J. Phys: C: Solid State Phys., Vol. 7. 1974. Printed in Great Britain. @ 1974 Far-infrared spectroscopy of bound and free electrons in III-V and II-VI semiconductors : I. Transitions between excited states observed in Zeeman spectroscopy of the shallow donor impurities P E Simmondst, J M Chamberlain?, R A Houltt, R A Stradlingt and C C Bradley$ t Clarendon Laboratory. Oxford University, Oxford $ National Physical Laboratory. Teddington, Middlesex Abstract. This paper is one of a series providing a comprehensive study of cyclotron resonance and impurity transitions for n-type GaAs, InP, CdTe and CdSe. In this paper, attention is concentrated on weak transitions which can be observed when the electrons are predomi- nantly bound to the donor states if the photon energy is insufficient to excite the electrons from the ground state of the donor. With all materials the most prominent lines observed are found to lie on the same curves of excitation energy against magnetic field. The following transitions are positively identified 2p- -+ 2s; 2p- , --f 3s; 2p- , + 3d- I ; 2p- --f 3d,. 2p0 + 3s, 2p0 + 3d+,.2s --f 3p+,,and3d-, + 3p-,.Thedependenceoftheselinesontemperature and impurity content is discussed and compared with the cyclotron resonance lines. 1. Introduction This paper is one of a series which discusses in detail an extensive series of experiments on the properties of bound and free electrons in III-V and II-VI semiconductors. Preliminary results of these experiments have already been published (Chamberlain et al 1972a, b) but the present papers will provide more detailed and conclusive results for a greater number of samples. With the compilation of additional data, a more complete interpretation is possible with the result that the original hypotheses have been modified in some respects. The purpose of this paper is to discuss a number of very weak lines which appear at temperatures where the shallow donors become occupied although the photon energies concerned are insufficient to excite electrons from the ground state of the impurities. These lines are attributed to transitions originating and terminating on excited states of the donors. Two of these lines were previously reported and, with all four materials investi- gated, were found to lie on the same two curves on a dimensionless diagram of excitation energy against magnetic field. From this field variation the two lines were identified as arising from 2p - to 2s and 2s to 3p + transitions (Chamberlain et al1972b). For GaAs, InP and CdTe the much stronger spectrum of lines from the Is ground state have already been discussed in detail elsewhere (Stradling et a2 1972, Simmonds et a1 1974). The cyclotron resonance masses and linewdiths which are measured in the same experiments will be discussed in a forthcoming paper. Prior to the report of excited state transitions in GaAs, InP, CdTe and CdSe, transi- tions between excited states had been suggested for InSb (Kaplan 1966) and for Te (Hulin 4164Excited states in Zeeman spectroscopy of shallow donors 4165 1970) and transitions from the higher energy components of the ‘valley-orbit’ split 1s states in silicon had been positively identified (Aggarwal 1964, Ottsenmeyer et a1 1964, Aggarwal and Ramdas 1965). At the same time very similar experiments to our own involving photoconductivity measurements with coherent far-infrared sources have detected weak lines from donors in germanium which mirrored the conduction band anisotropy (Gershenzon and Goltman 1972, Muro and Nisida 1973). These lines were interpreted in a similar manner to our own with the following transitions being pro- visionally identified: 2s + 2p*, 2s + 3p+, 2p, + 3d+l. One characteristic feature identifying the lines due to excited-state transitions is that they can be observed with photon energies insufficient to generate transitions from the ground state of a hydrogenic donor. Rather similar structure also showing a field dependence incompatible with cyclotron resonance has been reported in experiments with SnO, (Button et a1 1971)’ CdS (Cohn et a1 1971)’ CdSe (Button and Lax 1970) and ZnO (Button et a1 1972a, b). These lines may have a common origin with those found in the present experiments. The authors have recently become aware of some very comprehensive data on the excited-statetransitions inn-GaAs (Gershenzon et a1 1974) which complement and extend the experiments reported in this paper. Intrinsic light was used to narrow the observed lines in the latest experiments and fourteen different excited-state transitions were detected. Of these the following lines were positively identified : 2p0 + 2s (l), 2p - + 2p0 (2), 2p - , -+ 2s (3) and 2s + 2p + , (4) where the figures in parentheses refer to the designa- tion of the lines employed by Gershenzon et al. A weak line (5) was provisionally identified as 2p0 + 2p+ Line (3) is line B, of the present paper and was identified similarly. Line (10) is B,, line (1 1) is A, and line (12) is A,. From the calculations reported in the present paper it is clear that line (6) is 2p - -+ 3d - ,. The other lines in the paper by Gershenzon et a1 do not appear to fit any of the possible transitions between the n = 2 and n = 3 atomic states and consequently probably involve higher order states. However, it should be noted that there is some doubt concerning the position of the 3do state as the calcula- tions of Praddaude (1972) indicate that this state lies much higher in energy than the values of Lee et al (1973) used in the present paper. Praddaude’s values would suggest that line (9) is 2p+ -+ 3do and line (13) 2p- + 3d0. 2. Experimental techniques for the observation of the spectral lines The characteristics of the samples employed in the present experiments are given in table 1. The weak excited-state lines narrowed very rapidly with increasing sample purity so the highest-purity samples of each material were sought for the present study. For GaAs and InP, samples were therefore necessarily in the form of epitaxial films which have much higher chemical purity than bulk samples. It was barely possible to detect the excited-state transitions in transmission experiments as these lines are one or two orders of magnitude weaker than the lines arising from transitions originating from the ground state. With all materials the weak excited-state lines could best be detected by photo-