04-12-2012, 05:40 PM
Silicon Light Emitting Devices for Integrated Applications
[attachment=42517]
Abstract
Silicon crystallizes in the diamond structure, which consists of two
interpenetrating face-centered cubic lattices displaced from each other by one
quarter of the body diagonal. In semiconductors with zinc-blende structure
such as GaAs, the Ga and As atoms lie on individual sub-lattices, thus the
inversion symmetry of Si is lost in III-V binary compounds. This difference in
their crystal structures underlies the dissimilar electronic properties of Si and
GaAs.The energy band structure, in semiconductor in general, and Si particularly, is
derived from the relationship between energy and momentum of a carrier,
which not only depends on the crystal structure but also on the bonding
between atoms, their respective bond lengths, and the chemical species. The
band structure is often quite complex and can only be calculated theoretically.
Figure I.1-1 shows the dispersion relations for the energy E(k) of an electron or hole for wave vector length in the first Brillouin zone [1]. E(k) has maxima
or minima at the Brillouin zone center and zone boundary symmetry points,
but additional extremes may occur at other points in the Brillouin zone. The
valence band structure is much the same for many semiconductors and
exhibits a maximum at the Brillouin zone center or Γ point (i.e. at k=0). In
the case of Si the lowest point in the conduction band occurs away from high
symmetry points near the X point at the Brillouin zone boundary (along
<001>). The bandgap of Si is the difference between this point and the
valence maximum at point Γ. Because these two states have difference wave
vectors, Si is termed an indirect bandgap semiconductor.
Benefits of silicon optoelectronics
Photodetectors, waveguides, wavelength demultiplexers and modulators have
all been realized in silicon-based technology [5][6]. Solving the problem of
low efficiency of silicon-based efficient light emitters would greatly expand
the use of silicon as an optoelectronic material. Direct integration of
optoelectronic devices into microelectronic circuitry with single CMOS
processing would effectively reduce the manufacturing costs and increase
yields. That is because of the avoidance of expensive and exotic compound
semiconductor technologies. Furthermore, “If an all-silicon laser could be
created it would revolutionize the design of supercomputers and lead to new
types of optoelectronic devices…” [8].
Nanocrystal
Optical gain in silicon material, in nanocrystalline form, was reported for the
first time in [13], signaling the feasibility of a future true silicon laser [9]. The
densely packed nanocrystals of 3nm in size were embedded in thermal silicon
dioxide grown on silicon wafers by ion implantation and thermal annealing.
The nanocrystals formed a thin layer just below the surface of the matrix
material. The finding was explained by a population inversion of radiative
states associated with Si/SiO2 interface. The authors suggested that their gain
was a consequence of the high quality of the silicon nanocrystal-oxide
interface, which has many ‘surface states’ that emit light per nanocrystal.
Erbium doped [17]
Implantation of erbium in different matrix materials is the main fabrication
method for incorporating this rare earth element, which emits a standard
telecommunication wavelength at 1.54 μm by Er3+ intra-4f transitions. In
crystalline silicon, the equilibrium solid solubility of erbium is so low that
non-equilibrium concentration is necessary. EL devices were fabricated with
an emission peak at 1.535μm; in reverse bias breakdown, emission was far
more efficient than in forward bias. The mechanism of emission in reverse
bias was impact excitation of Er3+ by accelerated electrons across the
junction. Much more details on the electroluminescence of erbium-dope
silicon is found in [18], where it was concluded that optimization of the role
of the ligands, such as oxygen or fluorine, in the excitation and back transfer
processes, through band-gap states, crystal field engineering or coupling to
the lattice, would be the pathway to increase the quantum efficiency.
Solar cell technology
Making use of textured solar cell technology, a breakthrough in improving
efficiencies of silicon LEDs has been recently achieved by M. Green [20]. The
improved devices were diodes with a special arrangement of junction location
to reduce non-radiative recombination rate, and texturing the device top
surface with inverted pyramids, formed by anisotropic etching to expose
(111) crystallographic planes to increase the absorption of weakly absorbed
wavelengths. The latter is the major contribution to the improved
performance of the device. According to their report, the device is most
efficient at 200K but still gives approximately 1% power conversion efficiency
at room temperature. The emission peak follows the temperature dependence
of the silicon bandgap.
Summary
It goes without saying that a lot of progress has been achieved in the pursuit
for a true silicon light emitter. Many obstacles have been tackled, especially
the improvement of emission efficiency. Solutions to fiber optic silicon light
sources have also been dealt with, though a silicon-based display seems more
likely in the near future [16]. The orientation of the research ahead will
probably focus on a fast switching device so that high frequency modulation
is possible, in combination with higher efficiency; these favor a silicon laser.
