MAGNETOOPTICS OF NI-BOUND SHALLOW STATES IN ZNS AND CDS

Transition metals in semiconductors give rise to shallow states which cannot be described on the basis of their 3d wave functions. In this p aper a comparative study of shallow states associated with Ni in ZnS a nd CdS is presented. By means of high-resolution excitation spectrosco py under the influence of magnetic fields the determination of the ele ctronic origin of the observed fine structure becomes possible. The re sults give insight into the local binding properties as well as the im portant role of the exchange interaction. Excitation measurements of t he 3T1(P)-3T1(F) Ni2+ luminescence reveal weak lines on the low-energy onset of the Ni2+/+ charge-transfer band due to the formation of shal low states. The high sensitivity of these measurements allows Zeeman i nvestigations of these nonluminescent shallow states. In both ZnS and CdS all lines exhibit a diamagnetic shift towards higher energies, dem onstrating the effective-mass-like nature of the excited states. On th e other hand, the fine structure as well as the linear Zeeman behavior is quite different in both host materials indicating the formation of different shallow complexes. In ZnS a transient shallow acceptor stat e (Ni+,h) is formed. The zero-phonon-line (ZPL) doublet around 2.437 e V with a zero-field splitting of 1.4 meV develops into a singlet and a triplet (g=0.50) in a magnetic field. It is unambiguously explained b y the (Ni+,h(n=1)) ground state, taking into account the exchange inte raction. An additional ZPL shifted 25.3 meV towards higher energies is attributed to an excited (Ni+,h(n=2)) State. From the excited hole st ate as well as from an observed hole-transfer process between Ni2+ and Cu2+, centers the binding energy of the (Ni+,h) complex and the Ni2+/ + charge-transfer energy are determined to be 108 meV and 2.545 eV, re spectively. In CdS a deeply bound electron-hole pair (Ni2+,e,h) is for med. The ZPL at 2.1904 eV exhibits a zero-field splitting of 50 mueV a ttributed to the trigonal crystal field and an isotope shift of -37 mu eV/nucleon. In a magnetic field it splits into a triplet (g=2.26). The Ni-associated shallow complexes in ZnS and CdS differ in the localiza tion of the electron which depends on the deep Ni2+/+ acceptor level. In ZnS this level is deep in the band gap; in CdS it is close to the c onduction band.