Optical Spin Injection and Spin Lifetime in Ge Heterostructures

Abstract

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ∼37% and ∼85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ∼0.5 ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.

Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

“…Indeed the lifetime, τ , for electrons lying in the Γ valley of Ge is limited by the ultrafast scattering to the side valleys being hundreds of fs. 30,31 This process is much shorter than the spin relaxation time of CB electrons, τ es ∼ ns, 13,14,22,23 finally yielding circularly polarized luminescence. We recall that the observed luminescence polarization degree is ρ ∝ (1 + τ /τ es ) −1 .…”

“…14,26,27 Zone center valence band (VB) states transform with the same symmetry operations of atomic p orbitals and are characterized by a total angular momentum quantum number J=3/2 (for HH and LH) and J=1/2 (for SO states). The projection of the total angular momentum along the quantization axis, conveniently chosen to be parallel to the angular momentum of the exciting light beam, is J z = ±3/2 for HH, J z = ±1/2 for LH and SO band.…”

“…2,5,8 Only very recently, however, the quasi-direct behavior of Ge has sparked interest in its photonic properties, 9,10 and stimulated the use of various optical schemes aiming at addressing its spin physics. [11][12][13][14] In Ge, the absolute minimum of the conduction band (CB) is at the L point of the Brillouin zone, but there exists a local minimum at the zone-center Γ. At low temperature the former leads to an indirect energy gap of 0.744 eV, 15 the latter to a direct-gap of 0.898 eV.…”

“…15 Optical orientation via absorption of circularly polarized photons at the direct energy gap can then be exploited to readily investigate the spin properties of Ge. [16][17][18][19] Noticeably, spin-polarized electrons optically pumped in the Γ valley can conserve their spin during ultrafast scattering to the lower energy L valleys, 14 which are responsible for charge and spin transport. Such mechanism, which is absent in the widely studied III-V semiconductors, makes Ge spin dynamics very rich, but still poorly understood.…”

Strained germanium for applications in spintronics

“…The GeO 2 exhibits a low interfacial density of states to the bulk Ge, allowing for formation of high quality gates for control of spin and charge transport. Moreover, spin injection and spin relaxation in Ge have been extensively studied using electron spin resonance [20][21][22][23][24][25] and optical techniques [26][27][28][29]. However, in spite of all the recent progress in the Ge field and in contrast to Si, spin transport in Ge using nonlocal four-terminal techniques has only been observed at low temperatures to date [30][31][32].…”