# Silicon-Based Light Sources for Silicon Integrated Circuits

Authors: L. Pavesi
Journal: Advances in Optical Technologies
 (2008)

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## Abstract

Silicon the material per excellence for electronics is not used for sourcing light due to the lack of efficient light emitters and lasers. In this review, after having introduced the basics on lasing, I will discuss the physical reasons why silicon is not a laser material and the approaches to make it lasing. I will start with bulk silicon, then I will discuss silicon nanocrystals andEr3+ coupled silicon nanocrystals where significant advances have been done in the past and can be expected in the near future. I will conclude with an optimistic note on silicon lasing.


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## Body

## 1. Introduction

Recently, a large research effort has been
dedicated to the development of compact optoelectronic platforms. The
increasing interest in conjugating optical functionality to integrated circuits
stems not only from the potential of optics to overcome interconnect
bottlenecks imposed on electronic circuits by speed, power, and space
demands, but also from their flexibility for human
interface devices (e.g., displays and image recognition) and for a large set of
specific applications (including, e.g., biological sensors and compact optical
tomography apparatus). Optical signals are generally interesting in connection
to low power consumption, a key issue in the context of general trends such as
increased miniaturization and wireless and autonomous operation.From the industrial perspective, the preferred route
towards optoelectronic platforms is the upgrade of existing electronic
technology, in which Si has had no competitors since the 1960's. Compared to
Ge, Si has larger bandgap (1.12 eV), which allows higher operating temperature,
can be naturally structured with a companion insulator (SiO2), and is cheap and easily available. On the other hand, compound
semiconductor technologies, such as InP or GaAs, are not competitive with Si
essentially because of cost issues.While monolithic Si-compatible solutions have been
known since several years for many devices such as light detectors, waveguides,
and modulators, the lack of monolithic energy-efficient and cost-effective
CMOS-compatible light sources has hampered the development of optoelectronic
and photonic platforms. In this paper, we consider the state of the art of
Si-based light-emitting devices for Si integrated circuits. The reader is
encouraged to consult also the various review articles or books which have been
published on the topic of light emission in Si [1–8] as well as on hybrid
technology [9]. It is
worth to note that in this paper we will not discuss the hybrid approach based
on III–V semiconductors either bonded or heteroepitaxied on silicon, though
hybrid lasers have been integrated in silicon with very promising performances.


## 2. Basics of Light Amplification and Gain

A laser
requires three main components: an active material which is able to generate
and amplify light by stimulated emission of photons, an optical cavity which
provides the optical feedback to sustain the laser action, and a pumping
mechanism to supply energy to the active material. In the case of lasers based
on first-order optical processes, the pumping mechanism must be able to produce
a population inversion in the material. In an injection diode laser, the
pumping mechanism is provided by carrier injection via a p-n junction and the
optical feedback is usually provided by a Fabry-Perot cavity, [10, 11] although recently
whispering gallery resonators (microdisks) and photonic crystals are receiving
increasing attention.The light generation by electron-hole recombination in
semiconductors is quantified by the internal quantum efficiencyηint, which is the ratio between the number of generated photons and the
number of electron-hole pairs that recombine. This number is given by the ratio
of the electron-hole (e-h) radiative recombination probability over the total
e-h recombination probability, that is, by the fraction of all excited e-h
pairs that recombine radiatively. It is easy to demonstrate that ηint=τnr/(τnr+τr),
where τnr and τr are the nonradiative and radiative lifetimes, respectively. Thus, in
order to have a high ηint, either the radiative lifetime should be short (as in direct bandgap
semiconductors) or the nonradiative lifetime should be long (as in color center
systems).The property of amplifying light is given by the gain
spectrum of the material. For a bulk semiconductor, it is related to the joint
density of statesρ(ℏω),
the Fermi inversion factor fg(ℏω),
and the radiative lifetime:
(1)dΦ(ℏω)=drstim(ℏω)−drabs(ℏω)=λ28πτrρ(ℏω)fg(ℏω)Φ(ℏω)dz=g(ℏω)Φ(ℏω)dz,where g(ℏω) is the gain coefficient, dΦ is the change in the photon flux, drstim or drabs is the rate of stimulated emission or absorption at a given photon
energy ℏω,
respectively, fg(ℏω,EFe,EFh,T)=[fe(ℏω,EFe,T)−(1−fh(ℏω,EFh,T))], fe and fh are the thermal occupation functions for
electrons and holes, and Φ is the photon flux density. EFe and EFh are the quasi-Fermi
levels for electrons and holes, respectively. When
no external pumping is present, the Fermi inversion factor reduces to the
simple Fermi statistics for an empty conduction band and a filled valence band
(fg<0), and the gain coefficient reduces to the
absorption coefficient α.
When an external pump excites a large density of free carriers, the splitting
of the quasi-Fermi levels increases, and when EFe−EFh>ℏω the condition of population inversion is
satisfied and fg>0.    This means that (1) is positive and hence the system shows positive net optical gain (g>0). Note that in (1) a critical role is played by the
radiative lifetime: the shorter the lifetime, the stronger the gain.For an atomic system, the expression of the gain
coefficient reduces to(2)g(ℏω)=σem(ℏω)N2−σabs(ℏω)N1,where σem is the emission cross-section, σabs is the absorption cross-section, and N2 and N1 represent the density of active centers in the
excited and ground states, respectively. If σem=σabs, the condition to have positive optical gain is that N2>N1,
that is, the condition of population inversion.If a piece of active material of lengthL is used to
amplify light, one achieves light amplification whenever the material gain g is positive and larger than the propagation
losses αp of the light through the material, that is,  g>0 and g>αp.
If the system is forged as a waveguide of length L, and we call IT and I0 the intensity of the transmitted and the
incident beams, the amplification factor of the light is then(3)G=ITI0=exp[(Γg−αp)L]>1,where Γ is the optical confinement factor of the
optical mode in the active region.In a laser, optical feedback is usually provided by a
Fabry-Perot cavity so that the round-trip gain (the overall gain experienced by
a photon traveling back and forth across the cavity) can be larger than 1. This
condition is expressed by the relationG2R1R2>1,
where R1 and R2 are the back and front mirror reflectivities.


