Manipulation of electrical and ferromagnetic properties of photo-sensitized (Ga,Mn)As

We present the manipulation of magnetic and electrical properties of (Ga,Mn)As by the ad- sorption of dye-molecules as arst step towards the realization of light-controlled magnetic- semiconductor/dye hybrid devices. A signicant lowering of the Curie temperature with a cor- responding increase in electrical resistance and a higher coerciveeld is found for the GaM- nAs/uorescein system with respect to (Ga,Mn)As. Upon exposure to visible light a shift in Curie temperature towards higher values and a reduction of the coerciveeld can be achieved in photo- sensitized (Ga,Mn)As. A mayor change in the XPS spectrum of (Ga,Mn)As indicates the appearance of occupied levels in the energy range corresponding to the (Ga,Mn)As valence band states upon adsorption of uorescein. This points towards a hole quenching eect at the molecule-(Ga,Mn)As interface which is susceptible to light exposure. The discovery of the ferromagnetic semiconductor (Ga,Mn)As (1) with remarkable properties such as hole mediated ferromagnetism (2) has motivated an intense re- search activity in view of potential applications in spin- tronics. The material's most striking characteristic is the manifold possibilities to tune magnetic properties via the manipulation of the carrier density or the mod- ulation of the lattice strain. A non-volatile modulation of the carrier density can be induced by means of fer- roelectric gating (3). The use of conventional metal- lic gates in a classic field effect transistor geometry on the other hand allows for the continous tuning of the magnetization in the absence of magnetic fields (4). Strain controlled ferromagnetism in (Ga,Mn)As has been equally exploited to manipulate magnetic anisotropy in systems showing lithography-induced anisotropic strain relaxation (5, 6) and in GaMnAs/piezoelectric actua- tor hybrid structures (7, 8). In this study we present the effect of light-sensitive molecular layers adsorbed on thin (Ga,Mn)As epilayers. We use fluorescein and two of its derivatives to achieve an effective hole quenching in the magnetic semiconductor and show, as a proof of principle, the possibility to influence this hole quench- ing process by light excitation. Our result on GaM- nAs/organic dye hybrid systems is an important first step towards magnetic applications exploiting molecule- dependent, photon-controlled carrier modulation. The (Ga,Mn)As films used in this study are grown by molecular beam epitaxy (MBE) on (Ga,Mn)As sub- strates in two different MBE laboratories. The two sam- ple materials designated A and B with thicknesses of 50nm and 40nm exhibit Curie temperatures Tc of 68K and 48K, respectively. The Mn concentrations for sam- ples A and B are 5% and 8%, respectively. Further details regarding the epitaxial growth can be found in Ref.(7) and (8) for sample A and B, respectively. The molecular layers were adsorbed by immersing the (Ga,Mn)As films in a 2 mM solution of the molecules. This solution is prepared by dissolving the molecule pow- der in water at room temperature and then adjusting the pH to 7 with NaOH. Prior to the immersion in the so- lution the (Ga,Mn)As films are treated with an HF con- taining etch mixture (Original etch mixture: AF 87.5- 12.5 VLSI Selectipur, diluted 1:100 in water) for ap- proximately 10 seconds to clean the surface. After HF treatment the samples are rinsed in purified water and thereafter immediately placed in the molecule solution for about 12 hours. Finally, the samples are rinsed in wa- ter to remove most of the non-chemisorbed species and are left to dry in air at room temperature and protected from visible light. Magneto-transport measurements per- formed before and after the HF treatment confirm that the etching procedure does not modify the properties of the (Ga,Mn)As films. The light source employed in the illumination experiments is a standard HBO Hg lamp. No light-induced effects or heating have been observed in the magneto-transport properties in as grown (Ga,Mn)As films without molecules. The expected adsorption geometry of fluorescein and its chemical structure is illustrated in the inset of Fig. 1