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BYE. SEAISTYS (a), V. I. SUGAKOV (b), and 0. S. ZINETS (b) The retarded exciton Green's function G ( w ) is calculated for semiconductors with randomly distributed impurities. The expression of the exciton absorption band shape due to polariton scattering by charged impurities is obtained using a relation between the complex conductivity tensor ua;(w) and G(w). The band shape is of Lorentz type with r depending on thefrequency, T ( w ) = ( h / 2 ) v nc (ug2 + uil), where v is the velocity of the light generated exciton, cri2 and u& are the cross sections of elastic and inelastic Smattering by an isolated charged impurity, n, the charged impurity concentration. Numerical calculations concerning CdS and GaAs have been made. This band broadening mechanism may be dominant a t high impurity concentratians. Es wird die retardierte Greensche Funktion des Exzitons, G ( w ) , fur einen Halbleiter mit statistisch verteilten Storstellen berechnet. Unter Benutzung der Beziehung zwischen demTensor der komplexen Leitfiihigkeit, ua p(w) und G(w), wurde die Beziehung fur die Bandenform der Exzitonenabsorption gefunden, die durch Streuung yon Polaritonen an geladenen Storstellen verursacht wird. Die Bandenform ist vom Lorentz-Typ mit freqaenzabhingigem r ( w ) = (h/2) v nc (u& + o ; , ) , wobei v die Gescliwindigkeit des durch Licht eneugten Exzitons, und oil dessen Streuquerschnitte der elastischen und inelastischen S-Streuung durch das geladene Zentrum und n, die Konzentration der geladenen Storstellen sind. Quantitative Berechnungen wnrden fur CdS-und GaAs-Kristalle durchgefuhrt. Bei groBen Konzentrationen kann der betrachtete Mechanismus der Bandenverbreiternng dominieren.

The exciton-electron interaction process in the luminescence spectra which i s important at high concentrations of free carriers was introduced by Benoit B la Guillaume et al. (1). Later the problem was repeatedly discussed in theoretical (2), (3) and experimental (4) works.In the luminescence spectra the exciton-electron interaction displays a wide band whose intensity is proportional to the concentration of the free c a r r i e r s n.In this paper it is shown that the screening effects disturb the linear concentration dependence of the luminescence band intensity. In the region of high concentrations the intensity falls with increasing n. At n = n it reaches its maximum. Further growth of the concentration leads to a sharp decrease of the intensity and to a turning-off of the exciton-electron recombination process.

At high excitation levels a considerable concentration of free c a r r i e r s is available. This leads to observable changes of the luminescence spectra of the crystal at the absorption edge due to effects of exchange ( l ) , correlation (2, 3), plasmaron This paper is concerned with the luminescence from the electron-hole pair bound states. Besides screening effects, the interaction of excitons with plasma oscillations is taken into account considering the latter in terms of a fluctuative electric field of the free c a r r i e r gas.Let us consider a simple model of an isotropic crystal with a single valence band and a conduction band. The excitons are treated in the effective mass approximation.Due to screening the picture of bound exciton states is valid for r > 5 (3) (r = r /a where r = (3/41C n ) l l 2 , a is the Bohr exciton radius, n is the free c a r r i e r concentration). In such case the Debye-Huckel potential (6, 7) is a good approximation a t moderate temperatures. The ground state of the screened exciton is calculated by means of a variational procedure with the one parameter trial function (7). With increasing concentration the exciton binding energy E E (n) decreases while the plasmon energy increases (bw =h(471: e n/e l~)~'~, where E. is the dielectric con-