## 3. Limitation of Si for Light Emission and Amplification

Among the various semiconductor materials which have
been used to form LEDs and lasers, the absence of Si is striking. Let us review
why Si has not been used as a light-emitting material [4, 5, 7, 8]. Si is an indirect bandgap semiconductor (see Figure 1). As a consequence, the probability for a radiative recombination is low,
which in turn means that the e-h radiative lifetime is long (of the order of
some milliseconds). An e-h pair has to wait on average a few milliseconds to
recombine radiatively. During this time, both the electron and the hole move
around and cover a volume of the order of 10 μm3. If they encounter a defect or a trapping center, the carriers might
recombine nonradiatively. Typical nonradiative recombination lifetimes in Si
are of the order of some nanoseconds. Thus, in electronic grade Si, the
internal quantum efficiency ηint is about 10−6. This is the reason why Si is a poor luminescent material, that is,
the efficient nonradiative recombinations which rapidly deplete the excited
carriers. Many strategies have been researched over the years to overcome this
Si limitation, and some of which are based on the spatial confinement of the
carriers, and others on the introduction of impurities, the use of quantum
confinement, and the use of Si-Ge alloys or superlattices [4]. The fact that a slow emission, that is, a long
radiative lifetime, is also associated with low brightness of the source and
with the requirement of an external modulation scheme for high-speed data
transmission is to be noticed.Figure 1
Band structure
of bulk Si with the various possible transitions for an electron-hole pair:
radiative recombination, Auger recombination, and free-carrier absorption.In addition, two other phenomena limit the use of Si
for optical amplification (see Figure1). The first is a nonradiative
three-particle recombination mechanism where an excited electron (hole)
recombines with a hole (electron) by releasing the excess energy to another
electron (hole). This is called nonradiative Auger recombination mechanism
(Figure 1). This recombination mechanism is active as soon as more than one
carrier is excited. The probability of an Auger recombination is proportional
to the square of the number of excited carriers Δn and inversely proportional to the
bandgap energy [12].
For our discussion, this is a very relevant mechanism because the more excited
the semiconductor is, the more the Auger recombination is effective. The
probability for an Auger recombination in a bulk material is proportional to Δn2; we can thus write a nonradiative recombination lifetime due to Auger
as τA=1/CΔn2,
where C is a constant which depends on the doping of the material. For Si C∼10−30 cm6s−1 [7]. For Δn∼1019 cm3, τA =10 nanoseconds. The Auger recombination is
the dominant recombination mechanism for high carrier injection rate in Si.The second phenomenon is related to free-carrier
absorption (see Figure1). Excited carriers might absorb photons and thus
deplete the inverted population and, at the same time, increase the optical
losses suffered by the signal beam. The free-carrier absorption coefficient can
be empirically related to the Si free-carrier density nfc and to the light wavelength λ as αn∼10−18nfcλ2 at 300 K [7]. For nfc=1019 cm−3 and λ = 1.55 μm, αn = 24 cm−1. For heavily doped Si, these are the main limitations to lasing, while
for intrinsic Si this contribution can be exceedingly small unless nfc is very high as in a laser. In confined
systems, such as Si nanocrystals, this recombination mechanism is due to
confined carriers, and hence it is called confined carrier absorption.


## 4. Approaches to Si Light-Emitting Sources

In early 2000's, a series of papers appeared, which
questioned the common belief that Si cannot be used to form a laser [13–19]. In October 2004, the first
report on an Si laser appeared [20–22], while in February 2005 the first CW Raman laser integrated
in Si was reported [23–25]. Hybrid approaches became effective in 2006-2007
[9]. We summarize the
most relevant approaches towards light emission in Si in the following
list.
High-quality bulk Si inserted in a forward biased solar cell. This
approach has a demonstrated emission wavelength of 1.1 μm, and its system features include LED. The
advantages of such approach are demonstrated through a power efficiency of >1% at 200 K and through the highly efficient
electrical injection, and the disadvantages are demonstrated by a wavelength in
an unsuitable wavelength region. Evidence of optical gain has never been
reported in Si [15, 26].Stimulated Raman scattering in Si waveguides. The
demonstrated emission wavelength of such system is 1.6 μm, and its system features include CW
optically pumped Raman laser. Its advantages are demonstrated by the fact that
it is the only system where lasing has been clearly demonstrated in a cavity,
and the continuously tunable wavelength in near infrared. The disadvantage is
that no electrical injection is achievable according to fundamental mechanism
[24, 25].Nanopatterned Si. The
demonstrated emission wavelength of the system is 1.28 μm, and its system features include optically
pumped stimulated emission at cryogenic temperature. The advantages include
significant line narrowing and threshold behaviour at low pumping power, and
the disadvantages are manifested by a wavelength in an unsuitable wavelength
region, and the fact that the effect is demonstrated only at cryogenic
temperature and that electrical injection seems prohibitive [16].Dislocation loops formed by ion implantation in an Si p-n junction. The
demonstrated emission wavelength is 1.1 μm. The system features include LED with a
significant efficiency. The advantages are demonstrated through simple fabrication
method and efficient electrical injection, and the disadvantages through a
wavelength in an unsuitable wavelength region. The evidence of optical gain has
never been reported in Si [27].Si nanocrystals in dielectric (SiO2) matrix. The system's
demonstrated emission wavelength is 0.75 μm, and its system features include optical
gain at room temperature and efficient LED demonstrated in AC electrical
pumping regime. The advantages lie in the fact that it is CMOS-compatible and
easy to fabricate, as well as its demonstrated optical microcavity and
electrical injection scheme (AC). The disadvantages lay in
the fact that a wavelength does not match standard optical communication, and
that efficient bipolar electrical injection has not been yet achieved [13, 28].Er coupled to Si nanocrystals in a dielectric. The
demonstrated emission wavelength is 1.535 μm. The system features include internal gain
demonstrated in waveguides. The advantages lie in the fact that it is
CMOS-compatible and easy to fabricate, in addition to its demonstrated optical
cavity and wavelength being suitable for optical communication. The
disadvantages lie in the fact that overall gain has not been yet demonstrated
in waveguides, and that efficient electrical injection has not been yet
achieved [29].Strained germanium on Si. The system's
demonstrated emission wavelength is 1.55 μm. The system features are demonstrated by a
theory being able to predict high gain. The advantages lie in the fact that it
is easy to fabricate and has a wavelength suitable for optical communication.
The disadvantages are manifested by the fact that no experiments are in support
of the theory till now [30].On considering the CMOS compatibility, one should
mention that annealing procedure to nucleate the Si nanocrystals requires high
temperatures (usually in excess of1000°C). In a CMOS run, such high-temperature steps can only be introduced
at the beginning of the process. This implies that the standard CMOS steps are
to be performed after the nanocrystal fabrication. While this is an obviously
tight constraint, compatibility with standard CMOS is in principle possible
within this limitation. Otherwise one needs to consider other kinds of
processing such as layer bonding.


