ANGULAR INTENSITY MODULATION IN ANGLE-RESOLVED XPS AND AES OF NONCRYSTALLINE ULTRATHIN SURFACE-LAYERS - THE PHENOMENON AND ITS IMPLICATIONS

The well-known phenomenon of forward (elastic) scattering of x-ray photoelectrons and Auger electrons, which gives rise to prominent diffraction peaks for emission from monocrystalline materials, can lead to a much less prominent, yet significant, angular intensity modulation of emission from nominally non-order materials in which stoichiometric or other constraints enforce some short to medium-range order. A seemingly small intensity modulation can have a drastic effect on the determination of layer thickness or of effective attenuation length from angle-resolved measurements. This conclusion is arrived at by applying the known facts of forward scattering (summarized here) to the currently accepted structural model of oxide layers on the semiconductors Si and GaAs (serving as examples). The predictions are compared with experimental data on carbonaceous contamination in the native oxide layers on Si (111) and GaAs (001), obtained by angle-resolved XPS and AES and evaluated for angle-dependent effective attenuation length. It is concluded that angular intensity modulation can be expected to be a frequently occurring phenomenon with serious implications for the study of ultrathin layers and ultrashallow depth profiles by the electron spectroscopies XPS and AES.