A normal incidence scanning reflectometer of high precision

A near-normal incidence (~6 degrees ) reflectometer system is described that records continuously and directlythe reflectance R(omega) as a range of photon energies is scanned. The system has an absolute error of +/-2 x 10(-2) and a relative error of +/-2 x 10(-5). It incorporates a quartz light pipe rotating at 70 Hz which captures light from the incident and the reflected beam, respectively, during about 20% of its period of rotation in either case. A gating circuit separates the output signal of the photomultiplier into two channels, corresponding to the incident and the reflected beam, respectively. The signal corresponding to the incident beam is kept constant by a servo system which regulates the gain of the photomultiplier. The reflectance is thus proportional to the signal of the second channel, which is recorded as a function of photon energy. Portions of the reflectance spectrum of Ge are given as examples. No trace of a fine structure in the reflectance of Ge below 2 eV is found.