Baixe recent progress in lasers on silicon e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! NATURE PHOTONICS | VOL 4 | AUGUST 2010 | www.nature.com/naturephotonics 511 T he photonics market today is shared by several materials sys- tems, including compound semiconductors (indium phosphide, InP, and gallium arsenide, GaAs), elementary semiconductors (silicon, Si, and germanium, Ge), silica and rare-earth-doped glasses (glass fi bre, for example) and polymers. Each system targets particular applications or components. Today, the use of Si photonics is dwarfed by compound semiconductors and Si microelectronics, mostly due to the problems associated with making Si a host material for effi cient light emission, and thus subsequently realizing a laser. Fift y years ago the birth of the laser started a scientifi c and technological revolu- tion. Two years later, diode lasers were demonstrated in group iii–v compound semiconductors, and this was around the same time that Si-based transistor radios achieved mass popularity. Since then many scientists and engineers have researched lasing on Si substrates1. Rapid advances in Si photonics over the past two decades have been driven not only by the need for more complex, higher functionality and lower cost photonics integrated circuits, but also by pin count and power limits for communications, as summarized in the International Technology Roadmap for Semiconductors (ITRS)2. Electronics giants such as Intel, IBM, Hewlett Packard, STMicroelectronics, IMEC and Alcatel-Th ales have teamed up with research institutes around the world with support from government, industry and academia to drive progress in Si photonics. Th e current momentum and potential for making a useful laser in or on Si are signifi cant. Fundamentals At the time of the demonstration of the fi rst laser fi ft y years ago, the fundamental hurdle to realizing stimulated emission in Si was understood: optical transitions must obey the laws of conservation of energy and momentum, but these conditions are not satisfi ed simultaneously in crystalline Si. In direct bandgap materials (GaAs and InP, for example) radiative recombination occurs rapidly and effi ciently via a simple two-particle process, as shown by the simpli- fi ed band diagram in Fig. 1 (left ). Direct bandgap materials have a structure in which the lowest energy points of both the conduction and valence bands line up vertically in the wave vector axis; that is, they share the same crystal momentum. Th is is the principal reason why GaAs-, InP- and GaN-based materials have been the dominant material systems for semiconductor diode lasers since their fi rst demonstration in 1962. Si, like Ge, is an indirect bandgap material, and is not naturally capable of accomplishing effi cient radiative recombination. Free electrons tend to reside in the X valley of the conduction band, which is not aligned with free holes in the valence band (Fig. 1, right). Th erefore if a recombination is to lead to emission of a photon, a third particle must be involved to carry away the excess momentum, Recent progress in lasers on silicon Di Liang* and John E. Bowers Silicon lasers have long been a goal for semiconductor scientists, and a number of important breakthroughs in the past decade have focused attention on silicon as a photonic platform. Here we review the most recent progress in this fi eld, including low- threshold silicon Raman lasers with racetrack ring resonator cavities, the fi rst germanium-on-silicon lasers operating at room temperature, and hybrid silicon microring and microdisk lasers. The fundamentals of carrier transition physics in crystalline silicon are discussed briefl y. The basics of several important approaches for creating lasers on silicon are explained, and the challenges and opportunities associated with these approaches are discussed. which results in slow optical transition rates. A major non-radiative process is Auger recombination, in which an electron (or hole) is excited to a higher energy level by absorbing the released energy from an electron–hole recombination. Th e Auger recombination rate increases with injected free-carrier density and is inversely pro- portional to the bandgap. Free-carrier absorption (FCA) represents another major non-radiative process wherein the free electrons in the conduction band can jump to higher energy levels by absorb- ing photons. In high-level carrier injection devices (lasers and amplifi ers, for example) or heavily doped layers, free-carrier loss is orders of magnitudes higher than the material gain1. For both Auger recombination and FCA, the electrons pumped to higher energy levels release their energy through phonons, rather than by emitting photons. Th ey also have much shorter lifetimes (τnonrad) than those of radiative processes (τrad) in Si, resulting in an extremely poor inter- nal quantum effi ciency ηi of light emission, which is defi ned as3 τnonrad τnonrad + τrad ηi = and is generally of the order of 10–6. Consequently, semiconductor laser research over the past fi ft y years has primarily focused on com- pound semiconductor substrates, but now there is intense interest in lasers on Si. Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA. *e-mail: dliang@ece.ucsb.edu E n e rg y X L Wave vector Electrons Holes hv InP Si Direct recombination Free-carrier absorption Auger recombination Phonon Indirect recombination Г Figure 1 | Energy band diagrams and major carrier transition processes in InP and silicon crystals. In a direct band structure (such as InP, left), electron–hole recombination almost always results in photon emission, whereas in an indirect band structure (such as Si, right), free-carrier absorption, Auger recombination and indirect recombination exist simultaneously, resulting in little photon emission. FOCUS | PROGRESS ARTICLE PUBLISHED ONLINE: 30 JULY 2010|DOI: 10.1038/NPHOTON.2010.167 nphoton_.2010.167_AUG10.indd 511 10.7.19 8:33:27 AM © 20 Macmillan Publishers Limited. All rights reserved10 512 NATURE PHOTONICS | VOL 4 | AUGUST 2010 | www.nature.com/naturephotonics Th e recent and widespread availability of nanotechnology has allowed the traditional phonon-selection rule in indirect band- gap materials to be relaxed by breaking the crystal-symmetry or by phonon localization through the creation of nanostructures in crystalline Si. Th e motivation is to achieve quantum confi nement of excitons in a nanometre-scale crystalline structure4. A number of groups have reported enhanced light-emitting effi ciency and opti- cal gain in low-dimensional (that is, of the order of the de Broglie wavelength) Si at low temperatures. Th ey include porous Si5–8, Si nanocrystals9–12, Si-on-insulator (SOI) superlattices13 and photonic- crystal-like nanopatterns14, and Si nanopillars15,16. However, achiev- ing room-temperature continuous-wave (CW) lasing based on these temperature-dominated processes remains a challenge3,17,18. Despite being fundamentally limited by an indirect bandgap and low mobility, Si exhibits a number of important properties that make it a good substrate, if not necessarily a good gain medium for diode lasers. First, Si wafers are incredibly pure and have low defect density. Second, state-of-the-art 32 nm complementary metal–oxide–semiconductor (CMOS) technology is suffi ciently advanced to fabricate virtually all Si photonic components, which are mostly still in the micrometre regime. Both factors allow for Si waveguides with propagation losses that are typically one order of magnitude lower than compound semiconductor waveguides. Furthermore, Si has a high thermal conductivity, which is a very useful characteristic for an active device substrate. SiO2, the high- quality native oxide of Si, serves as a protective layer and a naturally good optical waveguide cladding, owing to its large refractive index diff erence from Si (Δn ~ 2.1). Th is is one of the major advantages of Si over Ge and other semiconductors for use in integrated circuits. Further loss-reduction in Si waveguides by oxidation19 and hosting rare-earth doping in SiO2 brings additional benefi ts to passive Si lightwave circuits. Although low waveguide loss does not change the ultralow band-to-band radiative emission effi ciency in Si, it improves the effi ciency of Si lasers that rely on a nonlinear eff ect such as Raman scattering. Silicon Raman lasers Th e Raman eff ect refers to the inelastic scattering of a photon by an optical phonon. When incident light is absorbed by an atom or mol- ecule at a vibrational state, the system energy is raised to an inter- mediate higher state. In most cases, the energy quickly drops back to the original vibrational state by releasing a photon with the same frequency, which is known as Rayleigh scattering, and is analogous to elastic scattering. Yet it is also possible to observe very weak (approximately one in ten million photons) additional components with lower and higher frequencies than the incident light due to the absorption or emission of optical phonons, namely the Stokes and anti-Stokes transitions, respectively. If a scattering medium is irradiated with pump and signal beams simultaneously, the pump beam excites the constituent molecules or atoms to a higher vibrational level, while the signal beam, which has a frequency resonant at the Stokes transition, triggers the gen- eration of another Raman Stokes photon. Th us, amplifi cation can be achieved through stimulation of the Stokes transition. Th is tech- nique is known as stimulated Raman scattering, and has enabled the realization of Raman glass fi bre amplifi ers with gain band- widths of over 100 nm. Th e Raman gain coeffi cient in Si is around fi ve orders of magnitude larger than that in amorphous glass fi bres because of the well-organized single-crystal structure20. However, Si waveguide loss is also several orders of magnitude higher than in glass fi bre, making fabrication of a low-loss Si waveguide one of the keys to realizing net Raman gain in Si. Furthermore, the tight optical confi nement in an SOI waveguide leads to an ultrasmall waveguide eff ective area, which in turn lowers the pump power threshold for stimulated Raman scattering. A pump with energy well below the Si bandgap is typically used to avoid elevating the electrons up to the conduction band and also to suppress FCA — both of which prevent lasing in Si. Initial studies demonstrated