### 4.1. Bulk Si Light-Emitting Diodes

The common belief that bulk Si cannot be a
light-emitting material has been severely questioned in a series of recent
works. An Australian group noticed that top-quality solar cells are
characterized by extremely long carrier recombination lifetimes of the order of
some milliseconds. That is, the recombination lifetime is of the order of the
radiative lifetime; henceηint is of the order of 1. Then, if the solar cell is biased in the forward
regime instead of the usual reverse regime, the solar cell could behave as a
very efficient light-emitting diode [15, 26].Figure2 shows a schematic of the device and a room
temperature emission spectrum. To increase the light extraction efficiency, the
LED surface was texturized so that most of the internally generated light was
impinging on the external surface of the cell with an incident angle lower than
the critical angle for total internal refraction. Thus, the light extraction
efficiency was increased from a few % being typical
of a flat surface to almost 100% for the texturized LED. Finally, to
reduce free-carrier absorption to a minimum, the electrodes, that is, the
heavily doped regions, were confined in very thin and small lines. By using
these three practices, a plug-in efficiency (ratio of the optical power emitted
from the LED to the electrical driving power) larger than 1% at 200 K was
achieved. Most interestingly, the turn-on voltage of the device was the same as
the forward bias of the solar cell, that is, less than 1 V.Summary of the
results of the Australian group on a bulk Si LED. (a) Sketch of the LED
geometry. (b) Luminescence spectrum (red), absorption spectrum (green), power efficiency
versus injected electrical power density (blue), and I-V characteristics
(inset) at room temperature, adapted from [15, 26].
(a)(b)The same research group published also a theoretical
paper [31], which
questioned one common belief that indirect bandgap materials could not show
optical gain because of parasitic absorption processes due to free carriers
[32]. Indeed they
demonstrated that optical gain is theoretically possible, and pointed out that
the most suitable energy region is the sub-bandgap region where processes
involving photons could help in achieving gain.These theoretical arguments have been partially
confirmed in a recent study where stimulated emission has been observed (see
Figure3) [33]. As the
limit to efficient light generation in Si is the short nonradiative lifetime,
the idea was to avoid carrier diffusion and to spatially localize free carriers
in a small device region where nonradiative recombination centers can be easily
saturated.Figure 3
Optical power
versus injected current for an LED containing SiO2 nanoclusters in the junction region (inset). Also shown are a few
electroluminescence spectra for different injection rate (arrows), adapted from
[33].Carrier localization was achieved by spin-on doping of
small silica nanoparticles at the junction of a p-n diode (Figure3) [33]. The current-voltage I-V
characteristic of the diode shows rectifying behavior with a clear threshold in
the light-current L-I characteristic. A change from a broad emission spectrum
characteristic of band-to-band emission below threshold to sharp peaks due to
stimulated emission above threshold is observed too. Stimulated emission is
observed for a two-phonon indirect transition as it was theoretically
predicted. Furthermore, when the injection current significantly exceeds the
threshold, a single peak dominates. All these results are very encouraging
since the proposed systems have excellent
electrical qualities as they are p-n junctions. One puzzling question is about
the reproducibility of this work since no other papers were appearing after
this result. The crucial question about the nature of the stimulated emission
(bulk- or defects-related) is still unanswered.Recently, another report of stimulated emission in
bulk Si has appeared [16]. Nanopatterning of a thin Si on insulator layer
allows to have a large effective Si surface, where a sizable density of
A' centers could pile up. These defect centers are
believed to play the role of active optical centers which can be optically
inverted. Indeed, very convincing experimental data of gain in these
nanopatterned films have been reported. The major caveat is that the gain is vanishing
as the temperature is raised; sizable gain is observed only for temperatures
lower than 80 K.Strong efficiency improvement of electroluminescence
has been independently reported by two groups. They have used carrier
confinement in extremely thin Si layers (few nm). In one work [17], lateral p+-p-n+ junctions have been developed where the p-layer has been thinned down
to 5 nm. Electroluminescence peaked at 1130 nm due to carrier confinement in
the thin p-region, and an efficiency of 1.4×10−4 was reported (0.1 μW optical power for mA injection current). In
another approach [18, 19],
a light-emitting field-effect transistor with lateral p-n junction was used to
inject carrier into an active layer made by ultrathin intrinsic Si layer,
thinned down using LOCOS processing. Performances similar to the one shown in
[17] have been
achieved.


