We use in this work a numerical method based on rigorous coupled wave analysis to calculate the diffraction efficiency in the case of a grating structure which can be a part of a Schottky based photodetector. The effects of the grating thickness, depth, period and filling factor on the diffraction efficiency are investigated. The thickness in particular is found to play a crucial role in determining the overall (sum of diffraction orders) diffraction efficiency since it is the parameter of importance when it comes to consider the degree of coupling between the surface plasmon-polaritons excited at both the air/metal and metal/semiconductor interfaces. Also the optimization of the grating parameters for a minimum reflectance is done at the particular wavelength λ = 1.5μm) considering three orders of diffraction. The effect of the grating profile on the diffraction efficiency is interpreted in terms of additional higher order Fourier harmonics added to a fundamental sinusoidal profile.