Doping dependence of high-field donor spectra in InSb

Experimental results on InSb concerning the direct observation of the ionising transition from the donor ground level to the N=0 Landau level and concerning the first excited state transition are reported (T=4.2K). Results are obtained on samples doped in the melt as well as by nuclear transmutations (N 1 =2*10 14 -5*10 15 cm -3 ). The observed shift of the energy of the ionising transition to higher values as the doping increases is explained on the basis of a level-broadening model (spatial fluctuations of the impurity potential); the energy of the first excited state transition remains unchanged with increasing doping as expected from the model. In the heavier doped samples (N 1 approximately 10 15 cm -3 ), the ionisation of the ground level can be observed at low enough magnetic fields where no magnetic freeze-out occurs. This shows that screening is not the predominant effect since in that case the energy of the ionising transition has to tend to zero.