### 4.2. Optical Gain in Si Nanocrystals

Another interesting approach to form light emitters
and amplifiers in Si is to use small Si nanoclusters (Si-nc) dispersed in a
dielectric matrix, most frequently SiO2
[4]. With
this approach, one maximizes carrier confinement, improves the radiative
probability by quantum confinement, shifts the emission wavelength to visible
and controls the emission wavelength by Si-nc dimension, decreases the confined
carrier absorption due to the decreased emission wavelength, and increases the
light extraction efficiency by reducing the dielectric mismatch between the
source materials and the air. Various techniques are used to form Si-nc whose size
can be tailored in the few nm range (Figure 4).Si
nanocrystals formation, structure, and luminescence spectrum.
(a)(b)Starting with an Si rich oxide, which can be formed by
deposition, sputtering, ion implantation, cluster evaporation, and so on, a
partial phase separation is induced by thermal annealing. The duration of the
thermal treatment, the annealing temperature, the starting excess Si content
are all determining the final sizes of the clusters, their dispersion in size
which can be significant, and the Si-nc crystalline nature. The size dispersion
is usually claimed as the source of the broad emission lineshape that at room
temperature is typical of the Si-nc emission spectra. In addition to size
dispersion, both size-selected deposition [34] and single Si-nc
luminescence experiments [35] demonstrate that Si-nc
emission is intrinsically broad due to the indirect nature of the emission. The
active role of the interface region in determining the optical properties of
Si-nc has been highlighted in a joint theoretical and experimental paper
[36]. The origin of
the luminescence in Si-nc is still unclear; many authors believe that it comes
from confined exciton recombination in the Si-nc [37], while others support a
defect-assisted recombination mechanism where luminescence is due to
recombination of carriers trapped at radiative recombination centers which form
at the interface between Si-nc and the dielectric [38] or even in the dielectric
[39]. One candidate
for these centers is the silanone bond which is formed by double Si–O bonds
[40]. The most
probable nature of the luminescence in Si-nc is a mechanism which involves both
recombination paths: excitons at about 800 nm and trapped carriers on radiative
interface state, which form in small sized nanocrystals, at about 700 nm.
Indeed, passivation experiments show that the intensity and lineshape of the
emission can be influenced by exposition to hydrogen gas or by further
oxidation [41].A number of papers reported observation of optical gain
in these systems [13, 42–49]. The observations of gain
by several different groups and on several differently prepared materials make
the observation solid. Figure 5 reports a summary of the most relevant data taken
on Si-nc formed by plasma-enhanced chemical vapor deposition (PECVD) [44, 45, 47]. Two techniques are reported here: the variable
stripe length method (VSL) which is sketched in the inset of Figure 5
and is
based on the one-dimensional amplifier model [45], and the pump-probe
technique which is based on the probe amplification in presence of a
high-energy and high-intensity pump beam [47]. In the VSL method, by varying the extent of the
pumped region (whose length is z), one measures the amplified spontaneous
emission (IASE) signal coming out from an edge of a waveguide
whose core is rich in Si-nc:(4)IASE(z)=Jsp(Ω)gmod(egmodz−1),where Jsp(Ω) is the spontaneous emission intensity emitted
within the solid angle Ω,
and gmod is the net modal gain of the material,
defined as gmod=Γgm−α.Summary of
various experimental proofs of gain in Si-nc. (a) Geometry used to measure the amplified spontaneous emission
(ASE); (b) ASE versus the pumping length
for two pumping powers; (c) ASE time decay
for the various pumping conditions indicated in the inset (L is the pumping
length); (d) luminescence, absorption,
and gain spectra at room temperature
for an Si-nc rich waveguide; (e) transmission spectra for various
pumping powers (the inset shows the experimental geometry used). Data have been
redrawn from [44, 45, 47].
(a)(b)(c)(d)(e)Data reported in Figure5 show that the ASE intensity
increases sublinearly with the pumping length when the pumping power is lower
than a threshold. For pumping power higher than threshold, the ASE signal
increases more than exponentially. This is a consequence of the pump-induced
switching from absorption (gmod<0) to gain (gmod>0).In addition, if time-resolved measurements are
performed (Figure5(c)), [45] the ASE decay lineshape
shows two time regimes: a fast decay within the first nanosecond, and a slow
time decay with typical time constant of few microseconds. It is well known
that Si-nc has time decay constant of some microseconds, so the appearance of a
nanosecond time decay is at first surprising. What is important is the fact
that the fast decay appears only if the pumping power and the excitation volume
are both large. If one decreases the excitation volume at high power or the
pumping power at large excitation volume, the fast decay disappears. This can
be understood if the fast decay is due to stimulated emissions. In fact, at
high pumping rate, three competitive paths open: stimulated emission, Auger
recombination, and confined carrier absorption. All of these could be the cause
of the fast decay. In particular, the Auger lifetime τA and the confined carrier absorption lifetime τCC can be modeled in an Si-nc by(5)τA=1CANex,τCC=12CCCNex,where CA and CCC are coefficients, and Nex is the density of excited recombination
centers. Nex is directly proportional to the pumping power
and not to the pumping volume. Thus, by decreasing the pumping length, the ASE
lineshape should be unchanged. On the other hand, by a simple rate equation
modeling [50], the
stimulated emission lifetime τse turns out to be(6)τse=43π(RNS)31ξσgcnph,where RNS is the average radius of the Si-nc, ξ is their packing density, σg is the gain cross-section, and nph is the photon flux density. Note that τse depends not only on the material properties (RNS, ξ, σg) but also on the photon flux density nph which exists in the waveguide. Also, nph depends in turn on the waveguide losses, the
Si-nc quantum efficiency, and the pumping rates. In addition, if the sample
shows gain, by increasing the excitation volume, nph exponentially increases; that is, τse decreases. τse shortens when either the pumping length or the
pumping power increases as both increase nph.
It is important to note that calculations show that the Auger lifetime in Si-nc
is in the interval of 0.1–10 nanoseconds
[51], which means that
Auger recombination is a strong competitive process
which should be always considered. In some Si-nc systems, due to either
material problems or poor waveguide properties or even both, Auger
recombination and confined carrier absorption might
prevail, and no optical gain could be observed.Figure5(d) shows
a summary of the wavelength dependence of the luminescence, absorption, and
gain spectra in a sample with 4 nm Si-nc [44]. It is seen that the gain spectrum is on the
high-energy side of the emission band and that absorption is negligible in the
region of gain and luminescence. These facts suggest a four-level model to
explain the gain where the levels can be associated with both different Si-nc
populations or with a radiative state associated with an Si=O double bond for
which optical excitation causes a large lattice relaxation of the Si=O bond
[52, 53] as in the silanone
molecule. A recent paper shows that oxide is needed to observe gain [54]. Indeed, Si nanocrystals
formed in Si nitride do not show gain, while Si nanocrystals formed in Si oxide
do show gain.Pump-probe measurements were attempted with
contradictory results [47, 55]. Our group was able to show probe amplification under
pumping conditions (see Figure 5(e)) [47], while another group reported pump-induced absorption
probably associated with confined carrier absorption [55]. Literature results show that the confined carrier
absorption cross-section σfc in Si-nc is at least one order of magnitude reduced with respect to
bulk Si [56]: σfc≈10−18 cm2 at 1.55 μm in P-doped Si-nc. This cross-section should
be further reduced at 700 nm due to the λ2 dependence of the confined carrier absorption.
Transmission measurements of a probe beam through an Si-nc slab deposited on a
quartz substrate show the typical interference fringes due to multiple
reflection at the slab interfaces (Figure 5). When the pump power is raised, the
transmission is increased and, at the maximum power used, net probe
amplification with respect to the input probe intensity in air is observed in a
narrow wavelength interval. Note that the probe amplification spectrum overlaps
the fast luminescence spectrum measured by time-resolved technique. Based on
these results, design of optical cavity for an Si-nc laser has been published
[57].In addition, very favorable results have been
published with respect to Si-nc-based LED, where turn-on voltage as low as few
volts can be demonstrated by using thin Si-nc active layers [58]. Electroluminescence in
these LEDs was due to impact excitation of electron-hole pairs in the Si-nc.
Improvements in the electroluminescence efficiency have been achieved by using
Si nanocrystals dispersed within a polymer matrix [59].Another recent work reports on an FET structure where
the gate dielectric is rich in Si-nc [28].
In this way, by changing the sign of the gate bias, separate injection of
electrons and holes in the Si-nc is achieved. Luminescence is observed only
when both electrons and holes are injected into the Si-nc. By using this pulsing
bias technique, high efficiency in the emission of the LED is achieved due to
the copresence of electrons and holes. Channel optical waveguide with a core
layer rich in Si-nc shows optical losses of only a few dB/cm mainly due to
direct Si-nc absorption and to scattering caused by the composite nature of the
guiding medium [60].
All these different experiments have still to be merged into a laser cavity
structure to demonstrate an Si-nc-based laser.


