Far-infrared studies at intermediate magnetic fields with the neutral shallow donors in GaAs and InP of transitions not involving the ground state

More than thirty sharp lines are observed in the far-infrared photoconductivity from the neutral shallow donors in n-GaAs and InP at wavelengths longer than the longest wavelength that will excite electrons from the ground (1s) state of the donor. Twenty-four of the lines observed can be positively identified as corresponding to transitions given by p to q where p and q are the principal quantum numbers involved and are given by 2, 3 and 4. The n=2 to 2 lines involving the 2s state (2p - -2s, 2s-2p + , 2s-2p 0 ) are sufficiently sharp that central-cell structure arising from the chemical shifts of the individual donor species can be observed. The central-cell splitting is found to agree well with that expected from measurements of the splitting on the 1s state. The inter-excited-state transitions are observed under much less restricted conditions of sample compensation, electric field bias, optical excitation and temperature than the broader peaks corresponding to transitions from negatively charged donor-ion (D - ) states to the conduction band states.