We show that the experimental results can be satisfactorily fitted by equation (3), shown by the solid lines in Fig. 5a,c,d, from which values can be extracted and are plotted in Fig. 5b as a function of temperature. By using the well-known g-factor ge=?0.44 for GaAs, we found that increases with increasing temperature and has a value around 60?ps at 80?K. The deduced values fall within the range of 20–150?ps typically reported for electron spin lifetime in GaAs bulk and quantum well structures27,28,29. The observed temperature dependence of the spin lifetime is also consistent with what is expected from the D’yakonov–Perel’ spin relaxation mechanism29,30. (We point out that deduced from the fitting is an effective averaged value, as the experimental results were obtained from ensemble electrons in GaAs with a distribution of depending on their proximity to the Bi2Te3/GaAs interface.) The fact that the surface spin current of the TI can be significantly altered and controlled by the spin injection from GaAs suggests that the TI can greatly benefit from the adjacent GaAs in such hetero or hybrid structures where one can exploit GaAs for its longer electron spin lifetime, improved performance at elevated temperatures and control of spin current in TI.



The bare substrates used in the growth of the TI film in Samples S1 and S2, namely the semi-insulating GaAs (111)B and GaAs (100) 2° off-cut substrate that are denoted as Sub.1 and Sub.2, respectively, were also studied as the reference samples.

Experimental techniques
Photocurrent was measured independently for both x and y directions without electric bias, under optical excitation with a wavelength-tunable, cw Ti-sapphire laser. The excitation T8 Fluorescent Lamps was modulated either in intensity by a mechanic chopper to generate helicity-independent photocurrent or in helicity by using a broadband electro-optic amplitude modulator to generate helicity-dependent photocurrent. Both helicity-independent and helicity-dependent photocurrent could be separately and selectively registered with the standard lock-in technique as described in Supplementary Note 9 and Supplementary Fig. 9. In the helicity-dependent photocurrent measurements, electro-optic amplitude modulator introduced a periodic variation of the laser polarization with each cycle starting from σ+ (left circularly polarized) to σy (linearly polarized along the y direction), then to σ? (right circularly polarized) and finally back to σ+ via σy. In cw-PL experiments, the samples were excited by the same laser beam as that used in the photocurrent measurements under the normal incident condition. The resulting PL emission from the GaAs substrate was collected in a backscattering geometry by a cooled Ge detector through a 0.8-m double grating monochromator. In the TR-PL experiments, a pulsed Ti-sapphire laser with a repetition rate of 76?MHz and a pulse duration of ～2?ps was employed. Transient PL was detected by a streak camera in combination with a 0.5-m single grating monochromator. The time resolution of the whole TR-PL system is 2?ps.