Wide-range optical spin orientation in Ge from near-infrared to visible light

Abstract

Ge-based spin-photodiodes have been employed to investigate the spectral dependence of optical spin orientation in germanium, in the range 400-1550 nm. We found the expected maximum in the spin polarization of photocarriers for excitation at the direct gap in (1550 nm) and a second sizable peak due to photogeneration in the L valleys (530 nm). Data suggest distinct spin depolarization mechanisms for excitation at and L, with shorter spin relaxation times whether the X point is involved. These devices can be used as integrated photon-helicity detectors over a wide spectral range. Spin-optoelectronics is a novel branch of semiconductor spintronics, aiming at adding a new degree of freedom to optoelectronics: the photon helicity. Exploiting the interplay between the photon angular momentum and the spin of electrons, integrated emitters (spin-LEDs) and detectors (spin photodiodes) of circularly polarized light have been proposed. Although GaAs is traditionally the material of choice for spin optoelectronics, because of its direct gap allowing for efficient conversion between light and spin polarization, recently Ge has attracted a considerable attention. In fact, thanks to the inversion symmetry of the crystal and the related absence of the D Yakonov-Perel spin scattering mechanism, Ge presents a longer spin coherence time than the one of GaAs. Spin manipulation [1], spin transport [2], spin optical pumping in the infrared [3][4][5][6][7], and electrical spin injection [8] in Ge have been reported. In our previous works [9,10], we demonstrated the room temperature operation of spin-PDs based on fully epitaxial Fe/MgO/Ge(001) heterostructures, working at 0.95 eV [11,12]. However, there is still a poor understanding of electrons and holes optical spin orientation in Ge, especially at photon energies much higher than the direct gap (0.8 eV). The optical spin orientation in Ge over a wide spectral range has been theoretically investigated by Rioux and Sipe [13]. However, no experimental data for excitation far from the point of the Brillouin zone (BZ) have been reported so far. Even for GaAs, only very recently the photon energy dependence of optical spin orientation well above the absorption edge has been reported [14].In this Rapid Communication, we report on the spectral dependence of the optical spin orientation in Ge at room temperature and over a wide spectral range (400-1550 nm, corresponding to 3.1-0.8 eV), via measurements of the helicitydependent photocurrent on Ge-based spin PDs. Surprisingly enough, the maximum sensitivity to photon helicity is observed far away from the direct gap (0.8 eV), where optical pumping is supposed to produce the highest initial spin polarization. Indeed, the helicity-dependent photocurrent variation indicates a first peak around 1500 nm (0.8 eV) and then an absolute maximum of about 10% at ∼530 nm (∼2.3 eV), corresponding to optical pumping in the L valley of the Ge band structure, * christian.rinaldi@polimi.it as shown in Fig. 1(a). Fitting of our data within a simple diffusive m...

Co - Fe - B/MgO/Djeffal1, Cadiz2, Stoffel3 et al. 2018Phys. Rev. Applied6040View full textAdd to dashboardCite

“…The variation mainly occurs for negative applied biases suggesting that the spin filtering of holes occurs. A similar observation was also reported by Rinaldi et al [28,45] on a device based on an electrode without PMA. Note that for excitation wavelengths λ = 980 nm and λ = 1310 nm, electrons and holes do not follow the same dynamics: electrons are photo-generated in the valley and are then very quickly scattered toward the L valley (approximately 200 fs [47]).…”

“…Let us now compare the evolution of | I /I | as a function of the applied bias when the excitation wavelength is changed from 980 to 1310 nm: | I /I | increases from about 0.06% to about 0.11% at V bias = −1 V. This may be partially due to an increased degree of optical spin orientation P s at 1310 nm as previously described by Rinaldi et al [45] following the k•p calculation by Rioux and Sipe [52] (at this particular wavelength, a polarization of both electrons and holes of about 45% has indeed been predicted, instead of 15% at 980 nm). Moreover, longer spin relaxation times for holes photo-generated at a lower energy are also expected to result in the larger | I /I | values.…”

“…The photocurrent exhibits a linear evolution with the laser power for all voltages considered here. [ 28,45]. However, the dependence of I /I as a function of µ 0 H does not follow the hysteresis cycle of the Co-Fe-B injector measured by SQUID [ Fig.…”

“…In order to estimate the maximum | I /I | that we could reach, an analytic model has been developed (see Appendix) showing that | I /I | max scales with P S (1/λ opt + 1/λ R )λ S for λ S ≪ λ opt or λ R , where P S is the photogenerated carrier spin polarization, λ S is the spin diffusion length, λ opt is the light absorption length, and λ R is the carrier recombination length. Considering λ opt ∼ 400 nm at a wavelength of 980 nm [45], an effective lifetime τ R = λ 2 R /D of about 10 µs [51], and a photo-generated hole spin polarization P S of about 15% at 980 nm [52], one can expect a maximum of | I /I | to be about 0.8% at RT. Therefore, the short hole spin relaxation time is a strong limitation that explains the weakness of the detected signal.…”

Bias Dependence of the Electrical Spin Injection into GaAs from Co−Fe−B/MgO Injectors with Different MgO Growth Processes

“…A. Electroluminescence circular polarization Figure 1(c) shows a typical cw EL spectra for a sputtering spin LED (annealed at 300°C) acquired under a bias of V ¼ 2.4 V for B ¼ 0.8 T at 25 K. We check to see that the magnetic circular dichroism is less than 1% at 0.8 T [20,37]. The EL polarization reaches P c ¼ 21.0 AE 1.5% for B ¼ 0.8 T at 25 K, and it is still about P c ¼ 20.0 AE 1.5% at 250 K (not shown).…”