Optical orientation of spins in GaAs:Mn/AlGaAs quantum wells via impurity-to-band excitation

The paper reports optical orientation experiments performed in the narrow GaAs/AlGaAs quantum wells doped with Mn. We experimentally demonstrate a control over the spin polarization by means of the optical orientation via the impurity-to-band excitation and observe a sign inversion of the luminescence polarization depending on the pump power. The $g$ factor of a hole localized on the Mn acceptor in the quantum well was also found to be considerably modified from its bulk value due to the quantum confinement effect. This finding shows the importance of the local environment on magnetic properties of the dopants in semiconductor nanostructures.

Figure 1

Photoluminescence spectra of the doped and undoped sample areas. The line (1) corresponds to an undoped part of the sample. The spectrum contains a quantum well $\left({\mathrm{X}}_{\mathrm{QW}}\right)$ and the bulk $\left({\mathrm{X}}_{3\mathrm{D}}\right)$ exciton lines, and the usual donor-acceptor (DA) and the electron-acceptor $\left(\mathrm{e}\text{−}{\mathrm{A}}^0\right)$ transitions. The spectrum (2) displays additionally an exciton localized on the ${\mathrm{Mn}}^0$ acceptor $\left({\mathrm{X}}_{\mathrm{Mn}}\right)$ and an optical transition of the electron to the neutral Mn $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$. Phonon sidebands of the DA and the $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ transitions are shown. The uppermost spectrum (3) displays the quantum-well-related recombination that is shifted in energy scale by $\mathrm{\Delta }$ as indicated in the figure; see the text.

Figure 2

Polarized photoluminescence spectra in the applied magnetic field of $3\mathrm{T}$ under HeNe laser excitation taken at different spots of the sample surface. The $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ transition energy fluctuates due to local variations in the ${\mathrm{Mn}}^0$ binding energy.

Figure 3

Recombination schemes for the shallow acceptor (left) and Mn (right). The ground state in magnetic field is denoted with black circles. The thicker vertical arrows correspond to higher transition probabilities.

Figure 4

Schematic outline of the polarized photoluminescence spectra for optical transitions of the electron to (a) Mn in bulk GaAs, (b) Mn in a quantum well, and (c) a shallow acceptor in bulk GaAs.

Figure 5

Zeeman splitting (experimental points and a linear fit) of DA transition occurring in the bulk GaAs substrate region and $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ transition in the quantum well. The experimental points are from the spectra in Fig. 6 for the maximums of ${\sigma }^{\pm }$ peaks.

Figure 6

Polarized photoluminescence spectra of bulk DA, $\left(\mathrm{e}\text{−}{\mathrm{A}}^0\right)$, and $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ transitions taken at different magnetic fields.

Figure 7

Polarized photoluminescence spectrum of the bulk DA line and $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ transition under ${\sigma }^{+}$ excitation $\hslash \omega \approx 1.53\mathrm{eV}$ measured at different pump power $I=0.1\text{–}30\mu \mathrm{W}$ in zero magnetic field. The upper (purple) and the lower (blue) dependencies of the polarization degree on photon energy were measured at 30 and $0.6\mu \mathrm{W}$ pump power, respectively. The lowermost curve is smoothed for clearness.

Figure 8

Polarized $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ photoluminescence vs pump power. The experimental points (squares and triangles) are fitted by Eq. (4) (solid and dashed lines).

Figure 9

Polarized photoluminescence spectra of $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ transition under ${\sigma }^{+}$ excitation measured in different spots on the sample in zero magnetic field.

Figure 10

Dependence of $\left(\mathrm{e}\text{−}{\mathrm{Mn}}^0\right)$ photoluminescence polarization on pump power measured at different spots on the sample under the pump with $787\mathrm{nm}$ wavelength. Inset shows two curves measured at the same spot but with different pump wavelength: circles, $810\mathrm{nm}$; squares, $788\mathrm{nm}$.

Figure 11

Sign reversal mechanism. At low pump power the negative polarization degree of the photoluminescence is due to an exchange interaction of optically oriented ${\mathrm{Mn}}^{-}$ and photoexcited electrons. At high pump power the photoexcited electrons keep their spin and provide the positive polarization sign.