Doping dependence of high-field donor spectra in InSb

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Abstract
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 (N1=2*1014-5*1015 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 (N1 approximately 1015 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.