In grazing incident diffraction, the incoming x-ray beam is of course always at narrow angles to the surface, so that surface sensitivity is achieved. However, for the detector there seem to be two options: In-plane, where the diffracted beam is measured also at an angle close to the surface, so that the scattering vector lies in the surface. And out-of-plane, where the detector moves away from the surface and the diffraction plane lies perpendicular to the surface. But what are the particular advantages and disadvantages of these two techniques?
I would say there is no particular advantage or disadvantage. At least not if you cannot specify additional criteria.
In the following answer I assume, you are interested in crystalline films, more precisely the relative crystal orientation with regard to a reference plane, the film plane? If you sample is powder this answer is superfluous.
I once did a little Mathematica illustration for a film containing polymer crystals (PEO). I attached a screenshot. You see your Substrate in the middle of the plotted Ewald sphere. The incident beam denoted K0 and the refracted beam K and the scattering vector Q. The latter is the one you are most interested in, since this will be perpendicular to the crystallographic plane responsible for the scattering.
The horizontal black line on the Ewald sphere is the position in which the in-plane detector can move while the vertical thick black line is the one the out-of-plane detector can move (normally this is a movable point detector tracing the depicted line). The blue and the green (different miller indices here, but those with the major intensity) circle are the cross-sections of the corresponding diffraction cone with the Ewald sphere. You only get a cone assuming you have a powder on the surface of the film (i.e. every crystallographic orientation is present!).
Accordingly if your detector can either move in-plane or out-of plane you will only be able to detect those crystallographic planes which are either perpendicular to the substrate (and tilted around the z axis, i.e., in-plane) or parallel to the substrate (and tilted around x axis, i.e, out-of-plane). For other crystallographic planes than the high intesity ones, the intensity and thus SNR might be very poor, although the geometric condition above would apply.
Thus if you are interested in any possible tilt of a specific crystallographic plane you would benefit from a device that is capable of tracing the whole Ewald sphere (takes a while) or ideally a 2D detector as is the case for GISAXS devices. So for the crystallographic plane as is in the provided picture (tilted around x and z) , neither the in-plane nor the out-of-plane detector would be able to detect it.
I would say there is no particular advantage or disadvantage. At least not if you cannot specify additional criteria.
In the following answer I assume, you are interested in crystalline films, more precisely the relative crystal orientation with regard to a reference plane, the film plane? If you sample is powder this answer is superfluous.
I once did a little Mathematica illustration for a film containing polymer crystals (PEO). I attached a screenshot. You see your Substrate in the middle of the plotted Ewald sphere. The incident beam denoted K0 and the refracted beam K and the scattering vector Q. The latter is the one you are most interested in, since this will be perpendicular to the crystallographic plane responsible for the scattering.
The horizontal black line on the Ewald sphere is the position in which the in-plane detector can move while the vertical thick black line is the one the out-of-plane detector can move (normally this is a movable point detector tracing the depicted line). The blue and the green (different miller indices here, but those with the major intensity) circle are the cross-sections of the corresponding diffraction cone with the Ewald sphere. You only get a cone assuming you have a powder on the surface of the film (i.e. every crystallographic orientation is present!).
Accordingly if your detector can either move in-plane or out-of plane you will only be able to detect those crystallographic planes which are either perpendicular to the substrate (and tilted around the z axis, i.e., in-plane) or parallel to the substrate (and tilted around x axis, i.e, out-of-plane). For other crystallographic planes than the high intesity ones, the intensity and thus SNR might be very poor, although the geometric condition above would apply.
Thus if you are interested in any possible tilt of a specific crystallographic plane you would benefit from a device that is capable of tracing the whole Ewald sphere (takes a while) or ideally a 2D detector as is the case for GISAXS devices. So for the crystallographic plane as is in the provided picture (tilted around x and z) , neither the in-plane nor the out-of-plane detector would be able to detect it.
(Assuming that the incident beam is grazing or thereabout) As you point out, keeping the detector also in grazing limits one to a probe of in-plane lattice vectors only. That is relevant for those questions limited only to in plane structure - such as trying to find whether a surface reconstruction is present, or perhaps evaluating the in-plane relaxation of a layer with respect to the substrate. Do your studies ask questions that require information about out of plane correlations? - for example even in the surface reconstruction, measuring the 'rod' associated with the reconstruction (getting to out of plane reciprocal space vectors) may give insight on number of layers participating in the reconstruction. Measuring the 'crystal truncation rod (CTR) or similar wanderings requires being able to scan the detector out of the grazing angle (and of course, the sample also needs to be oriented properly so that it remains at the constant incident angle). But - it all depends on what sort of system and questions are of interest.
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