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D-states (i.e. the states formed by a neutral donor, DO, binding an extra electron), Do interexcited state transitions (i.e. n+m where the principal quantum numbers n, m are greater than 1) and cyclotron resonance are all observed simultaneously in the far-infrared spectrum of n-GaAs and n-lnP. This observation constitutes the first unambiguous identification of D-states in a compound semiconductor. D-states have been observed in seven GaAs and two InP samples. A chemical shift is observed, but no central cell splitting is found even at the largest available fields (i.e. 24 T). D-states are studied under a variety of experimental conditions such as hydrostatic pressure, temperature and electric field bias. The application of hydrostatic pressure strongly enhances the D-intensity in VPE InP, but not in VPE GaAs. Effects attributable to the formation of D-complexes occur in GaAs when the temperature is reduced to 2 K. The intensity of the D-transition increases markedly with increasing electric field bias. Divergence between theory and experiment is evident when the dimensionless magnetic field parameter, y, exceeds unity, indicating the inadequacy of existing variational calculations in this regime. D-triplet and singlet transitions are tentatively identified in n-InSb.

The magneto-optical spectrum of the shallow donors in n-GaAs is studied using a far-infrared laser at photon energies greater than the binding energy of the donors. The transitions from the ( I s ) ground state into the Landau levels are not observed but, instead, series of transitions from the ground state into donor states associated with the individual Landau levels.The strongest series corresponds to the (000)-(ONO) transitions and t h e second strongest series to (000-(ON2) transitions where N is the index of the Landau level involved (and where the high-field notation is employed). The corresponding low-field notation is ls-2p+, Is-3d+*, ls-4f+,, ls-5g+, etc for the strongest series which can be followed up to the N=17 Landau level. Central cell structure due to the presence of several different donors can be observed on many of the lines associated with the N = 1 and N = 2 Landau levels. The ls-3p+ transition can be used to determine chemical shifts accurately for the donor species present, as this line avoids the overabsorption effects which can distort the stronger ls-2p+ line. A few weak lines remain unidentified.

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-2p0) 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.

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