Reduction of threading dislocations in N-polar GaN using a pseudomorphicaly grown graded-Al-fraction AlGaN interlayer

•Filter TDs in N-polar GaN with a pseudomorphicly grown AlGaN interlayer.•Without new generations of misfit TDs at AlGaN/GaN heterointerface.•An excellent optical property of N-polar GaN film with AlGaN interlayer.

We report on an improvement for the crystal quality of N-polar GaN with a pseudomorphicaly grown graded-Al-fraction AlGaN interlayer which introduces strain management in heterostructure and brings in the inclination and annihilation of threading dislocations (TDs). Significant blocking of screw and edge component TDs are observed by transmission electron microscopy, and the reduction of TDs densities are estimated by high resolution X-ray diffraction and plan-view cathodoluminescence measurement. Photoluminescence measurement shows a good optical property associated with the significant reduction of edge TDs. All the results show that insertion of graded AlGaN interlayer is a convenient method to achieve excellent crystal quality in N-polar GaN growth.