# Estimate screw dislocation density from XRD rocking curve: Problem with radian unit.

1. Apr 22, 2012

### gabby1981

Hi everyone.

Recently, I did (002) XRD scanning to get the rocking curve of a sample of GaN. From the measurement, I found that the FWHM of the sample is ~1.35 degree. I tried to convert it into radian unit using this formula: angle in radian=angle in degree x π /180 (degree). I got the answer around 4.74 radian. Then I used this value to estimate the screw dislocation density of the sample using this formula; screw dislocation density = FWHM2/4.35 x burger vector (0.5185). The answer is far off the reasonable value that typically reported for GaN. Could anyone help me?

2. Dec 3, 2013

### zvz

1.35° = 3.14159 Rad.*1.35°/180° = 0.023562 Rad. not 4.74 (you forgot to divide by 180, I presume). The screw-type dislocation density nstd is equal to the following (your formula is incomplete; also one needs to convert Burger vector bs to centimeters to get dislocation density in cm-2):

nstd = FWHM2/(4.35*bs2)=0.0235622/(4.35*(0.5185 nm*10-7 cm/nm)2) ≈ 5·1010 cm-2

It is a quite poor quality material, which is expected from the very broad XRD ω-scan.