GaN and its related Ⅲ-Ⅴ alloys have received much attention in recent years for their potential ability in short-wavelength light emitting diodes

laser diodes and photodetectors.However

due to the lack of a suitable lattice-matched bulk substrate

they are usually grown on Al

2

O

3

or SiC substrates with a high density of threading dislocations(TDs).The origin of these dislocations has been suggested to be due to a peculiar growth mode—mosaic structure which can be characterized by means of tilt and twist angles.For wurtzite GaN the mean tilt angle of mosaic structure is related to the FWHM of(0002) diffraction peak

which can be easily measured by using XRD.Unfortunately

the twist of lattice planes is difficult to be measured directly.High-resolution X-ray diffraction was used to determine the twist angle of mosaic structure in GaN epitaxial layers grown on sapphire by metal-organic chemical vapor deposition(MOCVD).Rocking curves of five planes were investigated

(0002);(10

1

3);(10

1

2);(10

1

1);and(20

2

1) respectively.Pseudo-Voigt function was used to simulate the rocking-curve of every plane.The twist angles of GaN layers were easily obtained by extrapolating the full width at half maximum(FWHM) of diffraction peak of the planes.The twist angles of the films measured directly by

φ

-scans of the(1

1

00) reflection in grazing-incidence X-ray diffraction(GIXRD) agree well with the extrapolated results.As far as we know

it's the first time that the extrapolated method was checked to be valid in this material

and the results are useful for the further study of GaN films.