### 4.3. Light Amplification in Er-Coupled Si Nanoclusters

The radiative transitions in the internal 4f shell of
erbium ions (Er3+) are exploited in the erbium-doped fiber amplifier (EDFA) [61], an all optical amplifier
which has revolutionized the optical communication technology. During the
nineties, several experimental efforts have been exerted in order to develop an
efficient and reliable light source by using Er3+ in Si [4].
The idea was to excite the Er3+, which emits 1.535 μm photons, by an energy transfer from the
electrically injected e-h pairs in a p-n Si diode. The most successful results
have been shown by the demonstration of room temperature emission with an
external quantum efficiency of 0.1% in an MHz modulated Er3+-doped Si LED [62]. The main problem associated with Er3+ in Si is the back transfer of energy from the Er3+ ions to the Si host, which causes a lowering of the emission
efficiency of the diode [63]. This is due to a resonant level which appears in the
Si bandgap due to the Er3+ doping and which couples with the Er3+ levels. In order to reduce this back-transfer process, it was proposed
to enlarge the bandgap of the Er3+ host so that the resonance between the defect level and the internal
Er3+ levels is lost [64]. Si-nc in an SiO2 dielectric were thus proposed as the host [65]. Indeed, it turns out that
Si-nc are very efficient sensitizers of the Er3+ luminescence with typical transfer efficiency as high as 70% and with
a typical transfer time of 1 microsecond [66]. In addition, the Er3+ are dispersed in SiO2, where they found their most favorable chemical environment. Quite
interestingly, the transfer efficiency gets maximized when the Si-nc are not
completely crystallized but still in the form of Si nanoclusters [67]. Some reports claim even
that the Er3+ can be excited through defects in the matrix [68]. Still under debate is the
number of Er ions that can be excited by a single Si-nc: a few or many ions.Figure6 summarizes the various mechanisms, and
defines the related cross-sections for this system. Excitation of Er3+ occurs via an energy transfer from photoexcited e-h pairs which are
excited in the Si-nc; the overall efficiency of light generation at 1.535 μm through direct absorption in the Si-nc is
described by an effective Er3+ excitation cross-section σexc. On the other hand, the direct absorption of the Er3+ ions, without the mediation of the Si-nc, and the emission from the Er
ions are described by absorption σabs and emission σem cross-sections, respectively. The typical radiative lifetime of Er3+ is of the order of 1 millisecond, with values as high as 7
milliseconds in carefully prepared samples, which is similar to the one of Er3+ in pure SiO2 [69]. Figure 7(a) reports the luminescence and absorption spectra measured in an Er3+-coupled Si-nc ridge waveguide at room temperature [70, 71].Figure 6
Diagram of the
excitation process ofEr3+ ions via an Si-nc, with the main related cross-sections. On the right,
the main internal energy levels of the Er3+ are shown.(a) Absorption
and luminescence spectra of anEr3+-coupled Si-nc waveguide. Adapted from [71]. (b) Signal
enhancement at 1.535 μm in an Er3+-coupled Si-nc waveguide versus the pumping power density by using top
pumping as shown in the inset. Adapted from [74].
(a)(b)Table1 summarizes the results for the various
cross-sections which are the results of an intensive study [72]. It is important to note
the five orders of magnitude increase in σexe and the fact that this value is conserved also when electrical
injection is used to excite the Si-nc [73]. If one places the Er3+ ions in an Si-nc ridge waveguide (see inset of Figure 7(b)), one can
perform experiments on signal amplification at 1.535 μm with the aim of demonstrating an Er-doped
waveguide amplifier (EDWA). The main advantage of an EDWA with respect to an
EDFA is the reduced size, the decreased pump power to achieve the same gain,
and the wide spectrum range to optically pump the system. A few groups have
performed such an experiment [70, 71, 74–76].Table 1
Summary of the various cross-sections related toEr3+ coupled to Si-nc. These values are the results of the LANCER
collaboration [71].
Erbium absorption cross-section at 1530 nmσabs = 6.6 × 10-21 cm2Erbium emission cross-section at 1530 nmσem = 5.7 × 10-21 cm2Erbium effective cross-section at 470 nmσexc = 2 × 10-16 cm2Erbium metastable level lifetime3 msErbium decay time4I11/2 to 4I13/2 transition2.38μsUpconversion coefficientCup for NEr<3×1020cm−37×10−17cm3/sSi-nc absorption cross-section at 470 nmσSi = 2 × 10-16 cm2Si-nc exciton lifetimeτab50μsTransfer coefficientCb1=2×10−15cm3/sThe most successful result was reported in [76] (see Figure 7(b)). In this
work, a very low Si-nc concentration has been used, and an internal gain of 7
dB/cm has been deduced. A successful experiment of pumping the EDWA with LED
was also reported [76]. In other experiments, with a large Si-nc concentration,
no or weak signal enhancement has been observed [70, 71, 75]. The reason is attributed
to the presence of a strong confined carrier absorption which introduces a loss
mechanism at the signal wavelength, and prevents the sensitizing action of the
Si-nc. Indeed, the energy transfer is in competition with confined carrier
absorption at the signal wavelength (see Figure 6). A confined carrier
cross-section of 10−18cm2 is usually assumed [66]. Propagation losses, saturation of Er3+ excitation, upconversion, and confined carrier absorption make the
proper design of EDWA difficult, where optical amplification can be observed.
The most relevant problem in the realization of an amplifier is related to the
fact that the coupling of Er with Si-nc is not complete. Indeed, most of the
reported works show that only up to 5% of the Er ions are coupled to the Si-nc, while
the other can be excited only through direct Er absorption [72]. This is the main problem
limiting the achievement of a net overall gain in the waveguide amplifier. In
addition, a flux-dependent effective excitation cross-section has been
demonstrated due to the distance-dependent coupling coefficient. Higher flux
yields lower excitation cross-section due to the saturation of the strongly
coupled ions. Thus, the main problem to obtain overall gain with this system
concerns the nanoengineering of the material composition, allowing the
production of materials with a high density of small sized Si-nc coupled
effectively to all the Er ions in the system. The small size is needed to
minimize the confined carrier absorption, while the high density is necessary
to increase the coupling with the Er ions [72]. Having obtained internal gain, electrically injected
LED [63, 77], and optical cavities [78], a laser
which uses the Er3+-coupled Si-nc system as active material seems feasible. With this
respect, it is worth noticing that toroidal microcavities formed in silica
doped with Er3+ have demonstrated optically pumped lasing at room temperature
[79].


