterize and compare the electrical characteristics before and after polarization.The transport characteristics of the device are controlled by ferroelectric polarization successfully.These results indicate that ferroelectric regulation of AlGaN/GaN HEMT microscopic transport characteristics is feasible,non-volatile and recoverable.Ferroelectric polarization can regulate the 2DEG concentration of AlGaN/GaN materials,and this regulation is non-volatile and recoverable.Preparing small size devices by in situ regulation and characterization of scanning probe technology is a highly efficient and precise method for controlling small size devices.This method omits the lithography and steaming processes in the traditional process and reduces the contamination of samples by high temperature and chemical reagents.The process is simple and the device structure is diverse,which has important reference significance for subsequent preparation and testing of small size GaN-based HEMT devices.