Ultrafast electron spin dynamics in ZnO and Zn1-xCoxO sol-gel thin films

We probe the electron spin dynamics in ZnO and Zn1-xCoxO sol-gel films with time-resolved Faraday rotation spectroscopy. Dephasing times T2* on the order of nanoseconds are observed at room temperature due to charge-separated states. In ZnCoO the effective electron Landé g factor rises with increasing Co concentration, providing the mean-field electron-Co exchange energy N0α = +0.25 ± 0.02 eV. Semiconductors are well-suited for ultrafast opto-spintronics. Polarized femtosecond laser pulses may be used to intitialize, manipulate and read out spin states in semiconductor systems ranging from three to zero dimensions. The wide-bandgap semiconductor ZnO possesses both a large bandgap of 3.4 eV and an exciton binding energy of 60 meV. These properties will be strongly beneficial to room-temperature operation of future spintronic devices made of this material. ZnO may also be readily doped with magnetic ions. Zn1-xCoxO has received extraordinary attention over the past decade, ever since reports of room-temperature ferromagnetism in this material began to appear. Remarkably, the magnitude of the Co-electron exchange energy N0 has never been measured for this diluted magnetic semiconductor. We use time-resolved Faraday rotation (TRFR) spectroscopy in the ultraviolet to directly probe the transient electron spin dynamics in chemically prepared ZnO and Zn1-xCoxO sol-gel films [1]. Figure 1 shows TRFR traces of an undoped ZnO sol-gel film, recorded at a temperature of T = 10 K and room temperature, along with traces collected from Zn1-xCoxO films at various Co concentrations. Surprisingly, TRFR signals not only persist up to room temperature, but the electron spin dephasing time T2 increases with rising temperatures, in stark contrast with previous observations for epitaxial ZnO films. Sol-gel films of Zn1-xCoxO show a similar temperature dependence of T2. The contrast with epitaxial films suggests that the anomalous T2 observed in these sol-gel ZnO films arises from their granularity. Hole traps at the surfaces of ZnO nanocrystals slow down electron-hole recombination and hence allow observation of extended electron spin coherence times. We have observed this effect in a previous study on the ultrafast spin dynamics in colloidal ZnO quantum dots [2] and believe that this mechanism is also active in the sol-gel films studied here. We confirmed this hypothesis by probing the carrier recombination dynamics in a regular pump-probe experiment recording differential transmission and comparing the results to the spin dynamics obtained by the TRFR measurements. EPJ Web of Conferences

Semiconductors are well-suited for ultrafast opto-spintronics.Polarized femtosecond laser pulses may be used to intitialize, manipulate and read out spin states in semiconductor systems ranging from three to zero dimensions.The wide-bandgap semiconductor ZnO possesses both a large bandgap of 3.4 eV and an exciton binding energy of 60 meV.These properties will be strongly beneficial to room-temperature operation of future spintronic devices made of this material.ZnO may also be readily doped with magnetic ions.Zn 1-x Co x O has received extraordinary attention over the past decade, ever since reports of room-temperature ferromagnetism in this material began to appear.Remarkably, the magnitude of the Co 2+ -electron exchange energy N 0  has never been measured for this diluted magnetic semiconductor.
We use time-resolved Faraday rotation (TRFR) spectroscopy in the ultraviolet to directly probe the transient electron spin dynamics in chemically prepared ZnO and Zn 1-x Co x O sol-gel films [1]. Figure 1 shows TRFR traces of an undoped ZnO sol-gel film, recorded at a temperature of T = 10 K and room temperature, along with traces collected from Zn 1-x Co x O films at various Co 2+ concentrations.
Surprisingly, TRFR signals not only persist up to room temperature, but the electron spin dephasing time T 2 * increases with rising temperatures, in stark contrast with previous observations for epitaxial ZnO films.Sol-gel films of Zn 1-x Co x O show a similar temperature dependence of T 2 * .The contrast with epitaxial films suggests that the anomalous T 2 * observed in these sol-gel ZnO films arises from their granularity.Hole traps at the surfaces of ZnO nanocrystals slow down electron-hole recombination and hence allow observation of extended electron spin coherence times.We have observed this effect in a previous study on the ultrafast spin dynamics in colloidal ZnO quantum dots [2] and believe that this mechanism is also active in the sol-gel films studied here.We confirmed this hypothesis by probing the carrier recombination dynamics in a regular pump-probe experiment recording differential transmission and comparing the results to the spin dynamics obtained by the TRFR measurements.We now turn to magnetically doped ZnO.Upon addition of Co 2+ to the ZnO sol-gel films, an increase in the precession frequency (i.e., an increase in g * ) is observed (Fig. 1).The temperature dependence of g * is shown in Figure 2. In undoped ZnO, g * remains nearly constant between T = 10 and 298 K (g * = 1.98 -2.00).In Zn 1-x Co x O, however, g * decreases with increasing temperature, approaching that of the undoped ZnO at high temperatures.These experimental results reflect the existence of exchange coupling between the photoexcited electrons and the Co 2+ dopants.A global fit of the data in Figure 2 yields the electron-Co 2+ exchange coupling parameter N 0 α = +0.25 ± 0.02 eV.The value reported here is independent of complications of earlier attempts using steady-state experimental techniques.Recent ab initio calculations on bulk Zn 1-x Co x O have suggested N 0 α = +0.34eV, which is in good agreement with our experimentally determined value.
The second striking observation in the TRFR traces of Fig. 1 is that electron spin dephasing is strongly accelerated by introduction of Co 2+ into the sol-gel ZnO films.With as little as x = 0.0001, T 2 * drops from 600 to 250 ps at T = 10 K.The accelerated dephasing upon addition of magnetic impurities is due to local fluctuations of the magnetization, which in turn arise from thermal fluctuations of <S x > and from microscopically inhomogeneous spatial distributions of the dopant ions (i.e., a breakdown of the virtual crystal approximation).The resulting linear dependence of 1/T 2 * on the Co 2+ concentration x observed in the experiment (data not shown) has been verified by a model based on these two effects.In conclusion, we have prepared ZnO and Zn 1-x Co x O films suitable for optical electron spin generation and detection using rapid solution techniques.Precise control over x marks a promising advance in the development of flexible, low-cost preparative methods for incorporation of oxide diluted magnetic semiconductors into ultraviolet optical microcavities [3] or related opto-electronic and opto-spintronic device structures.In both ZnO and Zn 1-x Co x O sol-gel films, the ensemble electron spin dephasing times T 2 * grow longer at elevated temperatures.This unprecedented property is attributed to inhibition of carrier recombination via thermally activated hole trapping.Through analysis of the electron's effective g factor as a function of Co 2+ concentration, the mean-field electron-Co 2+ exchange coupling parameter in Zn 1-x Co x O has been determined to be N 0 α = +0.25 ± 0.02 eV.

EPJFig. 1 .
Fig. 1.Time-resolved Faraday rotation curves for ZnO and Zn 1-x Co x O sol-gel thin films, recorded at a temperature of T = 10 K (left) and room temperature (right).Exponentially damped sinusoidal fits are displayed as dotted lines.