212
9 Semiconductor Spintronics
• Voigt geometry: This geometry is applied to characterize the seldom used edge
emitting spin LEDs (Fig. 9.8b). The selection rules applicable to this configuration
with QWs active medium are no longer suitable because the injected carriers are
spin polarized in the direction perpendicular to the quantification axis. In most
cases, the performance shown by Voigt geometry devices is below the results
obtained with surface-emitting devices. The main benefit is the possibility of
operation at magnetic remanence as most thin film ferromagnets easy axis is
in-plane.
• Oblique Hanle geometry: This detection method involves the application of an
oblique magnetic field B making an angle of roughly 45 degree with respect to
the horizontal axis. B induces a precession of the carrier’s spin with the Larmor
frequency and assigns a perpendicular component to the spin vector detectable
through the emitted light degree of circular polarization. This configuration is
a clever way to effectively detect spin injection from an in-plane ferromagnetic contact (Fig. 9.8c). The application of a small oblique B manipulates spins
sufficiently enough during transport.
9.4.3.2 GaAs-Based Spin LED
A spin LED is a LED configuration caped with a ferromagnetic spin injector
employed to polarize the spin prior to injection in the active medium of the LED
structure. Quite a lot of sophisticated semiconductor structures have been explored
after observing the fascinating phenomena that the spin information stored in the solid
state can successfully be converted to polarized light information. Most successful
one is a n–i–p heterostructure that includes heavily doped p-type and n-type regions
separated by a lightly doped ‘near’ intrinsic semiconductor region. It is feasible to
establish a confined potential such as QWs or QDs in the intrinsic region (Fig. 9.9).
The outline of fabrication technique is given in Fig. 9.10.
The p-doped region shows a doping gradient from the p+ substrate toward the
intrinsic active medium. While in the n-region, the layer in contact with the MTJ
spin injector is considerably doped with respect to the active medium. The doping
profile at the interface spin injector/semiconductor is to be tuned to adjust the Fermi
level pinning near the tunnel barrier region. The active medium consists of one or
several undoped quantum wells. InGaAs QWs may be a wise choice as absorption
of the emitted light can be avoided. Utmost attentions are to be given for selecting
the structural parameters.
9.4.3.3 Ge-Based Spin LED
To realize the room temperature, spin LED, investigations have been made for spin
injection in Ge. This type of spin-LED relies on a Fe/GeO 2 contact. However, a
strong magnetic field (around 4T) is required. The FM tunnel contacts expose spin
accumulation as given by three and four terminal device geometries (Fig. 9.11a, b).
9 Semiconductor Spintronics
• Voigt geometry: This geometry is applied to characterize the seldom used edge
emitting spin LEDs (Fig. 9.8b). The selection rules applicable to this configuration
with QWs active medium are no longer suitable because the injected carriers are
spin polarized in the direction perpendicular to the quantification axis. In most
cases, the performance shown by Voigt geometry devices is below the results
obtained with surface-emitting devices. The main benefit is the possibility of
operation at magnetic remanence as most thin film ferromagnets easy axis is
in-plane.
• Oblique Hanle geometry: This detection method involves the application of an
oblique magnetic field B making an angle of roughly 45 degree with respect to
the horizontal axis. B induces a precession of the carrier’s spin with the Larmor
frequency and assigns a perpendicular component to the spin vector detectable
through the emitted light degree of circular polarization. This configuration is
a clever way to effectively detect spin injection from an in-plane ferromagnetic contact (Fig. 9.8c). The application of a small oblique B manipulates spins
sufficiently enough during transport.
9.4.3.2 GaAs-Based Spin LED
A spin LED is a LED configuration caped with a ferromagnetic spin injector
employed to polarize the spin prior to injection in the active medium of the LED
structure. Quite a lot of sophisticated semiconductor structures have been explored
after observing the fascinating phenomena that the spin information stored in the solid
state can successfully be converted to polarized light information. Most successful
one is a n–i–p heterostructure that includes heavily doped p-type and n-type regions
separated by a lightly doped ‘near’ intrinsic semiconductor region. It is feasible to
establish a confined potential such as QWs or QDs in the intrinsic region (Fig. 9.9).
The outline of fabrication technique is given in Fig. 9.10.
The p-doped region shows a doping gradient from the p+ substrate toward the
intrinsic active medium. While in the n-region, the layer in contact with the MTJ
spin injector is considerably doped with respect to the active medium. The doping
profile at the interface spin injector/semiconductor is to be tuned to adjust the Fermi
level pinning near the tunnel barrier region. The active medium consists of one or
several undoped quantum wells. InGaAs QWs may be a wise choice as absorption
of the emitted light can be avoided. Utmost attentions are to be given for selecting
the structural parameters.
9.4.3.3 Ge-Based Spin LED
To realize the room temperature, spin LED, investigations have been made for spin
injection in Ge. This type of spin-LED relies on a Fe/GeO 2 contact. However, a
strong magnetic field (around 4T) is required. The FM tunnel contacts expose spin
accumulation as given by three and four terminal device geometries (Fig. 9.11a, b).