Magneto-optics of Ni-bound shallow states in ZnS and CdS.

Abstract

Transition metals in semiconductors give rise to shallow states which cannot be described on the basis of their 3d wave functions. In this paper a comparative study of shallow states associated with Ni in ZnS and CdS is presented. By means of high-resolution excitation spectroscopy under the influence of magnetic fields the determination of the electronic origin of the observed fine structure becomes possible. The results give insight into the local binding properties as well as the important role of the exchange interaction. Excitation measurements of the $^{3}$${\mathit{T}}_{1}$(P)${\mathrm{\ensuremath{-}}}^{3}$${\mathit{T}}_{1}$(F) ${\mathrm{Ni}}^{2+}$ luminescence reveal weak lines on the low-energy onset of the ${\mathrm{Ni}}^{2+/+}$ charge-transfer band due to the formation of shallow states. The high sensitivity of these measurements allows Zeeman investigations of these nonluminescent shallow states. In both ZnS and CdS all lines exhibit a diamagnetic shift towards higher energies, demonstrating the effective-mass-like nature of the excited states. On the 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 state (${\mathrm{Ni}}^{+}$,h) is formed. The zero-phonon-line (ZPL) doublet around 2.437 eV 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 by the (${\mathrm{Ni}}^{+}$,${\mathit{h}}_{\mathit{n}=1}$) ground state, taking into account the exchange interaction. An additional ZPL shifted 25.3 meV towards higher energies is at- tributed to an excited (${\mathrm{Ni}}^{+}$,${\mathit{h}}_{\mathit{n}=2}$) state. From the excited hole state as well as from an observed hole-transfer process between ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ centers the binding energy of the (${\mathrm{Ni}}^{+}$,h) complex and the ${\mathrm{Ni}}^{2+/+}$ charge-transfer energy are determined to be 108 meV and 2.545 eV, respectively. In CdS a deeply bound electron-hole pair (${\mathrm{Ni}}^{2+}$,e,h) is formed. The ZPL at 2.1904 eV exhibits a zero-field splitting of 50 \ensuremath{\mu}eV attributed to the trigonal crystal field and an isotope shift of -37 \ensuremath{\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 localization of the electron which depends on the deep ${\mathrm{Ni}}^{2+/+}$ acceptor level. In ZnS this level is deep in the band gap; in CdS it is close to the conduction band.