Spin noise spectroscopy of donor-bound electrons in ZnO

We investigate the intrinsic spin dynamics of electrons bound to Al impurities in bulk ZnO by optical spin noise spectroscopy. Spin noise spectroscopy enables us to investigate the longitudinal and transverse spin relaxation time with respect to nuclear and external magnetic fields in a single spectrum. On one hand, the spin dynamic is dominated by the intrinsic hyperfine interaction with the nuclear spins of the naturally occurring ${}^{67}$Zn isotope. We measure a typical spin dephasing time of 23 ns, in agreement with the expected theoretical values. On the other hand, we measure a third, very high spin dephasing rate which is attributed to a high defect density of the investigated ZnO material. Measurements of the spin dynamics under the influence of transverse as well as longitudinal external magnetic fields unambiguously reveal the intriguing connections of the electron spin with its nuclear and structural environment.

Figure 1

(a) Photoluminescence spectra recorded at 40 and 80 ps, respectively, after excitation with a picosecond laser pulse with a photon energy of 3.54 eV and a sample temperature of $T=4$ K. The inset shows the PL decay transient of the donor-bound $D^{0}X$ exciton. (b) cw-PL (black line) recorded with a higher resolution. The labels $D^{0}X$ and $D^{+}X$ mark the spectral positions of the neutral and ionized donor-bound exciton transition, respectively. The red dots are transmission measurements performed with a spectrally narrow laser.

Figure 2

Experimental spin noise setup. The light source is a frequency-doubled cw-Ti:sapphire laser. The linear polarized probe light is focused through the sample which is mounted in a helium cold-finger cryostat. The rotation of the linear polarized light is measured by a polarization bridge and a balanced receiver. The electrically amplified difference signal is digitized in the time domain and spectrally analyzed by a computer. The external magnetic field can be applied in transverse and longitudinal (not shown) direction.

Figure 3

(a) Spin noise spectrum recorded with a transverse magnetic field of $B_{\perp }=1\mathrm{mT}$ (black line). The background spectrum has been acquired at $B_{\perp }=10$ mT and the probe laser photon energy is 3.355 eV. The data is fitted by a model containing three spin noise contributions: an inhomogeneous contribution ${\gamma }_i$ centered at the Larmor frequency ${\nu }_L$ (red, dashed line), a homogenous contribution ${\gamma }_h^{\left(1\right)}$ (blue, dotted line) centered around zero frequency, and a broad homogenous contribution ${\gamma }_h^{\left(2\right)}$ (green, dashed dotted line) centered at the Larmor frequency ${\nu }_L$.

Figure 4

(a) Spin noise spectra recorded with different transverse magnetic fields. The background spectrum has been acquired at $B_{\perp }=10$ mT and the probe laser photon energy is 3.355 eV. The data is fitted by a model according to the description in the text. The inset depicts the change in the Larmor frequency (black solid line) and ${\gamma }_i$ (red, dashed line) for the inhomogeneous spin noise contribution. (b) Spin noise difference spectra for different longitudinal magnetic fields. The spectra are shifted such that the average spin noise power density in the range of 50–80 MHz is zero in order to compensate for experimental drifts. The data is fitted according the model described in the text. The inset compares the homogeneous longitudinal (${\gamma }_h^{\left(1\right)}$) and inhomogeneous transverse (${\gamma }_i$) spin relaxation rates in dependence of $B_{\parallel }$. The probe laser photon energy is 3.359 eV for these measurements.

Figure 5

Dependency of (a) the total spin noise power and (b) spin dephasing rate on the detuning relative to the $D^{0}X$ transition. The spectral position of the maximum spin noise power at 3.360 eV indicates that the major spin noise contribution originates from Al-donor-bound electrons.