Exciton absorption in germanium in the region of a straight junction

Abstract

1. The present communication reports the results obtained in an optical-absorption study of germanium in the region of a straight junction for a photon energy hu E G (E G ks the width of the forbidden band in a straight junction for ~< = 0). The investigations were carried out on a specimen with electron-type conductivity (carrier concentration N = 1.8 9 10 t4 cm -~, resistivity p ~. 20 ohm 9 cm) with crystallographic orientation [11~]. To attain the optimum condition [1] (Kd ~. 1) for observing the optical-absorption structure in a straight junction, the author has developed a special method of preparing thin single-crystal layers of certain semiconducting materials; by this method a specimen of thickness d = (4 • 0.3) ~ and diameter 10.5 mm was prepared on a glass substrate from a germanium platelet. By a special check on an X-ray diffraetometer it was confirmed that the specimen consisted of a perfect singte crystal similar to the initial material, Measurements of the transmission spectrum of the thin germanium specimen were made at the temperature of liquid nitrogen (77 ~ K) on a spectrometer having a lattice with a spectral resolution of 3 9 10 -~ eV (5 ~) in the region of 0.88 eV for a slit width of 30~. An uncooled lead sulfide photoresistor acted as radiation receiver. 2. The principal results of the measurements are given in the figure from which it can be seen that in the region of a straight junction in germanium the spectral transmission curve has two minima, which at a00 ~ K are submerged in the continuum and are not visible against the strong absorption background. The results can be explained on the basis of Elliott's theory [2] of optical absorption in germanium at the zone edge, taking into account the Coulomb interaction of electron and hole (exeiton absorption). The first minimum observed, Exl is attributed to the exciton absorption line, the position of which is determined as E x = 0.8867 • • 0.0001 eV. The half width of the line c= 5 9 10 -4 eV (9~), which gives a lifetime for the exciton r ~ 2 9 10 -~z see. The second minimum Ex, is identified with the transition from the split (as a result of deformation) valence zone V2 of the light holes to the discrete exciton level in accordance with the ideas of Kleiner and Both [8]. The position of the minimum is determined as Ext = 0.8892 9 0.0002 eV. The shape of the "deformation" absorption band is asymmetrical on the side of lower values of light quantum energy. The results obtained are in good agreement with the data in [4] for a thin germanium specimen (d = 4~) attached to a glass substrate. The threshold energy value E G corresponding to the appearance of the straight junction can be established with sufficient accuracy only from measurements of the absorption spectrum of an unstressed (deformation-free) specimen by comparing the experimental data with the theoretical curve for continuous absorption at the zone edge given by Elliott [2]. In the case of a deformed specimen such an estimate is made difficult owing to the marked effect of the neighboring "deformation" band. However, if at is assumed that the position of the edge is the growth point of the absorption coefficient immediately beyond the exeiton peak in the direction of greater energy values, then we obtain E G = 0.8874 eV. 6