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Advances in Optical Technologies

Volume 2008 (2008), Article ID 416926, 12 pages

http://dx.doi.org/10.1155/2008/416926

## Silicon-Based Light Sources for Silicon Integrated Circuits

Nanoscience Laboratory, Department of Physics, University of Trento, 38050 Povo, Trento, Italy

Received 15 January 2008; Accepted 30 April 2008

Academic Editor: D. Lockwood

Copyright © 2008 L. Pavesi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 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 and 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.

#### 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 (SiO_{2}), 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 efficiency , 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 , where and are the nonradiative and radiative lifetimes, respectively. Thus, in order to have a high , 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 states , the Fermi inversion factor , and the radiative lifetime: where is the gain coefficient, is the change in the photon flux, or is the rate of stimulated emission or absorption at a given photon energy , respectively, , and are the thermal occupation functions for electrons and holes, and is the photon flux density. and 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 , and the gain coefficient reduces to the absorption coefficient . When an external pump excites a large density of free carriers, the splitting of the quasi-Fermi levels increases, and when the condition of population inversion is satisfied and . This means that (1) is positive and hence the system shows positive net optical gain . 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 where is the emission cross-section, is the absorption cross-section, and and represent the density of active centers in the excited and ground states, respectively. If , the condition to have positive optical gain is that , that is, the condition of population inversion.

If a piece of active material of length is used to amplify light, one achieves light amplification whenever the material gain is positive and larger than the propagation losses of the light through the material, that is, and . If the system is forged as a waveguide of length , and we call and the intensity of the transmitted and the incident beams, the amplification factor of the light is thenwhere 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 relation , where and 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 . 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 is about . 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.

In addition, two other phenomena limit the use of Si
for optical amplification (see Figure 1). 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 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 ; we can thus write a nonradiative recombination lifetime due to Auger
as ,
where is a constant which depends on the doping of the material. For Si [7]. For cm^{3}, =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 Figure 1). 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 and to the light wavelength as at 300 K [7]. For and = 1.55 , = 24 . For heavily doped Si, these are the main limitations to lasing, while for intrinsic Si this contribution can be exceedingly small unless 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 , 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 , 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 , 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 m. 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 (SiO _{2}) matrix.* The system's
demonstrated emission wavelength is 0.75 m, 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 m. 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 m. 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 of ). 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; hence 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].

Figure 2 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.

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 Figure 3) [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.

Carrier localization was achieved by spin-on doping of small silica nanoparticles at the junction of a p-n diode (Figure 3) [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 was reported (0.1 W 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 SiO_{2}
[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).

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 ), one measures the amplified spontaneous
emission ( signal coming out from an edge of a waveguide
whose core is rich in Si-nc:where is the spontaneous emission intensity emitted
within the solid angle ,
and is the *net modal gain* of the material,
defined as .

Data reported in Figure 5 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 () to gain ().

In addition, if time-resolved measurements are performed (Figure 5(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 and the confined carrier absorption lifetime can be modeled in an Si-nc bywhere and are coefficients, and is the density of excited recombination centers. 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 turns out to bewhere is the average radius of the Si-nc, is their packing density, is the gain cross-section, and is the photon flux density. Note that depends not only on the material properties (, , ) but also on the photon flux density which exists in the waveguide. Also, 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, exponentially increases; that is, decreases. shortens when either the pumping length or the pumping power increases as both increase . 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.

Figure 5(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 in Si-nc is at least one order of magnitude reduced with respect to
bulk Si [56]: cm^{2} at 1.55 m in P-doped Si-nc. This cross-section should
be further reduced at 700 nm due to the 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 () 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 in Si [4].
The idea was to excite the , which emits 1.535 m 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 -doped Si LED [62]. The main problem associated with in Si is the back transfer of energy from the 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 doping and which couples with the levels. In order to reduce this back-transfer process, it was proposed
to enlarge the bandgap of the host so that the resonance between the defect level and the internal
levels is lost [64]. Si-nc in an SiO_{2} dielectric were thus proposed as the host [65]. Indeed, it turns out that
Si-nc are very efficient sensitizers of the luminescence with typical transfer efficiency as high as 70% and with
a typical transfer time of 1 microsecond [66]. In addition, the are dispersed in SiO_{2}, 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 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.

Figure 6 summarizes the various mechanisms, and
defines the related cross-sections for this system. Excitation of 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 m through direct absorption in the Si-nc is
described by an effective excitation cross-section . On the other hand, the direct absorption of the ions, without the mediation of the Si-nc, and the emission from the Er
ions are described by absorption and emission cross-sections, respectively. The typical radiative lifetime of 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 in pure SiO_{2} [69]. Figure 7(a) reports the luminescence and absorption spectra measured in an -coupled Si-nc ridge waveguide at room temperature [70, 71].

Table 1 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 and the fact that this value is conserved also when electrical injection is used to excite the Si-nc [73]. If one places the ions in an Si-nc ridge waveguide (see inset of Figure 7(b)), one can perform experiments on signal amplification at 1.535 m 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].

The 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 is usually assumed [66]. Propagation losses, saturation of 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 -coupled Si-nc system as active material seems feasible. With this respect, it is worth noticing that toroidal microcavities formed in silica doped with 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.

#### Acknowledgments

The collaboration with the MTLab of FBK-irst (Dr. Bellutti and Dr. Pucker) has allowed us to reach a detailed view of the field. The supports of EC (LANCER: FP6-033574; PHOLOGIC: FP6-017158), INTEL, and PAT are gratefully acknowledged.

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