## 4.1. Bulk Si Light-Emitting Diodes

The common belief that bulk Si cannot be a
light-emitting material has been severely questioned in a series of recent
works. An Australian group noticed that top-quality solar cells are
characterized by extremely long carrier recombination lifetimes of the order of
some milliseconds. That is, the recombination lifetime is of the order of the
radiative lifetime; henceηint is of the order of 1. Then, if the solar cell is biased in the forward
regime instead of the usual reverse regime, the solar cell could behave as a
very efficient light-emitting diode [15, 26].Figure2 shows a schematic of the device and a room
temperature emission spectrum. To increase the light extraction efficiency, the
LED surface was texturized so that most of the internally generated light was
impinging on the external surface of the cell with an incident angle lower than
the critical angle for total internal refraction. Thus, the light extraction
efficiency was increased from a few % being typical
of a flat surface to almost 100% for the texturized LED. Finally, to
reduce free-carrier absorption to a minimum, the electrodes, that is, the
heavily doped regions, were confined in very thin and small lines. By using
these three practices, a plug-in efficiency (ratio of the optical power emitted
from the LED to the electrical driving power) larger than 1% at 200 K was
achieved. Most interestingly, the turn-on voltage of the device was the same as
the forward bias of the solar cell, that is, less than 1 V.Summary of the
results of the Australian group on a bulk Si LED. (a) Sketch of the LED
geometry. (b) Luminescence spectrum (red), absorption spectrum (green), power efficiency
versus injected electrical power density (blue), and I-V characteristics
(inset) at room temperature, adapted from [15, 26].
(a)(b)The same research group published also a theoretical
paper [31], which
questioned one common belief that indirect bandgap materials could not show
optical gain because of parasitic absorption processes due to free carriers
[32]. Indeed they
demonstrated that optical gain is theoretically possible, and pointed out that
the most suitable energy region is the sub-bandgap region where processes
involving photons could help in achieving gain.These theoretical arguments have been partially
confirmed in a recent study where stimulated emission has been observed (see
Figure3) [33]. As the
limit to efficient light generation in Si is the short nonradiative lifetime,
the idea was to avoid carrier diffusion and to spatially localize free carriers
in a small device region where nonradiative recombination centers can be easily
saturated.Figure 3
Optical power
versus injected current for an LED containing SiO2 nanoclusters in the junction region (inset). Also shown are a few
electroluminescence spectra for different injection rate (arrows), adapted from
[33].Carrier localization was achieved by spin-on doping of
small silica nanoparticles at the junction of a p-n diode (Figure3) [33]. The current-voltage I-V
characteristic of the diode shows rectifying behavior with a clear threshold in
the light-current L-I characteristic. A change from a broad emission spectrum
characteristic of band-to-band emission below threshold to sharp peaks due to
stimulated emission above threshold is observed too. Stimulated emission is
observed for a two-phonon indirect transition as it was theoretically
predicted. Furthermore, when the injection current significantly exceeds the
threshold, a single peak dominates. All these results are very encouraging
since the proposed systems have excellent
electrical qualities as they are p-n junctions. One puzzling question is about
the reproducibility of this work since no other papers were appearing after
this result. The crucial question about the nature of the stimulated emission
(bulk- or defects-related) is still unanswered.Recently, another report of stimulated emission in
bulk Si has appeared [16]. Nanopatterning of a thin Si on insulator layer
allows to have a large effective Si surface, where a sizable density of
A' centers could pile up. These defect centers are
believed to play the role of active optical centers which can be optically
inverted. Indeed, very convincing experimental data of gain in these
nanopatterned films have been reported. The major caveat is that the gain is vanishing
as the temperature is raised; sizable gain is observed only for temperatures
lower than 80 K.Strong efficiency improvement of electroluminescence
has been independently reported by two groups. They have used carrier
confinement in extremely thin Si layers (few nm). In one work [17], lateral p+-p-n+ junctions have been developed where the p-layer has been thinned down
to 5 nm. Electroluminescence peaked at 1130 nm due to carrier confinement in
the thin p-region, and an efficiency of 1.4×10−4 was reported (0.1 μW optical power for mA injection current). In
another approach [18, 19],
a light-emitting field-effect transistor with lateral p-n junction was used to
inject carrier into an active layer made by ultrathin intrinsic Si layer,
thinned down using LOCOS processing. Performances similar to the one shown in
[17] have been
achieved.


