2, National Chiao Tung University, Hsin-Chu, , Taiwan
3, St. Bonaventure University, Olean, New York, United States
We have studied magneto-PL from ZnTe QDs embedded in ZnMnSe matrix grown by molecular beam epitaxy. The PL is due to radiative, spatially indirect transitions between holes confined in the ZnTe QDs and electrons that remain in the ZnMnSe matrix . The peak energy at T = 7 K exhibits a red shift with increasing magnetic field and saturates around 4 tesla, mirroring the magnetization of the paramagnetic ZnMnSe matrix. The observed red shift is due to the exchange interaction between the manganese and the carrier spins. The hole-Mn interaction comes from the hole wavefunction tail that penetrates into the ZnMnSe matrix. The saturation value of the red shift exhibits a strong dependence on the photon energy of the exciting laser beam. The red shift with excitation using 3.06 eV photons (energy above the matrix gap), has a saturation value of 4.5 meV; in contrast the saturation value using excitation with 2.54 eV (energy below the matrix gap but above the QD gap) is 14.5 meV. This strong dependence on energy of the exciting photon is understood as follows: Excitation with 3.06 eV photons generates electron-hole pairs in the ZnMnSe matrix. The holes are then captured by the ZnTe QDs. Once a QD becomes occupied by a hole, a barrier is created due to the Coulomb repulsion that prevents other holes to occupy the QD. Excitation with 2.54 eV photons do not have this limitation and result in multiple hole occupancy of the QDs. The Coulomb repulsion between holes in the same QD modifies the wavefunction in such a way that we have increased penetration into the magnetic ZnMnSe matrix, and therefore stronger Mn-hole spin-exchange interaction that gives us the larger PL energy red shift. We have calculated the hole wavefunction for single and double occupation using the linear variational method by constructing a matrix representation of the Hamiltonian operator in a basis of slater determinants of single particle harmonic oscillators. All matrix elements are solved analytically. The matrix is diagonalized to yield the hole wave function. The results of the calculation are in agreement with the experimental observations.
. B. Barman et al, Phys. Rev. B, 92, 035430 (2015).
This work is supported by NSF DMR 1305770.