up to 0.25 dB of stimulated Raman gain for a Stokes signal at 1,542.3 nm for SOI waveguides, using a 1,427 nm pump laser with a CW power of 1.6 W (ref. 21). Such high pump powers, however, induce another optical loss mechanism — two-photon absorption (TPA). TPA is a nonlinear loss mechanism in which two photons combine their energies to boost an electron in the valence band to the conduction band. Free carriers further induce FCA and dump more optical power inside the cavity. TPA increases with the number of photons in a waveguide, and therefore becomes a limiting factor when using high optical pump powers. Th e fi rst demonstration of a pulsed Si Raman laser22 overcame TPA by using a long delay together with a short optical pulse, thus allowing the carriers generated dur- ing TPA to recombine prior to the next pass of the optical pulse. Following demonstrations used a p-i-n (p-type/intrinsic/n-type layers) structure in the waveguide to sweep free carriers away under 0 10 20 30 40 50 60 Pump power (mW) L a se r o u tp u t (m W ) Wavelength (nm) 8 0 d B p-region n-region V bias Directional coupler Iinc Ring cavity Laser output Pump Bus waveguide zI p (0)I p (L) Si substrate Al contact Buried oxide n-regionp-region Si rib waveguides Al contact SiO2 passivation 1 μm 25V 10V 5V 0V 0 100 200 300 1,544 1,545 1,546 1,547 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 10 R e la ti v e s p e c tr a l p o w e r (d B ) a b c d Figure 2 | Low-threshold Si Raman racetrack ring laser. a, Schematic of a device with a p-i-n junction design. Ip(0) and Ip(L) are the pump power at the starting point and after a round trip in the cavity, respectively. The light propagation direction is given by z. b, SEM cross-section of a directional coupler and p-i-n junction region. c, Laser output power against coupled input pump power, showing a higher output power achieved at a higher reverse bias on p-i-n junction for a 3 cm cavity. The error bars here are derived from diff erent measurement traces. d, High-resolution spectrum showing a low- threshold Si Raman racetrack ring laser with a side-mode suppression ratio of over 70 dB. Figure reproduced from ref. 25, © 2007 NPG. PROGRESS ARTICLE | FOCUS NATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.167 nphoton_.2010.167_AUG10.indd 512 10.7.19 8:33:28 AM © 20 Macmillan Publishers Limited. All rights reserved10 NATURE PHOTONICS | VOL 4 | AUGUST 2010 | www.nature.com/naturephotonics 515 each containing ~400 devices lying aside a portion of a 150-mm- diameter hybrid Si racetrack laser wafer58. Th anks to this reduction in size, the fabrication of millions of microring lasers on a single Si wafer is now feasible. Sub-milliampere-threshold CW lasing has been experimentally demonstrated on a hybrid iii–v-on-Si integrated platform56 simi- lar to the hybrid microring laser discussed above. By lasing inside a compact microdisk iii–v cavity and coupling to an external Si waveguide, a good overlap between the optical mode and electrical gain results in threshold currents as low as 350 μA (ref. 59). An SOI waveguide is positioned underneath the iii–v microdisk to capture a small fraction of the evanescent light vertically (Fig. 6a). Four devices with slightly diff erent cavity lengths integrated onto the same Si waveguide (Fig. 6b) will have diff erent resonance wavelengths, allow- ing such a waveguide to be used as a wavelength-division multiplex- ing source60. Th e spectrum in Fig. 6c results from combining four devices with diameters of 7.632, 7.588, 7.544 and 7.5 μm. Th e devices are individually tuned to give an even spread inside one free spectral range (~24 nm)60. However, increasing thermal impedance causes laser performance to decrease dramatically60,61 with smaller diam- eters, which is a major hurdle in the realization of compact devices. Challenges and opportunities Lasers on Si are now a reality, and the recent progress in making Si lase — regardless of the particular lasing mechanism — is exciting. Although the gain in nanoscale Si at low temperatures diminishes quickly before reaching room temperature, and despite the gen- erated photon energy being larger than or similar to the Si band- gap, steady progress is being made. Erbium-doped Si nanocrystals push emission to telecommunication wavelengths, and the use of a metal–oxide–semiconductor structure results in an electrolumi- nescence effi ciency that is comparable to commercial iii–v LEDs. As long as a good trade-off solution between effi ciency and life- time can be found, lasers on Si will fi nd applications in relatively low-speed, large-volume optical interconnects. Materials scientists are pursuing a variety of heteroepitaxy techniques to fi nd a way to reduce the dislocation density in compound semiconductors on Si to a level that is low enough for good and reliable laser perform- ance. Th e recent demonstration of optically pumped Ge-on-Si lasers is exciting, and is focusing attention on how to increase the gain to levels comparable to that of iii–v materials. Hybrid iii–v-on-Si technology currently has the most advanced devices and the most advanced photonic integrated circuits on Si. Wafer-scale iii–v epi- taxial transfer up to 150 