Fig. 1. (Color online) Schematic diagrams of the layer structure for (a) series-A and (b) series-B samples. The (Al,Ga)As buffer layer plays the role of an energy barrier, and the low-temperature grown n-type GaAs in series-A samples will deplete the holes in (Ga,Mn)As film to some extent, while the heavily-doped p-type GaAs layers in series-B samples are used to suppress the hole depletion.

Fig. 2. (Color online) Temperature dependence of the magnetization for (a) series-A and (b) series-B samples. The magnetic field of 20 Oe is applied along the (Ga,Mn)As [–110] direction. An enhancement of T_C with the increase of Mn content can be clearly observed, while there is no obvious difference between the magnetic properties of these two series of samples.

Fig. 3. (Color online) (a) Longitudinal resistance as a function of temperature for series-A samples. (b) Temperature dependent longitudinal resistances of sample A4 under gate voltages, in which the peak shifts to lower (higher) temperature when the applied voltage is +2 V (–2 V).

Fig. 4. (Color online) (a) Schematic of the device structure. (b) Longitudinal resistance as a function of temperature for series-B samples. (c, d) Temperature dependent longitudinal resistances for samples B4 and B1 under various gate voltages, in which clear peak shifts can be observed.

Table 1. Detailed information of series-A and series-B samples.