Besides Joerg's suggestion of mis-cut substrates (which may be too expensive for use for thin film growth experiments) you could use Si wafers cut to (100). The first allowed peak in that direction is (400), which is at about 70 degrees 2theta for Cu K-alpha. That will still reflect any bremsstrahlung radiation from your x-ray tube, which can make a significant background. If you have a monochromator in your optics, that will cut it down, but you can still get artifacts, such as lambda/2 radiation reflecting from the (400) appearing to be a peak at the (forbidden) Si (200) position.
Or, maybe you can find a compromise between the Si substrate peak and defocusing with a smaller value of omega (theta - 2theta/2) than 3 degrees. Another possibility, if you are ready to rearrange your diffractometer, is to make the source-sample and sample-detector distances different, so that source, sample, and detector are on the focusing circle with theta not half of 2theta. I wonder if any of the manufacturers provide this geometry? ^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~ Peter W. Stephens Professor, Department of Physics and Astronomy Stony Brook University Stony Brook, NY 11794-3800 fax 631-632-8176