[attachment=42517]
Abstract
Silicon crystallizes in the diamond structure, which consists of two
interpenetrating face-centered cubic lattices displaced from each other by one
quarter of the body diagonal. In semiconductors with zinc-blende structure
such as GaAs, the Ga and As atoms lie on individual sub-lattices, thus the
inversion symmetry of Si is lost in III-V binary compounds. This difference in
their crystal structures underlies the dissimilar electronic properties of Si and
GaAs.The energy band structure, in semiconductor in general, and Si particularly, is
derived from the relationship between energy and momentum of a carrier,
which not only depends on the crystal structure but also on the bonding
between atoms, their respective bond lengths, and the chemical species. The
band structure is often quite complex and can only be calculated theoretically.
Figure I.1-1 shows the dispersion relations for the energy E(k) of an electron or hole for wave vector length in the first Brillouin zone [1]. E(k) has maxima
or minima at the Brillouin zone center and zone boundary symmetry points,
but additional extremes may occur at other points in the Brillouin zone. The
valence band structure is much the same for many semiconductors and
exhibits a maximum at the Brillouin zone center or Γ point (i.e. at k=0). In
the case of Si the lowest point in the conduction band occurs away from high
symmetry points near the X point at the Brillouin zone boundary (along
<001>). The bandgap of Si is the difference between this point and the
valence maximum at point Γ. Because these two states have difference wave
vectors, Si is termed an indirect bandgap semiconductor.
Benefits of silicon optoelectronics
Photodetectors, waveguides, wavelength demultiplexers and modulators have
all been realized in silicon-based technology [5][6]. Solving the problem of
low efficiency of silicon-based efficient light emitters would greatly expand
the use of silicon as an optoelectronic material. Direct integration of
optoelectronic devices into microelectronic circuitry with single CMOS
processing would effectively reduce the manufacturing costs and increase
yields. That is because of the avoidance of expensive and exotic compound
semiconductor technologies. Furthermore, “If an all-silicon laser could be
created it would revolutionize the design of supercomputers and lead to new
types of optoelectronic devices…” [8].
Nanocrystal
Optical gain in silicon material, in nanocrystalline form, was reported for the
first time in [13], signaling the feasibility of a future true silicon laser [9]. The
densely packed nanocrystals of 3nm in size were embedded in thermal silicon
dioxide grown on silicon wafers by ion implantation and thermal annealing.
The nanocrystals formed a thin layer just below the surface of the matrix
material. The finding was explained by a population inversion of radiative
states associated with Si/SiO2 interface. The authors suggested that their gain
was a consequence of the high quality of the silicon nanocrystal-oxide
interface, which has many ‘surface states’ that emit light per nanocrystal.
Erbium doped [17]
Implantation of erbium in different matrix materials is the main fabrication
method for incorporating this rare earth element, which emits a standard
telecommunication wavelength at 1.54 μm by Er3+ intra-4f transitions. In
crystalline silicon, the equilibrium solid solubility of erbium is so low that
non-equilibrium concentration is necessary. EL devices were fabricated with
an emission peak at 1.535μm; in reverse bias breakdown, emission was far
more efficient than in forward bias. The mechanism of emission in reverse
bias was impact excitation of Er3+ by accelerated electrons across the
junction. Much more details on the electroluminescence of erbium-dope
silicon is found in [18], where it was concluded that optimization of the role
of the ligands, such as oxygen or fluorine, in the excitation and back transfer
processes, through band-gap states, crystal field engineering or coupling to
the lattice, would be the pathway to increase the quantum efficiency.
Solar cell technology
Making use of textured solar cell technology, a breakthrough in improving
efficiencies of silicon LEDs has been recently achieved by M. Green [20]. The
improved devices were diodes with a special arrangement of junction location
to reduce non-radiative recombination rate, and texturing the device top
surface with inverted pyramids, formed by anisotropic etching to expose
(111) crystallographic planes to increase the absorption of weakly absorbed
wavelengths. The latter is the major contribution to the improved
performance of the device. According to their report, the device is most
efficient at 200K but still gives approximately 1% power conversion efficiency
at room temperature. The emission peak follows the temperature dependence
of the silicon bandgap.
Summary
It goes without saying that a lot of progress has been achieved in the pursuit
for a true silicon light emitter. Many obstacles have been tackled, especially
the improvement of emission efficiency. Solutions to fiber optic silicon light
sources have also been dealt with, though a silicon-based display seems more
likely in the near future [16]. The orientation of the research ahead will
probably focus on a fast switching device so that high frequency modulation
is possible, in combination with higher efficiency; these favor a silicon laser.