## 4.2. Optical Gain in Si Nanocrystals

Another interesting approach to form light emitters
and amplifiers in Si is to use small Si nanoclusters (Si-nc) dispersed in a
dielectric matrix, most frequently SiO2
[4]. With
this approach, one maximizes carrier confinement, improves the radiative
probability by quantum confinement, shifts the emission wavelength to visible
and controls the emission wavelength by Si-nc dimension, decreases the confined
carrier absorption due to the decreased emission wavelength, and increases the
light extraction efficiency by reducing the dielectric mismatch between the
source materials and the air. Various techniques are used to form Si-nc whose size
can be tailored in the few nm range (Figure 4).Si
nanocrystals formation, structure, and luminescence spectrum.
(a)(b)Starting with an Si rich oxide, which can be formed by
deposition, sputtering, ion implantation, cluster evaporation, and so on, a
partial phase separation is induced by thermal annealing. The duration of the
thermal treatment, the annealing temperature, the starting excess Si content
are all determining the final sizes of the clusters, their dispersion in size
which can be significant, and the Si-nc crystalline nature. The size dispersion
is usually claimed as the source of the broad emission lineshape that at room
temperature is typical of the Si-nc emission spectra. In addition to size
dispersion, both size-selected deposition [34] and single Si-nc
luminescence experiments [35] demonstrate that Si-nc
emission is intrinsically broad due to the indirect nature of the emission. The
active role of the interface region in determining the optical properties of
Si-nc has been highlighted in a joint theoretical and experimental paper
[36]. The origin of
the luminescence in Si-nc is still unclear; many authors believe that it comes
from confined exciton recombination in the Si-nc [37], while others support a
defect-assisted recombination mechanism where luminescence is due to
recombination of carriers trapped at radiative recombination centers which form
at the interface between Si-nc and the dielectric [38] or even in the dielectric
[39]. One candidate
for these centers is the silanone bond which is formed by double Si–O bonds
[40]. The most
probable nature of the luminescence in Si-nc is a mechanism which involves both
recombination paths: excitons at about 800 nm and trapped carriers on radiative
interface state, which form in small sized nanocrystals, at about 700 nm.
Indeed, passivation experiments show that the intensity and lineshape of the
emission can be influenced by exposition to hydrogen gas or by further
oxidation [41].A number of papers reported observation of optical gain
in these systems [13, 42–49]. The observations of gain
by several different groups and on several differently prepared materials make
the observation solid. Figure 5 reports a summary of the most relevant data taken
on Si-nc formed by plasma-enhanced chemical vapor deposition (PECVD) [44, 45, 47]. Two techniques are reported here: the variable
stripe length method (VSL) which is sketched in the inset of Figure 5
and is
based on the one-dimensional amplifier model [45], and the pump-probe
technique which is based on the probe amplification in presence of a
high-energy and high-intensity pump beam [47]. In the VSL method, by varying the extent of the
pumped region (whose length is z), one measures the amplified spontaneous
emission (IASE) signal coming out from an edge of a waveguide
whose core is rich in Si-nc:(4)IASE(z)=Jsp(Ω)gmod(egmodz−1),where Jsp(Ω) is the spontaneous emission intensity emitted
within the solid angle Ω,
and gmod is the net modal gain of the material,
defined as gmod=Γgm−α.Summary of
various experimental proofs of gain in Si-nc. (a) Geometry used to measure the amplified spontaneous emission
(ASE); (b) ASE versus the pumping length
for two pumping powers; (c) ASE time decay
for the various pumping conditions indicated in the inset (L is the pumping
length); (d) luminescence, absorption,
and gain spectra at room temperature
for an Si-nc rich waveguide; (e) transmission spectra for various
pumping powers (the inset shows the experimental geometry used). Data have been
redrawn from [44, 45, 47].
(a)(b)(c)(d)(e)Data reported in Figure5 show that the ASE intensity
increases sublinearly with the pumping length when the pumping power is lower
than a threshold. For pumping power higher than threshold, the ASE signal
increases more than exponentially. This is a consequence of the pump-induced
switching from absorption (gmod<0) to gain (gmod>0).In addition, if time-resolved measurements are
performed (Figure5(c)), [45] the ASE decay lineshape
shows two time regimes: a fast decay within the first nanosecond, and a slow
time decay with typical time constant of few microseconds. It is well known
that Si-nc has time decay constant of some microseconds, so the appearance of a
nanosecond time decay is at first surprising. What is important is the fact
that the fast decay appears only if the pumping power and the excitation volume
are both large. If one decreases the excitation volume at high power or the
pumping power at large excitation volume, the fast decay disappears. This can
be understood if the fast decay is due to stimulated emissions. In fact, at
high pumping rate, three competitive paths open: stimulated emission, Auger
recombination, and confined carrier absorption. All of these could be the cause
of the fast decay. In particular, the Auger lifetime τA and the confined carrier absorption lifetime τCC can be modeled in an Si-nc by(5)τA=1CANex,τCC=12CCCNex,where CA and CCC are coefficients, and Nex is the density of excited recombination
centers. Nex is directly proportional to the pumping power
and not to the pumping volume. Thus, by decreasing the pumping length, the ASE
lineshape should be unchanged. On the other hand, by a simple rate equation
modeling [50], the
stimulated emission lifetime τse turns out to be(6)τse=43π(RNS)31ξσgcnph,where RNS is the average radius of the Si-nc, ξ is their packing density, σg is the gain cross-section, and nph is the photon flux density. Note that τse depends not only on the material properties (RNS, ξ, σg) but also on the photon flux density nph which exists in the waveguide. Also, nph depends in turn on the waveguide losses, the
Si-nc quantum efficiency, and the pumping rates. In addition, if the sample
shows gain, by increasing the excitation volume, nph exponentially increases; that is, τse decreases. τse shortens when either the pumping length or the
pumping power increases as both increase nph.
It is important to note that calculations show that the Auger lifetime in Si-nc
is in the interval of 0.1–10 nanoseconds
[51], which means that
Auger recombination is a strong competitive process
which should be always considered. In some Si-nc systems, due to either
material problems or poor waveguide properties or even both, Auger
recombination and confined carrier absorption might
prevail, and no optical gain could be observed.Figure5(d) shows
a summary of the wavelength dependence of the luminescence, absorption, and
gain spectra in a sample with 4 nm Si-nc [44]. It is seen that the gain spectrum is on the
high-energy side of the emission band and that absorption is negligible in the
region of gain and luminescence. These facts suggest a four-level model to
explain the gain where the levels can be associated with both different Si-nc
populations or with a radiative state associated with an Si=O double bond for
which optical excitation causes a large lattice relaxation of the Si=O bond
[52, 53] as in the silanone
molecule. A recent paper shows that oxide is needed to observe gain [54]. Indeed, Si nanocrystals
formed in Si nitride do not show gain, while Si nanocrystals formed in Si oxide
do show gain.Pump-probe measurements were attempted with
contradictory results [47, 55]. Our group was able to show probe amplification under
pumping conditions (see Figure 5(e)) [47], while another group reported pump-induced absorption
probably associated with confined carrier absorption [55]. Literature results show that the confined carrier
absorption cross-section σfc in Si-nc is at least one order of magnitude reduced with respect to
bulk Si [56]: σfc≈10−18 cm2 at 1.55 μm in P-doped Si-nc. This cross-section should
be further reduced at 700 nm due to the λ2 dependence of the confined carrier absorption.
Transmission measurements of a probe beam through an Si-nc slab deposited on a
quartz substrate show the typical interference fringes due to multiple
reflection at the slab interfaces (Figure 5). When the pump power is raised, the
transmission is increased and, at the maximum power used, net probe
amplification with respect to the input probe intensity in air is observed in a
narrow wavelength interval. Note that the probe amplification spectrum overlaps
the fast luminescence spectrum measured by time-resolved technique. Based on
these results, design of optical cavity for an Si-nc laser has been published
[57].In addition, very favorable results have been
published with respect to Si-nc-based LED, where turn-on voltage as low as few
volts can be demonstrated by using thin Si-nc active layers [58]. Electroluminescence in
these LEDs was due to impact excitation of electron-hole pairs in the Si-nc.
Improvements in the electroluminescence efficiency have been achieved by using
Si nanocrystals dispersed within a polymer matrix [59].Another recent work reports on an FET structure where
the gate dielectric is rich in Si-nc [28].
In this way, by changing the sign of the gate bias, separate injection of
electrons and holes in the Si-nc is achieved. Luminescence is observed only
when both electrons and holes are injected into the Si-nc. By using this pulsing
bias technique, high efficiency in the emission of the LED is achieved due to
the copresence of electrons and holes. Channel optical waveguide with a core
layer rich in Si-nc shows optical losses of only a few dB/cm mainly due to
direct Si-nc absorption and to scattering caused by the composite nature of the
guiding medium [60].
All these different experiments have still to be merged into a laser cavity
structure to demonstrate an Si-nc-based laser.


