Impact Ionization of Indirect Excitons in Crossed Electric

Abstract

In a former paper I l l we reported the observation of impact ionization of optically generated indirect excitons by electric field accelerated free charge carriers in high magnetic fields in germanium. W e now present model calculations and measurements of current-voltage characteristics which enable us to give evidence about the dependence of the exciton binding energy on the quantum number of involved conduction band Landau levels,The current density J' in electric ( E ) and magnetic (2) fields in a non--+ degenerated isothermal semiconductor is given by I21 f = Moo fi + M10 3 x 2 + M20 fi*(g.E) .(1)Here, the MiO are the transport coefficients given by u pifor each sort of charge carriers n, p. u is the conductivity and p is the mobility of the charge carriers.In crossed fields and with illuminated samples we haveE is the sum of the Hall field E and the field of the concentration gradients of electrons and holes conditioned by the illumination, Ex is the applied electric field.For an n-type semiconductor ( n >> p) and low temperatures, where the = )-I are approximatively equal / 3 / , we get the x-component of mobilities p the mean current density Jx as follows:where b is the thickness of the sample, kT is the thermal energy, and q is the electron charge. The concentration n is split into the ground concentration n and the excess concentration 6n(y) produced by the illumination. The first term of ( 4 ) 0 Pockelsstr. 4 , D-3300 Braunschweig, FRG. ,Most of these results were give11 by oral presentation at the DPG Spring Conference, Munster 1989; Verhandl. DPG ( V I ) 24, 10 (1989) HL-4.2.