mm in diameter58 and individual iii–v dyes attaches to larger SOI wafers62 show high-volume manufacturability. Monolithic methods are typically preferred, although the highest quality SOI wafers today are made by wafer bonding. Th e ultimate reliability, performance, uniformity and cost of the hybrid approach are still unknown. New opportunities continue to appear. Companies are now con- sidering whether optical interconnects could be a possible solution to the problems of high power consumption and low bandwidths of electrical interconnects, while also achieving smaller intercon- nect delays, lower cross-talk and better resistance to electromag- netic interference. For example, it is challenging to extend the reach of a 10 Gb s–1 copper interconnect beyond 30 cm, even with sophisticated electronic processing. Placing a wavelength-division multiplexing optical communication system in or between micro- processors allows theoretically terahertz bandwidths for on- and off -chip interconnections. Low-cost optical interconnects will be manufactured in much higher volumes than they are today when applied to diverse applications such as high-defi nition display ports, memory server interconnects and on/off -chip intercon- nects. Th e emerging market of fi bre-to-PC devices using diplex- ers and triplexers, which require the integration of lasers and photodetectors with passive multiplexers and demultiplexers, could be a more immediate application of existing technology. For example, a hybrid Si integrated triplexer containing a 1,310 nm laser for upstream data transmission as well as 1,310/1,500 nm and 1,490/1,550 nm wavelength selective splitters and photodetectors for downstream digital and video reception was recently demon- strated by transferring two types of iii–v epilayers onto a single Si chip63. Quantum-well intermixing is a promising option that avoids bonding two diff erent iii–v materials simultaneously and so has enabled integration of hybrid lasers and modulators64. Over time, hybrid photodetectors will probably be replaced by mono- lithic Ge detectors. Th e ultimate integration scheme in practical r-metal p-metal p-InP n-InPMQW Si BOX a b c 0.1 1 10 100 1 10 100 1,000 10 100 1,000 T h re sh o ld c u rr e n t (m A ) R=0.98 R=0.9 R=0.8 R=0.3 Diameter (µm) Cavity length (µm) D = 15 µm Figure 5 | Compact hybrid Si microring lasers. a, Schematic of a hybrid microring laser with a Si bus waveguide. Expanded view shows the simulated fundamental transverse electric mode shifting towards the waveguide edge. Inset: cross-sectional SEM image of a microring laser, showing the diff erent layers and metal contacts. b, Calculated threshold current as a function of device cavity length for diff erent facet refl ectivities (R). Inset: top-view SEM image of a 15-μm-diameter device. c, A portion of a 150-mm-diameter iii–v-on-SOI wafer containing ring cavity lasers (left) and 1 cm2 chips containing 400 microring lasers (right). FOCUS | PROGRESS ARTICLENATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.167 nphoton_.2010.167_AUG10.indd 515 10.7.19 8:33:30 AM © 20 Macmillan Publishers Limited. All rights reserved10 516 NATURE PHOTONICS | VOL 4 | AUGUST 2010 | www.nature.com/naturephotonics optical interconnect systems is likely to be a combination of hybrid and monolithic approaches, thus taking full advantage of iii–v-, Ge- and Si-based materials. Integration with CMOS circuits can provide low cost, integrated control, signals processing and error correction. If Si photonics is to claim a large market in intrachip optical communication links, however, power consumptions must be reduced to 2 pJ bit–1 or lower65. Silicon Raman lasers are potentially ideal light sources for a variety of wavelength-sensitive regimes, owing to their unmatched wavelength purity and the possibility of extending the lasing wavelength into the mid-infrared region66. Example applications include high-resolution and ultrasensitive detection of molecules for trace gas analysis, pollution and toxic gas monitoring, bio- medical sensing, coherent free-space optical communications and metrology25. Achieving mode-hop-free CW tuning without sacrifi cing linewidth and extinction ratio is relatively straight- forward by tuning the emission wavelength of the pump laser. Th e sensing market is currently dominated by bulky and power- hungry solid-state and gas lasers, but Si Raman lasers will be very competitive in size and cost if a pump source can be integrated. Furthermore, recently demonstrated SiGe Raman amplifi ers and lasers bring extra fl exibility in the pump and signal wavelengths67. Th e higher carrier mobility in SiGe reduces the carrier lifetime and subsequently the FCA. Lasing realized in ring or disk resonators exhibits extremely useful resonance and nonlinear eff ects such as bistability. 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Additional information Th e authors declare competing fi nancial interests: details accompany the paper at www.nature.com/naturephotonics. FOCUS | PROGRESS ARTICLENATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.167 nphoton_.2010.167_AUG10.indd 517 10.7.19 8:33:33 AM © 20 Macmillan Publishers Limited. All rights reserved10