## 4.3. Light Amplification in Er-Coupled Si Nanoclusters

The radiative transitions in the internal 4f shell of
erbium ions (Er3+) are exploited in the erbium-doped fiber amplifier (EDFA) [61], an all optical amplifier
which has revolutionized the optical communication technology. During the
nineties, several experimental efforts have been exerted in order to develop an
efficient and reliable light source by using Er3+ in Si [4].
The idea was to excite the Er3+, which emits 1.535 μm photons, by an energy transfer from the
electrically injected e-h pairs in a p-n Si diode. The most successful results
have been shown by the demonstration of room temperature emission with an
external quantum efficiency of 0.1% in an MHz modulated Er3+-doped Si LED [62]. The main problem associated with Er3+ in Si is the back transfer of energy from the Er3+ ions to the Si host, which causes a lowering of the emission
efficiency of the diode [63]. This is due to a resonant level which appears in the
Si bandgap due to the Er3+ doping and which couples with the Er3+ levels. In order to reduce this back-transfer process, it was proposed
to enlarge the bandgap of the Er3+ host so that the resonance between the defect level and the internal
Er3+ levels is lost [64]. Si-nc in an SiO2 dielectric were thus proposed as the host [65]. Indeed, it turns out that
Si-nc are very efficient sensitizers of the Er3+ luminescence with typical transfer efficiency as high as 70% and with
a typical transfer time of 1 microsecond [66]. In addition, the Er3+ are dispersed in SiO2, where they found their most favorable chemical environment. Quite
interestingly, the transfer efficiency gets maximized when the Si-nc are not
completely crystallized but still in the form of Si nanoclusters [67]. Some reports claim even
that the Er3+ can be excited through defects in the matrix [68]. Still under debate is the
number of Er ions that can be excited by a single Si-nc: a few or many ions.Figure6 summarizes the various mechanisms, and
defines the related cross-sections for this system. Excitation of Er3+ occurs via an energy transfer from photoexcited e-h pairs which are
excited in the Si-nc; the overall efficiency of light generation at 1.535 μm through direct absorption in the Si-nc is
described by an effective Er3+ excitation cross-section σexc. On the other hand, the direct absorption of the Er3+ ions, without the mediation of the Si-nc, and the emission from the Er
ions are described by absorption σabs and emission σem cross-sections, respectively. The typical radiative lifetime of Er3+ is of the order of 1 millisecond, with values as high as 7
milliseconds in carefully prepared samples, which is similar to the one of Er3+ in pure SiO2 [69]. Figure 7(a) reports the luminescence and absorption spectra measured in an Er3+-coupled Si-nc ridge waveguide at room temperature [70, 71].Figure 6
Diagram of the
excitation process ofEr3+ ions via an Si-nc, with the main related cross-sections. On the right,
the main internal energy levels of the Er3+ are shown.(a) Absorption
and luminescence spectra of anEr3+-coupled Si-nc waveguide. Adapted from [71]. (b) Signal
enhancement at 1.535 μm in an Er3+-coupled Si-nc waveguide versus the pumping power density by using top
pumping as shown in the inset. Adapted from [74].
(a)(b)Table1 summarizes the results for the various
cross-sections which are the results of an intensive study [72]. It is important to note
the five orders of magnitude increase in σexe and the fact that this value is conserved also when electrical
injection is used to excite the Si-nc [73]. If one places the Er3+ ions in an Si-nc ridge waveguide (see inset of Figure 7(b)), one can
perform experiments on signal amplification at 1.535 μm with the aim of demonstrating an Er-doped
waveguide amplifier (EDWA). The main advantage of an EDWA with respect to an
EDFA is the reduced size, the decreased pump power to achieve the same gain,
and the wide spectrum range to optically pump the system. A few groups have
performed such an experiment [70, 71, 74–76].Table 1
Summary of the various cross-sections related toEr3+ coupled to Si-nc. These values are the results of the LANCER
collaboration [71].
Erbium absorption cross-section at 1530 nmσabs = 6.6 × 10-21 cm2Erbium emission cross-section at 1530 nmσem = 5.7 × 10-21 cm2Erbium effective cross-section at 470 nmσexc = 2 × 10-16 cm2Erbium metastable level lifetime3 msErbium decay time4I11/2 to 4I13/2 transition2.38μsUpconversion coefficientCup for NEr<3×1020cm−37×10−17cm3/sSi-nc absorption cross-section at 470 nmσSi = 2 × 10-16 cm2Si-nc exciton lifetimeτab50μsTransfer coefficientCb1=2×10−15cm3/sThe most successful result was reported in [76] (see Figure 7(b)). In this
work, a very low Si-nc concentration has been used, and an internal gain of 7
dB/cm has been deduced. A successful experiment of pumping the EDWA with LED
was also reported [76]. In other experiments, with a large Si-nc concentration,
no or weak signal enhancement has been observed [70, 71, 75]. The reason is attributed
to the presence of a strong confined carrier absorption which introduces a loss
mechanism at the signal wavelength, and prevents the sensitizing action of the
Si-nc. Indeed, the energy transfer is in competition with confined carrier
absorption at the signal wavelength (see Figure 6). A confined carrier
cross-section of 10−18cm2 is usually assumed [66]. Propagation losses, saturation of Er3+ excitation, upconversion, and confined carrier absorption make the
proper design of EDWA difficult, where optical amplification can be observed.
The most relevant problem in the realization of an amplifier is related to the
fact that the coupling of Er with Si-nc is not complete. Indeed, most of the
reported works show that only up to 5% of the Er ions are coupled to the Si-nc, while
the other can be excited only through direct Er absorption [72]. This is the main problem
limiting the achievement of a net overall gain in the waveguide amplifier. In
addition, a flux-dependent effective excitation cross-section has been
demonstrated due to the distance-dependent coupling coefficient. Higher flux
yields lower excitation cross-section due to the saturation of the strongly
coupled ions. Thus, the main problem to obtain overall gain with this system
concerns the nanoengineering of the material composition, allowing the
production of materials with a high density of small sized Si-nc coupled
effectively to all the Er ions in the system. The small size is needed to
minimize the confined carrier absorption, while the high density is necessary
to increase the coupling with the Er ions [72]. Having obtained internal gain, electrically injected
LED [63, 77], and optical cavities [78], a laser
which uses the Er3+-coupled Si-nc system as active material seems feasible. With this
respect, it is worth noticing that toroidal microcavities formed in silica
doped with Er3+ have demonstrated optically pumped lasing at room temperature
[79].


## 5. Conclusions

After seven years from the first observation of optical
gain in Si nanocrystals, a first Si laser has been demonstrated, though not
using Si-nc but the Raman effect. Various different approaches have been
however suggested to achieve electrically pumped lasing in Si. These cover a
wide spectral range from visible to infrared. These facts show that the
perspectives to achieve an injection laser in Si are nowadays more solid than
ever. We are quite optimistic that a laser will be realized in the near future
by using one of the various approaches presented here.



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*Source: 416926-2008-06-30.xml*