Suppression of the Auger recombination due to spin polarization of excess carriers andMn2+ions in the semimagnetic semiconductorCd

Abstract

The observation of a strong increase in the photoconductivity of Cdo95Mno05S under an applied magnetic field is reported. The effect, observed at low temperatures, is explained to arise due to the dramatic suppression of the Auger recombination processes involving the excitation of Mn + ions ( A,~T, &2& transitions within a 3d shell). The single-ion-assisted Auger recombination, which determines the lifetime of the excess carriers at low magnetic fields, become forbidden for the totally spin-polarized single ions, and at high magnetic field the cluster-assisted Auger recombination dominates.The identity of particles results in the dependence of the cross section of their mutual scattering (both elastic and inelastic) on the polarization of their spins, even if no spinorbit coupling is taken into account. Correspondingly, the spin polarization of carriers in a semiconductor due to an applied magnetic field can lead to a specific magnetic-field dependence of different kinetic coefficients. In ordinary semiconductors, however, it is difficult to observe this phenomenon in the pure form because the effect of the magnetic field on the orbital movement has usually the same order of magnitude (or even higher). The situation is more favorable in the semimagnetic semiconductors (SMSC's) (also referred to as diluted magnetic semiconductors). In these alloys the exchange interaction between the spin of a carrier and those of magnetic ions results in giant spin splitting of the carrier levels in the magnetic field (the effective g factor can reach a few hundred in the region of sufficiently low temperatures). ' Along with this, the effective masses of carriers in the wide-gap SMSC are high enough, cyclotron energies are small, and in the first approximation one can neglect the effect of the magnetic field on the orbital degrees of freedom. In this paper we report on the observation of an effect in such a semiconductor: the strong increase in the photoconductivity (PC) of Cdo 95Mnp psS in an applied magnetic field. This increase is solely determined by the spin polarization of the carriers and 3d electrons of manganese ions, which reduces drastically the recombination rate.The spin-dependent mechanism of recombination, which we propose to explain the observed PC increase, is the exchange Auger recombination with energy transferred to an excitation of a Mn ion. This channel of recombination is quite probable because the excitation energies of Mn from the ground state At (the total spin S= -', ) to excited states Tt and T2 (with S= -, ) are 2.43 and 2.58 eV, respectively, which is close to the band gap of Cdp95Mnpp5S. For the exact conservation of the total energy such an Auger recombination should be, generally speaking, phonon assisted. The direct experimental hint of the existence of these recombination processes is the fact that the photoexcitation of carriers in wide-gap SMSC's with x)0.01 results in the luminescence due to internal transitions of the 3d electrons of manganese. " Note also that the energy transfer from exc...

Carrier Dynamics and Magneto-Optical Properties of Cd1-xMnxS Nanoparticles

“…This is a universal behaviour that has been observed in DMS crystals, epilayers, quantum wells, quantum wires, and in self-assembled epitaxial quantum dots [33][34][35][36][37][38][39][40][41][42]. Although the precise mechanism of energy transfer from excitons to electrons in the Mn 2+ 3d 5 shell is still debated in the scientific community, [37,38,[42][43][44] the marked field dependence of this process indicates a spindependent excitation transfer as described by Nawrocki [45] and by Chernenko [42,43].…”

“…In the low excitation intensity the PL intensity curve behaves very different as in the high excitation intensity. Based on the approach of Nawrocki et al [45], Chernenko et al [42,43] E , become almost suppressed in samples with magnetic doping (see Fig. 6b).…”

“…For example, in self-organized Cd1-xMnxSe QD samples, the 4 T1 → 6 A1 emissions from the incorporated Mn 2+ ions are completely suppressed at a magnetic field values near 3-4 T [40]. According to Nawrocki model [45], this occurs because the Mn 2+ magnetization freezes out the electron population in the Ms = -5/2 ground state Zeeman sublevels of the Mn 2+ ions (labelled 6 A1) [2,42-45]. In this model, the transition of an electron from the conduction to the valence band occurs without change of its spin, and the transition is allowed if the total spin of the combined system of Mn-ion+electron is conserved.…”

Mn2+‐assisted recombination of trions in semimagnetic quantum dots

“…The selection rules for e-h pairs recombination were first proposed by Nawrocki et al [3]. The mechanism of energy transfer is related to the exchange interaction between band and 3d electrons.…”

Magnetic Field Influence on Auger Effect on Shallow Donors in CdF2:Mn2+Luminescence

“…According to the theoretical model of Nawrocki et al [4] it is given by where Ρeσ (Β) is the probability for the electron to be in the state with spin projection σ…”