In order to clarify the nature of the neutral Mn acceptor in GaP, we have carried out optical-absorption and electron-paramagnetic-resonance ͑EPR͒ experiments using both conventional and thermally detected EPR on semi-insulating GaP:Mn. In thermal equilibrium at low temperatures, all the manganese occurs in the charged acceptor state Mn Ga 2ϩ ͑A Ϫ ͒. By illumination with photon energies greater than 1.2 eV, it can be partially converted into the neutral charge state. The arising photostimulated EPR spectrum shows the characteristic of a tetragonally distorted center with an integer spin. The resonance lines are detectable only at temperatures below 7 K, and their linewidth of about 50 mT is due to the unresolved Mn-hyperfine splitting. We interpret the experimental data in terms of Mn Ga 3ϩ ions on strain-stabilized sites of tetragonal symmetry due to a strong T ⑀ Jahn-Teller coupling within the 5 T 2 ground state. Such a behavior is expected for a 3d 4 defect, as observed for the isoelectronic impurity Cr 2ϩ in GaAs, and other tetrahedrally coordinated semiconductors. The analysis of the EPR spectra thus verifies that, in GaP, the neutral charge state of the Mn acceptor is Mn Ga 3ϩ ͑A 0 ͒ in contrast to its behavior in GaAs and InP. ͓S0163-1829͑96͒06139-5͔

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In LEC-grown InP, about 30 sharp vibrational absorption lines are measured in the frequency region 2200 to 2350 cm -1 . All these lines are due to phosphorus-hydrogen stretching modes. Experiments on InP containing both hydrogen and deuterium finally proved that the line at 2202.4 cm -1 is due to a single hydrogen atom bonded to P in an indium vacancy (V In ) and that the line at 2315.6 cm -1 is due to the complex of four P -H bonds in an V In . In InP : H : D, this V In H 4 complex gives rise to six vibrational lines in the region of P -H modes and six lines in the region of P -D modes because of the five different types of V In H n D m complexes. The measured frequencies of these 12 lines are in excellent agreement with those obtained from ab initio calculations reported in the literature. Additional P-H complexes are discussed.

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We present an optical Fourier-transform-infrared photoluminescence (PL) and absorption study of the T2~E internal 3d transitions of Fe + in GaP. We analyze in detail the four zero-phonon lines of Fe + which appear at about 3300 cm in GaP. A fine structure, which originates from diferent iron isotopes, is resolved. Detailed PL and absorption spectra between 2500 and 4200 cm reveal many features in the Stokes and anti-Stokes phonon sidebands. With the help of temperaturedependent absorption measurements we are able to set up the complete level scheme for the internal 3d transitions of Fe + in GaP. We compare the experimental results to theoretical values obtained by crystal-field theory including spin-orbit coupling.

The local vibrational mode absorption lines at 3079.2 and 983.3 cm -1 , measured in semi-insulating LECgrown GaAs, are due to the stretching and wagging mode of one hydrogen bonded to a light impurity atom. The investigation of GaAs grown without boric oxide encapsulant delivers arguments that this impurity atom is nitrogen in the nitrogen-vacancy (N As -V Ga ) defect, which is responsible for the vibrational quadruplet line at 638 cm -1 .