The Preparation And Characterization Of N-polar GaN Thin Film

Due to the non-centrosymmetric property,the base plane（c plane）of the wurtziteⅢ-group nitride shows two types of structure.The first one is metal-polar（+c）,and the second one is nitrogen-polar（-c）.MostⅢ-group nitride devices are based on the metal-polar nitride.However,compared to the metal-polarⅢ-group nitride,the nitrogen-polar（N-polar）Ⅲ-group nitride has unique applications in optoelectronic,microwave and power electronic devices.Now,the growth of N-polar nitride is still difficult.The control of nitrogen polarity,the small growth window of nucleation,hexagonal defects and incorporation of impurity atoms need to be solved.In this paper,the research work has been divided into the following three parts.1.The nitridation process and the growth of nucleation layer are important for the growth of N-polar nitride.We found that increasing the thickness of Al N nucleation layer can change the polarity of Ga N epilayer into the Ga-polar one.When the thickness of Al N was below 150 nm,the polarity of Ga N was N-polar,and the morphology of Ga N was greatly influenced by the thickness of Al N.When the thickness of Al N reached 200 nm,the polarity of Ga N was metal-polar,and the surface of Ga N exhibited the step-flow morphology.We used the transmission electron microscope（TEM）to investigate those samples’Al N/Ga N interface,and found that inversion domains appeared in the Ga N layer of the samples whose Al N thickness was below 200 nm and the inversion domains only appeared upon the protrusions at the interface.The interface of the sample whose Al N thickness is 200 nm was very rough,with many triangular protrusions appearing on it,and no inversion domains were found in the Ga N layer.We inferred that the inversion domains induced by the triangular protrusions had occupied the whole Ga N layer,and hence the Ga N layer is Ga-polar.In addition,we found that increasing the temperature of nitridation and the growth temperature of Al N nucleation layer can improve the morphology but degrade the crystalline quality of Ga N epilayer.2.When X-ray diffraction is used to study the structurally imperfect single crystalline material,crystallites size and lattice strain usually cause the broadening of the X-ray diffraction profile.By fitting the X-ray diffraction data of the N-polar Ga N epilayer,we acquired crystallites size,lattice strain and the average tilt angle of crystallites.We found that increasing the misorientation angle can enlarge the lateral length and the average tilt angle,and can lower lattice strain.The lateral length was enlarged because shorter distance between neighboring steps promoted the diffusion of atoms on the surface and this led to the larger lateral dimension of the crystallites.The increase of the tilt angle meant that the misoriented substrate can degrade the-c direction of the N-polar nitride.The decrease of the lattice strain was due to the decline of the dislocation density.The images of optical microscope and atomic force microscope showed that the larger the misorientation angle,the lower the density of hexagonal defects,but the N-polar Ga N without hexagonal defects on the surface showed a higher surface roughness than the N-polar one with hexagonal defects did.Hall measurement results showed that the misorientation angle can improve the mobility of N-polar epilayer but could increase the background carrier concentration.3.Growing high-quality N-polar nitride on sapphire is still difficult.The density of dislocation in N-polar nitride reaches 10⁹ cm^-2,and the carrier concentration in unintentionally doped N-polar layer reaches 10¹⁸ cm^-3,and the electron mobility in N-polar HEMT is lower than that in metal-polar one.To avoid growing N-polar nitride,we designed a route to acquire the N-polar nitride from the metal-polar one.The route is that we first bond a substrate onto the surface of a metal-polar epilayer,and then remove the sapphire substrate,and finally we turn the bonding substrate and the metal-polar epilayer upside down and acquire the N-polar epilayer on the bonding substrate.A highly n-doped Ga N sacrificial layer was included in the metal-polar epilayer.To remove the sapphire substrate,we used the electrochemical etching to etch the epilayer,trying to lift-off the epilayer upon the sacrificial layer.We found the appropriate applied voltage was 25 V,and much higher voltage led to the layers below the sacrificial layer being etched,and thus led to lower etching selectivity.In addition,we acquired a small area of the N-polar epilayer using the supporting film,which paved the way for large area lift-off of the N-polar epilayer.